Tunable activity in electrochemical reduction of oxygen by gold polyaniline porous nanocomposites

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1 J Solid Stte Electrochem (21) 14: DOI 1.17/s x ORIGINAL PAPER Tunle ctivity in electrochemicl reduction of oxygen y gold polyniline porous nnocomposites Jixi Song & Junhu Yun & Fei Li & Dongxue Hn & Jingfeng Song & Li Niu Received: 26 Septemer 29 / Revised: 19 Decemer 29 / Accepted: 4 Ferury 21 / Pulished online: 9 Mrch 21 # Springer-Verlg 21 Astrct Oxygen electrochemicl reduction on gold polyniline (Au PANI) porous nnocomposite-modified glssy cron electrode in sic medi ws descried. The s-prepred Au PANI porous nnocomposite showed superior tunle ctivity for electrochemicl reduction of oxygen. The specific surfce re of Au PANI porous nnocomposites ws evluted to e out 11.3 m 2 g 1 through convenient voltmmetric pproch. Rotting ring-disk electrode experiments further demonstrted the numer of electrons exchnged in oxygen reduction incresed from 2e to 4e with incresing the trigger potentil from 3, to, 7 mv. The tunle ctivity in electrochemicl reduction of oxygen ws chieved s result of positive potentil-induced formtion nd reduction of Au surfce oxide. However, the tunle oxygen reduction rection is fit for pplying potentil in liner positive-going potentil sweep. Irreversile ORR tunility ws found fter more ctive surfce formed t 7 mv. To optimize the pplied potentil window on these Au-sed porous mterils hs potentil pplictions such s in electrochemicl sensing, fuel cells, or getting rid of the interference from the coexisted sustnces. Junhu Yun nd Li Niu contriuted eqully to this study. J. Song : J. Yun : F. Li : D. Hn : J. Song : L. Niu (*) Stte Key Lortory of Electronlyticl Chemistry, Chngchun Institute of Applied Chemistry, Chngchun 1322, People s Repulic of Chin e-mil: lniu@cic.jl.cn J. Song : J. Yun : F. Li : D. Hn : J. Song : L. Niu Chinese Acdemy of Sciences, Grdute University of the Chinese Acdemy of Sciences, Chngchun 1322, People s Repulic of Chin J. Yun (*) College of Chemistry nd Life Science, Zhejing Norml University, Jinhu 3214, Chin e-mil: jhyun@zjnu.cn Keywords Porous mteril. Gold polyniline nnocomposites. Oxygen reduction rection. Tunle electroctlysis. Gold oxide Arevitions ORR Oxygen reduction rection Au PANI Gold polyniline GCE Glssy cron electrode RRDE Rotting ring-disk electrode PANI Polyniline Au Gold NADH β-nicotinmide denine dinucleotide GOx Glucose oxidse Au PANI/ GCE Au PANI porous nnocomposites-modified electrode SDS Sodium dodecyl sulfte APS Ammonium peroxydisulte CV Cyclic voltmmetry EIS Electrochemicl impednce spectroscopy Introduction Nnostructured gold (Au) shows good performnces towrd ctlysis or electrochemicl ctlysis due to its high surfceto-volume rtio. Tiloring shpe or morphology of gold nnostructures hs ttrcted considerle ttention, owing to their tunle electronic, opticl, mgnetic, nd ctlytic properties nd corresponding to vrious fields of pplictions [1 6]. Thus, efforts were devoted to the preprtion of gold with verstile shpes [7 14]. Additionlly, nnoporous gold hd n unexpectedly high ctlytic ctivity for CO oxidtion [1] nd reduction of H 2 O 2 [16]. Delloying [17 19] ws used s generl route for fricting nnoporous Au, which involves the ppliction of toxic gent such s CN or corrosive concentrted HNO 3. Accordingly, greener nd

2 1916 J Solid Stte Electrochem (21) 14: simpler method for the synthesis of nnoporous Au ws ttempted to meet the need [16]. Polyniline (PANI) is one of the most importnt conducting polymers ecuse of its fcile preprtion, tunle conductivity, nd high environmentl stility. Composites contining PANI nd gold hve received gret del of ttention ecuse of their unique electronic nd opticl properties s well s extensive pplictions in diverse res. In prticulr, it ws reported tht PANI/Au multilyers [2] could electrochemiclly ctlyze the oxidtion of β-nicotinmide denine dinucleotide (NADH) nd detect DNA. PANI/Au hollow spheres [21] showed enhnced ioelectroctlytic ctivity for oxidtion of dopmine nd higher electronic conductivity compred with pure PANI hollow spheres. In ddition, PANI/Au nnoprticles composites hve een used s electron-trnsfer meditors for the ioelectroctlytic ctivtion of glucose oxidse towrd the oxidtion of glucose [22]. The oxygen reduction rection (ORR) [23 27] is one of the typicl systems for mesuring the ctlytic properties nd of gret importnce for fuel cell investigtions. However, reports on tuning the ctlytic ctivity for ORR re still very few [26, 28]. Although PANI/Au composites hve een widely studied, it hs not een reported tht PANI/Au nnocomposites with tunle ctivity of electrochemicl reduction of oxygen in sic medi. Here, the Au PANI porous nnocomposites were fricted ccording to our previous work. The specific surfce re of the Au PANI nnocomposites ws estimted c m 2 g 1 through voltmmetric pproch sed on the mount of chrge consumed during the reduction of the Au surfce oxide. The resulting Au PANI porous nnocomposites-modified electrodes (Au PANI/glssy cron electrode (GCEs)) were pplied for the ORR in sic medi. By pplying positive-going initil potentil to the modified electrode, the resulting Au PANI/GCEs reveled superior tunle oxygen reduction rection ctivity. Au oxidtion from the triggering potentil nd the synergistic effect of porous Au nd PANI in the nnocomposites plyed key roles. Moreover, irreversile ORR tunility ws found fter more ctive surfce generted t 7 mv. Experimentl Chemicls nd synthesis Before use, niline ws distilled t reduced pressure nd kept elow C. Chlorouric cid (HAuCl 4 3H 2 O), sodium dodecyl sulfte (SDS), mmonium peroxydisulte (APS), H 3 PO 4, NOH, K 4 [Fe (CN) 6 ], K 3 [Fe (CN) 6 ], nd KCl were nlyticl grde nd were used s received. The Au PANI porous nnocomposites were prepred ccording to our previous work [29]. Briefly,.167 g SDS ws dded to ml.1 mol L 1 H 3 PO 4 solution contining.1 mol L 1 niline. The solution ws ultrsonic gitted till SDS ws entirely dissolved. Then, 1. ml 1 mmol L 1 HAuCl 4 solution ws further dded dropwise nd kept stirring overnight t room temperture. The rowny green precipitte ws collected nd wshed with wter, ethnol, nd N-methyl-pyrrolidone vi centrifugtion, respectively. In control experiments, PANI ws polymerized vi dding 1 ml.4 mol L 1 APS to ml solution contining.1 mol L 1 niline nd.1 mol L 1 H 3 PO 4 nd stirring overnight. The green precipitte ws rinsed with wter, methnol N-methyl-pyrrolidone through centrifugtion, respectively. The gold nnoprticles were prepred ccording to Frens s report [7]. Briefly,. ml sodium citrte (1% y weight) ws dded into ml oiling gold chloride (.1% y weight). The wine red color solution ws cooled into room temperture fter min reduction. Mix the otined Au nnoprticles nd PANI physiclly nd centrifuged to form. mg/ml of dispersion. All glsswre used in the preprtion of Au-relted nnomterils ws soked with freshly prepred qu regi (extreme cre due to its powerful oxidtive ctivity) nd wshed thoroughly with wter. Chrcteriztions Trnsmission electron microscopy (TEM) imges were otined using Hitchi H-81 trnsmission electron microscope operted t 2 kv. TGA dt were collected on Pyris Dimond TG/DTA (PE, Americ). The smples were heted from 4 to 7 C t rte of 1 C/min under ir tmosphere. Cyclic voltmmetric (CV) mesurements were performed in conventionl three-electrode cell with coiled pltinum wire s the counter electrode nd n Ag AgCl (in sturted KCl) s the reference electrode on CHI 66 electrochemicl worksttion (CHI, USA). The working electrode ws re or chemicl-modified glssy cron electrode (d=3 mm). An EG & PARC model 636 rotting ring-disk electrode (RRDE) system nd n EG & PARC model 366 ipotentiostt were used for rotting ringdisk voltmmetry experiments. A rotting glssy cron disk pltinum ring electrode with disk dimeter of mm ws used s working electrode. The collection efficiency (N) of the ring electrode otined y reducing ferricynide t disk electrode ws.139. Electrochemicl impednce spectroscopy (EIS) were performed with Solrtron 147 ttery test unit nd Solrtron 12B frequency response nlyzer (Solrtron Inc., Englnd) for 1 mm K 4 [Fe(CN) 6 ]+ 1 mm K 3 [Fe(CN) 6 ] in.1 M KCl queous solution. A sinusoidl potentil modultion of ± mv mplitude ws superimposed on the open-circuit potentil. Applied frequency ws from 1 to.1 Hz.

3 J Solid Stte Electrochem (21) 14: The s-wshed precipitte ws dispersed in ethnol, nd then smll mount of the ethnol suspension ws dropped nd evported slowly in mient on cron-coted copper grids for TEM imging. For electrochemicl mesurements, totl of 1 μl of. mg/ml wter dispersed precipitte ws cst on the freshly polished GCE nd 3 μl for glssy cron disk pltinum ring electrode nd llowed to dry overnight in mient. Another 1 μl of.% of Nfion ws cst on the modified disk-ring electrode for stility nd dried. The modified GCE ws denoted s Au PANI/GCE. The conventionl PANI modified nd mixture of gold nnoprticles nd PANI-modified GCE were prepred following similr procedures to the ove Au PANI/GCE nd nmed s PANI/GCE nd Au+P/GCE, respectively. The nitrogen-sturted nd oxygen-sturted solutions were prepred y uling the electrolytes with pure N 2 or ultrhigh purity oxygen through the electrolyte solution for t lest 1 min, respectively. Results nd discussions Physicl chrcteriztion of Au PANI composites The resulted Au PANI composites dispersed in distilled wter re light green in color, s seen the photos in Fig. 1, which is indictive of the existence of PANI. Figure 1 is the digitl picture of the centrifuged nd dried resultnts dsored on the plstic centrifuge tue. See from the outside of the tue, it exhiits rown color with metl luster. TEM imge (Fig. 1c) shows tht the resulting nnocomposites depict n ppernce in shpe of microsheets with c nm in width of nnopores. The contrst long the edges shows tht the Au core is enwrpped with c. 8-nm-thick shell of polymer, s mrked in Fig. 1c. Therml properties of PANI nd Au PANI porous nnocomposites were mesured in ir tmosphere to investigte the components of the mterils otined. Figure 1d represents the TGA curves of PANI prepred with conventionl APS oxidtion rection (dotted line) nd Au PANI porous nnocomposite (solid line). It is oserved tht there is typicl three-step weightloss ehvior of PANI in Fig. 1d dotted line: first loss of wter or solvent followed y the second loss of dopnt nd low moleculr weight frgments of the polymer. And the finl loss results from the oxidtive degrdtion nd decomposition of the polymer ckone [3]. Heting Au PANI porous nnocomposites in ir lso represents the threestep weight loss of PANI, ut only lost 3% in the whole process nd left 97% Au component (Fig. 1d, solid line). It provides evidence tht PANI existed in smll weight percentge in the Au PANI porous nnocomposites while Au ws in dominnt percentge. Electrochemicl chrcteriztion of Au PANI nnocomposites In order to get surfce informtion out the Au porous mteril in Au PANI/GCE, 6 μl of 1 mg ml 1 Au PANI nnocomposites dispersion ws cst onto the surfce of GCE. After drying in mient for 12 h, cyclic voltmmetry were recorded in. mol L 1 H 2 SO 4 solution in the potentil rnge from 1 to 1,6 mv (vs. Ag AgCl) with the scn rte of 1 mv s 1. The reduction pek during the cthodic sweep ws used for clcultion s reveled in Fig. 2. Bsed on the mount of chrge consumed during the reduction of the Au surfce oxide monolyer nd previously reported reference vlue of 4 μc cm 2 ws used for the clcultion [16, 31 33] ccording to the two equtions [31], the specific c 1 d 1 7 Weight / wt% 98 2 Weight / wt% Temperture / o C Fig. 1 Photogrphs ( dispersion nd dry film) nd TEM imge (c) of Au PANI porous nnocomposites, d TGA curves of PANI (dotted line) nd Au PANI porous nnocomposites (solid line)

4 1918 J Solid Stte Electrochem (21) 14: I / ma surfce re of the Au PANI nnocomposites loding on GCE is estimted out 11.3 m 2 g 1, which is higher thn the previous report (i.e., 8.2 m 2 g 1 )[19]. tc ¼ dq de ¼ Idt de ¼ I ðde=dtþ. A ¼ tc C» E/ V vs. Ag AgCl Fig. 2 Voltmmogrm of Au PANI/GCE with the potentil rnge of 1 1,6mVin.molL 1 H 2 SO 4 solution. Scn rte, 1 mv s 1 ð1þ ð2þ Where τc is the totl cpcitnce, Q is chrge, E is potentil, I is current, t is time, A is the surfce re, nd C* is reference vlue. Tunle electrochemicl reduction of O 2 t Au PANI nnocomposites With the im t exploring the electrochemicl ctivity of the Au PANI for ORR, the Au PANI/GCEs were studied using cyclic voltmmetry in.1 mol L 1 NOH queous solution. The scnning potentil strted from certin positive point: 1, 3, 4,, nd 7 mv, respectively, to settled negtive potentil 6 mv. Figure 3 shows ech five sets of cyclic voltmmetric curves of Au PANI/GCE strting t successive series of incresing positive potentils in () O 2 - nd () N 2 -sturted.1 mol L 1 NOH solution. The rrows indicte the direction of the potentil scn. Both in O 2 - sturted nd N 2 -sturted.1 mol L 1 NOH solution, when the potentil limit ws 4 mv (curve C), it ws ccompnied y lrge reduction wve t c. 1 mv. And when widening the potentil window to mv (curve D) nd even to 7 mv (curve E), the reduction wves got etter resolved. It ws due to the removl of oxide formtion on the gold surfce [34, 3]. As reveled in Fig. 3 in presence of O 2, with incresing the initil potentil, from curve A to curve E, the corresponding reduction potentil of O 2 tunes positively from 413, 41, 31, 247, to 184 mv s well s the reduction current increse. The oviously incresed pek current nd positive shift in pek potentil suggests tht such Au PANI nnocomposites exhiit highly tunle electroreductive ctivity for O 2.This could e ttriuted to the influence of the formtion of surfce gold oxides [34, 3]. OH nions in the lkline solutions firstly dsored on the Au PANI nnocomposite electrode nd generted surfce Au oxides. While, surfce Au oxides would e electrochemiclly reduced to Au in the vicinity of.1 V t ckwrd potentil scn [36, 37]. Susequently, the Au electrochemicl ctlyzed O 2 t the more negtive potentil. The more positive potentil pplied, the more surfce Au oxides formed, which is more eneficil to O 2 reduction. Electrochemicl reduction of O 2 t re GCE, PANI, nd mixture of Au nnoprticles nd PANI To investigte the influences of morphology on the tunle electrochemicl reductions towrd oxygen, the susequent -2-2 A B C D E I / μa I / μa 2 A B C 2 4 D E Fig. 3 Cyclic voltmmogrms of Au PANI/GCE in () O 2 - nd () N 2 -sturted.1 mol L 1 NOH solution. A 1 6 mv, B 3 6 mv, C 4 6 mv, D 6 mv, E 7 6 mv. Scn rte, mv s 1

5 J Solid Stte Electrochem (21) 14: three control experiments were crried out. Firstly, cyclic voltmmetry ws pplied t re GCE in O 2 -sturted.1 mol L 1 NOH solution, s shown in Fig. 4, the corresponding O 2 reduction wves ehves in n opposite mnner to Au PANI, which shifts negtively from 31 to 42 mv with incresing of the strting point from 1, 3, nd 4 to mv (curve A to D). Especilly the initil potentil is settled from 7 mv, the reductive pek disppers oviously (curve E). PANI ws polymerized the APS s oxidnt nd yielded s fiers of 2 nm in dimeter on verge (inset of Fig. 4). The PANI fier-modified GCE (PANI/GCE) ws lso exmined side-y-side y CV mesurements in.1 mol L 1 O 2 -sturted NOH solution. As reveled in Fig. 4, the reductive wves re lmost round c. 43 mv (curve A to C). When the strting potentil is set s mv, only slight negtive shift is oserved in curve D. While further widening the potentil window to 7 6 mv (curve E), the PANI/GCE shows similr ehvior to the re GCE. It is well known tht PANI film does not hve electrochemicl ctivity in lkline solution. This reduction of O 2 could e ttriuted only to the contriution of re GCE, ut hindered y the cst PANI film. Gold nnoprticles were prepred through previous Frens s method [7]. Disperse the Au nnoprticles into the synthesized PANI fiers nd mke. mg/ml of dispersion. The ggregted PANI fiers were decorted with the otined Au nnoprticles of c. 24 nm in dimeter in sphere nd ellipsoid (inset of Fig. 4c). The mixture ws drop-coted on the GCE nd the resulting mixture-modified GCE (Au+P/GCE) ws mesured in O 2 -sturted.1 mol L 1 NOH solution t different strting potentils. See from the CV curves in Fig. 4c, the pek current of ORR enhnced with incresing the scnning potentils from curve A to E, respectively. However, no reduction potentil shift ws oserved. It proved the porous morphology nd the synergistic effect of Au nd PANI in the composites on the tunility of ORR to some extent. RRDE for oxygen reduction rection t Au PANI nnocomposites In sic medi, generlly, oxygen electroreduction cn proceed in two different pthwys [38]: the direct fourelectron reduction proceeds s O 2 þ 2H 2 O þ 4e! 4OH ð3þ While peroxide-intermedite-sed the two-electron pthwy cn e expressed s follows: O 2 þ H 2 O þ 2e! HO 2 þ OH HO 2 þ 2e þ H 2 O! 3OH ð4þ ðþ For further proof of higher rectivity on positive potentil polriztion, rotting ring-disk electrode technique ws employed to otin the numer of electrons ssocited with the O 2 reduction process on porous Au PANI in.1 mol L 1 NOH solution. As shown in Fig., the GC disk potentil ws scnned from 3 mv (, d), mv (, e), nd 7 mv (c, f) to 6 mv while the pltinum ring potentil ws held t 6 mv to oxidize the H 2 O 2 generted y O 2 reduction t the disk electrode. For ech electrode potentil window, 1/i hs liner reltionship with ω 1/2 to follow the Koutecky Levich eqution [39]: 1 i ¼ 1 i k þ 1 :62nFAD 2 = 3 o w 1 = 2 v 1 = 6 C o ð6þ where i k is the kinetic current in the sence of mss-trnsfer effect; n is the numer of electrons exchnged in the O 2 reduction rection; F is the Frdy constnt; A is electrode re; D ο is the diffusion coefficient of O 2 (1.9 1 cm 1 ); ω is the rotting rte; ν is the kinemtic viscosity ( cm 2 s 1 ), nd C o is the ulk concentrtion of O 2 (1.38 A B C D E c B C D E A I /μa 1 I /μa 1 I /μa 2 E D C 2 2 B A Fig. 4 Cyclic voltmmogrms of re GCE, PANI/GCE, c Au+P/ GCE in O 2 -sturted.1 mol L 1 NOH solution (A E) with different potentil rnge: (A) 1 6 mv, (B) 3 6 mv, (C) 4 6 mv, (D) 6 mv, (E) 7 6 mv. Scn rte, mv s 1. TEM imges of PANI nd Au+P re shown s inset of nd c, respectively

6 192 J Solid Stte Electrochem (21) 14: I / μa I / 1 - A - 1 d n=2 n=4 2 2 i l -1 / ma ω -1/2 (rps -1/2 ) ω -1/2 (rps -1/2 ) ω -1/2 (rps -1/2 ) Fig. Rotting ring-disk electrode voltmmogrms otined from porous Au PANI surfces in O 2 -sturted.1 mol L 1 NOH solution with different potentil rnge:, d 3 6 mv;, e 6; c, f 7 6 mv. The rotting rte ws vried from (1) 1,(2) 2,(3) 4 to (4) 8, nd () 1,2 rpm. Scn rte, mv s 1. The potentil of e n=2 n=4 I l -1 / ma c f n=2 n= I l -1 / ma the pltinum ring electrode ws set to 6 mv in order to oxidize H 2 O 2 to O 2.Echinset is Koutecky Levich plots otined from corresponding potentils windows. The solid line is fitted from the experimentl dt, nd the dshed lines re from the clculted dt considering the reduction of O 2 y two nd four electrons, respectively mol cm 3 )[38]. Tke the potentil window of 3 6 mv nd rotting rte of 1,2 rpm (Fig., d) for exmple firstly, the rtio of the ring to disk current, i R /i D,is.11 for the porous Au PANI-modified electrode. The clculted numer of electrons involved in the reduction of O 2 is found to e 2.4 ccording to the eqution n ¼ 4 2 i R i D N ðn ¼ :139Þ [4]. The n vlue suggests tht O 2 reduction on porous Au PANI surfce in.1 mol L 1 NOH solution is nerly 2e process to yield HO 2, which confirms the result cquired from the Koutecky Levich plot s shown in the inset of Fig.. In the sme wy, the n vlues were clculted to e 3.2 in scnning window of 6 mv (i R /i D =.4) nd up to 3. with initil potentil t 7 mv (i R /i D =.37) s nerly four-electron process to produce OH. It indictes the initil positive potentil-dependent kinetic ehvior of the O 2 electrochemicl reduction. Irreversiility of tunle ORR ctivity t Au PANI nnocomposites It is reported [34] tht the oxidtion region of Au rose t.6 to.8 V in lkline solutions, depending on ph. Irreversile ehvior of formtion nd reduction of oxide films t nole metls were lso previously dopted. Thus, the reproduciility of the Au oxide formtion nd reduction-relted tunility of ORR ws the concern in our present reserch. Figure 6 shows CVs of Au PANI nnocomposites-modified electrode in. mol L 1 H 2 SO 4 t the efore nd fter the series of ORRs. A positive shift of reduction pek nd more negtive onset of oxidtion potentil re oserved t the Au PANI/GCE. After electroctlyzing O 2 reduction, more gold oxide formed nd reduced result in the more ctive surfce mintined [41]. EIS cn provide useful informtion on the impednce chnges of the electrode surfce. The high fter efore 8-2 c I/ 1 - A -Z'' / KOhm 4 efore fter I /μa Z' / KOhm Fig. 6 CV curves in. M H 2 SO 4 nd Nyquist digrms in 1 mm K 4 [Fe(CN)6] + 1 mm K 3 [Fe(CN) 6 ] in.1 M KCl queous solution t open-circuit potentil of Au PANI/GCE efore nd fter series of ORRs in O 2 -sturted.1 mol L 1 NOH solution. c Another set of ORR CV grms fter finishing scnning within the ove different potentil rnges. Scn rte, mv s 1

7 J Solid Stte Electrochem (21) 14: frequency region is usully ssocited with the kinetics of the frdic process, nd the low-frequency region contins informtion regrding the diffusion of the species to the electrode surfce. The Nyquist digrms of Au PANI/GCE efore nd fter electroctlysis were mesured t different open-circuit potentils (Fig. 6). A very lrge semicircle in the high-frequency region ws oserved t fresh Au PANImodified GCE (squres). In contrst, the semicircle switched much smller fter the ove series of ORR scnning finished (strs), this promoted electron-trnsfer kinetics [42, 43] indicted more surfce ctive ws chieved y repeted ORR cycling. The ORR ctivity incresed with the higher upper potentil. However, once n oxide film hs een formed on Au t higher given potentil, the reductive pek of O 2 switched to more positive potentil rther thn recover to its originl potentil t the repeted lower upper limit. Another series of ORR ws repeted fter the CV scnning finished within the ove different potentil rnges. No remrkle tunle ctivity ws oserved in the following CV grms in Fig. 6c. However,similrpotentil pek for ORR locted t out 18 mv. So the tunle oxygen reduction rection is suitle for pplying potentil in liner positive-going direction. From nother spect, more ctive surfce gve rise to more fvorle ORR once the Au oxide formed extensively. Conclusions In summry, the Au PANI porous nnocomposites-modified electrode shows high tunle ctivity on electrochemicl reduction towrd O 2 y tuning the initil potentil in.1 mol L 1 NOH solution. Rotting ring-disk electrode voltmmetry nd Koutecky Levich plots quntittively proved the pplied initil scnning potentil influenced the pthwy involved in the reduction of O 2 in sic medi. Therefore, quntittive nd qulittive results further confirmed oth the trigger potentil-relted formtion nd reduction of Au surfce oxide nd the synergistic effects of porous Au nd PANI in the nnocomposites were significnt prmeters to control the tunle electrochemicl ctivity for ORR. However, irreversiility of tunle ORR ctivity ws found fter more ctive surfce formed t 7 mv. The tunle oxygen reduction rection is limited for pplying potentil in liner positive-going potentil sweep nd the upper potentil limit is not greter thn 7 mv. These porous nnocomposites could offer possiility to pply the mteril to specific systems, such s sensors or fuel cells, with high ctivity nd selectivity y optimizing the pplied potentil windows to eliminte the interference from the coexisted sustnces. Acknowledgments The uthors pprecite the useful discussions with Dr. Y. Zhng, Dr. Y. Shen, nd J. Zhi. And we re lso grteful to Ntionl Nturl Science Foundtion of Chin (NSFC, No nd 28274) for the finncil support. References 1. El-Syed MA (21) Acc Chem Res 34:27 2. Grzelczk M, Perez-Juste J, Mulvney P, Liz-Mrzn LM (28) Chem Soc Rev 37: Jin PK, Hung X, El-Syed IH, El-Syed MA (28) Acc Chem Res 41: Murphy CJ, Gole AM, Stone JW, Sisco PN, Alkilny AM, Goldsmith EC, Bxter SC (28) Acc Chem Res 41:1721. Skrlk SE, Chen J, Sun Y, Lu X, Au L, Coley CM, Xi Y (28) Acc Chem Res 41: Yn Y-M, Tel-Vered R, Yehezkeli O, Cheglkov Z, Willner I (28) Adv Mter 2: Frens G (1973) Nture 241:2 8. Jn NR, Gerhert L, Murphy CJ (21) J Phys Chem B 1:46 9. Sun XP, Dong SJ, Wng E (24) Angew Chem Int Ed 43: Hlder A, Rvishnkr N (26) J Phys Chem B 11: Li ZH, Rvine V, Rvine S, Grrigue P, Kuhn A (27) Adv Funct Mter 17: Huo ZY, Tsung CK, Hung WY, Zhng XF, Yng PD (28) Nno Lett 8: Lu X, Yvuz MS, Tun H-Y, Korgel BA, Xi Y (28) J Am Chem Soc 13: Hung X-J, Li C-C, Gu B, Kim J-h, Cho S-O, Choi Y-K (28) J Phys Chem C 112:36 1. Zielsek V, Jurgens B, Schulz C, Biener J, Biener MM, Hmz AV, Bumer M (26) Angew Chem Int Ed 4: Chndr D, Jen BK, Rj CR, Bhumik A (27) Chem Mter 19: Ding Y, Erlecher J (23) J Am Chem Soc 12: Dixon MC, Dniel TA, Hied M, Smilgies DM, Chn MHW, Allr DL (27) Lngmuir 23: Shulg OV, Jefferson K, Khn AR, D'Souz VT, Liu JY, Demchenko AV, Stine KJ (27) Chem Mter 19: Tin SJ, Liu JY, Zhu T, Knoll W (24) Chem Mter 16: Feng XM, Mo CJ, Yng G, Hou WH, Zhu JJ (26) Lngmuir 22: Grnot E, Ktz E, Bsnr B, Willner I (2) Chem Mter 17: Mruym J, In M, Ogumi Z (1998) J Electronl Chem 48: Zhng YR, Ashin S, Yoshihr S, Shirkshi T (23) Electrochim Act 48: Tin Y, Liu HQ, Zho GH, Ttsum T (26) J Phys Chem B 11: Zhi JF, Hung MH, Dong SJ (27) Electronlysis 19:6 27. Srpuu A, Nurmik M, Mndr H, Rosentl A, Lksonen T, Kontturi K, Schiffrin DJ, Tmmeveski K (28) J Electronl Chem 612: Cheng WL, Dong SJ, Wng EK (24) J Phys Chem B 18: Bu X, Yun J, Song J, Hn D, Niu L (29) Mter Chem Phys 116:13 3. Zhng LJ, Long YZ, Chen ZJ, Wn MX (24) Adv Funct Mter 14: Trstti S, Petrii OA (1991) Pure Appl Chem 63: Di X, Compton RG (26) Anl Sci 22: Soret TR, Strutwolf J, O'Sullivn CK (28) ChemPhysChem 9: Angerstein-Kozlowsk H, Conwy BE, Brnett B, Mozot J (1979) J Electronl Chem 1:417

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