Synthesis of in-situ luminescent ZnS nanoparticles

Synthesis of in-situ luminescent ZnS nanoparticles facile with CTAB micelles and their properties study Vaishali Shukla and Man Singh

Citation: AIP Conference Proceedings 1724, 020126 (2016); doi: 10.1063/1.4945246 View online: http://dx.doi.org/10.1063/1.4945246 View Table of Contents: http://aip.scitation.org/toc/apc/1724/1 Published by the American Institute of Physics

Synthesis of In-situ Luminescent ZnS Nanoparticles Facile with CTAB Micelles and Their Properties Study Vaishali Shukla a and Man Singh b a b

Centre for Nanoscience, Central University of Gujarat, Gandhinagar, India School of Chemical Sciences, Central University of Gujarat, Gandhinagar, India Telephone: 079-23260210, fax: 079-23260076 E-mail: [email protected], [email protected]

Abstract. Currently, the development of micelles route is thrust area of research in nanoscience for the control particle size and remarkable properties through chemical co-precipitation method. A 0.9 mM aqueous CTAB micellar solution plays a role as capping agent in the homogeneous solution of 0.5 M ZnSO4 and 0.5 M Na2S for synthesis, further precipitates purified with centrifugation in cold ethanol and millipore water to remove unreacted reagents and ionic salt particles. A resultant, white colored luminescent ZnS nanoparticle out with ~95% yield is reported. The ZnS nanoparticles have been examined by their luminescence properties, optical properties and crystal structure. The mean particle size of ZnS nanoparticles is found to be ~10 nm in various technical results and UV-absorption was 80 nm blue shifts moved from 345 nm (bulk material) to 265 nm, showing a quantum size impact. The X-ray diffraction (XRD) pattern shows the immaculate cubic phase. Photoluminescence (PL) investigates the recombination mechanism with blue emission from shallow electron traps at 490 nm in ZnS nanoparticles. An FTIR spectrum and Thermal gravimetric analysis (TGA) gives confirmation of CTAB – cationic surfactant on surface of ZnS nanoparticle as capping agent as well thermal stability of CTAB capped ZnS nanoparticles with respect to temperature. Key-words: ZnS nanoparticles (ZnS NPs), CTAB micelles, co-precipitation, Luminescence properties, Band-gap

INTRODUCTION Zinc sulfide (ZnS), a common II-VI semiconducting compound is a promising optoelectronics devices material in view of its wide direct band gap (3.5-4.00 eV) 1-2. However, a few perspectives, especially the factors controlling synthesis of nanomaterial in an aqueous solution of surfactant are still not extremely surely knew. Further efforts are being made to control the shape and size of nanoparticles (NPs) utilizing surfactant aggregates. Unfortunately, the use of surfactant self-assemblies to control the shape and size of NPs remains an amazingly troublesome undertaking, since the surfactant adsorption and aggregation forms itself is influenced by numerous thermodynamic factors. These factors will obviously affect NPs synthesis in aqueous micellar media 3. Capping on ZnS NPs with the surfactant like, cationic surfactant via micellar route is an imperative perspective to yield diverse nanostructures. There are only a few reports 3, 4 on systematic investigations of ZnS NPs using an aqueous micellar route that provides a detailed understanding of stabilization mechanism. It is well reported in literature 5 that the rate of adsorption of cationic surfactants is quick and the final amount adsorbed is higher than anionic and non-ionic surfactants. Along these lines, if adsorption is thought to be the criteria for the stabilization of NPs, then size, shape, and other properties of the NPs in cationic surfactant like CTAB must vary from those in anionic and non-ionic surfactants. The increase in band gap with the decrease in particles size is the most recognized part of quantum confinement in semiconductors. ZnS is a wide band gap semiconductor with band gap energy (E.g.) of 3.68 eV 6-7 . It has been generally used as a part of many optoelectronic devices, for example, blue-light-emitting diode 8, solar cells 9, and field emission devices 10. Their synthesis has been accomplished through different methods including hydrothermal synthesis 11, aqueous micelles 12, reverse micelles 13, sol–gel process 14, and spray pyrolysis 15. Nobel luminescence characteristic, for example, stable light emission with different colors were observed from ZnS NPs at room temperature. It has been accounted for that the different capped ZnS NPs have been one of the best efficient electroluminescent phosphor material being used and its synthesis and properties have widely been investigated. ZnS nanoparticles were prepared using simple precipitation method with micellar route depicted by Han et al. 4 and S. K. Mehta et al. 3 with a few alterations. In this paper, we gave an account of a fruitful synthesis of CTAB capped ZnS NPs with improved luminescence characteristics. We used X-ray diffraction (XRD), Florescence Spectroscopy, IR-Spectroscopic (FTIR), UV-Spectroscopy, Thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM) to characterized prepared nanoparticles. Keeping in mind the end goal to consider the impact of surfactant and surfactant concentration, cationic surfactant CTAB has been used to synthesize ZnS NPs. 2nd International Conference on Emerging Technologies: Micro to Nano 2015 (ETMN-2015) AIP Conf. Proc. 1724, 020126-1–020126-7; doi: 10.1063/1.4945246 Published by AIP Publishing. 978-0-7354-1371-9/$30.00

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EXPERIMENTAL SECTION Materials To synthesize CTAB (cationic surfactant) capped ZnS NPs, the following materials were used. Zinc Sulphate heptahydrate (ZnSO4•7H2O), Sodium Sulphide (Na2S•xH2O), and Cetyltrimethyl Ammonium bromide (CTAB), All the glass wares used during the experimental work were acid washed. The chemical reagents used were analytical reagent grade without further purification. Ultra-pure deionized water (Seralpur delta UV/UF setting, 0.055 mS/cm) was used in all synthesis steps.

Synthesis of CTAB capped ZnS nanoparticles The photoluminescent, CTAB capped ZnS NPs were synthesized in the aqueous solution of the CTAB (0.9 mM) was steady for quite a long time together. ZnS NPs were readied utilizing basic compound precipitation strategy. In run of the mill try different things with some change, The CTAB micellar solution containing Na2S·xH2O (0.5 M, pH 12.5) was added drop astute to another contains ZnSO4·7H2O solution (0.5M, pH 2.75, in 0.01M HCl), S2-/Zn2+ proportion of 2.0 with proceeding vigorous magnetic stirring in laboratory condition at RT. The solution was then permitted to remain for 60 min at the same condition. After the Na2S infusion, white voluminous precipitates were showed up and during process pH gradually increase from 3.0 to 8.0. During the reaction, H2S (g) was produced at low concentration which was colourless, highly flammable and gave pungent odor similar to that of rotten eggs odor. The H 2SO4 was utilized informally ZnS NPs whereas NaCl was removed by solubilizing in chilled water, conformed by silver test (AgCl gives white cloudy precipitates). The acquired dispersion was purified by dialysis against demineralised water or NaOH (0.01M) prompting dispersions with a pH of either 8.0 or 13.0 by centrifuge at 6000 RPM. The precipitates were dried in hot air oven at 700C.

Characterization of Nanoparticles The CTAB capped ZnS NPs were characterized using X-ray diffraction (XRD) patterns of the powdered samples using X-ray diffractometer, PTS 3000 by Rich-Seifert instrument with CuKα radiation (λ = 0.15418 nm) at room temperature. The size and morphology of the NPs were determined using scanning electron microscope (SEM, Carl Zeiss, Evo-18; 20 kV) images. FTIR spectra of dried powder were taken with Perkin Elmer spectrum – 65 series FTIR spectrometer. The optical absorption spectra of the NPs in deionized water recorded using UV-2060 plus spectrometer (Over: 200-600 nm). Fluorescence measurements were performed on a Hitachi-FP-6500 fluorescence spectrophotometer (Over: 300-500 nm). The thermal gravimetric analysis (TGA) was carried out with intercooler Perkin Elmer TGA-6000 thermometer.

RESULTS AND DISCUSSIONS CMC: Formation of CTAB Micelles Fig. 1(a), shows molecular structure and micelle structure of CTAB. Critical Micellar Concentration (CMC) is the key point for surface chemistry, which indicates the usually narrow range of concentration separating the limits, at below which most of the surfactant is in the monomeric state and above which all additional surfactant enters the Micellar state. 16

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Specific Conductivity (μS/cm)

25 23 21 19 17 15 13

20 ˚C

11

25 ˚C

9

30 ˚C

35 ˚C

7

40 ˚C

5

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Molar conc. (mM) FIGURE 1. (a) Structure formation of micelles (b) CMC determination of CTAB micelles

Fig. 1(b), shows the break points were quite significant as observed, specific conductivity increase almost linearly with CTAB concentration but with definite break points. However, a contrasting behavior has been obtained with CTAB, a positively charged surfactant with CMC is 0.9 mM in aqueous solution which is the similar value of literature, which is in between 0.92 - 1.0mM. 17

CTAB adsorption: FTIR analysis Fig. 2 (a) & (b), shows the symmetric and asymmetric –CH2 stretching, Vibrations of pure CTAB lie at 2921.8 cm-1 and 2847.6 cm-1; remained almost same in the presence of the ZnS nanoparticles within the experimental errors. The peaks at 1644.1 cm-1 and 1488.8 cm-1 for pure CTAB are attributed to –C-H scissoring vibrations of –N-CH3 moiety, which are shifted to 1625.2 cm-1 in the presence of the ZnS NPs. Also the peaks at 1284.1 cm-1 and 1245.3 cm-1 due to –C-N stretching are suppressed and significantly shifted to 1087 cm-1 and 1031 cm-1 in the presence of ZnS NPs. From FTIR spectroscopic results, it is clear that the peaks due to CTAB head group regions are shifted without any significant shift in hydrocarbon tail regions in the case of CTAB capped ZnS NP. These results confirm the stabilization of ZnS NPs by adsorption of CTA+ through head group region.18

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FIGURE 2. IR spectra of (a) pure CTAB and (b) CTAB capped ZnS NPs

IR spectra (Fig. 2) clearly shows the CTAB adsorption on the outer surface of ZnS NP (CTAB capped ZnS NP) via sulphur atom this would lead to the formation of a cationic self-assemblies via micelles which is energetically very stable. This in combination with the high affinity of the sulphur to ZnS indicates that bonding to the particle surface occurs through the cationic group as has been observed for metal particle dispersion stabilized with cationic surfactant.

Optical Absorption and Band-gap Fig. 3 (a), shows characteristic absorption peak appears at the maximum wavelength λmax=265 nm, which indicates 80 nm blue shift from the bulk ZnS material (345 nm) and this peak position reflect the bandgap of the synthesized CTAB capped ZnS nanoparticles. This blue shift absorption edge is due to the small size of the particle.

FIGURE 3. (a) Uv- Absorption spectrum (b) Band-gap of CTAB capped ZnS NPs

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According to plot (Fig. 3-b), the direct optical band-gap is 3.90 eV for CTAB capped ZnS nanoparticles. The obtained value of the optical band-gap of CTAB capped ZnS nanoparticles are higher than that of the bulk value of ZnS (3.68 eV). This blue shift of the band-gap takes place because of the quantum confinement effect 19.

Photoluminescence study (PL) Fig. 4 shows the PL spectrum of the redispersed ZnS NPs shows two intensity at 455 nm and 490 nm and one week emission at 425 nm. The interesting point is that the intensity shows reciprocal trends in spectrum; i.e, the emission that are strong in one become weak in the other and vice-versa. This type of behaviour can be attributed to change in shape and size of nanoparticles during separation and drying process as the luminescence spectra show size and shape dependent quantum confinement effect.

FIGURE 4. The room temperature PL spectra of CTAB capped ZnS NPs

The CTAB capped ZnS NPs solution are surface passivated by excess sulphide ions and surfactant monomers and show weak deep-trap (intense shallow-trap) emission at 425 nm, whereas due to removal of passivation after redispersion the defect related emission at 455 nm became more intense due to defects in nanocrystals. The peak at 490 nm has been assigned to the presence of sulphur vacancies in the lattice 20. ZnS nanocrystals contain excess of sulphur in aqueous solution and thus show weak emission due to sulphur vacancies.

Crystal structure and Morphology Fig. 5(a) shows crystal structure of CTAB capped ZnS NP, which exhibit the characteristic pattern corresponds to face-centered cubic (FCC) structure (JCPDS powder diffraction file no.5-0566). No other impurities such as oxides or organic compounds related to reactants were detected by XRD analysis indicating the phase purity of the ZnS NP product. Three diffraction peaks at 2θ values of 27.162, 49.102, and 56.577 correspond to , , and plane, respectively, of cubic ZnS. The peak broadening of the XRD clearly indicates that small nanocrystals of ZnS are present in the sample.

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FIGURE 5. (a) X-ray diffraction pattern (b) Morphology of CTAB capped ZnS NPs

Fig. 5(b) shows scanning electron micrograph which revealing most of the spherical nanoparticles at the left hand side shows several ZnS NPs among these single particles and agglomerates. During the drying step for sample preparation the particles tend to agglomerate which makes it difficult to determine size afterwards because it is often difficult to distinguish between a single particle and an agglomerate of two or three particles. Therefore, at 200 nm scale particle size vary from 59.76 nm to 61.63 nm which are obtained with SEM images. However, it was observed that bigger particles are often placed at the outer side of the sample and the smaller ones in the middle. This is also an affected of the drying process. During drying the diameter of the particle gets smaller and at its edges the particles are deposited on the grid, with the biggest particles depositing first on the grid because of their higher mass.

Thermal Gravimetric Analysis (TGA) In Fig. 6, a thermal gravimetric analysis (TGA) investigation of the powder showed, the CTAB does not desorb in a single step but that a decomposition of the molecule take place at the surface. In the first stage molecule defected with water which is adsorbed at the particle surface since the powder had been isolated from aqueous suspension, due to desorption of water molecules with 80-85 % weight loss was observed at around 1250C and in the second stage around 5-6 % weight loss was observed at around 3400C due to decomposition of cationic surfactant (CTAB) 21. Here, at the end 11.44 % weight loss was observed at around 1000 0C due to SO2 which has to take into account that the SO2 originates from the CTAB and the ZnS NPs.

FIGURE 6. Thermal gravimetric analysis (TGA) of CTAB capped ZnS NPs

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According to heat capacity formula: (Q = Cp·dT), where Cp is the specific heat capacity of a particular structure. Since, the CTAB capped ZnS NPs act as random formation of molecular self-assemblies. Thus, particular structure weight loss TGA curve shows amorphous material characteristic instead of metallic characteristic with respect to the time and temperature. Hence, there is a possibility that CTAB are capped around ZnS NPs in different fashion. The fact that the molecule decomposes rather than desorbs as a whole and the temperature which are necessary for this indicates that the CTAB is covalently bound to the surface. The fact that the SO2 molecule evaporates at temperature higher than 8000C confirms the assumption that the CTAB is attached to the surface via the sulphur atom.

CONCLUCION A Luminescent ZnS Nanoparticles facile with covalently bounded CTAB have been successfully synthesized by mean of self-assembly via micellar route, and FTIR spectra and TGA analysis sport the capped structure of nanoparticles via the sulphur atom. XRD images show formation of the small size FCC crystal structure, which may be due to nanosized particle distribution. However, a deep study and research has to be made in order to Photoluminescence study, the effect of luminescent properties is changed in the presence of the capping agent CTAB which show in defect related emission intensity peak. Thus, formation of CTAB capped ZnS NPs used as new fabricating material in opto-electronics devices.

ACKNOWLEDGMENTS Authors are thankful to Central instrumental facility lab, Central University of Gujarat, Gandhinagar; for infrastructural support. Also thankful to Department of Forensic Science, Gandhinagar, Gujarat; for SEM analysis and GFSU, Gandhinagar, Gujarat; for PL spectroscopy.

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