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Nano Today (2011) 6, 240264

a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / n a n o t o d a y

REVIEW

Noble metal nanomaterials: Controllable synthesis

and application in fuel cells and analytical sensors

Shaojun Guo a,b,1, Erkang Wang a,b,

a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, Jilin, Chinab Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China

Received 18 November 2010; received in revised form 25 January 2011; accepted 21 April 2011

Available online 24 May 2011

KEYWORDS

Metal nanomaterial;Nanocluster;Nanoelectrocatalyst;Fuel cell;Sensor

Summary Nobel metal nanomaterials (NMNs) with interesting physical and chemical prop-

erties are ideal building blocks for engineering and tailoring nanoscale structures for specific

technological applications. Particularly, effectively controlling the size, shape, architecture,

composition, hybrid and microstructure of NMNs plays an important role on revealing their new

or enhanced functions and application potentials such as fuel cell and analytical sensors. This

review article focuses on recent advances on controllable synthesis and fuel cell and sensing

applications of NMNs. First, recent contributions on developing a wet-chemical approach for

the controllable synthesis of noble metal nanomaterials with a rich variety of shapes, e.g.single-component Pt, Pd, Ag and Au nanomaterials, multi-component core/shell, intermetallic

or alloyed nanomaterials, metal fluorescent nanoclusters and metal nanoparticles-based hybrid

nanomaterials, are summarized. Then diversified approaches to different types of NMNs-based

nanoelectrocatalysts with the aim to enhance their activity and durability for fuel cell reactions

are outlined. The review next introduces some exciting push in the use of NMNs as enhanced

materials or reporters or labels for developing new analytical sensors including electrochemical,

colorimetric and fluorescent sensors. Finally, we conclude with a look at the future challenges

and prospects of the development of NMNs.

2011 Elsevier Ltd. All rights reserved.

Corresponding author at: Changchun Institute of Applied Chem-istry, Chinese Academy of Sciences, State Key Laboratory ofElectroanalytical Chemistry, 5625#, Renmin Street, Changchun, Jilin130022, China. Fax: +86 431 85689711.

E-mail address: [email protected] (E. Wang).1 Present address: Department of Chemistry, Brown University,

Providence, Rhode Island 02912, US.

Introduction

Nobel metal nanomaterials (NMNs) with interesting size-dependent electrical, optical, magnetic, and chemicalproperties, as a kind of modish materials, have beenintensively pursued, not only for their fundamental sci-entific interest, but also for their many technologicalapplications [112]. At present, size, shape, architecture,composition, hybrid and microstructure of NMNs are sev-eral important key parameters in determining, revealing

1748-0132/$ see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.nantod.2011.04.007
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Noble metal nanomaterials: Controllable synthesis and application in fuel cells and analytical sensors 241

and enhancing their functions and potential applications[4,5,810]. In principle, one can accurately tune the phys-ical and chemical properties of NMNs by controlling anyone of these parameters, but the flexibility and scope ofchange are highly sensitive to some specific parameters.For instance, Au nanoparticles (NPs) have size-dependentsurface plasmon resonance (SPR) property and generallyexhibit visible SPR absorption whereas gold nanorods (NRs),

gold nanocage and hollow gold nanospheres own strongnear-infrared (NIR) absorption [2,11,13]. These novel goldnanostructures with NIR absorption are very important forphotothermal therapy and bioimaging in the NIR regionbecause blood and soft tissue in the NIR region are rela-tively transparent in this region, so that collateral damageto surrounding healthy tissue is minimized; Pt nanomate-rials with high-index facets or complex morphologies (e.g.dendritic structure) or multi-compositions have been provento exhibit higher electrocatalytic activities toward smallmolecule oxidation and oxygen reduction reactions (twokey reactions in the field of fuel cell) than the commer-cial catalysts [1416]; Ag nanostructures with proper size,complex sharp structure or more edges and corners havehigher surface-enhanced Raman scattering (SERS) activitythan spherical Ag NPs [17]; certain noble nanoclusters (NCs)(Au, Ag and Pt in particular), consisting of several to roughlya hundred atoms and possessing sizes comparable to theFermi wavelength of electrons, can exhibit molecule-likeproperties and strong size-dependent fluorescent emission[18]. Thus, the control of these pivotal parameters pro-vides the good opportunity for enhancing their applicationpotentials in the fields of catalysis, electronics, photogra-phy, photonics, sensing, imaging, medicine and informationstorage, among others [5,16]. To date, diverse methods havebeen developed to synthesize NMNs in a variety of shapessuch as rod, wire, polyhedron, dendrite, dimer, belt, star,

and cage, etc. The research on NMNs has been flourish-ing in the last decades and many research papers as wellas some review papers have been dedicated to this topic[110]. However, at present, the majority of reviews arecentered on the research work of NMNs mainly before 2007.Therefore, a highlight review on latest significant develop-ments of NMNs (including their controllable synthesis andfuel cell and sensing applications) with a particular focus onthe last 3 years, will greatly appeal to the broad readershipinterested in nanochemistry, nanoenergy and nanoanalyticalchemistry.

With the increasing environmental concerns and accel-erated depletion of fossil fuels, the development of newtechnology for producing new alternative energy conversion

and storage devices such as solar cell, supercapacitor, andlithium ion battery, etc. is very important for solving thepresent energy crisis [19]. Fuel cell, as an environmentallyfriendly energy device, has been intensely studied becauseof their numerous advantages, which include high-energydensity, the ease of handling liquid, low environmentalimpacts and their possible applications to microfuel cell[16]. At present, tremendous research efforts have beendedicated to the fabrication of efficient fuel cells with highperformance. In all of the NMNs, Pt and Pt-based nanomate-rials are still indispensable and the most effective catalystsfor fuel cells. However, one of the major obstacles for fuelcell commercialization is the cost and reliability issues of

Pt nanocatalysts used [16]. Thus, design and synthesis ofadvanced catalysts to meet the requirements of reducing Ptloading amounts and meantime increasing the activity andstability of the Pt-based catalyst is highly desired [20,21].In order to achieve this goal, some important information,challenges or opportunities should be taken adequately intoconsideration and are now summarized briefly as follows:(1) the size of Pt-based nanocatalyst should be reduced to

a smaller size in order to provide higher electrochemicalactive area. (2) Controlling the shape of Pt-based catalyststo more complex morphologies (e.g. dendritic morphology)is an important avenue for greatly improving Pt electroactiv-ity. (3) The nanocatalysts had better own high-index facets,which can provide higher activity and stability for fuel cellapplication. (4) Another interesting route for greatly improv-ing Pt activity and stability is to design Pt-based bimetalor trimetal nanostructures with controlled architectures(e.g. core/shell, alloy and even both). (5) Design high-efficiency non-Pt multi-metal catalysts. (6) Searching newsupport materials with high conductivity, chemical stabilityand surface area. (7) Effectively controlling the uniform dis-tribution of NMNs on the advanced supporting materials withhigh conductivity. Inspired by these significant challenges,many scientists have explored some advanced strategies forobtaining highly active NMNs-based catalysts for fuel cellreactions.

On the other hand, nanoanalytical sensing system is arising interdisciplinary field, which combines the inherentcharacteristics of analytical techniques (e.g., high sensi-tivity, rapid detection and low cost, etc.) with uniqueelectronic, optical, magnetic, mechanical, and catalyticproperties of nanomaterials to become one of the mostexciting topics [22,23]. Particularly, with the gradualappearance of new or enhanced properties of NMNs, differ-ent analytical techniques or strategies have been developed

to construct high-sensitivity sensors for detecting diversetargets. Three notable techniques include electrochemi-cal, fluorescent and colorimetric sensing ones, respectively.(1) NMNs-based electroanalytical technique shows the enor-mous potentials for constructing enhanced platforms forchemical sensing and biosensing. This is because thatNMNs can effectively catalyze the redox processes of somemolecules of analytical interest due to their high con-ductivity, large surface area and good surface chemistryproperty, thus permit an improvement of the analyticalperformance (lower detection limit and shorter depositiontime) of voltammetric techniques in comparison to conven-tional electrodes [24,25]. (2) NMNs-based NCs, as a newclass of fluorophore, can be used as environmentally friendly

and biocompatible fluorescence probes for the detectionof low-concentration analytes, owing to its low toxicity,inexpensive and good biocompatibility [26,27]. (3) NMNs-based colorimetric methods (the change of SPR of Au orAg NPs) are also extremely attractive because they canbe easily read out with the naked eye, and allow onsite,real-time qualitative or semi-quantitative detection with-out complicated analytical instruments [28,29]. Therefore,NMNs-based analytical sensing devices exhibit good advan-tages and potentials for detecting different targets, whichhave important significance in the aspects of environmentalpollution, serious diseases, human health and food safety,etc.


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242 S. Guo, E. Wang

In this article, we will briefly review recent advancesin synthesis, fuel cell and analytical applications of NMNstogether with discussion on its major challenges and oppor-tunities for future NMNs research. However, due to theexplosion of publications in this field, this Review articlesis not intended to be comprehensive, rather, we would liketo discuss some salient features of synthesis, and fuel celland analytical sensing applications of NMNs.

Controllable synthesis of noble metalnanomaterials

The case of Au nanomaterials

Because of their high chemical stability, oxidation resis-tance, and good biocompatibility, gold nanomaterials havewide applications in a great number of fields, such as catal-ysis, electronics, photonics, chemical sensing and imaging,information storage, drug delivery and biological labeling[1]. The controllable synthesis of gold nanomaterials is aprerequisite for the advancement of nanoscience and nano-technology because the properties and functions of goldnanomaterials can be finely tuned to enhance their versa-tility by controlling the shape. Therefore, there has beenan explosion of interest in the fabrication of Au nanostruc-tures with specific shapes through a variety of methods,particularly related to the wet-chemical approaches. Amongthem, spherical gold NPs can be synthesized more easily.Since Brusts breakthrough about synthesis of small thiol-protected gold NPs, different methods have been reportedto prepare high-quality gold NPs. Several important reviewshighlighted the important advance on synthesizing gold NPs[1,23]. For instance, our group summarized some importanttrends in the preparation of gold NPs using different ligands

as protecting agents [23]. These typical protecting agentsinclude biomolecules, surfactant, ionic liquid (IL), polymerand dendrimer, etc. Generally speaking, citrate protectedgold NPs are now in common use. This is because that citratereduction method for gold NPs has some advantages such assimpleness, no need of expensive agents, uniform size, tun-able size range and favorable molecular engineering [30,31].But, the size range of gold NPs prepared via citrate reduc-tion method is hard to achieve >100 nm, which is importantfor application in the field of photonic crystals. In order tosolve this issue, recently, we developed a facile one-stepone-phase synthetic route to achieve size-controlled (from150nm to 1m) gold micro/nanoparticles with narrow sizedistribution by using o-diaminobenzene as a reducing agent

in the presence of poly(N-vinyl-2-pyrrolidone) (PVP) via asimple wet-chemical approach [32].

In addition to spherical gold NPs, anisotropic gold nano-materials are of considerable interest because they canexhibit more physical and chemical propertiesthan sphericalgold NPs. For instance, gold NRs (a widely research object)and gold nanocages have been facilely synthesized and usedas good NIR absorption materials for biomedical application.In this aspect, Xias group first employed silver nanocubes assacrificial template to synthesize gold nanocages in the pres-ence of PVP, and further reviewed some recent advanceson synthesis, property and application of gold nanocages[11]. Furthermore, gold NRs with controllable aspect ratio

Figure 1 TEM images of 3 nm Au NWs (A), 9 nm Au NWs (B),

and HRTEM images of 3 nm NW (C), 9 nm Au NW (D). Reprinted

from Ref. [35] with permission by American Chemical Society.

could be effectively synthesized via a typical gold seed-mediated growth process [33]. Although the above strategiescould be extended to synthesize gold NWs via the use ofsequential seed-mediated growth process, simple methodsfor controlled synthesis of high-quality ultrathin gold NWswas still a great challenge. Recently, Xia, Sun and Yang et al.independently demonstrated an organic route for synthesiz-ing ultrathin gold NWs in high yield [3436]. Figure 1 shows

the typical transmission electron microscopy (TEM) andhigh-resolution transmission electron microscopy (HRTEM)images of obtained ultrathin gold NWs. It is observed thatthese NWs have high quality and good crystallinity. Despitethe above significant success with respect to the bettercontrol of the dimension of the NWs, the above meth-ods for gold NWs usually required a lengthy reaction time.Recently, Feng et al. [37] presented a modified methodfor rapidly preparing ultrathin single-crystalline gold NWsin a solution of HAuCl4, oleylamine and triisopropylsilaneat room temperature within a few hours. Interestingly,these ultrathin NWs could easily self-assemble into two-dimensional network structures with quantities of closelypacked parallel NWs when they were deposited onto the

solid substrate. Two-dimensional (2D) gold nanostructuressuch as gold nanoplate [3842], nanobelt [43] and nanoplatewith star-like structure [44,45] have also been obtainedthrough different wet-chemical strategies. Typical exampleis from Kans work [45], which presented a general methodfor the synthesis of new star-like Au microplates in the pres-ence of PVP through introducing a temperature variationin the early stage of crystal growth. 3D gold nanostruc-tures have also received intense research interests due totheir high surface area and particular crystalline structure.Some typical 3D gold nanostructures include nanodendrite[46,47], micro/nanoflower [48,49], nanocage [2,11,5052],porous NPs [53,54], dimpled gold nanoplate [55] and gold


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Figure 2 SEM images of larger RD gold nanocrystals at dif-

ferent magnifications [scale bars: (A) 1m, (B) 500 nm, (C)

200 nm]. (D) RD geometrical models with typical orientations

corresponding to RD gold nanocrystals at the same positionsin (C). Reprinted from Ref. [56] with permission by American

Chemical Society.

polyhedron [5660], etc. Among them, polyhedral goldnanomaterials are more popular due to their important roleon revealing crystal facet-dependent physical and chemi-cal properties. For instance, Niu et al. [56] demonstrateda versatile seed-mediated growth method for selectivelysynthesizing single-crystalline rhombic dodecahedral, octa-hedral, and cubic gold nanocrystals. Figure 2 shows thetypical scanning electron microscopy (SEM) image (AC) andgeometrical model (D) images of typical rhombic dodecahe-dral (RD) nanocrystals. It is found that these nanocrystalshave monodisperse size. Another kind of interesting goldnanostructures is gold polyhedron with high-index facets[57,59,60], which is very important for enhancing the elec-trochemical activity toward target molecules. Prominentexample is from Xies work [57], which firstly reported thesynthesis of trisoctahedral gold nanocrystals enclosed by 24high-index facets (such as {2 2 1}) in high yield by a simplereduction of an aqueous solution of HAuCl4 with ascorbicacid (AA) in the presence of cetyltrimethylammonium chlo-ride (CTAC) at room temperature (Figure 3).

The case of Ag nanomaterials

Ag nanomaterials are of particular interest because Agnanostructures with different size and shape show a widerange of colors owing to their localized SPR (LSRP), whichis similar to Au nanostructure. Most importantly, an inter-esting LSRP property of silver nanostructures with diversemorphologies enable them to be used as excellent surface-enhanced Raman scattering (SERS) substrates because silverexhibits the best SERS effects compared with other metalssuch as gold and copper under certain conditions [61,62].Silver also exhibited other important applications includ-ing photonics, optical sensing, and biological labeling, etc.[63]. Therefore, developing advanced strategies for control-lable synthesis of Ag nanomaterials remains highly desirable.

Figure 3 (a) Typical large-area SEM image of the as-prepared

trisoctahedral gold nanocrystals. (b) Partially enlarged SEM

image of the nanocrystals. (c) XRD pattern of the nanocrystals.

(d) High-magnification SEM image of a single nanocrystals; inset:

model of an ideal trisoctahedron enclosed by {2 2 1} surfaces,

in the same orientation as the nanocrystals in the SEM image.

(e) Models of ideal trisoctahedra in different orientations cor-

responding to those of the nanocrystals marked with the same

number in (b). Reprinted from Ref. [57] with permission by

Wiley-VCH.

Until now, the simplest method for obtaining spherical AgNPs [64,65] is heating the mixture of AgNO3 and citrate (oradditionally adding AA) at 100 C whereas a typical methodfor the preparation of 3 nm Ag NPs generally need strongreducing agent (NaBH4) [66]. However, it is hard for obtainmonodisperse Ag NPs using the existed methods. Recently,Liang et al. [67] developed a polyol method for high-yieldsynthesis of monodisperse Ag nanospheres using polyethy-lene glycol (PEG) as both solvent and reducing reagent andPVP as a capping agent, which have good monodispersity inexisted aqueous and polyol synthetic systems.

The biggest breakthrough in the synthesis of Ag nanoma-terials is that Sun and Xia [68] reported a polyol process for

controlled preparation of Ag nanocubes. The as-prepared Agnanocubes have monodisperse size (175 nm). Later, theyfound [69] that the present polyol synthetic system could beextended to synthesize Ag nanocubes in large scale throughemploying NaHS-mediated process (0.1 g for one time). How-ever, it is hard to accurately tune the size of the as-obtainedAg nanocubes in a wide range. Recently, they [70] furtherexplored a single-crystalline seed-mediated polyol systemfor controllable preparation of Ag nanocubes with the sizerange from 30 to 200 nm. The synthesis of 1 D Ag NWs andNRs is another interesting research field that deserves spe-cial attention. The earliest method for the synthesis of AgNRs is Ag seed-mediated growth strategy in the presence


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244 S. Guo, E. Wang

Figure 4 (a) Low- and (b) high-magnification SEM images

of the as-prepared silver nanorices. (c) UV/vis spectrum of

nanorices. Reprinted from Ref. [84] with permission by Ameri-

can Chemical Society.

of soft template cetyltriethylammnonium bromide (CTAB)[71]. Later, Xias [72] and our group [73] developed a polyolsystem for controlled synthesis of silver nanobar and rect-angular silver NRs, respectively. Furthermore, a numberof methods can be effectively used for synthesizing high-quality Ag NWs. However, two most typical strategies arethe polyol system reported by Sun et al. [74] and aqueoussystem presented by Sun and Li group [75]. Ag nanoplate[76], triangular bipyramid [77], nanoprism [78], nanobelt

[79], nanoring [80], nanoflower [81], nanohorn [82], nan-odimer [83] and nanorice [84] have been facilely obtainedthrough different wet-chemical or polyol systems. Typicalexample is that Liang et al. [84] reported large-scale prepa-ration of silver nanorices with uniform shape in high yieldby a facile polyol route. Figure 4a and b shows low- andhigh-magnification SEM images of a typical sample. It isinterestingly found that a large quantity of rice-shaped NPswith a narrow size distribution was achieved. Figure 4cshows UV/vis spectrum of nanorices. Two peaks in the visibleand NIR region are observed, which are caused by transverseand longitudinal plasmon resonance, respectively.

The case of Pd nanomaterials

Palladium nanomaterials, well-known for its remarkablecapacity in hydrogen absorption, are widely used as primarycatalysts for the low-temperature reduction of automobilepollutants, hydrogenation reactions, hydrogen purification,petroleum cracking, water treatment and a range of organicreactions such as Suzuki, Heck, and Stille coupling, etc.[85,86]. Palladium nanomaterials also play a key role in fuelcell technology. Recent study further reveals that Pd nano-materials exhibit good surface-enhanced Raman scattering(SERS) and sensing activity [87]. In all of these applications,the size and shape of Pd nanomaterials are still critical

Figure 5 (a) Large-area and (b) enlarged TEM, (c) high-

angle annular dark-field scanning TEM (HAADF-STEM), and (d)

SEM images of as-synthesized concave Pd nanocrystals. (e)

High-magnification TEM image of a single concave tetrahe-

dron. Top-right and bottom-right insets show the corresponding

selected area electron diffraction (SAED) pattern and the ideal

structure model of the concave tetrahedron. (f) High-resolution

TEM image of the squared area indicated in (e). Reprinted from

Ref. [105] with permission by American Chemical Society.

parameters in order to maximize their performance. There-fore, controllable synthesis of Pd nanomaterials is highly

desired for tailoring their catalytic properties and also aprerequisite for achieving their high performance in variouscatalytic applications. At present, high-quality Pd nanoma-terials with different shapes have been facilely obtainedthrough a kinetics-controlled or thermally controlled pro-cess. These rich shapes include nanocubes [86,8890], NRs[9194], NWs [93,95,96], hollow structure [89,97], poly-hedron [98100], dendrite [92,101], nanoplate [102,103],porous Pd nanotube [104], concave tetrahedral/trigonalbipyramidal palladium nanocrystals [105], tetrahexahedral(THH) Pd nanocrystals [106] and nanothorn [107], etc. Forinstance, Huang and Zheng [93] reported a facile iodide ion-mediated high-yield synthetic strategy for the preparationof uniform Pd NWs and NRs with a high aspect ratio using PVP

as both the reductant and protecting agent. Later, throughintroducing a strong reductant (formaldehyde) and chang-ing solvent (benzyl alcohol) and temperature (100 C), they[105] further obtained the concave tetrahedral/trigonalbipyramidal palladium nanocrystals (Figure 5). Moreover,one notable method for large-scale synthesis of Pd NWs wasreported by Liang et al. [95], who demonstrated an in situsacrificial template route (ultrathin Te NWs) for obtainingultralong Pd NWs in ethylene glycol solvent. In addition,polyhedral Pd nanocrystals have also made great progressdue to their important role on catalytic applications. Forinstance, Xus group [99] developed a versatile method forselectively synthesizing single-crystalline rhombic dodec-


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Figure 6 SEM images of polyhedral palladium nanocrystals

synthesized under different conditions (scale bar: 200 nm). In

columns AE, the reaction temperatures are 30, 40, 50, 60,

and 80 C, respectively. In rows 15, 5 L of 100mM, 5L of

10 mM, 25L of 1 mM, 5L of 1 mM, and 5L of 0.1 mM KI solu-

tions were added to the growth solutions, respectively. In row

6, no KI was added. Reprinted from Ref. [99] with permission by

American Chemical Society.

ahedral, cubic and octahedral palladium nanocrystals, aswell as their derivatives with varying degrees of edge- andcorner-truncation in the presence of seeds and iodine ion(Figure 6). However, it is different to obtain polyhedralPd nanocrystals with high-index facets through the seed-mediated strategy. In order to achieve this goal, Tian et al.[106] reported for the first time the direct electrodepositionof THH Pd nanocrystals with {7 3 0} high-index facets usinga programmed electrodeposition method. It is interestinglyfound that the as-prepared THH Pd nanocrystals exhibitedhigher catalytic activity toward ethanol electro-oxidation in

alkaline media than Pd black.

The case of Pt nanomaterials

Because of their high catalytic activity, Pt nanomaterialshave been widely applied in many fields including fuel cells,sensors, and the petroleum and automotive catalysis [108].Given that Pt is a precious and rare metal, most of therecent efforts have emphasized the reduction of platinumutilization through increasing the catalytic efficiency of Ptcatalysts [8,108]. A number of studies reveal that the cat-alytic reactivity of platinum nanomaterials depends highlyon their morphology, and therefore the design of novel

Figure 7 Bright-field TEM image of the dendritic Pt NPs

(DPNs). The inset image is a dark-field TEM image of the DPNs.

Reprinted from Ref. [125] with permission by American Chemi-

cal Society.

platinum nanomaterials with unique morphologies has beengreatly intensified due to their potential for enhanced andnew properties and applications in the last decades [108].At present, diverse methods for synthesizing Pt nanoma-terials with different unique morphologies are in a greatsuccess. Some new Pt nanostructures are gradually updated,which include nanocube [109111], porous nanocube [112],NW [113116], nanotube [117119], hollow sphere [120],porous nanoshere [121], Y-shaped junction [122], poly-pod [111], dendrite [15,123129], polyhedron [111,130],nanohorn [131], macroporous Pt nanocage [132], THH Ptnanocrystals with high-index facet [14,133], and nanobox

[134], etc. For instance, Xia and coworkers [113,114]employed Fe3+-mediated kinetics process for effectivelyobtaining single-crystalline Pt NWs in a polyol system (evendirectly growing them on the solid substrate). Later, the syn-thesis of Pt NWs with widths of less than 2.5 nm and lengthsof over 30 nm was successfully demonstrated by Teng et al.via a phase-transfer process [115]. Recent result revealedthat electrospinning technique is a good method for mak-ing long (cm) Pt NWs of a few nanometers diameter [116].Dendritic Pt NPs have also received considerable interestsdue to their high surface area and excellent electrocat-alytic activity. For instance, Wang et al. [125] reported arapid, one-step, and efficient route for synthesizing Pt den-drite in high yield using Pluronic F127 block copolymer as

protecting agent and AA as a reducing agent, whose sur-face area was highest for unsupported Pt catalyst at thattime (Figure 7). Actually, Pt nanomaterials are very eas-ily to form dendritic morphology due to their particularcrystalline structure, which could be revealed by a num-ber of publications [15,123129]. The biggest breakthroughin the aspect of synthesis of Pt nanomaterials was thatSuns group [14] reported a facile electrochemical methodfor synthesizing unconventional THH Pt nanospheres withhigh-index surfaces for electrocatalytic applications. Fur-thermore, another interesting finding for electrochemicalsynthesis of Pt nanomaterials was shown by Yangs group[134], who reported an electrochemical synthesis of ultra-


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246 S. Guo, E. Wang

Figure 8 Representative TEM images of Pt nanoboxes at (a)

low- and (b) high-magnifications and (c) individual nanoboxs

imaged under various tilting angles with respect to the direction

of imaging beam. Inset in (a) shows the corresponding poten-

tial cycling profile. Reprinted from Ref. [134] with permissionby American Chemical Society.

fine Pt cubic nanoboxes from Pt-on-Ag heteronanostructures(Figure 8). Their results showed that these cubic nanoboxeshave an average edge length of about 6nm and a wallthickness of 1.5 nm, which is very important for enhancingmethanol oxidation reaction.

The case of multimetallic nanomaterials

Hybridization (such as designing core/shell, intermetal-

lic, heterostructured and alloy nanostructure) provides aneffective strategy for enhancing the functionality of metalnanomaterials. For instance, Au@Pd, Au@Pt, Pt@Pd andPd@Pt coreshell structured nanomaterials have shownsuperior catalytic properties which are not attainable bytheir monometallic counterparts [135]. Bimetallic alloyedNPs exhibited higher electrocatalytic activity and stabilitythan monometallic counterparts [136]. Accurately control-ling the size, shape, composition, pore and microstructureof multimetallic nanomaterials will provide better poten-tials for tuning their physical and chemical properties andenhancing their functions and application potentials. Atpresent, controllable synthesis of multimetallic nanomate-

rials is the hottest research topics in NMNs-based science.Early studies reveal that it is easy for noble metals withlow lattice mismatch to form core/shell NPs at differenttemperature or alloy NPs at high temperature. However,it remains a big challenge to synthesize bimetallic andtrimetallic nanomaterials with specific morphologies andstructures.

Recently, some advanced techniques such as oleylamine

oleic acid system in organic phase or wet-chemical route,provided the advanced process for controllable synthesisof bimetallic and trimetallic nanostructures. These typi-cal examples include alloy [137139], core/shell [140] andintermetallic NPs [141,142], nanocube [136,143148], NRs[149,150], NWs [151], nanotube [117,152,153], alloyed poly-hedron [148,154,155], core/shell polyhedron [156158],hollow cube [89], tadpole-like AuPd heterostructure [159],AuPd and PtPd dimer [160,161], core/shell nanodendrite[162169] and Au@Pt NPs assembling hollow sphere [170],etc. For instance, intermetallic compounds often exhibitbetter physical and chemical properties such as high cat-alytic activity, high magnetism, and excellent structuralstability at high temperatures than the constituent met-als and corresponding disordered alloys [141]. Based onthese novel characteristics, Chen et al. [141] reported asimple protocol employing the diffusion of newly producedCu atoms into pre-synthesized Au nanocrystals to preparemonodisperse intermetallic CuAu NPs in organic phase.Bimetallic core/shell NPs with well-defined morphologieshave been facilely obtained through two-step or one-stepepitaxial growth strategies. For instance, Yangs group [156]developed an epitaxial growth strategy for synthesizingdiverse polyhedral bimetallic NPs. Later, Lee et al. [157]further reported on one-step aqueous synthesis of bimetal-lic coreshell AuPd NPs with a well-defined octahedralshape in the presence of CTAC. More recently, Lu et al. [158]

developed a facile method for the high-yield fabricationof AuPd coreshell heterostructures with an unusual THHmorphology using Au nanocubes as the structure-directingcores (Figure 9). As shown in Figure 9, these high-qualityAuPd coreshell THH NPs have monodisperse size. Inaddition to the above intermetallic and core/shell nano-structure, alloyed bimetallic nanostructures have receivedwide interest due to their very high electrocatalytic appli-cations. One notable example is from Fangs work [148],which presented a new wet-chemical approach to preparemonodisperse Pt3Ni nanoctahedra and nanocubes termi-nated with {1 1 1} and {1 0 0} facets, respectively, usingplatinum(II) acetylacetonate and nickel(II) acetylacetonateas metal precursors (Figure 10). Another kind of impor-

tant bimetallic nanostructrues is hetergeneous bimetallicnanomaterials. One of most important contributions is thatLim et al. [162] demonstrated for the first time a two-stepprotocol for synthesizing Pt-on-Pd bimetallic nanodendritesusing octrohedral Pd nanocrystal as a seed (Figure 11). Ourgroup [151] further developed a multi-step aqueous routefor synthesizing ultralong Pt-on-Pd bimetallic NWs with den-dritic morphology. Later, Wang et al. [169] developed anorganic process for the controlled preparation of Au-on-Ptheterostructured dendritic NPs by overgrowing Au on Pt NPs.

Interestingly, scientists began to devise some new routesfor synthesizing multimetallic (3) nanomaterials with well-defined morphologies [171174]. In this year, Suns group


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Figure 9 (a) SEM images of the cubic Au nanocrystals used to serve as the cores. (b and c) SEM and TEM images of the THH AuPd

coreshell nanocrystals. (d) HAADF-STEM image of the THH AuPd coreshell nanocrystals. (e) Energy-dispersive X-ray spectroscopy

(EDS) line scan and elemental mapping image of a THH AuPd coreshell nanocrystal. (f) Schematic drawings of a THH nanocrystal

viewed from different angles. The axes projecting along the [100], [110], [111], and [7 3 0] directions are also shown. Reprinted

from Ref. [158] with permission by American Chemical Society.

[171,172] developed for the first time a overgrowth strategyfor obtaining Au or (Pd) core/(Pt/Fe) alloy shell trimetallicNPs using Au or Pd NPs as a seed, which exhibited enhancedelectrocatalytic activity and stability for oxygen reductionreaction (ORR). Later, they employed Pd@Au NPs as seed toprepare Pd/Au/FePt core/shell NPs [173]. Another interest-ing example was presented by Wang and Yamauchi [174],who found a one-step and effective autoprogrammed routeto the synthesis of all-metal Au@Pd@Pt triple-layeredcoreshell colloids, which consist of a Au core, Pd innerlayer and nanoporous Pt outer shell, in aqueous solution atroom temperature.

The case of noble metal nanoclusters

Few-atom fluorescent noble metal nanoclusters (NCs, Au, Agand Pt in particular), which fill the gap between the atomicscale and micro/nanoscaled metallic state, are an emergingresearch area. They have molecule-like characteristics andthis intermediate character gives rise to unique and size-dependent fluorescent properties that allow applications ofthese species in electrochemluminescence, catalysis, single

molecular spectroscopy, biological labeling, optical sensing,catalysis and biomedical technology [175,176]. Therefore,controllable synthesis of metal NCs with high quality is of

Figure 10 (ae) Images for Pt3Ni nanoctahedra. (fj) Images for Pt3Ni nanocubes. (a, f) Field-emission SEM images. (b, g) High-

resolution SEM images. (c) 3D image of an octahedron. (d, i) TEM images. (e, j) High-resolution TEM images of single nanocrystals.

(h) 3D image of a cube. Reprinted from Ref. [148] with permission by American Chemical Society.


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248 S. Guo, E. Wang

Figure 11 (A) TEM image of truncated octahedral Pd

nanocrystals synthesized by reducing Na2PdCl4 with AA in an

aqueous solution. The inset shows a geometrical model of

the truncated octahedron, where the green and yellow colors

denote the {1 0 0} and {1 1 1} facets, respectively. (B) HRTEM

image of a single truncated octahedron of Pd recorded along

the [0 1 1] zone axis and the corresponding Fourier-transform

pattern (inset). (C) TEM image of PdPt nanodendrites synthe-

sized by reducing K2PtCl4 with AA in the presence of truncated

octahedral Pd nanocrystal seeds in an aqueous solution. (D)

HAADF-STEM image of PdPt nanodendrites. Reprinted from

Ref. [162] with permission by the American Association for the

Advancement of Science.

paramount importance and highly desirable. To date, inorder to obtain high-quality metal NCs, some key factorsshould be taken into consideration. (1) The ligand shouldhave strong interaction with metal NCs. (2) The reducingcondition should be strict. Generally, strong reducing agents

or light irradiation or sonication should be employed toimprove the quantum yield (QY) of NCs. (3) Long aging timeis also important for obtaining high-quality NCs. Consider-ing these crucial factors, different approaches have beendevised to synthesize high-quality noble metal NCs in acontrollable manner. At present, Au and Ag NCs are morepopular due to their obvious molecule-like properties, syn-thetic accessibility and potential analytical applications. In

the aspects of Au NCs, small thiol molecules have firstlybeen employed to synthesize Au NCs via chemical reduc-tion of Au precursor in the presence of NaBH 4 [177181].Fluorescent emission of these Au NCs could be effectivelytuned from blue to near-IR regimes. Another interesting lig-and, poly(amidoamine) dendrimer, has also been employedto synthesize Au NCs with QY of >10% [182]. Furthermore,a ligand-induced etching process for preparing highly fluo-rescent Au NCs has been reported by Duan and Nie [183]and Lin et al. [184], who used hyperbranched and multi-valent polyethylenimine and reduced lipoic acid as ligands,respectively. However, the expensive or toxic agents wereusually employed in the above synthetic procedure. The useof bio-ligand can effectively reduce the possible toxicityof Au NCs and further achieve good biomedical applica-tions. An important result was given by Xie et al. [185], whoreported a simple, one-pot and green synthetic route forthe preparation of Au NCs at the physiological temperature(37 C) with red emission using bovine serum albumin (BSA)as a bio-ligand (Figure 12). Later, Lus group [186] foundthat lysozyme could also act as a good reducing and stabi-lizing agent for the preparation of highly fluorescent goldNCs.

Compared with Au NCs, Ag NCs are more easily synthe-sized via designing different ligands as stabilizing agents.Since the pioneering reports of Dickson and co-workerson small fluorescent Ag NCs [187,188], different ligands

have been explored for synthesizing Ag NCs, includ-ing poly(acrylic acid) derivatives [189], poly(methacrylicacid) (PMAA) [190192], poly(N-isopropylacrylamide-acrylicacid-2-hydroxyethyl acrylate) [193], zeolite [194,195], den-drimers [187], amine [196], DNA [197205], thiol [206,207],mercaptosuccinic acid [208] and lipoic acid [209], etc. Forinstance, Shang and Dong [190] reported report a new

Figure 12 Schematic of the Formation of Au NCs in BSA Solution. Reprinted from Ref. [185] with permission by American Chemical

Society.


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Figure 13 Use of two different DNA duplexes with inserted cytosine loops working as synthetic scaffolds to generate fluorescent

silver NCs for the identification of the sickle cell anemia gene mutation (black dots represent hydrogen bonds formed in base pairing

and black dashed lines the sugar-phosphate backbone). Reprinted from Ref. [199] with permission by American Chemical Society.

approach for the synthesis of fluorescent and water-solubleAg NCs, using the common polyelectrolyte PMAA as thetemplate under UV illumination. They found that pH ofsolution has an important role on the fluorescent emissionof obtained Ag NCs. The maximum intensity was obtainedat pH 4.5 whereas either lower or higher pH value led toweaker fluorescent intensity. Later, Rass group [191] foundthat visible light instead of ultraviolet could also initiatethe reduction of silver ion in the presence of PMAA andfurther lead to the formation of silver NCs in an environ-mentally friendly manner. More recently, Xu and Suslick[192] presented a new and easily controlled sonochemicalstrategy for the synthesis of water-soluble fluorescent AgNCs also in the presence of PMAA. Generally, at present,more investigations have focused on the use of DNA as scaf-folds for the synthesis of Ag NCs [197205]. These studiesreveal that the generation of these novel fluorescent NCsis highly DNA-sequence-dependent [198,203,204] and theirphotoluminescence (PL) emission band could be easily tunedthroughout the visible and near-IR range by simply changingthe sequence of the oligonucleotide. Recently, our group isamazing to find that hybridized DNA duplexes could alsobe developed as capping scaffolds for the generation offluorescent Ag NCs [199]. And, the formation of fluores-cent Ag NCs in hybridized DNA duplex scaffolds was highly

sequence-dependent, which could identify a typical single-nucleotide mutation, the sickle cell mutation (Figure 13).Another important result was from Pradeeps work [208],which reported the gram-scale synthesis of two luminescentsilver NCs with red/NIR- and blue-emitting by interfacialsynthesis, which was protected by small molecules contain-ing thiol groups. In addition, other metal materials such asPt [210,211] andCu [212], could also been made into fluores-cent NCs. For instance, Kawasaki et al. [210] demonstrateda simple, one-pot strategy for the synthesis of Pt NCs inN, N-dimethylformamide (DMF) solution in the absence ofany capping agents such as surfactant, polymer, or thiolate-organic compounds.

NMNs-based nanoelectrocatalysts for fuel cellapplication

Recent significant development in nanomaterials synthesishas led to the formation of various kinds of nanomateri-als with controlled size, shape, composition, interparticleinteraction and hybrid. These functional nanomaterials pro-vide good opportunity for developing highly active catalystsfor fuel cell reactions. Generally speaking, among them,Pt and Pt-based nanomaterials are still the most effectiveelectrocatalyst for fuel cell applications, which catalyze

hydrogen or small molecule oxidation at anode and ORRat cathode. Herein, from the perspective of enhancing theelectroactivity of catalysts, some recent push in developinghigh-efficiency Pt-based nanoelectrocatalysts for fuel cellapplications are highlighted.

Single-component Pt nanoelectrocatalysts with

controlled size and shape

Synthesis of Pt nanomaterials with controlled sizes andshapes is important for enhancing fuel cell applications.The spherical NPs with small size are usually employed aselectrocatalysts for catalyzing ORR and small molecule oxi-

dations since they offer high surface-area-to-volume ratios.Smaller the size of NPs is, their surface active area is higher.Thus, smaller size of NPs will facilitate the higher electroac-tivity for fuel cell reactions. Therefore, the commercialelectrocatalysts often employed small-size Pt or Pd NPs(3 nm). In addition to the generally recognized small size,recent trends reveal that shape also plays an important roleon enhancing the electrocatalytic activity of Pt nanomateri-als [14,109,110,129]. For instance, Wang et al. synthesizedPt nanocube, polyhedron and truncated cube by reaction ofplatinum(II) acetylacetonate with oleic acid and oleylaminein the presence of trace amount of Fe(CO)5 [109,110]. Theyfound that 7 nm Pt nanocubes with rich (1 0 0) facets were


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250 S. Guo, E. Wang

Figure 14 (A) Scheme of electrochemical preparation of THH Pt nanocrystals from nanospheres. (B) Low-magnification SEM imageof THH Pt nanocrystals with growth time of 60 min. (C and D) High-magnification SEM images of THH Pt nanocrystals viewed down

along different orientations, showing the THH shape. (E) Geometrical model of an ideal THH shape. (F) High-magnification SEM

image of a THH Pt nanocrystal, showing the imperfect vertices as a result of unequal size of the neighboring facets. Scale bars in

(C), (D), and (F), 100 nm. Reprinted from Ref. [14] with permission by the American Association for the Advancement of Science.

shown to be much more active than the other shaped PtNPs in H2SO4 solution, which was due to different adsorp-tions of sulfate ions on Pt (111) and (100) facets [109].Another important finding was given by Tian et al. [14], whofound that single-crystal Pt THH nanospheres enclosed by 24high-index facets such as (7 3 0), (21 0) and (52 0) surface

have a large density of atomic steps and dangling bonds,which exhibited much enhanced specific activity and sta-bility for electro-oxidation of small organic fuels than thecommercial C/Pt catalysts (Figure 14). Although the max-imization of high-index surfaces and abundant corner andedge sites should be the criteria for selection of an excel-lent nanocatalyst, the size of above Pt THH nanospheres isbig, which was disadvantage for enhancing Pt mass activity.In addition, changing the Pt nanospheres to more specificnanostructures such as Pt nanotube [117,118,213], Pt Y-shaped junction [122], Pt NWs [214,215], Pt hollow spherewith urchin-morphology [65], Pt dendrite [126,128] and Ptnanobox [134], etc. could also provide some avenues forreducing the amount of Pt used and improving the efficient

utility of Pt. For instance, Chen et al. [117] showed that Ptnanotubes obtained using Ag NWs as a sacrificial templateexhibited higher electrocatalytic activity and durability thanthat of commercial E-TEK catalyst and platinum black. Thisis because that Pt nanotubes could be less vulnerable to dis-solution, Ostwald ripening, and aggregation and eliminatethe carbon corrosion problem during fuel cell operation. Inorder to improve the activity of Pt nanotubes, they fur-ther employed a slightly modified procedure to generateporous Pt nanotubes with a subsequent investigation of theirORR activity and durability [213]. Their results revealedthat porous Pt nanotubes have better advantage for enhanc-ing the activity of ORR and methanol oxidation reaction

(MOR). Sun et al. [215] developed an aqueous phase routefor the synthesis of single-crystal Pt NWs on the nanospheresof a carbon black at room temperature, and found thattheir catalytic activity for ORR is much higher than thatof the state-of-the-art C/Pt catalyst. In addition, our groupfound that designing a hollow Pt nanosphere with urchin-

like morphology would also facilitate the electroactivityenhancement of ORR and MOR in H2SO4 solution [65].

Multi-component metal nanoelectrocatalysts

Recently, bimetallic nanoelectrocatalysts in the form ofcore/shell and alloy have attracted more interest in orderto achieve the goal of decreasing Pt loading amount whileenhancing its activity and durability. Core/shell M/Pt (Mdenotes transition metal) nanostructures are the mostwidely studied targets in all the bimetallic systems, whichare mainly classified as two kinds of nanostructures. Plat-inum monolayer or overlayer electrocatalysts, composed of

a Pt monolayer or overlayer deposited on a metallic NP core,are one of the most promising nanoelectrocatalysts for theORR [216,217]. For instance, Dongs group [216] employedunderpotential deposition (UPD) redox replacement strat-egy to design Pt overlayer-coated Au NP monolayer filmsand found that the obtained bimetallic nanomaterials exhib-ited high electrocatalytic activity. Later, Yoo and Park [217]extended this strategy for synthesizing electrocatalyticallyactive, bimetallic, core/shell NR arrays by coating an atom-ically Pt thin layer over a nanoporous gold NR array. Morerecently, Adzics group [218,219] found that Pd, Pd9Au1alloy, AuNi0.5Fe NP core/Pt monolayer shell electrocata-lysts showed more obvious advantage for enhancing the


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Figure 15 TEM (AB, DE), HAADF-STEM (C) and HRTEM (F) images of Pt-on-Pd bimetallic NWs with nanodendritic morphology.

FFT pattern (G) of the HRTEM image shown in F (circled part). Reprinted from Ref. [151] with permission by Royal Society of

Chemistry.

activity and stability for ORR, which can facilitate theiruse in automotive fuel cells. Another kind of interestingcore/shell Pt-based nanoelectrocatalyst is designing themorphology of Pt into more complex architecture with highsurface area (e.g. dendrite) [162170,220]. For instance,Xias group [162] developed a truncated octahedral Pdnanocrystal-mediated seed growth process to prepare PdPtbimetallic nanodendrites with high surface areas. ThesePdPt nanodendrites did not require special treatment andshowed a 2.5 times enhancement in ORR activity com-pared to commercial Pt/C catalysts. Later, our group [151]demonstrated a facile, wet-chemical strategy for the highyield (100%) synthesis of ultralong Pt-on-Pd bimetallic

NWs with the cores being Pd NWs and the shells beingmade of dendritic Pt, which exhibit high surface area andenhanced electrocatalytic activity toward MOR (Figure 15).In addition, Pt-on-Au bimetallic dendritic NPs exhibit someobvious advantages for fuel cell reactions because Au hasa higher chemical stability and durability as a catalystcomponent than Pd [135], and Au NPs can additionallyexhibit a SPR property. Based on this consideration, ourgroup [166] successfully reported a one-step, room tem-perature, and aqueous route for the synthesis of Pt-on-Aubimetallic dendritic NPs with noncompact nanobranches(BDNNNs) in high yield (100%). These BDNNNs have manybig gaps available among the Pt nanobranches, and thushave high catalytic activity for MOR. Designing bimetallic

porous nanotubes is an important avenue for improving theactivity and durability of fuel cell reactions due to theirmicrometer-sized length and thin wall. Typical example isfrom Dings work [221], who took the advantage of thesimple galvanic-replacement reaction between H2PtCl6 (orHAuCl4) and Ag, and successfully fabricated bimetallic Pt/Aunanotubes that showed high electrocatalytic activity towardformic acid oxidation with enhanced tolerance to CO poison-ing.

Adding a non-expensive transition metal (M) to Pt canchange the geometric (PtPt bond distance and coordi-nation number) and electronic structures of Pt, which isan important avenue for enhancing or tuning the electro-

catalytic activity of Pt [16]. Particularly, for the design ofalloyed NPs, one key point is to know that the role of Mshould provide enough sites for facilitating the adsorptionof OH group whereas the adsorption on the Pt sites shouldbe diminished. Furthermore, the electrocatalytic ability ofPt-based alloyed NPs is strongly dependent on the type andconcentration of M. For instance, Chen et al. [222] foundthat FePt NPs have composition-dependent ORR activitywith Fe63Pt37 < Fe58Pt42 < Fe54Pt46 < Fe42Pt58 Fe15Pt85>Pt.The crystal structure of alloy NPs is important for improv-ing the activity and stability of fuel cell. Typical example isfrom Suns contribution [223], which demonstrated that thefct-FePt NPs show higher activity and durability than the

fcc-FePt in the ORR condition. In addition, shape of alloyedbimetallic NPs played an important role on enhancing theactivity for fuel cell reactions. Particularly, recent researchemphasis has focused on the designed synthesis of Pt-(Co,Ni, Fe, Cu, Mn) nanocube [136,143,144,147,224], all ofwhich exhibited much higher electrocatalytic activity thancommercial catalysts. For instance, Kang and Murray [147]demonstrated cubic MnPt nanocubes have higher electro-catalytic activities for ORR and methanol and formic acidoxidation than the commercial E-TEK Pt catalyst in H2SO4solution due to their rich (1 0 0) facets (Figure 16). Inverse tothe above conclusion, alloyed bimetallic octahedron or trun-cated octahedron with rich (1 1 1) facets exhibited higherelectrocatalytic activity for ORR than commercial electro-

catalysts or even alloyed nanocube in HClO4 solution. Twotypical examples were from Yangs group [154] and Fangsgroup [148], both of whom demonstrated an organic phaseroute for synthesizing truncated octahedral Pt3Ni and octa-hedral Pt3Ni nanoelectrocatalysts. Specially, Zhang et al.[148] found that in HClO4 solution, the ORR activity onthe Pt3Ni nanoctahedra is about 5-fold higher than that ofnanocubes with a similar size. Another important advance onthe shape-dependent catalysis of bimetallic alloyed nano-electrocatalysts were reported by Xu et al. [152,225], whodeveloped a general approach to prepare nanotubular meso-porous bimetallic nanocatalysts (Pt/Cu and Pd/Cu) basedon a simple combination of room-temperature dealloying


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252 S. Guo, E. Wang

Figure 16 (A) ORR polarization curves for MnPt nanocubes, E-TEK C/Pt, and Pt black normalized to geometric area. (B) Mass

activity (as kinetic current densities at 0.8 V) for the three catalysts. (C, D) ORR polarization curves for cubic and spherical MnPt

nanocrystals in (C) 0.1M H2SO4 and (D) 0.1 M HClO4. (E) Formic acid and (F) methanol oxidation polarization curves for cubic MnPt

nanocrystals, spherical MnPt nanocrystals, and E-TEK C/Pt. Reprinted from Ref. [147] with permission by American Chemical

Society.

and galvanic-replacement reaction, which shows obviouslyenhanced electrocatalytic properties.

Besides the above nice bimetallic nanoelectrocatalysts,recently, trimetallic nanoelectrocatalysts began to become

a very hot research topic due to their more obvious syner-getic effect on the enhancement of catalytic activity andstability. Two important examples are from Suns group,who reported a unique approach for synthesizing core/shellstructured Pd/FePt NPs or Au/FePt or Pd/Au/FePt NPs[171173]. Then found that ORR activity on the Pd/FePtNPs is dependent on the FePt shell thickness, and thin FePtshell (1 nm or less) is both active and durable for ORR in0.1M HClO4 solution (Figure 17). More recently, Cui et al.[226] synthesized a family of PdAuCu heterostructured NPtubes catalysts by a one-step electrodeposition route ontoan anodic aluminum oxide template in anhydrous dimethylsulfoxide (DMSO) solution without the addition of any other

surfactants, which could enhance the activity and durabil-ity for fuel cell application, eliminate the support-effectproblem, and relax the Ostwald ripening and aggregation incontrast to the situation for NPs.

Pt-based hybrid electrocatalyst

Carbon nanomaterials such as carbon nanotubes (CNTs)and graphene are promising to replace the commercial-ized carbon black as the catalyst supports for the fuel cellapplication because they can effectively enhance the uti-lization, activity, and stability of the catalytic Pt NPs [227].At present, two kinds of important factors should be takensubstantially into consideration for obtaining high-efficiencycarbon nanomaterials supported metal NP electrocatalysts,which are the essential requirements for achieving the bet-


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Figure 17 HAADF-STEM (A), high-resolution HAADF-STEM (B), and elemental mapping (C) images of the 5 nm/1 nm Pd/FePt NPs.

Reprinted from Ref. [171] with permission by American Chemical Society.

ter performance in fuel cells. (1) Pt-based nanomaterialson the surface of carbon nanomaterials should have smallsize, controllable morphology, controllable density and uni-form distribution. (2) Carbon nanomaterials supports shouldhave high surface area, good electrical properties, and highelectrochemical stability under fuel cell operating condi-tions. Surrounded by these two key points, many scientistshave devoted their great efforts to synthesizing high-qualitycarbon nanomaterials supported Pt-based NP electrocata-lysts. Some recent typical examples include polymer as a

linker for synthesizing CNT/PtRu NPs hybrid electrocatalystsin polyol system [228,229], polymer as a linker for the prepa-ration of herringbone graphite carbon nanofibers supportedPtRu NPs [230], polyol heating approach to directly preparePt-Co/CNx nanotube electrocatalyst [231], flowerlike Pt NPclusters electrodeposited onto CNTs by using a three-stepprotocol [232] and IL-assisted preparation of CNT-supporteduniform Pt-based NPs [233236]. For instance, our group[233] described a convenient approach for the synthesis ofCNT/IL/Pt NPs hybrids using IL as linkers. Figure 18 illus-

Figure 18 Illustration of the procedure for preparing CNTs/ILs/Pt hybrids. DMF represents N, N-dimethylformamide. Reprinted

from Ref. [233] with permission by Wiley-VCH.


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254 S. Guo, E. Wang

Figure 19 TEM (AC) and HRTEM (D, E) images of GN/bimetallic nanodendrite hybrids at different magnifications. The circled

parts in panel D denote Pd NPs. FFT pattern (F) of the HRTEM image shown in panel E (circled part). Reprinted from Ref. [248] withpermission by American Chemical Society.

trates our concept. First, COOH-functionalized CNTs reactedwith amine-terminated methylimidazole in the presence ofa coupling agent (N, N-dicyclohexylcarbodiimide) to formIL-functionalized CNTs. Second, the counteranion (PtCl4

2)was coupled by using the imidazolium group on the sur-face of CNTs, which is a common cation in ILs. Third, theresulting complex compound was reduced by AA at roomtemperature to produce the CNTs/IL/Pt hybrids. The as-prepared three-component CNT/IL/Pt NPs hybrids exhibit

higher electrocatalytic activity toward MOR than thoseof E-TEK and CNTs/poly(diallyldimethylammonium chlo-ride) (PDDA)/Pt hybrid catalysts. Controlling the shape andmicrostructure of Pt nanomaterials on the surface of CNTs isalso an effective strategy for enhancing the performance ofhybrid electrocatalysts [237,238]. At present, there are twomain examples about synthesizing Pt nanomaterials withcontrolled shape supported on CNTs. One is from Yangsgroup, who presented an attractive method for the deco-ration of CNTs with Pt nanocube based on a noncovalentfunctionalization assembling strategy. The second work isabout faceted platinum nanocrystals supported on CNTs syn-thesized in the presence of the shape-controlling agent(NO2) through a solution chemistry route. Both of the above

hybrid electrocatalysts exhibited very high electrocatalyticactivity for fuel cell applications.

In contrast to one-dimensional carbon nanomaterials, therecent emergence of graphene nanosheets (GN, 2010 NobelPrize for Physics) has opened a new avenue for utilizing2D new carbon material as a support because of their highconductivity (103104 S/m), huge surface area (calculatedvalue, 2630 m2/g), unique graphitized basal plane structure,and potential low manufacturing cost [239]. Several groupshave explored advanced techniques for preparing GN/PtNPs hybrids and further used the hybrids for enhanced ORRand MOR. Generally, most of contributions are still concen-trated on synthesizing single-component Pt NPs supported

on GN [240247]. Although the obtained composite cata-lysts exhibited high electrocatalytic activity, their quality isstill not satisfying (e.g. nonuniform distribution of Pt NPs,wide size range of Pt NPs). In order to solve this prob-lem and further improve the activity of Pt, recently, ourgroup [248] reported a wet-chemical approach for the syn-thesis of GNs supported high-quality 3D Pt-on-Pd bimetallicnanodendrites, as shown in Figure 19. The electrochem-ical results revealed that the as-prepared GN/bimetallic

nanodendrite hybrids with controlled nanodendrite densityexhibited much higher electrocatalytic activity and stabilitytoward MOR than the platinum black and commercial E-TEKPt/C catalysts.

Noble metal nanomaterials for analyticalsensors

Noble metal nanomaterials for electrochemical

sensors

The development of nanoscience provides enormous oppor-tunities for analytical chemists. NMNs are one of most widely

used materials in electroanalytical investigations and havegood potentials for constructing electrochemical sensingplatforms with high sensitivity and selectivity to detect tar-get molecules based on different analytical strategies. Inour previous contributions, we have reviewed that Au NPsand inorganic nanomaterials could be used as enhancedelectrode materials for electrochemical sensing applications[22,23]. In order to further improve the sensitivity of elec-trochemical detection, it is very necessary to find betterelectrode materials for electroanalytical applications. Theclever combination of different nanoscaled inorganic nano-materials with good conductivity may open a new avenuefor utilizing nobel metal-based hybrid nanomaterials as


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Figure 20 a) Schematic of a GN FET. Anti-IgG was anchored to the GN sheet surface through Au NPs and functions as a specific

recognition group for the IgG binding. The electrical detection of protein binding (IgG to anti-IgG) was accomplished by FET and

direct current measurements. b) Schematic illustration of the GN FET biosensor fabrication process. GNs were firstly dispersed on

the electrodes and then decorated with Au NP-antibody conjugates through noncovalent attachment. Reprinted from Ref. [259]

with permission by Wiley-VCH.

enhanced elements for constructing electrochemical sensingplatform with high performance. This is because compos-ite nanomaterials could provide larger electrochemically

active surface areas for the adsorption of target moleculesand effectively accelerate the electron transfer betweenelectrode and detection molecules, which could lead toa more rapid and sensitive current response [249251].Recently, our group reported a series of metal-basedhybrid functional nanomaterials for constructing enhancedelectrochemical sensing platform for detecting differentmolecules [24,25,249,252254]. These typical hybrid nano-materials include CNT/silica coaxial nanocable supportedAu/Pt hybrid NPs [24], polyaniline nanofiber/high-density PtNP hybrids [25], graphene/Pt or Au NP hybrids [249,252]and high-density Au/Pt hybrid NPs supported on TiO2nanospheres [253,254]. In addition, several other groupshave also synthesized advanced hybrid functional mate-

rials for electroanalytical applications [250,251,255257].Several typical examples are followed. Huang et al. [250]demonstrated that Pd-NPs-loaded carbon nanofibers havebeen employed as enhanced materials for direct andmediator-less sensing H2O2 and NADH at low potentials.Zhus group [255] developed a self-assembly process forsynthesizing Au NP/C nanosphere hybrids. The as-preparedhybrid material could be conjugated with horseradishperoxidase-labeled antibody (HRP-Ab2) to fabricate HRP-Ab2AuNP/C bioconjugates, which could then be used as anamplified label for the sensitive protein assay. Nius group[258] presented a novel glucose biosensor based on immobi-lization of glucose oxidase in thin films of chitosan contain-

ing nanocomposites of graphene and gold NPs. The resultingsensing platform exhibited wide linear range and low detect-ing limit. More recently, Mao et al. [259] firstly reported

on the fabrication of a highly sensitive and selective fieldeffect transistors (FET) biosensor using thermally reducedGO sheets decorated with gold NP-antibody conjugate asamplified component for the ultrasensitive detection of pro-tein (Figure 20). This novel biosensor without any proteinengineering exhibited a high detection limit of (13 pM),whose performance (without optimization) is among thebest of carbon nanomaterial-based FET protein sensors.

Noble metal nanomaterials for colorimetric sensors

Colorimetric sensors are very attractive due to their sim-plicity, high sensitivity, low cost, easily read out with

the naked eye or concisely performed with UV/vis spec-trometry instead of the complex instruments and in somecases used at the point of use. Metal NPs are emergingas one kind of important colorimetric reporters becausetheir extremely high visible-region extinction coefficients(1081010 M1 cm1) are often several orders of magni-tude higher than those of organic dyes. In principle, thiscolorimetric sensing strategy relies on the fact that thedispersed gold NPs solution is red whereas the aggregatedgold NPs solution is purple even blue [260262]. And, inorder to improve the sensitivity and selectivity of col-orimetric sensors, engineering gold NPs with functionalmolecules having high recognition ability is also neces-


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256 S. Guo, E. Wang

sary. At present, significant research interest has beendirected toward gold NPs-based colorimetric assays forDNA, enzyme activity, small molecules, metal ions, carbohy-drates, and proteins, etc. using their unique SPR as sensingelements [261287]. In all the Au NPs-based colorimet-ric sensors, DNA-functionalized Au NPs as sensing elementshave received more interest because DNA has many uniquefunctions, which could be easily tuned by the binding of tar-

get molecules [261287]. The Mirkins group pioneered theuse of Au NPsthiolatedDNA conjugates [260262], whichled to a series of novel colorimetric sensors for the ultra-sensitive detection of polynucleotide [261], small molecule[263], enzyme activity [264266], DNA [267269], protein[270], and metal ion [271275], etc. For instance, Han et al.[267] developed a colorimetric bioassay which could screenfor potential triple helix specific DNA binders and simul-taneously determine their relative binding affinities usingDNA-functionalized Au NPs as sensing platform. Later, they[266] developed a new colorimetric technique for screen-ing endonuclease activity and determining the relativeinhibitory potency of potential inhibitors through monitoringthe kinetics of DNA-Au NPs aggregate dissociation. Despitethese successful demonstrations, considering that thiolated-DNA is usually expensive, searching new DNA-Au NPs systemfor colorimetric sensors is highly desirable for practicalapplications. In 2004, Li and Rothberg [276] reported anexciting result about DNA-Au NPs colorimetric assay sys-tem. They found that unlabeled single- and double-strandedoligonucleotides (unexpensive) have different propensitiesto adsorb on gold NPs in the colloidal solution. The forma-tion of double-stranded DNA would facilitate the desorptionof the single-stranded nucleic acids from the Au NPs,thus inducing the aggregation of the NPs. This aggregationresulted in a red-to-blue color change owing to the inter-particle coupled plasmon excitons in the aggregated states.

Followed by the inspiring report, some groups have exploreddifferent colorimetric strategies for detecting different tar-gets such as DNA [277], metal ion [278282], protein [283]and small molecules [284286] using unlabeled DNA modi-fied Au NPs as sensing elements [277287]. For instance, ourgroup [283] presented a simple and sensitive aptamer-basedcolorimetric sensor for thrombin detection using unmodi-fied Au NPs as sensing platform. As shown in Figure 21,when thrombin was added into unlabeled aptamer modifiedAu NPs solution, thrombin could interact with the aptameron the surface of Au NPs. Thus, aptamer is much moreinclined to fold into a structure of G-quadruplex/duplex,which could form a stable G-quadruplexthrombin com-plex. Finally, after high-concentration salt was added, the

color changes of the Au NPs could sensitively determinethe presence of target thrombin. According to the simi-lar principle, when unlabeled DNA interacted with othertarget molecules to form other stable complexes such asK+-stabilized G-quadruplex and THg2+T base pairs, etc.,the unmodified Au NPs-based colorimetric strategy could beused to detect other targets. Another important result aboutunmodified Au NPs as sensing platform was given by Xiaet al. recently [287], who developed a novel sensing strat-egy employing single-stranded probe DNA, unmodified goldNPs, and a positively charged conjugated polyelectrolyte todetect a broad range of targets including DNA sequences,proteins, small molecules, and inorganic ions. Inverse to

Figure 21 Au NPs colorimetric strategy for thrombin detec-

tion. Reprinted from Ref. [283] with permission by Royal Society

of Chemistry.

previous contributions, they found that in the presenceof conjugated polyelectrolyte and salt, double-strandedor otherwise folded DNA structures could protect AuNPs from aggregation, but single-stranded DNA could causespecifically the aggregation of Au NPs. Most importantly,the present report could be a general colorimetric approachto the specific and convenient detection of proteins, smallmolecules, and inorganic ions via employing the binding-induced folding or association of aptamers (Figure 22).

Au NPs engineered with other functional moleculescould also provide good sensing platforms for the detec-tion of target molecules. Prominent examples includecalsequestrin-functionalized gold NPs for the colorimet-

ric detection of Ca2+

[288], the color change of Au NPsinduced by the donoracceptor interaction between TNTand primary amines on the surface of Au NPs for thedetection of TNT [289], mercaptopropionic acid-modifiedgold NPs for the highly selective and sensitive detectionof Hg2+ ions in the presence of 2,6-pyridinedicarboxylicacid (chelating ligand) [290], gallic acid functionalized AuNPs for the detection of Pb2+ ion [291], gold NPs cappedwith 3-mercaptopropionate acid and adenosine monophos-phate for the detection of Hg2+ ion [292], the Au NPscolor change induced by the triple hydrogen-bonding recog-nition between melamine and a cyanuric acid derivativegrafted on the surface of gold NPs for reliable detec-tion of melamine [293], polythymine-modified gold NPs

for the detection of melamine also by forming triple H-bonds [294], Cu(I) (produced through the reduction of Cu2+

by sodium ascorbate)-catalyzed click chemistry betweenazide- and terminal alkyne-functionalized thiols modifiedgold NPs for the detection of Cu2+ [295], three-componentligands (glutathione, dithiothreitol and cysteine) modifiedgold NPs-based colorimetric assay for the label-free selec-tive detection of arsenic [296], and even the direct use ofunmodified gold NPs for analyzing melamine [297,298].

From an optical sensing point of view, silver NPs are alsogood candidates because Ag NPs likewise exhibit a distance-dependent color as Au NPs, and the extinction coefficientof Ag NPs is higher than that of Au NPs with the same size


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Figure 22 A new sensitive colorimetric assay for the detection of proteins, small molecules, and ions. Reprinted from Ref. [287]

with permission by PNAS.

[299]. Thus functionalization of Ag NPs with DNA moleculesand functional molecules provide a good possibility for selec-tive colorimetric biosensing and has recently been used todetect some important analytes [300304]. Our group [300]developed for the first time a sensitive, selective, simple,and label-free colorimetric assay using unmodified silver NPsprobe to detect enzymatic reactions (Figure 23). In our strat-egy, unreacted adenosine triphosphate (ATP) could protectAg NPs from salt-induced aggregation, whereas in the pres-ence of the enzymes (calf intestine alkaline phosphatase(CIAP) and protein kinase A (PKA)), the reaction product

of ATP [i.e., adenosine for CIAP and adenosine diphosphate(ADP) for PKA] could cause the aggregation of Ag NPs. Viaour method, dephosphorylation and phosphorylation couldbe readily detected by the color change of Ag NPs, with adetection limit of 1 unit/mL for CIAP and a detection limit of0.022 unit/mL for PKA, respectively. Xu et al. [302] utilizedunmodified DNA and Ag NPs to develop a novel colorimetricmethod for distinguishing ligands binding to homoadenine.They found that homoadenine could stabilize Ag NPs due toits a high affinity to Ag NPs. The presence of ligands wouldtake the adenine deoxynucleoside away from the surfaceof the Ag NPs. After adding the salt, Ag NPs would aggre-gate, accompanied by a color change from yellow to brown.

More recently, Kanjanawarut and Su [303] presented a DNAcolorimetric assay using unmodified Ag NPs and charge neu-tral PNA as probes based on their new finding that citrateprotected silver NPs undergo immediate aggregation in the

Figure 23 Enzymatic reactions with CIAP (A) and PKA (B), and

Ag NP-based enzyme colorimetric assay (C). Reprinted from Ref.

[300] with permission by American Chemical Society.


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Figure 26 Schematic illustration of fluorescence assays based

on the IFE of Au NPs. (A) Turn-on assay for cyanide and (B) turn-

off assay for H2O2. Reprinted from Ref. [317] with permission

by American Chemical Society.

and selective detection of cysteine based on the fact thatFCP would be moved away from the surfaces of Au NPsbecause of the strong interactions between the thiol groupof cysteine and gold NPs in the presence of cysteine [315],and turn-on model for sensitive cyanide detection basedon the dissolution of Rhodamine B-adsorbed gold NPs bycyanide [316]. (3) The third interesting scheme was theuse of the inner filter effect (IFE) of metal NPs, wheremetal NPs function as an absorber to modulate the emis-sion of the fluorophore. In addition, one important featurefor this sensing strategy is without the requirement of link ofthe absorber with the fluorophore. Typical example is fromShang and Dong [317], who demonstrated that Au NPs couldfunction as a powerful absorber in the IFE-based fluores-cent assays for the detection of CN and H2O2. Figure 26shows their two proof-of-concept sensing process. The firstassay worked in a turn-on mode upon the etching of Au NPsby CN, and the second one functioned in a turn-off modeupon the catalytic growth of Au NPs by H2O2. Later, theyfurther extended the present strategy to develop a turn-onfluorescent assay for cyanide based on the strong absorp-tion of Ag NPs to both excitation and emission light of ananion FCP [318]. (4) Metal-enhanced fluorescence (MEF, thatmeans that the emission of fluorophores within a certain dis-tance (510 nm) away from the metallic nanostructures can

be enhanced greatly), is another interesting sensing methodfor increasing the sensitivity of target molecules detection[319,320]. A recent contribution in this aspect was givenby Shtoyko et al. [319], who found that electrochemicallydeposited Ag fractal-like nanostructures on glass slide couldact as an excellent MEF substrate for immunoassays.

Conclusion, challenges and perspective

In summary, we provide recent advances on how researchon NMNs has become exciting in the aspects of control-lable synthesis, fuel cells and analytical sensing applicationsmainly in the last 3 years. These new research contribu-

tions include advanced techniques for the synthesis of NMNswith controllable size, shape, composition, hybrid, architec-ture and microstructure, etc., the design of high-efficiencyNMNs-based nanoelectrocatalysts for fuel cell and elec-trochemical sensors, and devising analytical strategies forconstructing NMNs-based colorimetric and fluorescent sen-sors. These research results reveal that NMNs can open manygood opportunities in an extremely multidisciplinary envi-

ronment for promoting the rapid developments of differentresearch fields. At present, one of the major challenges forthe deeper development of NMNs has been lack of a scal-able production method for synthesizing high-quality NMNswith controllable size and shape. Particularly, it is very dif-ficult to produce monodisperse small-size NMNs being richin high-index facets. In addition to single-component metalnanostructures, recently, controlled synthesis of bimetal-lic nanomaterials in the form of diverse morphologies hasbeen a very hot topic. However, effectively controlling theirmicrostructure is still a great challenge. The ultimate goalfor the controllable synthesis of NMNs is to develop somelow-cost, high-throughput, environmentally friendly tech-niques (has better all in one pot) for the preparation ofNMNs with desirable fine structure, such as size, shape, com-position, architecture and microstructure. Considering thisbig goal, the better understanding of NMNs growth mech-anisms may provide some ways and opportunities. Somein situ experimental techniques may provide new avenuesfor exploring the formation mechanism of NMNs synthesizedthrough different synthetic methods.

The requirement of highly active metal catalysts for fuelcell and electrochemical sensing applications has spurredtremendous interests in developing monodisperse, single-component, small-size NPs of Pt, Pd, and Au, as well asmultimetallic NPs and even metal NPs-based hybrids. How-ever, the improvement for fuel cell performance provided

by the majority of NMN-based catalysts is limited to eitheractivity or to stability, not to both. Therefore, in orderto meet the requirement of practical fuel cell reactions,the design of new metal catalyst will be oriented in syn-thesizing new monometallic or multimetallic nanostructurewith low cost and meantime high activity and durability.Considering this, three kinds of NMNs-based nanoelectrocat-alysts should be preferred. One is to design monodisperse,small-size metal NPs with rich high-index facets; secondis to synthesize multimetallic NPs with diverse morpholo-gies and compositions (had better also own rich high-indexfacets); third is how to control the uniform distribution andeffective connect of monometallic or multimetallic NPs withcontrollable size, shape and microstructure on the surface

of advanced supports. The corresponding hybrids with well-defined morphologies are very important for enhancing theirperformance in fuel cell applications. Another interestingaspect to say is that for electroanalytical chemist, it is verynecessary to search new NMNs-based electrode materialsfor constructing high-performance electrochemical sensorswith high sensitivity and selectivity, which preferred to beachieved through new analytical techniques. For analyticalchemist specialized in NMNs-based colorimetric and fluo-rescent sensors, the rewarding directions will focus on themolecular engineering of NMNs or new metal fluorescentmaterials for developing a series of analytical sensors betterthrough novel signal conversion mechanism.


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260 S. Guo, E. Wang

Acknowledgments

This work was supported by the National Natural Sci-ence Foundation of China (Nos. 21075116, 20935003and 20820103037) and 973 Project (2009CB930100,2011CB911002 and 2010CB933600).

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