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Electrochemical Synthesis of Carbon Fluorooxide Nanoparticles from 3C-SiC Substrates Sergei Alekseev, Dmytro Korytko, Maksym Iazykov, Sergei A. Khainakov, and Vladimir Lysenko J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06524 • Publication Date (Web): 11 Aug 2015 Downloaded from http://pubs.acs.org on August 20, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Electrochemical Synthesis of Carbon Fluorooxide Nanoparticles from 3C-SiC Substrates Sergei Alekseev∗†, Dmytro Korytko†, Maksym Iazykov‡, Sergei Khainakov§, and Vladimir Lysenko‡ †

Faculty of Chemistry, Taras Shevchenko National University of Kyiv, 64 Volodymyrska Str.,

Kyiv-01601, Ukraine ‡

Université de Lyon, Institut des Nanotechnologies de Lyon (INL), UMR-5270, CNRS, INSA de

Lyon, 7 av. Jean Capelle, Bat. Blaise Pascal, Villeurbanne, F-69621, France §

Departamentos de Química Física y Analítica y Química Orgánica e Inorgánica, Universidad de

Oviedo – CINN, 33006 Oviedo, Spain.

ABSTRACT Chemical nature of products, formed during electrochemical dissolution of polycrystalline 3C-SiC substrate in HF:ethanol mixture, was studied by means of FTIR spectroscopy, temperature-programmed desorption mass-spectrometry (TPD-MS), 1H, 19

13

C and

F NMR (solution as well as MAS), XPS, AFM and other characterization methods.

Simultaneous formation of two major products: porous SiC and carbon fluorooxide (CFO)

∗

Contact author, e-mail : [email protected]

ACS Paragon Plus Environment

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nanoparticles (NPs) with sizes of 1 – 10 nm is described. CFO NPs easily dissolves in polar organic solvents (ethanol, CH2Cl2, etc); their solutions demonstrate intense yellowish-green photoluminescence under UV excitation. A model of the CFO chemical structure based on relatively small graphene domains interconnected with partially fluorinated hydrocarbon groups and terminated by carboxylic acid (–CO2H), ethyl ester (–CO2C2H5), perfluorinated functional groups and polycarboxylated alkyl chains is proposed. Presence of carboxylates allows easy functionalization of the CFO NPs via amide chemistry. In particular, grafting of octadecyl groups makes CFO NPs soluble in hydrocarbons.

1. INTRODUCTION In the last decade, nanoparticles formed by IVth group elements, such as diamond1, graphitelike carbon2,3, silicon4 and silicon carbide5,6, attract a lot of attention of researchers, especially for their application as fluorescent probes in biology due to intense photoluminescence, resistance to photobleaching and low toxicity. Usually, a “top-down” approach consisting of partial oxidation and dissolution of bulk SiC in HF-based solutions is used to fabricate SiC nanoparticles (NPs) as well as nanostructured porous SiC. Chemical etching can be applied for porosification of SiC micropowders with further dispergation of the obtained highly-porous SiC in order to form the NPs. However, it requires rather harsh fabrication conditions (typically, HNO3:HF (1:3, v/v) mixture at 100oC)7-9. As for electrochemical etching of SiC wafers in HF:C2H5OH mixtures, it is smoothly carried out at mild experimental conditions with precise control of resulting SiC nanostructure morphology by setting up the etching parameters, such as electrolyte composition and current density10-14.


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Recently, a simple procedure of the electrochemical etching of large (> 50 nm) SiC NPs slurry to produce photoluminescent SiC quantum dots (< 10 nm) was reported15. Chemical transformations taking place during the electrochemical as well as chemical porosification of the SiC were never studied in details. Quantity of electrons (γ) generated under the dissolution of one SiC “molecule” was determined in some early works devoted to fabrication of porous SiC. The value γ = 6.9 mentioned in Ref16 allowed to suppose that the dissolution proceeds through overall reactions (1) and (2) having approximately the same rates: SiC + H2O + 6F− − 6e− = SiF62− + CO + 2H+

(1)

SiC + 2H2O + 6F− − 8e− = SiF62− + CO2 + 4H+

(2)

More recent and detailed work17 demonstrates that γ may be dependent on SiC polytype and doping level. For highly doped p-type C-face 6H-SiC it is close to 6 for low (j = 1 mA/cm2) and to 5 for high (j ≥ 10 mA/cm2) current densities. Low γ value was explained by authors of Ref.17 by evolution of hydrogen at higher currents according to the following reaction: SiC + H2O + 6F− − 4e− = SiF62− + CO + H2

(3)

Of course, the formation of hexafluorosilicate during the SiC etching seems doubtless, furthermore its intense bands at 738 at 482 cm−1 can be seen in the FTIR spectra of insufficiently washed samples of SiC nanomaterials, being often misinterpreted as bands of some surface species18,19. However, chemical compounds containing carbon atoms from the etched SiC materials should be in reality much more complicated than those described by the reactions (1-3) mentioned above. The first indicator of such complexity is yellow or brown colouration of electrolyte solution and of washing liquid commonly observed during and after electrochemical etching of any SiC polytype in HF-containing solutions.


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In the present work, numerous results describing chemical nature of the “soluble” product of the SiC electrochemical etching are reported. In particular, we demonstrate formation of relatively small (< 10 nm) carbon-based NPs or macromolecules with high content of carboxylic acid groups. Finally, critical analysis of some literature data on photoluminescent and biofunctional properties of SiC nanoparticles is performed, and it is demonstrated that the effects reported earlier are often caused by above-mentioned carbon-based NPs, making them especially promising for biological and other applications.

2. EXPERIMENTAL SECTION 2.1. Electrochemical etching of 3C-SiC and transformations of the resulted products Anodization of a low resistivity grade ( 100 K, 100_30 K, 30_10 K, 10_3 K and < 3 K) were redispersed in 70% ethanol and diluted to 2 ml volume.

2.2. Characterization Methods


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Transmission electron microscopy (TEM) observations were performed on Topcon EM-002B high resolution transmission electron microscope operating at 200 kV for the samples, prepared by drying of a drop of the colloidal solutions on a graphite grid. Atomic force microscopy (AFM) characterization was performed on Dimension V (Bruker) instrument using ultra-sharp silicon cantilevers (Nanosensors SSS-NCH) with a typical curvature radius of 2 nm and nominal spring constant of 42N/m. AFM images were acquired in a tapping mode at room temperature under ambient conditions for the particles deposited onto an atomically flat surface of muscovite mica discs (Agar Scientific). To make a deposition, a protocol similar to that used for negatively charged DNA molecules deposition was used20. Freshly cleaved mica was treated with a 50mM NiCl2 solution for 1 min, carefully rinsed with the DI water, afterwards incubated in the NPs solution (1 mg/ml) for 10 min, rinsed and dried in a stream of N2. Treatment of images and statistical data analysis (height and the lateral size distributions) was performed using the Gwyddion software. Zeta-potentials and size-distributions were measured by Zetasizer Nano ZS (Malvern) instrument at 173o backscatter geometry in a dynamic light scattering (DLS) measurement mode for 1 mg/ml solutions of NPs having refractive index near 1.5. UV-vis absorption spectra of studied solutions were measured on Unicam UV4 spectrometer in 10 mm quartz cuvettes. A solution of the same composition (solvent, buffers, etc.) as the studied solution with the NPs was always taken as a reference background. The photoluminescence spectra were recorded on Jasco FP-750 spectrofluorometer in 10×10 mm quartz cuvettes. Fourier transform infra-red (FTIR) spectra were recorded on Nicolet NEXUS 470 spectrometer in transmittance mode (32 scans, spectral range 400 – 4000 cm-1, resolution 4 cm-1). Samples


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were prepared as pellets in KBr or via deposition of NPs colloids in organic solvents onto the KBr window followed by drying. The interaction of CFO with HCl or NH3 was performed directly by keeping of the KBr window with deposited sample in saturated vapors of 35% HCl or 28% NH3 solutions for 20 sec. Temperature-programmed desorption mass-spectrometry (TPD-MS) measurements were carried out by heating of dry sample under high vacuum (pC=O

52.5

48.8

533.4

C–O

43.8

48.5

535

H2O

3.7

2.7


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The results of the C 1s peak deconvolution clearly indicate presence of abundant fraction of oxidized carbon species (i.e. carboxylic acid and ester groups as well as fluorinated carbon atoms) in the initial CFO sample. This is well consistent with the above-described data provided from FTIR, TPD-MS and NMR studies. Transformation of the CFO to CFO_hydr results in growth of the C 1s components related to CHxCy and perfluorinated species along with lessening of the C–O and (–CO2 + C–F) components. These changes can be related to the hydrolysis of ethyl ester groups and transformation of ≡C–F fragments to double bonds as well as to ≡C–OH groups. Despite of practical absence of –CO2C2H5 groups in the CFO_hydr sample according to the FTIR data, intensity of C 1s C–O component is rather high. Fragments –CF2– and –CF3 are known to be stable towards alkaline hydrolysis; that is why their fraction in the CFO_hydr sample increases.

4. Discussion 4.1. Chemical composition of the CFO species To understand the nature of anodic reaction causing the SiC etching, quantity of electrons (γ) generating under the dissolution of one SiC “molecule” was calculated from the values of (i) overall mass loss of the SiC wafer after etching and washing; (ii) mass of SiC_precip and (iii) the fact of silicon absence in the CFO product. The resulted quantity (γ = 6.09) correlates well with the literature data17 and indicates that the formal oxidation state of carbon in the reaction product should be close to “+2” (all silicon is assumed to form H2SiF6). However, possibility of CO2 formation according to the reaction (2) as well as participation of ethanol in the anodic process makes interpretation of this value rather complicated.


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In order to interpret elemental analysis data, first of all, the deficiency of mass appearing from the sum of percentages of C, H, N and F was related to oxygen since no other elements should be present in the samples and silicon amount is negligibly low. Secondly, in accordance with the FTIR data (see Fig. 7), all nitrogen in the CFO_C18 sample is considered as a part of aminooctadecyl (–NHC18H37) groups. Resulting brutto formulas, illustrating composition of the studied samples, as well as formal oxidation states (OS) of carbon atoms and degrees of unsaturation (DU) related to the quantity of carbons (NC) are presented in Table 3. Both OS and DU for CFO_C18 were calculated for the “core”, surrounded by –NHC18H37 groups, considered as univalent substituents with “-1” formal oxidation state. Table 3. Brutto formulas, formal oxidation states of carbon atoms and degrees of unsaturation. Sample

Formula

OS

DU/NC

CFO

C100.0H104.1F19.5O51.0

+0.174

0.382

CFO_C18 [C100.0H46.9F11.6O32.1](NHC18H37)17.8 +0.468

0.619

OS - oxidation state, formal charge on carbon atom (charges “+1” for H, “–1” for F, “–2” for O and “–3” for N were assumed). DU - degree of unsaturation, number of π-bonds and cycles in the molecule, DU = NC – NH/2 – NX/2 + NN/2 + 1, where NC,H,X,N – quantities of carbon, hydrogen, halogen and nitrogen atoms in a molecule). The parameters both OS and DU/NC increase from CFO to CFO_C18. Since no additional oxidants were present in the CFO_C18 synthesis, the increase in the OS can be related only to substitution of the ethoxyl groups by –NHC18H37 (elimination of HF and/or H2O causes no influence on the OS, but increases the DU). This assumption along with the amide nature of the octadecyl substituents in the CFO_C18, allows setting up the equations system giving us the “core-shell” brutto formulas (i.e. representations of the formulas as the “cores” surrounded by carboxylates or amide groups) of the CFO and CFO_C18 (Table 4).


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Table 4. “Core-shell” brutto formulas, OS and DU of core carbon atoms Sample

Formula

OS

CFO

[C100H89.1F26.9O27.1](CO2H)13.4(CO2C2H5)8.2 -0.079 0.312

CFO_C18 [C100.0H57.0F14.1O17.5](CONHC18H37)21.6

DU/NC

-0.079 0.536

As it can be seen from Table 4, reaction of the CFO with octadecylamine results not only in substitution of the –OC2H5 and –OH groups, but also in elimination of HF and H2O species from the cores. According to the UV-vis absorption data, the reaction results in growth of aromatic domains' size in the CFO_C18 and, at the same time, the overall light-absorbance decreases due to the sample's aromatic moiety “dilution” by the long-chain alkyl groups (Table 1). The DU/NC values of the core carbon atoms are relatively low for both of the studied samples, indicating considerable content of sp3-hybrid carbonic part in the cores with the attached carboxylates. For comparison, if NC >>1, DU/NC values equal to 0 (no double bonds or cycles) for alkanes, to 0.5 - for conjugated polyene hydrocarbons and graphane, 0.667 - for polyphenilene, 0.75 - for polyacenes (strips of condensed aromatic rings), to 1–(2n)-1 for Hterminated finite graphene hexagonal clusters Gn (where n is a number of concentric hexagonal rings) and to 1 - for pure carbon in any form. The formulas given in Table 4 can be partially confirmed by pH titration data. Shape of the pH-titration curve for the CFO_hydr sample (Fig. S11) is typical for one of polybasic acids, such as polymaleic acid34. Estimation of the acidic groups concentration for such acids is rather ambiguous due to an influence of acid molecule ionization on dissociation constant of the remaining –CO2H groups. Quantity of the carboxylic groups in the CFO_hydr sample appears to be equal to 5.28 mmol/g from the derivative maximum and to 6.17 mmol/g from the Gran plot


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of free OH– ions. These values (i.e. one acidic site per approximately seven carbon atoms) correlate well with the “core-shell” formulas of the CFO and CFO_C18 presented in Table 4 and assume complete hydrolysis of the –CO2C2H5 groups and relatively small changes of the core mass during the CFO_hydr preparation (unfortunately, no elemental analysis data are available for the CFO_hydr up to now). Titration of the untreated CFO samples does not result in any interpretable value of –CO2H concentration because of very slow equilibration rate in the alkaline media. Furthermore, high concentration of free F– ions was detected in the solution after titration.

4.2. Structural model of the CFO All aforementioned allowed us to state that the “CFO cores” contain following structural units: (i) relatively small (few rings) aromatic domains causing the UV-vis absorbance and the PL spectra reported above; (ii) labile to hydrolysis fluorine atoms giving a signal at -150 ppm in the 19

F NMR (Fig. 10); (iii) hydrolytically-stable –CF3, –CF2– and ≡C–F groups; (iv) some carbon

atoms bound to oxygen, probably, in the form of –OH, –OC2H5 (in this case the signals at 14.2 and 15.3 ppm in the 13C NMR (Fig. 9) should be related to ester and ether ethoxyls, respectively) or other –O– species; (v) alkyl backbones bearing the carboxylates and other substituents. Presence of the alkyl backbones follows from overall chemical composition of the CFO and the unsaturation degree of the CFO core (Table 4). Combination of “all the pieces of the puzzle” into a single formula gives us a representative structural model of the CFO depicted in Fig. 12. Presence of the aromatic fragments in the CFO allows assumption of a similarity between the CFO formation mechanism and “classical” synthesis of graphene on SiC facets under high-temperature vacuum annealing. That is why the


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hexagonal motifs made of carbon atoms are likely to be present in the CFO structure. The signal at -150 ppm in

19

F NMR spectrum is most likely corresponds to the aliphatic tertiary ≡C–F

fragments. Action of the relatively weak bases (such as diluted aqueous NaOH or C18H37NH2 in o-xylene) results in fluorine substitution and/or HF elimination from these fragments together with the aromatic domains growth. This feature is due to conjugation between the ≡C–F fragments and aromatic rings, and the ≡C–F fragments could be built into the surface of the “graphane” fragment of the hexagonal carbon motif, as it is shown in Fig. 12. EtO2C

OH

CO2H

CO2H EtO HO2C EtO EtO2C

HO2C

HO2C

F

F F

H HO2C

CO2Et

CF3

CF2

F

EtO

CO2Et

OH

HO

F

CO2H HO OEt

H

F

CO2H F

CF3

F F

H

H

CO2Et

CO2Et CO2H

HO2C

OEt

CF3

OH

CO2Et CO2H

CO2H

OH CO2Et

CF2

EtO2C

H

H

F

HO2C

CO2Et

F HO2C EtO

EtO2C

OEt

CO2Et CO2H

Figure 12. Structural model of the CFO NPs. Similarity of the FTIR spectra of the CFO and polymaleic acid24 allows us to represent the alkyl backbones as short polymaleic acid chains with partial substitution of tertiary H atoms by the –OH and –OC2H5 groups. Presence of such chains may cause some surfactant properties peculiar for the CFO_hydr sample in aqueous solutions. The structure represented in Fig. 12 is consistent with the spectral data provided in Figs. 8−10. In particular, tertiary (C3C–H)


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polymaleic acid and “graphane” protons could give rise to the broad signal at 3.6 ppm in 1H NMR spectrum (Fig. 8). The signals at 39.4 and 53.0 ppm in 13C NMR are likely to be related to polymaleic and graphane C3C–H carbons (Fig. 9). Low intensity signal of the C3C–O carbons coincides with the signal of the –OCH2 groups at 63.5 ppm, the signals of all the CFx groups and aromatic carbons are responsible for the broad peak at 90-140 ppm. The structure presented in Fig. 12 is an averaging of the “flat disk-like particle” and “dried globule” models, proposed above from the AFM observations. In the case of relatively large and thick particles (r = 15 nm, h = 2.5 nm) seen by the AFM, the network structure composed of several aromatic/graphane carbon pieces jointed by the alkyl backbones could be proposed.

5. Conclusions The results presented in this paper clearly demonstrate that the oxidative dissolution of SiC in HF can proceed in much more complicated way than it is described by the reactions (1) and (2) in the Introduction section. If formation of porous SiC and SiC nanoparticles is well-established, the question “what happens with carbon during the SiC dissolution?” is practically explored in our work for the first time. Indeed, formation of “carbon-enriched layers” on the HF-etched SiC surface was observed before11,35 and closely studied recently36,37, however, exact composition of a colored soluble etching products was, on our knowledge, not characterized up to now. It should be stated that the CFO NPs described in this article appear not because of the oxidation of ethanol during the SiC etching, but definitely due to transformations of the SiC itself. In order to prove this hypothesis, SiC electrochemical etching in ethanol-free HF-based aqueous solutions as well as chemical etching in HF:HNO3 mixture have been performed. As a result, corresponding etching products characterized by FTIR, UV-vis and PL spectra as well as


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zeta-potential values similar to those obtained for of the CFO have been formed (see the Figures S12-13 and brief discussion in the Supplementary Information). Formation of the “TEM-invisible” CFO species accompanying etching of the SiC calls into question a large set of the earlier published data concerning numerous studies of SiC nanoparticles obtained by our research group6,14,38,39 as well as by other researchers5,8,40. Since the experimental protocols used therein do not ensure complete separation of the CFO and small SiC NPs, the found effects may at least relate both to the CFO and to the SiC NPs. In particular, it concerns a large set of data on bio-functional properties of the nanoparticles made by electrochemical etching of SiC substrates in HF/EtOH and previously misassigned exclusively to the SiC NPs38,39. However, as published in our previous papers38,39,41,42, CFO species clearly demonstrate their amazing ability to easily penetrate inside the living cell's nuclei; their preferential

localization

inside

the

cell's

organelles

depending

on

their

chemical

functionalization; efficiency of their cell imaging application due to their intense photoluminescence as well as their promising therapeutic applications. Besides, formation of the “TEM-invisible” “semi-organic” products with the physicochemical properties and structure similar to the above-described CFO may accompany the processes of carbon-based nanoparticles syntheses2,3. PL, XPS and IR spectra reported in these works significantly resemble the CFO’s spectra, and despite of the nice TEM images of “true” carbon NPs with crystalline cores, no proofs of absence of the amorphous carbon-based species were provided. Of course, we do not claim the structural model of the CFO proposed in this work as an ultimate and unappealable one. As anyone can see on the example of graphene oxide, at least three different but complementary structural models of this practically important and


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extensively-studying compound were proposed in the last two decades23,29,30,43. However, we suppose that we were able to assess correct values of the CFO particle size and chemical functionalities from the large set of the data provided to reader’s attention. In particular, we want to emphasize the difference of the CFO structure from any other carbon nanomaterial already described in the literature. More detailed characterizations of the CFO’s physical, chemical and bio-functional properties as well as its possible applications are in the course. Summarizing all aforementioned, the CFO and similar inorganic-derived amorphous nanomaterials are expected to attract increasing attention in the nearest future.

Supporting Information Available Photos and powder XRD patterns of studied samples; FTIR spectrum, N2 adsorption isotherm, pore size distributions and adsorption parameters for SiC_precip; AFM images of the CFO NPs at different scale-lengths; UV-vis and PL spectra of different fractions of the CFO; FTIR spectra of the sample CFO_hydr before and after consequent treatment with NH3 and HCl; 1H and

13

C

MAS NMR spectra of the CFO; signal list for the 13C NMR of CFO solution; the XPS line F 1s; protocols of chemical and electrochemical CFO preparation in ethanol-free solutions, FTIR and UV-vis spectra of the corresponding samples. This material is available free of charge via the Internet at http://pubs.acs.org.” Acknowledgments The authors are grateful to Ms. Tetiana Serdiuk, Ms. Iulia Romanenko, Mr. Hamza Hajjaji, Mr. Alexander Kharin, Dr. Yuriy Zakharko, Dr. Tetyana Nychyporuk, Dr. Olivier Marty, Dr. Jean-Marie Bluet, Prof. Brice Gautier, Prof. Alain Geloen from INSA de Lyon, Dr. Virginie


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Monnier and Dr. Yann Chevolot from Ecole Centrale de Lyon, Ms. Elena Shamatulskaya, Dr. Svetlana Gryn, Dr. Boris Mischanchuk, Dr. Igor Zatovskiy, Prof. Valeriy Skryshevsky, Prof. Igor Komarov and Prof. Vladimir Zaitsev from Taras Shevchenko National University of Kyiv, Dr. Viacheslav Iablokov and Dr. Roland Barbossa from Universite Libre de Bruxelles (Belgium), Dr. Denis Shevchenko, Dr. Konstantin Artemenko and Prof. Jonas Bergquist from University of Uppsala (Sweden) for helpful discussion and help with experimental work and also IRSES European Project “Porous Silicon Carbide as a support for Co metal nanoparticles in Fischer–Tropsch synthesis” (proposal number: 319013) and Visby Swedish institute project 00814/2011 “Surface-Assisted Laser Desorption Ionization of biomolecules on modified porous Silicon/Silicon Carbide for their better analysis” for partial financial support. References (1) Barras, A.; Lyskawa, J.; Szunerits, S.; Woisel, P.; Boukherroub, R.; Direct Functionalization of Nanodiamond Particles Using Dopamine Derivatives. Langmuir, 2011, 27, 12451 - 12457. (2) Ray, S. C.; Saha, A.; Jana, N. R.; Sarkar, R.; Fluorescent Carbon Nanoparticles: Synthesis, Characterization, and Bioimaging Application. J. Phys. Chem. C, 2009, 113, 18546 - 18551. (3) Bao, L.; Zhang, Z.-L.; Tian, Z.-Q.; Zhang, L.; Liu, C.; Lin, Y.; Qi, B.; Pang, D.-W. Electrochemical Tuning of Luminescent Carbon Nanodots: From Preparation to Luminescence Mechanism. Adv. Mater. 2011, 23, 5801-5806. (4) Chirvony, V.; Chyrvonaya, A.; Ovejero, J.; Matveeva, E.; Goller, B.; Kovalev, D.; Huygens, A.; de Witte, P. Surfactant-Modified Hydrophilic Nanostructured Porous Silicon for the Photosensitized Formation of Singlet Oxygen in Water. Adv. Mater., 2007, 19, 2967-2972.


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S. Alekseev et al., Electrochemical synthesis of CFO NPs from 3C-SiC

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Electrochemical Synthesis of Carbon Fluorooxide ... - ACS Publications

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