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AZ9RBAYCAN KIMYA JURNALI № 4 2015
UDC 541.64:547.458.81
SYNTHESiS AND CHARACTERIZATION OF COBALT SULFIDE NANOSTRUCTURES
USING THIOUREA
O.O.Balayeva, A.A.Azizov, M.B.Muradov, A.M.Maharramov, G.M.Eyvazova, R.M.Alosmanov, Z.Q.Mamiyev, Z.A.Aghamaliyev
Baku State University Institute of Physics Azerbaijan NAS
Received 01.10.2015
Cobalt sulfide nanoparticles were synthesized through a successive ionic layer adsorption and reaction (SILAR) method at room temperature. Co(NO3)2-6H2O and thiourea [(NH2)2CS] aqueous solutions were used as precursors. Obtained cobalt sulfide nanomaterials were heated at 100°C temperature in a vacuum. By XRD analysis mixed phases of Co3S4, CoS1 097 and CoS2 were obtained with dominance of Co3S4 phase. The average size of cobalt sulphide nanoparticles in mixed phases obtained at 8 cycles of the reaction was estimated 9.8 nm. The optical band gap of the CoxSy nanomaterials was found 1.59 eV. By SEM analysis the obtained cobalt sulfide nanocrystals display a rice-like morphology.
Keywords: cobalt sulfide, nanocomposites, structural and optical properties.
Introduction
Sulfides of various transition metals show interesting electrical and optical properties such as semiconductivity, luminescence, and photoconductivity [1]. Cobalt sulfides a group of II-IV semiconductor materials can be used in solar devices [2], ultra-high-density magnetic recording material [3], anodes for Li-ion batteries [4] and catalysts for hydrodesulphurization and dehydro-dearomatization [5]. Cobalt sulphide has potential applications in solar selective coatings, IR detectors and as a storage electrode in photoelec-trochemical storage device [6]. Due to its proper band gap and good adsorption properties, CoS can be used as photocatalyst.
It is also one of the more complicated metal sulphide systems, with a number of phases and differing chemical compositions, including Co4S3, Co9S8, CoS, Co1-xS, Co3S4, Co2S3 and CoS2 [7]. Cobalt sulphide (CoS) is a semiconductor with band gap energy equal to 0.9 eV, however, Co3S4 has optical band gap of about 0.78 eV. Recently, cobalt sulfide has been synthesized by a hydrazine-assisted low-temperature hydrothermal preparation method [8]. One of the promising preparation methods of highly dispersed cobalt sulfide species is to use zeolite as a support because of its high surface area and homogeneous pore structure [9].
Cobalt sulfide nanopowders were prepared by a simple chemical reaction method at
room temperature [10]. The possibility of using cobalt sulfide powder as a cathode material in lithium rechargeable batteries was explored through electrochemical testing. Cyclic voltam-permetry measurements revealed that the Li/Co9S8 cell discharges in a two-step process. The discharge capacities were improved with increasing contents of electronic conductors in the electrodes.
The aim of this work was the synthesis and study of CoxS^ nanoparticles using cost effective and convenient stabilizing agent - func-tionalized nitrile butadiene rubber (F-NBR) for gaining advanced interesting properties. In this paper we prepared CoxS^/F-NBR nanocompo-sites by successive ionic layer adsorption and the reaction (SILAR) method in phosphorus containing polymer sorbent using thiourea. The reaction was carried out at room temperature. The results obtained from X-ray, UV-Vis, IR, SEM and EDX analysis are reported.
Experimental
Polymers containing -PO(OH)2 phos-phonic functional active groups were synthesized from the oxidative chlorophosphorylation reaction of NBR with PCl3 and oxygen. 10% solutions were prepared from NBR-26 with CHCl3. To carry out oxidative chlorophosphor-ylation reaction was used an apparatus consisting of a round four-necked flask equipped with
a mechanical stirrer, thermometer, reflux condenser and a bubbler for oxygen [11, 12]. The oxygen rate was 7 l/h and 110 ml PCl3 was added by drops to a stirred solution. The reaction was exothermal and the temperature was raised up to 450C in 18 hour by stirring. The function-alized polymer sorbents are dark brown powder. This powder sample was used for loading the nanoparticles.
The preparation of cobalt sulfide nano-particles was carried out by a SILAR method. 25 ml 0.5 M Co(NO3)2-6H2O solution was prepared as cobalt precursor. 0.2 g of F-NBR powders were added to the solution at room temperature. After 24 hours, polymers containing Co2+ ions were washed to remove unexchanged ions. The sulphirizing processes was carried out with 25 ml 1M SC(NH2)2 and stirred 24 hours. Samples were rinsed up with distillated water and this process was repeated in 6 and 8 cycles and air-dried. Obtained cobalt sulfide nanomaterials were heated at 1000C temperature in a vacuum.
In the experiment the below mechanism is postulated (1), (2). The nucleation processes occur in the first cycle and then the process goes with the formation of copper sulfide nano-particles:
SC(NH2)2+OH- ^OC(NH2)2+SH- , (1) SH-+OH-^S-2+H2O, (2)
Co2++S2-^CoS. (3)
Powder X-ray diffraction patterns of the samples were recorded by using of Bruker D2 X-ray diffractometer with CuXa-irradiation (X = 1.54060 A), UV-Vis absorption spectra and IR spectra were reflected where by SPECORD 250. SEM/EDX analysis was carried out on a Field Emission Scanning Electron Microscope JEOL JSM-7600F with Energy dispersive spectrometer X-max 50 and Electron Backscattered Diffraction System Nordlys Max from Oxford Instruments.
Results and discussions
All the measurements were done before and after annealing the samples at 100°C for 6 hour in vacuum.
Structural properties by X-ray diffracto-meter. The pattern in Fig. 1 corresponds to the formation of cobalt sulfide in thiourea. In this
case mixed phases of Co3S4, CoSi.097 and CoS2 were obtained with dominance of Co3S4 phase. If we look through to this figure there is indicating that, more of picks that correspond to mixed phases like CoS1097 and CoS2 were obtained after 6 cycles of the formation or the intensity increased. The diffraction peak at 20 value of 26.7, 38 and 55 respectively corresponds to the planes of (220), (400) and (330) indicating the formation of cubic Co3S4 [JCPDS no.42-1448] (Fig.1).
mwtoi/jLi M^mÂm JuAiWwau
I
4
A.
Mil MiWULf k^AMUi
20
30
60
70
40 50 20 (°)
Figure 1. XRD patterns of Co.rSv/F-NBR prepared using 1 M thiourea and 0.5 M Co(NO3)2-6H2O as precursors: a - 6 cycles and b - 8 cycles.
The average diameter of cobalt sulfide nanocrystallites were calculated by the Debye-Scherrer's equation (4)
^ a'k
D = pcoie • (4)
where, D is the particle diameter of nanocrystal-lite, a constant (0.9), X the X-ray wavelength (1.5418 A), and p is the half-width of the diffraction peak. The average size of cobalt sulphide nanoparticles in mixed phases obtained using thiourea at 8 cycles of the reaction was estimated 9.8 nm.
Optical properties. The UV-Vis spectra of cobalt sulfide nanomaterials were taken within the wavelength range 300-1100 nm. Band gap energy and transition type was derived from mathematical treatment of the data obtained from the optical absorbance versus wavelength data with the following relationship for near-edge absorption:
A = [k(hv - Eg)"/2] / hv , (5)
where v is the frequency, h is the Planck's constant; k equals a constant while n carries the value of either 1 or 4. The bandgap, Eg, could
be obtained from a straight line plot of (ahv)2/n as a function of hv. Extrapolation of the line to the base line, where the value of (ahv)2n is zero, will give Eg. If a straight-line graph is obtained for n = 1, it indicates a direct electron transition between the states of the semiconductors, whereas the transition is indirect if a straight-line graph is obtained for n = 4 [13, 14].
Figure 2. Optical band gap of cobalt sulfide nanomaterials obtained (a) - in 6 cycles, (b) -in 8 cycles, (c) - in 8 cycles (annealing at 1000C for 6 hour in vacuum).
For the determination of band gap we have considered the direct transition. A plot of (ahv) vs. hv (direct transition) of cobalt sul-fide/F-NBR nanocomposites and after heating are shown in Fig. 2. The band gap energy for every samples were obtained from the intercept of the energy axis after extrapolation of the straight lines section of (ahv) vs. hv curve. Optical band gaps obtained for direct transition are given in the Table.
Band gaps of Co^ obtained using tiourea (eV) (partic. size: 9.8 nm)_
6 cyc. 8 cyc. 8 cyc. (1000C)
1.62 1.59 1.72
It is seen that the energy band gap decreases, with increasing reaction cycles for obtained cobalt sulfide nanomaterials. After annealing the samples at 1000C for 6 hour in vacuum optical band gaps have decreased. This result may be happened due to the first grade destruction of polymer under the temperature. In this case the particles sizes didn't decrease, but
particles were analyzed more clearly by UV-Vis spectroscopy.
SEM and EDX analysis. Fig. 3 exhibits SEM and EDX images of prepared CoxS/F-NBR nanocomposites obtained using thiourea as sulfur precursor in 8 cycles. As shown in Fig. 3a, the obtained cobalt sulfide nanocrystals display a rice-like morphology with a length of ~500 nm and with a width of 140 nm. In sharp contrast on nanoparticles size with the XRD pattern. The coalescence of crystallites highly occurred to form larger particles to lower Gibb's free energy. During the nucleation and growth of cobalt sulfide nanoparticles, F-NBR act as a support material and suppress the aggregation of CoxSy nanoparticles.
Figure 3. SEM and EDX result of cobalt sulfide obtained using thiourea as sulfur precursor.
IR spectroscopy study. The IR spectra of CoxSy/F-NBR nanocomposite samples are shown in Fig. 4. Some differences such as increasing or decreasing of some bands intensity as well as the shift of peak position to the slightly lower wave numbers can be observed in the spectra.
The broad band at 2191 cm-1 can be as-
signed as the O-H stretching peak of the -PO(OH)2 group formed by the chlorophos-phonation reaction of the polymer. Upon the formation of CoxSj; nanoparticles this peaks shift to 2183 cm-1. Concentration and masses of attached atoms also influence the intensity and frequency of the peak. As masses of attached atoms increase, wavenumber decreases and as concentration decreases, intensity is also decrease. The 480 cm-1 peak corresponded to the S-S stretching [15] of the disulfide group indicating in Fig. 4a and 4b. The peak of PO at 977 cm-1 (Fig. 4c) shift to 970 cm-1 (Fig. 4a) and upon the formation of CoxSj; nanoparticles by thiourea. After heating, the peak of 989 cm-1 is shown with high intensity. The IR spectra also confirmed the stability of the prepared cobalt sulfide by the absence of any CoO peak at 580 cm"1 in the IR spectrum [16].
Figure 4. IR spectroscopy results of a -CoxS/F-NBR obtained in 8 cycles using thiourea, b - CoxS/F-NBR obtained in 8 cycles using thiourea (1000C), c - F-NBR.
Infra-Red, raman spectra and crystallo-graphic studies have shown that in thiourea there are almost equal contributions from the canonical forms I, II and III (Scheme 1).
Scheme 1 [17].
Thiourea is potentially capable of forming coordinate bonds through both sulphur and nitrogen even though the extremely low basicity of the ligand militates against the formation of nitrogen-metal bonds [17]. The symmetric stretching frequency of C=S (1446 cm-1) for thiourea is shifted to lower frequency (1430 cm-1) for the complex. Lower shift of C=S stretching frequency confirms the formation of metal-sulfur coordination bond [17] The weak peak at 2920 cm-1 shown in all samples are due to C-H stretching modes of the F-NBR carbon chain, indicating that the nanoparticles are covered by polymer macromolecules. The peak at 3429 cm-1 can be ascribed to the absorption of H2O in the all samples [18] (Fig. 4). Other corresponding bands as O-H (in P-O-H): 2854 cm-1, C-O (in P-C-O): 1109 cm-1 are shown on the sorbent spectra also changed after formation of nanoparticles.
Conclusion
Rice-like cobalt sulfide (CoxS^) nanoparticles were synthesized using Co(NO3)26H2O, Na2S-9H2O and thiourea [(NH2)2CS] as precursors by SILAR method at room temperature. By changing the reaction parameters sulfur sources different composition and crystal structure were obtained. Using Na2S as sulfur source cubic linnaeite (Co3S4) was formed and using thiourea as sulfur precursor, mixed phases of Co3S4, CoS1 097 and CoS2 were obtained with dominance of Co3S4 phase. The optical band gap of the CoxS^ and Co3S4 nanomaterials were found as 1.59 and 1.65 eV for different sulfur sources like thiourea [S=C(NH2)2] and sodium sulfide (Na2S9H2O) respectively. The influence of different sulfur sources on the formation of cobalt sulfides was discussed in detail.
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TIOKARBAMIDDON ISTIFADO ETMOKLO KOBALT SULFID NANOHISSOCIKLORININ
SiNTEZi VO TODQiQi
O.O.Balayeva, A.O.Ozizov, M.B.Muradov, A.M.M3h3rramov, G.M.Eyvazova, R.M.Alosmanov,
Z.Q.Mamiyev, Z.A.Agamaliyev
Kobalt sulfid nanohissaciklari ardicil ion qatinin adsorbsiyasi va reaksiyasi (AiQAR) metodu ila sintez olunmu§d ur. ilkin reagentlar kimi Co(NÜ3V6H2Ü va tiokarbamidin [(NH2)2CS] sulu mahlulundan istifada edilmi§dir. Alinmüj kobalt sulfid nanomateriallari 1000C temperaturda vakuumda qurudulmu§dur. Rentgen toz difraktometrindan (RTD) alinan naticalara göra Co3S4 üstünlüklü qari§iq fazali kobalt sulfid (Co3S4, CoSi.097 va CoS2) nanohissaciklari for-mala§mi§dir. Kobalt sulfide nanohissaciklarinin orta ölgüsü 8-ci tsikldan sonra 9.8 nm-dir. Optiki qadagan olunmu§ zonamn enerjisi 1.59 eV olmu§dur. Skanedici elektron mikroskopundan (SEM) alinan naticalara göra sintez olunmu§ kobalt sulfid nanohissaciklari düyü §akilli morfologiyaya malikdir.
Agar sözlar: kobalt sulfid, nanokompozitbr, qurulu§ v3 optiki xassabr.
СИНТЕЗ И ИССЛЕДОВАНИЕ НАНОСТРУКТУР СУЛЬФИДА КОБАЛЬТА С ИСПОЛЬЗОВАНИЕМ
ТИОМОЧЕВИНЫ
О.О.Балаева, А.А.Азизов, М.Б.Мурадов, А.М.Магеррамов, Г.М.Эйвазова, Р.М.Алосманов,
З.Г.Мамиев, З.А.Агамалиев
Наночастицы сульфида кобальта синтезированы методом последовательной адсорбции ионного слоя и реакции (ПАИСР) при комнатной температуре. Водные растворы Со(М03)2^6Н20 и тиомочевины [(ЫН2)2С8] были использованы в качестве первичных реагентов. Полученные наноматериалы сульфида кобальта были высушены при температуре 1000С в вакууме. Данные порошковой рентгеновской диффрактометрии показывают, что образуются смешанные фазы Со384, Со81097 и Со82 с доминированием фазы Со384. Средний размер наночастиц сульфида кобальта в смешанных фазах при 8 циклах реакции равен 9.8 нм. Ширина оптической запрещенной зоны наночастиц Со^ -1.59 эВ. Анализ РЭМ/ЭРС показал, что синтезированные материалы обладают рисоподобной морфологией.
Ключевые слова: сульфид кобальта, нанокомпозиты, структурные и оптические свойства.
