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Zinc blende versus wurtzite ZnS nanoparticles: Control of the phase and optical properties by tetrabutylammonium hydroxide

The influence of tetrabutylammonium hydroxide on the phase composition (cubic zinc blende versus hexagonal wurtzite) of ZnS nanoparticles was studied. The ZnS nanoparticles were prepared by a microwave-assisted solvothermal method, and the phase structure and optical properties along with the growth process of ZnS nanoparticles were studied. We report XRD, FE-SEM, EDXS, UV-vis and PL measurements, and first-principles calculations based on TDDFT methods in order to investigate the structural and electronic properties and the growth mechanism of ZnS nanostructures. The effects as well as the merits of microwave heating on the process and characteristics of the obtained ZnS nanostructures and their performance are reported.


Introduction
Increased interest in zinc sulfide (ZnS) nanomaterials has arisen mainly due to their applications in different technological fields such as optoelectronic luminescent devices and photovoltaic cells. [1][2][3][4][5] The synthesis of ZnS nanocrystals with tunable size and phase not only provides alternative variables in tailoring the physical properties of this semiconductor material, but is also vital to develop them as building blocks in constructing the future nanoscale optoelectronic devices using the so-called ''bottom-up'' approach whereby atoms and molecules selforganize into nano-sized crystals or more complex molecular assemblies. 6,7 ZnS can adopt three phases: cubic zinc blende, hexagonal wurtzite or the rarely observed cubic rock salt. 8 The cubic zinc blende structure of ZnS is the most stable form in the bulk which transforms into a hexagonal wurtzite structure at 1020 1C and melts at 1650 1C and both ZnS polymorphs have industrial applications. 9,10 In both cubic and hexagonal structures, Zn and S atoms are tetrahedrally bonded where the only difference is in the stacking sequence of atomic layers. Nevertheless, with decreasing particle size, the relative stability of two phases changes and low-temperature synthesis of small wurtzite ZnS nanoparticles have been reported, [11][12][13][14] and very recently, Kulkarni et al. 15 reported the ethylenediamine-mediated wurtzite phase formation in ZnS. Controlled fabrication of nanoparticles with different phases is desirable and necessary, which is however still a great challenge.
The kinetics of crystal growth strongly depend on the structure of the material, the properties of the solution, and the nature of the interface between the crystals and the surrounding solution. [16][17][18] In particular, the size dependence of the solid-solid phase transition temperature of ZnS nanoparticles has been the subject of intensive study, 11,19,20 but harnessing the thermodynamic performance of the nanoparticles in a controllable way remains a complicated matter. Phase control in the growth of ZnS crystals is important, because each phase has unique physical properties, for instance, the different phases show different lattice vibration properties and nonlinear optical coefficients. 21,22 As the current research moves toward nanoscale phenomena and technology, the exploration of facile and economic methods for the synthesis of ZnS nanostructures has been of great interest. However, the crystal structure of the nanoparticles strongly depends on the synthesis conditions. Zhang et al. 23 by combining molecular dynamics simulations and experimental measurements suggested that wurtzite particles with size smaller than about 7 nm, in vacuo, are more stable than zinc blende in vacuo at room temperature, however, lowering of the temperature of the zinc blende to wurtzite structure transition is possible for nanosized ZnS using modifiers, and these authors also have reported water-driven structure transformation in nanoparticles at room temperature. Within this framework, achievement of suitable control of the phase transition behavior of nanometer-sized ZnS materials would represent a significant progress on the way to their full exploitation in different areas of science and engineering.
In recent years, well-defined ZnS nanoparticles with various morphologies and structures, including nanotubes, nanorods, nanowires, nanocubes, nanospheres, nanoflowers and nanosheets, have been successfully synthesized and studied using a variety of methods. 9,[24][25][26][27][28][29][30][31] In addition to these techniques, the preparation of ZnS via solution chemical routes provides a promising option for the large-scale production of this material. Therefore, it is important to develop new environmentally friendly processing material methods with low cost, and with the possibility of formation of nanoscale materials with phase control and well-defined morphologies. Developing this phase-selective synthesis, in general, is crucial for the design of ZnS nanocrystals with novel tunable physical properties which is of great interest in nanotechnology.
The synthesis route of nanoparticles has a major influence on their size, shape and optical properties. Recently, a microwaveassisted procedure has been developed into an efficient method for the fabrication of nanomaterials, and is becoming very attractive in all areas of synthetic chemistry because it has some advantages over other synthetic methods, 32,33 because the application of microwave irradiation in chemical transformations often results in dramatic rate accelerations, enhanced yields, cleaner reactions [32][33][34][35][36] and improved material quality and size distributions in nanomaterials. [34][35][36][37][38][39][40] Some comprehensive reviews on microwave-assisted syntheses of nanomaterials have been published. 37,41 The mechanism associated with microwave effects in synthesis are not well understood, 38,42,43 and this is an open research field. In such cases, it is certain that quantum chemistry tools are extremely helpful in the rationalization and interpretation of reaction mechanisms at the atomic level, based on the characterization of key intermediates along the reaction pathways.
Here we report a rapid and economical microwave-assisted route for the preparation and phase control of ZnS nanoparticles by a simple microwave-assisted solvothermal (MAS) method and the presence of tetrabutylammonium hydroxide. The formation mechanism and the conditions under which the phase of the assynthesized ZnS (wurtzite and zinc blende) is obtained are discussed in detail. The obtained materials were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDXS), and ultraviolet-visible (UV-vis) and photoluminescence (PL) measurements. Targeting a better understanding of the experimental data, theoretical calculations were carried out by time-dependent density functional theory (TDDFT). In addition, this study gives practical guidance for controlling the phase in MAS growth of ZnS nanostructures, and the effects as well as the merits of microwave heating on the process and characteristics of the obtained ZnS powders are reported.

Materials
All reagents were of analytical grade and were used without further purification. ZnS nanostructures were synthesized by the MAS method with or without modifier assistance, in the presence of ethylene glycol (EG) at 140 1C and for short times.
In a typical procedure, 7.34 mmol of zinc acetate are dissolved in 25 mL of EG and heated to 80 1C (solution 1); 7.34 mmol of thiourea are separately dissolved in another 25 mL of EG (solution 2). With vigorous magnetic stirring, solution 1 is then quickly injected into solution 2. In the sequence, the solution was transferred into a Teflon autoclave, which was sealed and placed inside a domestic microwave-solvothermal system (2.45 GHz, maximum power of 800 W). The microwaveassisted solvothermal process was performed at 140 1C for 10 min. The resulting precipitate was washed with deionized water and ethanol to remove byproducts possibly remaining in the final product and the precipitates were finally collected and dried at 70 1C overnight. The synthesis was also performed with the assistance of a modifier. For this we conducted the same procedure described above with the addition of 15.44 mmol of tetrabutylammonium hydroxide (40%) in solution 1.

Characterization
The obtained powders were structurally characterized by XRD using a Rigaku-DMax/2500PC with Cu Ka radiation (l = 1.5406 Å) in the 2y range from 101 to 751 with 0.021 min À1 . The phase analysis by the Rietveld method 44 was carried out using the General Structure Analysis System (GSAS) software. 45 The morphologies of ZnS powder were observed by FE-SEM using a FEG-VP JEOL. The compositional analysis as well as the mapping of the elements in the analyzed samples was performed by means of energy dispersive X-ray spectroscopy (EDXS). Optical properties were analyzed by means of UV-vis absorption spectroscopy using a Cary 5G spectrophotometer (Varian, USA) in diffuse reflection mode.

Computational details and model systems
All calculations were carried out using the Gaussian 09 program package. 46 The structures of the reactants, intermediates, and products formed during the growth process for the model systems have been fully optimized by TDDFT using the B3LYP functional 47,48 employing the standard all-electron 6-311+G(d,p) basis set to describe the atoms. No imaginary frequencies were found for the optimized geometries. The solvent effect was evaluated with utilization of a polarized continuum model (PCM), 49,50 using a dielectric constant of 41.4 to simulate the EG.
Two model systems, Zn 4 S 4 and Zn 6 S 6 clusters, have been selected to represent the cubic zinc blende and the hexagonal wurtzite, respectively (see Fig. 1). For the Zn 4 S 4 and Zn 6 S 6 clusters, the local structure, excitation energy and oscillator strength for electronic transitions from the ground to excited states have been obtained by means of TDDFT calculations using a 6-311+G(d,p) basis set. Molecular orbital (MO) pictures were prepared using the Gaussian View 2.1 package 46  contour value of 0.030 and projected density of states (DOS) was analyzed using GaussSum. 51 To understand the role of the modifier in the synthesis, i.e. tetrabutylammonium hydroxide, the interaction energy with the surface of both Zn 4 S 4 and Zn 6 S 6 clusters has been obtained, following eqn (1): where DE inter is the interaction energy, E A/B is the total energy calculated for the modifier molecule adsorbed on the surface of Zn 4 S 4 or Zn 6 S 6 clusters, E A is the energy of the modifier molecule and E B is the energy of the clusters.  80-20. 52,53 No other diffraction peaks are found, which indicates that the products are pure ZnS. The diffraction peaks are significantly broadened because of the very small crystallite size. The mean crystallite size, T, was calculated from the Scherrer equation (eqn (2)): 54

Results and discussion
where l is the K a radiation; y is the Bragg diffraction angle; and b is the width of the peak at half the maximum intensity in radians (Table 1). The strain and grain size of both samples were calculated by the Williamson-Hall (W-H) method, according to the following equation (eqn (3)). 55 In particular, the W-H analysis is a simplified integral breadth method where straininduced broadening arises from crystal imperfections and distortion in the lattice. [56][57][58] Thus, the full width at half maximum (FWHM) may be expressed in terms of strain (e) which is estimated from the slope of the line and the crystallite size (T) from the intersection with the vertical axis, and other parameters have the same meaning as in eqn (2). The W-H plot of ZnS nanoparticles obtained by the MAS method is shown in Fig. 3(b and d).
In Q4 order to prove that compounds obtained by the MAS method are pure and feature a single-phase the Rietveld refinement method was employed in this study, with the   specific objective of analyzing and understanding whether there are differences in the structural arrangements and determining the size of the particles of ZnS nanoparticles obtained by the MAS method (see Fig. 3(a and c)). In Rietveld analysis, fitting parameters (R WP , R p , R exp and w 2 ) indicate good agreement between refined and observed XRD patterns for the samples obtained by the MAS method (see Table 2). These phases corresponding to the different polymorphs of ZnS can be clearly identified in this case, it was noted that the lattice parameters and unit cell volumes obtained for both zinc blende and wurtzite of the ZnS structures are very close to those published in the literature. 58 Our results clearly correspond to two different growth rates for two different directions, implying an irregular spherical-like particle shape promoting a simple way for phase control of ZnS nanoparticles during processing under the MAS conditions; on the other hand, the reaction yields much more hexagonal phase than the cubic phase. These results indicate that the crystal   Fig. 2). The FE-SEM micrographs in Fig. 4 illustrate the influence of the different preparation conditions on the morphology and size distribution of the prepared samples. FE-SEM images show that the synthesis route produces quite similar ZnS crystalline agglomerate nanoparticles as can be observed on the ZnS particle facets, as shown in Fig. 4(A and B). The interface characteristics and growth are strongly driven by the chemistry of the surface, which in turn contributes to the phase stability. 17,[61][62][63] The microwave process leads to rapid formation of a high density of nucleation sites and the growth of the ZnS nanoparticles. These nanocrystals have a strongly polarized surface, because of a high concentration of short and intermediate-range defects. In previous studies, 9 we suggested that the growth of ZnS obtained by MAS can be described via a nucleation-dissolutionrecrystallization mechanism, and this mechanism is responsible for the fast nucleation of the ZnS small particles which aggregate into a spherical morphology to minimize their surface energy. In general, the nucleation-dissolution-recrystallization mechanism occurs using the MAS method, 9,38,40,64 which is considered highly sensitive to relative rates of amorphous solid particle dissolution and nucleation of the crystalline phase. 17,38,[64][65][66][67][68] As a consequence, this mechanism involves the formation of a high concentration of aggregated nanoparticles with predominant growth controlled by the coalescence process. 39 EDXS reveals that the products contain Zn and S which are in excellent agreement with the stoichiometry of ZnS, indicating the purity of the sample processed by the MAS method. The EDXS spectra are also very similar to the spectrum of ZnS published by Ludi et al. 69 In recent years, considerable effort has been devoted to the synthesis of nanocrystals prepared by a solvothermal process using various modifiers to control and induce the growth of nanocrystals. 39,[70][71][72][73][74][75][76][77][78] The modifier plays a different role in each type of synthesis, and their effects are not completely understood. The use of a modifier in the chemical synthesis of nanomaterials has been employed to obtain new shapes with different sizes, which promotes the formation of materials with different chemical behaviour. [76][77][78] According to Lamer and Dinegar, 79 the precursor conversion reactions that limit the crystallization determine the temporal evolution of monomer concentration as well as the steady state supersaturation during the growth phase. In our case, the [Zn(SH) 4 ] 2À complex in solution can be considered as the growth unit for the ZnS nanostructures. [80][81][82] Therefore, we propose a growth process of ZnS nanostructures as shown in Fig. 5, and to further understand the phase control, cubic zinc blende vs. hexagonal wurtzite, we use the Zn 4 S 4 and Zn 6 S 6 clusters as model systems (see Fig. 1). These clusters serve as a base for the growth of larger crystals, and have been experimentally characterized by mass spectrometry. 83 The Zn-S distances calculated are 2.38 and 2.32/2.42 Å for the cubic zinc blende Zn 4 S 4 and hexagonal wurtzite Zn 6 S 6 clusters, respectively, which are in good agreement with other theoretical studies. [83][84][85][86][87][88] It is important to note that the experimental values of the lattice parameters reported for the bulk ZnS structures exhibit Zn-S bonding distances that are quite similar for wurtzite and zinc blende (B2.34 Å), due to the great similarity in the local coordination of the tetrahedral [ZnS 4 ] cluster in the lattice. 89 The theoretical infrared-and Raman-active modes for the Zn 4 S 4 and Zn 6 S 6 clusters are shown in Fig. 6. An analysis of the results yields two and five active infrared lines, while four and seven active lines in the Raman spectra are found for the Zn 4 S 4 and Zn 6 S 6 clusters, respectively.
The condensation of these complexes occurs through a mechanism of nucleophilic substitution, in which two original zinc complexes or monomers are found in the solution and react to form a dimer, with elimination of H 2 S and form sulfur bridges (intermediate II) [Zn-S-Zn] with stronger chemical bonds. From this intermediate II, there are two possible reaction pathways: (i) the intramolecular cyclic rearrangement of this dimer to form a cyclic structure, intermediate III, or (ii) the formation of a trimer, intermediate IV. This second step can be associated with the phase control along the growth process of ZnS nanocrystals. We have estimated differences in Gibbs free energies to study the reaction mechanism. This, however, is a crude approximation because medium effects are not taken into account. An analysis of the results shows that the reaction pathway for the formation at the nanoscale of ZnS suggests that With respect to structures shown in Fig. 1, the Zn 4 S 4 cluster has only one type of Zn atom, while the hexagonal phase (Zn 6 S 6 cluster) contains two different Zn atoms in this cluster that can be thought of as the unit cell of a wurtzite structure. More recently, a benchmark data set using DFT and TDDFT of geometrical parameters, vibrational normal modes, and lowlying excitation energies for these clusters has been reported by Azpiroz et al. 88 The calculated values of the interaction energy of the modifier molecule, tetrabutylammonium hydroxide, on the surface  of Zn 6 S 6 (with the active sites A and B) and Zn 4 S 4 clusters and the corresponding optimized geometries are given in Table 3.
An analysis of the results shows that the value of the interaction energies of the modifier are energetically favorable, at the A (À87.27 kcal mol À1 ) and B (À94.23 kcal mol À1 ) sites which are more favorable in the Zn 6 S 6 cluster than in the Zn 4 S 4 cluster (À30.87 kcal mol À1 ). In particular, theoretical calculations on the relative stability of both phases of the bulk ZnS have been previously studied by Yeh et al. 90 This result can explain that the tetrabutylammonium hydroxide modifier favors the formation of the hexagonal wurtzite phase with respect to the cubic phase. According to the literature, 9,39 ZnS exhibits an optical absorption spectrum governed by direct electronic transitions. UV-Vis diffuse reflectance spectroscopy was used to determine the band gap of these materials in order to better understand the differences in ZnS nanostructures (see Fig. 7(A and B)). The Kubelka-Munk method based on diffuse reflectance spectroscopy was employed to determine the band gap of these materials. 9 UV-vis absorption measurements illustrate a variation in the optical band gap values from 3.40 to 3.59 eV for ZnS nanoparticles, for the zinc blende structure and wurtzite structure, respectively. After the electronic absorption process, electrons located in the maximum-energy states in the valence band revert to minimum-energy states in the conduction band under the same point in the Brillouin zone. 39 In particular, the band gap values obtained for the samples are much lower than the expected band gap values of 3.68 eV for the zinc blende and 3.77 eV for the wurtzite, respectively, for the bulk ZnS reported by Fang et al. 2 The exponential optical absorption edge and the optical band gap energy are controlled by the degree of structural disorder in the lattice. 9,39,40 The decrease in the band gap value can be attributed to defects and local bond distortion as well as intrinsic surface states and interfaces which yield localized electronic levels within the forbidden band gap, due to electron transitions from the valence band to the conduction band. Fig. 7(C and D) show PL evolution of ZnS samples synthesized by a MAS method with or without any modifier assistance at 140 1C. The PL spectra of ZnS samples present a broad band covering the visible electromagnetic spectra in the range from 400 to 800 nm, with maximum emission at 485 and 530 nm, for the hexagonal structures and cubic structures respectively, when excited by a 350.7 nm laser line. This behavior is due to changes in the shape, crystal size, structure and orientation of ZnS crystals. The emission band profile is typical of a multiphonon process; i.e., a system where relaxation occurs by several paths involving the participation of numerous states within the band gap of the material. 9,39 In this scenario, we have investigated the electronic structure of the cubic zinc blende Zn 4 S 4 and hexagonal wurtzite Zn 6 S 6 clusters using the frontier molecular orbitals obtained from TDDFT calculations. the framework of Koopmans' theorem 91 are 7.15 and 6.95 eV for the Zn 4 S 4 and Zn 6 S 6 clusters, respectively. An analysis of the DOS for the cubic Zn 4 S 4 and hexagonal Zn 6 S 6 clusters shows that the HOMO orbital consists mainly of S 3p orbitals whereas the LUMO is composed of Zn 4sp hybrid orbitals (see Fig. 8).

Conclusions
In summary, we have demonstrated phase control, mediated by the presence of tetrabutylammonium hydroxide, in the growth of ZnS crystals by using a cost effective MAS method, and a very moderate temperature (140 1C) and a very fast reaction time are sufficient to produce nanostructures with a good degree of crystallinity. XRD, FE-SEM, EDXS measurements and theoretical calculations were extensively employed to investigate structural and surface chemical compositions along the growth process of the synthesized nanostructures. Our results strongly suggest that the crystal phase of the prepared ZnS can be controlled by a modifier, i.e. tetrabutylammonium hydroxide and a new route has opened up for constructing novel nanostructures, which gives a better understanding of the control of ZnS nanostructures and their optical behaviour at the atomic-level. This finding offers new possibilities and shows that theory can be a suitable partner with experiments in developing and rationalizing these properties at the atomic level which is very important for progress in nanotechnology.