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A novel and selective approach to obtain highly stable and capping nanospheres of crystalline cadmium sulfide (CdS) via solution growth in cadmium nitrate tetrahydrate-thiourea-ligand-pH buffer systems using a Taguchi experimental design is presented. CdS characterization is carried out by laser diffraction particle size analyzer, X-ray diffraction, scanning electron microscope/energy dispersive X-ray spectroscopy, and UV-Vis absorption spectroscopy. Synthesized nanospheres of crystalline CdS with an average crystal particle diameter of 37.5 nm and a capping layer thickness of 17.7–35.7 nm (73 to 115 nm as capped nanoparticles) keep size and form stable in aqueous solution. These crystalline CdS nanospheres exhibit a 2.46 eV band gap, as well as, an optical bandwidth comprises in the visible range with potential applications in solar cells.
© 2013 Optical Society of America
1. Introduction
Currently, faced with the high demand of renewable energy resources, solar cells of high efficiency have gained high priority in many countries due to the climate crisis. In this sense, cadmium sulfide (CdS) arises as a competitive material for solar cells because of its wide band gap (energy 2.42–2.53 eV), high absorption coefficient, high electron affinity, n-type conductivity, high photoconductivity, as well as, excellent luminescence [1–4]. CdS semiconductor nanoparticles or nanocrystals have proven to be one of the most promising materials for electronic application compared to CdS films or bulk materials [3]. Nanocrystals or quantum dots (QDs) are highly crystalline semiconductor nanoparticles. These are made up of 100–100 000 atoms and their typical dimensions are between nanometers to a few microns [5]. Currently, CdS nanocrystals are under intense investigation because they present interesting photophysical and photochemical properties arising from particle size effect [3,5]. Also, CdS nanocrystals have discrete electronic energy that gives rise to unique optical properties with many potential electronic and photonic applications, such as: DNA sensors, molecular labeling, photosensors, and solar cells [5–9]. However, these applications require a high control of CdS microstructure (size, particle size distribution, composition, and crystalline structure) and their optical and electrical properties. Therefore, it is crucial a good control of processing parameters during its synthesis. As a result, different research efforts are focused on the study and the development of synthesis techniques to produce CdS nanocrystals, such as: solvothermal or hydrothermal synthesis, evaporation processes, ultrasonic surfactant assisted method, solgel methods, and X-ray irradiation route [10]. Unfortunately, these techniques have many relevant disadvantages, for instance: (a) difficult to scale at industrial levels, (b) complicated to realize, (c) sophisticated equipment and complex procedures used, (d) seeds or templates needed to produce them, (e) long processing times required (up to 48 h for hydrothermal process), and (f) relatively high processing temperatures (up to 473.15 K for hydrothermal process) [10–13]. Solution growth (also named controlled precipitation, chemical deposition or chemical bath deposition-CBD) is a technique that allows the synthesis of CdS nanocrystals and other nanosized semiconductor materials under optimum processing conditions with the appropriate raw materials. Thereby, one of the major aspects to solve is the ability to synthesize nanocrystals of the required size with a controlled size distribution. On the other hand, the main difficulty of solution growth method is the dependence between the reaction parameters, the average size and the size distribution of generated particles that are not understood in detail. Generally, the reaction conditions cited in the literature are given in an empirical and intuitive manner [14]. Hence, Taguchi experimental designs have proved to be excellent tools for design and study of several materials allowing optimization of processing parameters [14–16]. These tools integrate statistical analysis that allows determining the quantitative effect of the processing parameters on one (analysis of variance, or ANOVA) or more than one (multiple analysis of variance, or MANOVA) specific property or response variable [15–17]. In this work, synthesis by solution growth technique is proposed for the production of highly stable nanospheres of crystalline CdS by using an L9 Taguchi experimental design. In addition, an alternative way to simultaneously control the particle size during the synthesis by solution growth and to modify the surface of nanoparticles based on the modification of reaction systems with small amino acids (glycine) as capping is suggested and compared with a commonly used ligand (sodium citrate) and without a ligand. The main goals of this study are: (a) to directly synthesize highly stable nanoparticles of crystalline CdS (nanocrystals) by solution growth in cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O)-thiourea (NH2CSNH2)-ligand-pH buffer systems, (b) to determine the quantitative effect of the processing parameters on the formation of crystalline CdS nanospheres, (c) to establish the optimal processing conditions for selective synthesis of stable and uniform nanospheres of crystalline CdS, and (d) to determine their optical properties.
2. Experimental procedure
2.1. CdS nanocrystals synthesis and processing parameters optimization
Based in an L9 Taguchi experimental design, synthesis of CdS was carried out by solution growth in cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O)-thiourea (NH2CSNH2)-ligand-pH buffer systems. An L9 Taguchi experimental design allows studying the effect of up to four parameters in three levels with a minimum number of trials (for this case nine), and optimizing the processing parameters. ANOVA provides insight into the optimal processing parameters and a means of estimating the percent contribution of each parameter tested on the variability in the measured quantities (for this particular case, the formation of nanoparticles of crystalline CdS). With an L9 Taguchi experimental design and analysis of variance (ANOVA), it is possible to study the effect of the processing parameters on the formation of crystalline CdS nanoparticles and to establish conditions that promote the selective formation of uniform nanospheres of crystalline CdS by solution growth method in the considered reaction systems. This array considers four processing parameters for the synthesis of CdS particles and they were varied at three levels, as follows: kind of ligand (without ligand, glycine, and sodium citrate), pH buffer (4 mL de borate, 2 mL de borate, and 2 ml de ammonium hydroxide/ammonium chloride), processing temperature (323.15, 338.15, and 353.15 K), and processing time (20, 30, and 40 minutes). For this study, the experimental design with two replicates per trial was accomplished. Both samples of each trial were subjected to analysis of variance and tested to determine the CdS nanoparticles formation and their characteristics. Table 1 shows the L9 Taguchi orthogonal array used for CdS synthesis by solution growth.
Table 1. L9 Taguchi orthogonal array for the synthesis of CdS particles.
For the current study, chemical reagents used in the reaction systems were supplied by Sigma-Aldrich. All chemical reagents for the production of solutions correspond to high purity grade materials and they were used, as received. Cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O) and thiourea (NH2CSNH2) were used as Cd+ and S− precursors, respectively. Glycine (C2H5NO2) and sodium citrate (C6H5Na3O7) were selected and used as ligands (modifier and complexing agents). These chemical agents were previously reported in the literature by other authors [10, 18] and were selected by their properties, such as: suitable agents and good ions release control in the processing of CdS films. Also, preliminary tests show that those ligands could have an important influence on: physicochemical properties of the chemical solutions used in the reaction systems (they can acts as particles surface modifier and solution modifier agents) and CdS particles characteristics (size, crystalline structure, and morphology). Finally, borate and ammonium hydroxide/ammonium chloride solutions were used as pH buffers to control the formation reaction of CdS particles.
CdS synthesis started with preparation of solutions of the corresponding chemical reagents. Solutions of 0.1 M Cd(NO3)2.4H2O, 1 M NH2CSNH2, 0.1 M C2H5NO2, and 0.2 M C6H5Na3O7 were individually prepared with deionized water, as diluent. For each trial, the amount of Cd precursor (4 mL of 0.1 M cadmium nitrate solution) and the amount of S precursor (5 mL of 1.0 M thiourea) were considered as constant parameters, and they were mixed with the pH buffer (2 mL of borate, 4 mL of borate, or 4 mL of ammonium hydroxide/ammonium chloride) and the ligand (2 mL of 0.1 M glycine or 5 mL of 0.1 M sodium citrate). Then, solution containing all chemical agents was placed in a volumetric flask with a capacity of 100 mL and diluted with deionized water until completing this volume. Subsequently, the solution was retained in a beaker and heated in a thermal bath at constant temperature for the formation of CdS particles under the processing conditions established for each trial in the L9 Taguchi experimental design.
2.2. Characterization of CdS
After the synthesis trials, synthesized particles were removed from remaining solutions by decantation and centrifugation. Then, obtained particles were washed with deionized water to remove the remaining chemical reagents and were prepared for their structural, chemical, and morphological characterizations by X-ray diffraction (XRD), scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS), and laser diffraction particle size analysis. Structural characterization and phase identification of synthesized particles were performed by XRD technique using a Philips X-ray diffractometer (Cu Kα radiation, anode excitation of 40 kV, and current of 30 mA). XRD analysis was configurated with a 0.02° step size and a rate/count time of 1° per minute, from 10 to 60 2θ (degree). The qualitative, quantitative, and structural analyses from XRD patterns corresponding to synthesized particles were performed by means of the X’pert High Score Plus™ computer software. Structural parameters were determined and confirmed using Crystal Maker® computer software. Average diameter of crystal particles of the crystalline CdS samples was calculated using Debye-Scherrer’s formula [19], given by:
where t is the diameter of crystal particle, λ is the wavelength of X-ray used (Cu = 1.54 Å), B is the full-width at half-maximum (radians), and θ is the scattering angle.Chemical and morphological characterizations were achieved in a Hitachi TM3000 Table-top SEM equipment provided with a Bruker XFlash MINSVE EDS device under environmental conditions at an acceleration voltage of 15 keV. To improve SEM images, previous to the SEM/EDS analysis, samples were prepared in an aqueous solution, then, placed on aluminum sample holder coated with graphite adhesive tape. After that, they were dried at environmental temperature to form a thin layer of CdS particles and to be in condition to carry out the microstructural characterization. Particle size of CdS and particle size distribution were measured in a laser diffraction particle size analyzer (Beckman Coulter model LS 13320 provided with a universal liquid module). For these measurements, CdS particles were suspended in deionized water. In addition, CdS samples were characterized in a Perkin Elmer Lambda 19 spectrophotometer in the 300–850 nm wavelength range to determine optical properties and band gap.
3. Results and discussion
3.1. Synthesis of micro and nanoparticles of CdS
CdS particles were synthesized by solution growth in the cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O)-thiourea (NH2CSNH2)-ligand-pH buffer systems, under the processing conditions established in the L9 Taguchi experimental design. During the synthesis, the presence of CdS particles was detected in the chemical solution by the formation of fine particles with a color ranging from light to bright yellow. Simultaneously, the formation of significant amounts of CdS particles was observed, and these results are presented in decreasing order, considering the kind of ligand, as follows: L2, L3, and L1 (without ligand); L5, L4, and L3 (glycine); and L8, L7, and L9 (sodium citrate). These results indicate a high reaction efficiency. Also, it is possible to visualize that the proposed approach can be used on large scale production for micro and nano-sized CdS particles. Figure 1 shows the obtained CdS particles by solution growth under the condition of L9 Taguchi experimental design.
Fig. 1 Photograph of CdS particles obtained for each trial under the conditions of L9 Taguchi experimental design.
3.2. Characterization and microstructural evolution of CdS
Qualitative and semi-quantitative analyses by EDS and SEM reveal the formation of micro and nanoparticles containing Cd and S, and these were associated with CdS formation. Additionally, results of the particle size analysis by laser technique confirm the production of CdS micro and nanoparticles. CdS microparticles present particle sizes from 1.2 to 2.2 μm under the conditions of L1, L3, and L8 trials. On the other hand, CdS nanoparticles with particle sizes from 73 to 127 nm were synthesized under the conditions of the L2, L4, L5, L6, L7, and L9 trials. Table 2 shows the main morphological and chemical characteristics of CdS particles obtained for each trial by solution growth under the experimental conditions.
Table 2. Main morphological and chemical characteristics of CdS particles.
According to microstructural characterization of synthesized samples, CdS with different morphologies can be obtained, such as: snow like materials, acicular nanoparticles, porous microspheres, and compact nanospheres. Figure 2 corresponds to SEM photomicrographs, and they show morphological characteristics of CdS micro and nanoparticles obtained for each trial by solution growth under the conditions of L9 Taguchi experimental design.
Fig. 2 SEM photomicrographs of CdS micro and nanoparticles obtained for each trial under conditions of the L9 Taguchi experimental design.
According to the kind of ligand, the morphological evolution of CdS particles obtained by solution growth under the experimental condition is, as follows: (a) reaction systems without ligand: CdS morphology changes from snow like materials (L3) to mixture of snow-like and small amount of particles (L1), and acicular nanoparticles (L2); (b) reaction systems containing sodium citrate: CdS morphology is modified from a mixture of snow-like material and acicular nanoparticles (L9) to acicular nanoparticles (L7), and particles with porous appearance (L8); and (c) reaction systems containing glycine: a mixture of snow-like materials and small amount of nanoparticles (L5), afterwhich CdS appears as snow-like materials and nanoparticles (L6), and finally only nanospheres (L4) with uniform sizes are synthesized. The analysis of SEM photomicrographs and EDS spectra corresponding to particles exposes different stages of the micro and nanostructural evolution of CdS. Contrary to the proposed mechanisms of the role of ligand or complexing agents (as ion release) in the solution growth process, the micro and nanostructural evolution study suggests that the formation of CdS nanoparticles in the systems containing glycine occurs on the surface of this chemical agent, as follows: (I) absorption of Cd+ and S− ions; (II) reaction between Cd+ and S− ions; (III) formation of nuclei and growth of CdS; and (IV) release of CdS particles capped with ligand agent to aqueous solution (chemical bath). Nevertheless, a more detailed study is needed to confirm the mechanisms of CdS particle formation in the systems containing glycine, and to elucidate and establish the formation mechanisms in the rest of the reaction systems. Given that the study of the mechanisms of CdS formation is beyond the scope of this paper, a comprehensive study of the mechanisms with additional results of the thermodynamics and the chemical kinetics of CdS formation will be published elsewhere. Lastly, based on SEM and EDS results, the formation of CdS nanospheres under the processing conditions of trial L4 is feasible, but these conditions do not necessarily correspond to optimal conditions. An analysis of the main effects is required and presented in the next paragraph.
3.3. Analysis of variance, main effects of the processing parameters, and optimization
ANOVA results indicate that under the experimental conditions, time has the strongest effect on the variability of crystalline CdS nanoparticles formation with a contribution of 43%, followed by temperature (29%), ligand (24%), and pH buffer (3%). Remain percent (1%) corresponds to the error term. According to statistical tools, the contribution percent due to the error term provides an estimate of the adequacy of the experiment. In this case, the low magnitude of error term (less than 5%) suggests that no important factors were omitted in the design of the experiment and no significant measurement errors were carried out [17].
The establishment of optimal processing parameters to promote the formation of crystalline CdS nanospheres for this case is based on the following condition: “the minimization of the response of each parameter improves the formation of crystalline CdS nanospheres”. Based on the optimization condition and the results of the main effect of the processing parameters on the formation of CdS nanospheres (Fig. 3), the optimal processing conditions to improve the nanoparticles formation of crystalline CdS by solution growth, are: 4 mL of cadmium nitrate tetrahydrate (0.1 M), 5 mL of thiourea (1 M), 5 mL of glycine (0.1 M), 4 mL borate buffer (pH 10), processing temperature of 338.15 K, and processing time of 40 minutes. In this particular case, the optimal conditions for the synthesis of crystalline CdS nanospheres corresponded to the processing parameters of trial L4. However, it was necessary to achieve the synthesis of CdS nanospheres under optimal conditions to validate the experimental results.
3.4. Validation test of crystalline CdS nanospheres formation
The optimal conditions for the formation of crystalline CdS nanospheres were validated by means of the synthesis of CdS under the optimal conditions, and their structural, chemical and morphological characterizations. Characterization results of the CdS particles obtained by solution growth under the optimal conditions are described in the next paragraphs.
XRD results reveal and confirm the formation of crystalline CdS nanospheres under the optimal processing conditions by solution growth synthesis method. Figure 4 shows a representative XRD pattern of the CdS nanospheres obtained by solution growth technique under optimal conditions. Qualitative and quantitative analyses indicate that XRD patterns of synthesized particles correspond to the CdS phase (JCPDS No. 00-041-1049; cadmium sulfide (CdS) or Greenockite, syn; hexagonal, a=b=4.14 Å, c=6.72 Å; and 28.183, 26.507, 47.840, 51.825). Nanospheres of crystalline CdS have the following relevant parameters: formula sum: Cd 1.00 S 1.00; formula mass: 144.4710 gmol−1; density: 4.807 gr/cm3 (theoretical density: 4.82 gr/cm3); hexagonal crystalline system with lattice parameters: a = b = 4.124 Å, c = 6.705 Å, and space group of P63mc. A summary of structural parameters calculated for cadmium sulfide (CdS, Greenockite, syn) synthesized as nanospheres in the current work is shown in Table 3. The average particle sizes of the crystalline CdS samples were calculated using Debye-Scherrer’s formula (1), and results indicate that CdS nanospheres are 37.5 nm in size.
Table 3. Relevant structural parameters of CdS nanospheres obtained under optimal conditions.
EDS semi-quantitative and qualitative analyses reveal the composition of CdS nanospheres obtained under optimal conditions, as follows: 49.15 and 50.85 atomic percent of S and Cd, respectively. These results can be associated to the formation of a stoichiometric phase of CdS. Figure 5 shows a representative SEM photomicrograph of CdS nanospheres obtained under optimal conditions and the corresponding EDS spectrum after processing. SEM photomicrographs (Fig. 5) indicate that the CdS nanospheres measure around 100 nm, and they preserve their size and remain stable after nine months of processing (Fig. 6). A more detailed study of particle size by duplicate with laser technique in a Coulter equipment and statistical analysis discloses that the mean particle size of CdS nanospheres is 115.50 nm, median particle size is 90.50 nm, and the mode particle size is 73.00 nm. Figure 7 illustrates the particle size distribution by duplicate of CdS nanospheres synthesized under optimal conditions.
Fig. 7 Particle size distribution by duplicate of CdS nanospheres obtained under optimal processing conditions.
The difference between particle sizes determined by XRD (37.5 nm), SEM (≈ 100 nm) and laser technique (73–106 nm) can be explained by the formation of a capping layer of glycine (amorphous phase) with 17.7–35.7 nm of thickness on CdS nanoparticles (crystalline phase) of 35.7 nm. The estimations of thickness of the capping layer of glycine were carried out under following considerations: (a) Given that XRD technique contemplates crystalline structures for the analysis, the contribution of amorphous phases (thickness of the capping layer of glycine) have little or null significance and can be ignored in the measurement of the diameter of crystal particles for CdS nanocrystals using Debye-Scherrer’s formula, (b) By SEM and laser technique, the determined particle sizes are whole particle sizes (particle/cover), and (c) Consequently, it is possible to establish and suggest a non destructive procedure to estimate the capping layer thickness (tc) by measuring the value of the average diameter of crystal particle (t) with XRD and the value of average particle size for the whole particle (tpc) by SEM or laser techniques, as follows:
3.5. Optical characterization
The average transmission spectrum of a representative CdS nanospheres sample obtained under optimal conditions is shown in Fig. 8(a). The curve exhibits a good response into the visible spectrum, with a value approximately of 60 % in the range from 500 to 850 nm.
In the same way, according to Fig. 8(b), it is clearly seen that the highest absorption is founded approximately at 320 nm, then it presents a fast decrease until 500 nm. This wavelength corresponds to photon energy of 2.48 eV, which is very close to the typical band gap of microcrystalline CdS. However, a more slowly increase in absorption band occurs at shorter wavelength and a weak shoulder is presented around to 450 nm. In semiconductors, the relation connecting absorbance with incident photon energy and optical band gap is represented by the Tauc relation [20, 21]:
where α is the absorption coefficient, hν is the energy of incident photon, A is a constant, and Eg is the band gap energy. Figure 9 corresponds to the plot of (αhν)2 versus hν (also named Tauc Plot), where the intercept of the graph on X-axis provides the value of band gap which is equal to 2.46 eV.Lastly, results of microstructural and optical characterizations for CdS nanospheres indicate that under optimal conditions of processing, solution growth approach offers superior advantages for the production of nanosized CdS particles compared with the established and previously reported methods [5–7,13], such as: better control of composition, size, crystalline structure and morphology, short processing times (40 minutes), and low temperature (338.15 K), all of which may be reflected ultimately in the processing costs, properties and better performance as window materials for solar cell.
4. Conclusion
Crystalline CdS micro and nanoparticles were successfully synthesized by solution growth in the cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O)-thiourea (NH2CSNH2)-ligand-pH buffer systems. L9 Taguchi experimental designs proved to be an excellent tool to study and optimize CdS nanocrystal formation by solution growth. Under the processing conditions, time has the strongest effect on the variability of CdS nanocrystals formation with a contribution of 43%, followed by temperature (29%), ligand (24%), and pH buffer (3%). The optimal processing conditions to improve the direct formation of capped CdS nanocrystal by solution growth are: 4 mL of cadmium nitrate tetrahydrate (0.1 M), 5 mL of thiourea (1 M), 5 mL of glycine (0.1 M) as ligand, 4 mL borate buffer (pH 10), processing temperature of 338.15 K, and processing time of 40 minutes. Under these conditions, highly stable and capping nanospheres of crystalline CdS with stoichiometric composition, density of 4.807gr/cm3, band gap of 2.45±0.05 eV, average diameter of crystal particle of 37.5 nm, capping layer thickness of 17.7–35.7 nm, and whole particle size (particle/cover) in the range of 73 to 115.5 nm are produced. The mechanism of formation of CdS nanospheres in the system containing glycine is given by means of the adsorption of ions (Cd+ and S−) on the ligand surface, followed by reaction between ions, nucleation, and growth of CdS, and lasty, the release of stabilized and capping nanospheres of crystalline CdS. Finally, the proposed approach of solution growth under optimal conditions offers a procedure for the direct synthesis of stable and capping nanospheres of crystalline CdS with a potential for high scale production. And according to the Fig. 8(a), the optical bandwidth (500–800 nm) allows guarantee a potential application of this type of nanospheres in solar cells, specifically, as window materials.
Acknowledgments
Authors gratefully acknowledge the Secretaría de Educación Pública of México for financial support under project contract PIFI-2011-29. We also express thanks to Laboratorio de Nanomateriales-FIC of the Universidad de Colima in Coquimatlán, Colima for technical assistance and facilities.
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