Effect of Imidazolium-Based Ionic Liquids on the Nanoscale

Dec 17, 2012 - Mark C. Siu , Samuel R. Anderson , Mahsa Mohammadtaheri , Taimur Ahmed , Sumeet Walia , Rajesh Ramanathan , Vipul Bansal. Advanced ...
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Effect of Imidazolium-Based Ionic Liquids on the Nanoscale Morphology of CuTCNQ (TCNQ = 7,7,8,8Tetracyanoquinodimethane) Metal−Organic Semiconductors Andrew Pearson,† Anthony P. O’Mullane,§ Suresh K. Bhargava,‡ and Vipul Bansal*,†,§,‡ †

NanoBiotechnology Research Lab (NBRL), ‡Centre for Advanced Materials & Industrial Chemistry, and §School of Applied Sciences, RMIT University, GPO Box 2476 V, Melbourne VIC 3000, Australia S Supporting Information *

ABSTRACT: We demonstrate for the first time the ionic-liquid-mediated synthesis of nanostructured CuTCNQ by the simple immersion of copper in a solution of TCNQ where the viscosity of the medium significantly impacts the corrosion−crystallization process and the final morphology of the material.

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INTRODUCTION Over the past several decades, significant attention1−9 has been directed toward the synthesis of metal−organic semiconducting materials based on charge-transfer complexes of metal-7,7,8,8tetracyanoquinodimethane, in particular, CuTCNQ, because of the observation of an interesting switching effect from high to low impedance state upon application of an external electric field or optical excitation.10−14 CuTCNQ has predominately been formed through chemical, electrochemical, and photochemical techniques in organic solvents such as acetonitrile.4,9,15−19 One of the simplest approaches that has been reported is that CuTCNQ can be formed through a spontaneous localized corrosion−crystallization process on a Cu substrate immersed in an acetonitrile solution containing TCNQ. The morphology of CuTCNQ can be controlled to an extent via controlling the reaction time, temperature, and TCNQ concentration.20,21 Recently it has been demonstrated that the applicability of CuTCNQ can be extended beyond that of switching and field emission whereby CuTCNQ modified with gold nanoparticles was shown to be an efficient photocatalytic material.15,16 It is therefore highly likely that the morphology of CuTCNQ will play a critical role in this new research direction. Ionic liquids (ILs) have been identified as attractive reaction media for the green synthesis of inorganic nanomaterials as a result of several interesting physical and chemical properties such as tunable viscosities, broad electrochemical windows, and negligible vapor pressures.22−24 Mei et al. have previously demonstrated the formation of different morphologies of organic−organic tetrathiafulvalene tetracyanoquinodimethane © 2012 American Chemical Society

(TTF-TCNQ) crystals in a solid−solid homogeneous reaction involving the mechanical grinding of precursors in different ILs.25 Nevertheless, with the increasing popularity of ILs as synthesis media, to the best of our knowledge the synthesis of CuTCNQ, an inorganic−organic charge-transfer complex, has yet to be undertaken in solvents such as ILs. In this Letter, we report the use of several imidazolium-based ILs as effective solvent media for the synthesis of CuTCNQ nanostructures. We demonstrate that the choice of an appropriate IL can have a dramatic effect on the heterogeneous corrosion−crystallization process on Cu substrates; therefore, different ILs result in the formation of different metal−organic nanoscale structures of CuTCNQ. The four ILs employed in this work all share the same counteranion (tetrafluoroborate [BF4]−), and all share a similar imidazolium cation (1-R-3methylimidazolium where R = ethyl, butyl, hexyl, or 2hydroxyethyl) and are simplified to [EMIM][BF4], [BMIM][BF4], [HMIM][BF4], and [HEMIM][BF4], respectively. The structures of these ILs are illustrated in Figure 1, where R denotes a different substituent as noted in the figure caption. In brief, these particular ILs were chosen for their similar chemical characteristics (in the case of [EMIM], [BMIM], and [HMIM]) such that only the physical properties of the IL, in particular, the viscosity, would vary greatly with increasing chain length. [HEMIM] was chosen to provide a comparison with an IL that contained different functional groups and hence Received: September 27, 2012 Revised: December 2, 2012 Published: December 17, 2012 8

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Figure 1. Cartoon depicting the structure of (1-R-3-methylimidazolium tetrafluoroborate ionic liquids where R = ethyl, butyl, hexyl, or 2hydroxyethyl) in [EMIM][BF4], [BMIM][BF4], [HMIM][BF4], and [HEMIM][BF4], respectively.

has different chemical properties in addition to different physical properties. The viscosity of the ILs follows the trend [EMIM] (43 mPa.s) < [HEMIM] (101.1 mPa.s) ≈ [BMIM] (103.8 mPa.s) < [HMIM] (202.4 mPa.s).

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EXPERIMENTAL SECTION

Materials. Copper metal foil (99% purity) was obtained from Chem Supply, 7,7,8,8-tetracyanoquinodimethane (TCNQ) was obtained from Fluka, and 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM][BF4]), and 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate ([HEMIM][BF 4]) were obtained from Ionic Liquid Technologies (IoLiTec). The copper foil was treated with a solution of 2 M nitric acid, rinsed three times with water, and dried with a flow of nitrogen before use. All other chemicals were used as received. Synthesis of CuTCNQ. The synthesis of CuTCNQ was performed simultaneously in four ionic liquids. To separate vials each containing 5 mL of a different ionic liquid (IL) ([EMIM][BF4], [BMIM][BF4], [HMIM][BF4], and [HEMIM][BF4]), 1 mg of TCNQ was added and vigorously sonicated until dissolved. The colors of the IL solutions were observed to change to green-black in the case of [EMIM][BF4], [BMIM][BF4] and [HMIM][BF4] and yellow-green in the case of [HEMIM][BF4]. Four small 1 cm × 1 cm squares of copper foil pretreated with dilute nitric acid were added to the different TCNQ/ IL solutions, and the reaction was allowed to proceed for 7 days. After 7 days, the TCNQ/IL solutions were observed to change color to dark red-brown in the case of [EMIM][BF4], [BMIM][BF4], and [HMIM][BF4] whereas little color change was observed in the case of [HEMIM][BF4]. In all cases, the growth of CuTCNQ could be clearly seen with the naked eye on the Cu substrate. The pieces of copper foil were then removed from the IL/TCNQ solutions, washed three times with deionized water, and dried with a flow of nitrogen. The pieces of copper foil were then characterized using SEM, XRD, FTIR, and Raman spectroscopy without further modification. The IL/ TCNQ solutions were stored and later examined by UV−visible absorbance spectroscopy. Instrumentation. Scanning electron microscopy (SEM) images were obtained on an FEI Nova NanoSEM at an operating voltage of 15 kV under high-vacuum conditions. X-ray diffraction (XRD) measurements were performed on a Bruker AXS D8 Discover with a general area detector diffraction system (GADDS). UV−visible spectroscopy (UV−vis) was performed on a Cary 50 Bio spectrophotometer, Raman spectroscopy (Raman) was performed on a PerkinElmer Raman Station 400F using 100% laser power and Raman focus, and FTIR spectra were obtained on a PerkinElmer Spectrum 100 using a diamond ATR universal sampling attachment.

Figure 2. SEM images of CuTCNQ structures synthesized in imidazolium-based ionic liquids.

appears as though the rods are composed of many smaller rods fused together. Of all the CuTCNQ structures obtained, these most closely resemble those obtained through conventional synthesis in acetonitrile;15 however, the cross-sectional dimensions are significantly smaller. Figure 2C shows the structures obtained in [HMIM][BF4] wherein many small rodlike growths are observed to decorate the majority of the surface in addition to sparse clusters of larger starlike structures. Finally, the structures grown in [HEMIM][BF4] shown in Figure 2D appear to assemble into large flowerlike clusters that are observed to range in size from 2 to 5 μm. Upon examination of higher-magnification images (Figure S-1), these flower structures appear to consist of many individual sub-100-nm CuTCNQ fibers. Both lower- and higher-magnification images of the obtained nanostructures in all four ILs are available in the Supporting Information (Figure S-1). To elucidate the structural differences observed in the different ILs, UV−visible absorbance spectroscopy was employed to monitor the consumption of TCNQ during the course of the reaction. Upon dissolving TCNQ in the respective ILs, distinct color changes were observed. Illustrated in Figure 3 are UV−visible spectra of TCNQ dissolved in the ILs before (black curves) and after its reaction with Cu to form CuTCNQ (red curves). The dominant feature present in all of the samples is the peak at 400 nm that can be attributed to dissolved TCNQ0, which corroborates well with TCNQ dissolved in acetonitrile.15 Interestingly, in [EMIM][BF4] (Figure 3A) and [BMIM][BF4] (Figure 3B), several minor peaks are observed from 650 to 800 nm, which can be attributed to the formation of TCNQ− and also corroborates well with what is observed in acetonitrile wherein the intensity ratio of the peaks associated with TCNQ0 and TCNQ− is comparable.15 The presence of peaks attributable to TCNQ− is interesting and indicates that [EMIM][BF4] and [BMIM][BF4] may have reduced TCNQ0 to TCNQ− in the absence of a Cu substrate. Significantly, these features are not observed for [HMIM][BF4] and [HEMIM][BF4] ILs. Upon introduction of

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RESULTS AND DISCUSSION Illustrated in Figure 2 are typical SEM images of the CuTCNQ structures obtained in the respective ILs. The structures obtained in [EMIM][BF4], as shown in Figure 1A, appear to resemble sharp needlelike structures with high aspect ratios that are several micrometers in length and less than 100 nm in diameter. Figure 1B illustrates the square-rod-like structures obtained in [BMIM][BF4] that have cross sections of ca. 200 nm × 200 nm and are once again several micrometers in length. At higher magnification (Supporting Information Figure S-1), it 9

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peak at 750 nm; however, at this stage this is not fully understood and warrants further investigation. Raman spectroscopy is a valuable technique for probing the difference between TCNQ0 and TCNQ− species in a material, as is shown in Figure 4 where the principle C−CN wing

Figure 3. UV−visible spectra of TCNQ in ILs before and after its reaction with Cu substrates to form CuTCNQ nanostructures. Figure 4. Raman spectra of CuTCNQ materials synthesized in different ILs and CuTCNQ synthesized in acetonitrile (designated CuTCNQ). Pristine TCNQ crystals are provided for comparison.

a Cu substrate into TCNQ dissolved in [EMIM][BF4] (red curve, Figure 3A), a reduction in the intensity of the peak attributed to TCNQ0 is observed along with a simultaneous increase in the peaks over the 650−800 nm range attributed to TCNQ− most likely as a result of CuTCNQ solubility in the IL. There is also a feature at ca. 490 nm that is due to a decomposition product, namely, dicyanotolluilocyanide (DCTC−), which is formed by the reaction of TCNQ− with trace water.26,27 Similarly, a reduction in TCNQ0 intensity is observed in [BMIM][BF4] when a Cu substrate is introduced (red curve, Figure 3B) along with an increase in the formation of DCTC− as evidenced by the peak at ca. 485 nm. However, with respect to [EMIM][BF4] the magnitude of these intensity changes is not as significant, which indicates that less TCNQ0 has reacted with the Cu substrate in [BMIM][BF4]. This corroborates well with low-magnification SEM images of CuTCNQ formed in [BMIM][BF4] (Figure S-1), which show a sparser coverage of CuTCNQ in comparison to that for the [EMIM][BF4] system (Figure S-1). This trend of lower TCNQ0 utilization is further demonstrated when the synthesis of CuTCNQ is carried out in [HMIM][BF4], where the lower-magnification SEM image (Figure S-1) shows much sparser CuTCNQ formation than seen in [BMIM][BF4]. We observe the corresponding decrease in the magnitude of change in the feature attributable to TCNQ0 (red curve, Figure 2C) as well as an absence of the peak at ca. 485 nm. Also the complete absence of TCNQ− features from 650 to 800 nm suggests that CuTCNQ is almost totally insoluble in [HMIM][BF4], which explains the absence of the DCTC decomposition product. Interestingly, in [HEMIM][BF4] upon reaction with a Cu substrate the peak attributed to TCNQ0 is observed to decrease in intensity, indicating a consumption of TCNQ0 similar to that observed for the other ILs; however, no feature is observed at ca. 485 nm but instead a large peak is observed at ca. 300 nm. This new feature may arise through the complexation of a reduced TCNQ species with [HEMIM][BF4]. There is also a broad

stretching vibrational mode at 1450 cm−1 in TCNQ0 is observed to shift to 1380 cm−1 for TCNQ− (i.e., in CuTCNQ). CuTCNQ synthesized in [EMIM][BF4], [BMIM][BF4], and[HMIM][BF4] demonstrates only the C−CN wing stretching vibrational mode at 1380 cm−1 attributable to TCNQ−. Given that DCTC− was formed during the course of the reaction using [EMIM][BF4] and [BMIM][BF4], the formation of CuDCTC on the Cu surface where CuTCNQ crystals were grown was excluded when compared to literature spectra.26 However, in the case of [HEMIM][BF4] the presence of both C−CN wing stretching vibrational modes attributable to TCNQ0 and TCNQ− at 1450 and 1380 cm−1, respectively, indicates that the mechanism of formation for CuTCNQ in IL [HEMIM][BF4] may be different. However, given that no features attributable to TCNQ0 were observed in the XRD (Supporting Information Figure S-2) and FTIR study (Figure S-3), this suggests that the presence of features attributable to TCNQ0 may be due to surface-bound TCNQ0 trapped by the IL, which is notoriously difficult to remove. FT-IR spectroscopy also confirmed that the as-synthesized CuTCNQ materials are phase I CuTCNQ, as demonstrated in the Supporting Information Figure S-3. As experimentally demonstrated, the physical properties of the IL, in particular, the viscosity, can have a significant effect on CuTCNQ nanostructures formed through the corrosion− crystallization process. Of the three alkyl imidazolium ILs, [EMIM][BF4] has the lowest viscosity and is also the least bulky. As a result, reacting species are able to diffuse easily through the medium, resulting in dense Cu substrate coverage with thin needlelike structures. Because of an increase in viscosity in IL [BMIM][BF4], the diffusion of reactant species becomes more difficult and may result in the formation of saturated reaction sites. As a consequence, the rodlike structures of CuTCNQ that are observed are larger and more 10

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Routes to Two Distinct Polymorphs and Their Relationship to Crystalline Films That Display Bistable Switching Behavior. Inorg. Chem. 1999, 38, 144−156. (2) Zhao, H.; Heintz, R. A.; Ouyang, X.; Grandinetti, G.; Cowen, J.; Dunbar, K. R. Insight into the Behavior of M(TCNQ)n (n = 1, 2) Crystalline Solids and Films: X-ray, Magnetic and Conducting Properties. NATO ASI Ser., Ser. C 1999, 518, 353−376. (3) Vickers, E. B.; Selby, T. D.; Thorum, M. S.; Taliaferro, M. L.; Miller, J. S. Vanadium 7,7,8,8-Tetracyano-p-quinodimethane (V[TCNQ]2)-Based Magnets. Inorg. Chem. 2004, 43, 6414−6420. (4) O’Mullane, A. P.; Fay, N.; Nafady, A.; Bond, A. M. Preparation of Metal-TCNQ Charge-Transfer Complexes on Conducting and Insulating Surfaces by Photocrystallization. J. Am. Chem. Soc. 2007, 129, 2066−2073. (5) O’Kane, S. A.; Clerac, R.; Zhao, H.; Ouyang, X.; Galan-Mascaros, J. R.; Heintz, R.; Dunbar, K. R. New Crystalline Polymers of Ag(TCNQ) and Ag(TCNQF4): Structures and Magnetic Properties. J. Solid State Chem. 2000, 152, 159−173. (6) Clerac, R.; O’Kane, S.; Cowen, J.; Ouyang, X.; Heintz, R.; Zhao, H.; Bazile, M. J., Jr.; Dunbar, K. R. Glassy Magnets Composed of Metals Coordinated to 7,7,8,8-Tetracyanoquinodimethane: M(TCNQ)2 (M = Mn, Fe, Co, Ni). Chem. Mater. 2003, 15, 1840−1850. (7) Wang, X.; Liable-Sands, L. M.; Manson, J. L.; Rheingold, A. L.; Miller, J. S. Decamethylnickelocenium Hydrogen-7,7,8,8-tetracyanoperfluoro-p-quinodimethandiide: Isolation of the Protonated Weak Base [HTCNQF4]. Chem. Commun. 1996, 16, 1979−1980. (8) Garcia-Yoldi, I.; Miller, J. S.; Novoa, J. J. Theoretical Study of the Electronic Structure of [TCNQ]22− (TCNQ = 7,7,8,8-Tetracyano-pquinodimethane) Dimers and Their Intradimer, Long, Multicenter Bond in Solution and the Solid State. J. Phys. Chem. A 2009, 113, 7124−7132. (9) Harris, A. R.; Neufeld, A. K.; O’Mullane, A. P.; Bond, A. M.; Morrison, R. J. S. Voltammetric, EQCM, spectroscopic, And Microscopic Studies on the Electrocrystallization of Semiconducting, Phase I, CuTCNQ on Carbon, Gold, And Platinum Electrodes by a Nucleation-Growth Process. J. Electrochem. Soc. 2005, 152, C577− C583. (10) Potember, R. S.; Poehler, T. O.; Benson, R. C. Optical Switching in Semiconductor Organic Thin Films. Appl. Phys. Lett. 1982, 41, 548−550. (11) Liu, H.; Liu, Z.; Qian, X.; Guo, Y.; Cui, S.; Sun, L.; Song, Y.; Li, Y.; Zhu, D. Field Emission and Electrical Switching Properties of Large-Area CuTCNQ Nanotube Arrays. Cryst. Growth Des. 2009, 10, 237−243. (12) Oyamada, T.; Tanaka, H.; Matsushige, K.; Sasabe, H.; Adachi, C. Switching Effect in Cu:TCNQ Charge Transfer-Complex Thin Films by Vacuum Codeposition. Appl. Phys. Lett. 2003, 83, 1252− 1254. (13) Liu, H.; Wu, X.; Chi, L.; Zhong, D.; Zhao, Q.; Li, Y.; Yu, D.; Fuchs, H.; Zhu, D. Tuning CuTCNQ Nanostructures on Patterned Copper Films. J. Phys. Chem. C 2008, 112, 17625−17630. (14) Liu, H.; Zhao, Q.; Li, Y.; Liu, Y.; Lu, F.; Zhuang, J.; Wang, S.; Jiang, L.; Zhu, D.; Yu, D.; Chi, L. Field Emission Properties of LargeArea Nanowires of Organic Charge-Transfer Complexes. J. Am. Chem. Soc. 2005, 127, 1120−1121. (15) Pearson, A.; O’Mullane, A. P.; Bansal, V.; Bhargava, S. K. Galvanic Replacement of Semiconductor Phase I CuTCNQ Microrods with KAuBr4 to Fabricate CuTCNQ/Au Nanocomposites with Photocatalytic Properties. Inorg. Chem. 2011, 50, 1705−1712. (16) Pearson, A.; O’Mullane, A. P.; Bhargava, S. K.; Bansal, V. Synthesis of CuTCNQ/Au Microrods by Galvanic Replacement of Semiconducting Phase I CuTCNQ with KAuBr4 in Aqueous Medium. Inorg. Chem. 2012, 51, 8791−8801. (17) Cui, S.; Liu, H.; Gan, L.; Li, Y.; Zhu, D. Fabrication of LowDimension Nanostructures Based on Organic Conjugated Molecules. Adv. Mater. 2008, 20, 2918−2925. (18) Liu, H.; Xu, J.; Li, Y.; Li, Y. Aggregate Nanostructures of Organic Molecular Materials. Acc. Chem. Res. 2010, 43, 1496−1508.

sparsely populated across the Cu surface. Further increasing the viscosity in IL [HMIM][BF4] results in the greatly restricted growth of CuTCNQ structures on the Cu surface. The formation of small rodlike growths with a few larger clusters further supports the notion that the viscosity of the medium in which the corrosion−crystallization process takes place plays a dramatic role in the morphology and density control of CuTCNQ structures as a result of the more inhibited movement of reactant species through the IL medium. For [HEMIM][BF4], a dense coverage of CuTCNQ nanostructures is again achieved. However, these structures are clustered into close-packed sites similar to what was observed in the more viscous ILs, which is possibly attributable to the participation of the 2-hydroxyethyl functional group on the imidazolium cation, restricting the movement of reactants through possible complex formation. However, as mentioned previously, this requires a more detailed examination.

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CONCLUSIONS We have demonstrated the synthesis of CuTCNQ nanostructures by the simple immersion of a copper substrate in several ionic liquids. Changes in the viscosity of the reaction medium, in which the corrosion−crystallization process occurs, is found to have significant effects on the size, shape, and density of the CuTCNQ structures that are obtained. Therefore, different ionic liquids offer an easy route to changing the nanostructural morphology of CuTCNQ. Interesting results were obtained when CuTCNQ was synthesized in IL [HEMIM][BF4], which possesses a different functional group than that in the other ILs employed. This indicates a more complex synthesis mechanism in which [HEMIM] is able to form a complex with one or more of the reactant species. A new facile route to the synthesis of different CuTCNQ nanostructures using ionic liquids as a reaction solvent offers potential in controlling their recently discovered properties in photocatalysis.

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ASSOCIATED CONTENT

S Supporting Information *

SEM, XRD, and FTIR. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 3 99252121. Fax: +61 3 99253747. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.B. thanks the Australian Research Council (ARC) for APD Fellowship and financial support through the ARC Discovery Project (No. DP0988099) and acknowledges the support of the Ian Potter Foundation in establishing a multimode advanced spectroscopy facility at RMIT University. A.O.M. thanks the ARC for financial support through a Future Fellowship (FT110100760). V.B., A.O.M., and S.K.B. thank the ARC for financial support through Linkage Project No. LP100200859 and Discovery Project No. DP110105125.

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REFERENCES

(1) Heintz, R. A.; Zhao, H.; Ouyang, X.; Grandinetti, G.; Cowen, J.; Dunbar, K. R. New Insight into the Nature of Cu(TCNQ): Solution 11

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(19) Zheng, H.; Li, Y.; Liu, H.; Yin, X.; Li, Y. Construction of Heterostructure Materials toward Functionality. Chem. Soc. Rev. 2011, 40, 4506−4524. (20) Duan, H. L.; Cowan, D. O.; Kruger, J. Electrochemical Studies of the Mechanism of the Formation of Metal-TCNQ Charge-Transfer Complex Film. J. Electrochem. Soc. 1993, 140, 2807−2815. (21) Liu, S.-G.; Liu, Y.-Q.; Wu, P.-J.; Zhu, D.-B. Multifaceted Study of CuTCNQ Thin-Film Materials. Fabrication, Morphology, and Spectral and Electrical Switching Properties. Chem. Mater. 1996, 8, 2779−2787. (22) Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176. (23) Skrabalak, S. E.; Chen, J.; Sun, Y.; Lu, X.; Au, L.; Cobley, C. M.; Xia, Y. Gold Nanocages: Synthesis, Properties and Applications. Acc. Chem. Res. 2008, 41. (24) Pearson, A.; O’Mullane, A. P.; Bansal, V.; Bhargava, S. K. Galvanic Replacement Mediated Transformation of Ag Nanospheres into Dendritic Au-Ag Nanostructures in the Ionic Liquid [BMIM][BF4]. Chem. Commun. (Cambridge, U.K.) 2010, 46, 731−733. (25) Mei, X.; Ouyang, J. Electronically and Ionically Conductive Gels of Ionic Liquids and Charge-Transfer Tetrathiafulvalene−Tetracyanoquinodimethane. Langmuir 2011, 27, 10953−10961. (26) Harris, M.; Hoagland, J. J.; Mazur, U.; Hipps, K. W. Raman and Infrared Spectra of Metal Salts of α,α-Dicyano-p-toluoylcyanide: NonResonant Raman Scattering in Tetracyano-p-quinodimethanide. Vib. Spectrosc. 1995, 9, 273−277. (27) Cehak, A.; Chyla, A.; Radomska, M.; Radomski, R. The Influence of Water and Oxygen on Stability of TCNQ Solutions. Mol. Cryst. Liq. Cryst. 1985, 120, 327−331.

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