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Gold Nanoparticle-Functionalized Carbon Nanotubes for Light-Induced Electron Transfer Process P. Pramod,† C. C. Soumya,† and K. George Thomas*,†,‡ †
Photosciences and Photonics Group, National Institute for Interdisciplinary Science and Technology (NIIST), CSIR, Thiruvananthapuram, 695 019, India ‡ School of Chemistry, Indian Institute of Science Education and Research - Thiruvananthapuram (IISER-TVM), CET Campus, Thiruvananthapuram, 695 016, India
bS Supporting Information ABSTRACT: The modified electronic properties at the heterojunctions of Au nanoparticledecorated single-walled carbon nanotubes (SWNTs) have been utilized for photoinduced electron transfer by anchoring a photoactive molecule, namely ruthenium trisbipyridine (Ru(bpy)32þ). A unidirectional electron flow was observed from the excited state of Ru(bpy)32þ to carbon nanotubes when the chromophores were linked through Au nanoparticles (SWNTAuRu2þ). In contrast, photoinduced electron transfer was not observed from the excited state of Ru(bpy)32þ neither to SWNT nor Au nanoparticles when these components were linked directly. The charge equilibration occurring at the SWNTAu heterojunctions, due to the differences in electrochemical potentials, result in the formation of a localized depletion layer at the bundled carbon nanotube walls, which may act as acceptor sites of electrons from *Ru(bpy)32þ. The charge separation in SWNTAuRu2þ nanohybrids was sustained for several nanoseconds before undergoing recombination, making these systems promising for optoelectronic and artificial photosynthetic device applications. SECTION: Nanoparticles and Nanostructures
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nderstanding the basic properties of carbon nanotube based one-dimensional systems15 has opened up newer possibilities for the design of nanoscale electronic devices,611 energy conversion systems,1221 and biological sensors.2224 For example, the intriguing electron accepting and transporting properties of single-walled carbon nanotubes (SWNTs) have been exploited for the design of nanohybrid systems for light energy conversion.1421 The use of SWNT-based nanohybrids for charge separation have been demonstrated by functionalizing them with appropriate donor groups,1417 as well as attaching semiconductor nanoparticles to their surface.18 An alternate approach is to use metal-semiconductor hybrid materials such as noble metal nanoparticle-decorated SWNTs10 for photovoltaic applications. The rationale behind this design strategy is based on the fact that the band structure at the metal nanoparticleSWNT interface is distinctly different from that of individual components.10 Since the metal and semiconductor possess different electrochemical potential, charge redistribution occurs at the contact junction so that the potentials are equilibrated, generally represented as band bending.25 When metals with higher work function (j) are doped on an n-type semiconductor, charge transfer occurs to the metal, resulting in the depletion of electrons at the semiconductor interface (Schottky-type potential barrier10). Theoretical as well as experimental investigations dealing with the modified electronic properties of the metalsemiconductor interface2527 and their use in catalysis have r 2011 American Chemical Society
been reported. Several methodologies for incorporating Ag, Au, and Pt nanoparticles onto the surface of SWNTs2831 and their potential applications as sensors and field effect transistors9,10 have also been demonstrated. While SWNTmetal nanoparticle systems are proposed for sensing applications, the possibility of utilizing these materials as components of light energy conversion systems has not been actively pursued. Herein, we investigate various light-induced processes between ruthenium trisbipyridine (Ru(bpy)32þ, a chromophoric system with remarkable photophysical and electrochemical properties3235) and SWNTs, both in the presence and absence of gold nanoparticles (Scheme 1). Ruthenium trisbipyridine bearing a thiol group (Ru2þC7SH) and triethylene glycol-protected Au nanoparticles were synthesized as per a recently published procedure.35 Ru2þC7SH molecules were functionalized on to the surface of Au nanoparticles by adopting a place exchange reaction (AuRu2þ). SWNTs (high-pressure CO conversion (HiPCO)) were purified by heating with concentrated HNO3 for 4 h to remove the transition metal impurities.36,37 The purified SWNTs were first converted to carboxylated SWNTs (SWNTCOOH) and further to corresponding acid chloride Received: February 8, 2011 Accepted: March 9, 2011 Published: March 15, 2011 775
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Scheme 1. Structures of Molecular Systems under Study and Synthesis of Functionalized Carbon Nanotubesa
a (a) SOCl2, DMF; (b) RuC7SH, THF, 70 °C, 36 h; (c) 2-mercaptoethylamine hydrochloride, pyridine, DMF, 70 °C, 24 h; (d) AuRu2þ, THF, rt, 36 h.
functionalized on SWNTs in the absence and presence of Au nanoparticles (SWNTRu2þ and SWNTAuRu2þ, respectively) by the pathways shown in Scheme 1. Details of the synthetic procedure adopted are provided in the Supporting Information. In both cases, the concentration of Ru(bpy)32þ was varied on the surface of SWNTs (vide infra). The different stages of the functionalization of SWNTs were monitored by analytical (thermal gravimetric analysis (TGA)), microscopic (transmission electron microscope (TEM)) and spectroscopic (Fourier transform infrared (FTIR), Raman, UVvisible) techniques. The presence of SWNTs was confirmed by TEM analysis, and representative images of SWNTRu2þ and SWNTAuRu2þ are presented in Figure 1. Au nanoparticles are linked to SWNT through a short flexible linker group, and the functionalized SWNTs are observed as relatively thin bundles in TEM images (represented as b-SWNT). It is clear from the TEM images that the Au nanoparticles (average diameter of 4.5 nm) are intimately attached onto the surface of b-SWNT (Figure 1BD). The highresolution TEM (HRTEM) images of SWNTAuRu2þ showed lattice fringes corresponding to both SWNT and Au nanoparticles, indicating that the crystalline nature of both nanotubes and nanoparticles are preserved during functionalization (Supporting Information; deformation of carbon nanotubes was observed upon prolonged exposure to an electron beam). Raman spectroscopy can provide detailed information on the structure and properties of carbon nanotubes.3,4 The normalized Raman spectra (excitation wavelength of 514 nm) of the pristine SWNTs and functionalized carbon nanotubes (SWNTCOOH, SWNTRu2þ, and SWNTAuRu2þ),
Figure 1. HRTEM images of functionalized SWNTs observed as relatively thin bundles: (A) SWNTRu2þ and (BD) SWNTAuRu2þ. Samples were prepared by drop casting a dilute solution on to Formvar coated copper grid.
(SWNTCOCl) and subsequently to SWNTSH by reacting with 2-mercaptoethylamine.36,37 Ru(bpy)32þ's were 776
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Figure 2. (A,B) Normalized Raman spectra of pristine (black), SWNTCOOH (red), SWNTRu2þ (blue), and SWNTAuRu2þ (green) at room temperature under air (excitation wavelength of 514 nm) and (B) RBM region at different stages of functionalization (G band of all the spectra were normalized). (C,D) Absorption and luminescent properties of unbound and bound Ru(bpy)32þ on SWNTs in degassed CH3CN: (C) absorption spectra of (a) Ru2þC7SH, (b) AuRu2þ, (c) purified SWNT, (d) SWNTRu2þ, and (e) SWNTAuRu2þ and (D) the corresponding luminescence spectra of (a) Ru2þC7SH, (b) SWNTRu2þ, and (c) SWNTAuRu2þ (excitation wavelength 453 nm). Inset of D) shows the luminescence lifetime profile of (a) SWNTRu2þand (b) SWNTAuRu2þ both at lower loadings of Ru(bpy)32þ in degassed CH3CN (excitation wavelength 440 nm).
∼120 carbon atoms per Ru(bpy)32þ for lower chromophore loading (SWNTRuL2þ and SWNTAuRuL2þ; Supporting Information).13 The UVvisiblenear-infrared (UVvisNIR) absorption spectra of SWNTRu2þ and SWNTAuRu2þ are presented in Figure 2C. The bands corresponding to Ru(bpy)32þ are distinct and remained more or less unperturbed when bound on SWNT, indicating the absence of any ground-state interaction. For example, the absorption spectra of Ru2þC7SH in acetonitrile possess two bands: a band centered around 288 nm corresponding to the ππ* transition of bipyridine ligand, and another at 453 nm originating from the metal-to-ligand charge transfer (MLCT).35 Both these bands remain unperturbed when Ru(bpy)32þ is functionalized on the surface of SWNT. The additional band at 530 nm observed in the case of SWNT AuRu2þ corresponds to the plasmon absorption of Au nanoparticles. This band was found to be red-shifted by ∼10 nm, compared to that in AuRu2þ (Figure 2C) due to the strong interaction between SWNT and Au nanoparticles. A decrease in NIR band gap transitions (van Hove singularities) was observed for SWNTRu2þ and SWNTAuRu2þ compared to purified SWNT. It is evident from the TEM images that both SWNT Ru2þ and SWNTAuRu2þ exist as thin bundles, and the nanotubenanotube interaction results in the suppression of NIR absorption.39 The luminescence of Ru2þC7SH is centered at 612 nm with a quantum yield of 0.07.35 We have found that the luminescence of Ru(bpy)32þ in SWNTRu2þ is more or less retained at lower loading of chromophores. More recently, Accorsi et al. have reported that the luminescence of the EuIII complex is unaffected when bound on an SWNT surface.40
measured at room temperature under air, are presented in Figure 2. The characteristic Raman bands of SWNTs, namely, RBM, D, G, and G0 bands, were retained during various stages of functionalization. The radial breathing mode (RBM) represent the tubular structure of carbon nanotubes and were observed at ∼218 and ∼277 cm1 in all the cases (Figure 2B). The ratio between the Raman intensity of the G-band (∼1590 cm1) to the D-band (∼1350 cm1) is a direct measure of the quality of carbon nanotubes with respect to the number of defects; a higher value of G/D ratio indicates that the SWNTs are of better quality.38 In the present case, the G/D ratio was calculated to be ∼85% for pristine carbon nanotubes and ∼83% for all the functionalized derivatives, ruling out the possibility of any defects occurring from covalent functionalization. Thus, the Raman spectroscopic results confirm that the integrity and tubular nature of SWNTs are well preserved during various stages of functionalization. The functionalization of Ru(bpy)32þ on SWNT was confirmed using FTIR by the presence of amide bands and carbonyl stretching of the thioester (Supporting Information). The FTIR spectrum showed characteristic peaks for SWNT COOH (1733 cm1, CO stretching), SWNTRu2þ (1682 cm1, thioester), and SWNTCONH(CH2)2SH (amide I band at 1686 cm1 and amide II band at 1535 cm1). Thermograms at both low and high loading of chromophores on functionalized SWNTs showed similarities with that of Ru2þC7SH (Supporting Information). TGA of functionalized SWNTs at low and high loadings of chromophores presents a loss of weight of ∼19% and ∼40% for SWNTRu2þ and ∼26% and ∼51% for SWNTAuRu2þ at 600 °C. This corresponds to the presence of ∼30 carbon atoms per Ru(bpy)32þ for high chromophore loading (SWNTRu2þ and SWNTAuRu2þ) and 777
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degassed and oxygen-saturated acetonitrile solutions. The spectral features of SWNTRu2þ at lower loading of chromophores are similar to those of Ru2þC7SH, which were totally quenched upon oxygen saturation (Supporting Information), indicating the absence of any excited state interactions. The band at 370 nm is assigned as the triplet excited state of the chromophore. In contrast, two additional peaks at 310 and 500 nm were observed at higher loadings of chromophores on SWNT along with the 370 nm band (trace ‘b’ in Figure 3A). Upon bubbling with oxygen, the triplettriplet absorption at 370 nm was quenched (trace ‘c’ in Figure 3A) leaving a residual absorption at 350 nm (vide infra). Complementary chemical and electrochemical methods have been previously adopted for the characterization of the redox products of Ru(bpy)32þ: reduction to Ru(bpy)31þ results in absorption peaks at 350 and 500 nm41,42 and a strong absorption at 310 nm41,43 upon oxidation to Ru(bpy)33þ. Comparing these results with the transient absorption spectra presented in Figure 3A, it is clear that the peaks originate through an excited-state redox reaction resulting in the formation of Ru(bpy)31þ and Ru(bpy)33þ. We have further probed the fate of these transient species by monitoring the absorption-time profile at 500 and 310 nm (Supporting Information). Both the transients followed monoexponential decay with a rate constant of 6.9 106 s1 (τ = 145 ns) in argondegassed solution, further suggesting that these species originate from the same redox process. Blank experiments carried out using a saturated solution of Ru2þC7SH do not produce any redox products on photoexcitation. Thus, based on luminescence lifetime and transient absorption studies, it is clear that when the concentration of the chromophore is high on the surface of SWNT, an interchromophoric quenching was observed through an electron transfer from *Ru(bpy)32þ to a ground state molecule due to its close proximity. Interchromophoric luminescence quenching was previously observed when (i) Ru(bpy)32þ chromophores are closely packed on Au nanoparticle surface due to light induced redox reactions35 and (ii) porphyrin molecules are linked to SWNTs due to exciplex formation.44 The main focus of the present investigation is to understand the role of Au nanoparticle-decorated SWNTs on light induced electron transfer process, when bound to a chromophore. Timeresolved transient absorption studies of SWNTAuRu2þ were carried out, both in argon-degassed and oxygen-saturated solutions, at lower and higher loadings of chromophores (Figure 3B). Interestingly, the transient absorption spectral profile of SWNTAuRu2þ is distinctly different from that of SWNTRu2þ and remained unaffected by varying the concentration of chromophores. In argon-degassed solutions, both lower and higher loadings of Ru(bpy)32þ yielded two transients: a sharp one at 310 nm and a broad band at 530 nm, apart from the triplettriplet absorption at 370 nm (trace ‘a’ in Figure 3B). Compared to the transient absorption of the oxidized product (Ru(bpy)33þ) at 310 nm,35,41,42 the bands at 370 and 530 nm got totally quenched upon bubbling with oxygen (trace ‘b’ in Figure 3B). The absorptiontime profile was monitored at 310 and 530 nm in argon degassed solution and found to exhibit monoexponential decay with a rate constant of 2.5 ( 0.05 106 s1, suggesting that these species originate from the same process (Supporting Information). In a recent study, Guldi, Prato and co-workers have characterized the reduced form of SWNT (HiPCO) by spectroelectrochemical and pulse radiolytic studies15 and found that the monoanion (SWNT•) possesses a broad absorption band at 530 nm. The transient corresponding
Figure 3. (A) Nanosecond transient absorption spectrum (355 nm laser pulse) of unbound and bound Ru(bpy)32þ on SWNTs recorded immediately after the pulse: (a) Ar-degassed solution of RuC7SH; (b) SWNTRu2þ having high loading of Ru(bpy)32þ in Ar degassed, and (c) oxygen-saturated solutions. (B) Nanosecond transient absorption spectrum (355 nm laser pulse) of SWNTAuRu2þ in degassed CH3CN recorded immediately after pulse: (a) high loading of Ru(bpy)32þ in Ar-degassed solution; (b) high loading of Ru(bpy)32þ in oxygen-saturated solution, and (c) low loading of Ru(bpy)32þ in Ardegassed solution.
Interestingly, the luminescence of Ru(bpy)32þ is quenched for (i) SWNTRu2þ having higher loading of chromophores and (ii) SWNTAuRu2þ nanohybrid systems having both at lower and higher loadings of chromophores (Figure 2D). The spectral overlap between the absorption of SWNT and the MLCT band of Ru(bpy)32þ prevented the selective excitation of the chromophore, and hence quantification of the emission yields by steady-state technique was difficult. The excited state interactions were further elucidated by following luminescent lifetime studies. Both Ru2þC7SH and SWNTRu2þ with lower loading of chromophores followed a monoexponential decay with an average lifetime (τ) of 960 ( 2 ns in acetonitrile (trace ‘a’ in inset of Figure 2D). In the case of SWNTRu2þ with higher loading of chromophores, a biexponential decay was observed (τ1 = 958 ( 2 ns; χ1 = 30% and τ2 = 4.5 ( 0.1 ns; χ2 = 70%; Supporting Information). These luminescence lifetime values are similar to that observed in AuRu2þ having higher loading of Ru2þ chromophores.35 In contrast, a biexponential decay was observed in the case of SWNTAuRu2þ, both at lower and higher loadings of chromophores (τ1 = 960 ( 2 ns; χ1 = 20% and τ2= 3.9 ( 0.1 ns; χ2 = 80%; trace ‘b’ in inset of Figure 2D). The lifetime of the long-lived component is similar to that of Ru2þC7SH, which can be attributed to the unquenched Ru(bpy)32þ bound on SWNT. The short-lived species arises as a result of excited state quenching of Ru(bpy)32þ, and nanosecond transient absorption studies were further carried out to elucidate the mechanistic pathways involved. The triplettriplet (TT) absorption of Ru2þC7SH has a characteristic band at 370 nm, which decays with a rate constant of kT = 1.10 106 s1 (τ = 909 ns) in acetonitrile (trace ‘a’ in Figure 3A).35 The corresponding bleach observed at 450 and 620 nm in the difference absorption spectra is attributed to the loss of ground state absorption and emission, respectively. Transient absorption studies of SWNTRu2þ, having lower and higher loadings of chromophores, were carried out both in argon778
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Scheme 2. Light-Induced Processes in SWNTRu2þ and SWNTAuRu2þ
to SWNT• was also observed on photoexcitation of SWNTbased dyads having ferrocene15 and pyrene16 as donors. On the basis of these results, it is clear that an efficient lightinduced electron transfer process occurs from the *Ru(bpy)32þ to gold nanoparticle-decorated SWNT, resulting in the formation of SWNT monoanion and Ru(bpy)33þ. On the basis of steady state and time-resolved studies, the light-induced processes observed in SWNTRu2þ and SWNTAuRu2þ are presented in Scheme 2A. From the luminescence lifetime studies, the rate constant for forward electron transfer (kET) was estimated as 2.55 108 s1. One of the promising features of SWNTAuRu2þ is its relatively slower back electron transfer rate constant (kBET = 2.5 106 s1), which was estimated based on transient absorption studies. Interestingly, the rate constant for the back electron transfer in SWNTAuRu2þ is 2 orders of magnitude slower than the forward electron transfer (kET/kBET = 102), which provides newer possibilities for designing energy conversion systems based on nanoparticle-decorated carbon nanotubes. Excited state of Ru(bpy)32þ can undergo reductive as well as oxidative quenching with donor/acceptor systems depending on the driving force of the reaction.41 Photoconductivity studies have shown that the oxidative quenching of *Ru(bpy)32þ is not observed when bound to the SWNT surface.45 Similar results were observed in the present case wherein the luminescence of Ru(bpy)32þ on SWNT is retained at lower chromophore loadings, indicating the absence of electron transfer. Thus, based on the luminescence lifetime and transient absorption studies,
it is concluded that photoinduced electron transfer is not observed from *Ru(bpy)32þ neither to Au nanoparticles35 nor SWNT when these components are linked directly. However, a unidirectional electron flow was observed from the *Ru(bpy)32þ to SWNT when linked through Au nanoparticles. It is well established that the heterojunctions of SWNTmetal10 as well as SWNTsemiconductor18 play a significant role in modulating the electronic properties of hybrid materials. In a recent report, Kauffman and Star have demonstrated that the potential barrier existing at the SWNTmetal nanoparticle interface is related to the work function (j) of the respective metal.10 For example, when Au metal (j ∼ 5.0 eV)10 is brought in contact with SWNT (j ∼ 4.7 eV)46 there will be a transfer of electrons from the SWNT to metal resulting in the depletion of electrons at the SWNT interface. In the present case the charge redistribution at the bundled SWNTAu nanoparticle interface, due to Fermi level alignment, results in the formation of a localized depletion layer on SWNT walls which acts as deep acceptor states10,25 (Scheme 2B). Thus the SWNTAu heterojunctions drive the electron transfer from the *Ru(bpy)32þ to carbon nanotubes. Further experiments (X-ray photoelectron spectroscopy (XPS), electrochemical measurements, etc.) are underway to gather insight on the nature of SWNTAu heterojunctions. Studies can be further extended by decorating SWNT with a variety of other metal nanoparticles (such as Ag, Pd, and Pt), which, in combination with appropriate molecular systems, may have the potential application for the design of efficient photovoltaic devices and chemical as well as biological sensors. 779
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In conclusion, photoactive hybrid nanomaterials were synthesized by functionalizing ruthenium trisbipyridine (Ru(bpy)32þ) chromophores onto SWNTs, both in the presence and absence of gold nanoparticles. On the basis of the steady-state and timeresolved studies, it is concluded that the electron transfer takes place from *Ru(bpy)32þ to Au nanoparticle-decorated SWNT. However, no electron transfer process was observed from the photoexcited chromophores to SWNT or Au nanoparticles34 when these components were linked directly. The electronic properties at the heterojunctions of SWNTAu nanoparticles are distinctly different from that of the isolated components due to the charge redistribution at the interface. The localized depletion layer at the bundled SWNT walls may act as acceptor sites for electrons from the excited chromophores, leading to forward electron transfer. The charge-separated intermediates in this multicomponent system are stable for several nanoseconds, and the high ratio of forward electron to back electron transfer (kET/kBET = 102) makes these hybrid nanosystems promising for energy conversion and optoelectronic applications. The intriguing electronic properties of the heterojunctions can be further modified by decorating SWNT with suitable metal nanoparticles and photoresponsive units, which can lead to the development of a new generation of photoactive hybrid nanomaterials.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Details on the experimental techniques; FTIR, TGA, and HRTEM characterization of functionalized SWNTs; and additional luminescent lifetime and nanosecond transient absorption figures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank CSIR (NW0035) and DST (SP/55/NM-75/2002), Government of India, for financial support, the HRTEM facility of NIIST, Trivandrum, India, Professor S. Sampath of the Indian Institute of Science, India, for allowing access to the confocal Raman microscope, and Professor Maurizio Prato of the Departmento di Scienze Farmaceutiche, Trieste, Italy, for gifting SWNTs. This is contribution NIIST-PPD-254 from NIIST. ’ REFERENCES (1) Iijima, S.; Ichihashi, T. Single-Shell Carbon Nanotubes of 1-nm Diameter. Nature 1993, 363, 603–605. (2) Bethune, D. S.; Klang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Cobalt-Catalyzed Growth of Carbon Nanotubes with Single-Atomic-Layer Walls. Nature 1993, 363, 605–607. (3) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (4) Vivekchand, S. R. C.; Govindaraj, A.; Rao, C. N. R. In Nanomaterials Chemistry: Recent Developments and New Directions; Rao, C. N. R., M€uller, A., Cheetham, A. K., Eds.; Wiley-VCH: Weinheim, Germany, 2007; pp 45118. (5) Ajayan, P. M. Nanotubes from Carbon. Chem. Rev. 1999, 99, 1787–1800. (6) Bachtold, A; Hadley, P.; Nakanishi, T.; Dekker, C. Logic Circuits with Carbon Nanotube Transistors. Science 2001, 294, 1317–1320. 780
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dx.doi.org/10.1021/jz200184j |J. Phys. Chem. Lett. 2011, 2, 775–781
