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Fluorinated Surface of Carbon Nanotube Buckypaper for Uniform Growth of CdS Nanoparticles Yuliya V. Fedoseeva, Vyacheslav Evgenievich Arkhipov, Eugene Maximovskiy, Artem Vladimirovich Gusel'nikov, Yuri L. Mikhlin, Konstantin S. Zhuravlev, Boris V. Senkovskiy, Stanislav Vasilievich Larionov, Lyubov G Bulusheva, and Alexander V. Okotrub J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04640 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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The Journal of Physical Chemistry

Fluorinated Surface of Carbon Nanotube Buckypaper for Uniform Growth of CdS Nanoparticles Yu. V. Fedoseeva*1,2, V. E. Arkhipov1, E. A. Maksimovskiy1, A. V. Gusel’nikov1, Yu. L. Mikhlin3, K. S. Zhuravlev4,2, B. V. Senkovskiy5,6, S. V. Larionov1, L. G. Bulusheva1,2, A. V. Okotrub1,2 1

Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Acad. Lavrentiev Ave., Novosibirsk 630090, Russia 2

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Novosibirsk State University, 2 Pirogova Str., Novosibirsk 630090, Russia

Institute of Chemistry and Chemical Technologies SB RAS, 50-24, Akademgorodok, Krasnoyarsk 660049, Russia

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A.V.Rzhanov Institute of Semiconductor Physics SB RAS, 13 Acad. Lavrentiev Ave., Novosibirsk 630090, Russia 5

St. Petersburg State University, 7-9 Universitetskaya Nab., St. Petersburg 199034, Russia 6

Physikalisches Institut, Universitätzu Köln, 77 Zülpicher Str., Köln 50937, Germany

Abstract: Interfacial interactions between CdS species and multi-walled carbon nanotubes (MWCNTs) determine the architecture and optical characteristics of CdS/MWCNT hybrids. The effect of fluorinated surface of MWCNT buckypaper on the decoration with CdS nanoparticles from an ammonia solution of cadmium(II) chloride and thiourea has been studied. Scanning electron microscopy showed that the fluorinated carbon surface provides more dense growth and uniform distribution of CdS nanoparticles. X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopies revealed a partial defluorination of the nanotubes and the formation of elemental sulfur as a result of a joint action of thiourea and ammonia. The carbon atoms freed of fluorine were found from density functional theory calculations to be active sites for attachment of Cd(II) complexes present in the chemical bath and subsequent growth of CdS nanoparticles. The obtained hybrid possesses dual photoluminescence from fluorinated MWCNTs and CdS nanoparticles. Under applied electric field, it emits blue light that is possible only for very small CdS nanocrystals. This work provides a concept for fabrication of CNT-based hybrid structures with high density of semi-conducting nanoparticles for optoelectronic applications.

Corresponding Author: E-mail: [email protected]. *

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1. Introduction For more than 10 years, the materials from CdS nanoparticles and carbon nanotubes (CdS/CNTs) have attracted attention of many researches due to a combination of high electrical conductivity of CNTs and size-tunable optical properties of CdS nanoparticles.1,2. The CdS/CNT hybrids are potential candidates for high-efficiency microwave absorption materials,3 photocatalysts for environmental remediation,4 and optoelectronic devices such as optical detectors, sensors, and photovoltaic systems.5 Since nanometer size of CdS crystals limits pathways for charge carriers, the combination with CNTs can improve their optoelectronic characteristics. Robel et al showed that the deposition of CdS quantum dots on the surface of single-walled CNTs (SWCNTs) resulted in a fast electron transfer from excited CdS nanoparticles to SWCNTs.6 Silva et al demonstrated the formation of ohmic contact and charge transfer from CdS nanoparticles to CNTs in hybrid materials.7 The photocurrent of CdS thin films modified with multi-walled CNTs (MWCNTs) was found to increase with respect to pure CdS film.8 This result authors explained with a higher efficiency either in charge collection or in charged pair production. Thus, CNTs can be effectively used as charge conductors and collectors in CdS-based optoelectronic devices. The CdS/CNT hybrids commonly possess photoluminescence (PL), which originates from CdS nanoparticles. However, the intensity of PL from CdS/CNTs is not so high, because nonradiative decay of excited state in CdS via transfer of photo-excited electrons from CdS to conduction band of a CNT.9 The insulating layers, such as silicon oxide or polymer, between CNTs and CdS nanoparticles can prevent this decay and enhance PL intensity of nanocomposites.10,11 Therefore, synthesis of new materials based on CdS and CNTs with improved PL properties is an actual task. Chemical bath deposition (CBD) is the simplest way to cover the CNT surface by CdS nanoparticles. Recently, we have shown that MWCNTs produced by catalytic chemical vapor deposition (CCVD) technique can be effectively decorated by CdS nanoparticles using the CBD from an aqueous ammonia solution of CdCl2 and thiourea (SC(NH2)2).12 An increase of the temperature of the reaction mixture and time of the synthesis led to the growth of CdS particles of big size and high crystallinity.13 CdS nanocrystals with an average size of less than 3 nm provided the blue photoemission of CdS-MWCNTs composites excited at the 325-nm wavelength.14 Quantum-chemical calculations revealed that Cd2+ ions and Cd(II) complexes should not interact with defect-free CNT walls, and nucleation and growth of CdS particles on the CNT surface more likely occur at edge states of graphene network.15 It was reported that the presence of oxygen-containing groups on the CNT surface assists adsorption of Cd2+ ions from an aqueous solution.16 These functional groups along with defect sites appearing because of acid treatment or air oxidation of CNTs improve the nucleation of CdS on nanotube surface.8,17 In contrast to the oxygenation, fluorination results in attachment of a greater 2 ACS Paragon Plus Environment

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quantity of foreign atoms to a CNT without destruction of its tubular structure.18 Since π-electrons of a CNT are involved in the formation of covalent C‒F bonds, electrical conductivity of the fluorinated CNT decreases with an increase of the fluorine loading.19 Electrically resistive fluorinated outer shells of a MWCNT located between CdS and conductive inner shells can decrease nonradiative decay rate of an excited state in CdS and increase the PL efficiency. Kim et al have reported that fluorinated double-walled CNTs (DWCNTs) are more effective for growth of cadmium selenide (CdSe) nanoparticles using the CBD method than the oxygenated and nonmodified DWCNTs.20 The decoration of fluorinated CNTs by CdS nanoparticles opens up new avenues in designing of materials consisting of CdS nanoparticles and CNTs for optoelectronic applications in visible light region. In the present study, we have used buckypapers from as-produced and fluorinated MWCNTs (F-MWCNTs) as substrates for growth of CdS nanoparticles from an aqueous ammonium solution of cadmium(II) chloride and thiourea during 5 and 15 min. Methods of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were applied to reveal the difference in the formation of CdS nanoparticles on the two substrates. Density functional theory (DFT) calculations were used to investigate the reactivity of fluorinated CNTs towards mixed cadmium complexes. Luminescence characteristics of nanocomposites consisting of CdS nanoparticles and F-MWCNTs were analyzed at excitations by UV laser and electrical field. 2. Materials & Methods 2.1. Synthesis. MWCNTs were synthesized in a horizontal reactor using aerosol-assisted CCVD. The details of the synthesis are described elsewhere.21,22 A 5 wt% solution of ferrocene in toluene was injected into the pumped quartz tubular reactor heated to 720 ºC using nitrogen as a carrier gas at a flow rate of 600 mL/min. An array of MWCNTs was grown on quartz tube walls of the reactor as a result of thermolysis of the reaction mixture. As-produced MWCNTs were estimated to be 600 µm in length and ~40 nm in diameter. A powder of the MWCNTs was treated by a mixture of nitric and sulfuric acids taken in a volume ratio of 1:3 at 90 ºC for 60 min. Then, the suspension was diluted with distilled water and filtered through a polyester membrane with a pore size of 1 µm under vacuum pumping. Thereafter, the MWCNTs were washed by distilled water three times to remove the residual acids. As soon as water was drained, the wet film together with the membrane was dried in an oven at 100 ºC for 2 h. The film was detached from the membrane in the form of buckypaper consisting of the MWCNTs.



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Figure 1. Schematic illustration of modification of CNTs by means of fluorination and decoration by CdS nanoparticles. The modification of the MWCNTs by means of fluorination and subsequent decoration by CdS nanoparticles are schematically presented in Figure 1. The fluorination of the MWCNTs was carried out using the procedure described in detail elsewhere.23 A piece of the MWCNT buckypaper was placed in a Teflon flask and held in saturated vapors over liquid BrF3 and Br2 (10 vol%) at 25 ºC during four days. Then, the flask content was dried by a flow of nitrogen for 48 h until the termination of Br2 evolution. An active molecular oxidizer BrF3 is used as fluorinating agent under mild conditions for synthesis of graphite fluorides24 and fluorination of CNTs, resulting in covalently attachment of fluorine atoms to walls of CNTs without destruction their tubular structure.25,26 The formation of CdS nanoparticles on the MWCNTs and F-MWCNTs was performed using the CBD method developed in ref. 12. Thiourea (SC(NH2)2) and cadmium(II) chloride (CdCl2) were separately dissolved in 7 wt% ammonia solution in water and heated to 40ºC. The concentrations of CdCl2 and SC(NH2)2 in the ammonia solution were 0.015 and 0.15 mol/L, respectively. Initial and fluorinated MWCNT buckypapers were fixed to Teflon holders, which were placed into a heated ammonia solution of CdCl2 at the same time. Almost immediately after submergence of the holder, the ammonia solution of SC(NH2)2 was added to the solution of CdCl2 and thoroughly stirred. Five or fifteen minutes later, the holders were taken out from the reaction mixture and washed by distilled water two times. Then, the buckypapers of MWCNTs and FMWCNTs with grown CdS nanoparticles (labelled as CdS/MWCNTs and CdS/F-MWCNTs) were removed from the holder and dried at ambient conditions. 2.2. Experimental section. Morphology of the samples was examined by SEM on a JEOL JSM 6700F microscope using secondary electron (SE) and back-scattered electron (BSE) modes. TEM analysis of the samples was performed on a JEOL 2010 microscope. 4 ACS Paragon Plus Environment

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NEXAFS and XPS spectra were recorded at the Berliner Elektronenspeicher ring für Synchrotronstrahlung (BESSY II) using radiation from the Russian-German beamline. The NEXAFS spectra near C K-edge were recorded in the total-electron yield mode with an experimental resolution better than 0.1 eV. The XPS spectra were measured at the energy of monochromatized synchrotron radiation equal to 850 eV. Electrons emitted normally to the sample surface were collected, and the angle between incident radiation and the analyzer was 55°. Photoelectron spectra originating from deeper probing depths were also recorded on a SPECS spectrometer (Germany) equipped with a PHOIBOS-150 MCD-9 hemispherical energy analyzer using a monochromatic Al Kα (1486.6 eV) radiation. The high-resolution core lines were measured at the constant pass energy of electron energy analyzer of 10 eV at the electron take-off angle of 90o. Fitting of the spectra was carried out after subtraction of a Shirley background using a Gaussian/Lorentzian function within Casa XPS 2.3.15 software. A HeCd laser operated at wavelength of 325 nm and an excitation power density of 0.5 2

W/cm excited the PL emission spectra. The measurements were performed at room temperature. Light emission induced by electric field was registered using a homemade set-up for measurement of field electron emission from nanomaterials.27 The sample was attached to a negative electrode, while indium tin oxide coated glass was used as an anode. The interelectrode distance was 500 µm. A rectangular-shape pulsed voltage of 1.5 kV with frequency of 200 Hz was applied to anode. 2.3. Computational details. Since MWCNTs have a big outer diameter of an average size 40 nm, the nanotube surface was modeled by a C73H21 graphene fragment, where hydrogen atoms saturated the dangling bonds at the fragment edges. Fluorinated graphene models with compositions C73F3H21, C73F5H21, and C73F7H21 were constructed based on this fragment. Three, five, and seven fluorine atoms were arranged on one side of the graphene fragment in such a way, that they separated one, three, and five non-fluorinated carbon atoms from others, correspondingly. Cd(II) complexes [(NH3)iCdSH]+ (i = 0−2) were located at central part of the non-fluorinated side of the models. The fluorinated graphene fragments with Cd(II) complexes were calculated using the threeparameter hybrid functional of Becke28 and Lee−Yang−Parr29 correlation functional (B3LYP method) included in the Jaguar 9.2 program package.30 Atomic orbitals were described using a LACVP** basis set, where polarization functions are included for all atoms except for cadmium. The geometry of the models was optimized completely using an analytical method to the gradient of 5×10-5 atomic units. The interaction energies (Eint) between Cd(II) complexes and backside fluorinated graphene fragments were calculated as Eint = Etot(model) − Etot(fluorinated graphene) − Etot(Cd), where Etot(model) – a total energy of optimized model, Etot(fluorinated graphene) – a total 5 ACS Paragon Plus Environment


energy of optimized isolated fluorinated graphene fragment, and Etot(Cd) – a total energy of optimized Cd(II) complex, respectively.

3. Results & Discussion 3.1 Structural aspects. SEM images of buckypapes after the CBD synthesis for 15 min are presented in Figure 2a–f. The image analysis detected CdS nanoparticles of nanometer size and big agglomerates of a size less than 1 µm (Figure 2a, d). To detect distribution of elements in the sample the signal from BSE was analyzed, since the intensity of BSE is strongly related to the 60

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Figure 2. SEM images of CdS/F-MWCNT (a–c) and CdS/MWCNT (d–f) taken using secondary electrons (a, c, d, f) and back-scattered electrons (b, e). HRTEM images of CdS/F-MWCNT (g, h). Duration of CdS growth was 15 min. Insets show diameter distribution of the CdS nanoparticles. 6 ACS Paragon Plus Environment

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atomic number of the element. The BSE SEM image of the CdS/F-MWCNT sample showed not only big bright spots, but also bright tubes (Figure 2b). A high intensity of the BSE signal was assumed to arise from CdS particles uniformly distributed along the F-MWCNTs. On the contrary, the BSE SEM image of the CdS/MWCNT sample showed mainly bright spots of big CdS particles, while bright tubes were not observed (Figure 2e). Analysis of particle size distribution revealed that the average size of CdS nanoparticles grown on F-MWCNTs is 15 nm, CdS nanoparticles formed on MWCNTs had the average size of 37 nm. CdS nanoparticles were discovered to be flat with a thickness of 5 and 15 nm for CdS grown on F-MWCNTs and MWCNTs, respectively. The HRTEM analysis indicated that some MWCNTs have opened ends through the acidic treatment (Figure 2g). The unexpected result was the narrowing of the inner cavities in some fluorinated MWCNTs (Figure 2h). It is likely, that the fluorinating agent was able to penetrate between the nanotube layers through the opened ends. This could allow both side fluorination of the layers thus increasing the interlayer distance and up to almost the complete disappearance of the hollow cavity. The deposition of CdS nanoparticles did not significantly change morphology of the MWCNTs and F-MWCNTs. The HRTEM images showed that CdS nanoparticles have a polycrystalline structure and are attached on the surface and ends of MWCNTs and F-MWNTs (Figure 2g–h). Small-size CdS nanoparticles may not be registered because of their instability under electron beam.31 3.2. XPS and NEXAFS study. XPS survey spectra of MWCNTs and F-MWCNTs showed the presence of C as a dominant element, O (3 at%), Fe from remaining catalyst (less than 1 at%), as well as 31 at% of F in the case of the F-MWCNTs. For the both CdS/MWCNT and CdS/FMWCNT samples, atomic concentrations of Cd and S were estimated to be about 1 at% and 4 at%, respectively. The concentration of O in the samples after the deposition of CdS was no more than 4 at%. The ХPS C 1s core lines of the F-MWCNTs in the initial buckypaper and that after the CBD were fitted by four peaks centered at 284.5, 286.0, 288.5, and 290.7 eV (Figure 3a). The first two peaks arise from nonfluorinated carbon atoms, namely the one at 284.5 eV corresponds to the sp2 hybridized carbon and that at 286.0 eV is assigned to the carbon atoms linked with CF groups (C*CF) and oxygen-containing groups. The components at 288.5 and 290.7 eV originate from carbon atoms bonded with one fluorine atom (CF) and two fluorine atoms (CF2).32 According to the fitting of the C1s spectra, a ratio between carbon and fluorine atoms complies with a stoichiometry of CF0.45. The CF2 groups were likely formed at the open ends or vacancy defects developed after the treatment of MWCNTs by acids. The C 1s spectra of the CdS/F-MWCNTs obtained after the CBD synthesis for 5 and 15 min showed a decrease and downshift of the components CF and CF2 relative to those in the F7 ACS Paragon Plus Environment


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MWCNTs spectrum (Figure 3a). This means a defluorination of the F-MWCNTs in the ammonia solution of cadmium(II) chloride and thiourea. The CF2 groups almost completely disappeared, while the CF groups partially remained in the samples. The CF2 to CF ratio of 0.4 in the FMWCNTs reduced by half after 5 min of the CdS deposition, indicating the higher reactivity of the CF2 groups in the CBD conditions. The quantity of F atoms per one C (F/C) decreased to 0.26 and 0.20 after the synthesis for 5 and 15 min, respectively. The above spectra were measured at a photon energy of 850 eV, with an estimated probing depth of the sample of ~1.5 nm. In order to obtain information about the chemical states of deeper shells of MWCNT samples, the Al Kα X-ray source with 1486.6 eV was used. In this case, the electron inelastic mean free path was ~2 nm. The XPS C 1s spectra of the F-MWCNTs and CdS/FMWCNTs (after 5 min of CdS deposition) measured at 1486.6 eV showed the F/C ratio of 0.50 and 0.20, respectively (Figure 3b). The concentration of F in the both samples is a bit larger than that in the spectra measured at 850 eV. This implies that the F-MWCNT surface was partially restored, while deeper carbon shells retained more fluorine owing to their less accessibility for the defluorinating agents. The similar phenomenon of the increase of the CF peak intensity with the probing depth was observed in the XPS C 1s spectra of the fluorinated SWCNT ropes.33 The XPS C 1s spectrum of the MWCNTs exhibited one asymmetrical peak centered at 284.5 eV, which is typical for graphite and CNTs (Figure 3c). No significant changes in the spectral profile were observed after the decoration of the MWCNTs by CdS nanoparticles. However, the deposition of CdS for 15-min induced a shift of the C 1s line to a higher-energy side by 0.2 eV. This can be explained by an upshift of the Fermi level due to a transfer of electronic density from CdS nanoparticles to the MWCNTs. The XPS F 1s spectrum of the F-MWCNTs was fitted by two components at 687.0 and 687.8 eV ascribed to the covalent C–F bonds in the CF and CF2 groups, respectively (Figure 3d). The CdS/F-MWCNT spectra exhibited single maxima at 686.6 eV corresponded to the CF groups and no high-energy components from the CF2 groups. The shift of the CF component to the lowenergy region in the spectra of the hybrids as compared to that in the F-MWCNT spectrum arises due to a lower concentration of fluorine as it has been previously observed for the CNTs with different fluorine loadings.19 The NEXAFS C K-edge spectrum of the MWCNTs showed sharp π*-resonance at 285.4 eV and σ*-resonance at 291.8 eV (Figure 4a). The energy positions of these peaks are typical for graphite and CNTs.34,35 The features appearing between these resonances in the range 286.5–290.0 eV are usually attributed to chemically functionalized carbon. In the case of the initial MWCNTs, weak features A at 287.4 eV and B at 288.5 eV more probably arise from carbon bonded to oxygen, mainly due to π*(C=O) and σ*(C–O) transitions.36 According to the XPS data, the concentration of 8 ACS Paragon Plus Environment

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Figure 3. XPS C 1s spectra of F-MWCNTs and CdS/F-MWCNTs after deposition of CdS for 5 and 15 min excited with photon energy of 850 eV (a) and 1486.6 eV (b). C 1s spectra of MWCNTs before and after deposition of CdS for 5 and 15 min (c), and F 1s spectra of F-MWCNTs before and after deposition of CdS for 5 and 15 min excited with photon energy of 850 eV (d). oxygen on the surface of the MWCNTs is not more than 3 at %. The decoration of MWCNT by CdS nanoparticles do not induce significant changes in the electronic structure of the MWCNTs. Only intensities of the peaks A and B slightly increase. That might be due to the formation of the C–Cd bonds between CdS clusters and defects in the MWCNTs or adsorption of SC(NH2)2 species. In the C K-edge spectrum of the F-MWCNTs, the intensity within 286.5–292.0 eV increased owing to the covalent C–F bonds formation (Figure 4b). After the 5-min location of the F-MWCNT buckypaper in the chemical bath used for CdS deposition, this intensity dropped that indicates a partial defluorination of the F-MWCNTs. The spectrum of the CdS/F-MWCNTs after the 15-min synthesis exhibited a sharp σ*-resonance, confirming recovering of graphitic structure of the MWCNTs, but the intensity of the π*-resonance strongly decreases and a new intense peak arose at 288.2 eV. We suggest that these spectral changes are because of interactions between CdS and MWCNTs. With increasing time of the CdS synthesis, the concentration of the bare carbon atoms is growing. These atoms can be active centers necessary for the nucleation and growth of CdS nanoparticles. The NEXAFS F K-edge spectra of the F-MWCNTs and CdS/F-MWCNT samples showed an intensive peak at 692.4 eV corresponding to K-edge and peaks C at 686.8 eV and D at 689.1 eV attributed to covalent C–F bonds (Figure 4c). Relative intensities of these peaks may be related with 9 ACS Paragon Plus Environment


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Figure 4. NEXAFS C K-edge spectra of MWCNTs, CdS/MWCNTs (a), F-MWCNT and CdS/F-MWCNTs (b). NEXAFS F K-edge spectra of F-MWCNT and CdS/F-MWCNTs (c). CdS were deposited for 5 min and 15 min. a local surrounding of CF groups.37 Actually, a NEXAFS investigation of DWCNTs fluorinated by different fluorination techniques found substantial differences in their pre-edge structures.38 As compared to the initial F-MWCNTs, the CdS/F-MWCNT hybrid obtained after the 15-min synthesis was characterized by stronger suppression of the C and D peaks. The surface chemical state of the deposited CdS nanoparticles was studied using the XPS Cd 3d and S 2p spectra. The shape of the S 2p spectra implies the presence of at least three sulfur species (Figure 5a). The first doublet with a component 2p3/2 located at 161.8 eV arises from sulfur in the CdS structure, while the second one at 163.0 eV can be ascribed to both S‒S bonding in Sn0 species or residual thiourea.39,40 The presence of the S‒S bonds on the surface of CdS nanoparticles grown on MWCNTs was previously explained by the S-rich surface of CdS and S‒S‒S linkages between surface atoms.15 A high-energy component at 168 eV is attributed to sulfate species, which may develop at the MWCNT edges during their treatment by sulfuric acid. Moreover, sulfate species can arise due to the reaction of CdS nanoparticles with water.15 The S component dominates in the spectrum of CdS/F-MWCNT and it is upshifted by 0.5 eV relatively to that component in the spectrum of the CdS/MWCNTs. This could be due to the formation of small-size particles with Srich surface and/or to the presence of elemental sulfur formed during the reaction. The related concentration of oxidized sulfur is lower in CdS grown on the F-MWCNTs than on the initial MWCNTs. The Cd 3d spectrum of the CdS/MWCNTs exhibited two doublets with the 3d5/2 components located at 405.8 and 407.4 eV (Figure 5b). The former doublet is assigned to CdS, whereas the origin of the second one is not uniquely defined. It can be interpret as CdO41, but more probably this 10 ACS Paragon Plus Environment

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Figure 5. XPS S 2p (a), Cd 3d (b) spectra of CdS/F-MWCNTs and CdS/ MWCNTs after the deposition of CdS for 5 min.

is due to charging effect, since the concentration of oxygen did not increase in the sample after the CdS deposition and this component was not observed in the previously studied CdS/MWCNT hybrids15. The spectrum of the CdS/F-MWCNTs was presented by a single doublet with the 3d5/2 component at 405.6 eV, indicating that cadmium has a chemical state of CdS. The component is downshifted by 0.2 eV and narrowed by 0.6 eV as compared to that in the spectrum of the CdS/MWCNT sample. This means that cadmium in the nanoparticles grown on the F-MWCNTs weaker interacts with sulfur than that in the CdS/MWCNT. This result agrees with the formation of the small-size particles of low crystallinity observed by microscopy methods (Figure 2). 3.3. Defluorination of F-MWCNTs. To understand chemical processes on the F-MWCNT surface in the chemical bath, we measured XPS spectra of samples after the 15-min location in aqueous solutions of CdCl2 (with and without ammonia), thiourea (with and without ammonia), as well as in aqueous ammonia solution. Only in the case of the ammonia-thiourea system, the FMWCNTs were markedly defluorinated and elemental sulfur was formed. The XPS C 1s spectrum of this reference sample detected a substantial reduction of the intensities of high-energy components originated from fluorinated carbons (Figure 6a) relatively to the spectrum of the initial F-MWCNTs. The F/C ratio estimated from the spectrum fitting is 0.20, which is equal to the value determined for the CdS/F-MWCNT composite obtained after the 15-min CdS synthesis. The S 2p spectrum of the reference sample was dominated by a doublet with the 2p3/2 component at 163.6 eV, which is assigned to elemental sulfur (Figure 6b). 11 ACS Paragon Plus Environment


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(a)

sp2

XPS C 1s hν=1486.6 eV

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S

XPS S 2p hν=1486.6 eV

C-CF

SOx C-F2

C-F

CF0.20

292

290 288 286 284 Binding energy (eV)

282

280 172

170 168 166 164 162 160 Binding energy (eV)

Figure 6. XPS C 1s (a) and S 2p (b) spectra of F-MWCNTs after being in aqueous ammonia solution of thiourea for 15 min. Scheme of redox reaction between fluorinated carbon nanotubes and thiourea in aqueous ammonia solution to give CNT and elemental sulfur (c).

Since the defluorination of F-MWCNTs did not proceed in the aqueous solution of thiourea without ammonia, alkaline medium is the necessary condition for the defluorination reaction. In such medium, the reductive action of thiourea toward fluorinated carbon increases, in other words, oxidation-reduction potential decreases. The proposed scheme of the process is presented in Figure 6c. Atom S of C=S group of thiourea is oxidized giving elemental sulfur. Previously, defluorination of the F-SWCNTs was observed in a solution of thiourea in dimethylformamide at 80 °C and 100 °C in the presence of pyridine.42 Authors suggested that thiourea moieties were attached to the SWCNTs partially substituting fluorine atoms with the formation of HF and C‒S bond. Here, however, the formation of the C‒S bonds was evidently not observed, but graphitic structure of the MWCNTs was partially restored. 3.4. Modeling of interaction between Cd(II) complexes and fluorinated graphene. It was shown previously that the CBD of CdS on the CNT surface from an aqueous ammonia solution of cadmium(II) chloride and thiourea is a heterogeneous process governed by a “molecule-bymolecule” mechanism.15 In the solution, Cd2+ ions bound to ammonia NH3 and thiourea SC(NH2)2 forming mixed-ligand complexes [Cd(NH3)iSC(NH2)2]2+ (i = 0−3) (Figure 7a).43,44 The complexes are hydrolyzed giving [Cd(NH3)iSH]+ species (Figure 7b), which may interact with defect sites on


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Figure 7. Formation of mixed complexes [Cd(NH3)iSH]+ in the chemical bath (a). The complex [Cd(NH3)2SH]+ as nucleus for CdS growth on the CNT surface (b), monovacancy in graphene network as active site for attachment of [Cd(NH3)2SH]+. The complex [Cd(NH3)2SH]+ bonded with a two-coordinate carbon atom near mono-vacancy in graphene lattice (c). Optimized geometry of fluorinated graphene fragment C73F3H21with [Cd(NH3)2SH]+ attached to the central carbon atom C surrounded by three CF groups at opposite graphene side (d). CNT surfaces (Figure 7c). These attached complexes can serve as the centers for nucleation and growth of CdS nanoparticles. To reveal the mechanism of CdS nucleation on the fluorinated CNT walls we invoked the DFT calculations. When the Cd(II) complexes were located above a fluorine atom of a fluorinated graphene fragment, the C‒F bond destroyed due to a strong attraction between Cd2+ cation and fluorine atoms. This resulted in elongation of the Cd‒S and Cd‒N bonds in the complexes. Experimentally we did not detect notable defluorination of the F-MWCNTs after their location in the aqueous solutions of CdCl2 with and without ammonia addition. We suggest that in our synthesis conditions, an interaction of the F-MWCNTs with thiourea causes removal of fluorine atoms from the outer side of nanotube walls, while fluorine on the opposite side remains intact. The reactivity of backside fluorinated graphene surface toward the [Cd(NH3)iSH]+ complexes was examined using the fragments with compositions of C73F3H21, C73F5H21, and C73F7H21, where one, three, and five bare carbon atoms were surrounded by three, five, and seven 13 ACS Paragon Plus Environment


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CF groups. An optimized structure of the fragment C73F3H21 with the [Cd(NH3)2SH]+ complex is presented in Figure 7d. The distance between Сd and C atoms is 2.39 Å and this value decreases to 2.34 and 2.38 nm in case of the [Cd(NH3)1SH]+ and [CdSH]+ complexes. An increase of the carbon area surrounding by CF groups to three and five atoms in the fragments C73F5H21, and C73F7H21 did not lead to the attachment of the Cd(II) complexes. Thus, a high reactivity of the C73F3H21 is related with a carbon atom isolated from the conjugated π-system of graphene by CF groups, which has an unpaired π electron readily binding to Cd atom. An interaction energy between the [Cd(NH3)iSH]+ complexes and fluorinated graphene C73F3H21 increases in absolute value from -0.59 to -2.98 eV, when the number of NH3 groups (i) changes from 2 to 0. For the comparison, the interaction energy between [Cd(NH3)iSH]+ complexes and a graphene fragment with a monovacancy varies from 3.392 to -5.196 eV, when i changes from 2 to 0.15 Despite the edge carbon atoms are more energetically and sterically favorable for the attachment of Cd(II) complexes than the defluorinated carbon atoms with localized π electrons, the amount of the last atoms can theoretically reach 50% for the CF composition, that can provide the highest density and homogeneity of the nucleation sites. According to the DFT calculations, we suggest that the removal of fluorine atom from the outer side of the F-MWCNTs promotes attachment of Cd(II) complexes and nucleation of CdS nanoparticles. CdS particles are more uniformly distributed and less agglomerated on the partially defluorinated surface of the F-MWCNTs, than on the as-produced MWCNTs, because the defluorinated tubes have a larger number of centers for the CdS nucleation. This presumption agrees well with the observation that the DWCNTs, chemically modified by fluorine, provide effective sites for the growth of CdSe nanoparticles.20 3.5. Photoluminescence properties. PL emission spectra of the CdS/F-MWCNT and CdS/MWCNTs hybrids and F-MWCNTs are compared in Figure 8a. The initial MWCNTs showed no PL signal since metallic conductivity of the nanotubes. The F-MWCNTs have an intense PL in a range from 380 to 700 nm with two bands centered at 430 and 540 nm. The dual-wavelength PL of a high intensity was previously observed for chemically modified nanocarbons and was attributed to the presence of aromatic domains with different sizes separated by functionalized carbon atoms.45 A lowering of the relative intensity of the band at 540 nm in the CdS/F-MWCNT hybrids can be attributed to a decrease of the amount of the smallest domains with the defluorination. The MWCNT defluorination during the process of CdS deposition causes also suppression of the total PL. A blue shift of the high-energy band by 14 nm in the spectra of CdS/F-MWCNTs is more probably provided by contribution from blue-green PL of CdS nanoparticles, since the CdS/MWCNT spectrum has the peak at 420 nm. Blue-green PL and electroluminescence previously 14 ACS Paragon Plus Environment

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Figure 8. PL spectra of CdS/F-MWCNTs and CdS/MWCNTs in comparison with the FMWCNT spectrum (a). Images of surface glow of CdS/MWCNT and CdS/F-MWCNT cathodes (15 min of CdS deposition) at an applied field of 3 V/µm (b). A scheme of a set-up electroluminescence measurements (c).

observed for the CdS/MWCNT composites have been attributed to the formation of small-size CdS nanoparticles.14 Light emissions from surfaces of the CdS/F-MWCNT and CdS/MWCNT hybrids, obtained after 15 min of the CdS deposition, were induced by electric field at 3 V/µm (Figure 8b). The registration of electro-induced light emission was done through a transparent anode covered by indium tin oxide (ITO). A scheme of a set-up for the electroluminescence measurements is presented in Figure 8c. The optical image of the CdS/F-MWCNT cathode showed blue spots, which were glowing on the surface of the sample when the electric field was applied. The nature of the light emission is ascribed to luminescence of CdS nanoparticles, which are located on the surface of the buckypaper, since the applied electric field induced electron emission accompanied by radiative recombination of electrons and holes in the CdS. Light spots registered on the CdS/MWCNT surface are blue-colored. In is known that CdS nanoparticles of size ~5 nm possess blue emission.46 In our case the CdS nanoparticles formed on the MWCNTs and F-MWCNTs have the average size of 37 and 15 nm, respectively, but they have polycrystalline structure and elongated along the tube surface. The HRTEM analysis showed that the size of CdS nanocrystals composing the big particles is about 5 nm, and they are likely responsible for blue luminescence of the hybrids. A comparison of the surface glow of CdS/MWCNT and CdS/F-MWCNT samples demonstrates that light spots registered on the CdS/MWCNT surface are less dense and some of them are red-colored. This 15 ACS Paragon Plus Environment


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difference is explained by the smaller size and more uniform distribution of CdS on the F-MWCNT surface as compared to those on the MWCNTs. 4. Conclusion For the first time, CdS nanoparticles were grown on buckypapers from as-produced and fluorinated MWCNTs using the CBD from an aqueous ammonia solution of CdCl2 and SC(NH2)2. The CdS nanoparticles on the F-MWCNTs have smaller size and they are more uniformly distributed, than the CdS nanoparticles on the MWCNTs. The observed partial defluorination of the F-MWCNTs and formation of elementary sulfur in the result of the synthesis were attributed to a chemical reaction in an ammonia solution of thiourea. The DFT calculations revealed that carbon atoms, from which fluorine was removed, interact with Cd(II) complexes via localized unpaired π electrons, thus creating the centers for CdS nanoparticle growth. The F-MWCNTs possess intensive blue-green luminescence, which is suppressed and shifted to blue region after the deposition of CdS nanoparticles. The use of the F-MWCNT buckypaper as the support for CdS nanoparticles opens up opportunities for the creation of flexible luminescent screens.

Acknowledgments We are grateful to Dr. M. A. Kanygin for SEM measurements. The work was supported by RFBR, research projects No. 14-33-50591 and 14-03-31359, Russian Federation President Grant MK3277.2017.2 and the bilateral Program “Russian-German Laboratory at BESSY”.

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