All-Carbon Nanosized Hybrid Materials: Fluorescent Carbon Dots

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All-Carbon Nanosized Hybrid Materials: Fluorescent Carbon Dots Conjugated to Multi-Walled Carbon Nanotubes Theodosis Skaltsas, Anastasios Stergiou, Demetrios D. Chronopoulos, Sihan Zhao, Hisanori Shinohara, and Nikos Tagmatarchis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02267 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 8, 2016

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The Journal of Physical Chemistry

All-Carbon Nanosized Hybrid Materials: Fluorescent Carbon Dots Conjugated to MultiWalled Carbon Nanotubes Theodosis Skaltsas‡, #, Anastasios Stergiou‡, #, Demetrios D. Chronopoulos‡, Sihan Zhao§, Hisanori Shinohara§ and Nikos Tagmatarchis‡*

‡

Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation,48

Vasileos Constantinou Avenue, 11635, Athens, Greece §

Department of Chemistry, Nagoya University, Nagoya 464-8602, Japan

#

These authors contributed equally.

1

ACS Paragon Plus Environment


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ABSTRACT. In this study, fluorescent carbon dots (CDs) were synthesized by following a hydrothermal route in which butane-1,4-diamine and maleic acid were employed in a Teflon autoclave reactor. The structure and morphology of the so-formed spherically shaped CDs was confirmed by a combination of spectroscopic and imaging techniques, such as NMR, ATR-IR, DLS, XRD and HR-TEM. Additionally, it was found that raw CDs possess numerous –NH2 functionalities located in their external periphery, responsible for their enhanced aqueous solubility as well as the excellent dissolution CDs showed in polar protic solvents. Moreover, these –NH2 units were utilized for covalently associating CDs with oxidized multi-walled carbon nanotubes (MWCNTs) yielding robust CDs-MWCNTs hybrids. Based on photoluminescence spectroscopy, electronic communications between the individual components of CDs-MWCNTs was evidenced by the quantitative quenching of the emission of CDs in the presence of MWCNTs as well as the shortening of the photoluminescence lifetime of CDs from 7.3 ns for raw CDs to 300 ps for CDs-MWCNTs. Finally, the redox properties of CDs-MWCNTs were evaluated by electrochemistry measurements, allowing to determine the electrochemical band gap of the hybrid material to be 1.2 eV.

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Introduction In the last two decades an explosion of research in low-dimensional all-carbon nanosized materials has been witnessed,1-4 starting with the zero-dimensional fullerenes, proceeding with the one-dimensional carbon nanotubes and two-dimensional graphene and recently realizing the carbon dots. Of exceptional importance is the potential of those carbon nanomaterials, due to their novel structural, electronic and morphological characteristics, for applications in the fields of nanoelectronics, energy conversion and storage, sensing and catalysis.5,

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Particularly, the

photoluminescence properties of fullerenes, nanotubes and graphene as well as their hybrids with organic chromophores and semiconducting nanoparticles, have been exploited in photovoltaics, bioimaging and photocatalytic hydrogen production and environmental remediation.7-9 However, the youngest member of the nanocarbon family, carbon dots, has yet to be comprehensively examined, not only for fully understanding their photoluminescence properties, but also for revealing new and establishing existing paradigms of functions and applications.10, 11 Carbon dots, abbreviated herein as CDs, were firstly identified as fluorescent impurities in dispersions of nanotubes derived from arc-discharge.12 Generally, CDs can be prepared by either top-down or bottom-up approaches, with the latter offering the advantage of being more economic and less time consuming as compared with the former. Representative top-down methods for obtaining CDs include arc-discharge,12 laser ablation,13-15 and electrochemical oxidation16-19 of a graphite target. On the other hand, numerous CDs were synthesized by hydrothermal/solvothermal treatment20-22 and microwave-assisted irradiation23,

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of small

organic molecules (i.e. bottom-up methodologies). Notably, the latter approaches are the most commonly employed, since they are facile, low-cost and environmentally benign to synthesize large quantities of CDs. Moreover, a variety of carbon rich materials and mixtures can be 3



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employed for the carbonization process and growth of CDs, while, with the coexistence of heteroatom containing compounds the preparation of doped-CDs with tunable photophysical properties can be accomplished.25-31 Regardless of the synthetic route followed, the size of CDs ranges 1-10 nm and importantly, the periphery of their surface is saturated with functional groups such as hydroxyls, carboxylic acids and amines suitable for post-modification and the preparation of novel hybrid nanomaterials. In general, CDs possess tunable light absorption and show strong excitation-dependent broadband photoluminescence (i.e. 300-600 nm) features, rendering them suitable candidates in imaging32 and optoelectronic devices.33 Additionally, the high solubility, excellent photostability, small size, biocompatibility and chemical inertness of CDs are beneficial parameters when applications are concerned.34-36 Although the fluorescence mechanism of CDs is not fully understood, with controversial assignments, predominantly due to the diverse preparation protocols followed (i.e. composition of material) and the large distribution of particle sizes (i.e. heterogeneity of material), it is commonly accepted that the emission of CDs is mainly governed by quantum confinement effects and/or functional groups present on the periphery of the surface of the CDs.37 Nevertheless, it was recently shown that CDs can efficiently act as either electron donors or acceptors depending on the species they combine and their relative energy potentials. At the same time it was described that, chemical modifications to CDs lead to significant changes in the photoluminescence behaviour (i.e. wavelength and lifetime).38 Hence, without surprise, charge-transfer phenomena have been exploited in CD-based ensembles featuring for example a perylenediimide moiety as an acceptor unit. Specifically, electrostatic interactions were utilized in order to integrate a positively charged perylenediimide with anionic CDs and subsequently revealed electronic interactions between the two species in both the ground and

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excited states that gave rise to a charge-separated state.39 In a similar fashion, Coulombic interactions were employed to bring into contact CDs with graphene oxide and methylviologen and the ensembles formed found to feature notable charge-transfer activity.40,

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Furthermore,

CDs were supramolecularly combined with single-walled carbon nanotubes, in which the components of the so-formed ensemble function as electron donors and acceptors, respectively.42 However, with the recent exemption of a nanocomposite of CDs/MWCNTs, which was described without being thoroughly characterized,43 the covalent association between CDs and nanotubes has yet to be performed and fully evaluated. Such a connection significantly differs from non-covalent ones,44 electrostatic and/or van der Waals, in terms that the two components forming the hybrid species are tightly bound together via robust and stable covalent bonds, and therefore, effective electronic interaction between them may occur. Herein, firstly the preparation of fluorescent CDs by a hydrothermal process employing butane-1,4-diamine and maleic acid is demonstrated. Then, moving a step forward, the covalent attachment of CDs onto oxidized multiwalled carbon nanotubes, forming CDs-MWCNTs hybrid materials, was achieved. Thorough characterization and investigation of the properties of raw CDs as well as of CDs-MWCNTs was performed, revealing not only the morphological and structural characteristics of CDs as well as their photoluminescent and redox behavior, but also identifying the occurrence of electronic interactions between CDs and MWCNTs within the hybrid species. Experimental Section Materials and Reagents

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All chemicals and solvents were purchased from Aldrich and used without further purification. Short multi-walled carbon nanotubes (95% CNTs, diameter 8-15 nm, length ~500 nm) were acquired from Nanostructured & Amorphous Materials Inc and used as received. Instrumentation 1

H and

13

C NMR spectra were recorded in a 300 MHz Varian instrument operated by Vjnmr

software, with TMS used as internal standard and D2O as solvent. Steady state UV-Vis electronic absorption

spectra

were

recorded

on

a

Perkin-Elmer

(Lambda

19)

UV-Vis-NIR

spectrophotometer. Steady state emission spectra were recorded on a Fluorolog-3 JobinYvonSpex spectrofluorometer (model GL3-21). Picosecond time-resolved fluorescence spectra were measured by the time correlated single photon counting (TCSPC) method on a Nano-Log spectrofluorometer (Horiba JobinYvon), by using a laser diode as an excitation source (NanoLED, 375 nm) and a UV-Vis detector TBX-PMT series (250–850 nm) by Horiba JobinYvon. Lifetimes were evaluated with the DAS6 Fluorescence-Decay Analysis Software. Mid-infrared spectra in the region 500-4500 cm-1 were obtained on a Fourier Transform IR spectrometer (Equinox 55 from Bruker Optics) equipped with a single reflection diamond ATR accessory (DuraSamp1IR II by SensIR Technologies). A drop of the solution was placed on the diamond surface, followed by evaporation of the solvent, in a stream of nitrogen, before recording the spectrum. Typically, 100 scans were acquired at 2 cm-1 resolution. Thermogravimetric analysis was performed using a TGA Q500 V20.2 Build 27 instrument by TA in a nitrogen (purity >99.999%) inert atmosphere. Dynamic light scattering measurements were performed on a ALV/CGS-3 Compact Goniometer System (ALV GmbH, Germany), equipped with a JDS Uniphase 22mW He–Ne laser, operating at 632.8 nm, interfaced with a ALV-5000/EPP multi-tau digital correlator with 288 channels and a ALV/LSE-5003 light 6


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scattering electronics unit for stepper motor drive and limit switch control. The scattering intensity and correlation functions were measured at 90o. Correlation functions were collected for ten times and were analyzed by the cumulant method and the CONTIN software, which provides the apparent hydrodynamic radii distributions by Laplace inversion of the correlation function and by aid of the Stokes–Einstein relationship. Electrochemical studies were performed using a standard three-electrode cell. Glassy carbon was used as a working electrode and platinum wires were used as counter and pseudo-reference electrodes. Bu4NPF6 (98%) was recrystallized three times from acetone and dried in a vacuum at 100oC before being used as an electrolyte. Before each experiment, the cell was purged with Ar for 30 seconds. Measurements were recorded using an EG&G Princeton Applied Research potentiostat/galvanostat Model 2273A instrument connected to a personal computer running the PowerSuite software. The working electrode was cleaned before each experiment through polishing with a cloth and 6, 3 and 1 mm diamond pastes. AFM topological images were obtained by using an atomic force microscope (Bruker Dimension FastScan) operated in tapping mode with probe scanning rate of 1 ~ 10 Hz. HRTEM measurements were carried out at RT under a pressure of 10-6 Pa by using a JEM-2100F (JEOL) high-resolution field-emission gun TEM operated at 80 keV. HRTEM images were recorded by using a charge-coupled device with an exposure time of typically 0.5 ~ 1 s. Samples were bath-sonicated in methanol for ~ 30 s before they were drop-cast onto STEM150Cu grids (Lacey/carbon support film type B, Okenshoji Co.). The X-ray diffraction pattern was collected using a SuperNova-Agilent Technologies X-ray generator equipped with a 135 mm ATLAS CCD detector and a 4-circle kappa goniometer (CuKα high intensity X-ray micro-focus source, λ=1.5418 Å), operated at 50 kV and 0.8 mA. The specimen-to-film distance was set at 117 mm and the exposure time was set to 180 sec. The X-ray pattern, initially viewed

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using the program CrysAlisPro, was displayed and measured with the aid of the program iMosFLM. Synthesis of CDs Maleic acid (1.16 gr, 1 mmol) and butane-1,4-diamine (1.6 gr, 2 mmol) were suspended in distilled water (3 mL), introduced in a Teflon autoclave reactor and heated for 3 hours at 250 oC. Then, the reaction mixture was left overnight to cool-down slowly and afterwards the solvent evaporated to dryness. Subsequently, the yellow-brown powder was redispersed in distilled water and left standing overnight. During that time precipitation occurred, the supernatant then was pipetted and concentrated in a rotary evaporator. The as-derived solid residue was redispersed in ethanol, centrifuged for 5 min at 4000 rpm and the supernatant was filtered through a PTFE membrane filter (pore size 0.1 µm). The dark yellow filtrate was concentrated in a rotary evaporator and then dried for 4h at 130 oC to furnish a yellow-brown powder at 31% yield. Oxidized MWCNTs Pristine MWCNTs (50 mg) were suspended in oleum (25 mL, 20% SO3) and the mixture was stirred under nitrogen atmosphere for 18 h. Subsequently, a mixture of oleum/HNO3 (25 mL, 1:1 vol/vol) was slowly added, while stirring in an ice bath to avoid increasing the temperature. Then, the reaction mixture was heated at 65 oC under stirring for 2 h. After cooling down to room temperature, the highly acidic mixture was carefully treated with distilled water (150 mL) and the suspension was filtered through a PTFE membrane (pore size 0.1 µm). Oxidized MWCNTs (ox-MWCNTs) were collected, washed extensively with deionized water, methanol and dichloromethane to remove any acidic residues, until the pH of the filtrate was neutral. 8


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Preparation of CDs-MWCNTs 20 mg of ox-MWCNTs were dispersed in 30 mL of SOCl2 and refluxed overnight at 75 oC. Subsequently, the dispersion was evaporated in a rotary evaporator and the solid residue obtained was redispersed in dry THF, followed by rotary evaporation. The latter process was repeated three times in order to remove the excess SOCl2. Then, 100 mg of CDs dissolved in 10 mL of dry N,N-dimethylformamide (DMF) were introduced and the mixture was heated for 5 days at 85 oC. After that period, the reaction mixture was filtered through a PTFE membrane (pore size 0.1 µm) and washed several times with DMF, methanol and water in order to remove unreacted CDs. Results and Discussion The preparation of CDs was based on facile and simple approach. According to Scheme 1, maleic acid and butane-1,4-diamine in 1:2 molar ratio were suspended in distilled water and heated up at 250 oC for 3 hours in a Teflon autoclave reactor. The high temperature applied, together with the pressure build up within the autoclave reactor, was critical for the efficient formation of CDs possessing a carbogenic core. Furthermore, since the diamine employed in the reaction scheme was in excess, the surface of CDs was decorated with primary amines. The incorporation of –NH2 units all around the periphery of the core of CDs is advantageous since it allows functionalization developments, based on post-condensation reactions with diverse materials bearing terminated carboxylic acid moieties toward the preparation of advanced CDbased hybrids, while offering at the same time high solubility in polar protic solvents.

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Scheme 1. Preparation of fluorescent carbon dots upon thermal condensation of maleic acid with butane-1,4-diamine in an autoclave.

On the other hand, since pristine MWCNTs do not possess any acidic functionalities capable for reacting with the aforementioned amine-decorated CDs, oxidation and introduction of numerous –COOH units at the tips of MWCNTs was accomplished. Subsequently, ox-MWCNTs were converted to the corresponding acyl chloride derivatives, which according to Scheme 2, employed directly to the reaction with the free amine units located onto CDs, forming CDsMWCNTs hybrid materials via stable and robust amidic bonds.

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Scheme 2. Preparation pathway for CDs-MWCNTs hybrids. Initially, MWCNTs were oxidized, then, the carboxylic acids located at the tips were activated to the corresponding acyl chlorides, which were eventually condensed with the primary amines present onto the periphery of the surface of raw CDs.

Under the hydrothermal pressurized conditions applied, the condensation of the carboxylic acid groups of maleic acid with the amine moieties of butane-1,4-diamine was initially verified by ATR-IR spectroscopy. In this context, the ATR-IR spectrum of CDs was comprised of typical stretching vibrations due to aliphatic C-H, amidic carbonyl moieties and N-H groups of the soformed polyamides at ~2937, 1692, and 3491 cm-1, respectively (Figure 1). Furthermore, Kaiser test confirmed the presence of free amines on the surface of CDs (Supporting Information, Figure S1a) and allowed their quantitative determination. Evidently, the loading of –NH2 groups

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on the surface of CDs was calculated as 1200 µmol per gram of CDs. The success of the CDs preparation was further justified by 1H NMR studies performed in D2O, which revealed signals appearing in the aromatic region (i.e. 6.7-8.0 ppm), while simultaneously, the characteristic signals of maleic acid’s cis C=C double bond were absent (Supporting Information, Figure S2a). These findings warranted the occurrence of a carbonization process during the synthetic procedure, in which maleic acid acts as the carbon source for the construction of CDs. In addition, 13C NMR spectroscopy supported the presence of both aromatic and aliphatic carbons in the structure of CDs, however, likewise 1H NMR observations, the intensity of the signals in the aromatic carbon region is relatively low demonstrating that the carbonization process is indeed partial, yielding CDs with limited extended aromatic sites (Supporting Information, Figure S2b). Conjugating CDs to ox-MWCNTs resulted on retaining the characteristic IR fingerprints as previously identified for raw CDs, particularly those associated with the amidic carbonyl vibrations, however, broadened owing to the presence of additional carbonyl amides, while, vibrations derived from the carboxylic acid groups on ox-MWCNTs appearing at 1714 cm-1 were absent (Figure 1). Importantly, the Kaiser test for the CDs-MWCNTs hybrid material was negative, revealing only a minute amount of free –NH2 groups, accounting for barely 13 µmol/gr (Supporting Information, Figure S1b), as compared with the corresponding amount calculated for raw CDs (i.e. 1200 µmol/gr). Hence, the success of the condensation reaction toward the preparation of CDs-MWCNTs was confirmed.

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Figure 1. ATR-IR spectrum of raw CDs (blue line) as compared with the corresponding spectra of maleic acid (red line), butane-1,4-diamine (black line), CDs-MWCNTs (brown line) and oxMWCNTs (green line).

Next, dissolution and stability assays for CDs in different solvents were performed. It was found that CDs were soluble in polar protic solvents such as methanol, ethanol and DMF as well as water, due to their amino-rich exterior surface contributing to multiple hydrogen-bonding interactions, while they were insoluble in dichloromethane, toluene, petroleum ether and ethyl acetate. The highest solubility of CDs was observed in water and methanol, reaching 155.3 and 116.5 mg/mL, respectively. The molar absorptivity (ε) of CDs in water was calculated to be 81.64 L g-1 m-1 at 340 nm, by simply applying the Beer-Lambert law with the aid of a titration curve (Supporting Information, Figure S3). Briefly, a known concentration of a CDs aliquot in water (i.e. 1 mg mL-1) was diluted 2, 4 and 8 times. The intensity of the absorption at 340 nm of all four samples was recorded by UV-Vis spectroscopy and divided by the optical path length (i.e. 10-2 m). The calculated A/b values were plotted versus the concentration of each sample that

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was tested and the four points were fitted linearly. The slope of the fitted line corresponds to the molar absorptivity value (ε) of the CDs according to the Beer – Lambert law. In contrast, upon conjugation of CDs to ox-MWCNTs, the solubility of CDs-MWCNTs was decreased. Even in the polar protic solvents of methanol and water, where raw CDs showed highest solubility values, the dissolution of CDs-MWCNTs was minimum, while, the hybrid was completely insoluble in other organic solvents. However, that behavior of CDs-MWCNTs can be rationalized by considering that the free amines on the surface of CDs were no longer available for inducing and/or enhancing solubility but rather condensed with the carboxylic groups of oxMWCNTs. Initial insight on the morphology of raw CDs and CDs-MWCNTs hybrids, in the solid state, was delivered by high-resolution transmission electron microscopy (HR-TEM) assays. Spherical and quite uniform particles with diameter ranging between 2-15 nm for raw CDs were observed (Figure 2a). Careful examination revealed the lattice spacing of CDs to approximately be 0.35 nm, a value that corresponds to the {002} facet of graphitic carbon, in accordance with similar findings in the literature.45 Additionally, the crystallinity of CDs was evaluated by XRD measurements, in which the characteristic {002} peak was found at 19.2o corresponding to an interlayer spacing of 0.46 nm (Figure 2a, inset). The increase in the lattice spacing of CDs compared to the one evaluated from TEM is attributed to amorphous material that environs the carbogenic graphite segment containing core of CDs.45 The latter was further confirmed by Raman spectroscopy, revealing high fluorescence to dominate the corresponding spectrum of CDs. Markedly, TEM imaging of CDs is not a straightforward process mainly due to the relatively low contrast of CDs against the substrate of the grid (i.e. carbon film) utilized. In addition, further complications arose during the imaging of CDs-MWCNTs. Evidently, as shown

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in Figure 2b, aggregates of MWCNTs were identified, covered with amorphous matter containing CDs at sites with higher contrast. Then, HR-TEM images of ox-MWCNTs were also obtained, which unlike the ones owed to the hybrid material, clearly show elongated objects due to MWCNTs (Figure 2c). In order to shed light on the morphology of CDs-MWCNTs, AFM measurements were also conducted. In agreement with TEM, extended species similarly to MWCNTs were identified, however, with rough surface of varied width, ascribed to the existence of CDs (Figure 2d).

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Figure 2. Representative HRTEM images of (a) raw CDs (inset: XRD profile), (b) CDsMWCNTs hybrids, and (c) ox-MWCNTs. (d) AFM image of CDs-MWCNTs hybrids.

Dynamic light scattering (DLS) is a powerful tool for gathering information directly related with the size and distribution of nanosized materials in the liquid phase. Hence, an aqueous dispersion of CDs was measured by DLS and the average hydrodynamic radius (Rh) was found to be 6 nm, in full agreement with the findings from TEM analyses. Interestingly, two months later the measurement was repeated and found that the Rh was increased to around 200 nm, implying the formation of larger aggregates with the elapse of time. On the other hand, regardless that oxMWCNTs are elongated species and not spherical, an Rh value of 12 nm was estimated from DLS, which is only indicative of the small size of ox-MWCNTs. Nevertheless, the Rh of CDsMWCNTs was found to be 37 nm, significantly bigger as compared to the one of raw CDs, hence denoting the successful conjugation of CDs to ox-MWCNTs toward the formation of allcarbon nanostructured CDs-MWCNTs hybrids. Subsequently, the loading of CDs onto MWCNTs was determined by thermogravimetric analysis (TGA). Evidently, ox-MWCNTs showed a minute weight loss of 4% in the temperature range 220-450 oC under inert atmosphere, owed to the loss of the incorporated oxygenated species, while raw CDs lost approximately 85 % of their initial mass in the same temperature range (Figure 3). Remarkably, raw CDs showed a three-step weight loss at 206, 275 and 425 oC, according to the derivative of their thermograph. This sequential weight loss is ascribed to the heterogeneous nature of CDs, containing an amine-rich surface, to which the first step of thermal decomposition at 205 oC is owed, covered by an amorphous polyamidic matter, due to which the

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second step of the thermal decomposition at 275 oC is assigned, with a highly carbonaceous core possessing limited extended graphitic segments (cf. see TEM and XRD analysis), due to which the third step of the weight loss at 425 oC is allocated. On the other hand, for CDs-MWCNTs a 33% weight loss in the temperature range 220-520 oC was observed, attributed to the thermal decomposition of CDs (Figure 3). Interestingly, CDs exhibited a slightly increased stability toward decomposition upon heating when conjugated to MWCNTs, starting to degrade at 352 o

C, contrasting the 204 oC observed as the starting point for the thermal decomposition of raw

CDs. The latter is rationalized by considering the hydrophilicity of raw CDs due to the presence of numerous –NH2 units, which allows the incarceration of water molecules and/or other protic solvents in their structure, something that fails to occur in the CDs-MWCNTs hybrid material where the amine-functions participate in the formation of amidic bonds.

Figure 3. Thermographs of ox-MWCNTs (black line), CDs-MWCNTs (blue line), raw CDs (red line), and their derivatives (blue and red dotted lines, respectively), as obtained under inert atmosphere.

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The optical properties of raw CDs were examined by steady-state UV-Vis and photoluminescence spectroscopy. On the electronic absorption spectrum of CDs (Figure 4a), two shoulders in the UV region were discernable; the first at 270 nm and the second at 340 nm, where the former was attributed to π-π* transitions of the C=C bonds and the latter to n-π* transitions of the C=O groups and/or the trapping of the excited states energy by the surface states.46-48 The photoluminescence excitation spectrum of raw CDs was characterized by a broad band centred at 340 nm (Figure 4a), proving not only that the maximum emission of CDs can be achieved upon the particular excitation wavelength, but most importantly stressing that the specific CDs can emit when excited to a rather large range of wavelengths from 270 nm to 400 nm. In fact, the emission of CDs found to vary with the excitation wavelength, since the peaks of their emission diverged from ca. 410 to 480 nm as the excitation wavelength changed from 300 to 400 nm (Figure 4b). The latter is justified by considering the size distribution of CDs, according to which the photoluminescence properties change. However, the maximum intensity of the emission peak at 420 nm was achieved when the CDs were excited by 340 nm laser light. In an attempt to determine the photoluminescence mechanism of CDs, an additional experiment was set. Five different aqueous solutions of CDs were prepared, by adjusting the pH value at 1, 4, 7, 10 and 12, respectively, while keeping constant the concentration. In this way, the amine groups located on the surface of CDs were protonated at acidic pH values, hence, changes were expected at the fluorescence spectra in acidic pH values, as compared with the neutral or alkaline pH conditions where the amine units present on the surface of CDs should be deprotonated. Hence, in alkaline pH, a slight blue-shift (ca. 4 nm) of the CDs emission band was observed combined with a small intensity decrease. On the contrary, at acidic pH values, the intensity reduction was larger, while a 10 nm red-shift was also observed (Supporting Information, Figure

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S4a). Furthermore, it was evident that the red-shift of the emission, when the excitation wavelength increased, was more intense at neutral and alkaline pH values, in comparison with the red-shift of the emission when the acidic CDs dispersions were examined. Namely, a ca. 18 nm shift was discernable as the excitation varied in the range 300-380 nm, at pH 1 and 4 (Supporting Information, Figures S4a and b), while the corresponding shift for the same excitation range at pH 7, 10 and 12 was found ca. 37 nm (Supporting Information, Figures S4cf). Collectively, these findings implied that the excitation dependent emission is a result of the trapping of the excited states energy by the surface states. On the other hand, the intensity of the emission should be rather attributed to a synergistic effect resulted from both the surface and the carbonaceous core of CDs, particularly when considering that the intensity of the emission, although differed for pH values between 1 and 12, it was never fully quenched.45 The photoluminescence properties of CDs-MWCNTs were also examined and compared with those of raw CDs. To this end, the characteristic emission of CDs at 420 nm, upon excitation at 340 nm, was found red-shifted by ca. 10 nm and quantitative quenched in CDs-MWCNTs (Figure 4c), for samples possessing equal absorbance at the excitation wavelength 340 nm, suggesting strong electronic interactions between the two components in the excited states. Furthermore, the fluorescence quenching of CDs in CD-MWCNTs was supportive of electron and/or energy transfer as the decay mechanism for the singlet excited state of CDs. Then, on the basis of the time-correlated single-photon counting method, the fluorescence lifetime profiles for CDs were acquired and scrutinized. The analysis of the time-profile of the fluorescence decay at 375 nm for the singlet excited state of CDs was exclusively mono exponentially fitted with a lifetime of 7.3 ns. On the other hand, two components were identified for the CDs-MWCNTs hybrid material; a slower one with 5.8 ns lifetime, attributed to non-interacting CDs, and a faster

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one with 300 ps lifetime, corresponding to the fluorescence quenching of the emission intensity of the singlet excited state of CDs in the CDs-MWCNTs (Figure 4d). Then, by comparing the photoluminescence lifetime value for CDs-MWCNTs with the one of raw CDs, the quenching rate constant (kSq) of the singlet excited state of CDs in CDs-MWCNTs was determined to be 3.2x109 s-1. Moreover, the corresponding quantum yield (ΦSq) was calculated to be 0.96. At this point, it should be mentioned that a blank experiment was carried out, in which raw CDs were simply mixed with ox-MWCNTs (i.e. without forming covalent amide bonds), forming a noncovalent CDs/MWCNTs ensembles. Only a 25% quenching of CDs emission was observed in the non-covalent CDs/MWCNTs ensemble, contrasting the quantitative one found in the covalently formed CDs-MWCNT hybrid material (Supporting Information, Figure S5).

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Figure 4. (a) UV-Vis absorption (red line) and photoluminescence excitation (black line) spectra of raw CDs obtained in H2O. (b) Fluorescence emission spectra of raw CDs, obtained in H2O, upon different excitation wavelengths. Inset: Digital image of fluorescent CDs, obtained in H2O upon excitation at 340 nm. (c) Photoluminescence spectra, and (d) lifetime decays of raw CDs (red line) and CDs-MWCNTs (black line).

Lastly, the redox properties of CDs were assessed. Cyclic voltammetry (CV) studies were performed versus the ferrocene/ferrocenium couple (Fc/Fc+ 0.33 V) in DMF at 100 mV s-1 scan rate, using a glassy carbon working electrode, a platinum wire as counter and pseudoreference electrodes and Bu4NPF6 as supporting electrolyte. Raw CDs exhibited one irreversible oxidation at +0.54 V and one quasi-reversible reduction at -1.26 V (Figure 5). Based on that, the electrochemical band-gap of CDs was evaluated to be 1.80 eV. Cyclic voltammetry studies have not revealed clear observations for the electrochemical behavior of CDs after conjugation with MWCNTs, therefore differential pulse voltammetry (DPV) assays were performed for the CDsMWCNTs hybrids. In the oxidative DPV run, a negative shift by 100 mV for the oxidation process due to CDs was observed in CDs-MWCNTs, demonstrating that CDs were easier oxidized when interacting with the MWCNTs in the hybrid material. Additionally, in the reductive DPV run, a weak broad peak at -0.77 V related with the reduction of MWCNTs in the CDs-MWCNTs was observed. Hence, the electrochemical band gap for the CDs-MWCNTs hybrid is calculated to be 1.2 eV. In Table 1, all photophysical parameters and redox potentials for CDs-MWCNTs are collected.

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Table 1. Emission values, fluorescence lifetimes and redox potentials for raw CDs and CDsMWCNTs hybrids.

Photophysical properties Redox propertiesb Sample

Lifetime (ns) Emissiona (nm)

a

τ1

τ2

Eox

CDs

420

7.3

-

+0.54d

CDs-MWCNTs

430c

5.8

0.3

+0.44e

Obtained in H2O upon excitation at 340 nm.

b

E1red -1.26d -0.77e

E2red -1.66e

Eg 1.80f 1.20 f

Studies were performed in a standard three-

electrode cell, with glassy carbon as working electrode and platinum wires as counter and pseudoreference electrodes, employing 0.1 M Bu4NPF6 as electrolyte in DMF. Values are shown in volts versus Fc/Fc+. c Quenched as compared with the emission of raw CDs. d Data from CV. e Data from DPV. f Band-gap values in eV.

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Figure 5. (a) Cyclic voltammogram for raw CDs obtained at a scan rate of 100 mV/s. (b) Oxidative and reductive DPV runs for raw CDs (red line) and CDs-MWCNTs (black line), obtained at a scan rate of 25 mV/s. All measurements conducted in dry and deaerated DMF with 0.1M Bu4NPF6 as supporting electrolyte.

Conclusions Summarizing, fluorescent carbon dots (CDs) were prepared by a hydrothermal method employing butane-1,4-diamine and maleic acid. The so-produced CDs showed excellent solubility in protic polar solvents as well as water. Comprehensive characterization of the CDs, via NMR, ATR-IR, TGA, DLS, XRD and HR-TEM, revealed the presence of numerous –NH2

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units located in the external periphery of spherically shaped CDs. The loading of the amine functionalities on CDs was evaluated to be 1200 µmol per gram of CDs by employing the Kaiser test. Then, the amine-rich CDs were condensed with acyl chloride activated MWCNTs yielding the all-carbon CDs-MWCNTs hybrid material. The covalent association of CDs and MWCNTs resulted on the formation of hybrid species strongly bound together via robust and stable amide bonding, allowing effective electronic communication between the individual species to take place. The photophysical and redox properties of CDs-MWCNTs were examined by detailed absorption, photoluminescence and electrochemistry assays. Based on the optical studies, particularly the quantitative photoluminescence quenching of CDs by the presence of MWCNTs, meaningful insight regarding electronic interactions between the two moeities, within the CDsMWCNTs hybrid material, was identified. Furthermore, time-resolved photoluminescence measurements gave rise to two lifetimes for CDs-MWCNTs, a slow one at 5.8 ns due to noninteracting CDs – the singlet excited state of raw CDs was exclusively mono exponentially fitted with a lifetime of 7.3 ns – and a fast one at 300 ps, attributed to the fluorescence quenching of the emission intensity of the singlet excited state of CDs in CDs-MWCNTs. Eventually, the oxidation of CDs accompanied by the reduction of MWCNTs within the hybrid material was ascertained by a combination of cyclic and differential pulse voltammetry, allowing the determination of the electrochemical band gap to be 1.2 eV. Evidently, such all-carbon nanostructured hybrid materials possess the appropriate characteristics to be utilized as ideal candidates in energy conversion applications with particular emphasis to solar light harvesting as well as in photocatalysis. Further explorations along those lines are in progress in our group.

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Supporting Information. Spectroscopic data for raw CDs and reference CDs/MWCNTs ensembles are presented in the supporting information section. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

*Tel + 30 210 7273835; Fax + 30 210 7273794, [email protected]. Author Contributions

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

These authors contributed equally.

ACKNOWLEDGMENT We thank Prof. Kenichiro Itami at the Department of Chemistry, Nagoya University, Japan, and Dr. Yuhei Miyauchi at the Institute of Advanced Energy, Kyoto University, Japan, for using the AFM apparatus. We also thank Dr. Evangelia Chrysina at the Institute of Biology, Medicinal Chemistry and Biotechnology, National Hellenic Research Foundation, Greece for her assistance with the XRD measurements. REFERENCES 1.

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Table of Contents Graphic

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All-Carbon Nanosized Hybrid Materials: Fluorescent Carbon Dots

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