Interaction between poly(vinyl pyrrolidone) PVP and fullerene C60 at the interface in PVP-C60 nanofluids–A spectroscopic study

Fourier transform infrared and Raman bands shows a discernible enhancement in band intensity of C–H stretching, C=O stretching, C–N stretching, C–H2 bending, and C–H2 in-plane bending in PVP molecules in the presence of C60 molecules. Amplification in intensity is ascribed to microscopic interactions results when a donation of nonbonding electron (n) occurs from a “>N–C=O” entity of PVP into a lowest unoccupied molecular orbital of the C60 molecule in PVP-C60 charge transfer (CT) complex. The C=O stretching band intensity (integrated) Vs C60 content plot exhibits a peak near a critical 13.9 μM C60 value owing to percolation effect. Light emission spectra show that even a small addition of 4.63 μM C60 able to suppress the band intensity by ~23% as a result of an energy loss. The integrated band intensity also decreases through a peak near 13.9 μM when plotted against the C60-content. In correlation to the vibration spectra, the maximum effect observed both in light emission and excitation spectra suggests a percolation effect in the CT complex. Exhibition of percolation threshold in C60-PVP donor-acceptor complex will be helpful in optimizing the photovoltaic properties vital for solar cell applications.


Introduction
Since inception, fullerene (C60)an allotrope of carbon has been attracting most to the research community around the globe owing to its distinctive physico-chemical properties. Spanning of electronic absorption bands in the large part of UV-Vis spectrum, excellent electron acceptor quality, low reorganization energy, and ability to generate efficient reactive oxygen species like singlet oxygen in solution make C60 an attractive ingredient for various applications such as antibacterial activity, solar cells, photodynamic therapy, and photo-catalyst. Donor-acceptor systems where C60 act as electron sink are of potential applications in molecular electro-active devices and work as model compounds for artificial photosynthesis. It is reported that the electron transfer from an organic compound to C60 molecule via photo-irradiation is an essential requirement for designing of organic solar cells [1][2][3] . In organic photo-voltaics, C60 and derivatives act as acceptors due to their high electron accepting ability, high electron conductivity, and great potentiality to promote charge separation at the donor/acceptor interfaces [4][5][6][7][8][9][10]. The photo-conversion efficiency of solar cells can be improved in presence of C60 cluster as they can minimize the direct interaction between the excited sensitizer and redox couple [4]. In recent years, work on polymer/C60 bulk hetero-junction photovoltaic cells are bringing attention because of their potential inexpensive fabrication route, light weight and high rate of production [6,9]. C60 based ultrathin layers are used in organic light emitting devices as a backlight for liquid crystal displays [7,8]. Presence of C60 molecules leads to increase the photoconductivity of conducting polymers and organo-metallic compounds owing to photo-induced charge transfer processes useful for photovoltaic applications [5,10] In spite of wide applications of C60 based materials, the limited solubility and prone to aggregation in most aqueous & non-aqueous solvent greatly hinders the progress in most of the proposed applications. In this regards, various methods have been proposed to improve the solubility and stability of C60 in solution media. The development of C60 dyad systems and their assembly via surface modification route is proved to be an excellent method as the C60 moiety in such donor-acceptor systems almost retains its redox and various photo-physical properties. In this route, a stabilizing agent is added so that C60 molecules can fully coated/encaged by a thin surface layer which facilitates solubilization/dispersion process in a solvent [11][12][13][14][15][16][17][18][19][20]. We have used poly(vinyl pyrrolidone) PVP as surface modifier to develop PVP-C60 nanofluids (NFs) in n-butanol and water [11,19,20]. PVP polymer is a non-toxic biocompatible material with high complexing ability [11][12][13][14][15][16][17][18][19][20]. Also, PVP molecules has the ability under copymerization reaction with poly(vinylidene fluoride), poly(vinyl alcohol), polyacrylonitrile, poly(vinyl chloride), and poly(vinyl formal) in many inorganic or organic solvents. It has been used extensively as a solubilizing agent and to form rheo-optical NFs. The lone pair of electrons present on O and N atoms of the pyrrolidone and ability to undergo keto-enol tautomerism helps PVP to sollubilize C60 molecules in water or in many non-aqueous solvent as they form donor-acceptor type complex [11,19,20].
There are lots of literatures available to support the existence of interfacial interaction between a donor and acceptor using vibration and photoluminescence (PL) spectroscopy. In a paper, Hartestein et al. [21] reported that C-H stretching band intensity is enhanced in 4-nitrobenzoic acid as largely as 10 4 times when a thin silver over layer (0.2-6 nm) forms over it. As it is pointed out by Konarev et al., [22] a complex formation of C60 with an organic electron donor gives rise to only a very small 1-2 cm -1 shift in some of the C60 bands owing to an unfavorable steric effect. Shifting of C=O stretching frequency to the lower value by 60 cm -1 has been observed as a result of formation of bond between >C=O group and Pt in a CT complex [23]. In another report, Xian et al. [24] have observed a size-dependent interaction of PVP with Pd NPs. For example, Pd NPs (~20 nm diameter) bond to PVP on both the O 2and N 3sites over >N-C=O moieties in pyrrolidone rings. Finer Pd NPs of diameter ~5 nm adsorb PVP merely on the O 2sites. As much red-shift as 4 cm -1 has been observed in the C=O stretching frequency in a PVP-Pd complex when PVP monomer to Pd molar ratio (φpm) was taken to be 1.0. This value had been increased to as large as ~22 cm -1 on φpm o 0.005. As also reported by Grace and Pandian [25], only a weak coordinative chemical bonding appears via C=O moieties of PVP with a nanogold in this example of NFs, showing a marginally decreased C=O (PVP) stretching frequency by 10 cm -1 . Ramakanth and Patnaik [26] reported that C60 quenches light-emission of pyrene by as much as 90 % at 0.05 mM content. Sluch et al. [27] suggested that pyrene loses light emission intensity drastically to a low value in presence of C60 molecule due to its high electron affinity.
In this paper we report a study on the vibrational and luminescence spectra of PVP-C60 NFs in nbutanol. The vibrational properties of the synthesized NFs were studied to verify the existence of interaction between PVP and C60 molecules using Fourier Transform Infrared (FTIR) and Raman spectroscopy. The photoluminescence (PL) properties were studied in terms of light emission and excitation spectra.

Materials and method
We used C60 powder (99.9% purity) and PVP (K-25) which were procured from Alfa Aesar and Aldrich chemicals, respectively. The two analytical grade solvents which are used for developing NFs are Toluene and n-butanol. In order to develop NFs we first prepare two precursor solutions (PS) of optimized concentrations. We named those two solutions as PS-1 and PS-2. PS-1 solution has a C60 concentration 1.39 mM and the PVP which was named as PS-2 has 1.1 M. As our aim was to prepare NFs of various concentrations, we added different volumes of PS-1 to five different batches of the PS-2 using syringe. The mixture solutions were stirred magnetically at room temperature. It was then sonicated at 50 0 C for 20 min by fixing the frequency at 20 kHz and power at 250 W. Then we remove toxic toluene solvent by keeping them in a vacuum oven maintained at 3 kPa pressure and 110 0 C for a period of ∼10 h. In this manner we obtained solid samples of PVP-coated C60. AS our motto was to develop NFs in an aqueous medium, we added deionized water to the solid PVP-surface modified samples of C60 and then sonicated at 50 0 C to develop aqueous C60 NFs. We adopted a procedure to confirm the final C60 concentration in our developed aqueous PVP solutions as described in literatures [11,19,20].

Characterizations
Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet Corporation, Model NEXUS-870) was used to study the vibrational spectra of prepared nanofluids. Attenuated total reflectance mode was selected while recoding the vibrational spectra. We used sample holder made of ZnSe crystal. Computerassisted Perkin-Elmer (Model-LS 55) luminescence spectrometer was used to record emission and excitation spectra of samples. The instrument was fed with a red sensitive PMT detector (RS928) and a high-energy pulsed xenon discharge lamp was used as an excitation source (average power 7.3 W at 50 Hz). Raman spectrometer (Renishaw, 514 nm Argon ion laser, 40mW) was used to record Raman spectra of our samples.

FTIR and Raman bands in PVP-C60 NFs
We studied FTIR and Raman bands in illustrating the donor-acceptor type interaction between C60 and PVP solubilized in n-butanol. Figure 1 shows typical FTIR bands in C60:PVP NFs consisting of (a) 0, (b) 4.63, (c) 9.26, (d) 13.9, and (e) 18.52 µM C60 with 40.0 g/L PVP in n-butanol. As expected, an increase in the C60 dosage clearly appears in selective IR bands either in their frequencies or peak intensities. The band intensity is enhanced markedly in the C−H stretching bands shown at 2963, 2935, and 2874 cm -1 , C=O stretching band at 1666 cm -1 , two C-N stretching bands at 1497 and 1,291 cm -1 , C-H2 bending at 1463, 1438, and 1321 cm -1 , and C-H in-plane bending at 1223 and 1378 cm -1 in PVP molecules in the presence of C60 molecules. Such features of microscopic interfacial interactions arise in a PVP-C60 nanofluid when a donation of lone pairs of electron occurs from a ">N-C=O" moiety into a LUMO (t1u) band of the C60 molecule in a CT complex [16][17][18].  Furthermore, the 1665 cm -1 C=O stretching band of parent PVP is decreased progressively to 1659 cm -1 upon loading-up C60 up to 13.9 μM with a markedly enhanced peak intensity in a spectrum in figure  1d. As plotted in Fig. 1B, this band exhibits a peak in its integrated peak intensity at this critical 13.9 μM C60 value owing to a percolation effect. It is a result of a weak interfacial interaction of PVP with the C60 molecules in the NFs [11,19,20,[23][24][25]. The various FTIR and Raman bands observed in a bare sample 40 g/L PVP in n-butanol and that modified with a selective 18.52 μM C60 dosage in the form of a nanofluid are given in Table-1. In a close correlation with the various literatures' results on reinforced PVP using different kinds of metal particles [23][24][25]28,29], our studies of the synthesized PVP-C60 NFs explicitly reveal that the PVP molecules are bonding over a nascent C60 nanosurface only weakly through the ">N-C=O" moieties (PVP) owing to primarily of steric restrictions.  interface interaction with the C60 molecules as it is reflected in a small red-shift of 5-8 cm -1 in the respective bond stretching frequencies besides effectively enhanced band intensities. Small Raman shift characterizes only a weak CT interaction taking place between the two PVP and C60 entities which appear in presumably small joint assemblies. Further, we studied the integrated Raman band intensity (Iint.) for the C=O stretching vibration in PVP in the PVP-C60 NFs in n-butanol. A plot made in Fig. 3 reveals that the Iint exhibits a peak near a Cp1 = 13.2 μM C60 value, which compares well to a value 13.9 μM C60 found from the IR bands.

Emission and excitation spectra in PVP-C60 NFs
Now let us analyze light emission spectra observed in the PVP-C60 NFs which contain (a) 0, (b) 4.63, (c) 9.26, (d) 13.9, and (e) 18.52 µM C60 along with 40.0 g/L PVP in n-butanol. Figure 3 compares the spectra measured over 350-550 nm by exciting the samples at a common λem = 325 nm in order to comment on the effects of the variation of the C60 dosages on the emission bands. A virgin sample before adding any C60 exhibits a strong light emission with λmax lying at 395 nm in bare PVP polymer dispersed in n-butanol. This band refers to the n ← nπ* transition from C=O moieties in exfoliated PVP configurations as it is also observed earlier in PVP solution [19,20,[29][30][31][32][33]. Even a small addition of only 4.63 μM C60 suppresses this band significantly so as it appears in a broad band (Band-1) which is red-shift to 401 nm with a nearly ~23% decreased peak intensity. This is a result of an energy loss of the light-emission which confers an nelectron transfer from pyrrolidone group to the electron deficient C60 in a PVP-C60 CT complex. As large as ~67% fall in the emission intensity of bare PVP polymer occurs in this region when the C60 dosage is increased to 18.52 μM C60 in a sample of 40.0 g/L PVP in n-butanol. ` As shown in the inset of figure 3, a deconvolution of light emission spectrum in the virgin PVP describes two overlapping bands of λmax lying at 400 and 435 nm. The band at 435 nm (Band-2) occurs from the n ← nπ* transition from >N-C moieties in exfoliated PVP configurations. We also studied normalized emission band intensity (Iem) in the two bands over the 0−18.52 μM C60-contents. As portrayed in figure 4, the Iem-value falls down with C60-content showing a small plateau over 8-14 μM C60 before it drops rather rapidly when the localized electron density has decreased over the C60 surfaces. To analyze whether the light-emission in PVP-C60 NFs follows the excitation process from the energy levels of the same optical species in a sample, we studied the excitation spectra by irradiating the samples in the emission band under identical conditions. Figure 5 shows the results of the excitation spectra which were scanned by irradiating the different samples at a common emission wavelength λem = 400 nm. As expected, also the excitation spectrum loses its intensity over 290-420 nm on the presence C60 of different dosages from 4.63 to 18.52 μM in a 40.0 g/L PVP in n-butanol. Both the emission and excitation spectra follow the same trend of the intensity loss as a function of the C60 content. These results confer unambiguously that the light emission and excitation spectra arise from the same optical entities of energy levels in the two regions. The spectra shown in figure 5A comprise at least three distinct bands overlapping on another. For example, a deconvolution of a typical spectrum (c) exhibits the three distinct bands of 334, 355, and 386 nm of the peak-values. The integrated band intensity (Iex) also in the excitation spectrum decreases through a peak near Cp1 = 13.9 μM when plotted against the C60-content ( Figure 5B). The results confer a correlation in the light absorption, emission and excitation spectra in the present PVP-C60 NFs which all show a maximum effect at a common C60 content of ~13.9 μM (a percolation threshold).

Conclusion
The surface interaction between C60 nanoparticle and PVP molecule in PVP-C60 NFs was studied with the help of vibration and PL spectra. Amplification in the intensity some of the vibration band of PVP molecules in the presence of C60 molecules is ascribed to an interfacial interaction which arise when a donation lone pairs of electron occurs from a ">N-C=O" moiety of PVP into the C60 molecule in a CT PVP-C60 complex. Red-shift in the emission band of PVP molecule in presence of C60 molecule occurs as a result of an energy loss due to CT from PVP to C60 molecule. Demonstration of percolation effect in the vibration property is well agreed with PL result in NFs. Existence of donor-acceptor interaction in a hybrid system is useful of photovoltaic applications.