ftir spectroscopy method for investigation of co-ni...

Journal of Chemical Technology and Metallurgy, 52, 5, 2017

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FTIR SPECTROSCOPY METHOD FOR INVESTIGATION OF Co-Ni NANOPARTICLE NANOSURFACE PHENOMENA

Ivan Zahariev1, Mehmet Piskin2, Emre Karaduman2, Dimka Ivanova1, Ivania Markova1, Ludmil Fachikov1

Received 09 February 2017Accepted 31 May 2017

Journal of Chemical Technology and Metallurgy, 52, 5, 2017, 916-928

1University of Chemical Technology and Metallurgy 8 Kliment Ohridski, Sofia 1756, Bulgaria E-mail: [email protected] Technical University Istanbul, Turkey

INTRODUCTION

The nanomaterials are intensively studied because of their unusual and unique properties determining their application in various fields of human activity, including energy, biotechnology, medicine, nanoelectronics, com-munications, etc. [1 - 16]. Their properties significantly change with decrease of the particle size as compared with those of the bulk materials or single molecules. This determines their application in catalysis, electron-ics, optics, biology, photography and magnetism [1 - 8].

A necessity of control techniques stems out with the creation of new materials based on nanosized particle not only because of their size and shape, but also because of their unique properties. Conventional techniques for nanomaterials characterization referring

to the determination of the morphology, the structure, the specific surface area, the chemical and phase com-position, the magnetic parameters are developed. X-ray diffraction method (XRD) and electron microscopy in its varieties as SEM, TEM, HRTEM are used to study the nanomaterials.

The elaboration of methods to study the various nanostructures (nanoparticles, nanowires, nanofibers, nanotubes) [9 - 20] is of great importance. The spectral methods (electromagnetic waves including ultraviolet (UV), UV-visible (Vis), Infrared (IR) and Raman spec-troscopy occupies an important place [21 - 27] are among them. UV-Vis and IR spectroscopy are used to establish the presence of functional groups on the nanomaterials surface, while Raman spectroscopy is applied to deter-mine the structural type of the nanomaterials crystal

ABSTRACT

The Co-Ni nanoparticles examined are synthesized through a borohydride reduction with NaBH4 in aqueous solutions of chloride salts containing a different ratio of Co and Ni (1:1, 4:1 and 1:4, correspondingly) and also in the course of a template synthesis with graphite as a support in presence of β-cyclodextrin. The morphology, the el-emental and phase composition of the synthesized Co-Ni nanoparticles are studied by SEM, EDS and XRD analyses. FTIR spectroscopy investigations carried out provide to elucidate the atom /molecule groups formed in the Co-Ni nanoparticles and their carbon-containing nanocomposites. The different shape and position of the bands of absorp-tion at the relevant wavenumber [cm-1] identify the mode of vibrations (symmetric and asymmetric stretching and bending vibrations) of the created chemical bonds arising at the nanoparticle surface such as C-OH, CO-OH, C-H2, C=O, BO3, BO4, free OH, H-OH (H2O), CoO, NiO. The FTIR spectra reported illustrate also the effect of the differ-ent Co:Ni ratios studied and that of the support used. The data obtained show that FTIR spectroscopy is a sensitive method suitable for studying Co-Ni nanoparticles and their carbon-containing nanocomposites surface phenomena.

Keywords: FTIR spectroscopy, borohydride reduction, Co-Ni nanoparticles, template synthesis, carbon-containing nanocomposites.

Ivan Zahariev, Mehmet Piskin, Emre Karaduman, Dimka Ivanova, Ivania Markova, Ludmil Fachikov

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lattice and the presence of impurities.The study of the interface phenomena referring to

the formation atomic/molecular groups on the nanosized particles surface and the corresponding chemical bonds between them [28] is of particular importance. IR spec-troscopy is highly sensitive regarding the identification of atomic/molecular groups. It is also suitable to study nanosized objects synthesized in various ways, includ-ing that of wet synthesis (borohydride reduction in an aqueous solution of metal salts). The modern advance of infrared spectroscopic technique determines it as a reli-able method, which is easy to implement for characteri-zation of nanomaterials and investigation of phenomena that occur on the interface of the nanosized particles.

IR spectroscopy can be successfully applied to investigate metallic (Co, Fe, Ni, Cu) nanomaterials (na-noparticles and nanowires). It is possible to characterize the surface state of nanosized objects synthesized by a wet synthesis (borohydride reduction in an aqueous solu-tion of metal salts) [28] by IR spectroscopy with Fourier transformation in the range of 4000 cm-1 - 400 cm-1. The IR spectra of the synthesized nanomaterials give new meaning and contemporary interpretation of the results obtained, which is a key to their application as high-tech products for the microelectronics and nanoelectronics.

The present work reports the synthesis of Co-Ni nanoparticles through a chemical borohydride reduction and the study of their carbon-containing nanocomposites by FTIR spectroscopy aiming to identify the chemical bonds arising at the nanosurface.

EXPERIMENTAL

Synthesis of intermetallic Co-Ni nanoparticles at and their carbon-containing nanocomposites

Intermetallic Co-Ni nanaoparticles are synthesized through a borohydride reduction method with 0.2 М NaBH4 in a mixture of aqueous solutions of 0.1 М CоCl2.6H2O and 0.1 M NiCl2.6H2O respectively at a ratio Cо:Ni = 1:1, 4:1, 1:4. To reach the ratio Co: Ni = 1:1 chosen according to the phase diagram of the binary Co-Ni system we used 25 ml 0,1M CоCl2.6H2O and 25 ml 0,1M NiCl2.6H2O and to reach the ratios of Co: Ni = 4:1 and 1:4 the quantities are respectively 40 ml

0.1М CоCl2.6H2O/10 ml 0,1M NiCl2.6H2O and 10 ml CоCl2.6H2O/40ml 1M NiCl2.6H2O ml. To fully complete the reduction process the quantity of the reducing agent stabilized with NaOH is 50 ml 0.2 M NaBH4 in the all cases. Citric acid (С6Н8О7) is used as a stabilizing ligand. We used a quantity of 0, 44 g.

The synthesis is carried out in a double-wall cell to keep a constant temperature (by a thermostat) ensuring a consecutively introducing of the initial solutions and continuously mechanical stirring of the reaction mixture with a magnetic stirrer. The experiments are run at room temperature and atmospheric pressure. The reduction pro-cess is completed in 2 minutes adding the reducing agent drop wise. Fine powder precipitates are obtained. They are filtrated, washed with a distillate water and alcohol and dried in a vacuum oven during 24 hours at 100оС.

Intermetallic Co-Ni core/carbon shell nanoparticles are synthesized through the same borohydride reduction method at the same technological conditions but by the help of template technique using a carbon-containing support. In this way carbon-containing nanocompisites with Co-Ni nanoparticles are in-situ obtained. Fluorinated graphite (CF) in the presence of β-cyclodextrin (β-CDx) is used as a support. The used quantities of CF and β-CDx are respectively 0,363 g /100 ml. In this case the ratio 20 % CF: 20 % β-CDx: 60 % Co-Ni nanoparticles is realized.

EquipmentThe morphology of the synthesized Co-Ni core/

carbon shell nanoparticles is investigated by the help of Scanning electron microscopy (SEM). The SEM im-ages are made with a JEOL JSM 6390F (Japan) SEM microscope at accelerating voltage of 20 kV in three regimes: secondary electron image (SEI image), a back reflex electron composition image (BEC image), and a split/shadow image, the so-called a combined regime (SPI image). The SEI images give information for the investigated sample surface, while the BEC images – for its chemical composition. The dark regions of the BEC images characterize the existence of atoms with a smaller atom number, while the light region brings information for an existence of atoms with bigger atom number. The elemental dispersitive analysis (EDS) is made using the same SEM microscope with an appliance for Energy-


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dispersive X-ray spectroscopy. The phase composition of the investigated samples is determined by X-ray diffraction (XRD). The X-ray diffraction patterns of all samples are collected within the 2θ range from 10o to 95o with a constant step 0.03o and counting time 1 s/step on Philips PW 1050 diffractometer using CuKα radiation.

The intermetallic Co-Ni nanoparticles synthesized through a chemical reduction with NaBH4 in a mixture of water solutions of the corresponding metal salts including using a support of graphite in the presence of β-CDx are characterized by the help of Infrared spectroscopy with Fourier transformation using FTIR spectrophotometer EQUINOХ 55 (Bruker) in mid-IR region of the spectrum – in the interval from 4000 to 400 cm-1 and FTIR spectra of are collected.

RESULTS AND DISCUSSION

FTIR study of Co-Ni nanoparticles synthesized at a different ratio Co:Ni (samples 1, 2 and 3)

Fig. 1 presents compared FTIR spectra of Co-Ni nanoparticles synthesized respectively at a ratio Co:Ni = 1:1 (sample 1), Co:Ni = 4:1 (sample 2) and Co:Ni = 1:4 (sample 3). The experimental data for the bands of absorption and the mode of the bond vibrations in the formed atom groups at the relevant wavenumber (cm-1) are given in Table 1.

It is registered the very slight bands of absorption with peaks respectively at 3743.0 cm-1 (sample 1), 3834.0 and 3751.0 cm-1 (sample 2), and 3703 (sample 3). These bands characterize stretching vibrations in C-O bonds in C-OH groups. It could be seen that in the case of sample 2 and sample 3 this band is shifted to lower frequencies compared to the same band for sample 1.

The broad absorption band with a peak at around at 3390 cm-1 (sample 1), 3402 cm-1 (sample 2) and 3386 cm-1 (sample 3) could be assigned to stretching mode of vibrations of O-H bonds in CH-OH groups resulting from the used citric acid (C6H8O7) and also to asymmet-ric stretching mode of vibrations ν1(OH) of O-H bonds in free OH groups, while the sharp and well expressed absorption bands at 2879 and 2822 cm-1 (sample 1), 2881 cm-1 and 2825 cm-1 (sample 2) and 2882 cm-1 (sample 3) are revealed to stretching mode of vibrations of C-H bonds in CH2 groups. These bands observed in the frequency interval from 3400 to 2750 cm-1 characterize the chemical bond vibrations typical for the citric acid (C6H8O7) used as a complexing agent during the Co-Ni nanoparticle synthesis.

The band of absorption with a peak at 2360 cm-1 (sam-ple 1), 2359 cm-1 (sample 2), and 2361 cm-1 (sample 3) re-fers to asymmetric stretching mode of vibrations ν1(OH) of O-H bonds in free OH groups. The sharp band with a peak at about 1627 cm-1 can be also detected (sample 1). This

Fig. 1. FTIR spectra of Co-Ni nanoparticles synthesized at a different ratio Co:Ni:S1-Co:Ni = 1:1, S2 - Co:Ni = 4:1, S3 - Co:Ni = 1:4.


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band is typical for banding mode of vibrations ν4(OH) of H-O-H bonds in H2O molecules adsorbed on the na-noparticle surface. In the case of sample 2 and sample 3 this band is shifted to lower frequency (respectively 1613.9 cm-1 and 1624 cm-1).

Very slight band is appeared only for sample 2 at 1757 cm-1, which characterizes stretching vibratuions of C=O

bonds in CO-OH group (carboxyl COOH group). According the FTIR spectrum in Fig. 1 in the fre-

quency region of 1500 - 400 cm-1 three modes of vibra-tions are revealed. They are typical for B-O bonds in BO3 and BO4 groups forming different structural units:

- asymmetric stretching mode of vibrations ν3 (BO3) in BO3 groups in the frequency region of 1400 to 1200 cm-1,

Table 1. Experimental data for the bands of absorption and the mode of the bond vibrations.

Absorption Frequency, ν [cm-1] Chemical

bond Atom group

Mode of vibration

Sample 1 Co:Ni=1:1

Sample 2 Co:Ni=4:1

Sample 3 Co:Ni=1:4

Sample 4 Co:Ni=1:1 CF+β-CDx



C

Graphite

ν, Stretching

-

-

-

3900.3

3900.0

3949.0

C-O

O-H

C-OH

OH (Free)

ν, Stretching

νas, Asymmetric

stretching

- -

3743.0 - -

3390.2 -

- 3834.0 3751.0

- -

3402.0 -

3872.0 3833.0

- 3703.0

- 3386.0

-

3856.4 - -

3736.5 -

3381.6 -

3880.0 - -

3760.0 3740.0

- -

3823.0 -

3748.9 3727.0/ 3490.4 3456.0

C-H C-H2 ν, Stretching

2879.7 2822.4

2881.0 2825.6

2882.0 2824.0

2881.0 2829.5

2894.6 2824.5

2885.0 2800.0

O-H

OH

(Free)

νas, Asymmetric

stretching 2368.4 2359.8

2361.9 2361.1 2360.5

2360.7

C=O

H-O-H

CO-OH carboxyl

H2O molecules

ν, Stretching

δ, Bending

-

1627.6 -

1757.0 -

1613.9

-

1624.0 -

-

1650.3 1600.0

-

1620.0 -

-

1644.2 -

B-O

BO3

νas,

Asymmetric stretching

1411.2 1346.0 1270.0

- -

- 1398.6 1274.0 1250.0

-

1422.6 - - - -

1399.9 1330.0 1271.0

- -

1397.3 -

1270.0 -

1217.0

1426.6 1335.0 1281.0

- -

C-O

B-O

C-OH

BO4

ν, Stretchning

νs,

Simmetric stretchning

1029.0 - - -

864.4 -

- -

980.0 978.6 865.0

-

- - - - -

1080.0 -

990.0 880.0

- 750.0

-

- -

914.0 -

862.5 773.0

1038.0 1010.0 982.0 950.0 850.0

- -

B-O

BO3 BO4

δ, Bending

678.3 670.0

- -

570.0 551.0 528.0

- 466.0

- 674.8

- -

649.0 -

520.0 -

450.0

- -

667.9 - - - - -

448.0

- 671.7

- -

560.0 - -

500.0 -

- -

668.6 560.0 540.0 499.0

- -

671.9 590.0 559.0 540.0 510.0 504.3 490.0

- 458.5

B-O

Co-O Ni-O

BO2

CoxOy NixOy

δ,

Bending ν,

Stretching

431.8

- - -

431.8

- - -

- - - -

436.6

- - -

- -

421.5

438.0 430.0

-


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respectively, for sample 1 with a peak at 1411 cm-1, 1346 cm-1 and 1270 cm-1, for sample 2 with a peak at 1398 cm-1, 1274 cm-1 and 1250 cm-1 and for sample 3 with a peak at 1424 cm-1 and 1271 cm-1;

- symmetric stretching mode of vibrations ν1(BO4) in BO4 groups in the frequency region of 1000 to 850 cm-1, respectively for sample 1 with a peak at 1029 cm-1 and 864 cm-1, for sample 2 and sample 3 with a peak at 980 cm-1 and 865 cm-1 ;

- bending mode of vibrations ν4(BO3) and ν4(BO4) in BO3 and BO4 groups in the frequency region of 800 to 400 cm-1, respectively for sample 1 with a peak at 670 cm-1, 570 cm-1, 551 cm-1, 528 cm-1, 466 cm-1, and 431 cm-1 observed in the three investigated samples, for sample 2 with a peak 649 cm-1, 520 cm-1, 450 cm-1

and 434 cm-1 and for the sample 3 with a peak 661 cm-1, 449 and 422 cm-1 ;

The stretching vibrations at frequencies higher than 1000 cm-1 are characterized by brief and very strong B-O chemical bonds in the rigid BO3 group, while the stretching vibrations at frequencies lower than 1000 cm-1 are typi-cal for the weaker and longer chemical bonds in the BO4 group. The BO3 and BO4 groups participate in the forma-tion of pirobarate, dibarate, triborate and tetraborate units (structural units with a different number of BO3 and BO4 groups) [28]. The FTIR spectroscopy study of the synthe-sized Co-Ni nanoparticles in 1400 - 400 cm-1 proves the creation of chemical bonds between boron and ogygen atoms, as well as the formation of BO3 and BO4 groups connected differently in boroxol rings (B3O4,5)x and in structures of dibirate, triborate and tetraborate units.

The bands observed in the region of 700 to 400 cm-1 characterize the bending mode of vibrations ν4(BO3) and ν4(BO4) of B-O bonds respectively in BO3 and BO4 groups.

The sharp and slight band of absorption at around 670 cm-1 corresponds to bending mode of vibrations Co-O bonds in CoO and Co3O4 oxides, while that one at 570 cm-1 corresponds to bending mode of vibrations Ni-O bonds in NiO oxides (sample 1). In the frequency region of 700 to 400 cm-1 it could be seen slight ex-pressed absorption bands at 670 cm-1, 570 cm-1, 551 cm-1, 528 cm-1, 466 cm-1, and 431 cm-1 (sample 1), 649 cm-1, 520 cm-1, 450 cm-1 and 434 cm-1 (sample 2) and 661 cm-1, 449 and 422 cm-1 (sample 3) that are characteristic for

bending mode of vibrations ν4(BO3) and ν4(BO4) in BO3 and BO4 groups, as well as they could be a result of the overlapping of absorption bands characterizing banding mode of vibrations of B-O bonds in BO3 and BO4 groups and stretching mode of vibrations of metallic-oxygen bonds in metallic CoO, Co3O4, and NiO oxides.

As it is mentioned above it could be seen in Fig.1 that the bands of absorption characterizing sample 2 and sample 3 are shifted to the lower frequencies in the frequency region of 1500 to 400 cm-1. The observed shift of absorption bands to the lower frequencies compared to those in the case of a ratio Co:Ni = 1:1 (sample 1) it may be due to an arise of BO4 groups in the system with diborate and triborate units which change the boron co-ordination from 3 (in BO3 groups) to 4 (in BO4 groups).

The FTIR spectra of samples 1, 2 and 3, compared in Fig. 1, look identical in shape. It could be said that the different ratio of Co:Ni (1:1, 4:1, 1:4) doesn’t influence the position of the absorption bands.

FTIR study of Co-Ni nanoparticles template synthe-sized at a different ratio Co:Ni using a fluorinated graph-ite (CF) as a support in the presence of β-ciclodextrin as a capping agent during the synthesis (samples 4, 5 and 6)

Compared FTIR spectra of Co-Ni nanoparticles template synthesized at a different ratio Co-Ni using a fluorinated graphite (CF) as a support in the presence of β-ciclodextrin as a capping agent during the synthesis, respectively sample 4 (Co:Ni = 1:1), sample 5 (Co:Ni = 4:1) and sample 6 (Co:Ni = 1:4) are shown in Fig. 2, while the experimental data for the bands of absorption and the mode of the bond vibrations in the formed atom groups at the corresponding wavenumber (cm-1) are given in Table 1 presented above.

It could be seen in Fig. 2 that the FTIR spectra of Co-Ni nanoparticles (Co:Ni = 1:1, 4:1 and 1:4) using graphite as a support in the presence of β-cyclodextrin during the synthesis (respectively samples 4, 5 and 6) seem similar to those of the samples 1, 2 and 3 that pre-sent Co-Ni nanoparticles synthesized at the same ratios (Co:Ni = 1:1. 4:1 and 1:4) but without using a support during the synthesis (Fig. 1).

In the case of carbon-containing Co-Ni nanoparticles nanocomposites slight expressed bands of absorption


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characterizing chemical bond vibrations in graphite and β-cyclodextrin and also in citric acid are observed. They appear in the frequency region of 3900 - 3700 cm-1 with a maximum at the corresponding frequency as follows: narrow slight absorption bands respectively at 3990 cm-1

(samples 4 and 5) and 3949 cm-1 (sample 6) which be-long to the stretching vibrations of C=C bonds resulting from the graphite used as a support during the synthesis and also absorption bands at 3856 cm-1 and 3736 cm-1 (sample 4), 3880 cm-1, 3760 cm-1, 3740 cm-1 (samples 5) and 3823 cm-1, 3748.9 cm-1 and 3727 cm-1 (sample 6) that characterize the C-H bond vibrations in CH and CH2 groups due to the citric acid and β-cyclodextrin. Wide bands respectively at 3381 cm-1 (sample 4) and 3456 cm-1 (sample 6) which is caused by the stretching vibrations of O-H bonds in the primari hydroxyl groups (C-6-OH) or in the secondary hydroxyl groups (C-2-OH) connected by the intermolecular hydrogen bonds are registered.

The bands of absorption observed in the FTIR spectra with a maximum respectively at 2881 cm-1 and 2829 cm-1 (sample 4), at 2894.6 cm-1 and 2824.5 cm-1 (sample 5) and at 2885 cm-1 and 2800 cm-1 (sample 6) belong to the stretching vibrations of C-H bonds in CH and CH2 groups. These bands are probably overlapped by the bands that are characteristic for the stretching vibrations of CH–OH bonds as a result from the used citric acid C6H8O7.

The sharp and narrow bands with a maximum re-spectively at 2361 cm-1 (sample 4), 2360.5 cm-1 (sample 5), 2360.7 cm-1 (sample 6) reveal asymmetric stretching mode of vibrations ν1(OH) of O-H bonds in free OH groups and also stretching vibrations of C–H bonds due to the used of β-cyclodextrin during the synthesis;

The absorption bands with a maximum respectively at 1650 cm-1 (sample 4), 1620 cm-1 (sample 5) and 1644 cm-1 (sample 6) characterize the bending vibrations mode ν4(OH) of H-O-H bonds in H2O molecules adsorbed. These bands could be also due to stretching vibrations mode of the C=O bond in the carboxyl group (COOH) of citric acid (very slow band), as well as belong to the stretching vibrations of the C=C bonds in the benzene ring of the β-cyclodextrin. They could be also due to the graphite used as a support during the template synthesis.

In the frequency region of 1400 to 1200 cm-1 the very slight absorption bands of the bending mode of vibra-tions δ(C-H) of C-H bonds in the primary and secondary hydroxyl groups of β-cyclodextrin are revealed - these are the bands with a maximum at 1399 cm-1, 1330 cm-1, and 1271 cm-1 (sample 4), respectively at 1397 cm-1, 1270 cm-1 and 1217 cm-1 (sample 5) and 1426.6 cm-1, 1335 cm-1 and 1281 cm-1 (sample 6).

At the frequency lower that 1200 to 1000 cm-1 the very slight absorption bands describe the stretching mode of vibrations ν(C-O) of C-O bonds in the hydroxyl

Fig. 2. FTIR spectra of Co-Ni nanoparticles template synthesized at a different Co:Ni ratio using graphite as a support in the presence of β-ciclodextrin during the synthesis: S4-Co:Ni = 1:1, S5-Co:Ni = 4:1, S6-Co:Ni = 1:4.


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group of β-cyclodextrin - these are the bands at 1080 cm-1

and 990 cm-1 (sample 4), at 914 cm-1 (sample 5), and at 1038 cm-1, 1010 cm-1, 982 cm-1 and 950 cm-1 (sample 6).

The absorption bands in the frequency region of 950 cm-1 to 600 cm-1 could be assigned to the bending vibra-tions of the C-H bonds – these are the slight expressed bands at 880 cm-1, 750 cm-1, 667 cm-1 (sample 4), respec-tively at 862.5 cm-1, 773 cm-1, 668 cm-1 (sample 5) and at 850 cm-1, 671 cm-1, 590 cm-1 (sample 6).

The narrow sharp and slight expressed band with a maximum at 560 cm-1, 500 cm-1 , 436 cm-1 (sample 4),

respectively at 560 cm-1, 540 cm-1, 490 cm-1 (sample 5) and 559 cm-1, 540 cm-1, 510 cm-1, 504 cm-1, 490 cm-1, 458 cm-1 could be a result of the overlapping of absorption bands characterizing bending mode of vibrations of B-O bonds in BO3 and BO4 groups and stretching mode of vibrations of metallic-oxygen bonds in CoO, Co3O4, and NiO oxides.

It could be seen that the FTIR spectra of samples presented Co-Ni nanoparticles template synthesized at a different ratio Co:Ni (1:1, 4:1 and 1:4) using graphite as a support in the presence of β-ciclodextrin during the synthesis in the frequency region of 4000 to 1200 cm-1 are

Fig. 3. FTIR spectra of Co-Ni nanoparticles synthesized at a ratio Co:Ni = 1:1: S1 – through a brohydride reduc-tion, S4 – template synthesized using graphiteas a support in the presence of the β-ciclodextrin during the synthesis.

Fig. 4. FTIR spectra of Co-Ni nanoparticles synthesized at a ratio Co:Ni = 4:1: S2–through a brohydride reduction, S5–template synthesized using graphiteas a support in the presence of the β-ciclodextrin during the synthesis.


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almost similar as regards the absorption band position. The bands of absorption typical for the stretching and bending mode of vibrations respectively of C-H, O-H, C=O, C=C bonds in the corresponding groups characterizing citric acid, graphite, β-ciclodextrin using during the nanoparticle synthesis are appeared. These FTIR spectra distinguish by shape and intensity in the interval of 3000 to 1400 cm-1.

The absorption bands at 2800 and 2360 cm-1 in the FTIR spectrum of a sample 4 are sharp and clearly ex-pressed compared to the FTIR spectra of samples 5 and 6. These bands correspond to symmetrical stretching vibrations of O-H bonds in C-OH and CH2-OH groups characteristic for the polysaccharide structure of β-CDx and also in adsorbed Н2О molecules and confirm the use of the capping agent (β-ciclodextrin) during the synthesis.

There are some differences in the FTIR spectra in the interval of 1400-400 cm-1, which are due to the generation of BO3 and BO4 groups formed as a result of the boro-hydride reduction process with NaBH4 used for the wet synthesis of the Co-Ni nanoparticle in aqueous medium.

The bands under 500 cm-1 are also distinguished. They are sharp and slight expressed and characterize metal-oxygen bond vibrations in CoO and NiO oxides. These bands could give information about the pres-ence of bigger quantity of CoO oxides due to the bigger

quantity of Co (sample 5), respectively bigger quantity of NiO oxides due to the bigger quantity of Ni. It is difficult to say that exactly because these bands could be also overlapped with the bands corresponding to the banding vibrations of B-O bonds in BO3 and BO4 groups.

According the Fig. 2 presenting compared FTIR spectra of the synthesized samples 4, 5 and 6 when graph-ite is used as a support in the presence of β-ciclodextrin during the synthesis it could be said that the ratio of Co:Ni influences feebly the mode of the absorption bands characterizing the O-H, B-O, C=C bond vibrations in the corresponding atom groups described above.

In Figs. 3 - 5 are compared the FTIR spectra of Co-Ni nanoparticles synthesized at the same ratio Co:Ni, but respec-tively through a chemical borohydride reduction with NaBH4 and when a template synthesis through the same chemical borohydride reduction is applied using graphite as a support in the presence of the β-ciclodextrin during the synthesis.

The bands of absorption observed in the FTIR spectra with a maximum respectively at 2881 cm-1 and 2829 cm-1 (sample 4), at 2894.6 cm-1 and 2824.5 cm-1 (sample 5) and at 2885 cm-1 and 2800 cm-1 (sample 6) belong to the stretching vibrations of C-H bonds in CH and CH2 groups. These bands are probably overlapped by the bands that are characteristic for the stretching vibrations of CH–OH

Fig. 5. FTIR spectra of Co-Ni nanoparticles synthesized at a ratio Co:Ni = 1:4: S3 – through a brohydride reduction, S6 – template synthesized using graphite as a support in the presence of the β-ciclodextrin during the synthesis.


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bonds as a result from the used citric acid C6H8O7.On the basis of the compared FTIR spectra shown

in Figs. 3 - 5 a conclusion could be done that the graph-ite support lead to change of the spectrum mode (band shape and band intensity), but the band position is slowly influenced. The shift to higher frequencies is observed that means the short and strong chemical bonds (in BO3 groups) are creatide much more compared to the BO4 groups that are characterize by longer and weaker chemical bonds. This shift is typical for the nanostate and confirms that the nanostate is a state of shrinking matter.

Morphology and phase composition of the inves-tigated samples

The samples that are studied by FTIR spectroscopy are also investigated by SEM and XRD analyses. Fig. 6 presents SEM images, respectively made of the samples 1 (Co:Ni = 1:1), sample 2 (Co:Ni = 4:1) and sample 3 (Co:Ni = 1:4) at a magnification x1000 in SPI regime and show the morphology of the synthesized Co-Ni nanoparticles, while Fig. 7 is SEM images made at the same magnification x1000 in SPI regime of Co-Ni nanoparticle synthesized using graphite as a support in the presence of β-cyclodexrin at the same ratios Co:Ni = 1:1 (sample 4), Co:Ni = 4:1 (sample 5) and Co:Ni = 1:4 (sample 6).

Looking at the Fig. 6 presenting SEM images of Co-Ni nanoparticles synthesized at different ratio of Co-Ni, respectively Co:Ni = 1:1, 4:1 and 1:4 it could be observed clearly the difference in the morphology of the sample 1, 2 and 3. When the Ni content is the same (Co: Ni = 1:1 and 4:1) the morphology is almost the same (Fig 6a and Fig.6b), but when the Co content is the same (Co: Ni = 1:1 and 1:4) the morphology is different (Fig. 6a and Fig. 6c). The morphology of the sample 3 (Fig. 6c) is homogeneous, consisting of uniformly distributed particles of the same size.

The graphite used (Fig. 7a) as a support is charac-terized by a flake-like structure that has influenced the morphology formation of the intermetallic Co-Ni nano-particles during their synthesis. The morphology of the carbon-containing nanocomposies based on the Co-Ni nanoparticle is similar to that of the graphite: small spherical and irregular in shape particles can be seen

Fig. 6. SEM image at a magnification x1 000 in SPI regime of Co-Ni nanoparticles synthesized at a different ratio Co:Ni: a - Co:Ni = 1:1 (sample 1), b – Co:Ni = 4:1 (sample 2), c - Co:Ni = 1:4 (sample 3).

a)

b)

c)


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such as the flake-like particles of the graphite.The structure of the β-cyclodextrin (β-CDx) used as

a capping agent during the synthesis represents hollow sphere with functional groups situated on its surface – sugar molecules bounded together in a ring (cyclic oligosaccharides creating a cone shape. The β-CDx cov-ers the template synthesized nanoparticles and prevents their aggregation.

It can also be seen that the ratio of Co:Ni also influ-ences the nanoparticle formation. As in the previous case of samples 1, 2 and 3 presenting Co-Ni nanoparticles synthesized respectively at a ratio of Co: Ni = 1:1, 4:1 and 1:4 (Fig. 6) in this case when Co-Ni nanoparticles are synthesized using graphite as a support in the pres-ence of β-CDx during the synthesis it could be seen a

light difference in the morphology of the sample 4, 5 and 6 (Fig. 7). May be it is due to the graphite support that strongly influences the nanoparticle formation. When the Ni content is the same (Co: Ni = 1:1 and 4:1) the morphology of sample 4 and 5 distinguishes (Fig. 7b and Fig. 7c), but when the Co content is the same (Co: Ni = 1:1 and 1:4) the morphology of the sample 4 and 6 almost is the same (Fig. 7b and Fig. 7d). The morphology of the samples 4 and 6 is homogeneous, consisting of uniformly distributed particles of the same size, while the morphology of the samples 5 is characterizes by smaller and bigger nanoparticles irregular in shape presenting nanoparticle aggregates.

When fluorinated graphite is used as a support the SEM images presented in Fig. 6 show a morphology

a)

b)

c)

d)

Fig. 7. SEM image at a magnification x1 000 in SPI regime of the used graphite support and Co-Ni nanoparticle synthesized at a different ratio Co:Ni using graphite as a support in the presence of β-cyclodexrin: a - graphite sup-port, b - Co:Ni = 1:1 (sample 4), c - Co:Ni = 4:1 (sample 5), d - Co:Ni = 1:4 (sample 6).


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characterizing by flake-shape particles. This morphol-ogy is typical for the morphology of the graphite itself. The synthesized intermetallic nanoparticles are of a type Co-Ni core/carbon shell.

Template synthesis through a borohydrate reduction using a carbon (graphite CF and β-cyclodextrine) proves to be an effective way to in situ obtain new carbon based composites with active intermetallic Co-Ni nanoparticles exhibiting magnetic behavior. Prepared are fine Co-Ni powder, which have morphology typical for alloys and magnetic properties to be used successfully in biomedicine.

In Fig. 8 XRD spectra of the synthesized Co-Ni nanoparticles at a ratio Co: Ni=1:1, 4:1 and 1:4 in-cluding using graphite as a support in the presence of β-cyclodextrin during the synthesis are presented.

Since both cobalt and nickel have the same face-centered cubic (fcc) phases and their character peaks appear at almost the same diffraction angle, it is not easy to distinguish these two metals. It can be seen that all samples have the same character peaks: a broad promi-nent peak at around 2θ value of 45o corresponding to both Co (fcc) and Ni (fcc) phases. The peak at 2θ value of 34o describes Co hexagonal centered phase (hcp). The XRD analysis proves the formation of oxide phases of

Co and Ni. The peaks at around 2θ of values 12o and 37o are assigned to Co3O4 phase. The peaks at around 2θ of value 62o are related to NiO phase.

When the content of Ni is more (ratio Co:Ni=1:4), the peaks characterizing respectively the phases of Co (hcp) (2θ≈34o), Co (fcc), Ni (fcc) (2θ≈45o) and NiO (2θ≈62o) are more intensive than that in the other cases of ratio Co:Ni = 1:1 and 4:1.

Looking at the XRD patterns compared in Fig. 8 it could be seen clearly the influence of the content of Co, respectively Ni on the phase formation and phase composition of the synthesized Co-Ni nanoparticles and their carbon-containing nanocomposites. The peaks at 2θ≈34o (Co (hcp) phase), 2θ≈ 45o (Ni (fcc) phase), and 2θ≈62o (NiO phase) prove exactly this conclusion.

The XRD spectra of Co-Ni nanoparticles and their carbon-containing composites show that during the synthesis of Cо-Ni nanoparticles at a different ratio of Cо:Ni (1:1, 4:1, 1:4) and also when using a carbon-based support two main phases of Co (hcp,fcc) and Ni(fcc) are formed that are in accordance with the phase diagram of the binary Co-Ni system. A phase of graphite is also detected at around 2θ of value 2θ≈26o. The impurity of CO, Co3O4 and NiO phases is due to oxidizing processes.

Fig. 8. XRD patterns of Co-Ni nanoparticles synthesized at a different ratio Co:Ni without a support: S1 - Co:Ni=1:1, S2 - Co:Ni=4:1, S3 - Co:Ni = 1:4, and whit a support: S4 -Co:Ni = 1:1, S5 - Co:Ni = 4:1, S6 - Co:Ni = 1:4.


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CONCLUSIONS

On the basis of the FTIR study of the Co-Ni nano-particles synthesized through a borohydride reduction at a different ratio Co: Ni (1:1, 4:1, 1:4) and also applying a template synthesis using graphite as a support in the presence of β-ciclodextrin during the synthesis it could be said that the Infrared spectroscopy is a sensitive and suitable method for investigation of the occurring na-noparticle surface phenomena and to establish the atom groups formed on the nanoparticle surface. The FTIR spectra prove the chemical bonds created in the surface atom groups.

The Infrared spectroscopy can catch the differences in the FTIR spectra due to different ratio Co:Ni and in the case when a support is used for the nanoparticle tem-plate synthesis. Both the shape and position of the bands of absorption at the relevant wavenumber in the spectra give information concerning the mode of vibrations of the chemical bonds created between the corresponding atoms in the atom groups situated on the nanoparticle surface, re-spectively egarding the occurred nanosurface phenomena.

AcknowledgementsThe authors acknowledge the financial support for

this investigation provided by the National Science Fund at the Ministry of Education and Science – Bulgaria under the Contract DN07/29 – 16.12.2016 (respectively Scientific Research Center at the UCTM-Sofia under the Contract No 881-10.01.2017).

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