The influences of the concentrations of “green capping agents” as stabilizers and of ammonia as an activator in the synthesis of ZnS nanoparticles and their polymer nanocomposites

Thokozani Xaba; Makwena J. Moloto; Mundher Al-Shakban; Mohammad A. Malik; Nosipho Moloto; Paul O’Brien

doi:10.1515/gps-2016-0089

Article Open Access

The influences of the concentrations of “green capping agents” as stabilizers and of ammonia as an activator in the synthesis of ZnS nanoparticles and their polymer nanocomposites

Thokozani Xaba
Thokozani Xaba obtained a BSc (Hons) and an MSc in Material Chemistry at the University of Zululand as well as an Advanced Diploma in Higher Education (ADHE, i.e. teaching diploma) from the University of the Free State. She has recently completed her PhD, which will be confirmed in April 2017, with the Vaal Univesity of Technology (VUT). She has been working as a chemistry lecturer at VUT since 2009. She is currently involved in research projects based on nanomaterials and polymer science, and has been supervising a number of postgraduate students since 2011.
, Makwena J. Moloto
Makwena J. Moloto is currently an associate professor at the Department of Chemistry at VUT. He has published about 50 peer reviewed articles in his research areas of inorganic nanochemistry and polymer chemistry. His research interests lie in the study of semiconductor nanoparticles, their incorporation in polymer fibres, and various explorations of their biological and biomedical applications as well as water treatment. He currently serves as a reviewer in a number of international materials and nanotechnology journals.
, Mundher Al-Shakban
Mundher Al-Shakban is a PhD student at the School of Materials at the University of Manchester. He holds a BSc in Physics (1999) from University of Basrah, Iraq, and an MSc degree in Physics (2006) from the University of Basrah, Iraq. Since 2014, he has been studying for a PhD under the supervision of Professor Paul O’Brien CBE, FRS. His research focuses on the development of precursors for the preparation of semiconductor nanoparticles and thin films for photovoltaic and optoelectronic applications.
, Mohammad A. Malik
Mohammad A. Malik completed his PhD degree at the University of London in 1990, and then worked with Professor Paul O’Brien FRS initially at Queen Mary, University of London (1990–1995), then Imperial College (1995–2000), and currently The University of Manchester (2000–2014). He has been involved in various fields of research and has over 100 peer-reviewed articles on topics, including single-source molecular precursors for II/VI, III/V, III/VI, and IV/VI semiconductors, MOCVD, AACVD, CBD, and the colloidal synthesis of nanoparticles.
, Nosipho Moloto
Nosipho Moloto is currently an associate professor at the School of Chemistry, University of the Witwatersrand, Johannesburg. She obtained her PhD from the University of the Witwatersrand, South Africa. She has won a number of recognitions, including the first prize in the Women of Science Award. Her current research interests include synthesis of novel semiconductor nanocrystals as well as hybrid nanostructures and their use in biosensors, solar cells and chemical sensors.
and Paul O’Brien
Paul O’Brien FRS is Chair of Inorganic Materials Chemistry in the Schools of Chemistry and of Materials at the University of Manchester; he has been a Research Dean, Head of the School of Chemistry, and is now Head of the School of Materials. He has received distinguished awards, which include the Potts Medal (Liverpool) and the A. G. Evans Memorial Medal Lecture (Cardiff), the Kroll, Sir Colin Humphreys, the Platinum medals of the IOMMM, and the first Peter Day Award from the RSC. His research focuses on developing new chemical processes for semiconducting thin films and nanoparticles, especially those comprising metal chalcogenides.

Published/Copyright: October 5, 2016

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From the journal Green Processing and Synthesis Volume 6 Issue 2

Abstract

The green synthetic route for the synthesis of semiconductor nanoparticles has received special attention recently due to its features, such as low cost and environmental friendliness. The (Z)-2-(pyrrolidin-2-ylidene)thiourea was used as a ligand and allowed to react with zinc acetate to form the ZnS nanoparticles through the homogeneous precipitation method. The polyvinyl pyrrolidone and polyethylene glycol were employed as capping agents, whereas the ammonium solution was used as an activator for the stabilizers. The effects of the concentration of the capping agents on ZnS nanoparticles were investigated. The poly ethylene glycol (PEG)- or (polyvinylpyrollidone) PVP-capped zinc sulfide nanoparticles were then incorporated with polydadmac to form the polymer nanocomposites. The results from various sophisticated instruments, such as spectrophotometer (UV-Vis), photoluminescence (PL), Fourier transform infrared (FTIR), X-ray powder diffraction (XRD), transmission electron microscope (TEM), and scanning electron microscopy (SEM), reveal that the concentrations of capping agents and ammonium solution has great effects on nanomaterials.

Keywords: activator; ammonium solution; PEG; polydadmac; PVP; zinc sulfide nanoparticles

1 Introduction

Green chemistry deals with the design of chemical products and processes that eradicate the utilization and generation of hazardous substances; it is an important topic many fields, such as chemical industry, research laboratories, and teaching laboratories, due to the health, environmental and economic interests involved in its study [1], [2], [3]. This technology is also a multi-disciplinary assimilation of nanotechnology, biotechnology, and physical methodology in medicinal systems [4]. The incorporation of green chemistry principles to nanotechnology is one of the key issues in nanoscience research. At present, a special need exists to develop environmentally friendly and sustainable methods for the synthesis of nanoparticles, which use nontoxic chemicals, environmentally benign solvents, and renewable materials, to evade contrary effects in medical applications [5].

Thiosemicarbazones are well-known chelating agents that bond through the sulfur and azomethine nitrogen atoms, and have been studied in the past years for their biological characteristics [6]. These chelates and their derivatives have been reported to have vigorous potential as antibacterial, antitubercular, antimalarial, anticancer, and antiviral drugs [7].

Semiconductor nanoparticles, especially II–VI semiconductors, have gained great interest in the past years due to their unique optical and electronic properties as well as potential applications [8], [9]. Among the family of II–VI semiconductor substances, ZnS nanomaterials are the foremost candidates that have been extensively studied due to their favorable electronic and optical properties for optoelectronic applications. ZnS nanomaterials are important semiconducting nanomaterials with a wide optical band gap and have been commonly utilized in numerous optoelectronic devices, such as blue-light-emitting diodes, flat-panel displays, infrared windows, optical sensors, optical coating, solar cells, and field emission devices [10], [11], [12], [13]. The synthesis of ZnS nanoparticles has been achieved through various methods, including hydrothermal process, electro-spinning technique, colloidal chemical treatment method, reverse micelles, sol–gel process, and spray pyrolysis [14], [15], [16], [17], [18], [19], [20].

Poly(diallydimethylammonium chloride) is a cationic linear polymer normally used in the treatment of drinking water and wastewater because of its benefits compared with inorganic coagulants [21]. Its flocculation property is due to high charge density, which stimulates the agglomeration of suspended particle, making it an effective component in flocculating, decolouring, killing algae, and removing organics, such as humus [22]. Many reports have confirmed that the addition of inorganic nanoparticles into polymers can improve its mechanical properties, such as the tensile strength, modulus, or stiffness, via reinforcement mechanisms defined by theories of nanocomposite materials [23]. The fabrication of polymer-based nanocomposites has gained much interest in both academia and industry because of the improved thermal and mechanical properties of the nanocomposites. These nanocomposites reveal significant improvement in numerous applications, such as drug delivery, medical and agricultural technologies, energy storage, environmental remediation, electromagnetic absorption, sensing, transportation, and safety [24].

Shamsipur and Rajabi reported the synthesis of zinc sulfide quantum dot using chemical co-precipitation technique in the presence of 2-mercaptoethanol as a capping molecule. These synthesized ZnS nanoparticles were used as optical sensors for the detection of cyanide ions in aqueous solutions [25]. The synthesis and characterization of PVP-encapsulated ZnS nanoparticles were reported by Ghosh et al. [26]. The optical properties of the ZnS nanoparticles with varying ageing time at the reaction temperature, concentrations of PVP and S²⁺ ions were investigated and a mechanism for the formation of PVP-ZnS nanoparticles under varying PVP/Zn²⁺ mole ratio was recommended. Among the semiconductor polymer-based nanocomposites, ZnS-polymer nanocomposites have received much attention. Recently, Tiwari and co-workers synthesized nanostructured polymer-semiconductor hybrid materials, such as ZnS-poly(vinyl alcohol), ZnS-starch, and ZnS-hydroxypropylmethyl cellulose, using a facile aqueous method [27].

The present work describes the synthesis ZnS nanoparticles through the homogeneous precipitation route. PVP and PEG were used as capping molecules and their effects, including the role of ammonium hydroxide solution, were recognized. The PEG-capped ZnS nanoparticles were then allowed to react with polydadmac to form the polymer nanocomposites. Their optical absorption, luminescence properties, size distribution, and bond distribution were characterized by using spectrophotometer (UV-Vis), photoluminescence (PL), and Fourier transform infrared (FTIR), the structural and morphological properties were examined using X-ray powder diffraction (XRD), transmission electron microscope (TEM), and scanning electron microscopy (SEM).

2 Materials and methods

2.1 Materials

Pyrrolidone, thiourea, zinc acetate dihydrate 99%, 1 m ammonium hydroxide, poly ethylene glycol (PEG), polyvinylpyrollidone (PVP), and 20 wt% poly(diallydimethylammonium chloride) solution (polydadmac), methanol, and acetone were reagents from Sigma-Aldrich (Manchester, UK) and were all used without further purification.

2.2 Experimental

2.2.1 Synthesis of the zinc sulfide nanoparticles:

The (Z)-2-(pyrrolidin-2-ylidene) thiourea ligand was prepared according to the procedure described previously [28], which was based on the reaction of pyrrolidone with thiourea. During the synthesis of ZnS nanoparticles, the zinc acetate dihydrate (5 mmol) in warm 50% methanol (20 ml) was mixed with (20 ml) warm 50% methanolic solution of the ligand (10 mmol) in a 100 ml one-necked flask and refluxed for 1 h on a water bath at 70°C. The ZnS solution was then transferred into an activated 1%, 3%, or 5% PEG solution, which was adjusted from pH=5.6 to pH=11 with 1 m ammonium hydroxide solution. The same procedure was repeated with PVP. The blank determination was also performed using distilled water instead of the capping agent. The ZnS nanoparticles were separated from the solution using centrifuge technique, washed with methanol, and dried in an open air. The uncapped (i.e. free ZnS nanoparticles), blank, as well as the PEG and PVP capped ZnS nanoparticles were technically characterized with various instrumental techniques.

2.2.2 Preparation of the polymer nanocomposites:

The synthesized ZnS nanoparticles (~3 mg) were dispersed in 5 ml of distilled water. Exactly 5 ml of 2% polydadmac solution, which was prepared from 20 wt% stock solution by dilution methods, was added to the nanoparticle solution. The beaker with the mixture was then sealed with a foil and placed in an ultra-sonic bath for 4 h to ensure complete incorporation. The polymer nanocomposite mixture (5 ml) was then transferred into a petri dish with a clean glass slide, which was washed with acetone as shown in Scheme 1. The solution inside the dish was allowed to evaporate inside the fume hood for 24 h to form the polymer nanocomposite films.

Scheme 1:

Capping process of ZnS nanoparticles and the fabrication of polymer nanocomposite films.

2.3 Characterization

The FTIR spectrum of the nanoparticles was recorded on a Perkin Elmer Thermo Scientific iD5-ATR spectrometer. The powder sample was placed on a sample holder, and the spectrum was recorded. UV-1800 Shimadzu spectrophotometer and Gilden Fluorescence were used to measure the optical properties of ZnS nanoparticles. The particles were dissolved in distilled water, after which the solution was placed in a quartz cuvette with 1 cm path length. XRD patterns of the powdered samples were obtained on a Phillips X’Pert chemistry research diffractometer using secondary monochromated Cu Kα radiation (λ=1.54060 Å) at 40 Kv/30 mA. Measurements were taken using a glancing angle of incidence detector at an angle of 2° for 2θ values over 10–80 in steps of 0.0167 with a scan speed of 0.0452. High-resolution transmission electron microscopy (HRTEM) was performed using a Tecnai F30 FEG TEM instrument at an accelerating voltage of 300 kV. TEM samples were prepared by placing 1 or 2 drops of ZnS nanoparticles dissolved in toluene on lacey carbon copper grids. SEM analysis was performed using a Philips XL 30FEG, and EDX was carried out using a DX4 instrument.

3 Results and discussion

3.1 ZnS nanocrystals

The (Z)-2-(pyrrolidin-2-ylidene) thiourea was used as the ligand to synthesize zinc sulfide nanoparticles through the homogeneous precipitation method. PEG and PVP were used as capping molecules to ensure the stability of the nanomaterials. The effects of the concentrations of these capping agents were investigated. We found that when the pH was adjusted from acidic to the basic form with ammonium solution, the functional groups of the capping molecule were activated to attract the nanocrystalline materials into their surface; this also resulted in the formation of stable aqueous dispersions ZnS nanoparticles with high concentration and stronger plasmonic response.

3.1.1 FTIR analysis

The FTIR spectral measurements for all the samples of ZnS nanoparticles were conducted in the scan range between 500 cm⁻¹ to 4000 cm⁻¹ at room temperature. The FTIR spectra are shown in Figure 1. The FTIR spectra of the free/uncapped, blank, PEG-capped ZnS nanoparticles showed the same absorption band at 3369 cm⁻¹, which can be attributed to the O–H stretching vibration of water molecules [29]. However, the other absorption peak of the free ZnS nanoparticles in Figure 1 [A] (a) shifted toward the lower frequency compared with the other samples. The absorption peaks at 1523 cm⁻¹ in Figure 1 [A] (a) and at 1554 cm⁻¹ for Figures 1 [A] (b) to (e) were also assigned to the O–H bending of water molecules. The vibration peaks were observed at 1409 cm⁻¹ in all the FTIR spectra, except for the free ZnS, which may be due to the adsorbed nitrate ions from the ammonium solution. The absorption bands around 1339 cm⁻¹ for all the FTIR spectral measurements corresponded to -CH₂ bending. The absorption peak at 992 cm⁻¹ of the free ZnS and peaks at 827 cm⁻¹ of the other FTIR spectra indicated the presence of resonance interaction between vibrational modes of sulfide ions in the crystal [30]. The medium and strong bands at 682 cm⁻¹ from FTIR spectra of the blank, PEG-capped ZnS nanoparticles and at 581 cm⁻¹ from the free ZnS nanoparticles were assigned to the ZnS band, corresponding to the metal sulfide bond [31]. Similar features were observed on the spectra of the PVP-capped nanoparticles in Figure 1 [B].

Figure 1:

FTIR spectra of the free/uncapped (a), blank (b) 1% (c), 3% (d), and 5% (e) PEG- [A] and PVP- [B] capped ZnS nanoparticles.

3.1.2 Optical properties

Optical absorption and the photoluminescence spectroscopy are powerful, non-destructive techniques that are used to explore the optical behavior of semiconductor nanoparticles and their nanocomposites. The essential property of semiconductors is the band gap, which is the energy separation between the filled valence band and the empty conduction band. The optical excitation of electrons across the band gap is strongly permitted, generating a rapid increase in absorption at the wavelength corresponding to the band gap energy. This phenomenon in the optical spectrum is referred to as the “optical absorption edge.” The optical properties of the semiconductor materials are directly defined by the shape and size of the products. The position of the peak in the UV-Vis absorption spectrum of the semiconductor nanoparticles depends on their size.

The UV-Vis absorption spectra of uncapped/free, blank, PEG-capped, and PVP-capped ZnS nanoparticles are shown in Figures 2 [A] and 3 [A]. The sharp absorption peak was observed at 355 nm in the UV-Vis spectrum of the uncapped ZnS nanoparticles, indicating the red shift of 10 nm compared with the absorption peak of 345 nm for bulk ZnS material [32], [33]. However, the UV-Vis absorption spectra for the blank, PEG-capped, and PVP-capped ZnS nanoparticles were highly blue shifted compared with the uncapped ZnS nanoparticles and the bulk ZnS material. This magnificent shift was the consequence of the quantum size effect, which indicated a change in band gap along with exciton features that can be used as a measure of particle size and size distribution. It is also noted that when the concentration of the capping agent becomes high, the absorption peaks turn out to be weaker as represented in Figure 2(A) and in Figure 3(A) (d) and (e). These observations signify that at higher concentrations of capping molecules, the nanoparticles tend to hide themselves inside the surface of the stabilizing agent, because the extent of the capping molecule is bigger than that of the nanomaterial.

Figure 2:

Absorption [A] and emission [B] spectra of the free/uncapped (a) blank and (b) ZnS nanoparticles capped with (c) 1%, (d) 3%, and (e) 5% PEG.

Figure 3:

Absorption [A] and emission [B] spectra of the free/uncapped (a) blank and (b) ZnS nanoparticles capped with (c) 1%, (d) 3%, and (e) 5% PVP.

The band gap energy (E_g) can be computed from the UV-Vis spectra using a Tauc plot of (αhv)² as a function of hv, and by extrapolation of the linear part of the curve to the energy axis according to the literature [34], [35]

(1)αhv=A(hv−Eg)n

where α is the absorption coefficient, hv is the photon energy, E_g is the direct band gap energy, and B is a constant as shown in eq (1). The small n is an exponent that depends on the type of the transition. Given that our transitions are direct, we take n=2. Figures 4 (A) and (B) show the Tauc plots of the blank, PEG-capped, and PVP-capped ZnS nanoparticles at different concentrations. The optical band gaps were established from the extrapolation, as indicated by the spotted lines. From the obtained results, it can be clearly observed that the synthesized ZnS nanoparticles displayed an increase in the band gap energy by more than 0.30 compared with the ZnS bulk material. The particle size of capped ZnS nanoparticles can be calculated using the calculated band gap values and the following expression (Eq. (2)) [36], [37]:

Figure 4:

Tauc plot of the free/uncapped (a) blank (b) ZnS nanoparticles capped with (c) 1%, (d) 3%, and (e) 5% PEG (A) and PVP (B).

(2)R(Eg)=0.32−2.90Eg−3.493.50−Eg

where R is the diameter of the particle in nanometers, and E_g is the band gap in electron volts. The band gap and the calculated particle sizes of the samples are shown in Table 1.

Table 1:

Optical band gap, blue shift, and particle size of the ZnS nanoparticles.

Samples	Band gap (eV)	Blue shift ΔE (eV)	Particle size (nm) from
Samples	Band gap (eV)	Blue shift ΔE (eV)	UV	TEM
Free or uncapped	3.94	0.35	3.69	5.23
Blank	4.00	0.41	3.50	4.03
ZnS/PEG (1%)	4.05	0.46	3.36	3.51
ZnS/PEG (3%)	4.13	0.54	3.17	3.46
ZnS/PEG (5%)	4.19	0.60	3.05	2.72
ZnS/PVP (1%)	4.07	0.48	3.31	4.15
ZnS/PVP (3%)	4.13	0.54	3.17	3.18
ZnS/PVP (5%)	4.17	0.58	2.99	3.01

In order to study the influence of ammonium solution, PEG, PVP and their concentration effects using the photoluminescence technique, all the measured parameters such as temperature, sample volume, and exciting wavelength, were kept constant. Figures 2 [B] and 3 [B] show the photoluminescence (PL) spectra of the uncapped, blank, PEG, and PVP capped ZnS nanoparticles with an excitation wavelength of 240 nm. The PL spectrum of the pure ZnS nanoparticles was quite broad and had a strong intensity compared with the other spectra with an intensive emission peak centered at 335 nm. The photoluminescence spectra showed that, as the concentration of the capping molecules increased, the intensity of their peaks decreased. The spectra also revealed that at higher concentrations of the capping molecule (Figures 2 [B] (e) and 3 [B] (e)), the emission peaks became red shifted to 338 nm and 339 nm compared with the spectra at the lower concentrations. The later results demonstrate that the capping agents do not affect the structure of the nanoparticles; however, if too much of the capping agent is used, it can affect the optical properties of ZnS nanoparticles.

3.1.3 Structural properties

3.1.3.1 XRD analysis

XRD is a technique that is used to confirm the structure and crystalline phases of the nanocrystalline materials. Figure 5 shows the XRD pattern of the blank, PVP-capped, and PEG-capped ZnS nanoparticles at various concentrations. The XRD pattern in Figure 5 (a) showed diffraction peaks at 2θ values of 26.70°, 28.39°, 33.04°, 36.13°, 47.37°, 58.96°, and 69.31°. The peaks were identified to originate from (100), (103), (0117), (0122), (110), (2017), and (0235) planes, respectively, which correspond to the cubic zinc-blende phase of ZnS respectively. These spectra were well matched with the standard JCPDS data card number 01-072-9259. Figures 5 (b) and (e), which are the spectra that represent the lowest concentrations of the capping molecules, revealed two additional weak peaks between (0117) and (0122) plane compared with the blank ZnS nanoparticles, indicating the growth of the hexagonal phase nanoparticles. These results prove that these nanomaterials contain both hexagonal and cubic phase, as supported by JCPDS data Card No. 04-007-1600. The XRD spectra that represent the highest concentrations of the stabilizers to the ZnS nanoparticles are shown in Figures 5 (c), (d), (f), and (g). The additional peaks were observed at around 31.61°, 34.26°, 56.48°, 62.68° and 67.71°, corresponding to (0115), (0119), (0210), (0225), and (0232) planes from these spectra, thus matching very well with JCPDS data Card No. 01-074-5013. The absence of the earliest two peaks at (100) and (103), as observed in Figure 5 (a), confirms that the spectra in Figures 5 (c), (d), (f), and (g) exhibit hexagonal phase nanostructures.

$Figure 5: X-ray diffraction patterns of the blank (a) ZnS nanoparticles capped with (b) 1% PVP, (c) 3% PVP, (d) 5% PVP, (e) 1% PEG, (f) 3% PEG, and (g) 5% PEG.$

Figure 5:

X-ray diffraction patterns of the blank (a) ZnS nanoparticles capped with (b) 1% PVP, (c) 3% PVP, (d) 5% PVP, (e) 1% PEG, (f) 3% PEG, and (g) 5% PEG.

3.1.3.2 TEM analysis

The size and morphology of the zinc sulfide nanoparticles capped with different concentrations of PEG were determined by TEM measurements. Figures 6(A), (B) and (C) show TEM images, which are spherical or pseudo-spherical particles, with more or less uniform size ranging from 4.03 nm to 3.46 nm. These results indicated that the size of the particles decreased when the concentration of the capping molecule increased. The TEM results in Figure 6(D) mainly consist of agglomerated nanoparticles with an average particle size of 2.72 nm compared with the other TEM images. These later results indicated that, as the concentration of the capping molecule increased, the particles became more aggregated and the sizes of the particles became larger.

Figure 6:

TEM images of blank (A) ZnS nanoparticles capped with (B) 1%, (C) 3%, and (D) 5% PEG.

3.2 Deposition of ZnS polymer nanocomposites

The ZnS polymer nanocomposites were prepared by mixing zinc sulfide nanoparticles in a solution form with the PDD polymer solution. The mixture was ultra-sonicated and transferred into a dish with the glass substrates to form the polymer nanocomposites.

3.2.1 Optical absorption of the ZnS nanocomposites

Figure 7 (A) (a) shows the absorption peak of the polydadmac (i.e. polymer, PDD) at 340 nm, which seems to disappear in the absorption spectra of ZnS-PEG-PDD nanocomposites. The photoluminescence measurements that were carried out using the same excitation wavelength of 280 nm at room temperature are revealed in Figure 7 (B). A similar trend that occurred in the absorption spectra in Figure 7 (A) was observed in Figure 7 (B). The emission spectra showed the same intense and common peaks at 399 nm. The spectrum of the pure PDD revealed another small peak at 420 nm, which seemed to disappear once the polymer was incorporated with ZnS-PEG nanoparticles.

Figure 7:

Absorption [A] and emission [B] spectra of the PDD (a) and PDD incorporated with the blank (b) ZnS nanoparticles capped with (c) 1%, (d) 3%, and (e) 5% PEG.

3.2.2 Structural properties of the ZnS nanocomposites

3.2.2.1 XRD analysis

Figure 8 displays the XRD patterns of pure PDD unified with the uncapped and PEG-capped ZnS nanoparticles. The XRD of pure PDD sample in Figure 8 (a) exhibits diffraction peaks at 29.26°, 31.62°, 34.15°, 45.61°, and 66.43°. Figures 8 (b) to (e) show a common peak at 38.11°, which is an indication of the presence of ZnS nanoparticles. The appearance of the two diffraction peaks in Figures 8 (b) and (c) from the diffraction pattern of the uncapped and 1% PEG-capped ZnS nanoparticles incorporated with PDD, which are not available in Figure 8 (a), are also observed at 42.74° and 81.99°. The disappearance of the peaks from the diffraction pattern of the pure PDD indicates that, at low concentration of the capping agent, the nanoparticles in PEG-ZnS-PDD nanocomposite have a strong influence compared with the pure polymer. Figures 8 (d) and (e) reveal diffraction peaks that are similar to the peaks of the pure PDD, thus signifying that at higher concentration of the capping molecules, the polymer tends to have a strong influence over the PEG-ZnS-PDD nanocomposites.

$Figure 8: X-ray diffraction patterns of the PDD (a), PDD incorporated with the blank (b) ZnS nanoparticles capped with (c) 1%, (d) 3%, and (e) 5% PEG.$

Figure 8:

X-ray diffraction patterns of the PDD (a), PDD incorporated with the blank (b) ZnS nanoparticles capped with (c) 1%, (d) 3%, and (e) 5% PEG.

3.2.2.2 SEM analysis

SEM micrographs of the polydadmac that are merged with the uncapped and PEG-capped ZnS nanoparticles are depicted in Figures 9 (A) to (D), respectively. As can be clearly seen, the ZnS-PEG capped nanoparticles attached to the surfaces of the polymer in Figures 9 (A) to (C) compared with Figure 9 (D), revealing a slightly attachment of the particles on the surface of the polymer. The later results reveal that, as the concentration of the capping molecule is increased, the stabilizers and polymers tend to supress the nanoparticles, making it difficult to be identified them. These results are in agreement with the optical absorption results and TEM analysis of the ZnS nanoparticles.

Figure 9:

SEM micrographs of PDD incorporated with the uncapped ZnS nanoparticles (A) 1% PEG (B), 3% PEG (C), and 5% PEG (D) ZnS nanoparticles.

4 Conclusion

The simple synthesis of the ZnS nanoparticles capped with green capping molecules by homogeneous precipitation technique, and the preparation of the polymer nanocomposite have been reported. The FTIR spectra confirmed the formation of ZnS nanoparticles; these also proved that the capping agent has a great influence on the formation of nanoparticles, because the capped nanoparticles shifted toward the higher frequencies compared with the uncapped ZnS nanoparticles. It has been noted that at pH=11, the branches of the capping agents responsible for binding with metal ions became more active and further attracted the ZnS nanoparticles. The UV-vis absorption peaks for the blank and the capped nanoparticles exhibited blue shift from the bulk and uncapped nanoparticles. PL results showed that increasing the concentration of the capping can affect the optical properties of ZnS nanoparticles. The XRD patterns revealed different phases as the concentration of the capping molecule increased. The morphology of the capped ZnS nanoparticles has been identified by TEM analysis, revealing the spherical shape particles. The SEM images proved that, indeed the incorporation of the nanoparticles to the polymer, has been achieved. Due to the O–H that is present in ammonium solution, the results revealed that the ammonium solution can be used to activate the functional groups of the capping agents to make it possible for them to cover the nanoparticles, while using it as a capping molecule at the same time.

Funding source: National Research Foundation

Award Identifier / Grant number: TTK13071722088

Funding statement: The authors would like to acknowledge the National Research Foundation (Grant/Award Number: TTK13071722088 “Thuthuka Grant Holder”) and Vaal University of Technology for the funding as well as the University of Manchester for providing the necessary facilities.

About the authors

Thokozani Xaba

Thokozani Xaba obtained a BSc (Hons) and an MSc in Material Chemistry at the University of Zululand as well as an Advanced Diploma in Higher Education (ADHE, i.e. teaching diploma) from the University of the Free State. She has recently completed her PhD, which will be confirmed in April 2017, with the Vaal Univesity of Technology (VUT). She has been working as a chemistry lecturer at VUT since 2009. She is currently involved in research projects based on nanomaterials and polymer science, and has been supervising a number of postgraduate students since 2011.

Makwena J. Moloto

Makwena J. Moloto is currently an associate professor at the Department of Chemistry at VUT. He has published about 50 peer reviewed articles in his research areas of inorganic nanochemistry and polymer chemistry. His research interests lie in the study of semiconductor nanoparticles, their incorporation in polymer fibres, and various explorations of their biological and biomedical applications as well as water treatment. He currently serves as a reviewer in a number of international materials and nanotechnology journals.

Mundher Al-Shakban

Mundher Al-Shakban is a PhD student at the School of Materials at the University of Manchester. He holds a BSc in Physics (1999) from University of Basrah, Iraq, and an MSc degree in Physics (2006) from the University of Basrah, Iraq. Since 2014, he has been studying for a PhD under the supervision of Professor Paul O’Brien CBE, FRS. His research focuses on the development of precursors for the preparation of semiconductor nanoparticles and thin films for photovoltaic and optoelectronic applications.

Mohammad A. Malik

Mohammad A. Malik completed his PhD degree at the University of London in 1990, and then worked with Professor Paul O’Brien FRS initially at Queen Mary, University of London (1990–1995), then Imperial College (1995–2000), and currently The University of Manchester (2000–2014). He has been involved in various fields of research and has over 100 peer-reviewed articles on topics, including single-source molecular precursors for II/VI, III/V, III/VI, and IV/VI semiconductors, MOCVD, AACVD, CBD, and the colloidal synthesis of nanoparticles.

Nosipho Moloto

Nosipho Moloto is currently an associate professor at the School of Chemistry, University of the Witwatersrand, Johannesburg. She obtained her PhD from the University of the Witwatersrand, South Africa. She has won a number of recognitions, including the first prize in the Women of Science Award. Her current research interests include synthesis of novel semiconductor nanocrystals as well as hybrid nanostructures and their use in biosensors, solar cells and chemical sensors.

Paul O’Brien

Paul O’Brien FRS is Chair of Inorganic Materials Chemistry in the Schools of Chemistry and of Materials at the University of Manchester; he has been a Research Dean, Head of the School of Chemistry, and is now Head of the School of Materials. He has received distinguished awards, which include the Potts Medal (Liverpool) and the A. G. Evans Memorial Medal Lecture (Cardiff), the Kroll, Sir Colin Humphreys, the Platinum medals of the IOMMM, and the first Peter Day Award from the RSC. His research focuses on developing new chemical processes for semiconducting thin films and nanoparticles, especially those comprising metal chalcogenides.

Acknowledgments

The authors would like to acknowledge the National Research Foundation (Grant/Award Number: TTK13071722088 “Thuthuka Grant Holder”) and Vaal University of Technology for the funding as well as the University of Manchester for providing the necessary facilities.

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Received: 2016-5-14

Accepted: 2016-8-15

Published Online: 2016-10-5

Published in Print: 2017-4-1

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Articles in the same Issue

https://doi.org/10.1515/gps-2016-0089

Keywords for this article

activator; ammonium solution; PEG; polydadmac; PVP; zinc sulfide nanoparticles

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