Determination of Precise Redox Properties of Oxygen-Doped Single

Oct 28, 2015 - Single-walled carbon nanotubes doped with a limited amount of oxygen (O-doped SWNTs) are expected to be novel materials due to the appe...
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Determination of Precise Redox Properties of Oxygen-Doped SingleWalled Carbon Nanotubes Based on in Situ Photoluminescence Electrochemistry Tomonari Shiraishi,† Gergely Juhász,‡ Tomohiro Shiraki,† Naoto Akizuki,§ Yuhei Miyauchi,§ Kazunari Matsuda,§ and Naotoshi Nakashima*,†,∥ †

Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡ Department of Chemistry, Graduate School of Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550, Japan § Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan ∥ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, Japan S Supporting Information *

ABSTRACT: Single-walled carbon nanotubes doped with a limited amount of oxygen (O-doped SWNTs) are expected to be novel materials due to the appearance of red-shifted new emission and enhancement of the luminescence quantum yields compared to those of pristine SWNTs, which are of importance for the development of high performance biosensors, imaging materials, and optical devices. The appearance of the new optical properties is due to the change in the electronic states induced by the oxygen doping (Odoping) of the SWNTs, thus quantitative analysis of the electronic states of the O-doped SWNTs is crucial. In this study, we have successfully determined the precise electronic states of the O-doped SWNTs based on the in situ photoluminescence (PL) electrochemical method. The measurements revealed the presence of at least two distinct O-doping sites with unique optical and electrochemical properties for all four studied chiralities. The electrochemical measurements also showed that shifts in the valence and conduction band resulting from the O doping are on the order of 0.02−0.03 eV, which is much lower than the red shift of the photoluminescence peak. This behavior agrees with the theoretical simulations using the density functional based tight binding (DFTB) method. This study suggests that the doped sites on the SWNTs act as a neutral quantum dot trapping exciton generated on the tubes.

1. INTRODUCTION

Chemical modification of the SWNTs has been utilized by focusing on (i) modification of the electronic properties of the SWNTs,9 (ii) improved solubilization in solvents,10 and (iii) metallic and semiconducting SWNT sorting using their chemical reactivity differences.11,12 The typical modification processes have a serious drawback that is disruption of the sp2 structure and π-conjugation of the SWNTs by the chemical reactions.13 In 2010, however, Ghosh et al. reported that atomically doped SWNTs with a limited amount of oxygen (Odoped SWNTs) showed a new emission peak (E11*) with an ∼150 meV redshift compared to the original one (E11).14 Moreover, Miyauchi et al. demonstrated that the quantum yield

From the viewpoints of both fundamental and applications, considerable attention has been focused on the design of nanomaterials with unique structures and functions; especially, precise control of such nanostructures1,2 is important for the development of next-generation nanomaterials. Single-walled carbon nanotubes (SWNTs) that were made of a rolled-up graphene sheet with one-dimensional extended π-conjugated structures have outstanding electrical, mechanical, thermal, and optical properties. The electronic properties (i.e., Fermi level, band gap, and redox potentials) of the SWNTs are uniquely dominated by their structures including the tube diameters and chiral angles that are specified by a pair of integers (n,m), the so-called chiral index.3,4 The isolated SWNTs provide photoluminescence (PL) in the near-infrared (IR) region,5 which is applicable for their use as materials for biosensors,6 imaging materials,7 and optical devices.8 © XXXX American Chemical Society

Special Issue: Kohei Uosaki Festschrift Received: August 12, 2015 Revised: October 17, 2015

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V-670), PL spectrofluorometer (HOLIBA JOBIN YVON, NanoLog-3), ultrapure water system equipped with an Elix-5 kit (Millipore Co.), atomic force microscope (TOYO Technica, Agilent 5500), and potentiostat (TOHO Technical Research Co., PS-06). Preparation of O-Doped SWNTs. The O-doped SWNTs dispersed in a D2O solution were prepared by an ozone treatment.13 Briefly, the SWNTs (10 mg) were added to a D2O solution (40 mL) of a micelle solution of SDBS (0.2 wt %). The mixture was sonicated for 60 min (bath-type) and for 40 min (tip-type sonicator operating at 100 W with a 3 mm microtip) and then ultracentrifugated at 138 000g for 4 h. A 0.75 mL aliquot of ozone-containing D2O was added to the prepared SWNT dispersion (2 mL), and to complete the reactions, light was irradiated overnight on the resulting mixtures using a desk lamp (∼5 mW cm−2). As the control samples, the nondoped SWNT (pristine SWNT) dispersions were prepared by the same procedure without using the ozonedissolving D2O solution. Fabrication of a Modified Electrode. The O-doped SWNT solution (1 mL) was mixed with a D2O solution (111 μL) of Na-CMC (1 wt %) to replace the SDBS with Na-CMC, then filtered 7 times through a membrane filter (molecular weight cutoff, MWCO: 10 000) to remove any excess SDBS. In order to remove any excess Na-CMC, the O-doped SWNTs were then precipitated by ultracentrifugation at 900 000g for 1 h and redispersed in water by sonication for 1−2 min. A 50 μL aliquot of the resultant O-doped SWNT dispersion was placed on a cleaned indium tin oxide-coated transparent quartz glass (ITO) electrode followed by heating at ∼40 °C to fabricate a SWNT/Na-CMC film on the electrode. Finally, a 10 μL portion of a PDDA aqueous solution (20 wt %) was placed on the film, then rinsed with water to remove any excess PDDA to obtain the O-doped SWNT/Na-CMC/PDDA-modified ITO electrode. The pristine SWNT/Na-CMC/PDDA-modified ITO electrode was also prepared in a similar manner. In Situ PL Spectroelectrochemical Measurements. The electrolyte for the measurements was a 0.3 M NaCl aqueous solution containing 30 mM Na2HPO4 (pH 8). All the electrochemical measurements were conducted in an aqueous system using a Ag/AgCl (sat’d KCl) reference electrode and a coiled Pt wire counter electrode under an Ar atmosphere. The SWNTs/Na-CMC/PDDA-modified ITO electrode was vertically placed in a quartz cell with a 10 mm path length. The cell was filled with the electrolyte solution in which the reference and counter electrodes were placed so as not to interfere with the light path. In the fitting procedure to obtain the spectral integrated PL intensities of each peak, we analyzed the data taking into account the emergence of the charged exciton (trion) formation.26−28 Calculations. The calculations were performed using the DFTB+ program (ver 1.2) with the “mio” Slater-Koster set.29 The nanotube models were calculated as a 3D periodic system with at least a 4.0 nm separation between the tubes in the x and y directions. The size of the unit cell was optimized in the z direction. Since the typical length of the nanotubes is generally much longer than 10−100 nm, periodic calculations are a good approximation and also help to reduce any possible edge effect close to the caps of the nanotubes. For all the models, one oxygen dopant per unit cell (approximately 4 nm tube) was considered. The calculations were performed with a 1 × 1 × 4 Monkhorst−Pack grid.30 The unit cell size as well as the

of the O-doped SWNTs increased by 18 times that of the pristine SWNTs.15 These findings revealed that the new emission emergence and the quantum yield enhancement are able to be achieved by the limited chemical modification that has not yet been fully clarified. Presently, free exciton trapping on the modified sites of the O-doped SWNTs has been considered for such a phenomenon. Actually, from the emission intensity dependence of the E11* on the temperature15 and excitation light intensities16 the PL from the E11* is favorably observed in an atmosphere where the free exciton is able to migrate more freely along the nanotube axis. However, the fundamentally important issues are (i) determination of the electronic states in O-doped SWNTs and (ii) how the modified moieties become exciton trapping sites. The electronic states of the O-doped site have been investigated based on theoretical calculations.17−20 Mu et al. carried out calculations utilizing the GW method and the Bethe−Salpeter equation (GW+BSE), in which the O-doping induces an ∼10 meV shift in the electronic states, and the resulting large Stokes shift is important for the E11* emission.18 Ma et al. compared the PL of single O-doped SWNTs to that of nondoped SWNTs at low temperature and observed a wide range of shifts in the PL peaks. Using the density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, they reported a correlation between the observed multiple PL peaks and the calculated electronic states of the Odoped sites with different chemical structures.19 A very recent report of Ma et al. also suggested that similar O-doped SWNTs are capable of a single-photon generation and potentially useful for photonic devices.21 Although theoretical examinations have been carried out, there has been no report describing the experimentally determined electronic states of the O-doped SWNTs, which is crucial for a deep understanding of the unique optical properties of the O-doped SWNTs. In this study, we have succeeded in determining for the first time the precise electronic states (oxidation and reduction potentials and Fermi level) of the O-doped SWNTs based on the in situ PL spectroelectrochemical method that we previously developed to determine the precise electronic states of the SWNTs with the (n,m) chirality.22−25 This method enables us to simultaneously evaluate the electronic states of nondoped and O-doped SWNTs with different chiralities based on the analysis of the PL from E11 and E11*, respectively. In order to explain the experimental results, we carried out theoretical calculations using the density functional-based tight binding (DFTB) method. As a result, we revealed that the Odoped region becomes the exciton trapping site through potential shifting of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the SWNTs.

2. MATERIALS AND METHODS The SWNTs ((6,5)-rich CoMoCAT), sodium dodecyl benzenesulfonate (SDBS), D2O, carboxymethylcellulose sodium salt (Na-CMC, polymerization degree (n) = 600−800), and poly(diallyldimethylammonium chloride) (PDDA) aqueous solution, were purchased from SouthWest Nanotechnologies, Cambridge Isotope Laboratories, Wako Pure Chemical, Kishida Chemical, and Aldrich, respectively, and were used as received. The instruments used in this study are as follows: bath-type sonicator (AS ONE, US-1R), tip-type sonicator (MISONIX, XL-2000), ultracentrifuge (Hitachi, himac CS 100 GXL), visible-near IR absorption spectrophotometer (JASCO, B

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epoxide groups on the doped tube relate to the strongest and second strongest PLs, respectively.16 On the basis of this report, the observed peaks of E11*a and E11*b are assigned to the PLs from the ether-structured and the epoxide-structured site, respectively. In Situ PL Spectroelectrochemical Measurements of the O-Doped SWNTs. The in situ PL spectroelectrochemical measurements of the O-doped SWNTs were conducted according to our reported procedure.22 Figures S3 and S4 (Supporting Information) show the positively and negatively applied potential-dependent PL spectra, in which the spectra were obtained by excitation at 520 nm (see Supporting Information, Figures S3a and S4a), 570 nm (Figures S3b and S4b), 650 nm (Figures S3c and S4c), and 670 nm (Figures S3d and S4d). PL spectral fitting was carried out for the convoluting spectra in order to remove the effect of spectral overlap and the trion (see the Supporting Information, Figure S5). As shown in Figures S6 and S7, the PL intensity plots normalized by the initial value were well fitted by the Nernst equation, which agreed with the previous reports.22 As typical results, the change in the PL spectra at the 570 nm excitation is shown in Figure 2b−e. A similar experiment was carried out using one more sample prepared in a similar manner and confirmed the reproducibility. We thus successfully determined the oxidation and reduction potentials of the O-doped SWNTs from the potentials at the half PL intensity. Table 1 summarizes the results (average of the two samples). The obtained interesting features are as follows. (i) The HOMO and LUMO of the O-doped (n,m) SWNTs shifted in the negative and positive directions, respectively, compared to those of the corresponding (n,m) nondoped tubes, while the (n,m) chirality dependence of such shifts was not significant. (ii) The change of the optical band gaps resulting from the O-doping was greater than the change in the electrochemical band gaps. (iii) The Fermi levels were insensitive to the O-doping, and they remained almost identical after the O-doping to those of the corresponding nondoped tubes. (iv) Two distinct new redox states were observed, namely, a smaller shift for the (n,m)a SWNTs and a larger shift for the (n,m)b SWNTs. The Eox and Ered of the E11 transition of the O-doped SWNTs and the pristine SWNTs are compared (see Supporting Information, Table S1). The E11 peak of the Odoped tubes has no significant shift compared to the E11 peak of the nondoped tubes. This behavior has been previously explained by Ghosh et al.,14 and it shows that the doping sites act as neutral quantum dot trapping excitons generated on the pristine areas of the tube.15 The doping process does not affect the electronic states on the nondoped areas of the tube, and the electronic effects are significant only around the Odoping site. We found that the observed Eox and Ered were different between the E11 and E11*, which eliminates the possibility that the PL quenching of the E11* sites causes electrochemical quenching of the free excitons on the E11 sites of the O-doped SWNTs. These results suggest that our PL spectroelectrochemical method can be used to independently evaluate the electrochemical potentials on the nondoped site and the O-doped sites. The measured Eox and Ered correspond to the valence band maximum (VBM), namely, the HOMO, and the conduction band minimum (CBM), namely, the

position of the carbon atoms were optimized with a convergence criteria of 5 meV. The electronic converging criterion was 0.27 eV for the SCF cycles, and convergence could be reached in all cases without Fermi broadening (T = 0 K). Broyden mixing was applied using the mixing parameter of 0.2.31

3. RESULTS AND DISCUSSION Preparation and Characterizations of O-Doped SWNTs. The O-doped SWNTs were prepared according to a reported procedure.15 Prior to the in situ PL spectroelectrochemical measurements, the SDBS that wrapped on the Odoped SWNTs was replaced with Na-CMC which is an electrochemically inert matrix in our measurement condition, and cast on an ITO electrode to form a film, and then a cationic polymer, PDDA, was added to fabricate an ion-complexed film, which is insoluble in water.22 The exchange reaction from the SDBS to Na-CMC was confirmed by vis/near-IR absorption and PL spectroscopies; namely, a spectral redshift32 was observed in both spectra due to the change in permittivity around the O-doped SWNTs (Supporting Information, Figure S1), while the E11* emission behavior remained even after the exchange reaction, indicating a successful exchange reaction. We measured an AFM image of the O-doped SWNTs embedded in the films and found that many SWNTs are individually dispersed in the film (Supporting Information, Figure S2). The optical properties of the pristine and O-doped SWNTs on the ITO electrode were evaluated from the twodimensional (2D) PL mapping of the cast film (Figure 1), in

Figure 1. 2D-PL mapping of the film containing isolated O-doped (n,m) SWNTs/Na-CMC on an ITO electrode.

which four clear PL spots from the E11 transitions were observed, which were assigned to the SWNTs with the chiralities of (7,3) (E11: λex = 520 nm, λem = 1005 nm), (6,5) (E11: λex = 570 nm, λem = 1003 nm), (8,3) (E11: λex = 670 nm, λem = 976 nm), and (7,5) (E11: λex = 650 nm, λem = 1045 nm). The O-doped SWNTs in the film on the ITO were found to provide two new peaks that are assignable to the E11*a and E11*b. The corresponding emission peaks for each chirality are as follows: (7,3) (E11*a: λem = 1163 nm, E11*b: λem = 1279 nm), (6,5) (E11*a: λem = 1153 nm, E11*b: λem = 1273 nm), (8,3) (E11*a: λem = 1155 nm, E11*b: λem = 1275 nm), and (7,5) (E11*a: λem = 1157 nm, E11*b: λem = 1275 nm). Ma et al. reported that the PL wavelengths are determined by the chemical structures of the O-doped sites, in which the ether and C

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Figure 2. (a) Schematic illustration of the PL spectroelectrochemical measurements of the O-doped SWNTs. (b, d) Applied potential dependence of the PL spectra of the film containing isolated O-doped SWNTs on an ITO electrode and (c, e) normalized PL intensities of the (6,5) (blue lines), (6,5)*a (red lines), (6,5)*b (green lines)SWNTs. The potential was applied to the electrode by an arbitrary step (c) from 0 to 1 V vs Ag/AgCl and (e) from 0 to −1 V vs Ag/AgCl. The excitation wavelength is 570 nm. The normalized PL intensity of the (6,5)SWNTs in the (c) oxidation and (e) reduction processes is plotted as a function of the applied potential. We determined the oxidation and reduction potentials of the (6,5), (6,5)*a, and (6,5)*b SWNTs from the potentials at the half PL intensity.

Table 1. Determined Electronic Properties of the O-Doped SWNTs ((n,m)*a and (n,m)*b)a

a

For comparison, data of the nondoped (n,m) SWNTs are presented.

Figure 3. (a) Oxidation and (b) reduction potentials and (c) Fermi levels of the O-doped SWNTs of the E11 (blue), E11*a (red), and E11*b (green).

D

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and ΔEopt were 0.014 and 0.168 eV, respectively, between the E11*a and the E11, and 0.163 and 0.264 eV, respectively, between the E11*b and the E11, in which ΔEelectro was ∼0.13 eV lower than that of the ΔEopt in both the E11*a and the E11*b. On the basis of the electrochemically determined redox potentials, the observed gap difference between the E11 and the E11* could arise from the change in the electrostatic interaction of the electrons and holes of the excitons at the O-doped sites. Generally, the exciton binding energy on the SWNTs becomes a few hundred meV due to the lower shielding effect featured by the one-dimensional structure.5 On the other hand, the exciton on the O-doped SWNT is constrained at the zerodimensional (0D) doped sites, and the exciton binding energy should increase due to the enhancement of the electrostatic interaction between the electrons and holes by the spatial restriction. Theoretical Calculation. As described, the in situ PL spectroelectrochemical measurements revealed that the Odoping induces the potential shifts in the HOMO and the LUMO, which creates the exciton trapping sites showing the highly efficient emission. We also examined the observed potential changes based on theoretical simulations. The simulation models of the SWNTs were generally short tubes with a few nanometers length. However, the average distribution of the chemical modification sites of the Odoped SWNTs is one oxygen atom doping per a 20 nm tube length, whose size scale differs from the typical simulation models of the SWNTs. Therefore, we selected the density functional based tight binding (DFTB) method to calculate the O-doped SWNT models. The DFTB method is a semiempirical calculation method independently comparable to the DFT calculations but with the capability to calculate much larger systems. It has been successfully used to describe bioinorganic,34−36 catalytic, and nanocarbon systems.23,37,38 We have previously used the DFTB method to predict the redox potentials of a selection of pristine SWNTs with a wide range of diameters.23 Since realistic modeling of the doping effects requires an accurate simulation of large unit cells, the DFTB is an especially fitting method to describe oxygen doping effect on the SWNTs. To predict the redox properties, we used the band structure around the Fermi level of the (n,m) SWNTs, and we assumed a linear correlation of the energy of the CBM and VBM with the oxidation and reduction potentials, respectively. While this approach neglects several important contributions, such as reorganization energies and solvent effects, a previous study showed that important trends like family behavior could be reproduced over a wide range of nanotubes. Ghosh et al.14 have already pointed out that the ozone chemisorption may be followed by a Criegee rearrangement, and an ether-type adduct is the most probable candidate as the product of the ozonization. This reaction mechanism was later supported by the DFT-GGA level calculations of Johnson and his coworkers.39 Therefore, we compared the band structure of the pristine and O-doped (with C−O−C ether bonds) SWNTs with the chiralites of (n,m) = (6,5), (7,3), (7,5), and (8,3). The band structure of the O-doped SWNTs around the Fermi level is shown in Figure 5, with the most representative numbers in Table 2. The lowest energy conductance band and highest valence bands are doubly degenerated in the pristine SWNTs, of which the degeneracy is removed by the doping. In all the cases, the new bands shift toward lower energies. Since the shift of both the conductance and valence bands is relatively small and uniform, the smaller band gaps in the O-doped

LUMO, respectively, because the potentials were determined by the PL quenching at the transition between the first subbands of the SWNTs. Figures 3a and b show the potential diagrams of the HOMO and LUMO for the E11, E11*a, and E11*b of the O-doped (6,5) SWNTs, in which the E11*a and the E11*b showed negative shifts of 0.012 and 0.112 eV, respectively, for the HOMO and positive shifts of 0.002 and 0.051 eV, respectively, for the LUMO compared to the potential values of the E11. These results show that the O-doping on the SWNTs narrows the band gap by shifting the potential levels closer to each other. Such behavior was also observed for the O-doped SWNTs with (n,m) = (7,3), (7,5), and (8,3). The shift in the electronic states of the O-doped SWNTs would be due to the splitting of the degenerated HOMO and LUMO levels by symmetry breaking that originated from the O-doping.18 The determined potentials are assignable to the HOMO that is located at the most negative position and to the LUMO that is located at the most positive position. The present study provides direct evidence about the creation of the exciton trapping sites triggered by the O-doping-induced potential shifts, which is consistent with the reports that the exciton confinement on the doped sites may create a quantum dots-like trapping site on the SWNTs.15,16 The difference in the potential shifts of the E11*a and the E11*b is due to the structural differences in the ether and epoxide moieties that generated sp2 and sp3 hybridizations, respectively.33 A comparison between the electrochemical band gap (ΔEelectro = Eox − Ered) and the optical band gap (ΔEopt) was conducted, and the result is shown in Figure 4, in which

Figure 4. Energy shifts of the optical (upward triangles) and electrochemical (downward triangles) band gaps of the E11 (blue), E11*a (red), and E11*b (green).

ΔEelectro had a 0.2 eV lower value than the ΔEopt in the E11. We have already reported that the ΔEelectro values of the nondoped (n,m) SWNTs reflect solvation of the charged tubes, and an ∼0.2 eV difference between the ΔEelectro and ΔEopt is observed for the nondoped tubes.25 A similar solvation effect can be considered on the E11 transition for the O-doped SWNTs. For the O-doped SWNTs, the measured differences in the ΔEelectro E

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order of 0.02−0.06 eV, or if it is purely attributed to the level splitting, it is in the 0.03−0.04 eV range. In both cases, the energies are reasonably close to the experimentally observed 0.05 eV for (n,m) = (6,5), (7,3), and (7,5) but significantly underestimate the band gap change of 0.11 eV for (n,m) = (8,3). As a comparison, Wang et al.17 calculated the band structure of the pristine and O-doped (6,5) SWNT and found a red-shift of 0.09 eV, which overestimated the electrochemical data, but it is still well below the optical gap of 0.145 eV. All these calculations and the electrochemical data point in the direction that the actual redshift of the band gap is significantly smaller than the observed optical redshift and the explanation of the effect used by Ghosh et al.14 are not sufficient. Calculations of Spataru et al.40 showed that the dynamic effects from acoustic plasmons can be responsible for a significant decrease in the band gap of doped SWNTs. However, in this mechanism the decrease of the band gap would be larger than ΔEopt; therefore, it cannot fully explain our observations. Other mechanisms, such as a strong Stoke shift of adatom-related states, as suggested by Mu et al.,16 or the brightening of the dark states are the most likely explanations. More detailed research using more accurate methods to predict the optical properties of the O-doped SWNTs, including the assignation of dark and bright excitons, will be discussed in a separate manuscript in the near future.

4. CONCLUSION We have determined the precise oxidation and reduction potentials as well as Fermi levels of the O-doped SWNTs for the first time based on the analysis of the results obtained by the in situ PL spectroelectrochemistry; namely, this method allowed us to separately determine the E11 of the nondoped area and E11* of distinct O-doped sites on the O-doped SWNTs by analyzing the PL quenching induced by electrochemical oxidation and reduction based on the Nernst equation analysis. The significant advantage of the present method is that we could simultaneously observe different O-doping sites on the SWNTs with several different chiralities and could determine their electronic properties. Interestingly, we found that the O-doping induced a negative shift in the HOMO and positive shift in the LUMO of the SWNTs. The theoretical calculations using the DFTB method well explained the experimental results. Finally, we would like to emphasize that our presented method is powerful enough to determine the precise redox potentials of many functionalized SWNTs including the O-doped SWNTs, which is highly important to reveal the fundamental features of such nanomaterials.

Figure 5. Band structure of the pristine (black line) and O-doped (red line) SWNTs.

Table 2. Energy Shift and Splitting of the Frontier Bands at the Γ and Z Points as the Result of O-Dopinga (6,5) Γ conduction valence Z conduction valence

(7,3)

(7,5)

shift splitting shift splitting

−0.039 0.031 −0.009 0.034

E (eV) −0.047 −0.028 0.032 0.026 −0.003 −0.016 0.039 0.026

shift splitting shift splitting

−0.026 0.038 −0.025 0.028

0.162 0.041 −0.118 0.033

(8,3) −0.022 0.029 −0.026 0.027

−0.016 0.032 −0.029 0.022

−0.014 0.036 −0.036 0.022

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07841. Figures showing vis/near-IR absorption and PL spectra of the pristine and O-doped SWNTs before and after the exchange reaction. Applied potential dependence of the PL spectra of the film containing isolated O-doped SWNTs on an ITO electrode. Table showing the electronic properties of the pristine SWNT (PDF)

a

Splitting is defined as the splitting of the then-degenerate bands; shift is defined as the average shift in the split band compared to the pristine tube.

SWNTs are mostly the result of the splitting of the bands. The accurate calculation of the absolute band energies is a challenging task and may require the simulation of the interaction between the SWNTs and polymer-wrapping/ solvents. Therefore, it is not surprising that the individual shifts in the valence and conduction band energies are not matching with the change in the oxidation and reduction potentials. The calculated redshift of the band gaps is on the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F

DOI: 10.1021/acs.jpcc.5b07841 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Author Contributions

(15) Miyauchi, Y.; Iwamura, M.; Kawazoe, T.; Ohtsu, M.; Matsuda, K.; Mouri, S. Brightening of Excitons in Carbon Nanotubes on Dimensionality Modification. Nat. Photonics 2013, 7, 715−719. (16) Iwamura, M.; Akizuki, N.; Miyauchi, Y.; Mouri, S.; Shaver, J.; Gao, Z.; Congnet, L.; Lounis, B.; Matsuda, K. Nonlinear Photoluminescence Spectroscopy of Carbon Nanotubes with Localized Exciton States. ACS Nano 2014, 8, 11254−11260. (17) Suggs, K.; Person, V.; Wang, X.-Q. Band Engineering of Oxygen Doped Single-Walled Carbon Nanotubes. Nanoscale 2011, 3, 2465− 2468. (18) Mu, J.; Ma, Y.; Yin; Liu, H. C.; Rohlfing, M. Photoluminescence of Single-Walled Carbon Nanotubes: The Role of Stokes Shift and Impurity Levels. Phys. Rev. Lett. 2013, 111, 137401. (19) Ma, X.; Adamska, L.; Yamaguchi, H.; Yalcin, S. E.; Tretiak, S.; Doorn, S. K.; Htoon, H. Electronic Structure and Chemical Nature of Oxygen Dopant States in Carbon Nanotubes. ACS Nano 2014, 8, 10782−10789. (20) Ohfuchi, M. Ab Initio Study on Oxygen Doping of (5,4), (6,4), (6,5), and (8,6) Carbon Nanotubes. J. Phys. Chem. C 2015, 119, 13200−13206. (21) Ma, X.; Hartmann, N. F.; Baldwin, J. K. S.; Doorn, S. K.; Htoon, H. Room-temperature Single-photon Generation from Solitary Dopants of Carbon Nanotubes. Nat. Nanotechnol. 2015, 10, 671−675. (22) Tanaka, Y.; Hirana, Y.; Niidome, Y.; Kato, K.; Saito, S.; Nakashima, N. Experimentally Determined Redox Potentials of Individual (n,m) Single-Walled Carbon Nanotubes. Angew. Chem., Int. Ed. 2009, 48, 7655−7659. (23) Hirana, Y.; Juhász, G.; Miyauchi, Y.; Mouri, S.; Matsuda, K.; Nakashima, N. Empirical Prediction of Electronic Potentials of SingleWalled Carbon Nanotubes with a Specific Chirality (n,m). Sci. Rep. 2013, 3, 2959. (24) Hirana, Y.; Tanaka, Y.; Niidome, Y.; Nakashima, N. Strong Micro-Dielectric Environment Effect on the Band Gaps of (n,m)Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2010, 132, 13072−13075. (25) Hirana, Y.; Niidome, Y.; Nakashima, N. Effect of Charge of a Matrix Polymer on the Electronic States of Single-Walled Carbon Nanotubes. Bull. Chem. Soc. Jpn. 2012, 12, 1262−1267. (26) Lampert, M. A. Mobile and Immobile Effective-Mass-Particle Complexes in Nonmetalic Solids. Phys. Rev. Lett. 1958, 1, 450−453. (27) Matsunaga, R.; Matsuda, K.; Kanemitsu, Y. Observation of Charged Excitons in Hole-Doped Carbon Nanotubes Using Photoluminescence and Absorption Spectroscopy. Phys. Rev. Lett. 2011, 106, 037404. (28) Park, J. S.; Hirana, Y.; Mouri, S.; Miyauchi, Y.; Nakashima, N.; Matsuda, K. Observation of Negative and Positive Trions in the Electrochemically Carrier-Doped Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2012, 134, 14461−14466. (29) Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, Th.; Suhai, S.; Seifert, G. Self-consistent-charge Densityfunctional Tight-binding Method for Simulations of Complex Materials Properties. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 7260−7267. (30) Monkhorst, H. J.; Pack, J. D. Special points for Brillonin-zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (31) Johnson, D. D. Modified Broyden’s Method for Accelerating Convergence in Self-consistent Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 38, 12807−12813. (32) Kato, Y.; Inoue, A.; Niidome, Y.; Nakashima, N. Thermodynamics on Soluble Carbon Nanotubes: How Do DNA Molecules Replace Surfactants on Carbon Nanotubes? Sci. Rep. 2012, 2, 733. (33) Piao, Y.; Meany, B.; Powell, L. R.; Valley, N.; Schatz, G. C.; Wang, Y.; et al. Brightening of Carbon Nanotube Photoluminescence through the Incorporation of sp3 Defects. Nat. Chem. 2013, 5, 840− 845. (34) Zheng, G.; Lundberg, M.; Jakowski, J.; Vreven, T.; Frisch, M. J.; Morokuma, K. Implementation and Benchmark Tests of the DFTB Method and its Application in the ONIOM Method. Int. J. Quantum Chem. 2009, 109, 1841−1854.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Grants-in-Aid for Science Research (Scientific Research on Innovative Areas, No. 26620069 for NN) and Nanotechnology Platform Project (Molecules and Materials Synthesis) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and by The Japan Science and Technology Agency (JST) through its Center of Innovation Science and Technology based Radical Innovation and Entrepreneurship Program (COI Program).

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REFERENCES

(1) Fujigaya, T.; Nakashima, N. Fuel Cell Electrocatalyst Using Polybenzimidazole-Modified Carbon Nanotubes As Support Materials. Adv. Mater. 2013, 25, 1666−1681. Fujigaya, T.; Nakashima, N. Noncovalent Polymer Wrapping of Carbon Nanotubes and the Role of Wrapped Polymers as Functional Dispersants. Sci. Technol. Adv. Mater. 2015, 16, 024802. (2) Ariga, K.; Yamauchi, Y.; Rydzek, G.; Ji, Q.; Yonamine, Y.; Wu, K. C.-W.; Hill, J. P. Layer-by-layer Nanoarchitectonics: Invention, Innovation, and Evolution. Chem. Lett. 2014, 43, 36−68. (3) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (4) Hamada, N.; Sawada, S.; Oshiyama, A. New One-dimensional Conductors: Graphitic Microtubules. Phys. Rev. Lett. 1992, 68, 1579− 1581. (5) Wang, F.; Dukovic, G.; Heinz, L. E. The Optical Resonances in Carbon Nanotubes Arise from Excitons. Science 2005, 308, 838−841. (6) Li, C.; Shi, G. Carbon Nanotube-based Fluorescence Sensors. J. Photochem. Photobiol., C 2014, 19, 20−34. (7) Welsher, K.; Liu, Z.; Sherlock, S. P.; Robinson, J. T.; Chen, Z.; Daranciang, D.; Dai, H. J. A Route to Brightly Fluorescent Carbon Nanotubes for Near-infrared Imaging in Mice. Nat. Nanotechnol. 2009, 4, 773−780. (8) Avouris, P.; Freitag, M.; Perebinos, V. Carbon-nanotube Photonics and Optoelectronics. Nat. Photonics 2008, 2, 341−350. (9) Banerjee, S.; Hermraj-Benny, T.; Wong, S. S. Covalent Surface Chemistry of Single-Walled Carbon Nanotubes. Adv. Mater. 2005, 17, 17−29. (10) Kumar, I.; Rana, S.; Cho, J. W. Cycloaddition Reactions: A Controlled Approach for Carbon Nanotube Functionalization. Chem. Eur. J. 2011, 17, 11092−11101. (11) Toyoda, S.; Yamaguchi, Y.; Hiwatashi, M.; Tomonari, Y.; Murakami, H.; Nakashima, N. Separation of Semiconducting Single-Walled Carbon Nanotubes by Using a Long-Alkyl-Chain Benzenediazonium Compound. Chem. - Asian J. 2007, 2, 145−149. (12) Do, Y.-J.; Lee, J.-H.; Choi, H.; Han, J.-H.; Chung, C.-H.; Jeong, M.-G.; Strano, M. S.; Kim, W.-J. Manipulating Electron Transfer between Single-Walled Carbon Nanotubes and Diazonium Salts for High Purity Separation by Electronic Type. Chem. Mater. 2012, 24, 4146−4151. (13) Qin, S.; Qin, D.; Ford, W. T.; Herrera, J. E.; Resasco, D. E.; Bachilo, S. M.; Weisman, R. B. Solubilization and Purification of Single-Wall Carbon Nanotubes in Water by in Situ Radical Polymerization of Sodium 4-Styrenesulfonate. Macromolecules 2004, 37, 3965−3967. (14) Ghosh, S.; Bachilo, S. M.; Simonette, R. A.; Beckingham, K. M.; Weisman, R. B. Oxygen Doping Modifies Near-Infrared Band Gaps in Fluorescent Single-Walled Carbon Nanotubes. Science 2010, 330, 1656−1659. G

DOI: 10.1021/acs.jpcc.5b07841 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (35) Hou, G.; Cui, Q. QM/MM Analysis Suggests That Alkaline Phosphatase (AP) and Nucleotide Pyrophospha-tase/Phosphodiesterase Slightly Tighten the Transition State for Phos-phate Diester Hydrolysis Relative to Solution: Implication for Catalytic Promiscuity in the AP Superfamily. J. Am. Chem. Soc. 2012, 134, 229−246. (36) Hou, G.; Cui, Q. Stabilization of Different Types of Transition States in a Single Enzyme Active Site: QM/MM Analysis of Enzymes in the Alkaline Phosphatase Superfamily. J. Am. Chem. Soc. 2013, 135, 10457−10469. (37) Ohta, Y.; Okamoto, Y.; Irle, S.; Morokuma, K. Rapid Growth of a Single-Walled Carbon Nanotube on an Iron Cluster: DensityFunctional Tight-Binding Molecular Dynamics Simulations. ACS Nano 2008, 2, 1437−1444. (38) Lin, C. S.; Zhang, R. Q.; Niehaus, T. A.; Frauenheim, Th. Geometric and Electronic Structures of Carbon Nanotubes Adsorbed with Flavin Adenine Dinucleotide: A Theoretical Study. J. Phys. Chem. C 2007, 111, 4069−4073. (39) Yim, W.-L.; Johnson, J. K. Ozone Oxidation of Single Walled Carbon Nanotubes from Density Functional Theory. J. Phys. Chem. C 2009, 113, 17636−17642. (40) Spataru, C. D.; Lèonard, F. Tunable Band Gaps and Excitons in Doped Semiconducting Carbon Nanotubes Made Possible by Acoustic Plasmons. Phys. Rev. Lett. 2010, 104, 177402.

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DOI: 10.1021/acs.jpcc.5b07841 J. Phys. Chem. C XXXX, XXX, XXX−XXX