Critical Analysis of Phase Evolution, Morphological Control, Growth

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Review

Critical analysis of phase evolution, morphological control, growth mechanism and photophysical applications of ZnS nanostructures (0D-3D): A review Ashish Tiwari, and Sanjay J. Dhoble Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01463 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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Crystal Growth & Design

Critical analysis of phase evolution, morphological control, growth mechanism and photophysical applications of ZnS nanostructures (0D-3D): A review Ashish Tiwari a,*, S.J. Dhobleb a

Department of Chemistry, Government Lahiri College, Chirimiri 497449, India

b

Department of Physics, RTM Nagpur University, Nagpur, 440033, India

*Corresponding author – e-mail: [email protected] Abstract ZnS nanostructures are prominent and promising candidate of class II-IV semiconductor materials that can be prepared by sophisticated techniques. Transition of the material from bulk to nanosize brings forth drastic changes in various properties particularly the photophysical properties. In recent years the research areas have been focused in modifying and manipulating the morphologies of ZnS nanostructures for fabricating photocatalysts, photonic devices, biolabeling agent, optical sensors, detectors, and other novel applications. This review article addresses phase evolution (theoretical modeling approach and experimental validation), morphological control, growth mechanism based on thermodynamic considerations, surface energy driven models, kinematics, template directed growth etc. and understanding of the photophysical properties of ZnS based on the dimension of nanostructures (zero dimensional to three dimensional). A broad overview is presented for various synthesis techniques from the aspect of different morphologies (peculiar morphologies such as nanosaws, nanospines, nanoswords, nanocircles, cauliflower like structure etc.) and phase control of the nanostructures followed by discussion of the possible growth mechanism. Some of the novel techniques such as photochemical method, direct templating route, nucleation doping strategy have been included. The structural changes occurring with incorporation of various transition metal ions into ZnS host and the dependence of fascinating photophysical properties on the different reaction conditions and parameters along with recent advancement in their applications have been introduced in the later sections. The parameters have been discussed and analyzed for tuning the various luminescent properties based on dimension of the ZnS nanoparticles. We tried to summarize the current status of the research, discuss the issues and concerns in the present scenario and provide suggestions for further exploration in other potential directions.

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Introduction ZnS nanostructures have been extensively researched since 1994 after the pioneer work of Bhargava et al. where they reported that doped semiconductor nanocrystals can simultaneously yield high luminescent efficiencies and lifetime shortening. This study cascaded the investigation of new class of materials in the nano dimension range having altogether different property from its bulk counterpart.1 In last two decades the development in the field of ZnS nanostructures have been exponentially grown and the studies have stimulated enormous curiosity among scientists of various disciplines such as chemistry, physics and biologists because of their inexplicable optical properties and extensive novel applications, including optoelectronic devices such as LEDs, electroluminescence, laser technology, waveguides, sensors, photocatalysis, solar energy conversion, photo degradation of pollutants and biodevices.2 ZnS is chemically more stable and can act as an appropriate semiconductor host matrix for different dopant ions due to its wide energy band gap. ZnS is nontoxic and exists as two structural polymorphs, wurtzite (hexagonal W) and sphalerite (cubic/zinc blende ZB) having band gap of 3.77 eV and 3.72 eV respectively. Both polymorphs are having tetrahedral arrangements of Zn and S but differing in stacking sequence of atomic layers in cubic and hexagonal structures. The wurtzite phase is thermodynamically metastable and forms at higher temperature than sphalerite. It is noteworthy that wurtzite ZnS is more expedient than the sphalerite phase with respect to the optical properties. Hence, it is essential to carefully monitor the synthesis conditions so that wurtzite ZnS can be protected from entities that would induce phase transformation. Reducing the size of the particle result in significant increase in surface-to-volume ratio therefore surface energy greatly affects the phase stability of the sphalerite and wurtzite polymorphs. The surface energy and subsequently the crystal structure of the ZnS nanoparticles strongly depend on synthesis conditions. In recent years, the material scientists are interested in self-assembly and organization of nanostructures into complex structures (higher order transformation) for nanodevice fabrication. The morphologies of ZnS that can be fabricated in nanoscale/nanodimension have been probably the most exuberant amongst other inorganic semiconductors. Till now, the related research has made great progress and excellent reviews have been done by Fang et al.3, 4 ( that embraced extensive study based on synthesis-property-application of diverse range of ZnS nanostructures and also on ZnS


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nanoarrays) but novel developments in this field have not been summarized after 2011 thus we have tried to cover literature till September 2016. We think it is appropriate to provide this review to further incite and motivate researchers to develop novel ZnS nanostructures for exploring their new applications. In this review article we have tried to establish relationship and interdependence, between size, morphology and photophysical properties of ZnS nanostructures based on the concept of Nanoarchitectonics i.e. arranging nanoscale structural units in a required configuration.5 The article emphasizes on design, synthesis, fabrication and functionalization of ZnS nanostructures according to their dimensionalities. This review article is certainly not an exhaustive review of all ZnS based nanostructures that have been so far reported rather we have tried to focus on highlighting characteristic features involved in their formation. It offers a systematic and extensive report regarding the concerns on tuning the dimensionality and phase transformation. Meanwhile some novel synthesis techniques have been highlighted for their merits of facile production, energy conservation, and green fabrication. The article describes favorable conditions required for phase stability, novel and unconventional synthetic techniques have been accounted, more complex hierarchical structures and peculiar morphologies such as nanosaws, nanospines, nanoswords, and nanocircles have also been included. Growth mechanisms that dominate structural transformations of these nanostructures and parameters that affect the photophysical properties and recent applications in various fields of science and technology were delicately summarized. Inorganic-organic hybrid ZnS nanostructures have not been included in the article as they have been described as ZnS nanocomposites elsewhere.6 Dimensionalities of ZnS nanostructures Dimensionalities of the nanostructures are viable from theoretical as well as experimental point of view in current research and the properties of nanomaterials are strongly dependent on it. Precisely ZnS nanostructures can be broadly categorized as (i) 0D, (ii) 1D, (iii) 2D and (iv) 3D nanomaterials. Although several methods have been reported for the fabrication of these nanostructures but the major challenge is to control their assembly into precise and predictable dimensioned nanoarchitectures. Zero dimensional (0D) ZnS nanostructures comprises of very small objects which are not having large anisotropic aspect ratios. It might include quantum dots, nanocrystals,



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nanoparticles, and nanospheres. It is difficult to differentiate clearly 3D ZnS nanostructures from 0D. One dimensional (1D) ZnS nanostructures include nanorods, nanowires, nanofibers, nanowhiskers, and nanotubes. Two dimensional (2D) ZnS nanostructures comprise of nanosheet, nanobelts, platelets, diskettes and thin films. The novel and exotic properties of theses nanostructures make them a potential candidate in various practical applications. Three dimensional (3D) ZnS nanostructures can include mesoporous structures, core shell nanoparticles, hollow nanospheres, nanopods (tetrapods), nanoflowers, hierarchical nanostructures etc. Figure 1 depicts the electron micrographs of several ZnS nanostructures and the variety of nanostructures (0D to 3D) can be obtained by tuning the reaction conditions.

Figure 1. The electron micrographs of the various morphologies of ZnS nanostructures a. quantum dots, b. nanowires/nanorods, c. nanosheets, d. nanosaws, e & f nanotubes, g. coreshell nanoparticles, h. nanotetrapods, i. T-shaped nanostructures, j. nanoflowers, k. nanospindles, l. hierarchical nanostructures. Phase Evolution The thermodynamic stability, depression in the melting point temperature and other properties of materials changes drastically with size of the nanostructures which is accompanied by structural transformations at lower temperatures. There can be several


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parameters that can control structural transformation such as aggregation, interfacial energies surfactants, solvents, templates etc. As far as phase stability is concerned ZnS nanostructures are indispensably unstable and can suffer reversible and irreversible post-fabrication. Moreover solid–solid phase transformations are susceptible to alteration in the thermal and chemical environment.7 Computational approaches, simulations and advanced theoretical modeling were used by several workers to explore the analogy between size and shape that determine the crystallinity of ZnS nanostructures. An excellent review on the use of computational techniques in modeling and predicting possible structures for ZnS has been done by Hamad et al.8 Barnard et al. reported that morphology of sphalerite ZnS nanoparticles, core–shell structure and mixing ratio of crystalline/amorphous particles significantly influence their thermodynamic stability.9 The findings indicated that the lowest energy shape of zinc blende nanoparticles is a rhombic dodecahedron, enclosed by stoichiometric (non-polar) {220} facets and if higher energy (polar) facets are introduced it can bring about size-dependent transformation between ZB, amorphous and core–shell crystalline/amorphous nanoparticles. Zhang et al. employed molecular dynamics (MD) simulations at 300 K with periodic boundary conditions and determined structures having minimum energy surface.10 The studies indicated that in vacuum small sized ZnS nanoparticles were thermodynamically more stable in the wurtzite phase as compared to the sphalerite phase. It was observed that as the average particle size reduced to ~ 20 nm the phase transition temperature decreases significantly. Moreover adsorption of water molecules onto ZnS nanoparticles resulted in more stability of sphalerite over wurtzite. This theoretical assumption is well supported by experimental synthesis of ZnS hierarchical nanostructure by Silambarasan et al.11 They reported the formation of wurtzite crystal structure at 70 °C and concluded that the reduction in size of the nanoparticles alters the surface energy and consequently reduces the phase transformation temperature. Moreover L-threonine (surface passivating molecule) adsorbed on the surface of ZnS helped effectively in decreasing the phase transformation temperature. Porta et al. reported that the phase purity of ZnS can be modified by suitable adjustment in experimental conditions influenced by the presence of tetrabutylammonium hydroxide.12 Recently Khalkhali et al. studied structural and configurational evolutions of ZnS NPs using MD methods.13 The results obtained from simulation experiments revealed that when the



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particle size of relaxed ZnS NPs exceeds 3 nm, three well defined regions can be identified viz. a crystalline core, a distorted network of 4-coordinated atoms environing the crystalline core, and surface structure consisting of a network of 3-coordinated atoms. Based on different structural analyses (radial distribution function (RDF), angular distribution (AD), Honeycutt-Andersen indices, root mean square displacement (RMSD) and coordination number (CN) calculations) it was established that stability and size of these regions were highly dependent on the crystal structure and size of the ZnS NPs. Very recently Khalkhali et al. have utilized different structural analyses(radial distribution function, angular distribution, tetrahedral order parameter, and root-mean-square displacement) and compared the structural evolution of bare and hydrated ZnS NPs having diameter in the range of 1-5 nm.14 The study revealed that the surface of hydrated nanoparticles is more stressed as compared to the bare nanoparticles and previously observed three phase structure of bare nanoparticles is not formed in the hydrated state and this under-coordination is compensated by adsorption of water molecules in hydrated nanoparticles. The polarity and the dipole moment also significantly changes in hydrated state. Feigl et al. reported that rapid phase prediction could be achieved through ab inito calculation of the thermodynamic properties, equilibrium morphologies, subsequent mapping of these morphologies and their response with thermodynamical factors like size, temperature and pressure and different chemical regimes.15 In an another article Feigl et al. employed DFT calculation with a shape-dependent thermodynamic modeling technique based on the Gibbs free energy (G) of a nanoparticle, to explore the relationship between size, shape and stability of wurtzite ZnS nanostructures.16 The study indicated that wurtzite ZnS nanostructures exhibit a preference toward anisotropy and nanorods are thermodynamically preferred over quasi-spherical nanoparticles. In a similar investigation Feigl et al. found nonequilibrium structures can be formed in S-rich environment and encouraging Znterminated surfaces to form will increase the influence of thermodynamics over morphological selectivity, resulting in formation of low energy equilibrium or quasiequilibrium arrangement.17 The study highlights the tailoring of experimental conditions to target specific needs. These theoretical studies suggest that role of supersaturation of sulphur on the shape of these particles along with temperature can be introduced in the model as a “tunable” parameter for optimizing the shape of the particles under various conditions.


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Changing the thermochemical conditions and comparing metastable shapes it is possible to get a more realistic sampling of the configuration space and observe the impact of different experimental conditions. New thermodynamical models (using ab initio thermodynamics) can be built by taking the ground state energies and more complex chemical reservoir such as water or any other relevant precursor or initiator can be included. This will open up doors to extend the studies to have new DFT surface energies for surfaces passivated with different molecules. Chen et al. reported that solvothermal synthesis of ZnS:Mn2+ nanowires prepared in the strong basic condition (pH-10.2) adopt the wurtzite phase at lower temperature.18 The influences of temperature, reaction time, concentration of precursors and pH on size and phase constitution of ZnS nanoparticles were investigated by Zhang et al.19 Precipitation kinetics plays an important role in stacking. Factors and synthetic conditions that promote slow precipitation favors growth of wurtzite phase while fast precipitation favors growth of sphalerite phase. They suggested that nano-wurtzite is more stable than nano-sphalerite at all pH conditions investigated. Datta et al. reported that the stability and phase transformation in ZnS nanocrystallites can be accomplished by adjusting the concentration of Cu ions which modifies the surface energy.20 Wang et al. reported phase transformation in ZnS nanobelts, based on the results of in situ high-pressure synchrotron X-ray-diffraction measurements, theoretical calculations and HRTEM investigations.21 They suggested that the particular morphology of wurtzite ZnS nanobelts depicts the most-favorable low-energy surface structure, which predominantly control the formation and structural stability of nanobelts. They emphasized that certain morphology increases the stability of metastable phases by altering the surface-energy density. The presence of organic solvent molecules assist in phase transformation by lowering the difference in free energy of formation between the wurtzite and sphalerite phase of ZnS. Acharya et al. reported probably for the first time, phase transition of ZnS from zinc blende to wurtzite at 170 °C in the presence of ethylenediamine (EN).22 In most of the reports on the synthesis of ZnS using surfactant molecules either sphalerite phase was obtained or a complex intermediate phase was formed which required high annealing temperature for phase transformation into wurtzite. This method has an advantage that phase transformation can be easily tuned with respect to the concentration of EN. DFT calculations done by them supported the structural and morphological



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transformation of ZnS based on the concept of surfactant mediated transformation. The calculation showed that when N atoms (provided by amine moiety of EN) passivated the surface atoms of ZnS, a rodlike geometry having wurtzite as the preferred structure is obtained. Kole et al. reported a controllable synthesis of several nanostructures of ZnS and synthesized direct phase pure wurtzite ZnS nanostructures by hydrothermal method at 220 °C.23 In this method post annealing treatment or post hydrothermal treatment was not needed to eliminate the en molecules. It was found that at 1: 3 molar ratio of Zn:S precursors, pure WZ ZnS beltlike structures were obtained having growth along the [001] directions. Ding et al. reported the synthesis of ZnS nanobelts through a vapor-liquid-solid (VLS) process and reported that by adjusting the synthesis conditions, the phase of ZnS nanostructures can be controlled.24 The Au catalyst particles having dimension less than 50 nm and low deposition temperature of 680-750 °C favored the formation sphalerite ZnS nanobelts. Wurtzite ZnS nanobelts were deposited at high temperature ca. 750 °C and unaffected by particle size of Au catalyst. Several studies have been made that reported the lowering of phase transition temperature from the sphalerite to the wurtzite phase for ZnS nanostructures using surfactants. Deng et al. prepared well crystallised wurtzite ZnS by annealing the precursors, which were synthesized by forming intercalates between ethylenediamine and ZnS crystallites solvothermally in an inert atmosphere at 350 °C.25 Wurtzite-type ZnS nanobelts decorated with a ZnS– diethylenetriamine (DETA) hybrid intermediate were synthesized by Yao et al. solvothermal reaction.26 The findings indicated that the volume ratio of DETA : water significantly affected the structure, composition, shape, and phase evolution of the samples. The concentration of precursor solvent strongly influenced the phase of ZnS nanostructures as reported by Li et al.27 They synthesized shape controlled ZnS nanocrystals by thermolysis of single-source precursor zinc ethylxanthate (Zn(exan)2) with octylamine (OA) or trioctylphosphine (TOP) as precursor solvent. It was found that there was simultaneous phase transformation from wurtzite to sphalerite as the concentration of TOP was increased in the solution. Tiwary et al. reported the phase transformation in ZnS at nanoscale by applying “top to down” approach of reducing particle size through mechanical impact using a high energy ball mill.28 The report suggested that high ball to power ratio yields high pressure along with a localized temperature rise that influences and promotes sphalerite to wurtzite


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phase transformation. Hou and Gao reported a solvothermal method to synthesize sphalerite heart-like ZnS nanoparticles and the wurtzite ZnS nanospheres by using sodium dodecyl benzene sulfonate (SDBS) and alginic acid, respectively. It is probably the first report of phase transformation or control of the phase crystallinity of ZnS by modifying the additives.29 Tong et al. reported L-Cysteine assisted synthesis of ZnS nanostructured spheres assembled from ZnS nanocrystals with the controllable crystal phase and morphology.30 LCysteine as a sulfur source significantly contributed in the formation of ZnS nanospheres. ZnS nanostructured spheres congregated from various ZnS nanocrystals, such as nanosheets, QDs, nanorods, and multimorphology nanocrystals (as shown in figure 2).

Figure 2. TEM micrographs of ZnS nanostructures (NSs). (a) ZnS NSs at low magnification; (b) a single sphere; (c) HRTEM image on the edge of a sphere. (d) ZnS NSs at low magnification; (e) a single sphere; (f) edge of a sphere at higher magnification. (g) dispersed multimorphological ZnS nanocrystals at low magnification; (h) HRTEM image of a single spherical nanocrystal of sphalerite ZnS; (i) HRTEM image of a single ZnS nanocrystal in which the phase transformation from sphalerite to wurtzite is not complete; (j) HRTEM image of a single nanorod of wurtzite ZnS. Reproduced with permission from ref. 30. Copyright 2007 American Chemical Society. Zong et al. reported that, solid–solid phase transition (depending on the size of nanocrystal) is associated with increase of the surface energy.31 Nanocrystals having higher surface



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energies would contribute to increase the phase transition pressures (as shown in figure 3), since a higher pressure is needed in overcoming the extra surface energy.

Figure 3. X-ray diffraction patterns collected at various pressures for different Mn2+ doped ZnS samples. The peaks derived from the gasket are distinguished from reflections of Zn1−xMnxS by using ‘‘G’’ markings. (a) X-ray diffraction patterns for the sample doped with 0.85% mole per cent of Mn2+ content. The right inset shows X-ray diffraction patterns collected at 17.7 GPa pressures for the sample doped with 0.85% mole per cent of Mn2+ content. (b) X-ray diffraction patterns for the sample doped with 1.26% mole per cent of Mn2+ content. The right inset shows X-ray diffraction patterns collected at 18.3 GPa pressures for the sample doped with 1.26% mole per cent of Mn2+ content. Reproduced with permission from ref. 31. Copyright 2011 Elsevier Ltd. Synnot et al. reported a microwave assisted synthesis of ZnS-Ag-In NPs and found that addition of Ag at an optimum concentration of >4% can bring about phase transition from sphalerite to wurtzite. The addition of these elements has different roles in modifying the properties of ZnS.32 ‘In’ serves to enhance the photocatalytic activity by virtue of its n- type doping and creation of additional S-defects and metallic silver particles formed during the microwave irradiation act on the surface of the ZnS NPs due to which the surface energy is significantly affected and subsequently, leads to phase transition. Novel Synthesis techniques Recently several novel methods have applied to synthesize ZnS nanostructures in different dimensions. Some of the state of the art techniques have been discussed below. Microwave assisted methods Microwave assisted solvothermal (MAS) method has evolved as a new technique for fabricating nanostructures. The obvious merits of the technique are kinetic enhancement, low


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synthesis temperature, less time consumption, particle size control and aggregation process.33 Porta et al. synthesized ZnS nanoparticles by MAS method at 140°C. They investigated the effect of the precursors (chloride, nitrate and acetate of Zn2+) on the structure, surface chemical composition and optical properties of ZnS nanoparticles.34 Yao et al. have synthesized ZnS nanospheres by microwave irradiation and reported that no template agent was required for spontaneous assembling of ZnS nanoparticles into a hierarchical structure.35 Saravanan et al. studied the synthesis of sphalerite ZnS quantum dots.36 ZnS nanoparticles doped with Ag+, Cu2+, Ce3+ and Sn4+ were synthesized via a microwave-assisted heating process by Yang et al.37 The study indicated that the irradiation time does not significantly influence on particle size of ZnS nanoparticles. Lu et al. employed zinc 2-ethylhexanoate as a novel zinc precursor to synthesize ZnS:Mn2+ nanoparticles.38 Electrostatic repulsion due to anions bounded on the surface of the nanoparticles hindered coalescence of particles and hence homogenous, well dispersed nanoparticles were formed having average size of 3 nm. Poormohammadi-Ahandani and Habibi-Yangjeh prepared nearly spherical Cu-doped ZnS nanoparticles in water using microwave irradiation.39 It is a template free method to produce nanoparticle and the size of the as prepared ZnS NPs was found to be dependent on Cu2+ content. Similarly a template-free, microwave-irradiation-assisted growth of ZnS nanorods has been described by Limaye et al.40 This is probably the first report on the synthesis of ZnS nanorods using microwave irradiation. ZnS nanorods having a high aspect ratio with dimensions of ∼50 nm diameter and more than 1 µm in length were obtained. Melt salt synthesis method The composite molten salt method is a novel approach for synthesizing nanostructures, having merits such as being single step, easy scale-up, cheap. Li et al. were first to report the synthesis of ZnS nanoparticles involving self-assembly of ultrafine particles (2–6 nm) by the composite molten salt method.41 The synthesis of ZnS nanoparticles involved the reaction between a Zn(NO3)2 and Na2S in a solution of eutectic composite salts at 200 °C and ambient pressure, without employing any organic dispersing agent or capping molecule. ZnS nanoparticles having hexagonal prism morphology had been synthesized by molten-salt method by Liu et al. by heating ZnS nanoparticles and NaCl powder at 800 ℃.42 Side length and height of the obtained products were between 50-100 nm and 50-200 nm respectively. This method was not extensively applied for synthesis as energy consumption requires 200℃ or even 800℃, which is not so eco-



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friendly. Besides, the size distribution of products is always not so uniform as compared to wet chemical methods. Therefore, a proper modification on evaluation of this method is necessary. Spray Pyrolysis Spray pyrolysis is an inexpensive, simple and robust synthetic method that involves a droplet-toparticle conversion process and has been used to manufacture a variety of nanostructures. Okuyama et al. synthesized ZnS fine particles by an ultrasonic spray-pyrolysis method at 600 °C.43 It was found that by changing the concentration of the metal nitrates in the starting aqueous solution particle size can be varied. Nanosized zinc sulfide particles of 20-40 nm in diameter has been synthesized by Lengorro et al. using electrospray pyrolysis.44 ZnS:Ni2+ hollow microspheres and nanoparticles were synthesized by Bang et al. by using USP technique where facile control of morphology can be achieved by changing reaction temperatures.45 Liu et al. employed the droplet to vapor to particle conversion process and synthesized for the first time sub 10 nm ZnS nanoparticles that have not been previously reported by spray pyrolysis technique.46 It was found that factors such temperature and residence time controlled the small size of the particles (2–7 nm). Nemade and Waghuley recently synthesized ZnS nanoparticles having ~8 nm size by a flame-assisted spray pyrolysis route for studying the LPG sensing response of ZnS nanoparticles at room temperature.47 Solvothermal method Solvothermal method has been utilized as the most successful solution techniques for the synthesizing nanostructures with controlled morphologies. The merits of this method are low cost, energy efficient, low temperature processing, mild synthesis conditions, high yield and phase purity. The morphology of the nanostructures can be varied by tuning the experimental parameters such as precursors, reaction temperature, reaction time and the solvent.48 This approach is much more simpler than those based on solid phases e.g. CVD. Meanwhile there could be issues regarding the crystallinity of the products which might not be good as compared to solid reaction methods. However by improving the solvothermal conditions (like high pressure) good results can be obtained. ZnS nanoparticles having less than 3 nm average diameters were synthesized by Li et al. via solvothermal reaction at 120°C.49 they reported that size of the as prepared nanocrystals can be changed by adjusting the reaction temperature and time.


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Zhou et al. synthesized wurtzite ZnS by solvothermal process between 160-200 °C.50 Very thin plates having square- or rectangle like morphology (2D nanosheets) were obtained (as shown in figure 4) having lateral dimensions in the range of 1-2 µm. The mild nature (low temperature) of the synthesis method resulted in ZnS nanostructures having no striated fringes and well-resolved 2D lattice planes.

Figure 4. TEM image of ZnS thin plates with square- or rectangle like morphology. Reproduced with permission from ref. 50 . Copyright 2005 American Chemical Society. Jiang et al. solvothermally synthesized phase- and shape controlled ZnS nano- or microstructures using Zn(NCS)2(C5H5N)2 as single precursor. The products having multimorphologies (nanospheres, microflowers, and microspheres) were obtained by using different solvents and tuning other experimental conditions.51 The quasi spheres having diameter of ~80 nm were obtained. It was interesting to note that pure wurtzite form was formed when the temperature was quenched to RT after solvothermal synthesis. Dong et al. used oleic acid for stabilizing solvothermally synthesized ZnS:Mn nanocrystals.52 The results indicated that the crystallinity of ZnS: Mn doped nanocrystals strongly depended on the growth temperature. A thio Schiff-base-assisted solvothermal process had been reported by Shakouri-Arani et al. to synthesize ZnS nanoplates (without using any surface active agent) mediated by propylene glycol at 140 °C as shown in figure 5.53 They found that, phase crystallinity changed from wurtzite to sphalerite as the solvent was changed from propylene glycol to 1-butanol.



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Figure 5. SEM of ZnS at 120 C shows large number of quasi hexagonal plates. Reproduced with permission from ref. 53. Copyright 2013 The Korean Society of Industrial and Engineering Chemistry. Kaur et al. reported the formation ZnS nanocrystals by solvothermal method using 4,4′dibenzyldisulfide (DBDS = (C7H7)2S2)) as a new temperature controlled in-situ source of S2− ions organosulfur compounds at 220 °C.54 The results indicated formation of microspheres consisting of pure sphalerite ZnS NCs of 3–6 nm size without the using any template/capping agent. Mendil et al. prepared very recently different ZnS nanostructures by solvothermal synthesis.55 They reported solvent induced phase transformation from sphalerite to wurtzite without affecting other reaction conditions such as temperature, process time etc. Different concentrations of solvent mixture consisting of ethylenediamine (EN) and distilled water (W) were used. The results indicated that at higher EN concentration (75%-100%) pure wurtzite ZnS were formed. The material scientists are aiming to synthesize multimorphological nanostructures within one rationally designed system. Very recently Zhang et al. solvothermally synthesized a series of nanostructures that included nanoparticles (0D), ultrathin nanobelts (2D) and branched nanotetrapods (3D) by judiciously controlling the concentration of EN.56 The role of EN in multimorphological and anisotropic growth of these nanostructures is multifunctional i.e. it acted as a ligand and complexing agent both. This approach can open doors for controlled synthesis of other II–VI nanostructures in which EN can be replaced by short carbon-chain amine agents. Multimorphological ZnS nanostructures including nanoparticles, nanobelts, single-crystal nanowires and layered ZnS(EN)0.5 hybrid structures were solvothermally synthesized very recently by Han et al.57 The study indicated that EN modulates the growth pattern by acting as structure-directing and surface-passivating coordination agent and hence controlling the phases and morphologies. The addition of EN after formation of ZnS precursors lead to the anisotropic growth along the [001] axis of the ZnS clusters transforming it to 1D wurtzite nanowires. But adding EN before the formation of the ZnS clusters resulted in formation of layered ZnS (EN) 0.5


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hybrid structures (EN serving as the bridging blocks) through the connection of wurtzite ZnS (110) slabs. Solid State method Ngo utilized solid-state reaction method to prepare the doped and undoped ZnS polycrystallites.58 They used surfactant SPAN-80 as capping agent but it was involved only in emulsifying process. This approach is different as compared to wet chemical method where the surfactant is directly involved in the synthesis. It stimulated about 20-times stronger photoemission with narrow full- width. Wurtzite ZnS complex spheres assembled with branched nanorods solvothermally synthesized at 110°C has been reported by Chai et al.59 The results indicated the formation of urchin- like structures consisting of nanorods and spheres assembled with branched nanorods as depicted in figure 6. The dimension of the trunk and branches were 10 nm and 6 nm respectively.

Figure 6. (a and b) FESEM and (c) TEM images of the sample obtained at 110 °C for 45 min. (d and e) TEM images of the sample obtained at 110 °C for 1.5h. Reproduced with permission from ref. 59. Copyright 2014 Elsevier B.V. Hydrothermal Method The hydrothermal method for nanostructure synthesis is of considerable interest for practical applications. It is based on the fact that water and aqueous solutions can dissolve/dilute practically insoluble materials at high temperature and pressure. It is an environmentally friendly



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technique and can be utilized to tailor nanostructures without using other structure directing agents. Thioglycolic acid (TGA)-assisted hydrothermal method to fabricate well-crystallized ZnS nanoclusters has been described by Salavati-Niasari et al.60 They used a new precursor, bis(salicylaldiminato)zinc(II); [Zn(sal)2] and reacted it thioacetamide (CH3CSNH2) for synthesizing ZnS nanoclusters having spherical morphology and diameter ranging from 50 to about 150 nm. Kim et al. reported anionic precursor-mediated morphology-controlled synthesis of ZnS nanostructures without using organic surfactants via a hydrothermal method. Cauliflower architectures comprising of many small ZnS spheres were formed (shown in figure 7) when sodium sulphide was used as sulphide source but in case of thioacetamide as sulphide source, hedge apple-shaped nanostructures were obtained comprising of radially arranged nanocubes.61

Figure 7. FE-SEM images of the products obtained with different anionic precursors (a) cauliflower like (b) rice grain (c) hedge apple like (d) broccoli-like nanostructures. Reproduced with permission from ref. 61. Copyright 2016 Elsevier B.V. ZnS:Mn nanoparticles were synthesized by EDTA-assisted hydrothermal method by Vishwanath et al.62 The nanocrystals were of sphalerite phase, nearly spherical, homogenous and having diameter of ~4nm. Chen et al. hydrothermally synthesized wurtzite ZnS nanorods at180 °C mediated by ethylenediamine (EN). They also investigated the effects of various parameters like concentrations of solvent and reactant, synthesis temperature, and reaction times on crystallinity and morphologies of ZnS nanostructures. The study not only reveals the phase evolution of ZnS from wurtzite to sphalerite system but also morphological transformation of nanorods to nanograins.63 Hydrothermal method was utilized for structural transformation of fluffy nanostructures to ZnS:Mn2+ microtubes by Chen et al.64 They investigated the morphology


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evolution process from nanowires to fluffy structures and finally to ZnS:Mn2+ microtubes through time-dependent experiments by prolonging the reaction time gradually from 2 to 8 hours at 100 °C. Ethylenediamine plays a decisive role in phase control of the nanostructures. It was revealed by X-ray diffraction patterns that wurtzite phase of the product was obtained in the presence of ethylenediamine while in the absence of EN sphalerite phase was formed. ZnS nanoparticles and nanorods having single-phase were hydrothermally synthesized by Zhou et al. 65

The ZnS nanostructures so obtained were having microstructural defects like stacking faults

(SFs) and twin boundaries in some ZB ZnS nanoparticles having dimension of ~5 nm and SFs, ZB/WZ ZnS nanotwins in ZnS nanorods. Highly ordered ZnS nanowire arrays were synthesized by Dai et al. using AAO template with double diffusion through hydrothermal method.66 Homogenous ZnS nanowires (diameter of about 60 nm) were synthesized in the holes of the AAO template as depicted in figure 8.



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Figure 8 (a) SEM image of AAO template (b) SEM images of ZnS nanowires embedded in AAO (c) and(d) SEM images of ZnS nanowires (e)TEM and HRTEM images of ZnS nanowires. Reproduced with permission from ref. 66. Copyright 2013 Elsevier B.V. Nucleation-doping strategy In the traditional method for synthesis of metal ion doped ZnS QDs, the dopant ions and competitive host ions were added in the same reaction system and hence cannot be controlled and optimized easily. In order to overcome this issue, a new synthetic method was developed, termed as nucleation-doping strategy, to prepare semiconductor quantum dots through organometallic routes.67 The characteristic of the nucleation-doping strategy was decoupling the doping from nucleation, thus allowing the growth of QDs in a controlled manner. The products


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so obtained were having pure dopant emission with high efficiency. Xiao et al. synthesized ZnS : Mn/ZnS (core/shell) QDs in aqueous solution having uniform spherical shape and reported that additional growth ZnS shell on ZnS:Mn QDs enhances the emission intensity of Mn2+ in Mn:ZnS/ZnS core/shell QDs as surface defects were eliminated.68 Yu et al. studied the influence of the synthesis conditions such as different Mn2+ concentrations, molar ratio of TGA: (Zn+Mn) and thickness of the ZnS shell on the luminescent properties of thioglycollic acid stabilized ZnS:Mn2+/ZnS QDs by nucleation drop strategy.69 Results indicated that the ZnS:Mn2+/ZnS QDs were water-soluble having improved fluorescence properties. Xu et al. synthesized Mn-doped ZnxCd1−xS nanocrystals having a thin ZnS layer on the MnS core followed by additional growth of a CdS layer having variable thicknesses using nucleation-doping strategy.70 They reported an enhancement of PL quantum yield by 29%. Liquid-solid-solution technique (LSS) In this system three phases are formed viz. sodium linoleate (solid), linoleic acid (liquid), and the aqueous metal ions solution.71 A spontaneous phase transfer of the metal ions (based on ion exchange) occurs across the interface of sodium linoleate (solid) and the aqueous solution (solution). This subsequently results in the formation of metal linoleate whereas sodium ions enter into the aqueous phase. Son et al. used a liquid-solid-solution LSS technique for the preparation of homogenous ZnS:Mn nanocrystals having a diameter of 7.3±0.7 nm.72 Photoluminescence enhancement of ZnS:Mn nanoparticles synthesized by a liquid-solid-solution method was reported by Jung et al. It was found that an effective passivation layer was formed by lithium and hence suppressing the nonradiative recombination transitions.73 Photochemical method Photochemistry can also allow the production of ZnS nanostructures at room temperature has been recently reported by Gonzalez et al.74 The hydrogen transfer photochemical reaction between acetone and isopropanol generated 2-hydroxyl-isopropyl radicals which were used as reducing agents to synthesize ZnS QDs. It is the first report on room temperature synthesis of wurtzite ZnS QDs involving ketyl radicals. ZnS-amorphous carbon nanotubes (ACNTs/ZnS) composites was photochemically synthesized by Fang et al.75 Single Source Precursor Method A single-source precursor is ideal for synthesis of ZnS nanostructures as it is easier to control the



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stoichiometry and undesirable side reactions can be restricted. The desired final product is of high purity and residues if obtained can be easily removed as gaseous by-products or removed by washing with solvents. Zn(S2CNEt2)2 molecular precursors were utilized for fabricating singlecrystal ZnS NWs by Barrelet et al.76 Maji et al. reported the formation of mesoporous zinc sulphide formed via a single precursor Zn(SOCCH3)2Lut2 complex having average particle size of 5nm and diameter of pore ~ 4.7 nm.77 Palve et al. prepared ZnS nanocrystallites by pyrolysis of

Zn(cinnamtscz)2

and

ZnCl2(cinnamtsczH)2

(cinnamtsczH

=

cinnamaldehydethiosemicarbazone) as single source precursors at 515 °C.78 It was found that on pyrolysis of Zn(cinnamtscz)2 ZB ZnS, was obtained while WZ ZnS was obtained from pyrolysis of ZnCl2 (cinnamtsczH)2. Sun et al. described a low temperature (110 °C) synthesis of wurtzite ZnS nanoparticles (NPs) from zinc diethyldithiocarbamate (Zn(DDTC)2) as single-source molecular precursor using microwave-assisted technique.79 A number of factors such as proper solvent, MW irradiation power and single-source precursor contributed in rapid and low temperature synthesis of WZ ZnS NPs. Han et al.

reported the synthesis of ZnS hollow or solid spheres and polyhedral ZnS

nanoparticles from the precursor (Zn(C2H5OCS2)2).80 The report suggested that in similar reaction conditions the yield of products using EG or water as solvent is very low as compared to the yield obtained using dimethylformamide (DMF). DMF possesses hydrophobic and hydrophilic groups, which readily dissolves Zn(S2COC2H5)2 and hence provide a proper chemical environment resulting in formation of ZnS hollow spheres. Ultrasonication Xu et al. reported ultrasonic irradiation method to prepare ZnS nanoparticles without any stabilization with organic materials.81 The authors claimed that products obtained were of the crystalline state as compared to the glassy structures which generally tend to precipitate in wet chemical method. Zhao et al. reported the synthesis of different morphologies like submicrospheres, nanosheets and nanorods of ZnS nanostructures using a single block copolymer by solution method.82 ZnS submicrospheres were obtained by autoclaving at 105 °C, ZnS nanosheets were formed by ultrasonication followed by leaving the reaction mixture at room temperature where as long-time continuous ultrasonication and autoclaving at 105 °C resulted in the formation of homogenous ZnS nanorods. Senthilkumaar et al. synthesized undoped and ZnS:Mn nanorods capped with mercaptoethanol using ultrasonication.83 The obtained products


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were having wurtzite structure and diameters of the nanorods were in the range of 20-50 nm. They suggested that growth of the nanorods can be achieved by suitably adjusting reaction condition such as addition of precursor, reaction temperature and reaction time. Sen et al. synthesized core-shell type manganese-doped zinc sulphide nanoparticles (ZnS:Mn/ZnS) having average size of ~2.3 nm by a low-temperature ultrasound controlled wet chemical route.84 Goharshadi et al. applied ultrasound irradiation assisted by different ionic liquids (ILs) of bis (trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium to produce extremely small ZnS NPs having diameter of 1 nm without utilizing any surfactant.85 It was found that average crystallite size of ZnS NPs remained same when ILs were used suggesting that sonication has greater effect than ILs. Korotchenkov et al. reported the synthesis of ZnS:Mn nanoparticles by a sonochemical (SCH) process and compared it with results obtained by synthesis of ZnS:Mn nanoparticles by a chemical (CH) process.86 The size, crystallinity and morphology of the NPs were found to be dependent on the sonication parameters like power, temperature of solution etc. The average particle size was estimated to be 3.7 ±1.5 nm by chemically synthesized method and 6.5 ± 3.5 nm by sonochemical method. Zhu et al. reported one-step synthesis of well defined mesoporous sphalerite ZnS having high specific surface area via sonochemistry-assisted salt-extraction (SASE) route.87 In this method NaNO3 precipitated along with ZnS inhibited the growth of the ZnS nanoparticles and wormhole-like mesopores were obtained. Very few reports are available on the synthesis of mesoporous zinc sulfide with tunable pore size. Rana et al. first reported the sonochemical synthesis of mesoporous ZnS with BET surface area of 210 m2g-1 and average pore diameter of 2.8 nm, respectively.88 Sun et al. employed cotemplate approach in which dodecylamine was used as a template and an alcohol with varying carbon chain was used as an auxiliary template. Mesoporous ZnS having narrow distribution of particles with an average pore diameter of 7.57 nm and the BET surface area of 354 m2 g-1 respectively was formed after removal of template.89 Reverse micelle technique Reverse micelle technique (i.e. water-in-oil (W/O) microemulsion) is a prominent method to control size and shape of semiconductor nanoparticles. It is flexible, reproducible, safe and simple, also produces various nanostructures (having narrow size distributions) by suitably controlling experimental conditions such as surfactant, molar ratio of water:surfactant,



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concentration of precursors, co-surfactant and synthesis temperature.90 The method involve aggregation of nanocrystals within the water pools/core of the micelle.

Whiffen et al.

synthesized ZnS nanoparticles in the reverse micelle medium using cyclohexane as an oil phase, Triton X-100 as a non-ionic surfactant and n-pentanol as a co-surfactant.91 They reported the formation of 3-5 nm sized sphalerite phase NPs, as revealed by TEM observations. Cao et al. synthesized ZnS:Mn2+ NPs along with coating ZnS shell by inverted micelles method.92 Yang et al. reported the synthesis of ZnS:Mn and ZnS:Mn/ZnS core-shell NCs. The average size of dispersion of core/shell-NCs in thioglycollic acid (TGA) was reported to be ∼11 nm, according to DLS.93 Thermal Evaporation In this method condensed or material in powdered form is evaporated at a high temperature followed by condensation of the vapor phase under proper deposition conditions (at certain temperature, pressure, substrate etc.) to obtained required nanostructures. Zhang et al. synthesized in bulk single-crystal ZnS nanowires by thermal evaporation.94 The results showed that the nanowires had a wurtzite phase and were structurally uniform along with thin amorphous surface. Yuan et al. reported the bulk synthesis of ZnS nanobelts, nanowires, and nanoparticles by thermal evaporation of ZnS powders and subsequent deposition onto silicon substrates using Au as catalyst.95 the temperature difference of the substrates and concentration of ZnS vapor induced the single crystal to grow along two optimized direction ([001] and [010]) in different manner and hence resulted in the formation of various ZnS nanostructures. Xu et al. synthesized ZnS nanobelts using thermal evaporation method.96 The results indicated that an amorphous surface layer of oxide was formed during stage of temperature reduction. Velumani and Ascencio reported the formation of sphalerite ZnS nanorods having preferential orientation along a particular axis by evaporation technique.97 Liu et al. fabricated wurtzite ZnS nanobelts having width in the range 200- 600 nm via thermal evaporation method.98 ZnS nanowires were grown by Lin et al. by thermal evaporation of commercial ZnS powder onto the Au-coated silicon substrate.99 They reported bulk formation of densely packed arrays of nanowires, having approximately 80-100 nm diameter and 10 µm in length. Fang et al. synthesized nanorods, nanowires, nanosheets and nanobelts of ZnS by evaporation of ZnS nanopowder.100 Four different temperature zones and the catalyst were the critical parameters which influenced the


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different morphologies of the ZnS nanostructures. The report suggested that the nanorods and nanowires grew along [100] whereas nanosheets and nanobelts grew along [002] direction. Pol et al. synthesized of ZnS NPs via solid state thermal reaction between zinc powder and S powders at 650 °C.101 The morphological investigations revealed closely packed ZnS particles having 50-300 nm diameter. Chen et al. used thermal evaporation method to fabricate a novel self-assembled ZnS branched architecture. The trunks and the branches consisted of a single phase WZ ZnS.102 Li et al. synthesized uniform ZnS nanobelts in bulk quantities by thermal evaporation method using Au catalyst.103 The as deposited products consisted of wire-like nanostructures which were several tens of micrometer in length, and several hundreds of nanometers in width. Ma et al. fabricated 3D nanosaws, nanobelts (as shown in figure 9) and nanowindmills of wurtzite ZnS by thermal evaporating ZnS powders.104 The unusual morphologies were fabricated by adjusting the growth direction between different axis and facets.

Figure 9. SEM images of ZnS- Comb like and Sheet like structures. Reproduced with permission from ref. 104. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim



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Single-phased ZnS nanowires with a thin layer coating of graphitic carbon was obtained Shen et al. by thermally evaporating ZnS and SnS powders.105 1D nanostructure were formed with high aspect ratio, having length up to 100 µm and diameters in range of 50-120 nm. Yan et al. reported a hydrogen-assisted thermal evaporation method for synthesis of ZnS nanosaws, and accounted the effect of stacking fault on saw teeth formation .106 The nanosaw consisted of a stem, a middle layer, and saw-teeth. The ZnS nanosaw stem grew along the [10͞10] direction and had wurtzite phase, whereas the saw-tooth that grew along the [111] direction had mixed phases. The intrinsic stacking faults were crucial in formation of saw-tooth and resulted in transformation of the phase from WZ to ZB in the WZ stem. Lan et al. reported the controlled synthesis and characterization of single crystalline ZnS nanocombs via a two-step method. Au coated ZnS nanobelts were subsequently annealed using SnO2/C resulted in comblike morphology.107 There was an epitaxial growth of nanorods on the (00±1) side surfaces of the ZnS nanobelts however the growth of the trunk nanobelts was along [210] direction. Patternassembled structures consisting of tubular ZnS nanostructures have not been commonly reported. Shen et al. reported the synthesis of self-assembled three-dimensional 3D dandelion like ZnS nanostructures via thermal evaporation of ZnS and Ge powder. The study revealed formation of well-faceted hexagonal cross sections of ZnS submicrotubes, formed by the coalescence of adjacent ZnS dandelions.108 Li et al. reported the synthesis of ZnS nanoparticles without using sulphide precursors by reacting metallic salts with dimethylsulfoxide (DMSO). The results indicated the formation of zinc blende phase, having mean size of 3 nm.109 Shen et al. reported heteroepitaxial growth of ZnS nanowires on Zn3P2 to fabricate well-aligned crystal orientated nanowire arrays by thermal evaporation method.110 The results suggested that the ZnS nanowire arrays were single-crystalline wurtzite ZnS preferential growth along the [0001] direction, having uniform diameters in the range of several tens of nanometers and lengths in range of 500 nm. Complex 1-D hierarchical heterostructures were fabricated by wrapping ZnS nanowires with highly dense SiO2 nanowires has been reported by Shen et al.111 Hierarchical leaf-like structure having core of ZnS nanowires and with gradual decrease of diameter along its axis were formed. Six fold symmetry ZnS heptapods and threefold symmetry ZnS tetrapods were fabricated by Shen et al. via a thermally evaporating ZnS and SiO mixture.112 Self-assembly of several aligned ZnS nanowires having growth along [0001] direction produced these complex 3 D structures (as depicted in figure 10).


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Figure 10 a Low-magnification SEM image of a product. High-magnification SEM images of (b–e) the sixfold symmetry ZnS heptapods and (f–i) the threefold symmetry ZnS tetrapods. Reproduced with permission from ref. 112. Copyright 2007 American Institute of Physics. Wu et al. fabricated fishbone-like ZnS nanostructures by thermally evaporating ZnS powders and depositing it onto ITO glass substrate.113 It was composed of prominent spines having lengths of several microns and several secondary branches having width ranging from 50-100 nm and length upto 500 nm extending along both sides of the spines. The growth mechanism for the formation of this nanostructure can be explained on the basis of the fact that due to high free energy in the side of spine, In and Sn vapor can be condensed on either sides and act as nucleating sites for secondary growth and hence resulting in formation of nanostructures with complex morphology as shown in figure 11.



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Figure 11.

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Growth schematic of ZnS fishbone-like nanostructures. Reproduced with

permission from ref. 105. Copyright 2009 Elsevier B.V. Stoillo et al. prepared micro- and nanostructures having several morphologies of both pristine ZnS and doped ZnS:In via VS method.114 Nanowires, nanoribbons and rods were obtained for pure ZnS, but the dopant In induced the formation of hierarchical structures and nanoplates as shown in figure 12. This shows that In assisted in the lateral growth of the ZnS low dimensional structures. The results indicated that both swords and ribbons grew along [10͞10] and [0001] directions hence the non polar faces {͞12͞10} were left which constituted the largest surfaces in the nanostructures.

Figure 12.

Structure morphologies obtained in the higher deposition temperature range (850–

900 °C): (a and b) for sample B; (c) for sample C; and (d) for sample D. (e) Sketch of the growth of swords and ribbons. Reproduced with permission from ref. 114. Copyright 2013 Elsevier B.V. Fang et al. utilized thermal evaporation strategy for the synthesizing multiangular branched ZnS nanostructures with needle-shaped tips (as shown in figure 13).115 They found that growth of these unique ZnS nanostructures could be easily tuned by slowly reducing the temperature from 1050 to700 °C .


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Figure 13. SEM micrographs of the as-grown ZnS nanostructures, showing that they are composed of multiangular architectures with needle shaped tips. (a and b) Low-magnification SEM images. (c and d) High-magnification SEM images verifying the multiangular and needleshaped structures. Reproduced with permission from ref. 115. Copyright 2008 American Chemical Society. Large scale production of ZnS microrods on SiO2/Si substrate was reported by Trung et al. via thermal evaporation of ZnS nanopowders.116 The optical photoemission can be tuned by controlling the oxidation process which can bring about conversion of ZnS microrods into ZnS/ZnO heterostructures. These nanostructures can find potential application in photonics and gas sensing. Several unusual, novel and complex 3D ZnS structures have been obtained such as nanosaws, dart shaped nanoribbons, diskettes, nanoribbon arrays.117-119 Chemical vapor deposition Chemical vapor deposition (CVD) involves coating of heated objects in a chamber by passing a precursor gas or gases and variety of materials can be deposited with very high purity. ZnS nanostructures were synthesized by Shang et al. through a CVD technique followed by bombarding Cu ion at an accelerated voltage of 15 keV.120 ZnS nanostructures were made of regular hexagonal pallets densely stacked along the (0001) direction, and ion implantation caused damage to the surface of these ZnS nanostructures and the morphology became rough. The method of ion implantation can selectively infuse the desired element into the host matrix, more effectively than traditional techniques, in which in situ doping has failed due to the lattice



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mismatch or undesired chemical reactions. Zinc-blende and wurtzite ZnS nanobelts were synthesized via chemical vapor deposition at 1050 °C, using Au as catalyst and graphite as reducing agent by Hu et al.121Electron microscopy results revealed that at 600 °C three different crystalline phases of ZnS nanobelts were formed viz. ZB–ZnS having growth along [001], W– ZnS having growth along [0-1 1 0] and mixed phase ZB-twins and W–ZnS. Moon et al. reported that highly crystalline single-phased ZnS (wurtzite) nanowires can be grown by chemical vapor deposition along [001] direction [110]. The results indicated formation uniform of ZnS nanowires having diameters in the range between 10-30 nm and lengths up to several hundred microns.122 Chang et al. reported the synthesis of ZnS nanowires on silicon substrates via a chemical vapor deposition method.123 The results indicated the formation of single phase ZnS nanowires (sphalerite), having base diameter ranging between 320–530 nm and diameter of tips in the range of 20–30 nm. Liu et al. reported large scale and rapid growth (15– 20 min) of ZnS:Mn/Cd 1D nanostructures using Zn and S precursors via a CVD method.124 ZnS nanostructures (nanowires and nanoribbons) grown on Au particle-filled anodic alumina oxide (AAO) templates were length in several tens of microns and have a high aspect ratio, while ZnS nanostructures grown onto catalyst-free AAO template were having cosh- or branch-like morphology. Zhai et al. reported epitaxial growth ZnS tetrapods on sphalerite CdSe nanocrystals by CVD technique.125 The results indicated that sizes of the nanopods can be easily changed by tuning the distances between the substrates and precursors. Direct templating route Jiang et al. fabricated ordered ZnS nanowires having diameter of 5 nm a by a direct templating method in reverse hexagonal liquid crystal synthesized by using oligo(ethylene oxide)oleyl ether amphiphiles, n-hexane, n-hexanol/i-propanol (2:1), and water.126 An in situ formation (SISF) technique was utilized to prepare arrays of ZnS nanowires by γ-irradiating the reaction mixture at room temperature. Su et al. employed DNA molecules as template to synthesized ZnS nanoparticle wires.127 The mean size of the ZnS nanoparticles were 6.5 nm and was having zinc blende phase. .Circles of ZnS nanowires were synthesized by Gao et al. using genome DNA of Escherichia coli as templates. It can be seen from TEM image (figure 14) that the nanoparticles were clinging on the DNA strands and forming ZnS nanocircles.128 ZnS and (CdxZn1−x)S nanocrystals were grown on on polydiacetylene (PDA) Langmuir films by Upcher et al.129 and arrays of oriented NCs co-


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aligned with PDA were formed. The result suggested that NCs underwent structural adaptation and the structural mismatch significantly affected the orientation, phase, and size of the nucleating NCs.

Figure 14 (a) Typical TEM images of circular ZnS nanowires ;(b) represent the higher magnified TEM image of ZnS nanocircles. Reproduced with permission from ref. 128. Copyright 2014 American Chemical Society. The various merits and demerits of the important synthetic techniques can be summarized as shown in table 1.



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Table 1. The merits and demerits of some important techniques for synthesizing ZnS nanostructures Techniques Reverse micelle route Sol gel

Microwave Template-assisted growth Hydrothermal/Solvothermal

Ultrasonic Assisted method Combustion synthesis CVD/PVD Spray pyrolysis Combustion synthesis Thermal Evaporation Single source Precursor Laser ablation

Merits narrow size distribution, Core/shell NCs, reproducibility monosized particles reproducible no requirement for special equipment high-efficiency , rapid formation narrow size distribution better control over shape and size, growth direction, and array density low cost, high yield and large scale production extremely high temperatures and pressures can be achieved for a short duration, better control over growth rate of nanocrystals, no use of templates. short process time, simple, low cost ultrapurity (No organic impurities), good structural control highly pure & homogenous deposition on large area, good reproducibility short process time, simple, low cost 1D nanostructures, mass production, high deposition rate morphology control, template free, reproducibility wide range of nanostructures

Demerits high temperatures, pressures, use of biohazard substances longer processing time , deposited in a silica matrix not suitable for bioimaging applications pressure regulation, unwanted side reactions, not suitable for large scale reaction solvent extraction, calcination, removal of the template expensive autoclaves, difficulty in controlling the reaction time spherical nanoparticles are produced agglomeration and low crystallinity complicated instrumentation, high cost, low productivity, synthesis of multicomponent materials is difficult wastage of solution, non uniform deposition agglomeration and low crystallinity high processing temperature and long growth duration high decomposition temperature, use of inert atmosphere. production of heterogeneous reactants difficult to control nucleation in vapor phase growth


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Growth Mechanism Morphology controlled synthesis and their architectural control is crucial for fabricating nanostructures for potential applications. The growth mechanisms responsible for formation of various ZnS nanostructures can be complicated due to involvement of several factors such as synthesis methods and different growth conditions. In modeling of surface-supported nanostructure formation, the mobile adatoms on the surface act as a major source of building material to fabricate nanostructures specially 1D nanostructures. Some well established growth mechanisms for ZnS nanostructures includes template-assisted growth, thermal evaporation process, evaporation-condensation approach, vapor-liquid-solid (VLS) and vapor-solid (VS) growth, epitaxial growth, homoepitaxial growth, heteroepitaxial growth, conversion from other nanostructures, plasma-assisted metal organic chemical vapor deposition (MOCVD), and hydrothermal/solvothermal reaction, oriented attachment mechanism. Kinetic-control of the growth pattern along various crystallographic directions leads to formation of different shapes of nanostructures. Gautam et al. explained the growth mechanism of a core-shell structure in which In constituted the cores and formation of the shell by ZnS nanotubes.130 The results suggested that approximately 70% of the branches grew along [0001] direction with an uneven flat end, while the rest 30% grew along the [10͞10] direction as shown in figure 15. The growth mechanism of heterostructures was established by the principles of carbothermal CVD.



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Figure 15 (a) A XRD pattern of the as-prepared heterostructures. The standard peak positions for wurtzite ZnS and In are shown at the panel top. (b, c, d) SEM images of the heterostructures at different magnifications. (e) A high-magnification SEM image showing the ZnS branches quasialigned at the heterostructure tip. Reproduced with permission from ref. 130. Copyright 2008 American Chemical Society. The growth mechanism of nanostructures and their shape control are significantly dependent on the surface passivating agents used during the synthesis. They act as surface directing agent and the functional groups can provide a proper chemical microenvironment to direct the specific nucleation for the growth of a particular morphology. Zhang et al.56 elaborately explained the influence of EN on the growth behavior of ZnS nanostructures. Lamellar shaped ZnS were obtained by Zhao et al.82 and the shape control was strongly affected by the use of triblock copolymer surfactant Pluronic P123. Lu & Walker explained a combination of ion-by-ion growth and cluster-by-cluster deposition for formation of ZnS nanoflowers which resulted from seeded


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and unseeded growth on -COOH- terminated self assembled monolayers (SAMs).131 Based on the formation of the stacking faults with the growth of the stem, a defect inducing growth model was suggested by Yan et al. to explain the formation of ZnS nanosaw.132 Intrinsic stacking faults caused the change of phase from WZ to ZB in the WZ stem resulting in formation of step-like boundaries. The ZB ZnS segment on the step-like boundary worked as nucleating site and leads the subsequent growth of the saw-tooth. Gu et al. explained the mechanism of formation of MWCNT/ZnS heterostructures. The ultrasonication modified and activated the surface of MWCNT, offering suitable nucleation site for growth process and oriented aggregation of ZnS nanocrystals heterostructures.133 Furthermore Gu et al. reported the formation of spherical ZnS nanostructures based on the mechanism of oriented aggregation in which no templates were used and the formation of hollow and solid sphere was due to H2S gas bubbles formed during reaction.134 Vapor-solid (VS) and vapor-liquid-solid (VLS) growth mechanisms have been extensively implemented to explain the growth of diverse range of 1D nanostructures. On the basis of growth kinetics, the VS mechanism give emphasis on diffusion of reactant species to surface and preferentially getting incorporated at sites having high surface energy and this feed and maintain strategy helps in uninterrupted growth of nanostructures. In VLS mechanism the nanodroplets of liquid metal/alloys serves as catalytic site for adsorbing and dissolving reactant species and hence direct nanowires growth whose diameter can be controlled. Hao et al. synthesized two types of ZnS nanostructures (periodically twinned sphalerite ZnS nanowires (PTNWs) and asymmetrical polytypic nanobelts (APNBs)), using VLS process catalyzed by Au. The results indicated that sphalerite ZnS straight strips were embedded in wurtzite nanobelts (parallel to the belt axis) throughout the entire length.135 The formation of the two types of ZnS nanostructures is based on modulating parameters (depending on the nature of catalyst droplets and side surface planes) of mass diffusion process . Li et al. discussed the heteroepitaxial growth of ZnS nanowires arrays on single crystal CdS nanoribbon substrate via catalyst assisted VLS growth process.136 Several ZnS nanostructures, including nanorods, nanowires, nanowhiskers and hierarchical nanostructures, have been synthesized by Fang et al. via Sn-catalytic vapor liquid- solid process.137 The results indicated that Sn droplet acted as the catalytic active site. The morphology of nanostructures was significantly affected by catalyst source materials, their sizes, and deposition substrate.



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The formation of ZnS tricrystal whiskers with {1013} twin planes have been done by Zhu et al. Epitaxial growth of ZnS nanospine arrays (oriented along three [0001] directions) were done on the three sections of the tricrystal (as shown in figure 16).138

Figure 16. SEM images of nanostructures grown from the ZnS tricrystal via three-dimensional epitaxy: (a) arrays of nanospines viewed from the ridge side of the tricrystal; (b) arrays of nanospines viewed from the groove of the tricrystal. Reproduced with permission from ref. 138. Copyright 2009 Tsinghua University Press and Springer-Verlag. Kang et al. synthesized Mn/Fe doped and co-doped ZnS nanowires and nanobelts on Au-coated Si substrates through CVD method and explained the growth mechanism via vapor-liquid-solid (VLS) process.139 The nanostructures were having different growth directions depending on the total pressure. Moore et al. reported a new method for orientation aligned growth of ZnS nanowires. ZnS nanowires were grown on a buffer layer of CdSe (grown on a Si (111) substrate).140 An essential parameter in controlling the growth pattern of 1D nanostructures is the


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slow and continuous supply of monomers. Xiong et al. described formation of urchin-like ZnS nanospheres self assembled from ZnS nanorods due to the orientation-directed (synergistic) effect of solvents ethanol amine and water.141 Yang et al. ligand-induced in-situ aggregation strategy for fabricating medium-sized ZnS nanospheres (NPs) comprising of small nanoparticles. In this method the secondary and complete structures of target materials can be simultaneously tuned.142 pH dependence and variation of particle size was very recently reported by Yin et al. 143 It was found that the average size for ZnS nanostructures synthesized at pH of 3, 5, 7 and 9, were 41.2, 40.4, 21.1 and 11.4 nm respectively. It was proposed that greater nucleation takes place at higher pH, hence small sized particle were readily formed. Coordination of ammonia molecules on the surface of the nucleated ZnS nanocrystals decreases the surface energy and retards the growths of nanocrystals. Yao et al. fabricated MW induced spontaneously assemble of ZnS nanospheres into a hierarchical structure without involving any template.144 They reported that the hierarchical nanospheres structure were loose, because sphalerite ZnS is weakly susceptible to MW irradiation, had it been more susceptible the stronger hot spots or hot surfaces of ZnS particles would have resulted in formation of tightly assembled nanostructures The other growth patterns can be summarized in table 2.



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Table 2. Growth patterns of ZnS nanostructures ZnS nanostructure ZnS nanowaveguides (rectangular crosssectional nanowires) ZnS nanobelts ZnS:Ga nanowalls

Method

Growth mechanism

Direction

pulsed laser vaporization

VLS(Vapor-liquidsolid)

(100) ; (001)

VLS

[01͞10]

104

[0001]

146

-

147

-

82

[001]

62

(111)

148

thermal evaporation thermal evaporation

ZnS fluffy nanospheres

wet chemical

ZnS submicrospheres

sonochemical

ZnS nanorods

hydrothermal

Volmer-Weber growth hollowing process based on Ostwald ripening random Brownian motion driven particle collision 1D-growth mechanism

Ref. 145

ZnS; Cu+–Al3+ nanorods

surfactant free wet chemical

Ostwald-ripening and orientedattachment based coarsening

ZnS nanowires

slow precipitation

diffusion model

(111), (200)

149

ZnS nanorods

solid state annealing

catalytic-assisted technique

(008)

150

ZnS hollow nanospheres

hydrothermal

Kirkendal effect

-

151

ZnS nanotubes

hydrothermal

diffusion and consumption process

-

152

ZnS nanopods

metal-organic chemical vapor deposition

heterostructure epitaxial growth

[0001]

125

ZnS:Cu nanorods

solvothermal

twinning and stacking faults

[002]

19

ZnS branched architecture

thermal evaporation

self assembly

[2 1͞ ͞10]

103


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ZnS:Mn nanowires

solvothermal

solvent coordination molecular template SCMT mechanism and the effective monomer model

ZnS heptapod and tetrapod

thermal evaporation

self-assembly of ZnS nanowires

ZnS nanostructures(Cauliflower like and hedge apple)

hydrothermal

ZnS nanobelts

thermal evaporation

Ostwald-ripening and orientedattachment based coarsening silicon-induced vapor–liquid–solid process

[001]direction

143

[0001] orientations

112

-

61

[0001]

154

Intercrossed sheet-like ZnS:Ga nanostructures grown vertically via a combined mechanism on the polycrystalline ZnS:Ga films was reported by Lu et al.146 as shown in figure 17. Figure 17 (a) Typical SEM image of ZnS:Ga nanowalls. The inset in panel a is a high magnification SEM image showing the nanowalls as thin as 15 nm. (b) The tilted SEM image of the ZnS:Ga nanowalls . Reproduced with permission from ref. 146. Copyright 2009 American Chemical Society. Changes in photophysical properties Transition from bulk to nano regime is always accompanied by spectral shift better known as ‘blue shift’. There are several factors that induce this phenomenon such as band-gap expansion in the nanoparticles, large surface to volume ratio, quantum confinement effects, different activator ion concentration, codopants, energy transfer, variations in the measurement temperature, coating or surface passivation, polycrystalline phases, impurities and recording spectra at different excitation wavelengths. The different growth pattern of the ZnS nanostructures varies with capping agent and hence the photoluminescence (PL) intensity and



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photoluminescence quantum yield (PLQY) changes accordingly. Enhancement in the PL intensity is generally observed for particles having good crystalline structure (as lattice defects are minimized), thus reducing nonradiative recombination of electron–hole pairs. In addition to this as the concentration of surfactant is significantly increased (beyond the optimum value), the the surface of the particle might deform thereby creating some nonradiative defect centers. pH strongly affects the PL properties of ZnS nanostructures. It is more favorable that coordination of thiols with ZnS nanoparticles occurs at a particular pH and resulting in enhancement of PL intensity. Chhatterjee et al. reported that PL of mercaptoethanol and cysteine capped ZnS NPs nanoparticles was found to be strongly affected by the concentration and nature of the surfactant.155 2.3% and 2.0% PLQY was reported for mercaptoethanol and cysteine capped NPs, respectively when the molar ratio of the precursors (Zn2+:surfactant) was 1:1.5. The finding suggested that for given particle size, PLQY of ME-capped ZnS NPs is more as compared to Lcysteine-capped ZnS NPs. This can be accounted due to the presence of sulfur vacancy defects in L-cysteine, which were absent in ME capped ZnS NPs. Annealing can significantly affect the PL properties as reported by Song et al.156 The subcrystal boundaries and the surface of polycrystalline particles are centers for non-radiative recombination for electron- hole pairs and annealing significantly minimizes these recombination centers. They further reported that particle sizes of the ZnS spheres also affect the PL intensity. Annealing ZnS microspheres at 550–800 °C, two emission sites were observed, one at 454nm due to trapped luminescence originating from the surface states and the other at 510 nm that might be associated with stoichiometric vacancies. There is a considerable red shifting of the emission wavelength from 454 nm of pristine ZnS to 508 nm of ZnS:Cu nanostructures. Li et al. reported that synthesis at a high pH and appropriate molar ratio of constituents significantly affect PL intensity.157 The results indicated that ZnS NPs synthesized at pH 12 were brighter than NPs synthesized at pH 8 as can be seen from figure 18. Dynamic properties of charge carries and electronic energy levels of Cu-doped ZnS nanocrystals along with the electronic and optical properties for device applications has been reviewed by Corrodo et al.158


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Figure 18. The PL spectra of two ZnS QDs samples with the same composition of MPA:Zn:S = 8:4:1, synthesized at different pH, 8 and 12. Reproduced with permission from ref. 157. Copyright 2007 Institute of Physics. Sooklal et al. reported that ZnS:Mn2+ nanocrystals in which the Mn2+ ions were incorporated in the lattice, blue (related to ZnS) and orange emissions (due to dopant) were observed 435 nm and 590 nm respectively but when distribution of Mn2+ ions was outside the ZnS crystals, 590 nm emission was not observed rather a new peak at 350 nm was observed, along with quenching and blue shifting of 435 nm peak to 390 nm.159 The nature and concentration of dopants in semiconductor nanoparticles play decisive role in luminescence efficiency and emission band/peak position. Harish et al. reported that fluorescence intensity of ZnS:Ce3+-Cu2+ (codoped) NCs was 18 times more than that of pure ZnS NCs.160 It was found that as the concentration of Cu ions were increased beyond an optimum value there was a gradual decrease in PL intensity which might be associated with the intraionic nonradiative relaxation between the neighboring Ce and Cu ions. Zhou et al. reported room temperature PL studies for 1D doped ZnS nanostructures. The results revealed strong emissions were obtained in blue, green and yellow–orange range as depicted in figure 19.161 For sample containing 0.14% Mn, 0.17% Cu and 0.75% Fe the emission band appeared around 468 nm arising due to Zn vacancies. The emission peak at 515 nm might be associated with transition from the conduction band of ZnS to t2 excited state of Cu2+(d9). The emission band centered at 622 nm (orange-red) light nm might be attributed to Cu impurities. No visible emission band was observed for Fe dopant. The study indicated that Fe ions can effectively deactivate PL intensity (visible region) of the ZnS and suppresses the role of copper.



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Figure 19. PL spectra of the ZnS 1D nanostructures. (a) Mn, Cu co-doped; (b) Mn, Cu, Fe codoped. Reproduced with permission from ref. 161. Copyright 2008 Elsevier B.V. Xiao et al. demonstrated that PL properties could be tuned by controlling the molar ratio of precursors, and pure excitonic emission can be obtained.162 it was observed that the trap state emission (ascribed to sulfur vacancies) can be eliminated if excess of sulfur precursors was used during the synthesis, leading to pure excitonic emission. The temporal evolutions of PL QYs (as evident from figure 20 and 21) indicated that, by increasing the molar ratio of S: Zn, the trapstate emission gradually vanishes whereas excitonic emission gradually increased.


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Figure 20. Temporal evolution of PL spectra of the ZnS NCs obtained with different molar ratios (a) S:Zn = 1:1 and (b) S:Zn = 2:1. Reproduced with permission from ref. 162. Copyright 2011 Elsevier B.V.

Figure 21. Temporal evolution of PL QYs of ZnS NCs synthesized with different S:Zn molar ratios (a) S:Zn = 1:2, (b) S:Zn = 1:1, and (c) S:Zn = 2:1. Reproduced with permission from ref. 162. Copyright 2011 Elsevier B.V. Lu et al. synthesized ZnS:Mn2+ nanophosphors by microwave irradiation method and studied the d–d transitions of Mn2+ ions in ZnS nanoparticles in the PLE spectrum.163 Five absorption peaks were observed in the region from 400–550nm (as shown in Figure 22 (inset)). The peaks appeared at 453, 474, 485, 495 and 517 nm corresponding to the 6A1(S) →4E (4D), 6A1(S) →4T2 (4D), 6A1(S) →4A1, 4E (4G), 6A1 (S) → 4T2 (4G), and 6A1 (S) →4T1 (4G) transitions, respectively. The appearance of these additional excitation peaks were attributed to high-lying excited states of Mn2+.



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Figure 22. PL excitation spectrum of 2-ethylhexanoic acid coated ZnS:Mn2+ nanoparticles prepared using 200W microwave power. Inset shows the fine structure in the spectrum at the wavelength region 400–550 nm. Reproduced with permission from ref. 163. Copyright 2009 Elsevier B.V. Table 3 gives a comparison of synthesis methods in terms of pH, particle size, reaction temperature, morphology, the dependence of synthesis technique and PL properties of different nanostructures. The structural changes that occurred by incorporation of different metal ions can be easily visualized.


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Table 3. Photophysical properties of ZnS and doped ZnS nanostructures Surface Sample

Route

Phase

pH

Passivation/

T °C

t

Morphology

Size

PL (nm)

2h

nanowire

L 500 nm

566

Ref.

Solvent

ZnS: Mn

hydrothermal

W

¯

ethylene diamine and ethanol

100

64

D 10 nm 4h

microtubes

-

470# without EN

ZnS:Mn

ZnS: Mn

ZnS :Mn

organometallic

-

Methacrylic acid

-

-

NPs

590

inorganic

-

Na(PO3)n

-

-

NPs

450

hydrothermal

ZB

-

-

100

10

-

20nm

600

precipitation

-

-

-

RT

6

-

4 nm

610

solvothermal

W

-

n-octylamine and Cyclohexane

164

165

D-2.1nm 170

3

NW

L-50–

575

166

150nm.

395 ZnS

wet chemical

¯

¯

2-ME

100

426

-

167

480 531

ZnS:Ag

co-precipitation

ZB

8.5

L- Cysteine

-

2.6 nm

L-arginine, ZnS

wet chemical

ZB

>8

L-cysteine, L-histidine,

/ZnO

reverse micelle

Reflux

20

-

5.3-11.7

C-573

nm

H-575

RT

4 nm

ZB

300

6 nm

10

ZnO

169

M-576

ZB

ZB

168

A-575

L- methionine

ZnS:Mn

490

350

-

NPs

8.4 nm

W

400

100 nm

W

525

100 nm


582 588

170


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ZnS:Co 2+ ZnS:Co

3+

ZnS

co-precipitation

composite molten salt

ZB

Critical Analysis of Phase Evolution, Morphological Control, Growth

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