ARTICLE pubs.acs.org/JPCC
Growth Mechanism and Optical Properties Determination of CdS Nanostructures Gajanan Pandey*,† and Supria Dixit‡ † ‡
Department of Applied Chemistry, Babasaheb Bhimrao Ambedkar University, Lucknow-226025 (U.P.) India Department of Physics, Gwalior Institute of Information Technology, Gwalior-475051 (M.P.) India ABSTRACT: CdS nanocrystalline materials have been prepared by reaction of Cd(II) ions with an alkaline solution of thiocarbamide in the aqueous solution phase. The effect of reaction time and surfactants (cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS)) on morphology and size of the products has been investigated. Dramatic shape and size variations have been observed by varying the surfactant and reaction time. CdS nanocrystals formed in various reactions were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) to determine the crystallinity, phase, composition, size, and morphology. The optical properties of the thus prepared samples were determined by UVvis absorption spectra. A possible formation mechanism of CdS nanocrystals has also been discussed in this article.
’ INTRODUCTION Semiconductor nanoparticles have versatile applications including biological labels1,2 and optoelectronic transistor components.3 Since the optical, electronic, and many other physical properties of nanostructured semiconductors strongly depend on size, size distribution, and crystal structures,4 the development of well-controlled synthetic routes for tailoring nanocrystal morphology, elucidating the growth mechanism, and optimization of physical parameters for generation of welldefined nanostructures are the important issues before the nanomaterials chemists. CdS is an important well-studied semiconductor, having a Bohr’s radius of 2.4 nm5 and a direct band gap of 2.42 eV6 at room temperature, it has been used in photovoltaics, light emitting diodes, and other optical devices due to its nonlinear properties.7,8 In the recent years, a number of studies have been performed for shape and size manipulation of CdS nanocrystals. For example, CdS nanowires have been synthesized by laser ablation with metal catalyst as seeds,9 by thermal evaporation,10,11 by a solution route,12 by templating,13,14 by a solvothermal process,1520 r 2011 American Chemical Society
etc. CdS nanorods were prepared by a cation exchange process21 and by a solvothermal process2225 using various capping agents or a passivator to control and induce the growth of nanocrystals. Further, solvothermal routes have been applied by a number of researchers to generate varieties of shapes, sizes, and morphologies like triangular and hexagonal CdS nanocrystals,2628 nanoflowers and nanotrees,29 belts,30,31 twinrods, tetrapods, etc.32 Apart from wet chemical synthesis, discussed above, for solid phase synthesis of CdS nanocrystals a mechanical alloying process has been adopted to avoid toxicity of metal organic precursors and high temperature, used in solution phase synthesis.33 However, after all, the wet chemical synthetic routes have their own merits because a number of physical and chemical parameters can be manipulated to tailor the nanostructured materials of desired shape, size, and distribution which are highly desired for optical and electronic applications, and therefore Received: February 17, 2011 Revised: June 19, 2011 Published: August 02, 2011 17633
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The Journal of Physical Chemistry C understanding the growth mechanism in solution phase synthesis is a great challenge before materials chemists. Furthermore, it is worth noting that in the aforesaid wet chemical routes the CdS nanoparticles were prepared in organic phases. Unfortunately, organic phase synthetic routes are not only more costly but also not eco-friendly. Since the CdS nanostructures prepared in organic phases are stabilized by hydrophobic capping agents, they cannot be used in biological applications, where hydrophilicity is a prime requirement. Alternatively, nanoparticles can be synthesized in aqueous phases because they are simple, reproducible, economic, and eco-friendly. CdS nanostructures have been prepared in the aqueous phase by a number of researchers,3436 but either the precursors used in the previous processes are very complex and toxic or high-temperature and complicated synthetic steps are involved. To understand the fundamentals of growth behavior of metal sulfide nanoparticles (NPs) in a watersurfactant system, extensive structural, kinetic, and thermodynamic studies and the effect of additives on micellization have been performed in the recent years.3740 However, due to conflicting opinions regarding factors controlling synthesis and stabilization of NPs in aqueous surfactant solution, it is quite difficult to scale-up a general method for NP synthesis using a surfactant. The interesting aspect which should be studied in the micellar medium is related to particle size control by adsorption of surfactant onto the particle surface. The self-aggregation of NPs is prevented by surfactant coating at the NP surface due to changed interparticle potential.39 Further, it has been shown that the stabilizers (surfactants) have some preferred chemical moiety which binds the NP surface, hinders their unlimited growth, and eventually generates a preferred size of particles regulated by the arrangement of surfactant molecules around the inorganic nanosized core.41,42 Recently, Bakshi and co-workers used a series of cationic Gemini surfactants as capping stabilizing agents to synthesize PbSe and CuSe NPs with clear coreshell (surfactant) at relatively mild temperature of 85 °C.43 They revealed that the strong affinity of surfactant for the NP surface (derived from FTIR) was the driving force for the monolayer formation in the form of a shell. The size of the NP core increased, while the thickness of the shell decreased as the hydrophobicity of surfactant increased. Although a number of studies have been performed in the recent years to understand the growth process of CdS nanocrystals in the aqueous solution phase in the presence of surface capping agents,4446 it is virtually unknown to observe the growth behavior of CdS nanoparticles in the presence of cationic and anionic surfactants at mild temperature in the similar reaction conditions. Hereunder in our effort we report an easy and inexpensive route of synthesis of CdS nanocrystals in the aqueous phase in the presence of cationic and anionic surfactants, cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), at mild temperature, 90 °C, and products are analyzed at different stages of reaction time to understand the growth process.
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’ EXPERIMENTAL SECTION
Double distilled water and ethanol (obtained from Fisher Scientific) were used as solvents. A.B. Synthesis of CdS Nanoparticles. A solution (50 mL) of cadmium acetate (5 mmol; 1.33 g in 50 mL of double distilled water) was prepared. To an aqueous solution of thiourea, TU (20 mmol; 1.52 g in 100 mL of double distilled water), an aqueous solution of 103 NaOH was added, and pH was maintained to 9.5. The alkaline solution of TU was added to a Cd(II) solution, and the color of the resultant solution turned yellowish. The solution mixture was refluxed at 90 °C for 1 h in constant supply of water cooling. The yellowish suspension thus formed was cooled at room temperature, centrifuged, and washed with doubled distilled water and ethanol repeatedly to remove impurities, and the product was dried in air. To observe the effect of surfactant capping on the morphology, CdS NPs were synthesized in the presence of a cationic surfactant CTAB. To the alkaline solution mixture of 1:4 Cd(II): TU (same as prepared in previous reaction), 1 mmol CTAB (0.365 g in 20 mL double distilled water) solution was added. The reaction mixture was refluxed on a water bath for 0.5 h at 90 °C in constant supply of water cooling. After 0.5 h of reaction, a suspension was formed which was ultracentrifuged and washed with double distilled water and ethanol 23 times to remove organic impurities. Without altering the reaction parameters, the same reaction was performed for a longer time in the presence of CTAB, and the products were separated after 2, 3, and 5 h of reaction periods. To compare the effect of ionic charge on the polar headgroup of the surfactants on shape and size of CdS NPs, the above reaction was performed in the presence of an anionic surfactant SDS. Keeping the other reaction parameters (Cd(II):TU molar ratio, reaction temperature, pH of solution) the same, the reaction was performed in the presence of 1 mmol of SDS (0.288 g in 20 mL of double distilled water) solution for 1 h, and the resultant product was separated by ultracentrifugation, purified by repeat washing with double distilled water and ethanol, and dried in air. The physical observance, i.e., color variance of the suspensions in each reaction, was observed and analyzed carefully in the present investigation. A.C. Characterization. For analysis of the crystal structure and phase of the above-prepared samples, the X-ray diffraction pattern was recorded by Panalytical’s X’Pert Pro X-ray diffractometer in 2θ range 2080° using Cu KR radiation in step sizes of 0.02 degree (d = 1.541 Å). Scanning electron microscopy, equipped with energy dispersive X-ray (SEM and EDX) images, was observed on a Quanta 400 (ESEM with EDX from FEI Company) instrument. Transmission electron microscopy (TEM) analysis was carried out on an H-7500 (Hitachi) instrument applying an accelerating voltage at 120 kV. FTIR spectra of pure and adsorbed surfactants (on CdS NP) were recorded on Perkin-Elmer Spectrum RXI. Optical properties of materials were studied by the UVvis absorption spectrum by a UV2450 (Shimadzu) spectrophotometer.
A.A. Materials. All the chemicals used in this investigation were analytical reagent grade. Metal precursor, cadmium acetate (Cd(CH3COO)2 3 2H2O), was purchased from BDH. Thiourea (NH2CSNH2), cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and sodium hydroxide were purchased from Fisher Scientific. All the chemicals used in this investigation were used as received without further purification.
’ RESULTS AND DISCUSSION The CdS NPs have been prepared by reaction of Cd(II) ions with alkaline solution of thiocarbamide in the absence of any surfactant, in the presence of a cationic surfactant CTAB, and in the presence of an anionic surfactant SDS. The isolated products were in moderate to high yields. The synthetic conditions, 17634
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Table 1. Summary of Reaction Conditions and Resulting Morphologiesa dimension (nm) determined by S. No.
surfactant
reaction time (h)
particle size calculated
TEM and SEM/morphology
by XRD patterns (nm)
yield (%)
1
1
30 ( 15.3/triangular, staggered triangles, star-like mixed
31.81
87
2
SDS
1
80 ( 4.5/spherical
81.29
85
3
CTAB
1/2
15 ( 8.2/spherical
17.69
68
4
CTAB
2
25 ( 6.6/triangular, staggered triangles
24.97
72
5
CTAB
3
37 ( 4.4/star-like
36.51
75
6
CTAB
5
52 ( 2.8/Geodesic spheres
53.49
90
a
Note: The concentration of Cd(II) ions was 5 mmol in 50 mL of double distilled water; TU was 20 mmol in 100 mL of double distilled water; amount of SDS and CTAB was 1 mmol in 20 mL of double distilled water each; reaction temperature was 90 °C in each case.
where D is crystallite size; λ is the wavelength of the Cu KR line (1.541 Å); θ is the angle between the incident beam and reflection lattice; and β is the full width at half maxima (fwhm) of the diffraction peak. The particle sizes calculated using the above formula are presented in Table 1, which are in good agreement with particle sizes determined by TEM and SEM observations. The lattice constants were calculated using the formula49 1=a2 ¼ 4=3ðh2 þ hk þ k2 Þ=a2 þ ð1=c2 Þ
Figure 1. XRD patterns of various samples. (a) CdS NPs synthesized without use of any surfactants for 1 h of reaction time; (b) in the presence of SDS for 1 h of reaction time; (c) in the presence of CTAB for 5 h of reaction time; (d) in the presence of CTAB for 3 h of reaction time; (e) in the presence of CTAB for 2 h of reaction time; and (f) in the presence of CTAB for 1/2 h of reaction time.
% yield, physical data, and resulting morphologies of the products are summarized in Table 1. The phase and crystallinity of as-prepared CdS materials were determined by XRD analysis, and XRD patterns are shown in Figure 1(af). All the diffraction peaks on the curves are in good agreement with standard hexagonal wurtazite CdS due to the presence of (100), (002), (101), (102), (110), (103), and (112) planes (JCPDS file No. 41-1049). The broadness of XRD lines indicates reduced particle size as well as some inhomogeneous particles distribution. In the XRD patterns, the intensity of the (002) peaks is stronger than all the other peaks, suggesting CdS NPs have a strong preferential orientation along the [001] direction.47 The crystallite sizes were calculated using Scherrer’s formula48 D ¼ 0:91λ=β cos θ
and the lattice parameters were calculated as a = 3.40 Å and c = 5.52 Å which are also close to the value reported in the above JCPDS file within experimental error. The phase compositions of as-synthesized CdS crystals were determined by EDX spectra shown in Figure 2. As evident by EDX patterns, the products are composed of Cd and S in the ratio 1:1. Further structural analysis of CdS NPs was performed using TEM and SEM analysis. Various hierarchical architectures were formed by varying the experimental parameters, e.g., reaction time and the surfactants. CdS NPs were formed by reacting Cd(II) ions with in situ formed S2 ions in the aqueous solution phase. To observe the study of the effect of surfactants on morphology and size of CdS NPs, the reactions were performed in the presence of surfactants and without any surfactant. Figure 3 shows the TEM and SEM images of CdS NPs prepared in the aqueous phase in the absence of any surfactant at 1 h of reaction time at 90 °C. It is evident from the TEM and SEM micrographs that irregular-shaped CdS particles (triangular, staggered triangles, star-like mixed particles), ranging in size from 30 to 45 nm, were formed in the absence of any surface stabilizer. In the presence of a cationic surfactant (1 mmol of CTAB), spherical coreshell like CdS particles with an average size distribution of 15 ( 8.2 nm were formed (Figure 4) when the reactions were performed at 90 °C for 1/2 h. In the SEM images (Figure 4a and b), aggregates of spherical particles are observed, whereas in the TEM images (Figure 4cf), isolated particles are seen. When the CdS NPs were synthesized in the presence of an anionic surfactant (1 mmol of SDS), perfectly spherical coreshell structures, with regular size and size distribution (average size distribution 80 ( 4.5 nm) particles, were formed at 1 h of reaction time (Figure 5); however, particles are comparatively larger than previous cases, i.e., without surfactant (Figure 3), and in the presence of CTAB (Figure 4). To obtain a better understanding of formation and evolution of CdS nanostructures, time-dependent experiments were further carried out in the presence of the same amount of cationic 17635
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Figure 2. EDX patterns of various samples. (a) CdS NPs synthesized without use of any surfactants for 1 h of reaction time; (b) in the presence of SDS for 1 h of reaction time; (c) in the presence of CTAB for 1/2 h of reaction time.
Figure 4. (a,b) SEM images and (cf) TEM images of CdS NPs formed in the presence of CTAB for 1/2 h of reaction time at 90 °C.
Figure 3. (a) SEM image and (bd) TEM images of CdS NPs formed in the absence of any surfactant for 1 h of reaction time at 90 °C.
surfactant (1 mmol of CTAB) in an aqueous reaction medium at 90 °C. The shape evolution of nanoarchitectures is shown in Figure 6. It is obvious that primary nanocrystallites, formed at the early stage of reaction time, tended to aggregate to form triangular/star-shaped CdS (size 25 ( 6.6 nm) with pale edge
and dark centers, when reaction time was prolonged for 2 h at the same reaction temperatures, 90 °C (Figure 6). The exterior edge of nanostructures is loosely packed compared to the interior, indicating there is a remarkable variation in intrinsic density. On further increase of reaction time to 3 h, star-like nanostructures, with a narrow size distribution (37 ( 4.4 nm), were formed (Figure 7). Both the SEM and TEM analysis support the formation of star-like, almost monodispersed particles at 3 h of reaction time. Without altering other reaction parameters, when the reaction was further prolonged for 5 h, almost identical shaped geodesic sphere-like CdS nanocrystals, with narrow size 17636
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Figure 5. (a,b) SEM images and (c,d) TEM images of CdS NPs formed in the presence of SDS for 1 h of reaction time at 90 °C.
Figure 7. (a) SEM image and (be) TEM images of CdS NPs formed in the presence of CTAB for 3 h of reaction time at 90 °C.
If the reaction was performed in the absence of any surfactant, similar structures were formed at 1 h of reaction time, but the products were a mixture of triangular/staggered triangular/ multitriangular stacking structures with a wide size distribution (30 ( 15.3 nm). This observation indicates that the original nature of CdS structures is based on the shape evolution of triangular-shaped structures without any surfactant, and the presence of CTAB in the reaction medium stabilizes the CdS NPs by adsorption of surfactant on the NP surface, preventing their unlimited growth,50 so that the products formed on increasing reaction time become narrower and narrower in size distribution. Also, the monodispersity increased when the reaction time was prolonged, as the CdS NPs formed at 5 h of reaction time were comparatively more narrow in size distribution than those at 3 h of reaction time in the presence of CTAB. It is interesting to note that in the presence of SDS comparatively larger (80 ( 4.5 nm) coreshell like CdS NPs, having core and shell diameter ∼65 and ∼15 nm, respectively, were formed at smaller time, 1 h. It is well-known that in an alkaline medium S2 ions are generated by hydrolysis of thiocarbamide.51 Figure 6. (a) SEM image and (bh) TEM images of CdS NPs formed in the presence of CTAB for 2 h of reaction time at 90 °C.
distribution (even narrower than 3 h), have been formed, but now the size of the particles was increased to 52 ( 2.8 nm (Figure 8). It has been found that initially formed spherical crystallites at 1/2 h reaction time in the presence of CTAB (diameter 15 ( 8.2 nm) attained triangular/staggered triangular sheet-like structures having an average dimension of 25 ( 6.6 nm, on prolonged heating for 2 h at 90 °C. On further heating for 3 h, it seems that three to four triangular sheets are stacked (one below the other), forming star-like morphology, and simultaneously sizes increased to 37 ( 4.4 nm. However, when reaction time was increased from 3 to 5 h, many triangular sheets were stacked together forming geodesic sphere-like nanostructures; however, now the size of the particle was increased, and size distribution was narrower than 3 h (52 ( 2.8 nm).
The capping groups or surfactants play an important role in nanocrystals growth. At high temperatures, the surfactant molecules are dynamically adsorbed to the surface of growing crystals, thereby stabilizing the particles in solution and mediating their growth.52 In solution phase synthesis of nanostructures, the reaction can proceed either in a kinetic or in a thermodynamic regime.53,54 Several reaction parameters, such as monomer concentration, reaction temperature, reaction time, and capping groups, influence the type of growth regime. The thermodynamic growth regime is driven by high energy (temperature), yielding isotropic nanocrystals, whereas in the kinetic condition, low temperature and high monomer concentration are the 17637
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Figure 8. (a) SEM image and (bf) TEM images of CdS NPs formed in the presence of CTAB for 5 h of reaction time at 90 °C.
governing factors yielding anisotropic structures. Optimizing the reaction parameters (temperature, monomer concentration, amount of surfactant, etc.), cubic phase (zinc blende) or hexagonal (wurtzite) growth can be achieved.5559 In the present investigation, 1:4 Cd(OAc)2:TU in the presence of 1 mmol of surfactants, at comparatively mild temperature (90 °C) was used to generate CdS particles, and therefore wurtzite CdS NPs resulted. Cd(II) ions react with S2 ions, forming CdS molecules which are associated, nucleated, and grown to make CdS particles. At the suitable stage, the surfactant molecules are adsorbed on the surface of nanoparticles (terminating or capping the nanoparticles surface), diminishing their high surface energy, such that the energetic wells that nanoparticles occupy are effectively lowered. It has been now well established that FTIR spectroscopy is an effective tool in understanding the adsorption mode of longchain alkyl cationic and anionic surfactants.60,61 Figure 9 represents the FTIR spectra of free and adsorbed CTAB on CdS NPs. Clearly, a broad peak in the range of 34103432 cm1 due to OH stretching has been observed in all samples because of some absorbed moisture. The spectrum of pure CTAB shows the symmetric and asymmetric stretching of CH2 vibrations of the alkyl chain corresponding to 2849 and 2918 cm1, respectively, and remained almost the same (between 2853 and 2855 cm1; symmetric, 29212923 cm1; asymmetric) in the presence of CdS NPs. The peaks at 1484 and 1462 cm1 correspond to
Figure 9. (a) FTIR spectrum of pure CTAB, (b) CdS NPs in the presence of CTAB for 1/2 h of reaction time at 90 °C, (c) CdS NPs in the presence of CTAB for 2 h of reaction time at 90 °C, (d) CdS NPs in the presence of CTAB for 3 h of reaction time at 90 °C, and (e) CdS NPs in the presence of CTAB for 5 h of reaction time at 90 °C. 17638
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asymmetric, and symmetric CH scissoring vibrations of the NCH3 moiety62 of free CTAB have been shifted at 15401552 cm1 and suppressed in the presence of CdS NPs, indicating the interaction of surfactant through the headgroup. The spectrum of pure SDS (Figure 10) shows the two main doublet absorption bands at 28502921 cm1 and 12191259 cm1 corresponding to the aliphatic group and the sulfate group.62 The doublet peaks due to the aliphatic group remain almost the same (28522921 cm1) in the presence of CdS NPs. A broad peak at 3424 cm1 due to OH stretching has been observed in the spectrum of CdS NPs in the presence of SDS because of some absorbed moisture. In SDS, the peaks at 1084 and 1018 cm1 have been assigned to the SO stretching vibration and the weakly bonded sulfate group,42,63 which is significantly shifted to 1063 and 976 cm1 in the presence of CdS NPs. Therefore, from FTIR results, it is clear that peaks due to surfactant headgroup regions are shifted significantly without any significant shift in the hydrocarbon tail region. The results confirm the adsorption of surfactants on CdS NPs through the headgroup. When surfactants are dissolved in water, at concentration lower than critical micellar concentration (cmc), the surfactant
Figure 10. (a) FTIR spectrum of pure SDS and (b) CdS NPs in the presence of SDS for 1 h of reaction time at 90 °C.
behaves as a strong electrolyte, whereas above the cmc the monomer forms the aggregates, called micelles, and the process of aggregation is affected by temperature, solvents, and the presence of other entities. In the presence of nanoparticles, the process of micellization takes place prior to that of free micelles.39 The driving force for early micellization in the presence of some additives (the CdS NPs under study) has been attributed to the screening of surface charge of micelles.64 It has been further reported that the surfactants (cationic as well as anionic) adsorb on the surface of particles as micelle-like aggregates, and these aggregates can form even at the concentration lower than cmc due to the interaction between the polar group and NPs. The nature of the interaction depends specifically on the polar headgroup.42 In aqueous medium, surfactants with a cationic headgroup control the shape and size of CdS NPs by binding S2 ions, generated by reaction of thiocarbamide (NH2CSNH2) with the OH ion.65 The adsorption of surfactant via ion pairing passivates the surface of NPs once formed. Since the pH of resulting solution is 9.5, it is expected that the synthesized particles are negatively charged due to an excess of S2 ions on the particle surface and therefore draw the cationic surfactant (CTAB) by long-range electrostatic attraction, inducing the CTAB adsorption through the headgroup. This expectation is supported by FTIR results also. However, the hydrophobic tail cannot prefer an aqueous medium, and therefore a counter layer should be oriented in the opposite way; i.e., attachment of hydrophobic tails between two layers and thus the hydrophilic group (head) is oriented in aqueous environment, forming a bilayer structure of surfactant over the NP surface. Close inspection of the shell (CdS NPs synthesized in the presence of CTAB for 1/2 h of reaction time; Figure 4) indicates that it is ∼4.5 nm thick, which is very much in agreement with the bilayer thickness of CTAB (4.4 nm). Surfactants with anionic headgroups adsorb on the surface of formerly generated NPs by donating their lone pairs to empty d orbitals of metal ions and therefore govern their size and shape. Possibly, the Cd(II) ions form a CdSDS complex in the presence of SDS. The S2 ions decompose the metal complex, forming CdS molecules. The coordination of Cd(II) ions with SDS never “block the route” of S2 ions for CdS formation, but it insures the availability of Cd(II) ions in a systematic way because the S2 ions decompose the CdSDS complex, forming CdS. On the other hand, in the case of CTAB due to CTA+S2 ion pairing, [S2] decreases on the surface of the NP, and therefore CdS formation is retarded. The adsorption of spherical micelle aggregates of SDS on the CdS NP surface attain shell-like structure over CdS NPs (Figure 5).66,67 This hypothesis is supported by FTIR results also. To explore the reaction mechanism of nanoparticle growth in various sizes and shapes, we attempted to fit our data to the model for classical crystallization by Ostwald Ripening (OR). OR
Table 2. Optical Properties and Bandgap of CdS NPs Synthesized in the Various Reaction Conditions S. No.
surfactant
reaction time (h)
color of dispersion
peak position due to excitonic transition (nm)
band gap Eg/V
1
-
1
yellow
498
2.69
2
SDS
1
orange
520
2.45
3 4
CTAB CTAB
1/2 2
lemon yellow yellow (lighter than S.No. 1)
478 493
3.01 2.80
5
CTAB
3
yellow (deeper than S.No. 4)
506
2.59
6
CTAB
5
yellow (deeper than S.No. 5)
512
2.51
17639
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Figure 11. UVvis spectra of various samples. (a) CdS NPs synthesized without use of any surfactants for 1 h of reaction time; (b) in the presence of SDS for 1 h of reaction time; (c) in the presence of CTAB for 5 h of reaction time; (d) in the presence of CTAB for 3 h of reaction time; (e) in the presence of CTAB for 2 h of reaction time; (f) in the presence of CTAB for 1/2 h of reaction time.
occurs when initially smaller, more soluble crystallites dissociate and resulting solutes species (ions or molecules) contribute to growth of larger nanoparticle products. Growth by this route was formally described by the LifshizSlyozovWagner (LSW) theory68,69 d ¼ at 1=n þ d0 where t is time; a is temperature-dependent materials constant; d is average particle diameter; and d0 is the diameter at time t = 0. According to this model, n varies from 2 to 4 depending upon what controls the nanoparticle growth. When n = 2, growth is governed by surface diffusion at the soluteliquid interface; when n = 3, growth is controlled by volume diffusion of ions in solution; and when n = 4 growth is controlled by dissolution kinetics of initially formed species. The size data determined by various analyses (XRD and TEM) give n ∼ 2, indicating that surface diffusion at the solidliquid interface is the controlling factor for shape evolution of various nanostructures in the present investigation.36 During the course of the reaction, the color variation at the different time stages has primarily been identified by visual inspection, and UVvis spectra of resultant dispersions were recorded. The observations are summarized in Table 2. The as-prepared triangular, star-shaped, and geodesic sphere shaped CdS NPs (2060 nm) are considerably larger than the excitonic Bohr’s radius, and therefore we cannot expect quantum
confinement. However, quantum confinement can be possible near the tip edge of nanostructures (triangles, stars, and geodesic sphere etc.) which are smaller than or comparable to the Bohr’s radius of CdS because the optical properties of nonspherical nanostructures are governed by the lowest possible dimensions of nanocrystals.70 In the case of as-prepared various nanostructures having a thin edge tip exhibiting quantum confinement, it can be speculated that the band gap energy will be increased from the inner wider portion to the outer sharp edge tip of nanocrystals due to position-dependent quantum-size effect.71 Therefore, to reveal any possible shape-dependent optical properties, the UVvis absorption spectra of as-prepared CdS NPs have been measured and presented in Figure 11. The absorption shoulders of CdS particles formed at 1/2, 2, 3, and 5 h of reaction time in the presence of CTAB at 90 °C were observed at 478, 493, 506, and 512 nm, respectively, due to 1S1S excitonic transition.72 The absorption shoulder of the CdS nanocrystal synthesized without any surfactant at 1 h of reaction time was obtained at 498 nm; the absorption edge is somewhat broad, indicating a wide size distribution.73 In the case of CdS NPs formed in the presence of SDS at 1 h of reaction time at 90 °C, the absorption shoulder was found at 520 nm. It is evident by the relative blue shift from the bulk74 that the position-dependent quantum size effect has actually been found in our synthesized 17640
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The Journal of Physical Chemistry C CdS nanocrystals case, even though the sizes are comparatively larger but have well-defined shape with sharp edge and tip. The bandgap of nanodispersion was calculated using the Tauc relation75
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’ AUTHOR INFORMATION Corresponding Author
*E-mail: pandeygajanan@rediffmail.com. Phone: +91-5222998126. Fax: +91-522-2444888.
ðεhνÞ2 ¼ PðEg hνÞ where ε, h, ν, Eg, and P are the molar extinction coefficient, Plank’s constant, frequency of light, bandgap of nanoparticles, and arbitrary constant, respectively. The linear part of the (εhν)2 verses hν plot was used to calculate the band gap value. Compared with that of bulk CdS (2.42 eV),76 the calculated band gap values clearly indicate the presence of quantum-size effects in the prepared CdS nanoparticles.
’ ACKNOWLEDGMENT The corresponding author (GP) is grateful to All India Council for Technical Education (AICTE), New Delhi, for providing RPS project grant. We gratefully acknowledge DRDE Gwalior for SEM and EDX analysis. The authors are also thankful to Prof. A. K. Shrivastava, School of Studies in Physics, Jiwaji University, Gwalior, for useful discussions and suggestions. ’ REFERENCES
’ CONCLUSION In summary, we have presented an easy and low cost synthetic process of CdS nanostructures in the aqueous solution phase at low temperature, 90 °C, in the presence of cationic and anionic surfactants, and a plausible growth mechanism has been discussed. The products were comprehensively analyzed by TEM, SEM, and EDX for phase, crystallinity, composition, size, and morphology determination. A thorough study of shape and size variation, with variation in reaction time and surfactant, has been done. The irregular, mixed shape and size CdS NPs attained regular shape and size in the presence of surfactants, CTAB and SDS. Surfactants adsorb on the CdS NP surface as micelle-like aggregates. In the aqueous solution phase, since the hydrophobic tail cannot prefer the aqueous medium, the surfactant head is oriented in the aqueous phase forming a bilayer of surfactant over the NP surface. In the case of CdS NPs, synthesized for 1/2 h of reaction time in the presence of CTAB, coreshell-like structure is formed due to the formation of a bilayer over CdS NPs. When the CdS NP was synthesized in the presence of SDS, clear coreshell like morphology was formed due to adsorption of spherical micelle aggregates of SDS over the CdS NP surface. The adsorption of surfactants on the CdS NP surface is supported by FTIR results. Further, in the presence of CTAB, smaller sized star-shaped CdS particles were formed, whereas in the presence of SDS, comparatively large sized spherical particles were formed. The size of CdS NPs increased with an increase in reaction time, and a dramatic shape conversion from spherical to triangular, star-shaped, and geodesic spheres has evolved on prolongation of reaction time from 1/2 to 2, 3, and 5 h in the presence of CTAB. Classical crystallization, due to Ostwald ripening (described by LSW theory), was suitably fitted for particle growth, and trend of size variation data, determined by XRD and TEM, resulted n ∼ 2, indicating surface diffusion at the solidliquid interface, was the controlling factor for particle growth in the present investigation. Relative blue shifts in the UVvis spectra of CdS NPs, formed in the various reaction conditions, indicated that the position-dependent quantum size effect was responsible for relative blue shifts, although the particles were larger than excitonic Bohr’s radius but having a well-defined shape with a sharp edge tip. For further insight into understanding the shape evolution of various nanostructures in aqueous media at mild temperatures, variation in M:TU molar ratio, concentration variation of surfactants, tail size, and structure of surfactant might play important roles in shape and size evolution of NPs, which will be undertaken in our future work.
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