1. Introduction
The rediscovery of carbon nanotubes (Iijima, 1991) has inspired extensive research activity. These materials have extremely high surface areas, large aspect ratios, remarkably high mechanical strength, and can have electrical and thermal conductivities that are similar to that of copper (Ebbesen et al., 1996). They come in two forms: single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). SWNTs have diameters ranging from 1.2 to 1.4 nm. MWNTs have larger overall diameters, with sizes depending on the number of concentric walls within the structure. Like graphite, carbon nanotubes are relatively non-reactive, except at the nanotube caps which are more reactive due to the presence of dangling bonds. The reactivity of the carbon nanotube side walls’ π-system can also be influenced by tube curvature or chirality (Okpalugo et al., 2005). In particular, their remarkable structure-dependent properties have attracted great attention due to their potential applications in heterogeneous catalysis (Planeix et al., 1994), use as substrates for destruction of cancer cells (Kam et al., 2005) and applications for biological and chemical sensing (Poh et al., 2004). Carbon nanotubes require chemical modification in aqueous solution environments to make them more amenable for attachment of reactive surface species. In the case of attaching metal nanoparticles to the carbon surface, functionalization is necessary to avoid agglomeration of the metal. Sensor applications involve the tethering of chemical moieties with specific recognition sites for the detecting ultra-trace analytes (Dai, 2002). Surface functionalization is also necessary for depositing high-loading, catalytically active metal nanoparticles on them (Xing et al, 2005).
Great attention has been paid to attaching functional groups onto carbon nanotube surfaces (Holzinger et al., 2001; Kim et al., 2004; Chen et al., 2005; Park et al., 2006) and probing the electronic structure resulting from post-nanotube-synthesis preparations. To understand the changes that result from surface functionalization strategies, well-defined characterization of the carbon nanotube’s surface chemistry and structure is needed. The ability to get an accurate detailed picture of the tethered functional groups that attach to the solid surface using aqueous solution preparation methods is important for controlling carbon nanotube surface composition composition.
We have developed an array of analytical methods to probe the surface composition of carbon nanotubes during various stages of nanomaterial synthesis in our laboratory. Summarized herein are three case studies. In the first study, sonochemically functionalized MWNTs were probed by X-ray photoelectron spectroscopy (XPS) revealing a consecutive, first-order attachment mechanism. In the second study, extended X-ray absorption fine structure (EXAFS) and attenuated total reflection infrared (ATR-IR) spectroscopy were used to examine tethered Pt nanoparticles on functionalized MWNTs. In the third study, we functionalized high pressure carbon monoxide (HiPco) SWNTs to produce carboxylic acid (COOH-SWNT), maleic anhydride (MA-SWNT), and nitroso (NO-SWNT) attached SWNTs in order to examine the effects of the tethered groups on the solid surface point-of-zero charge (PZC). The PZC is defined as the aqueous solution pH value at which the degree of surface protonation and hydroxylation are equal, which results in an electrostatically neutral charge at the electrical double layer interface (Brown et al., 1999). SWNTs were used in the PZC studies due to their relative ease for surface functionalization with specific moieties.
2. Experimental
In the first case study, MWNTs produced from chemical vapor deposition were obtained from Nanolab, Inc. (Waltham, MA). The as-purchased MWNTs (95% purity, ~30 nm in diameter) were put into a mixture solution of HNO3 and H2SO4 in an Erlenmeyer flask. The concentrations of both acids were 8.0 M. The flask was placed in an ultrasonic bath (Fisher Scientific, 130 W and 40 kHz) maintained at 60º C. Sonication was performed for 1, 2, 4 and 8 hrs. The sonochemically treated MWNTs were then separated from the acids in a centrifuge (Thermal IEC Centra CL2), and thoroughly washed using doubly distilled, deionized water prior to analysis (Xing et al., 2005).
The chemical oxidation states and surface compositions of the resulting sonochemically treated MWNTs and Pt electrocatalysts were analyzed by XPS using an ion-pumped Perkin-Elmer PHI ESCA 560 system using a PHI 25-270AR double pass cylindrical mirror analyzer. An Mg Kα anode operated at 15 kV and 250 W with photon energy of
In the second case study, MWNT-Pt nanoparticle structural analysis was performed using EXAFS. Finely dispersed Pt nanoparticles (3.5 nm in diameter) tethered onto MWNTs were prepared via sonicating MWNTs in HNO3/H2SO4 for 2 hrs followed by reducing the Pt salt precursor, K2PtCl4 (Xing, 2004). Spectra were obtained from the 12-BM BESSRC Advanced Photon Source (APS) beamline at the Argonne National Laboratory and the X18B beamline at the National Synchrotron Light Source (NSLS) at the Brookhaven National Laboratory to analyze the Pt LIII edge (11.564 keV) of the Pt nanoparticles tethered to the carbon nanotube surface. A spectrum of a 5
The experimental procedure for the third case study (McPhail et al., 2009) was as follows: (1) COOH-SWNTs were prepared by refluxing in H2SO4/HNO3 according to Lu et al. (2007). A 30.0 mL solution of 3:1 concentrated nitric-to-sulfuric acid ratio was added to a 100-mL round-bottom glass flask along with 60.1 mg of p-SWNTs. The mixture was refluxed at 338 K for 12 hrs, with constant magnetic stirring, under N2 atmosphere. (2) NO-SWNTs were prepared using an electrochemical functionalization procedure based on the description made by Wang et al. (2005). SWNT sheets were prepared by sonicating them in 1% Triton X-100 (The Chemistry Store.com Inc.; St. Cayce, SC) solution followed by vacuum filtration with Millipore Teflon filter paper (0.2
Isoelectric point measurements at the solid-liquid interface were made on MA-SWNT, NO-SWNT, COOH-SWNT and p-SWNT surfaces using a method described by Park and Regalbuto (1995). Twelve solutions in the range of pH = 1.0-12.0 were made using dilute aqueous solutions of NaOH and HCl. A 1.8 mL aliquot of each solution was pipetted into polyethylene vials and allowed to equilibrate for 1 hr. The initial pH of each solution was then recorded. A 2.0 mg amount of the SWNTs to be examined were added to each vial, which were then capped and shaken with a vortex mixer to settle the SWNTs. After an additional 12-hr equilibration period, the final pH at the SWNT solid surface was measured for each vial using a spear-tip semisolid electrode. Finally, initial pH values versus final pH values were plotted.
3. Results and discussion
The sonochemically functionalized MWNTs were characterized and quantified by XPS. XPS is an effective surface sensitive method for quantifying the extent (or level) of surface oxidation (Huefner, 2003). The distribution of oxygen containing functional groups (-C-O-, -C=O, and O-C=O) is also often characterized by deconvoluting the C 1s spectral envelope to obtain quantitative information, based on differences in XPS binding energy (BE) (Datsyuk et al., 2008).
Fig. 1 (left-hand panel) shows the narrow scan spectra of the C 1s region of sonochemically treated and untreated MWNTs. The XPS spectrum shows distinct carbon peaks, representing the major constituents of the oxidized MWNT surface. The dominant peak structure for the C 1s core level at a BE of 284.4 eV corresponds to the bare, untreated MWNT surface (Ago et al., 1999; Suzuki et al., 2002). C 1s core level shifts at 287.6 and 288.3 eV indicate that the moieties consist of CO-/C=O and COO- respectively, in agreement with literature values reported for these groups tethered onto the MWNTs (Langley et al., 2005). Intensities of the high BE states increased due to oxidation as sonication ensued. The CO-/C=O and COO- concentrations were quantified relative to the graphitic carbon peak. The C 1s line broadening with extra feature developments were attributed to the surface oxidation of MWNTs where C atoms bond to more O atoms as a result of the sonochemical treatment. The population of the oxidized groups (CO-, C=O, and COO-) relative to the MWNT carbon were quantified via plotting the sum of their C 1s peak areas relative to that of the graphitic MWNT carbon as a function of sonochemical treatment time. The increase in surface oxidation measured from the integrated C 1s peak areas of the ([CO-] + [COO-])/[C] tracks well with the overall increase in XPS atomic percent oxygen. A greater uptake of oxygen by the surface carbon atoms corresponds to a higher population density of COx functional groups detected by the XPS.
The kinetic model for the oxidation process is shown in Fig. 1 (right-hand panel). A stochastic addition mechanism obeying a consecutive 1st-order mechanism was revealed. Here, we report the first detailed mechanistic delineation of the carbon nanotube oxidation process. Evolution of the high binding energy peak intensities during sonication shows a consecutive, single-step first order O-attachment mechanism, leading to the carboxylate. This scheme is consistent with a report made by (Chiang et al., 2011) showing CO to be an intermediate species, which could be oxidized quickly to other forms, usually COO under acidic environment. Sonication creates defect sites on the sidewalls that allow for O atom attachment (Li et al., 2006). Differential equations describing the mechanism are as follows:
Least squares fittings show rate constants of k1 (C
In examining the Raman D-to-G integrated peak area ratios (Fig. 2A; left-hand axis), the disordered sp3 state increased with longer sonochemical treatment. The largest increase occurred between 0 and 1 hr of sonication with a plateau reached at 2 hrs. Noteworthy is the fact that the plateau of the relative Raman D-to-G band intensities (left-hand axis) coincided with a plateau of the atomic percent mole fractions of oxygen (right-hand axis), obtained from normalizing XPS high-resolution energy scans of the O 1s core level (Fig. 2A; right-hand axis), at 2 hrs. The population of sp3-hybridized carbon increased relative to the sp2-hybridized carbon during sonication, accompanying the creation of sidewall defects to which the functional groups attached. Thus, the groups covalently bonded to the surface with moieties directly forming from C atoms within the graphene sheets. The growth rate of sp3-to-sp2 Raman intensities with sonication time (Fig. 2B) was also consistent with that of our consecutive 1st-order kinetic model. The density of the surface functional groups was directly (albeit not linearly) related to sonication time; 2 hrs of sonication resulted in optimal Pt nanoparticle dispersion. Upon deposition of the Pt nanoparticles, the Raman line shapes and relative D-to-G band intensities remained unchanged. The presence of these peaks verified that the carbon nanotubes remained largely intact during the oxidation procedure and after deposition of Pt nanoparticles.
To examine the local structure of the nanoparticles, EXAFS was performed on the Pt deposited on the 2-hr sonochemically treated carbon nanotubes. The Pt-CNT samples were examined as a dry powder-like form instead of aqueous solution phase to get a stronger signal. EXAFS is an oscillatory feature in the X-ray absorption above the absorption edge of the target atoms and is defined as the fraction deviation in the absorption coefficient:
with
The XPS Pt 4f7/2 core level of the Pt-CNT (not shown; prepared using a 2-hr sonochemical treatment), referencing the graphitic C 1s orbital at a BE equal to 284.4 eV (Ago et al., 1999; Suzuki et al., 2002), had a BE= 71.4 eV, indicating that Pt was predominantly in the metallic (zero) oxidation state (Fleisch et al., 1986). XPS signals from the C 1s and O 1s and Pt 4f levels and from no other elements were observed. The asymmetry observed in the 4f5/2 level at ~78 eV indicated a small population of PtO or PtO2, which was masked by much larger signal from metallic Pt. The lack of insufficient signal from the Pt oxide (PtOx) hampered precise determination of the stoichiometric proportions of PtO and PtO2. Hence, EXAFS was needed for clearer structural elucidation.
In comparing the FTs of the EXAFS Pt LIII edge of Pt nanoparticles deposited on the –COO- and –C=O functionalized MWNTs, the first nearest neighbor atom was observed at ~1.78 Å in the Pt-MWNT sample instead of the expected distance of ~2.78 Å for Pt-Pt interactions in its zero oxidation state (Fig. 3A). The latter distance was observed for a standard PtO2 powder used for comparison. This result was consistent with the XPS core level shift for the Pt 4f7/2 orbital observed at 71.4 eV, denoting metallic Pt (Fleisch et al., 1986). The low
clearly not with Pt-Pt, denoted by the ~1.78 Å position. A FT of a reference PtO2 powder is shown in Fig. 3B. The location of its 1NN, signifying Pt-O, is seen at
Fig. 4 shows ATR-IR difference spectra of 2 hr sonochemically treated carbon nanotubes before and after Pt nanoparticles were tethered to these surfaces. The C-O ester features, denoted by peaks (2) and (3) in the 2-hr sonicated MWNTs with no Pt deposited, were replaced by a broad single band with a center at 1092 cm-1 after Pt nanoparticle attachment. This change in IR envelope shape indicated a strong interaction of ester O with the Pt nanoparticles. The carbonyl O band at 1700 cm-1 (before Pt nanoparticle deposition) were replaced by two peaks absorbing at 1712 and 1629 cm-1, indicative of Pt nanoparticles interactions with carbonyl O. Pt binding with the carbonyl O was evident from the absorbance shift from a single feature at 1700 cm-1 to two peaks at 1712 and 1629 cm-1. Bands from the ester C-O stretches were still present with vibrational stretches at 1160 cm-1. From Fig. 4, it was clear that the Pt loading of the oxidized MWNTs dramatically altered the absorbance signal from the C-O stretches in the 1300-to-900 cm-1 region. The carbonyl C=O signal at 1700 cm-1 was less affected although there was a shift to higher frequency at 1712 cm-1 along with the emergence of another stretch at 1629 cm-1, indicative of multiple binding sites for the Pt nanoparticles. Hence, ATR-IR data showed that the ester O peaks present before tethering Pt nanoparticles were radically altered after the Pt deposition, denoting their involvement in the coordination of the Pt nanoparticles to create the nanostructure. Based on this IR result and the EXAFS analysis, we propose two Pt-MWNT surface structures. Attachment can occur via carboxylate ions in which the O atoms effectively have equal bond order and participation in the Pt binding in the form of COO(Pt) (Fig. 3a). Pt nanoparticles can also coordinate to ester O atoms bound to the carbon nanotube surface, bridging between two carbons and serving as a binding site for the Pt nanoclusters in the form of C(=O)CO(Pt) (Fig. 3b). According to Petroski and El-Sayed (2003), because the d band of Pt is close to the Fermi level, electron density to form new bonds would come from the C=O group rather than the Pt. Hence, shifts in the C=O stretch would be sensitive to coordination with Pt (peak 1 in Fig. 4) as observed.
In our final case study, variations in the measured PZC were seen between differently functionalized SWNT structures (McPhail et al., 2009). Fig. 5 shows a plot of final versus initial pH values of solutions to which various SWNT samples were added. A plateau (horizontal dashed lines) in the plot indicates the PZC for each specifically-functionalized carbon nanotube.
The PZC values in this series of functionalized carbon nanotubes indicated a relatively acidic surface, amenable for adsorption of anionic (metal nanoparticle) precursors. The PZC values for the SWNTs were in ascending order: COOH-SWNTs (1.2) < MA-SWNTs (2.0) < p-SWNTs (3.5) < NO-SWNTs (7.5). Lowering of the p-SWNTs PZC compared to other studies (Matarredona et al., 2003) was attributed to our use of smaller radius (~0.7 nm) SWNTs. The COOH groups, due to its acidity, lowered the PZC to a greater extent than the MA groups (by 0.8 pH units). The PZCs were found to be tunable within 6.3 pH units by functionalizing them with various moieties of different electron withdrawing/donating character. The moieties markedly affected the PZCs. There is an obvious correlation of PZC with electron distribution, emanating from attached moieties along the SWNTs sidewalls.
In the context of electrophilic aromatic substitution (EAS) reactions, nitroso groups are known to be electron withdrawing, maleic anhydride groups are lightly electron releasing, and carboxylic acid groups are strongly electron releasing, which can be quantitatively described by Hammett sigma constants (
Here, we note a new observation: greater σ values coincide with a greater propensity to be hydroxylated, thereby increasing the PZC. The greater electron donating character of the moiety led to an increased degree of surface hydroxylation. Quantitatively, the
4. Conclusions
In summary, we have demonstrated the utility of XPS for delineating MWNT oxidation kinetics, EXAFS (coupled with XPS and ATR-IR) for elucidating nanoparticle-MWNT interfacial structure, and the dependence of PZC on the electron withdrawing/donating character of moieties attached to SWNTs. Sonication of MWNTs is a facile functionalization technique as it lowers the surface activation energy barrier resulting in low temperature functionalization and reduction in surface physical damage. The process greatly reduces the functionalization time to as low as 2 hrs. Sonochemical treatments tend to create dangling bonds on the surfaces of carbon nanotubes, which progressively oxidize to hydroxyl (OH), carbonyl (CO), and carboxyl (COOH) functional groups (Al-Aqtash and Vasiliev, 2009). Kinetic studies uncovered a stochastic functionalization mechanism involved in the preparation of MWNTs for nanoparticle attachment. EXAFS, coupled with XPS and ATR-IR data, was pivotal in the elucidation of ester-like O atoms found to play an important role in synthesizing Pt nanoparticle-MWNT structures. Controlled surface functionalization on SWNTs can influence its PZC, an important variable for Coulombic attachment of structures onto the surface. The above described surface analytical methods, performed on MWNTs and SWNTs as benchmarks, may well be applicable for examining aqueous solution functionalization processes on newly emerging carbon nanomaterials, i.e., graphene and graphene oxides (Liu et al., 2008; Geim, 2009; Yan and Chou, 2010), for advanced technological applications.
Acknowledgments
We gratefully acknowledge support from the Faculty Research Creative Activity Committee (FRCAC) of Middle Tennessee State University awarded in 2011.
References
- 1.
Ago H. Kugler T. Cacialli F. Salaneck W. R. Shaffer M. S. P. Windle A. H. Friend R. H. 1999 Work Functions and Surface Functional Groups of Multiwall Carbon Nanotubes.,103 8116 8121 . - 2.
Al-Aqtash N. Vasiliev I. Y. 2009 Ab initio Study of Carboxylated Graphene. J. Phys. Chem. C,113 12970 12975 . - 3.
Brown G. E. Jr Henrich V. E. Casey W. H. Clark D. L. Eggleston C. Felmy A. Goodman D. W. Grätzel M. Maciel G. E. Mc Carthy M. I. Nealson K. Sverjensky D. A. Toney M. F. Zachara J. M. 1999 Chemical Interactions of Metal Oxide-Aqueous Solution Interfaces," ,99 77 174 . - 4.
Brukh R. Mitra S. 2007 Kinetics of Carbon Nanotube Oxidation. ,17 619 623 . - 5.
Carey F. A. 2002 , 4th ed.; Kluwer Academics/Plenum Publishers: New York. - 6.
Chen G. X. Kim H. S. Park B. H. Yoon J. S. 2005 Controlled Functionalization of Multiwalled Carbon Nanotubes with Various Molecular-Weight Poly (l-lactic acid). ,109 22237 22243 . - 7.
Chiang Y. C. Lin W. H. Chang Y. C. 2011 The Influence of Treatment Duration on Multi-walled Carbon Nanotubes Functionalized by H2SO4/HNO3 Oxidation. Appl. Surf. Sci.,257 2401 2410 . - 8.
Dai H. 2002 Carbon Nanotubes: Synthesis, Integration, and Properties. ,35 1035 1044 . - 9.
Datsyuk V. Kalyva M. Papagelis K. Parthenios J. Tasis D. Siokou A. Kallitsis I. Galiotis C. 2008 Chemical Oxidation of Multiwalled Carbon Nanotubes. ,46 833 840 . - 10.
Dewar M. J. S. Pyron R. S. 1970 Nature of the Transition State in Some Diels-Alder Reactions. ,92 3098 3103 . - 11.
Ebbesen T. W. Lezec H. J. Hiura H. Bennett J. W. Ghaemi H. F. Thio T. 1996 Electrical Conductivity of Individual Carbon Nanotubes. ,382 54 56 . - 12.
Fleisch T. H. Mains G. J. 1986 Photoreduction and Reoxidation of Platinum Oxide Surfaces. ,90 5317 5320 . - 13.
Frenkel A. I. Hills C. W. Nuzzo R. G. 2001 A View from the Inside: Complexity in the Atomic Scale Ordering of Supported Metal Nanoparticles. ,105 12689 12703 . - 14.
Geim A. K. 2009 Graphene: Status and Prospects. ,324 1530 1534 . - 15.
Hansch C. Leo A. Taft R. W. 1991 A Survey of Hammett Substituent Constants and Resonance and Field Parameters. v.,91 165 195 . - 16.
Holzinger M. Vostrowsky O. Hirsch A. Hennrich F. Kappes M. Weiss R. Jellen F. 2001 Sidewall Functionalization of Carbon Nanotubes. .,40 4002 4005 . - 17.
Huefner S. 2003 Springer: Berlin. - 18.
Hull R. V. Li L. Xing Y. Chusuei C. C. 2006 Pt Nanoparticle Binding on Functionalized Multiwalled Carbon Nanotubes. ,18 1780 1788 . - 19.
Iijima S. 1991 Helical Microtubules of Graphitic Carbon. ,354 56 58 . - 20.
Kam N. W. S. O’Connell M. Wisdom J. Dai H. 2005 Carbon Nanotubes as Multifunctional Biological Transporters and Near-infrared Agents for Selective Cancer Cell Destruction.” ,102 11600 11605 . - 21.
Kim B. Sigmund W. M. 2004 Functionalized Multiwall Carbon Nanotube/Gold Nanoparticle Composites. ,20 8239 8242 . - 22.
Kojima I. Miyazaki E. Iwao Y. 1982 Field Emission Study of VIII Transition Metals. III. Adsorption of Ethylene and Acetylene on Platinum. , 10,27 41 . - 23.
Lamber R. Jaeger N. I. 1993 Electron Microsopy Study of the Interaction of Nickel, Palladium and Platinum with Carbon. III: Formation of a Substitutional Platinum-carbide in Ultrafine Platinum Particles. ,289 247 254 . - 24.
Langley L. A. Villanueva D. E. Fairbrother D. H. 2005 Quantification of Surface Oxides on Carbonaceous Materials. ,18 169 178 . - 25.
Li J. L. Kudin K. N. Mc Allister M. J. Prud’homme R. K. Aksay I. A. Car R. 2006 Oxygen-Driven Unzipping of Graphitic Materials. , 96, 176101. - 26.
Linert W. Lukovits I. 2007 Aromaticity of Carbon Nanotubes. ,47 887 890 . - 27.
Liu L. Ryu S. Tomasik M. R. . Stolyarova E. Jung N. Hybertsen M. S. Steigerwald M. L. Brus L. E. Flynn G. W. 2008 Graphene Oxidation: Thickness-Dependent Etching and Strong Chemical Doping. Nano Lett.,8 1965 1970 . - 28.
Lu X. Imae T. 2007 Size-Controlled In Situ Synthesis of Metal Nanoparticles on Dendrimer-Modified Carbon Nanotubes. J. Phys. Chem. C,111 2416 2420 . - 29.
Lukovits I. Kármán F. Nagy P. M. Kálmán E. 2007 Aromaticity of Carbon Nanotubes. ,80 233 237 . - 30.
Matarredona O. Rhoads H. Li Z. Harwell J. H. Balzano L. Resasco D. E. 2003 Dispersion of Single-Walled Carbon Nanotubes in Aqueous Solutions of the Anionic Surfactant NaDDBS. ,107 13357 13367 . - 31.
Mawhinney D. B. Naumenko V. Kuznetsova A. Yates J. John T. 2000 Infrared Spectral Evidence for the Etching of Carbon Nanotubes: Ozone Oxidation at 298 K. ,122 2383 2384 . - 32.
Mc Phail M. R. Sells J. A. He Z. Chusuei C. C. 2009 Charging Nanowalls: Adjusting the Carbon Nanotube Isoelectric Point via Surface Chemical Functionalization. ,113 14102 14109 . - 33.
Mercuri F. Sgamellotti A. 2009 First-principles Investigations on the Functionalization of Chiral and Non-chiral Carbon Nanotubes by Diels-Alder Cycloaddition Reactions. ,11 563 567 . - 34.
Newville M. 2001 IFEFFIT: Interactive XAFS Analysis and FEFF Fitting. ,8 322 324 . - 35.
Okpalugo T. I. T. Papakonstantinou P. Murphy H. Mc Laughlin J. Brown N. M. D. 2005 High Resolution XPS Characterization of Chemical Functionalised MWCNTs and SWCNTs. ,43 153 161 . - 36.
Park J. Regalbuto J. R. 1995 A Simple, Accurate Determination of Oxide PZC and the Strong Buffering Effect of Oxide Surfaces at Incipient Wetness. ,175 239 252 . - 37.
Park M. J. Lee J. K. Lee B. S. Lee Y. W. Choi I. S. Lee S.-g. 2006 Covalent Modification of Multiwalled Carbon Nanotubes with Imidazolium-Based Ionic Liquids: Effect of Anions on Solubility. ,18 1546 1551 . - 38.
Petroski J. El -Sayed M. A. 2003 FTIR Study of the Adsorption of the Capping Material to Different Platinum Nanoparticle Shapes. ,107 8371 8375 . - 39.
Planeix J. M. Coustel N. Coq B. Brotons V. Kamblar P. S. Dutartre R. Geneste P. Bernier P. Ajayan P. M. 1994 Application of Carbon Nanotubes as Supports in Heterogeneous Catalysis. .,116 7935 7936 . - 40.
Poh W. C. Loh K. P. Zhang W. D. Sudhiranjan Ye J. S. Sheu F. S. 2004 Biosensing Properties of Diamond and Carbon Nanotubes. ,20 5484 5492 . - 41.
Ravel B. Newville M. 2005 ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-ray Absorption Spectroscopy Using IFEFFIT. ,12 537 541 . - 42.
Lee S. et al. 2010 Characterization of Multi-walled Carbon Nanotubes Catalyst Supports by Point of Zero Charge, ,doi:10.1016/j.cattod.2010.10.031 - 43.
Suzuki S. Watanabe Y. Ogino T. Heun S. Gregoratti L. Barinov A. Kaulich B. Kiskinova M. Zhu W. Bower C. Zhou O. 2002 Electronic Structure of Carbon Nanotubes Studied by Photoelectron Spectromicroscopy. , 66, 035414-1-035414-4. - 44.
Wang Y. Malhotra S. V. Owens F. J. Iqbal Z. 2005 Electrochemical Nitration of Single-wall Carbon Nanotubes. ,407 68 72 . - 45.
Xing Y. 2004 Synthesis and Electrochemical Characterization of Uniformly-Dispersed High Loading Pt Nanoparticles on Sonochemically-Treated Carbon Nanotubes,108 19255 19259 . - 46.
Xing Y. Li L. Chusuei C. C. Hull R. V. 2005 Sonochemical Oxidation of Multiwalled Carbon Nanotubes. ,21 4185 4190 . - 47.
Zhang J. Hongling Z. Qing Q. Yang Y. Li Q. Liu Z. Guo X. Du Z. 2003 Effect of Chemical Oxidation on the Structure of Single-Walled Carbon Nanotubes. ,107 3712 3718 . - 48.
Zhang Y. Toebes M. L. van der Eerden A. O’Grady W. E. de Jong K. P. Koningsberger . D. C. 2004 Formation, Characterization, and Magnetic Properties of Fe3O4 Nanowires Encapsulated in Carbon Microtubes. ,108 18509 18519 . - 49.
Zorbas V. Smith A. L. Xie H. Ortiz-Acevedo A. Dalton A. B. Dieckmann G. Draper R. K. Baughman R. H. Musselman I. H. 2005 Importance of Aromatic Content for Peptide/Single-walled Carbon Nanotube Interactions. ,127 12323 12328 .