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Fabrication and characterization of oriented carbon atom wires assembled on gold
Chemical Physics Letters 469 (2009) 284–288
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Fabrication and characterization of oriented carbon atom wires assembled on gold Kuan-Hong Xue a,*, Shao-Peng Chen a, Lin-Xia Wang a, Ri-Bing Wei a, Shi-Min Xu b, Li Cui b, Bin-Wei Mao b, Zhong-Qun Tian b, Chun-Hua Zen b, Shi-Gang Sun b, Li-Jun Wu c, Yi-Mei Zhu c a
Chemistry Department, Nanjing Normal University, 122 NingHai Road, Nanjing 210097, China State Key Laboratory of Physical Chemistry of Solid Surfaces & College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China c Brookhaven National Laboratory, Upton, NY 11973, USA b
a r t i c l e
i n f o
Article history: Received 27 November 2008 In final form 20 December 2008 Available online 27 December 2008
a b s t r a c t Carbon atom wires (CAWs) are of the sp-hybridized allotrope of carbon. To augment the extraordinary features based on sp-hybridization, we developed an approach to make CAWs be self-assembled and orderly organized on Au substrate. The self-assembling process was investigated in situ by using scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). The properties of the assembled film were characterized by voltammetry, Raman spectroscopy, electron energy loss spectroscopy (EELS), and the contact angle measurements. Experimental results indicated that the assembled CAW film was of the good structural integrity and well organized, with the sp-hybridized features enhanced. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Although the sp-hybridized allotrope of carbon has long been known [1–4], progress of its research is slow due to lack of an available method to obtain pure samples on a large scale. Anyway, intrigued by the extraordinary properties arising from the molecular structure of sp-hybridization, researchers have been making continuous efforts to explore its synthesis and the possible applications of the linear carbon [5–12]. Earlier, we reported a new method capable of massively producing linear carbon allotrope from pyrolysis of potato starch [13]. To highlight the characteristic morphology of the linear shape and the size of the diameters on an atomic scale, and to avoid the argument about whether the linear chain is constructed from cumulative double bonds (@C@C@)n, or alternative single- and triple-bonds (–C„C–)n, we termed our product carbon atom wires (CAWs) rather than using published nomenclature, such as ‘carbyne’, ‘polyyne’, ‘alkynes’ or ‘poliynes’. We found the CAW modified electrode to possess some extraordinary properties, such as remarkable catalytic activity in the redox of some biological molecules, like dopamine, ascorbic acid, and adrenaline, etc. [14]. The pristine CAWs produced by pyrolysis are random, tangled wires. It is known that their amazing features based on sp-hybridization, can be augmented [15] and utilized in some attractive applications, such as molecular electronic devices [16,17], if their carbon chains are organized and oriented. Kavan et al. synthesized partly oriented polyyne by electrochemical reduction of poly(tetrafluoethylene) spatially aligned by mechanical stretching or friction deposition. They found that the molecular
* Corresponding author. Fax: +86 25 83598448. E-mail address: [email protected] (K.-H. Xue). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.12.075
chains of treated polyyne had larger conjugation length than those obtained from ordinary non-oriented precursors [18]. Here, we report an approach to orient CAWs by use of a technique to fabricate a self-assembled monolayer (SAM) [19–22]. Among various SAMs, those derived from alkanethiols or dialkyl disulfides chemisorbed on gold(0) surface in a solution have been the most extensively investigated due to the convenience of the operation and the good quality of the resulting film [23–27]. To add the mercapto-group on to their carbon chains, CAWs were treated with concentrated nitric acid to produce the carboxylic-acid group which could condense selectively with the amino group of b-aminoethanethiol, present in excess, to yield the CAW-thiol, as in the following scheme
HOOC—ðCÞn —COOH þ 2NH2 ðCH2 Þ2 SH
CH2 Cl2 ;NðEtÞ3
!
DCC; sufficient time
HSðCH2 Þ2 HNOC—ðCÞn
—CONHðCH2 Þ2 SH þ 2H2 O
ð1Þ
or
HOOC—ðCÞn —COOH þ NH2 ðCH2 Þ2 SH —ðCÞn —CONHðCH2 Þ2 SH þ H2 O
CH2 Cl2 ;NðEtÞ3
!
DCC; limited time
HOOC ð2Þ
where DCC (N,N0 -dicyclohexyl carbodiimide) was used to promote the condensation reaction, while CH2Cl2 and N(Et)3 (triethylamine) acted as a solvent, and to maintain the alkalinity of the reaction system, respectively [28]. The self-assembling process of CAW-thiol was investigated in situ by using scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). The properties of the assembled film were characterized by
K.-H. Xue et al. / Chemical Physics Letters 469 (2009) 284–288
voltammetry, Raman spectroscopy, electron energy loss spectroscopy (EELS), and the contact angle measurements.
2. Experimental 2.1. Preparation 2.1.1. Carbon atom wires (CAWs) Carbon atom wires were prepared by the pyrolysis of potato starch in the presence of a ferric metal as a catalyst within a tube furnace at 500–800 °C in a stream of Ar and H2 [13]. The product was treated with concentrated HNO3 to remove the catalyst and oxidize CAWs, producing carboxylic-acid groups at the end of carbon chain. 2.1.2. CAW-thiol The CAW-thiol connected with ASH groups at both ends of the carbon chain was synthesized as Eq. (1) by a condensation reaction in a 50 mL flask, equipped with a reflux condenser, containing 10 mg of HNO3-treated CAWs, 20 mg 2-aminoethyl mercaptan, 10 ml CH2Cl2, 1 ml triethyl amine and 0.2 g DCC (N,N0 -dicyclohexyl carbodiimide) at 60 °C. The condensation reaction continued for about 24 h until there was no band at 1725 cm1 on the FT-IR spectrum, which corresponds to the C@O stretching mode in the carboxylic-acid group, in the sample. To remove the excess reactants, byproducts, and other impurities, the product was filtered and ultrasonically washed in absolute ethanol for 4–5 times until the solution became neutral. When the condensation reaction was stopped at 6 h after starting, less than half of all the carboxylic-acid groups had time to complete the condensation, and thereby another kind of CAW-thiol with only one ASH group connected to the end of carbon chain was obtained, as shown in Eq. (2). The two kinds of CAW-thiols have different surface properties when they are self-assembled on gold (shown later). To obtain a powder product, the final filter mass was dried in a vacuum oven at 40 °C for 12 h. To prepare an aqueous solution for self-assembly, the final filter mass was put into 10 ml doubly distilled water, treated ultrasonically for half an hour, allowed to settle quiescently for 12 h, and then centrifuged in 10 000 rpm for 20 min. The upper clear solution after centrifugation was taken for self-assembling.
285
(1 1 1) facets immediately. At all potentials of Au (1 1 1) electrode, the tip’s potential was kept at 150 mV to prevent its oxidization. 2.2.3. EQCM An electrochemical quartz crystal microbalance (EQCM) of QCA917 SEIKO EG&G was used to monitor the mass change at an electrode of quartz crystal covered with 300 nm-thick deposit of gold during the assembling process. 2.2.4. Voltammetry Electrochemical measurements were performed in an undivided cell with three electrodes by using CHI Model 440B (CH Instruments, Shanghai). A Pt coil and the saturated calomel electrode (SCE) were used as the counter- and reference-electrodes, respectively. The working electrode was of gold. 2.2.5. EELS EELS spectra were acquired in the diffraction mode with the JEOL 3000F transmission electron microscope, equipped with a field emission gun and a Gatan energy filter, at the accelerating voltage of 300 kV, with a collection angle of 4 mrad and 0.2 eV per pixel. To prepare a sample for the measurement, a gold film was deposited on a single-crystal silicon (1 0 0) wafer by argonion sputtering and then annealed with a hydrogen flame to produce a contaminant-free reconstructed Au (1 1 1) surface. The gold coated silicon wafer was kept in a clear CAW-thiol solution for 24 h, and, the assembled film formed was detached together with the gold layer from the silicon wafer by treating the sample with concentrated HNO3, and then, put on a free Cu grid without any supporting film. The EELS was taken at the area of the free CAWthiol film without gold attached. 2.2.6. Raman spectroscopy Raman spectra were recorded with Labram HR800 from France Jobin Yvon, using an Ar ion laser of 514 nm as the exciting radiation with the data-acquisition time of 600 s. The grating and slit width were 600 grooves/mm and 200 lm, respectively. 2.2.7. Contact angle Drop Shape Analysis System DSA 100 from KRÜSS GmbH was used for measuring the contact angle.
2.2. Characterization 3. Results and discussion 2.2.1. IR FT-IR spectra were taken in 32 scans by the spectrograph Nexus 670 FT-IR (Nicolet) with a resolution of 4 cm1. A CAW sample was ground with 100 times its bulk of pure potassium bromide (KBr) and pressed into a disk for the measurement. To remove moisture, the sample and KBr were dried in a vacuum oven at 150 °C for 48 h. 2.2.2. STM Measurements with the electrochemical scanning tunneling microscopy (STM) were carried out on Nanoscope IIIa from USA Digital Instrument with A scanner using a constant current mode. The tip was made of tungsten etched by 2 M KOH solution. All images were acquired from atomically flat Au (1 1 1) facets of a small bead produced by melting a gold wire that was welded to a gold plate serving as the working electrode. A gold ring was used as the counter electrode and a mini Ag/AgCl (sat. KCl) electrode as the reference electrode. The initial potential of Au (1 1 1) electrode was controlled at 200 mV and the Au (1 1 1) reconstruction surface was clearly seen. When the CAW-thiol solution was dropped into the 50 mM KClO4 solution, the electrode potential changed to +200 mV and we noted that the CAW-thiol adsorbed onto Au
3.1. Preparation for CAW-thiol Fig. 1 shows the infrared spectra (IR) of CAWs in powders before (a), and, after (b) reacting with excess b-aminoethanethiol for 24 h. On both of the curves, the band at either 1725 cm1 (curve a) or 1606 cm1 (curve b) is assigned to the C@O stretching mode, but the former belongs to the carboxylic-acid group and the latter to amide, while the band at 2572 cm1 of curve b refers to SH stretching [29]. These IR spectra demonstrate the realization of a condensation reaction between the CAWs and b-aminoethanethiol, with the formation of CAW-thiol, as expressed in Eq. (1). 3.2. Self-assembling process To investigate the self-assembling process of CAW-thiol in situ, an electrochemical quartz crystal microbalance (EQCM) and a scanning tunneling microscope (STM) were applied to measure the mass of assembled film and examine the surface morphology of a gold electrode, as illustrated in Fig. 2, at different times during the assembly. By use of STM, we found that some stripes appeared almost immediately after the CAW-thiol solution was added, and
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gradually became spots, as shown in Fig. 2B. The appearance change of CAW-thiol assembled on the Au (1 1 1) electrode can be illustrated by the following presumption: At the beginning in the assembling process, there was much free space and therefore the linear assembled CAW-thiol molecules were lay horizontally on the electrode surface, and so, they appeared in strips as in Fig. 2A. As more and more CAW-thiol was assembled, there were fewer and fewer unoccupied areas on the surface, so that the assembled molecules gradually were forced to stand vertically, and consequently, only the ends of the molecular chains could be sighted as in Fig. 2B. From the size of stripes and spots, it was proposed that the CAW-thiol molecules were clustered into bundles. By using an EQCM, the mass change Dm of a gold coated quartz crystal, i.e., the mass of assembled film was obtained from the following Sauerbrey equation [30],
Df ¼ S Dm Fig. 1. Infrared spectra of CAWs before (a) and after (b) the reaction with excess baminoethanethiol for 24 h.
the stripe numbers increased quickly on the surface of the Au (1 1 1) electrode later on. After assembling for 22 min, the electrode surface was covered by the stripes almost completely, as shown in Fig. 2A. Then, with the time passing, the appearance of stripes
ð3Þ
where Df was the frequency change of the quartz crystal, while S was a proportionality constant, equal to 5.45 ng cm2 Hz1 for the electrode used in the research. In the EQCM measurement, the crystal resonant frequency decreases quickly at the first stage of assembling, then, much more slowly starting from 200 min, and after 14 h of assembly, the resonant frequency change Df basically becomes constant at 114 Hz, indicating 621 ng cm2 of CAW-thiol assembled.
Fig. 2. STM images on the surface of an Au (1 1 1) electrode at 200 mV (vs. Ag/AgCl, saturated KCl) in 50 mM KClO4 solution containing CAW-thiol after the self-assembling of CAW-thiol for 22 min (A), and, 3 h 12 min (B).
K.-H. Xue et al. / Chemical Physics Letters 469 (2009) 284–288
3.3. Characterization of the assembled film Electrochemical measurements, such as those of heterogeneous electron-transfer rate and differential capacitance are often applied to characterize the properties of the assembled films [19,31]. Exploring the property change on the surface of a gold electrode at different times during the assembly of CAW-thiol, we measured cyclic voltammograms (CVs), as shown in Fig. 3A, in 1.0 M KCl containing an electrochemical probe species 1.0 mM FeðCNÞ3 6 with the scan rate of 100 mV s1, and obtained the capacitances (shown in Fig. 3B) calculated from CVs (not presented) in a blank solution only containing 1.0 M KCl with the scan rates of 10, 50, and 100 mV s1 respectively. In Fig. 3, the peak current (Fig. 3A) and capacitance (Fig. 3B) drop sharply in the first 100 s of assembling, revealing facilitation of the process at the stage. Although the quantity of CAW-thiol assembled at the early assembling time was very small, indicated by the EQCM measurement, the electrode’s surface was almost completely covered by a thin layer of molecules that effectively hindered the passage of electric current because they lay on the gold surface, as shown in Fig. 2A. From Fig. 3, electron-transfer was substantially blocked after 5 h assembly as the assembled molecules stood vertically upwards (see Fig. 2B), and the assembled film was more densely packed; thus, the rate of assembly slowed down, verified by the EQCM measurements as well. After 24 h, the Faradaic current, capacitance (Fig. 3) and the resonant frequency change of the quartz crystal of EQCM
287
remained about constant, implying no more unoccupied positions left and a densely packed assembled film formed on the gold surface. At the peak current potential Ep,c 0.208 V for the couple 2 FeðCNÞ3 6 =FeðCNÞ6 in Fig. 3, the Faradaic reduction current at the gold electrode assembled for 24 h, is less by a factor of 2.5 103 than that at 0 h, i.e., bare gold, and the capacitance at 24 h is independent of the scan rate (10, 50, and 100 mV s1, in Fig. 3B) and electrolyte (1.0 KCl and HClO4, not shown). All these experimental results imply that the CAW film assembled on the gold electrode for 24 h are well organized and to block the penetration of the probe ions almost completely. Raman spectroscopy is sensitive to the hybridization state in carbonaceous materials and often used to identify the sp bonding structure of linear carbon allotropy [4,32–34]. Fig. 4 shows the Raman spectra of pristine pyrolytic product in powder (a), and, thiolCAWs in an assembled film (b) and an aqueous solution (c). On curve b, the Raman band around 2000 cm1, referring to the stretching mode of C„C and indicating the presence of sp-hybridization, is remarkably enhanced, while the D (around 1391 cm1) and G (around 1579 cm1) bands, typical features of amorphous sp2/sp3 carbon, are much weaker than the curves of a and c respectively. These properties suggest that the self-assembling process makes CAWs display the characteristics arising from the sphybridization bonding structure more remarkable than the case before the treatment. This inference was confirmed by the findings from electron energy loss spectroscopy (EELS) measurements depicted in Fig. 5. Fig. 5 shows the EELS spectra of assembled CAW-thiol, in which the band around 21.8 eV at the low-energy region (Fig. 5A), coming from the plasma resonance of all valence electrons (p + r plasmon), and the bands around 280.9, 292.3, and 318.7 eV at the K-shell excitation region (Fig. 5B), resulting from excitation of electrons in the ground-state 1s core levels to the vacant p* and r* antibonding states, shift respectively to lower energy values than the corresponding values of graphite. These changes are thought to be the characteristics of linear carbon allotropy [4,35]. 3.4. Designing of the film surface The surface of assembled film can be designed to have various properties by connecting different functional groups at the end of carbon chain of molecules assembled. This tactics can be illustrated
Fig. 3. (A) Cyclic voltammograms in 1.0 M KCl containing 1:0 mM FeðCNÞ3 with 6 the scan rate of 100 mV s1, and, (B) the capacitance calculated from CVs with the 1 scan rate of 10, 50, and 100 mV s in 1.0 M KCl, on a gold electrode at different times of the assembling of CAW-thiol.
Fig. 4. Raman spectra of a pristine pyrolytic product in powder (a), and, thiol-CAWs in an assembled film (b) and aqueous solution (c).
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4. Conclusion We developed an approach to construct an assembled film of carbon atom wires, another allotropic form of carbon based on sp-hybridization. The assembled CAW film produced by the approach was well organized and its special features arising from the sp-hybridized structure were enhanced. The surface of assembled film could be designed to have various properties by connecting different functional groups at the end of carbon chain of the molecules assembled. The linear carbon is of an one-dimensional conjugated system with plenty of delocalized electrons along its molecular chain. It can be utilized as the molecular wire in molecular electronic devices and thus, the approach reported has the potential application in many fields [4,12,16]. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 20473039) and State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University, China) (No. 200405). Work at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Science, under Contract No. DE-AC02-98CH10886. We thank Siber Hegner China (A Division of Siber Hegner Ltd.) for the contact angle measurements. References [1] [2] [3] [4]
Fig. 5. EELS spectra of the low-energy region (A), and, the K-shell excitation region (B) taken at the assembled CAW film.
[5] [6] [7] [8] [9] [10] [11] [12] [13]
through a contact angle measurement in which a static water droplet (2 lL) dropped on the surfaces of self-assembled CAW-thiols prepared by the condensation, depicted in Section 2.1, for 24 h and 6 h, and the optical-contact-angle records of 101.5° and 62.2° were obtained respectively. The experimental result shows that the surface of the self-assembled CAW-thiol prepared by the condensation for 24 h is hydrophobic, while that of 6 h condensation is hydrophilic. Why is the surface property of these two kinds of film so different? It is because that when the amount of b-aminoethanethiol used is in excess and the reaction time is sufficiently long in the preparation of CAW-thiol, the condensation reaction will follow chemical Eq. (1), and, the ASH groups are attached at the both ends of carbon chains. During the assembling process of the CAW-thiol prepared by the condensation for 24 h, one ASH group, in each carbon chain, forms the SAAu on the Au substrate, while another ASH group remains free, oriented upward on the assembled film, forming a hydrophobic surface. The presence of ASH group on the surface of the assembled CAW-thiol film is clearly shown by the band around 2572 cm1 s on curve b of Fig. 1. When the reaction time for condensation is less than 6 h, less than a half of ACOOH groups could have time to react with b-aminoethanethiol, i.e., the reaction taking place in the system in the case will follow Eq. (2), rather than Eq. (1). Thus, free ACOOH rather than free ASH is left on the surface of assembled CAW-thiol film, which can be conformed by the presence of the band around 1725 cm1 and absence of the band around 2572 cm1 on its IR spectrum, hereby forming a hydrophilic surface.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
[30] [31] [32] [33] [34] [35]
A.G. Whittaker, P.L. Kintner, Science 165 (1969) 589. R.B. Heimann, J. Kleiman, N.M. Salansky, Nature 306 (1983) 164. P.P.K. Smith, Peter R. Buseck, Science 229 (1985) 485. R.B. Heimann, S.E. Evsyukov, L. Kavan (Eds.), Carbyne and Carbynoid Structures, Kluwer Academic Press, Dordrecht, 1999. R.J. Lagow et al., Science 267 (1995) 362. L. Kavan, Chem. Rev. 97 (1997) 3061. F. Cataldo, Carbon 42 (2004) 129. H. Tabata, M. Fujii, S. Hayashi, Carbon 44 (2006) 522. R.H. Baughman, Science 312 (2006) 1009. A. Sun, J.W. Lauher, N.S. Goroff, Science 312 (2006) 1030. R.E. Martin, F. Diederich, Angew. Chem., Int. Ed. 38 (1999) 1350. S. Eisler, A.D. Slepkov, E. Elliott, T. Luu, R. McDonald, F.A. Hegmann, R.R. Tykwinski, J. Am. Chem. Soc. 127 (2005) 2666. K.-H. Xue, F.-F. Tao, W. Shen, Ch.-J. He, Q.-L. Chen, L.-J. Wu, Y.-M. Zhu, Chem. Phys. Lett. 385 (2004) 477. K.-H. Xue, J.-M. Liu, R.-B. Wei, S.-P. Chen, Chem. Phys. 327 (2006) 319. C.S. Casari et al., Appl. Phys. Lett. 90 (2007) 013111. Z. Crljen, G. Baranovic, Phys. Rev. Lett. 98 (2007) 116801. J.G. Kushmerick, J. Naciri, J.C. Yang, R. Shashidhar, Nano Lett. 3 (2003) 897. L. Kavan, J. Hlavaty´, J. Kastner, H. Kuzmany, Carbon 33 (1995) 1321. M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 (1987) 3559. G.M. Whitesides, P.E. Laibinis, Langmuir 6 (1990) 87. A. Ulman, Chem. Rev. 96 (1996) 1533. J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chem. Rev. 105 (2005) 1103. R.G. Nuzzo, D.L. Allara, J. Am. Chem. Soc. 105 (1983) 4481. P.E. Laibinis, G.M. Whitesides, D.L. Allara, Y.-T. Tao, A.N. Parikh, R.G. Nuzzo, J. Am. Chem. Soc. 113 (1991) 7152. S.-W. Tam-Chang, H.A. Biebuyck, G.M. Whitesides, N. Jeon, R.G. Nuzzo, Langmuir 11 (1996) 4371. N. Nishi, D. Hobara, M. Yamamoto, T. Kakiuchi, J. Chem. Phys. 118 (2003) 1904. G.G. Baralia, A. Pallandre1, B. Nysten, A.M. Jonas, Nanotechnology 17 (2006) 1160. Z.F. Liu, Z. Shen, T. Zhu, S.F. Hou, L.Z. Ying, Z.J. Shi, Z.N. Gu, Langmuir 16 (2000) 3569. E. Pretsch, T. Clerc, J. Seibl, W. Simon, Tables of Spectral Data for Structure Determination of Organic Compounds (Translated from the German by K. Biemann), The second English edn., Springer-Verlag, Berlin Heidelberg, 1989 (I5-I280). T.W. Schneider, D.A. Buttry, J. Am. Chem. Soc. 115 (1993) 12391. M. Yuan, S. Zhan, X. Zhou, Y. Liu, L.Y. Lin, Z. Zhang, J. Hu, Langmuir 24 (2008) 8707. A. Lucotti et al., Chem. Phys. Lett. 417 (2006) 78. H. Tabata, M. Fujii, S. Hayashi, Chem. Phys. Lett. 420 (2006) 166. J. Kastner, H. Kuzmany, L. Kavan, F.P. Dousek, J. Kiirti, Macromolecules 28 (1995) 344. V.V. Korshak, Y.P. Kudryavtsev, V.V. Khvostov, M.B. Guseva, V.G. Babaev, O.Y. Rylova, Carbon 25 (1987) 735.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Fabrication and characterization of oriented carbon atom wires assembled on gold Kuan-Hong Xue a,*, Shao-Peng Chen a, Lin-Xia Wang a, Ri-Bing Wei a, Shi-Min Xu b, Li Cui b, Bin-Wei Mao b, Zhong-Qun Tian b, Chun-Hua Zen b, Shi-Gang Sun b, Li-Jun Wu c, Yi-Mei Zhu c a
Chemistry Department, Nanjing Normal University, 122 NingHai Road, Nanjing 210097, China State Key Laboratory of Physical Chemistry of Solid Surfaces & College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China c Brookhaven National Laboratory, Upton, NY 11973, USA b
a r t i c l e
i n f o
Article history: Received 27 November 2008 In final form 20 December 2008 Available online 27 December 2008
a b s t r a c t Carbon atom wires (CAWs) are of the sp-hybridized allotrope of carbon. To augment the extraordinary features based on sp-hybridization, we developed an approach to make CAWs be self-assembled and orderly organized on Au substrate. The self-assembling process was investigated in situ by using scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). The properties of the assembled film were characterized by voltammetry, Raman spectroscopy, electron energy loss spectroscopy (EELS), and the contact angle measurements. Experimental results indicated that the assembled CAW film was of the good structural integrity and well organized, with the sp-hybridized features enhanced. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Although the sp-hybridized allotrope of carbon has long been known [1–4], progress of its research is slow due to lack of an available method to obtain pure samples on a large scale. Anyway, intrigued by the extraordinary properties arising from the molecular structure of sp-hybridization, researchers have been making continuous efforts to explore its synthesis and the possible applications of the linear carbon [5–12]. Earlier, we reported a new method capable of massively producing linear carbon allotrope from pyrolysis of potato starch [13]. To highlight the characteristic morphology of the linear shape and the size of the diameters on an atomic scale, and to avoid the argument about whether the linear chain is constructed from cumulative double bonds (@C@C@)n, or alternative single- and triple-bonds (–C„C–)n, we termed our product carbon atom wires (CAWs) rather than using published nomenclature, such as ‘carbyne’, ‘polyyne’, ‘alkynes’ or ‘poliynes’. We found the CAW modified electrode to possess some extraordinary properties, such as remarkable catalytic activity in the redox of some biological molecules, like dopamine, ascorbic acid, and adrenaline, etc. [14]. The pristine CAWs produced by pyrolysis are random, tangled wires. It is known that their amazing features based on sp-hybridization, can be augmented [15] and utilized in some attractive applications, such as molecular electronic devices [16,17], if their carbon chains are organized and oriented. Kavan et al. synthesized partly oriented polyyne by electrochemical reduction of poly(tetrafluoethylene) spatially aligned by mechanical stretching or friction deposition. They found that the molecular
* Corresponding author. Fax: +86 25 83598448. E-mail address: [email protected] (K.-H. Xue). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.12.075
chains of treated polyyne had larger conjugation length than those obtained from ordinary non-oriented precursors [18]. Here, we report an approach to orient CAWs by use of a technique to fabricate a self-assembled monolayer (SAM) [19–22]. Among various SAMs, those derived from alkanethiols or dialkyl disulfides chemisorbed on gold(0) surface in a solution have been the most extensively investigated due to the convenience of the operation and the good quality of the resulting film [23–27]. To add the mercapto-group on to their carbon chains, CAWs were treated with concentrated nitric acid to produce the carboxylic-acid group which could condense selectively with the amino group of b-aminoethanethiol, present in excess, to yield the CAW-thiol, as in the following scheme
HOOC—ðCÞn —COOH þ 2NH2 ðCH2 Þ2 SH
CH2 Cl2 ;NðEtÞ3
!
DCC; sufficient time
HSðCH2 Þ2 HNOC—ðCÞn
—CONHðCH2 Þ2 SH þ 2H2 O
ð1Þ
or
HOOC—ðCÞn —COOH þ NH2 ðCH2 Þ2 SH —ðCÞn —CONHðCH2 Þ2 SH þ H2 O
CH2 Cl2 ;NðEtÞ3
!
DCC; limited time
HOOC ð2Þ
where DCC (N,N0 -dicyclohexyl carbodiimide) was used to promote the condensation reaction, while CH2Cl2 and N(Et)3 (triethylamine) acted as a solvent, and to maintain the alkalinity of the reaction system, respectively [28]. The self-assembling process of CAW-thiol was investigated in situ by using scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). The properties of the assembled film were characterized by
K.-H. Xue et al. / Chemical Physics Letters 469 (2009) 284–288
voltammetry, Raman spectroscopy, electron energy loss spectroscopy (EELS), and the contact angle measurements.
2. Experimental 2.1. Preparation 2.1.1. Carbon atom wires (CAWs) Carbon atom wires were prepared by the pyrolysis of potato starch in the presence of a ferric metal as a catalyst within a tube furnace at 500–800 °C in a stream of Ar and H2 [13]. The product was treated with concentrated HNO3 to remove the catalyst and oxidize CAWs, producing carboxylic-acid groups at the end of carbon chain. 2.1.2. CAW-thiol The CAW-thiol connected with ASH groups at both ends of the carbon chain was synthesized as Eq. (1) by a condensation reaction in a 50 mL flask, equipped with a reflux condenser, containing 10 mg of HNO3-treated CAWs, 20 mg 2-aminoethyl mercaptan, 10 ml CH2Cl2, 1 ml triethyl amine and 0.2 g DCC (N,N0 -dicyclohexyl carbodiimide) at 60 °C. The condensation reaction continued for about 24 h until there was no band at 1725 cm1 on the FT-IR spectrum, which corresponds to the C@O stretching mode in the carboxylic-acid group, in the sample. To remove the excess reactants, byproducts, and other impurities, the product was filtered and ultrasonically washed in absolute ethanol for 4–5 times until the solution became neutral. When the condensation reaction was stopped at 6 h after starting, less than half of all the carboxylic-acid groups had time to complete the condensation, and thereby another kind of CAW-thiol with only one ASH group connected to the end of carbon chain was obtained, as shown in Eq. (2). The two kinds of CAW-thiols have different surface properties when they are self-assembled on gold (shown later). To obtain a powder product, the final filter mass was dried in a vacuum oven at 40 °C for 12 h. To prepare an aqueous solution for self-assembly, the final filter mass was put into 10 ml doubly distilled water, treated ultrasonically for half an hour, allowed to settle quiescently for 12 h, and then centrifuged in 10 000 rpm for 20 min. The upper clear solution after centrifugation was taken for self-assembling.
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(1 1 1) facets immediately. At all potentials of Au (1 1 1) electrode, the tip’s potential was kept at 150 mV to prevent its oxidization. 2.2.3. EQCM An electrochemical quartz crystal microbalance (EQCM) of QCA917 SEIKO EG&G was used to monitor the mass change at an electrode of quartz crystal covered with 300 nm-thick deposit of gold during the assembling process. 2.2.4. Voltammetry Electrochemical measurements were performed in an undivided cell with three electrodes by using CHI Model 440B (CH Instruments, Shanghai). A Pt coil and the saturated calomel electrode (SCE) were used as the counter- and reference-electrodes, respectively. The working electrode was of gold. 2.2.5. EELS EELS spectra were acquired in the diffraction mode with the JEOL 3000F transmission electron microscope, equipped with a field emission gun and a Gatan energy filter, at the accelerating voltage of 300 kV, with a collection angle of 4 mrad and 0.2 eV per pixel. To prepare a sample for the measurement, a gold film was deposited on a single-crystal silicon (1 0 0) wafer by argonion sputtering and then annealed with a hydrogen flame to produce a contaminant-free reconstructed Au (1 1 1) surface. The gold coated silicon wafer was kept in a clear CAW-thiol solution for 24 h, and, the assembled film formed was detached together with the gold layer from the silicon wafer by treating the sample with concentrated HNO3, and then, put on a free Cu grid without any supporting film. The EELS was taken at the area of the free CAWthiol film without gold attached. 2.2.6. Raman spectroscopy Raman spectra were recorded with Labram HR800 from France Jobin Yvon, using an Ar ion laser of 514 nm as the exciting radiation with the data-acquisition time of 600 s. The grating and slit width were 600 grooves/mm and 200 lm, respectively. 2.2.7. Contact angle Drop Shape Analysis System DSA 100 from KRÜSS GmbH was used for measuring the contact angle.
2.2. Characterization 3. Results and discussion 2.2.1. IR FT-IR spectra were taken in 32 scans by the spectrograph Nexus 670 FT-IR (Nicolet) with a resolution of 4 cm1. A CAW sample was ground with 100 times its bulk of pure potassium bromide (KBr) and pressed into a disk for the measurement. To remove moisture, the sample and KBr were dried in a vacuum oven at 150 °C for 48 h. 2.2.2. STM Measurements with the electrochemical scanning tunneling microscopy (STM) were carried out on Nanoscope IIIa from USA Digital Instrument with A scanner using a constant current mode. The tip was made of tungsten etched by 2 M KOH solution. All images were acquired from atomically flat Au (1 1 1) facets of a small bead produced by melting a gold wire that was welded to a gold plate serving as the working electrode. A gold ring was used as the counter electrode and a mini Ag/AgCl (sat. KCl) electrode as the reference electrode. The initial potential of Au (1 1 1) electrode was controlled at 200 mV and the Au (1 1 1) reconstruction surface was clearly seen. When the CAW-thiol solution was dropped into the 50 mM KClO4 solution, the electrode potential changed to +200 mV and we noted that the CAW-thiol adsorbed onto Au
3.1. Preparation for CAW-thiol Fig. 1 shows the infrared spectra (IR) of CAWs in powders before (a), and, after (b) reacting with excess b-aminoethanethiol for 24 h. On both of the curves, the band at either 1725 cm1 (curve a) or 1606 cm1 (curve b) is assigned to the C@O stretching mode, but the former belongs to the carboxylic-acid group and the latter to amide, while the band at 2572 cm1 of curve b refers to SH stretching [29]. These IR spectra demonstrate the realization of a condensation reaction between the CAWs and b-aminoethanethiol, with the formation of CAW-thiol, as expressed in Eq. (1). 3.2. Self-assembling process To investigate the self-assembling process of CAW-thiol in situ, an electrochemical quartz crystal microbalance (EQCM) and a scanning tunneling microscope (STM) were applied to measure the mass of assembled film and examine the surface morphology of a gold electrode, as illustrated in Fig. 2, at different times during the assembly. By use of STM, we found that some stripes appeared almost immediately after the CAW-thiol solution was added, and
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gradually became spots, as shown in Fig. 2B. The appearance change of CAW-thiol assembled on the Au (1 1 1) electrode can be illustrated by the following presumption: At the beginning in the assembling process, there was much free space and therefore the linear assembled CAW-thiol molecules were lay horizontally on the electrode surface, and so, they appeared in strips as in Fig. 2A. As more and more CAW-thiol was assembled, there were fewer and fewer unoccupied areas on the surface, so that the assembled molecules gradually were forced to stand vertically, and consequently, only the ends of the molecular chains could be sighted as in Fig. 2B. From the size of stripes and spots, it was proposed that the CAW-thiol molecules were clustered into bundles. By using an EQCM, the mass change Dm of a gold coated quartz crystal, i.e., the mass of assembled film was obtained from the following Sauerbrey equation [30],
Df ¼ S Dm Fig. 1. Infrared spectra of CAWs before (a) and after (b) the reaction with excess baminoethanethiol for 24 h.
the stripe numbers increased quickly on the surface of the Au (1 1 1) electrode later on. After assembling for 22 min, the electrode surface was covered by the stripes almost completely, as shown in Fig. 2A. Then, with the time passing, the appearance of stripes
ð3Þ
where Df was the frequency change of the quartz crystal, while S was a proportionality constant, equal to 5.45 ng cm2 Hz1 for the electrode used in the research. In the EQCM measurement, the crystal resonant frequency decreases quickly at the first stage of assembling, then, much more slowly starting from 200 min, and after 14 h of assembly, the resonant frequency change Df basically becomes constant at 114 Hz, indicating 621 ng cm2 of CAW-thiol assembled.
Fig. 2. STM images on the surface of an Au (1 1 1) electrode at 200 mV (vs. Ag/AgCl, saturated KCl) in 50 mM KClO4 solution containing CAW-thiol after the self-assembling of CAW-thiol for 22 min (A), and, 3 h 12 min (B).
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3.3. Characterization of the assembled film Electrochemical measurements, such as those of heterogeneous electron-transfer rate and differential capacitance are often applied to characterize the properties of the assembled films [19,31]. Exploring the property change on the surface of a gold electrode at different times during the assembly of CAW-thiol, we measured cyclic voltammograms (CVs), as shown in Fig. 3A, in 1.0 M KCl containing an electrochemical probe species 1.0 mM FeðCNÞ3 6 with the scan rate of 100 mV s1, and obtained the capacitances (shown in Fig. 3B) calculated from CVs (not presented) in a blank solution only containing 1.0 M KCl with the scan rates of 10, 50, and 100 mV s1 respectively. In Fig. 3, the peak current (Fig. 3A) and capacitance (Fig. 3B) drop sharply in the first 100 s of assembling, revealing facilitation of the process at the stage. Although the quantity of CAW-thiol assembled at the early assembling time was very small, indicated by the EQCM measurement, the electrode’s surface was almost completely covered by a thin layer of molecules that effectively hindered the passage of electric current because they lay on the gold surface, as shown in Fig. 2A. From Fig. 3, electron-transfer was substantially blocked after 5 h assembly as the assembled molecules stood vertically upwards (see Fig. 2B), and the assembled film was more densely packed; thus, the rate of assembly slowed down, verified by the EQCM measurements as well. After 24 h, the Faradaic current, capacitance (Fig. 3) and the resonant frequency change of the quartz crystal of EQCM
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remained about constant, implying no more unoccupied positions left and a densely packed assembled film formed on the gold surface. At the peak current potential Ep,c 0.208 V for the couple 2 FeðCNÞ3 6 =FeðCNÞ6 in Fig. 3, the Faradaic reduction current at the gold electrode assembled for 24 h, is less by a factor of 2.5 103 than that at 0 h, i.e., bare gold, and the capacitance at 24 h is independent of the scan rate (10, 50, and 100 mV s1, in Fig. 3B) and electrolyte (1.0 KCl and HClO4, not shown). All these experimental results imply that the CAW film assembled on the gold electrode for 24 h are well organized and to block the penetration of the probe ions almost completely. Raman spectroscopy is sensitive to the hybridization state in carbonaceous materials and often used to identify the sp bonding structure of linear carbon allotropy [4,32–34]. Fig. 4 shows the Raman spectra of pristine pyrolytic product in powder (a), and, thiolCAWs in an assembled film (b) and an aqueous solution (c). On curve b, the Raman band around 2000 cm1, referring to the stretching mode of C„C and indicating the presence of sp-hybridization, is remarkably enhanced, while the D (around 1391 cm1) and G (around 1579 cm1) bands, typical features of amorphous sp2/sp3 carbon, are much weaker than the curves of a and c respectively. These properties suggest that the self-assembling process makes CAWs display the characteristics arising from the sphybridization bonding structure more remarkable than the case before the treatment. This inference was confirmed by the findings from electron energy loss spectroscopy (EELS) measurements depicted in Fig. 5. Fig. 5 shows the EELS spectra of assembled CAW-thiol, in which the band around 21.8 eV at the low-energy region (Fig. 5A), coming from the plasma resonance of all valence electrons (p + r plasmon), and the bands around 280.9, 292.3, and 318.7 eV at the K-shell excitation region (Fig. 5B), resulting from excitation of electrons in the ground-state 1s core levels to the vacant p* and r* antibonding states, shift respectively to lower energy values than the corresponding values of graphite. These changes are thought to be the characteristics of linear carbon allotropy [4,35]. 3.4. Designing of the film surface The surface of assembled film can be designed to have various properties by connecting different functional groups at the end of carbon chain of molecules assembled. This tactics can be illustrated
Fig. 3. (A) Cyclic voltammograms in 1.0 M KCl containing 1:0 mM FeðCNÞ3 with 6 the scan rate of 100 mV s1, and, (B) the capacitance calculated from CVs with the 1 scan rate of 10, 50, and 100 mV s in 1.0 M KCl, on a gold electrode at different times of the assembling of CAW-thiol.
Fig. 4. Raman spectra of a pristine pyrolytic product in powder (a), and, thiol-CAWs in an assembled film (b) and aqueous solution (c).
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4. Conclusion We developed an approach to construct an assembled film of carbon atom wires, another allotropic form of carbon based on sp-hybridization. The assembled CAW film produced by the approach was well organized and its special features arising from the sp-hybridized structure were enhanced. The surface of assembled film could be designed to have various properties by connecting different functional groups at the end of carbon chain of the molecules assembled. The linear carbon is of an one-dimensional conjugated system with plenty of delocalized electrons along its molecular chain. It can be utilized as the molecular wire in molecular electronic devices and thus, the approach reported has the potential application in many fields [4,12,16]. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 20473039) and State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University, China) (No. 200405). Work at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Science, under Contract No. DE-AC02-98CH10886. We thank Siber Hegner China (A Division of Siber Hegner Ltd.) for the contact angle measurements. References [1] [2] [3] [4]
Fig. 5. EELS spectra of the low-energy region (A), and, the K-shell excitation region (B) taken at the assembled CAW film.
[5] [6] [7] [8] [9] [10] [11] [12] [13]
through a contact angle measurement in which a static water droplet (2 lL) dropped on the surfaces of self-assembled CAW-thiols prepared by the condensation, depicted in Section 2.1, for 24 h and 6 h, and the optical-contact-angle records of 101.5° and 62.2° were obtained respectively. The experimental result shows that the surface of the self-assembled CAW-thiol prepared by the condensation for 24 h is hydrophobic, while that of 6 h condensation is hydrophilic. Why is the surface property of these two kinds of film so different? It is because that when the amount of b-aminoethanethiol used is in excess and the reaction time is sufficiently long in the preparation of CAW-thiol, the condensation reaction will follow chemical Eq. (1), and, the ASH groups are attached at the both ends of carbon chains. During the assembling process of the CAW-thiol prepared by the condensation for 24 h, one ASH group, in each carbon chain, forms the SAAu on the Au substrate, while another ASH group remains free, oriented upward on the assembled film, forming a hydrophobic surface. The presence of ASH group on the surface of the assembled CAW-thiol film is clearly shown by the band around 2572 cm1 s on curve b of Fig. 1. When the reaction time for condensation is less than 6 h, less than a half of ACOOH groups could have time to react with b-aminoethanethiol, i.e., the reaction taking place in the system in the case will follow Eq. (2), rather than Eq. (1). Thus, free ACOOH rather than free ASH is left on the surface of assembled CAW-thiol film, which can be conformed by the presence of the band around 1725 cm1 and absence of the band around 2572 cm1 on its IR spectrum, hereby forming a hydrophilic surface.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
[30] [31] [32] [33] [34] [35]
A.G. Whittaker, P.L. Kintner, Science 165 (1969) 589. R.B. Heimann, J. Kleiman, N.M. Salansky, Nature 306 (1983) 164. P.P.K. Smith, Peter R. Buseck, Science 229 (1985) 485. R.B. Heimann, S.E. Evsyukov, L. Kavan (Eds.), Carbyne and Carbynoid Structures, Kluwer Academic Press, Dordrecht, 1999. R.J. Lagow et al., Science 267 (1995) 362. L. Kavan, Chem. Rev. 97 (1997) 3061. F. Cataldo, Carbon 42 (2004) 129. H. Tabata, M. Fujii, S. Hayashi, Carbon 44 (2006) 522. R.H. Baughman, Science 312 (2006) 1009. A. Sun, J.W. Lauher, N.S. Goroff, Science 312 (2006) 1030. R.E. Martin, F. Diederich, Angew. Chem., Int. Ed. 38 (1999) 1350. S. Eisler, A.D. Slepkov, E. Elliott, T. Luu, R. McDonald, F.A. Hegmann, R.R. Tykwinski, J. Am. Chem. Soc. 127 (2005) 2666. K.-H. Xue, F.-F. Tao, W. Shen, Ch.-J. He, Q.-L. Chen, L.-J. Wu, Y.-M. Zhu, Chem. Phys. Lett. 385 (2004) 477. K.-H. Xue, J.-M. Liu, R.-B. Wei, S.-P. Chen, Chem. Phys. 327 (2006) 319. C.S. Casari et al., Appl. Phys. Lett. 90 (2007) 013111. Z. Crljen, G. Baranovic, Phys. Rev. Lett. 98 (2007) 116801. J.G. Kushmerick, J. Naciri, J.C. Yang, R. Shashidhar, Nano Lett. 3 (2003) 897. L. Kavan, J. Hlavaty´, J. Kastner, H. Kuzmany, Carbon 33 (1995) 1321. M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 (1987) 3559. G.M. Whitesides, P.E. Laibinis, Langmuir 6 (1990) 87. A. Ulman, Chem. Rev. 96 (1996) 1533. J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chem. Rev. 105 (2005) 1103. R.G. Nuzzo, D.L. Allara, J. Am. Chem. Soc. 105 (1983) 4481. P.E. Laibinis, G.M. Whitesides, D.L. Allara, Y.-T. Tao, A.N. Parikh, R.G. Nuzzo, J. Am. Chem. Soc. 113 (1991) 7152. S.-W. Tam-Chang, H.A. Biebuyck, G.M. Whitesides, N. Jeon, R.G. Nuzzo, Langmuir 11 (1996) 4371. N. Nishi, D. Hobara, M. Yamamoto, T. Kakiuchi, J. Chem. Phys. 118 (2003) 1904. G.G. Baralia, A. Pallandre1, B. Nysten, A.M. Jonas, Nanotechnology 17 (2006) 1160. Z.F. Liu, Z. Shen, T. Zhu, S.F. Hou, L.Z. Ying, Z.J. Shi, Z.N. Gu, Langmuir 16 (2000) 3569. E. Pretsch, T. Clerc, J. Seibl, W. Simon, Tables of Spectral Data for Structure Determination of Organic Compounds (Translated from the German by K. Biemann), The second English edn., Springer-Verlag, Berlin Heidelberg, 1989 (I5-I280). T.W. Schneider, D.A. Buttry, J. Am. Chem. Soc. 115 (1993) 12391. M. Yuan, S. Zhan, X. Zhou, Y. Liu, L.Y. Lin, Z. Zhang, J. Hu, Langmuir 24 (2008) 8707. A. Lucotti et al., Chem. Phys. Lett. 417 (2006) 78. H. Tabata, M. Fujii, S. Hayashi, Chem. Phys. Lett. 420 (2006) 166. J. Kastner, H. Kuzmany, L. Kavan, F.P. Dousek, J. Kiirti, Macromolecules 28 (1995) 344. V.V. Korshak, Y.P. Kudryavtsev, V.V. Khvostov, M.B. Guseva, V.G. Babaev, O.Y. Rylova, Carbon 25 (1987) 735.