Synthesis and characterization of castor oil and ricinoleic acid capped CdS nanoparticles using single source precursors

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Synthesis and characterization of castor oil and ricinoleic acid capped CdS nanoparticles using single source precursors Ginena Bildard Shombe a,b, Egid Beatus Mubofu a, Sixberth Mlowe b, Neerish Revaprasadu b,n a b

Chemistry Department, University of Dar es Salaam, P.O. Box 35061, Dar es Salaam, Tanzania Department of Chemistry, University of Zululand, Private Bag X1001, KwaDlangezwa 3886, South Africa

art ic l e i nf o

a B S T R AC T

Article history: Received 2 September 2015 Received in revised form 21 October 2015 Accepted 14 November 2015

We report the synthesis of castor oil and ricinoleic acid capped CdS nanoparticles by the thermolysis of piperidine (1) and tetrahydroquinoline (2) dithiocarbamate complexes of cadmium(II) at temperatures varying from 190 °C to 300 °C. Reaction parameters such as time and temperature were varied to study their effect on the properties and morphology. The optical properties of CdS were typical of particles that displayed quantum confinement effects. X-ray diffraction studies revealed the existence of both cubic and hexagonal phases depending on the reaction conditions. Ricinoleic acid capped CdS gave cubic phase particles whereas castor oil capped CdS gave both cubic and hexagonal phases dependent on the reaction temperature and the type of complex used. The morphology of the particles varied from oval-short rods to spherical shaped particles with sizes ranging from 10 to 22 nm. Rhodamine B (RhB) dye photodegradation studies of a representative CdS nanoparticles’ sample have been carried out in the presence of halogen light and studied using UV–visible spectroscopy. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Castor oil Ricinoleic acid Optical properties Cadmium sulfide Photocatalysts

1. Introduction Among the various semiconductors, cadmium sulfide nanoparticles have been given considerable attention by researchers and scientists due to their size dependent unique optical and electronic properties. These properties make them useful in optoelectronic applications including solar cells [1], light emitting diodes [2], nonlinear optics [3] and heterogeneous photo catalysis [4]. In the last few decades, considerable effort has been expended towards developing synthetic methods of preparation for CdS and other II–VI semiconductor nanoparticles [5–9]. Despite the many synthetic approaches reported in the literature, a persistent challenge is the need to develop synthetic methods which are environmentally friendly and economically viable. Recent focus has been on the development of greener methods for nanoparticle synthesis [10–12]. One approach is the use of non-toxic solvents and/or capping groups in the synthetic methodology [13–15]. Passivating agents are frequently used to inhibit nanoparticle overgrowth and aggregation and also control the structural characteristics of the resultant particles in a precise manner [16]. Common capping agents used include amines, phosphines and thiols [17–22]. However, most of these capping agents are toxic, expensive and require complicated synthetic processes [23,24]. As n

Corresponding author. E-mail address: [email protected] (N. Revaprasadu).

a result, the need of employing greener methods of synthesis which use renewable and bio-based capping agents has grown over the years. Olive oil [25], oleic acid [26], anacardic acid [13] and castor oil [11] are examples of greener capping agents and dispersants that are already reported in literature. Castor oil, a naturally occurring triglyceride, has been recently investigated for its use in nanoparticles synthesis [27–30]. The oil consists of different fatty acids with ricinoleate making about 90% of the total fatty acid chains [31]. Castor oil is extracted from the seeds of the castor oil plant, Ricinus communis. It is obtained as a colorless to very pale yellow liquid with a distinct taste and odor, and has a boiling point of 313 °C [30]. Due to its easy availability, low cost, non food competition, high boiling point, high viscosity and environmental considerations, castor oil seems to have a huge potential as a green capping agent. Hydrolysis of the oil gives ricinoleic acid, a mono-unsaturated 18-carbon fatty acid, as the major component [32]. Unlike other fatty acids, ricinoleic acid has a hydroxyl functional group on C-12 which makes it and castor oil in general more polar than most fat acids. In the current work, castor oil and its major constituent, ricinoleic acid are used as greener capping agents and dispersants in the synthesis of CdS nanoparticles using heterocyclic dithiocarbamate complexes as single source precursors. To our knowledge this is the first report of the thermolysis of single molecular precursors in castor oil and ricinoleic acid. The prepared nanoparticles were characterized and a representative sample tested as catalysts in the photodegradation of RhB dye using fluorine lamp as the source of light. We hope

http://dx.doi.org/10.1016/j.mssp.2015.11.011 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

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that this work will lead to more studies on the use of greener solvents for nanoparticle synthesis.

2. Experimental 2.1. Chemicals Castor oil was extracted from castor seeds; ricinoleic acid was isolated from castor oil. Rhodamine B dye (495%), tri-n-octylphosphine (TOP) 90%, piperidine (99%) and 1,2,3,4, tetrahydroquinoline (98%) were purchased from Sigma-Aldrich. Petroleum ether (90%), potassium hydroxide (90%), anhydrous magnesium sulfate, sulfuric acid (98%), hexane, chloroform, methanol (99.5%), carbon disulfide (99.5%), sodium hydroxide (98%), cadmium chloride monohydrate (99%) and acetone were purchased from Merck chemicals. All chemicals were used as purchased without any further purification. 2.2. Instrumentation Infrared spectra were recorded on a Bruker FT-IR tensor 27 spectrophotometer directly on small samples of the compounds in the range 200–4000 cm
1. Optical measurements were analyzed using a Varian Cary 50 UV–visible spectrophotometer in which the samples were placed in silica cuvette (1 cm path length), using hexane as a reference solvent. Photoluminescence of the particles was analyzed using a Perkin-Elmer, LS55 Luminescence spectrometer. The samples were placed in a quartz cuvette (1 cm path length) and all measurements were done at room temperature. Samples were prepared by placing a drop of dilute solution of nanoparticles on Formvar-coated grids (150 mesh) for TEM and holey carbon grids for HRTEM. The samples were allowed to dry completely at room temperature and viewed using a JEOL 1400 TEM and JEOL 2100 HRTEM. viewing was done at an accelerating voltage of 120 kV (TEM) and 200 kV (HRTEM), and images captured digitally using a Megaview III camera; stored and measured using soft imaging systems iTEM software (TEM) and Gatan camera and Gatan software (HRTEM). Powder diffraction patterns were recorded in the high angle 2θ range of 20–70° using a Bruker AXS D8 diffractometer equipped with a nickel filtered Cu Kα radiation (λ ¼1.5418 Å) at 40 kV, 40 mA and at room temperature. The scan speed and step sizes were 0.2 min
1 and 0.01314 respectively. 2.3. Extraction of castor oil and isolation of ricinoleic acid Castor oil was extracted from castor seeds using a method reported by Akpan et al. 2006, in which hexane was used as the extracting solvent in a soxhlet apparatus [33]. Its isolate, ricinoleic acid was isolated from the oil by a method reported by Vaisman et al. 2007 [34]. In a typical process, 60 g of KOH in 500 ml of ethanol were added in 250 g of castor oil and refluxed. After 3 h, 1.5 L of distilled water acidified by 50 ml conc. H2SO4 in 150 ml of H2O was added. Two layers were formed in which the organic layer was collected, washed with warm distilled water and dried over MgSO4. The mixture was filtered to yield ricinoleic acid. 2.4. Synthesis of precursors The method used in the preparation of the ligands and complexes is similar to that reported in our previous work [22]. 2.5. Synthesis of nanoparticles 0.5 g of complex (1) or (2) was dissolved in 6.0 mL of castor oil.

The mixture was then injected into 6.0 g of hot castor oil in a three neck flask at 190 °C /230 °C /270 °C /300 °C under nitrogen gas flow. After the reaction time of 30 min/1 h and 2 h, an aliquot of the sample was taken, to which methanol was added resulting in the formation of a flocculent precipitate. The precipitate formed and the solvent were separated by centrifugation, and dispersed in hexane to give yellow castor oil capped CdS nanoparticles. Similar procedures were followed using ricinoleic acid (RA) as both a dispersing medium and a capping agent. 2.6. Photodegradation The photocatalytic activity of representative samples of the asprepared materials was evaluated by the degradation of RhB dye under halogen (fluorine) light irradiation of fluorine lamp. Castor oil capped CdS nanoparticles prepared from complex (1) at 190 °C (CdS 1), and 300 °C (CdS 2); and ricinoleic acid capped CdS nanoparticles prepared from complex (1) at 190 °C (CdS 3), and 300 °C (CdS 4) were used for this study. After the base line setting using water as a blank solvent, absorbance of RhB solution was measured. About 0.015 g of CdS 1 was added in 20 ml of RhB solution and placed in a sealed black box equipped with fluorine lamp as the source of light. At given time intervals, a sample of the reaction mixture was taken and optical absorption spectra were recorded to determine the degradation rate of RhB. Control experiments; without the addition of the catalyst and with a catalyst in absence of light were also carried out. The same procedure was repeated for CdS 2, 3 and 4.

3. Results and discussion 3.1. Characterization of the precursors Piperidine dithiocarbamate and tetrahydroquinoline dithiocarbamate ligands and their corresponding Cd (II) complexes synthesized were pure and obtained in good yields. White precipitates of piperidine dithiocarbamate ligand and its corresponding cadmium complex (1) were obtained while pale yellow powders were obtained for the case of tetrahydroquinoline dithiocarbamate ligand and its cadmium complex (2). The compounds are air stable, easy to synthesize and soluble in some organic solvents such as chloroform. Characterization of piperidine and tetrahydroquinoline dithiocarbamate ligands and their corresponding cadmium (II) complexes by 1H NMR, IR, TGA and CHN have been reported previously [22]. 3.2. Castor oil capped CdS nanoparticles The optical absorption spectra of CdS nanoparticles synthesized from complex (1) at different reaction temperatures (190–300 °C) are shown in Fig. 1. The excitonic absorption maxima bands from all temperatures are blue shifted relative to the peak absorption of bulk CdS indicating quantum confinement effects [35]. The band gap energies of the materials at each temperature can be calculated from Tauc plots [36]. The calculated band gap for a 190 °C sample estimated from the Tauc plot (Inset Fig. 1) is 2.25 eV. A slight red shift is observed from the spectra on moving from lower to higher temperatures suggesting an increase in the size of the particles as the reaction temperature is increased (Tauc plots not shown for clarity of the figures). The corresponding PL spectra (Fig. 2) display strong band edge emissions with the maxima at ca. 480 nm, a considerable blue shift as compared to the bulk CdS [35]. The discrepancies observed in PL and UV–vis spectra could be due to several reasons such as agglomeration and/or surface defects to mention few. In both cases, the effect of reaction time was

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Fig. 1. UV–vis absorption spectra of castor oil capped CdS nanoparticles synthesized using complex (1). Inset shows a Tauc plot for 190 °C sample.

Fig. 2. PL spectra of castor oil capped CdS nanoparticles synthesized using complex (1). The excitation wavelength used was 420 nm.

negligible as little or no changes were observed in the absorption and emission spectra of aliquots taken after 30 min, 1 h and 2 h of the reaction time. The effect of temperature was however significant. Similar trends in the optical measurements were observed when complex (2) was used as a precursor; the band gap estimated using Tauc plot showed a value of 2.24 eV (190 °C) (ESI Fig. S1 and S2). CdS nanoparticles emitted in the high energy region of the spectra, revealing the quantum effects. The synthesis of CdS nanoparticles using complex (1) resulted in the formation of spherical, short rods and oval shaped particles (Fig. 3). At 190 °C, ill defined and agglomerated particles are observed (Fig. 3a). The particles appear spherical and more defined in shape at 230 °C with an average size of 15.53 73.18 nm. Increasing the reaction temperature to 270 °C produces elongated particles in the form of short oval-rods with an average size of 15.6 73.74 nm whereas at 300 °C, oval shaped particles with an average size of 17.96 71.95 nm are observed. A slight increase in particle size is observed as the temperature is raised with a reduction in particle agglomeration. The HRTEM image of a single particle synthesized at 300 °C shows lattice fringes with a D- spacing of 3.4 Å corresponding to the [100] plane of hexagonal CdS (Fig. 3e). The fast Fourier transform (FFT) pattern also shows bright

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spots confirming the highly crystalline nature of the particles. The average size and distribution of particles synthesized at 300 °C is shown in Fig. 3(f). A similar trend was observed when complex (2) was used as a precursor (Fig. 4). At lower temperatures (190 °C and 230 °C) small particles were observed which tended to agglomerate, while more defined particles were produced when the complex was thermolysed at 270 °C and 300 °C. Oval-rod shaped particles with an average size of 13.36 71.82 nm are observed at 270 °C while spherical particles are produced at 300 °C with an average size of 15.7 7 1.73 nm. The agglomeration of the particles explains the broad absorption peaks observed in the UV–vis spectra of the particles. The particle size histogram shows formation of relatively monodispersed nanoparticles at 300 °C. The powder X-ray diffraction (p-XRD) patterns of castor oil capped CdS nanoparticles synthesized at various temperatures are shown in Figs. 5 and 6. CdS has two different crystal patterns; the metastable cubic phase and the thermodynamic stable hexagonal phase [37]. Both cubic and hexagonal phases of CdS were obtained depending on the temperature of the reaction and the type of complex used. On thermolysing complex (1) in castor oil at 190 °C, 230 °C, 270 °C and 300 °C, hexagonal CdS nanoparticles were obtained at all temperatures as confirmed by the presence of (100), (002), (101), (110), (103), and (112) reflections (card #: 01-0800006) (Fig. 5) in the XRD pattern. Synthesis of hexagonal CdS at lower temperature has previously been reported [22]. Complex (2) produced cubic phase at 190 °C and 230 °C while at 270 °C and 300 °C, the hexagonal phase of CdS was obtained (Fig. 6). The influence of temperature on the structural phase transformation of CdS crystals cannot be ignored since temperature plays an important role in the structural phase transformation processes [38]. Previous studies have reported the phase transition from cubic to hexagonal in CdS semiconductor nanoparticles and thin films with respect to temperature, revealing the hexagonal phase to be dominant at higher temperatures [39–42]. This is in a good agreement with our results in which cubic phase CdS was synthesized at lower temperatures while the more stable hexagonal CdS was synthesized at higher temperatures. Stacking faults which are dependent on stacking fault energies of the compound has also been reported to be responsible for phase transformation [43]. The stacking fault energy of CdS is very low [44] such that it is easier to achieve phase transition in CdS compounds. It has further been reported that phase transition depending on nanoparticle size can also occur [45]. For CdS, the hexagonal wurtzite structure is obtained for diameters greater than 5 nm where as for very small sizes (ca. 3 nm) the cubic zinc blende structure is observed. In the current study the observed structural characteristics of the synthesized materials might be attributed by metastability of the cubic phase, size change of the particles or surface properties. These results suggest that the mechanism at which the complexes decompose are different. However, the existence of a hexagonal CdS phase solely from XRD patterns cannot be excluded because of the large similarity between cubic and hexagonal CdS structures. 3.3. Synthesis of ricinoleic acid capped CdS nanoparticles Ricinoleic acid capped CdS nanoparticles were prepared by the thermolysis of complex (1) and complex (2) in ricinoleic acid at 190 °C, 230 °C, 270 °C and 300 °C. The absorption results are blue shifted typical of nanometric materials (Fig. 7). An estimated band gap (2.37 eV) for a 190 °C sample using Tauc plot is typical for CdS nanoparticles. A slight red shift with increasing the reaction temperature is also observed in this case. The corresponding PL spectra display a band edge emission at ca. (480–495) nm (Fig. 8). An obvious increase in FWHM is observed as the temperature increases, which could be attributed to the increase in

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50 nm

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30 20

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12.0-13.9 14.0-15.9 16.0-17.9 18.0-19.9 20.0-21.9 Particle size (nm)

Fig. 3. TEM images of castor oil capped CdS nanoparticles synthesized from complex (1) at (a) 190 °C, (b) 230 °C, (c) 270 °C and (d) 300 °C; (e) HRTEM image of the particles synthesized at 300 °C (Insert: FFT of CdS nanoparticle), and (f) Histogram showing particle size distribution at 300 °C.

nanoparticle size. The optical measurements for CdS from complex (2) have similar features (ESI Figs. S3 and S4). TEM analysis for ricinoleic acid capped particles was done only for the particles synthesized from complex (1) at 270 °C and 300 °C. Spherical particles were produced at both temperatures with the average particle size been 21.127 3.57 nm at 270 °C and 22.6773.58 nm at 300 °C. Reduction of particle agglomeration and a slight increase in particle size is observed as temperature is increased (Fig. 9). XRD analysis of the particles from complex (1) at 190 °C, 230 °C, 270 °C and 300 °C confirmed the synthesis of cubic phase CdS identified by the (111), (220), and (311) planes [card #: 01-0800019] (Fig. 10). Similar results were obtained when complex (2) was used. For ricinoleic acid capped CdS nanoparticles, the cubic phase is dominant at all temperatures. The synthesis of cubic phase CdS is also an evidence of quantum confinement effect as cubic phase can be observed only in nanocrystalline CdS in contrast to hexagonal phase which can be present in both bulk and nanocrystalline CdS [46]. The results also show that the nature of the capping group evidently modifies the structural and surface properties of the nanoparticles [47,48]. The structural modification

of the as-synthesized nanoparticles as a result of changing the capping agent is not clear and is rarely/not reported in literature. Infrared analysis was done on the synthesized particles in order to confirm the mode of bonding between the capping material and the CdS nanoparticles. Fig. 11a shows the spectra of ricinoleic acid in which, the aliphatic hydroxyl group (–OH) and carbonyl group of the carboxylic acid are indicated by the peaks at 3456 and 1736 cm
1 respectively. The peaks at 3006 and 2924 cm
1 represent the sp2 and sp3 –C–H stretches respectively. The decrease in intensity of the peaks representing the carbonyl and hydroxyl functionalities in the spectra of the capped material (Fig. 11b) reveals the chemical interaction of the CdS nanoparticles and the ricinoleic acid. The interaction between ricinoleic acid and CdS nanoparticles can therefore be explained by resonance between the two O-atoms of the carboxylic acid which causes ricinoleic acid to be chemisorbed to surfaces of CdS nanoparticles forming CdS–O bonds through both atoms of the carboxylic group. These suggest that carbonyl and OH group present in both castor oil and ricinoleic acid bind and thus allowing growth process of CdS nanoparticles.

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Particle size (nm) Fig. 4. TEM images of castor oil capped CdS nanoparticles using complex (2) at (a) 190 °C, (b) 230 °C, (c) 27 °C and (d) 300 °C; and (e) a histogram showing distribution of particles synthesized at 300 °C.

3.4. Photocatalytic degradation To study the catalytic activity of the synthesized materials, the initial absorbance of the dye in the absence of the catalyst ( Ao) and the absorbance of the dye after irradiation at a certain time interval ( A ) were recorded. Castor oil and ricinoleic acid capped CdS nanoparticles synthesized at 190 °C and 300 °C from complex (1) were used as representative samples to decompose RhB dye in the presence of fluorine light. The control UV–vis spectra of RhB degradation solution in absence of CdS nanoparticles and presence of CdS nanoparticles without the light are shown in the ESI Figs. S5 and S6, respectively.

The degradation rate of RhB dye using CdS 1 as a photocatalyst was observed to reach 67% after 60 min of reaction time (Fig. 12). This might be attributed by small size of the particles (9.53 nm) which causes an increase in surface area and the number of surface atoms. Therefore, the number of active surface sites available for photogenerated charge carriers to react with adsorbed molecules to form free radicals increases [49]. The large surface area to volume ratio of 0.62959 for CdS 1, has much more reactive sites as compared to CdS 2 (0.33406), CdS 3 (0.42735) and CdS 4 (0.26467), the comparison plot is shown in ESI Fig. S10. The degradation efficiency of the catalyst appeared to decrease with further increase of exposure time. As the concentration of the dye decreased, the

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(103)

(102)

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Fig. 5. XRD patterns of castor oil capped CdS nanoparticles synthesized using complex (1).

300 C

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CdS 2-4 are shown in ESI (Fig. S7 and S9). The mechanism of degradation by semiconductor based photocatalysts has been explained elsewhere [50]. The results show that CdS nanospherical nanoparticles exhibit light photocatalytic degradation activity which could be due to its suitable band gap, large surface area to volume ratio, absorption edge and phase structure [51], resulting in photogenerated electrons and holes with reduction and oxidation abilities.

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Fig. 8. PL spectra of ricinoleic acid capped CdS nanoparticles synthesized using complex (1).

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2θ (deg) Fig. 6. XRD patterns of castor oil capped CdS nanoparticles synthesized using complex (2).

CdS nanoparticles capped with castor oil and ricinoleic acid were successfully prepared from piperidine and tetrahydroquinoline dithiocarbamate complexes of cadmium (II) metal. Transmission electron microscopy (TEM) analysis revealed the synthesis of oval-short rods and spherical shaped particles with average sizes ranging from 10 to 22 nm. The degree of agglomeration of the particles decreased as the temperature of the reaction was increased. An increase in size of the particles with increasing reaction temperature was also observed. Optical properties of the particles showed evidence of quantum confinement effect. Broad absorption band edges were observed for castor oil and ricinoleic acid capped particles which are probably due to agglomeration of the particles. Powder X-ray diffraction technique for particles capped with ricinoleic acid revealed the synthesis of cubic CdS at all reaction conditions. Both cubic and hexagonal phase were obtained for castor oil capped particles depending on the reaction temperature and type of the complex used. A representative CdS nanoparticles' sample analyzed showed photocatalytic ability for removing dye pollutants.

Acknowledgments

Fig. 7. UV–vis absorption spectra of ricinoleic acid capped CdS nanoparticles synthesized using complex (1). Inset shows a Tauc plot for 190 °C sample.

presence of CdS nanoparticles in the solution became pronounced and the shifting of RhB peak to lower wavelength was observed (Fig. 12). The degradation rate of RhB using CdS 2 (17.96 nm), CdS 3 (14.04 nm) and CdS 4 (22.67 nm) was not as fast as that of CdS 1 probably due to size difference of the particles. As shown in Fig. 13, the catalytic activity of CdS 1 was relatively higher compared to that of CdS 2 and the activity of CdS 3 was also higher than that of CdS 4. UV–vis spectra of RhB degradation solution for

The authors are grateful to the Bank of Tanzania (B.O.T), National Research Foundation (NRF (Grant no. 64820)), Department of Science and Technology (DST) South Africa, Tanzania (COSTECH) and South Africa (NRF) Project for financial support. The authors also thank University of Kwa-Zulu Natal for electron microscopy (TEM and HRTEM) measurements.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2015.11.011.

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Wavenumber (cm ) Fig. 11. IR spectra for (a) ricinoleic acid and (b) ricinoleic acid capped CdS nanoparticles.

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Please cite this article as: G.B. Shombe, et al., Materials Science in Semiconductor Processing (2015), http://dx.doi.org/10.1016/j. mssp.2015.11.011i