N,N-B i s(2-hydroxyethy l)-2-am i noethanesu lf on i c Ac i d-ass i sted L i qu i d-phase Growth o f Au@Pd Core­She ll Nanopart i c l es w i th H i gh Cata l yt i c Act i v i tyWe i Zhang,Hu i p i ng Zhao,Zhong Lu,Fengx i Chen,*and Rong Chen*Schoo l o f Chem i stry and Env i ronmenta l Eng i neer i ng,Wuhan Inst i tute o f Techno l ogy,Wuhan430073,P.R.Ch i na(E-ma il:rchenhku@w i ,f xchen@w i )Au@Pd core­she ll nanopart i c l es were success f u ll y synthes i zed v i asequent i a l reduct i on o f Au(III)and Pd(II)sa l ts w i th BES at roomtemperature.The Au@Pd nanopart i c l es exh i b i ted s i gn ificant l y h i ghercata l yt i c act i v i ty f or var i ous Suzuk i react i ons than monometa lli c Pd or Aunanopart i c l es.S i ze-dependent cata l yt i c act i v i ty was a l so observed,i.e.,the Au@Pd nanopart i c l es o f<10nm showed h i gher act i v i ty.REPRINTED FROMVol.44No.102015p.1371–1373CMLTAGOctober5,2015The Chemical Society of JapanN ,N -B i s(2-hydroxyethy l )-2-am i noethanesu lf on i c Ac i d-ass i sted L i qu i d-phase Growtho f Au @Pd Core ­She ll Nanopart i c l es w i th H i gh Cata l yt i c Act i v i tyWe i Zhang,Hu i p i ng Zhao,Zhong Lu,Fengx i Chen,*and Rong Chen *Schoo l o f Chem i stry and Env i ronmenta l Eng i neer i ng,Wuhan Inst i tute o f Techno l ogy,Wuhan 430073,P .R.Ch i na(E-ma il :rchenhku @w i ,f xchen @w i )Au @Pd core ­she ll nanopart i c l es were success f u ll y synthe-s i zed v i a sequent i a l reduct i on o f Au(III)and Pd(II)i ons w i th N ,N -b i s(2-hydroxyethy l )-2-am i noethanesu lf on i c ac i d (BES)at room temperature.The i r s i ze and morpho l ogy cou l d be changed by vary i ng the BES concentrat i on and pH va l ue o f the react i on system.The Au @Pd core ­she ll nanopart i c l es exh i b i ted s i gn ifi-cant l y h i gher cata l yt i c act i v i ty f or var i ous Suzuk i react i ons than monometa lli c Pd or Au nanopart i c l es.The s i ze-dependent cata l yt i c act i v i ty was a l so observed,i .e.,the Au @Pd core ­she ll nanopart i c l es o f <10nm showed h i gher act i v i ty.Au ­Pd b i meta lli c nanopart i c l es w i th d ifferent m i crostruc-tures (e.g.,a ll oy or core ­she ll )have been reported to possess super i or cata l yt i c per f ormance as compared to the i r monome-ta lli c counterparts f or a var i ety o f react i ons (e.g.,deoxygenat i on o f octano i c ac i d,1U ll mann coup li ng o f ary l ch l or i des i n water,2,3ox i dat i on o f CO 4and a l coho l s,5and d i rect synthes i s o f H 2O 26).Among them,the Au ­Pd core ­she ll nanopart i c l es w i th Pd atoms concentrated on the sur f ace (denoted as Au @Pd)are more i mportant f or react i ons that cannot be cata l yzed by Au nano-part i c l es.The Au ­Pd b i meta lli c nanopart i c l es have been genera ll y synthes i zed v i a chem i ca l reduct i on o f the correspond i ng meta l sa l ts w i th reduc i ng agents (e.g.,H 2,hydraz i ne,a l coho l ,ascorb i c ac i d,CTAC,or graphene ox i de)i n the li terature.7­11In the present work,a f ac il e li qu i d-phase growth route i s app li ed to prepare the Au @Pd nanopart i c l es at room temperature i n BES aqueous so l ut i on.BES i s a Good ’s bu ffer genera ll y used f or research i n b i o l ogy and b i ochem i stry (Scheme 1).It acts as both a m il d reductant and a stab ili zer dur i ng the synthes i s.To the best o f our know l edge,th i s i s the first report f or the use o f BES to grow the Au @Pd nanopart i c l es f rom aqueous so l ut i on.A typ i ca l procedure f or mak i ng the Au @Pd nanopart i c l es (samp l e S1)was as f o ll ows:Pr i mary Au nanopart i c l es were first prepared by add i ng H[AuC l 4]so l ut i on (125¯L,50mM)i nto a round-bottom flask conta i n i ng 5mL o f BES so l ut i on (200mM,pH 7.5)and 16mg o f po l y(v i ny l pyrro li done)(average M w :55000).A f ter adjust i ng the pH va l ue to 3by HC l so l ut i on (1M),the m i xed so l ut i on was st i rred at room temperature f or 5h to get a purp l e-red Au co ll o i d.PdC l 2so l ut i on (1mL,25mM)was then added,f o ll owed by adjust i ng the pH va l ue to 3w i th NaOHso l ut i on (2M).The who l e m i xture was v i gorous l y st i rred at room temperature f or another 12h to get a dark-brown co ll o i d conta i n i ng the Au @Pd nanopart i c l es.The Au @Pd nanopart i c l es were co ll ected by centr if ugat i on and washed w i th ethano l and de i on i zed water five t i mes be f ore character i zat i on.Samp l e S1was character i zed by TEM,HRTEM,and STEM-EDX,wh i ch were taken on a Ph ili p Tecna i G20E l ectron M i croscope at an acce l erat i ng vo l tage o f 200kV .TEM i mages i n F i gures 1A ­1C showed that samp l e S1were ma i n l y spher i ca l nanopart i c l es o f 15­20nm (average s i ze:ca.17nm).The dark core and the li ght she ll were d i scernab l e f or i nd i v i dua l nano-part i c l es,i nd i cat i ng the f ormat i on o f the core ­she ll nano-structure.HRTEM i mage (F i gure 1D)revea l ed l att i ce f r i nges o f Au(111)p l anes i n the core and Pd(111)p l anes i n the she ll (d 111=0.237nm f or f ace-centered cub i c Au i n JCPDS #04-0784and d 111=0.224nm f or f ace-centered cub i c Pd i n JCPDS #46-1043).STEM-EDX a l so con firmed the core ­she ll structure o f S1.As shown i n F i gures 2B and 2C,the Au X-ray s i gna l or i g i nates f rom the center o f spher i ca l nanopart i c l es wh il e the Pd s i gna l f orms a sheath around spher i ca l nanopart i c l es.Th i s e ffect i s best ill ustrated i n the co l or map that over l ays the Au and Pd X-ray s i gna l s (F i gure 2D).A ll th i s ev i dence support that the Au @Pd nanopart i c l es have been success f u ll y prepared.It has been we ll demonstrated w i th HEPES i n prev i ous work that hydroxy and tert i ary am i ne groups can reduce nob l e meta l sa l ts to f orm nanopart i c l es (HEPES i s another Good ’s bu ffer,BESNSO 3HHOHON NSO 3HHOHEPESScheme 1.Chemi ca l structures o f two Good ’s bu ffers.F i gure 1.(A ­C)TEM i mages o f samp l e S1;(D)HRTEM i mage at the edge o f the s i ng l e nanopart i c l e i n (C).Received:June 23,2015|Accepted:July 26,2015|Web Released:October 5,2015CL-150600Chem.Lett.2015,44,1371–1373|doi:10.1246/cl.150600©2015The Chemical Society of Japan |1371represent i ng 4-(2-hydroxyethy l )-1-p i peraz i neethanesu lf on i cac i d).12­17BES has the same f unct i ona l groups as HEPES (i .e.,su lf on i c ac i d,tert i ary am i ne,and hydroxy groups as shown i n Scheme 1).There f ore,the Au(III)and Pd(II)sa l ts were sequent i a ll y reduced w i th BES as a m il d reductant to generate the Au @Pd nanopart i c l es through ep i tax i a l growth o f the Pd she ll on pr i mary Au seeds i n our BES-conta i n i ng synthet i c system.In add i t i on,BES can i nteract w i th Au or Pd meta l s v i a f unct i ona l groups (e.g.,su lf onate and am i ne groups),wh i ch he l p stab ili ze the resu l t i ng nanopart i c l es.The s i ze and shape o f the Au @Pd nanopart i c l es cou l d be changed by tun i ng the BES concentrat i on and pH va l ue o f the react i on system (samp l es S2­S6i n Tab l e 1).When the concen-trat i on o f BES was 100mM (S2),un if orm spher i ca l Au @Pd nanopart i c l es o f 4­11nm (average s i ze:ca.8nm)were obta i ned (F i gures S1a and S1b i n Support i ng In f ormat i on).When the BES concentrat i on was i ncreased to 300mM (S3),the obta i ned Au @Pd nanopart i c l es st ill preserved the spher i ca l nanostructure but w i th a broader part i c l e s i ze d i str i but i on o f 7­20nm (average s i ze:ca.10nm)(F i gures S1c and S1d).However,f urther i ncreas i ng the BES concentrat i on to 400mM s i gn ificant l y changed the morpho l ogy o f the resu l t i ng Au @Pd nanopart i c l es (S4),wh i ch cons i st o f a m i xture o f part i c l es w i th a var i ety o f shapes.Interest i ng l y,many regu l ar tetrahedra l nanopart i c l es o f ca.12nm were f ormed i n S4(F i gures S1e and S1f ).These resu l ts i nd i cate that the BES concentrat i on exerts great i n fluence on the s i ze and morpho l ogy o f the Au @Pd nanopart i c l es v i a chang i ng the nuc l eat i on and growth rate o f nanopart i c l es.At l ower pH va l ue (e.g.,pH 2f or S5),the resu l t i ng Au @Pd nanopart i c l es were most l y spher i ca l w i th narrow s i ze d i str i but i on o f 15­20nm (F i gure S2a).However,at h i gher pH va l ue (e.g.,pH 5f or S6),non-un if orm spher i ca l Au @Pd nanopart i c l es w i th broad s i ze d i str i but i on o f 10­40nm were obta i ned (F i gure S2b).It has been reported that the pH va l ue o f the so l ut i on s i gn ificant l y a ffects the s i ze and shape o f Au and Pd nanostructures.17­20Th i s work a l so revea l s that the Au @Pd nanopart i c l es cou l d be tuned by contro lli ng the pH va l ue o f the react i on system.Suzuk i coup li ng react i on between 4-i odopheny l methy l ether w i th pheny l boron i c ac i d was chosen to eva l uate the cata l yt i c act i v i ty o f the resu l t i ng Au @Pd nanopart i c l es.The react i on was genera ll y carr i ed out as f o ll ows:Pheny l bor i c ac i d (1.5mmo l ),4-i odopheny l methy l ether (1mmo l ),K 2CO 3(2mmo l ),tetrabuty l ammon i um brom i de (TBAB,1mmo l ),and the Au @Pd nanopart i c l es (0.005mmo l o f Pd)were added i nto 9mL o f ethano l/H 2O (1:2by vo l ume)so l ut i on.The react i on m i xture was re fluxed at 86°C f or 3­5m i n and then coo l ed down to room temperature.The product was extracted tw i ce w i th d i ethy l ether and dr i ed w i th anhydrous Na 2SO 4.A f ter be i ng concentrated i n a rotary evaporator,the product was pur ified by co l umn chromatography per f ormed on s ili ca ge l (200­300mesh)w i th petro l eum ether as e l uent to get the i so l ated y i e l d and i dent ified by compar i ng the 1H NMR spectra w i th standard data,wh i ch were recorded on a Bruker-Avance II 400MHz NMR spectrometer.As shown i n F i gure 3,the Au @Pd nanopart i c l es (S1and S2)exh i b i ted s i gn ificant l y h i gher cata l yt i c act i v i ty than mono-meta lli c Au or Pd nanopart i c l es w i th s i m il ar s i zes (TEM i mages o f Au and Pd nanopart i c l es shown i n F i gure S3).The i so l ated y i e l ds o f the coup li ng product on S1,S2,Pd and Au nano-cata l ysts were respect i ve l y 66%,83%,15%,and neg li g i b l e.The d ifferent y i e l ds on S1and S2a l so i mp li ed s i ze-dependent cata l yt i c act i v i ty o f the Au @Pd core ­she ll nanopart i c l es f or Suzuk i coup li ng react i on.The Au @Pd core ­she ll nanopart i c l es (S1)were f urther tested to cata l yze other Suzuk i coup li ng react i ons.As shown i n Tab l e 2,both ary l brom i des (Entr i es 1­3)and i od i des (Entr i es4F i gure 2.(A)TEM i mage o f samp l e S1;(B)Au mapp i ng;(C)Pd mapp i ng;(D)Over l apped i mage o f Au and Pd mapp i ng.Tab l e 1.E ffects o f the BES concentrat i on and pH va l ue on the synthes i s o f Au @Pd core ­she ll nanopart i c l es a Samp l e BES /mM pH S i ze d i str i but i on (average s i ze)/nm Morpho l ogy S1200315­20(17)spher i ca l S210034­11(8)spher i ca l S330037­20(10)spher i ca l S440037­20(14)po l yhedra l S5200215­20(17)spher i ca l S6200510­40(35)po l yhedra laThe genera l synthet i c procedure descr i bed f or S1i n the ma i n text was f o ll owed except the above spec ific parameters.B(OH)2+I OCH 3F i gure 3.Iso l ated y i e l ds f or Suzuk i coup li ng between 4-i odopheny l methy l ether and pheny l boron i c ac i d on d ifferent nanocata l ysts.1372|Chem.Lett.2015,44,1371–1373|doi:10.1246/cl.150600©2015The Chemical Society of Japanand5)effic i ent l y reacted w i th pheny l boron i c ac i d to produce the correspond i ng coup li ng products w i th exce ll ent y i e l ds w i th i n 6­25m i n.In sharp contrast,on l y neg li g i b l e to l ower y i e l ds were obta i ned on Pd nanopart i c l es under otherw i se i dent i ca l con-d i t i ons.The cata l yt i c resu l ts c l ear l y revea l that the Au@Pd nano-part i c l es possess s i gn ificant l y h i gher cata l yt i c act i v i ty than pure Pd nanopart i c l es.The promot i ona l effect o f Au i nd i cates synerg i st i c effects between the Au core and the Pd she ll.Th i s may be re l ated to charge trans f er between Au and Pd,wh i ch then changes the e l ectron i c structure and cata l yt i c act i v i ty o f the act i ve Pd spec i es.21,22In summary,the Au@Pd core­she ll nanopart i c l es have been success f u ll y prepared at room temperature w i th the ass i stance o f BES that acts as both a reductant and a stab ili zer dur i ng the synthes i s.The Au@Pd nanopart i c l es exh i b i ted s i gn ificant l y h i gher cata l yt i c act i v i ty f or Suzuk i coup li ng react i on than monometa lli c Pd or Au nanopart i c l es.S i ze-dependent cata l yt i c act i v i ty was a l so observed f or the Au@Pd nanopart i c l es.Our growth route to the Au@Pd nanopart i c l es prov i des an env i ron-menta ll y f r i end l y and energy-effic i ent“green”synthet i c proto-co l,wh i ch cou l d be eas il y extended to the l arge-sca l e synthes i s o f core­she ll nanostructures conta i n i ng other trans i t i on meta l s.Th i s work was supported by H i gh-tech Industry Techno l ogy Innovat i on Team Tra i n i ng Program o f Wuhan Sc i ence and Techno l ogy Bureau(No.2014070504020243),Nat i ona l Natura l Sc i ence Foundat i on o f Ch i na(Nos.21371139,21171136,and 21571146)and Natura l Sc i ence Foundat i on o f Hube i Prov i nce (No.2014CFB795).Support i ng In f ormat i on i s ava il ab l e e l ectron i ca ll y on J-STAGE. 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