as an important member of ii  vi group semiconductors of wide band gap , zinc sulfide ( zns ) has been extensively investigated . the material is utilized in a wide range of applications , e.g. photocatalysts , photoconductors , optical sensors , optical coatings , electrooptic modulators , field effect transistors , electroluminescent materials , solid - state solar window layers , light - emitting materials , etc . . hexagonal wurtzite and cubic sphalerite , which show band gaps of 3.77 ev   and 3.72 ev , respectively at room temperature ( rt ) . the hexagonal phase is thermodynamically metastable and is only stable at temperatures higher than 1,020c   whereas the cubic phase is thermodynamically stable at rt . however , based on results of molecular dynamics simulations as well as thermodynamic analysis , zhang et al . reported that nanoparticles of wurtzite zns is thermodynamically more stable than those of sphalerite zns in vacuum . in the last decade , many reports on the production of zns are for sphalerite zns rather than wurtzite zns , plausibly due to the metastable nature of the latter . nevertheless , it has been reported that hexagonal zns nanocrystals can be produced at a relatively low temperature through solvothermal reactions . it is known that zns can transform spontaneously from hexagonal to cubic structure upon contact with some organic molecules ( e.g. pentafluorothiophenol , thiophenol , 1-decanethiol , 1-hexanethiol , benzoic acid and cyanoacetic acid ) at ambient temperature . in terms of optical properties , it is hence meaningful to find ways that wurtzite zns can be protected from entities that would induce phase transformation . the fabrication of complex three - dimensional ( 3d ) architectures of zns ( assembled by nanostructured zns as building blocks ) with controlled morphology , orientation and dimensionality has attracted much attention in the past decade because the structures may provide opportunities to exploit novel properties such as high surface area and surface permeability . zns assemblies having superior nature with 3d structures such as solid and hollow nanospheres , hierarchical structures   and submicrotubes   have been reported . recently , zns nanostructures with different morphologies and sizes were fabricated through low cost , simple and efficient hydrothermal method . for example , quantum - sized zns nanocrystals with quasi - spherical and rod shapes were synthesized using alkylamine , bundles of wurtzite zns nanowires were synthesized using hydrazine hydrate , two - dimensional wurtzite zns nanostructures was fabricated from ethylenediamine , spherical nanostructures comprised of zns nanocrystals were obtained in aqueous solutions , and zns nanoparticles and nanorods were fabricated with controlled crystallinity through a solvothermal approach involving the change of solvent used for synthesis . in this paper , we report the synthesis ( at 150c ) of a novel 3d zns core the metastable wurtzite zns is encapsulated by a shell of sphalerite zns so that the wurtzite zns core can be protected from the outside environment . the results show that the approach is efficient and the outcome highly reproducible . to the best of our knowledge , the fabrication of such a core all the reagents used for the synthesis of the core  shell material were of analytical grade ( purchased from nanjing chemical industrial co. ) and used without further purification . first , 14.87 g zinc nitrate [ zn(no3)26h2o ] and 40.0 g naoh were dissolved in deionized water to form a 100.0 ml solution . then 3.0 ml of the solution was mixed with 5.0 ml of deionized water and 25.0 ml of absolute ethanol ( c2h5oh ) , followed by the addition of 5.0 ml of aqueous ammonia ( 25% ) . before being transferred into a teflon - lined autoclave , an appropriate amount of sublimed sulfur powder ( in zn : s molar ratio of 3:1 , 1:1 and 1:3 ) subsequently , the autoclave with its content was kept in an oven at 150c for 24 h. at the end of the hydrothermal treatment , the as - obtained solid material was separated using a centrifuge , and thoroughly washed with absolute ethanol and deionized water ( 3 cycles ) . the as - obtained materials were examined on an x - ray powder diffractometer ( xrd ) at room temperature ( rt ) for phase identification using cu k radiation ( model d / max - ra , rigaku ) . the morphology of samples was examined over a high - resolution transmission electron microscope ( hr - tem , jeol-2010 , operated at an accelerating voltage of 200 kv ) and a field emission scanning electron microscopy ( fe - sem , fei sirion 200 , operated at an accelerating voltage of 5 kv ) . visible ( uv  vis ) absorption ( cary , usa ) and photoluminescence ( pl , excitation source : he - cd laser , 325 nm ) at rt . 1 , the x - ray diffraction ( xrd ) results of the solid products show zns of hexagonal wurtzite ( jcpds no . the strong and shape peaks assignable to wurtzite zns indicate good crystallization and grain formation . in contrast , the broad peaks of sphalerite zns show the size effect of nanocrystallites . using the scherrer formula with deal consideration to the fwhm ( full width at half maximum ) of peaks , the average grain size of sphalerite zns is estimated to be around 10 nm . hence , despite bulk wurtzite zns is metastable below 1,020c , stable wurtzite zns can be fabricated under hydrothermal conditions with temperature as low as 150c . the results of xrd also indicate that the crystal structure of the final products is strongly related to the zn : s molar ratio . the product obtained at zn : s of 3:1 is a mixture of sphalerite and wurtzite zns , whereas at zn : s of 1:3 , the product is mainly sphalerite zns . the xrd pattern of standard wurtzite and sphalerite structures of zns are also shown the high - resolution transmission electron microscopy image of the product synthesized at zn : s of 3:1 shows core / shell quasi - spheres with external diameters of around 500 nm ( fig . 2 ) . the cores ( with size of about 200 nm ) seem to be in the course of evolution from irregular to hexagonal shape the insets of fig . 2 show the selected - area electron diffraction ( saed ) patterns recorded at ( a ) region of both core and shell and ( b ) region of shell only . it is clear that the former suggests the presence of sphalerite and wurtzite phases , while the latter the sole presence of sphalerite phase , which is consistent with the xrd results . the insets are the saed patterns recorded on region of a both core and shell and b shell only of the nanostructures field emission scanning electron microscopy and energy dispersion spectroscopy ( eds ) were employed to characterize the product fabricated with zn : s = 3:1 . shell nanostructures are oval and the surface is tightly packed with nanoparticles . according to the eds data ( fig . 3b ) , the obtained product is comprised of zn and s elements with zn : s molar ratio close to 1:1 ( i.e. the stoichiometry of zns ) . a fe - sem image and b eds spectrum of the product obtained at zn : s of 3:1 although the exact mechanism for the formation of the core first , we know that without the introduction of sulfur powder but following the same procedure , wurtzite zno is the final product . also , in an alkali solution and under high pressure ( condition of hydrothermal treatment in an autoclave ) , sulfur is transformed to s and so3 through a disproportionation reaction : it is possible that cubic and hexagonal zns are formed through two different ways : ( 1 ) hexagonal wurtzite zns is formed from hexagonal wurtzite zno via s substitution of o under the conditions of hydrothermal treatment ; in other words , zno acts as a template for the formation of wurtzite zns ; ( 2 ) cubic sphalerite zns nanoparticles are formed through a facile reaction between s and zn . at zn : s of 1:3 , path ( 2 ) dominates . visible ( uv  vis ) absorption and photoluminescence of the obtained materials fabricated at zn : s = 3:1 ( core  shell nanostructured zns ) and zn : s = 1:3 ( cubic sphalerite zns ) . as shown in the uv  vis absorption spectra ( fig . 4a ) , the two materials exhibit excitonic absorption at ~291 nm ( with a tail at 301 nm ) and 310 nm . compared to the corresponding peak of bulk zns ( onset is at 340 nm ) , there is a modest blue shift possibly due to quantum - size effects . the long absorption tail ( between 400800 nm ) in the case of cubic sphalerite zns ( zn : s = 1:3 ) was due to scattering by the crystals . in addition , with the rise of zn : s ratio from 1:3 to 3:1 , a red shift from 291 nm ( or 301 nm ) to 310 nm was observed . from the tem images , one can see that the average size of the wurtzite zns cores is the biggest when zn : s ratio is 3:1 . we hence attribute the red shift to the increase in average radius of the nanocrystallites . in fig . 4b , the pl spectra of core  shell zns ( obtained at zn : s = 3:1 ) and cubic sphalerite zns ( obtained at zn : s = 1:3 ) exhibit strong emissions at 545 nm and 524 nm , respectively , along with an identical weak shoulder at about 617 nm . the low - energy luminescence at 617 nm could be due to defects other than schottky vacancies inside the nanostructures . liu et al . related the green emission observed over sphalerite zns to some self - activated centers , probably vacancy states or interstitial states of the peculiar structure . such understanding could be helpful to explain the 545-nm emission observed in the present study . furthermore , compared to the pl spectrum of sphalerite zns , the strong emission center of core the results demonstrate that the excitonic absorption and pl emission can be tuned by simply controlling the zn : s ratio . a uv  vis absorption and b pl spectrum of the product obtained at zn : s of 3:1 ( core  shell nanostructured zns ) and 1:3 ( cubic sphalerite zns ) in summary , novel core  shell material comprised of cubic sphalerite ( shell ) and hexagonal wurtzite ( core ) zns were synthesized at 150c using zinc nitrate and sulfur as source materials via a simple hydrothermal method . shell nanostructures might involve two steps : ( 1 ) the formation of wurtzite zns using zno ( hexagonal ) as template , and ( 2 ) the formation of a shell of sphalerite zns nanoparticles via a facile reaction between s and zn . by simply controlling the zn : s ratio , one can tune the optical properties of the material such as excitonic absorption and pl emission . we would like to acknowledge the foundation of national laboratory of solid state microstructures , nanjing university ( grant no . 2010zz18 ) , the national high technology research and development program of china ( grant no . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
novel core  shell nanostructures comprised of cubic sphalerite and hexagonal wurtzite zns have been synthesized at 150c by a simple hydrothermal method . 
 the results of hr - tem and saed investigation reveal that the cores of hexagonal wurtzite zns ( ca . 
 200 nm in average diameter ) are encapsulated by a shell of cubic sphalerite zns . 
 the fe - sem image of the nanomaterials shows a surface tightly packed with nanoparticles ( < 10 nm in size ) . 
 the optical properties of the fabricated material have been studied in terms of ultraviolet  visible absorption and photoluminescence . 
 furthermore , a possible mechanism for the fabrication of the core 
 shell nanostructures has been presented .