ordered self - assembly of nanoscale building blocks , such as nanoparticles , nanorods , nanoribbons , and so forth , into complex architectures has recently become a hot topic in material research fields .
remarkable progress has been made in the self - assembly of highly organized building blocks of metals [ 1 - 4 ] , semiconductors [ 5 - 8 ] , copolymers , and organic  inorganic hybrid materials   based on different driving mechanisms , such as ostwald ripening , kirkendall effect , and self - assembly of nanoscale blocks through hydrophobic interactions .
however , controlled organization into curved hollow structures from the primary building units , for example sheets , remains a challenge for materials self - assembly .
the ability to assemble primary units into hollow structures is in great demand not only because of their role in better understanding the concept of self - assembly with artificial building blocks but also due to its great potential for technological applications .
nickel hydroxide ( ni(oh)2 ) , as one of the most important transition metal hydroxides , has received increasing attention due to its extensive applications , especially as a positive electrode active material , in alkaline rechargeable ni - based batteries .
it has been reported that the capacity of the positive electrode could be significantly increased when nanophase ni(oh)2 was added to micrometer - size spherical ni(oh)2 .
further efforts have focused on searching for new synthetic methods of ni(oh)2 nanocrystals with high quality and various exciting morphologies .
1d , 2d , and 3d nanostructures of ni(oh)2 , including nanorods , nanoribbons , nanotubes , nanosheets , and superstructures patterns [ 23 - 28 ] , have been fabricated successfully by a variety of methods . nickel oxide ( nio ) is a very prosperous inorganic material which was widely applied in the fields of smart window , electrochemical supercapacitor , battery cathodes , catalyst , etc .
, there are only limited reports concerning the synthesis of ni(oh)2 and nio hollow architectures and their interesting properties . for example , wang s group synthesized hollow architectures of ni(oh)2 with unusual form and hierarchical structures by using styrene - acrylic acid copolymer ( psa ) latex particles as the templates .
hierarchically porous -ni(oh)2 microspheres constructed with nanoflakes were recently prepared with the help of hexamethylenetetramine ( hmta ) as the basic source , exhibiting small blue shift compared with the bulk counterpart .
reported the fabrication of hierarchical ni(oh)2 monolayer hollow - sphere arrays with a fine structure of nanoflakelets by an electrochemical strategy based on a polystyrene ( ps ) sphere colloidal monolayer .
such hierarchically structured hollow - sphere arrays have demonstrated a tunable optical transmission stop band in the visible - near - ir ( vis  nir ) region from 455 to 1855 nm , depending on the hollow - sphere size and the fine structure .
however , hollow structures prepared from hard templating routes ( e.g. ps latex particles ) usually suffer from disadvantages related to high cost and tedious synthetic procedures , which may prevent them from being used in large - scale applications .
thus , it still remains a challenge to develop simple approaches to synthesize hierarchical ni(oh)2 and nio hollow architectures .
herein we describe a facile hydrothermal route to synthesize highly ordered 3d flower - like hierarchical-ni(oh)2hollow architectures with a high yield .
the morphology - retained nio hollow architectures with porous structure were readily obtained by thermal decomposition of the as - obtained-ni(oh)2products .
finally , the optical property of nio sample was investigated with the help of uv  vis spectrum .
in a typical synthesis , 1 mmol of nicl26h2o was dissolved in 5 ml of deionized ( di ) water , followed by an addition of 15 ml of ethanol and 5 ml of co(nh2)2solution ( 2 mol l ) under vigorous stirring .
then , 2 ml of nh3h2o ( 35% by v / v ) was added dropwise into the above solution to form a clear blue solution .
the autoclave was sealed and heated in an oven at 120 c for 12 h and then allowed to cool to room temperature .
the resulting pale green slurry was rinsed with di water several times to remove soluble impurities .
the product was dried in an oven at 50 c for 8 h to get the sample of-ni(oh)2 . to obtain nio
the as - prepared sample of-ni(oh)2was calcined in air for 4 h. the phase purity of the products was examined by x - ray powder diffraction ( xrd ) using a rigaku d / max 2500 diffractometer at a voltage of 40 kv and a current of 200 ma with cu - k radiation (  = 1.5406  ) , employing a scanning rate 0.02/s in the 2ranging from 30 to 80. scanning electron microscopy ( sem ) images and energy dispersive x - ray ( edx ) analysis were obtained using a hitachi s-4300 microscope ( japan ) .
transmission electron microscope ( tem ) images and the corresponding selected area electron diffraction ( saed ) pattern were taken on a hitachi-600 transmission electron microscope at an accelerating voltage of 200 kv .
high - resolution transmission electron microscope ( hrtem ) images were carried out for the as - prepared sample using jeol jem-2010 transmission electron microscope at an accelerating voltage of 200 kv .
the size distribution of the sample was measured using a scale on the magnified sem micrographs .
thermogravimetric ( tga ) and differential scanning calorimetric ( dsc ) analyses were carried out on a netzsch sta-409 pc thermal analyzer with a heating rate of 10 c minin flowing oxygen atmosphere .
room - temperature uv  vis absorption spectrum was recorded on a shimadzu uv-1601 pc uv  vis recording spectrophotometer .
in a typical synthesis , 1 mmol of nicl26h2o was dissolved in 5 ml of deionized ( di ) water , followed by an addition of 15 ml of ethanol and 5 ml of co(nh2)2solution ( 2 mol l ) under vigorous stirring .
then , 2 ml of nh3h2o ( 35% by v / v ) was added dropwise into the above solution to form a clear blue solution .
the autoclave was sealed and heated in an oven at 120 c for 12 h and then allowed to cool to room temperature .
the resulting pale green slurry was rinsed with di water several times to remove soluble impurities .
the product was dried in an oven at 50 c for 8 h to get the sample of-ni(oh)2 . to obtain nio
the phase purity of the products was examined by x - ray powder diffraction ( xrd ) using a rigaku d / max 2500 diffractometer at a voltage of 40 kv and a current of 200 ma with cu - k radiation (  = 1.5406  ) , employing a scanning rate 0.02/s in the 2ranging from 30 to 80. scanning electron microscopy ( sem ) images and energy dispersive x - ray ( edx ) analysis were obtained using a hitachi s-4300 microscope ( japan ) .
transmission electron microscope ( tem ) images and the corresponding selected area electron diffraction ( saed ) pattern were taken on a hitachi-600 transmission electron microscope at an accelerating voltage of 200 kv .
high - resolution transmission electron microscope ( hrtem ) images were carried out for the as - prepared sample using jeol jem-2010 transmission electron microscope at an accelerating voltage of 200 kv .
the size distribution of the sample was measured using a scale on the magnified sem micrographs .
thermogravimetric ( tga ) and differential scanning calorimetric ( dsc ) analyses were carried out on a netzsch sta-409 pc thermal analyzer with a heating rate of 10 c minin flowing oxygen atmosphere .
room - temperature uv  vis absorption spectrum was recorded on a shimadzu uv-1601 pc uv  vis recording spectrophotometer .
the phase structure and purity of the as - synthesized samples were examined by powder xrd .
1that all of the diffraction peaks can be indexed to a pure hexagonal structure of-ni(oh)2(jcpds no : 14 - 0117 ) .
xrd pattern of the as - obtained-ni(oh)2sample the morphologies of as - synthesized products were examined by sem and tem .
figure 2 shows the sem images of the -ni(oh)2 products . clearly , the products consist of a high yield of fairly uniform particles with the average size of about 4.5 m in diameter ( fig .
2b and c , which reveal that all the -ni(oh)2 particles have 3d flower - like hierarchical morphology .
those 3d flower - like architectures are built from several dozen of nanosheets with a thickness of 1020 nm and a width of 0.52.5 m .
the surface of the sheets assembled into the hierarchical micro - architectures was very smooth , probably due to ostwald ripening .
inset ofa : the size distribution of the as - synthesized sample;etem image of one typical hierarchical hollow architectures;fhrtem image taken from the age of the hexagonal phase-ni(oh)2sheets and the corresponding selected - area electron diffraction ( saed ) pattern ( lower left corner ) the morphologies and structures of as - synthesized samples were further characterized by tem . as shown in fig .
2e , tem observations demonstrate that the products are flower - like structures similar to the sem observation .
the remarkable feature of the hollow architectures is the obvious contrast between the dark edge and the pale center , as reported for other hollow particles with a central cavity . to further obtain structural information for the well - aligned sheets , high - resolution tem ( hrtem ) images and the corresponding selected area electron diffraction ( saed ) patterns were also recorded on single sheet . in a hrtem image ( fig .
2f ) taken from the edge of a sheet , the lattice fringes are clearly visible with a spacing of 0.27 nm , which is in good agreement with the spacing of the ( 01 - 10 ) planes of-ni(oh)2(jcpds no : 14 - 0117 ) .
the saed and hrtem analyses reveal that the building units are single - crystal . in order to reveal the formation process of the 3d flower - like hollow architectures in more detail ,
time - dependent experiments were carried out and the resultant products were analyzed by tem .
the representative tem images of the products prepared at certain reaction time intervals are shown in fig .
3 . under the present synthetic conditions , nanoparticles and some ultra - thin nanosheets can be obtained as a result of aggregation and growth after treatment for 2 h ( fig .
when the reaction time was prolonged to 6 h , besides flower - like hollow architectures , some underdeveloped flower - like hollow architectures also existed in the as - synthesized samples , as shown in fig .
after the reaction was further prolonged to 12 h , fully developed 3d flower - like hierarchical hollow architectures similar to that shown in fig .
tem images of the as - synthesized samples with treatment times ofa2 h , b6 h , andc12 h at 120 c .
sem images of the as - synthesized sample obtained at 120 c for 12 h : dwithout urea;eandfwithout ammonia;gschematic illustration of the formation of-ni(oh)23d flower - like hollow architectures in addition , the roles of urea and ammonia were found to be very important for the formation feature of 3d flower - like hollow architectures . in a control experiment , when no urea was added under the same reaction conditions , the products take on a flake - like shape ( fig .
3d ) rather than 3d flower - like hierarchical hollow architectures , while the ammonia was absent , only honeycomb - structured micro - architectures can be obtained , as shown in fig .
3e and f. on the basis of the above results in the present study , we believe that urea , ammonia , and reaction time play important roles in the formation of 3d flower - like hollow architectures
. the formation of 3d flower - like hierarchical hollow architectures may result from the combined roles of urea , ammonia under the appropriate reaction condition .
the chemical reaction in the process to obtain ni(oh)2 3d flower - like hollow microarchitectures could be formulated as follows:(1)(2)(3)(4 ) most probably , the bubbles of co2 gas produced in the reaction with the participation of co(nh2)2 must have played a key role , since no other templates / surfactants / emulsions were used in this work .
a possible formation process involving the assembly - then - assemble mechanism can be schematically illustrated in fig .
ni in solution reacts first with nh3 to form a relatively stable complex , [ ni(nh3)6 , because of its strong affinity to ni at room temperature .
afterwards , the complex was decomposed and released nh3 to provide oh ions for the formation of ni(oh)2 by a hydrothermal treatment . at the same time , with the participation of co(nh2)2 , many micrometer / sub - micrometer co2 bubbles are produced in the system at 120 c ( step a ) .
the freshly crystalline nanoparticles are unstable because of their high surface energy and tend to aggregate and form higher nanoparticles , driven by the minimization of interfacial energy . in our synthesis , the formation of [ ni(nh3)6 complex would sharply decreased the free ni concentration in the solution , which resulted in a relatively low reaction rate of ni ions with oh ions .
a slow reaction rate caused the separation of nucleation and growth steps , which is crucial for high - quality crystal synthesis .
as a result , the sheet - like high crystalline ni(oh)2 was firstly formed ( step b ) , which may be related to the nature of the initial crystal structures .
then the self - assembly and ostwald ripening process occurs around the gas / liquid interface of co2 and water , and finally 3d flower - like hierarchical hollow architectures ( step c ) . here
, co2 bubbles decomposed from co(nh2)2 can act as soft templates to induce the self - assembly of nanosheets on their surfaces .
a similar gaseous bubble has also been used as a template for tio2 and vooh hollow nanostructures , which is different from the assembly - then - inside - out evacuation mechanism in the formation of fe3o4 hollow spheres .
our time - dependent experiments support the above aggregation - then - assembly mechanism ; it is found that the assembly process occurs after the formation of the nanosheets .
the thermal behavior of flower - like hierarchical-ni(oh)2hollow architectures was investigated with tg and dsc measurements ( fig .
a tg curve showed that-ni(oh)2started to decompose ( weight loss ) at about 285 c .
the total weight loss was measured to be ~22% which is larger than the theoretical value ( 19.4% ) calculated from the following reaction:(5 ) the powders exhibit thermogravimetric transitions that are likely due to the loss of physical absorbed and structural water .
the initial weight loss from 30 to 140 c is attributed to the loss of surface adsorbed water and ethanol .
the weight loss in the range of 140365 c is due to the removal of the crystalline water molecules .
after 365 c , the weight loss continued but gradually slowed at 400 c and almost ceased at 500 c . as a consequence , the stable residue can reasonably be ascribed to nio .
the temperature range of the endothermic peak in the dsc curve fits well with that of weight loss in the tg curve , corresponding to endothermic behavior during the decomposition of-ni(oh)2to nio .
differential scanning calorimetric analysis ( dsc ) and thermogravimetric analysis ( tg ) curves of-ni(oh)23d flower - like hollow architectures the nickel hydroxyl can easily be transformed to nio upon heat treatment .
figure 5shows the xrd patterns of the flower - like hierarchical-ni(oh)2hollow architectures heated at various temperatures .
all the diffraction peaks can be indexed to the face - centered cubic ( fcc ) nio phase ( jcpds no .
no peaks due to-ni(oh)2are observed , suggesting that-ni ( oh)2is completely converted to nio after being heated for 4 h , which is also confirmed by tg and dsc studies .
notably , when increasing calcination temperature to 500 c , all the peaks belonging to nio cubic phase were markedly sharpening with high intensities , which suggested that the crystallinity of nio phase was higher at high calcination temperature than that obtained at low calcination temperature .
xrd patterns of the as - obtained-ni(oh)2samples calcined at different temperatures for 4 h : ( a ) 300 c and ( b ) 500 c the corresponding sem images and eds patterns are presented in fig .
6a , after annealing for 4 h in air , the morphology and structure of the flower - like hierarchical hollow architectures were sustained very well , which may due to the in situ conversion of -ni(oh)2 nanosheets to nio nanosheets .
in addition , the nanocontact between particles may also stabilize the 3d flower - like structure mechanically against collapse or fracture . the magnified sem image shown in fig .
this kind of porous structure was formed due to the dehydration and decomposition of ni(oh)2 during heating .
6d demonstrates that the as - prepared sample contains ni and o , and the atomic ratio of ni and o is ~44.01:40.14 , which agrees well with the stoichiometry of nio .
the au peaks come from the thin gold layer for conductive coating ( the signal of c is from the conductive adhesive ) . shown in the inset of fig .
6d is the saed pattern that was recorded from the individual nanosheet , which can be indexed to the face - centered cubic structure with phase purity .
it is interesting and surprising that the porous nanosheet still exhibits an almost single - crystalline diffraction pattern . here
, heat treatment may provide the energy to make the nio particles self - assembled with high orientation and kept the former single - crystalline nature of the sheets .
asem image of double typical 3d flower - like hierarchical nio architectures;b  cthe corresponding enlarged sem images of the area marked with a red rectangle .
insetcis a high - magnification tem image of a sheet;deds result of the as - obtained-ni(oh)2samples calcined at 500 c for 4 h. inset ofdshows saed pattern of the nio nanosheet the uv  vis absorption spectrum of the sample is presented in fig .
the strong absorption in the uv region can be observed , which is attributed to the band gap absorption of the as - synthesized nio sample . in principle , the optical band gap energy eg for a semiconductor can be estimated by the equation : ( 6 ) where  is the absorption coefficient , h is the photon energy , b is a constant relative to the materials n is either 2 for direct inter - band transition or 1/2 for indirect inter - band transition .
the band gap of the as - synthesized nio samples was about 3.57 ev by the extrapolation of the above equation , which shows obvious red - shift compared with that of the bulk nio ( 4.0 ev ) .
such a difference could be contributed to their spherical hollow hierarchical architectures and the small thickness of the sheets with porous structures , in which the interactions between the connected building blocks led to a quantum size effect .
no linear relation was found for n = 1/2 , indicating that the as - prepared nio samples have a direct band gap .
the 3d flower - like hierarchical-ni(oh)2hollow architectures have been synthesized by a facile hydrothermal route in the presence of urea and ammonia .
the 3d flower - like hollow architectures with the size of several micrometers are composed of nanosheets of 1020 nm in thickness .
the results indicated that the reaction time , urea and ammonia play important roles in the formation of 3d flower - like hierarchical-ni(oh)2hollow architectures . by calcining the as - prepared flower - like-ni(oh)2hollow architectures , hierarchical nio crystallites with porous single - crystalline nanosheets
the optical absorption band gap of the as - obtained nio samples is determined to be 3.57 ev . due to the unique architectures ,
the as - obtained products may have potential applications in water treatment , electrode , sensors , catalysts , biomarkers , microelectronics , energy storage , and other related micro / nanoscale devices due to their unique architectures .
this work was financially supported by the national natural science foundation of china ( nos . : 50573090 and 10672161 ) and beijing municipal natural science foundation ( no .
 three - dimensional ( 3d ) flower - like hierarchical-ni(oh)2hollow architectures were synthesized by a facile hydrothermal route . 
 the as - obtained products were well characterized by xrd , sem , tem ( hrtem ) , saed , and dsc - tga . 
 it was shown that the 3d flower - like hierarchical-ni(oh)2hollow architectures with a diameter of several micrometers are assembled from nanosheets with a thickness of 1020 nm and a width of 0.52.5 m . 
 a rational mechanism of formation was proposed on the basis of a range of contrasting experiments . 
 3d flower - like hierarchical nio hollow architectures with porous structure were obtained after thermal decomposition at appropriate temperatures . 
 uv  
 vis spectra reveal that the band gap of the as - synthesized nio samples was about 3.57 ev , exhibiting obviously red shift compared with the bulk counterpart . 
Introduction
Experimental Section
Synthesis of 3D Flower-Like Hierarchical
Characterization
Results and Discussion
Conclusions
Acknowledgments