nanoscale hybridization of functional organic and inorganic components realizes a spectrum of applications , which rely on the design of welldefined , multifunctional materials . noblemetallic nanoparticles ( nps ) have been important constituents for such hybrid materials because of their excellent applicability in catalysis , gas sensors , energy conversion and fuel cells , surfaceenhanced raman scattering , and printable conductive ink.1 the large surfacetovolume ratios and specific crystalline structures of nps result in a large share of atoms located on the surface , which can significantly influence the material properties.2 , 3 for example , in catalysis , ultrafine nps are typically more reactive than bulk metal counterparts . however , they are prone to aggregation due to high surface energy , and recovering of nps after completion of the reaction by simple filtration is difficult . over the past decade , research into the development of hierarchical nanostructures has focused on ensuring higher activity and reusability , especially for pd nanoparticle ( pdnp ) catalysts.4 , 5 
  hollow spherical nanostructures form a unique class of hierarchical architectures due to their high specific surface areas , enabling high metal payload and excellent catalytic activity.6 however , most of these approaches are prone to catalyst leaching or degradation from repeated cycles , thus impairing the reaction yields and limiting the reuse . moreover , multistep modifications such as chemical etching and thermolysis create additional problems of processes and energy costs . therefore , exploration of facile and efficient methods to design and fabricate such hierarchical structures for pdnps is of critical importance . another important potential applications of noble metals is in fabrication of conductive materials by printed electronics.7 , 8 these printed materials are employed in photovoltaics , transistors , displays , batteries , antennas , and sensors.8 however , metal nps in conductive inks are typically stabilized by surfactants and polymer additives . the stabilizers need to be removed before or during the sintering process , either by bulk heating or selective welding,9 in order to decrease the resistance at the junction . printed patterns of pd were applied as metallization catalysts for conductive patterns , since pd is an excellent catalyst for electroless plating of various metals.10 , 11 , 12 typically , fabrication of pd patterns requires reduction of pd photocatalyst printed on suitable substrates.13 
  herein , we present a unique functionalized graft copolymer , poly(nvkcomah)g(nh(ppo10copeo31 ) [ nvk = nvinylcarbazole , mah = maleic anhydride , ppo = polypropylene oxide , peo = polyethylene oxide ] , which facilitates controlled assembly of ultrafine pdnps . miyaura reactions , where the quantitative production of coupling compounds was achieved with trace ( 500  mol  % ) pd . furthermore , the catalyst could be easily recycled for at least five cycles without the loss of high activity . in the approach taken here , a functionalized graft copolymer , poly(nvkcomah)g(nh(ppo10copeo31 ) ( scheme  s1 in the supporting information ) , was used to produce extended wellformed polymer@pd nanowire networks . in a typical synthesis , a 120  m ( cf3coo)2pd/tetrahydrofuran ( thf ) solution ( 2  ml ) was added to a 40  m poly(nvkcomah)g(nh(ppo10copeo31)/thf solution under an n2 atmosphere . for reduction of pd , nabh4 in methanol ( 4  ml , 4  mole  equiv to pd ) was subsequently added under an n2 atmosphere , and the solution was stirred for 1  h. the resulting complex solution was poured into methanol , and the precipitate was filtered and dried under vacuum . the pd content incorporated onto the hollow spheres was estimated as 21  wt  % by thermogravimetric analysis ( figure  s2 in the supporting information ) . figure  1 shows electron microscopic images of the material generated according to this strategy . the samples were prepared by spincasting a thf solution of the product ( 3  mg  ml ) on aluminum foil and copper grids with carbon coating for scanning electron microscopy ( sem ) and transmission electron microscopy ( tem ) , respectively . the sem image ( figure  1  a ) shows a networked structure consisting of spherical particles with the average diameter of about 100  nm . the magnified sem image ( figure  1  b ) shows raspberrylike spheres consisting of pdnps with the average diameters of about 8  nm connected to each other to form the networked structure . the spheres are hollow , as shown in figure  1  b including a sphere with defects . sem  ( a ,  b ) , tem  ( c ) , hrtem  ( d  f ) , and selected area electron diffraction ( saed ) pattern  ( inset in d ) images of the pd nanowirenetwork . tem analysis was performed to investigate the assembly of the pdnps on the hollow spheres . it is quite evident from figure  1  c that the pdnps have coalesced with each other , which is a result of strong interactions . the scanning tunneling electron microscopy ( stem ) image ( figure  s3 in the supporting information ) also shows that the pdnp layer has grown over the polymer shell , producing closed nanoshells , interconnected in the nanowirenetwork fashion . it should be mentioned that uncontrolled aggregation leading to large clusters did not occur despite the strong interactions between pdnps . the nanowirenetwork is twodimensional , as confirmed by the atomic force microscopy ( afm ) image ( figure  s3 ) . the pd in the nanowirenetwork is almost monocrystalline , as confirmed by the selected area electron diffraction ( saed ) pattern ( inset in figure  1  d ) , which shows clear spots . the pd nanowirenetwork contains ( 553),14f,14 g ( 310),14c ( 111),14c,14e,14 g and ( 221)14c,14 g highindex facets along the border as shown in figure  1  d.4a , 5 g , 14 various defective crystalline structures were also observed in the highresolution ( hr ) tem images . figure  1  e indicates the presence of kinks formed between the coalescence of pd nanoparticles , which increase the coarseness of the nanostructure . figure  1  f shows twined ( 111 ) and ( 200 ) planes originated from the coalescence of the pdnps . figure  s5 in the supporting information shows the intrafacial dislocation in the lattice planes . these characteristic structures such as highindex facets are known as very reactive sites enhancing the catalytic performance of the pd material as described latter . the powder xray diffraction ( xrd ) pattern of this network ( figure  2 ) the crystallite size was estimated as 10  nm from the full width at half maximum ( fwhm ) of the ( 111 ) diffraction by applying scherrer 's equation ( d=/cos , where d is the average particle size in  ,  is fwhm of the diffraction peak in radian ,  is the peak position ( 2=38 ) ,  is the wavelength of the radiation ( 1.54056   for cuk radiation ) , and  is the shape factor of 0.89 for spherical crystals ) . the identical sizes that were calculated from the sem image and the xrd diffraction also support the monocrystalline nature . a plausible mechanism for the formation of the pd nanowirenetwork is illustrated in figure  3 . the functionalized graft copolymer poly(nvkcomah)g(nh(ppo10copeo31 ) , carries carboxylic acid groups capable of coordinating with pd ions . the resulting ionic structure around the stem chain has poor affinity with thf , whereas the polyether chains have high affinity with thf . chemical reduction with nabh4 produces pdnps from pd embedded on the sphere as seeds , forming polymer@pd nanospheres . the dynamic light scattering ( dls ) profiles in figure  3 support this mechanism . the hydrodynamic diameter ( d 
 h ) of poly(nvkcomah)g(nh(ppo10copeo31 ) in thf is 13  nm ( figure  3  a ) , and the complexation with pd results in an increase of d 
 h to 60  nm ( figure  3  b ) , due to the electrostatic repulsion forces between pd ions . the np formation via reduction concentrates the pd in solution onto the sphere surface , thus leading to their expansion ( d 
 h=130  nm ) ( figure  3  c ) . upon drying , the stiff shell consisting of pdnps maintains its shape , whereas the inner polyether chain shrinks towards the shell . as a result the selfassembly of pdnps into the nanowirenetwork can be ascribed to the strong interparticle interactions originating from the high surface energy and elasticity of pdnps.15 
  plausible mechanism of hollow sphere formation with dynamic light scattering ( dls ) profiles in thf . we then employed the pd nanowirenetwork to the mizoroki  heck ( table  1 ) and the suzuki 2 ) reactions , since these reactions have excellent applications in synthetic organic chemistry and the fabrication of electronic materials.15 , 16 first , the mizoroki  heck reaction of iodobenzene ( 1  a ) and nbutyl acrylate ( 2 ) was carried out with the nanowire network ( 500  mol  % of pd to 1  a ) in 1butanol in the presence of triethylamine ( et3n ) for neutralization and tetrabutyllammonium acetate ( tbaa)17 as an efficient base at 100  c . the coupling proceeded smoothly to give nbutyl cinnamate ( 3  a ) in a 98  % yield ( entry  1 , table  1 ) . the reactions of 1  a and pmethoxy iodobenzene ( 1  b ) with styrene ( 2 ) also proceeded smoothly to give the corresponding stilbenes , 3a and 3b , in 85 and 99  % yields ( entries  5 and 6 ) , respectively . moreover , aryl bromides may also be used as substrates for this coupling . when the reaction of bromobenzene ( 4  a ) and 2 was performed in the presence of sodium acetate and tetrabutylammonium bromide ( tbab ) , the reaction gave 3a in an 85  % yield . both electronrich and electrondeficient aryl bromides ( 4  b  d ) may also be reacted with 2  b to give the corresponding cinnamates , 3b3d , in 9095  % yields ( entries  710 ) . we ascribed the excellent catalytic activity enabling catalysis with trace amounts of pd to the characteristic structures such as highindex facets observed in the hrtem images . [ a ]  1 ( 1.0  mol  equiv ) , 2 ( 2.0  mol  equiv ) , et3n ( 3.0  mol  equiv ) , tetrabutyllammonium acetate ( tbaa , 0.2  mol  equiv ) , pd ( 500  mol  % ) , 1buoh , 100  c , 24  h. [ b ]  4 ( 1.0  mol  equiv ) , 2 ( 2.0  mol  equiv ) , naoac ( 0.6  mol  equiv ) , tetrabutylammonium bromide ( tbab , 0.2  mol  equiv ) , pd ( 500  mol  % ) , 100  c , 24  h. [ c ]  pd concentration detected by icpms in the supernatant of the reaction mixture was below 1  ppb . [ a ]  reaction conditions : phenylboronic acid ( 0.55  mmol ) , aryl bromide ( 0.50  mmol ) , k2co3 ( 2  equiv ) , tetrabutylammonium bromide ( tbab , 5  mol  % ) , pd ( 500  mol  % ) , etoh / h2o ( v / v=1/1 ) , 80  c [ b ]  pd concentration detected by icpms in the supernatant of the reaction mixture was below 1  ppb [ c ]  the catalyst was recycled by filtration and washing with etoac . miyaura coupling reaction was also examined with the polymer@pd nanowirenetwork ( table  2 ) . this crosscoupling reaction was also carried out using 500  mol  % of the pd nanowirenetwork as the catalyst in a mixed solvent of water and ethanol ( v / v=1/1 ) under reflux conditions . the yield was only 53  % but was enhanced considerably to > 99  % by the addition of 5  mol  % tbab ( run  1 ) , served as a phase transfer catalyst.18 , 20 table  2 shows that the nanowirenetworks are highly active for various electronrich and electrondeficient aryl bromides in a similar manner as the mizoroki  heck reaction ( entries  59 ) . moreover , less reactive aryl chlorides may also be used as the substrates for this coupling . crosscoupling reaction of chlorobenzene ( 6  a ) and phenylboronic acid was performed under identical conditions , and the reaction gave 5  a in 90  % yield . both electronrich and electrondeficient chlorides ( 6  b  c ) may also be reacted to give the corresponding biphenyls , 5  b5  c , in 95  % yields . figure  s6 and s7 in the supporting information show plots of conversion versus reaction time for the mizoroki  heck and suzuki  miyaura reactions , in which the conversions increase with time reaching quantitative yields in 24 and 6  h , respectively . the high activity of the polymer@pd nanowirenetwork towards aryl chlorides identical to the aryl bromides and aryl iodides can be presumably ascribed to the steric restrictions on approach of the substrates towards the pd nanowirenetwork.19 the affinity of the polymer@pd nanowirenetwork towards the reaction solvents and the substrates , for example , 1buoh , etoh / h2o , and bromobenzene , is very low as confirmed by the negligible swelling ratios ( < 5  % ) . this low affinity , made the approach as the ratedetermining step in spite of the excellent chemical activity originating from the high active sites . this low affinity of the polymer@pd nanowirenetwork is also a possible reason for the improvement of the yield by the addition of tbab . since the nanowirenetwork efficiently promoted the mizoroki  heck and suzuki  miyaura coupling reactions , recycling of the polymer@pd nanowirenetwork was investigated for both reactions under identical conditions . after the reactions in entries  1 of tables  1 and 2 were completed , the nanowirenetwork was recovered by simple filtration and washing with ethyl acetate owing to the microscaled size , and reused for the identical reactions . the catalysts of both reactions were reused five times without the loss of catalytic activity , as indicated in entries  24 in both tables  1 and 2 . the negligible leaching of pd species was confirmed in the reaction mixtures after the 5th  reuse cycles of the pd network catalysts by inductively coupled plasmamass spectrometry ( icpms ) analysis ( entries  4 in tables  1 and 2 ) . the concentrations of pd species in the supernatants of both reactions were 1  ppb , which is as low as the icpms detection limit and lower than previously reported catalyst systems that plausibly proceeded with ppmorder of leached pd species.20 the ease of redeployment of the catalyst and the quantitative conversions over several efficient cycles testifies to the versatility of the pd nanowirenetwork as an active immobilized catalyst . a hot filtration test was conducted to investigate the leaching behavior for the reaction of the 4  a and phenylboronic acid conducted under identical conditions with entry  1 in table  2 ( figure  s8 in the supporting information).21 the reaction mixture was hotfiltered at 80  c after 1  h to remove the insoluble pd network . as a result , moreover , the tem images of the reused catalysts ( figure  s9 and s10 in the supporting information ) indicate that the pd nanowirenetwork maintained the tertiary nanostructure during the reactions . these results indicate that this catalysis proceeded on the surface of the pdnps or with the undetectable amount of leached pd species , which was quantitatively captured via release catch mechanism.22 heterogeneous catalysts often suffer from leaching of the active metal species during reactions , and eventually lose their catalytic activity . leaching is also responsible for product contamination with metals and loss of the noble metal . the negligible leaching of pd species from this catalyst is an important advantage for maintaining high catalytic activity , as well as avoiding outflow of and contamination by pd . the polymer@pd nanospheres ink in thf ( 20  mg  ml ) was prepared by dispersing the polymer@pd nanosphere isolated via precipitation into methanol . the resulting ink was inkjetprinted on a cleaned glass substrate as a grid pattern with a 50  m spacing and a 15  m line width in a 11  cm square ( figure  4  a ) . figure  4  b shows the printed pd nanowirenetwork grids on a text , indicating the excellent visibility . the hybridization with the graft copolymer realized printing of the narrow pattern with the stable ink . the printed patterns showed excellent horizontal conductivity of 7.510  s  m owing to the efficient connection of pdnps existing on the shell even without annealing or removal of the organic parts . optical images of inkjetprinted grids ( 11  cm squares ) of polymer@pd nanosphere ink on glass substrate : a )  magnified image with inset image of the ink and b )  visibility of the university logo underneath the printed squaregrids of polymer@pd nanosphere . we have successfully developed a polymer@pd nanowirenetwork material by utilizing poly(nvkcomah)g(nh(ppo10copeo31 ) as the template for complexationinduced phase separation and spontaneous coassembly . this pd nanowirenetwork has the advantages of a facile preparative protocol , excellent catalytic activity and recyclability in mizoroki  heck and suzuki  miyaura coupling reactions , and application for printable and conductive materials . we anticipate that this facile fabrication procedure can be extended to the fabrication of various nano hybrid materials with designed functionalities for catalysis , energy applications , and printable electronics . complete experimental procedures , extra schemes , hrtem , spm , stem , kinetic profiles , hot filtration test , and polymer@pd nanosphere conductive ink preparation can be found in the supporting information . as a service to our authors and readers , this journal provides supporting information supplied by the authors . such materials are peer reviewed and may be reorganized for online delivery , but are not copyedited or typeset . technical support issues arising from supporting information ( other than missing files ) should be addressed to the authors .
abstracta facile onepot approach for preparing hierarchical nanowirenetworks of hollow polymer@pd nanospheres is reported . 
 first , polymer@pd hollow nanospheres were produced through metalcomplexationinduced phase separation with functionalized graft copolymers and subsequent selfassembly of pdnps . 
 the nanospheres hierarchically assembled into the nanowirenetwork upon drying . 
 the pd nanowirenetwork served as an active catalyst for mizoroki  heck and suzuki  miyaura coupling reactions . as low as 500  mol  % pd was sufficient for quantitative reactions , and the origin of the high activity 
 is ascribed to the highly active sites originating from highindex facets , kinks , and coalesced structures . 
 the catalyst can be recycled via simple filtration and washing , maintaining its high activity owing to the micrometersized hierarchical structure of the nanomaterial . 
 the polymer@pd nanosphere also served as a printable conductive ink for a translucent grid pattern with excellent horizontal conductivity ( 7.5105  s  m1 ) .