Surfactant‐Templated Synthesis of High‐Performance Noble Metal Electrocatalysts: A Case of Dioctadecyldimethylammonium Chloride

Mesoscopic structures of noble metal (NM) nanomaterials are important parameters that remarkably affect physicochemical properties and electrocatalytic performances. To date, various synthesis strategies have been proposed to prepare NM nanomaterials with well‐defined mesoscopic structures. Specifically, amphiphilic surfactants that chemically composed of hydrophobic tail(s) and hydrophilic head(s) have been utilized as functional templates to precisely engineer mesoscopic structures of NM nanomaterials and further optimize their electrochemical performance. Compared with traditional surfactants, dioctadecyldimethylammonium chloride (DODAC) is a functional amphiphilic surfactant that can assemble into vesical and cylinder micelles and thus template precise synthesis of NM nanomaterials with various advanced structures. In this review, recent developments in DODAC‐templated synthesis of high‐performance NM electrocatalysts with a specific focus on the findings in the group are presented. Three kinds of advanced NM mesoscopic structures, including one‐dimensional (1D) nanowires, three‐dimensional (3D) mesoporous nanospheres (MSs), and sophisticated hollow/asymmetric MSs, are described in detail. Meanwhile, the applications in various electrocatalytic reactions are presented to highlight high activity, selectivity, and stability of NM electrocatalysts. This review provides some insights into both synthesis and application exploration of high‐performance NM electrocatalysts by templated synthesis of amphiphilic surfactants.


Introduction
To meet the demand of sustainable energy development and environmental protection, it is necessary to develop clean renewable energy technology, such as fuel cells and hydrogen production. [1]Electrocatalysis offers a green route to achieve above aim.However, related electrochemical reactions are kinetically limited by higher reaction activation energies, which result in a great waste of electric energy.Introducing appropriate electrocatalysts is a highly efficient approach to reduce the reaction activation energies and guarantee the electrocatalytic reactions proceed as quickly as possible.Over the past few years, noble metal (NM) electrocatalysts have been considered and widely utilized for a large number of reactions, for example, hydrogen evolution reaction (HER), [2] oxygen reduction reaction (ORR), [3] oxygen evolution reaction, [4] alcohol oxidation reaction (AOR), [5] formic acid oxidation reaction (FAOR), [6] carbon dioxide reduction reaction (CO 2 RR), [7] and nitrate reduction reaction (NITRR). [8]In practice, an ideal electrocatalyst should be endowed with low cost and excellent performance in terms of activity, stability, and selectivity.Hindered by the scarce reservation, expensive cost, and inadequate performance fall short of the industrial requirements for NM electrocatalysts, limiting the commercialization of related energy conversion technologies.Generally, overall chemical reaction takes place at the surface/ interface of electrocatalysts; their performances are strongly dependent on the surface structures and compositions of active NM sites. [9]In terms of the principle that performance enhancement can be afforded by increasing number of exposed active sites and intrinsic activity, a variety of synthetic strategies have Mesoscopic structures of noble metal (NM) nanomaterials are important parameters that remarkably affect physicochemical properties and electrocatalytic performances.To date, various synthesis strategies have been proposed to prepare NM nanomaterials with well-defined mesoscopic structures.Specifically, amphiphilic surfactants that chemically composed of hydrophobic tail(s) and hydrophilic head(s) have been utilized as functional templates to precisely engineer mesoscopic structures of NM nanomaterials and further optimize their electrochemical performance.Compared with traditional surfactants, dioctadecyldimethylammonium chloride (DODAC) is a functional amphiphilic surfactant that can assemble into vesical and cylinder micelles and thus template precise synthesis of NM nanomaterials with various advanced structures.In this review, recent developments in DODAC-templated synthesis of high-performance NM electrocatalysts with a specific focus on the findings in the group are presented.Three kinds of advanced NM mesoscopic structures, including one-dimensional (1D) nanowires, three-dimensional (3D) mesoporous nanospheres (MSs), and sophisticated hollow/asymmetric MSs, are described in detail.Meanwhile, the applications in various electrocatalytic reactions are presented to highlight high activity, selectivity, and stability of NM electrocatalysts.This review provides some insights into both synthesis and application exploration of high-performance NM electrocatalysts by templated synthesis of amphiphilic surfactants.
been proposed to acquire advanced NM electrocatalysts with different structures and compositions. [10]Among various synthetic systems established currently, surfactant template method has attracted widespread attention to precisely control sophisticated nanostructures of NM electrocatalysts.Surfactant template synthesis refers to surfactant molecules as the structural template, whose hydrophilic groups interact with metal precursors while hydrophobic groups enable confined in situ nucleation growth of NM electrocatalysts. [11]Benefiting from the diversity of surfactant molecules and adjustable reaction parameters, various micelle structures were formed controllably. [12]As a consequence, the surfactant-templated strategy is widely popular in the synthesis of NM electrocatalysts. [13]In general, critical packing parameter (P, P = v/al) was well accepted as the criterion to predict the micelle structure, where the v is the volume of the hydrophobic part, a represents the equilibrium area occupied per hydrophilic group when the interfacial energy of the surfactant aggregate is in minimum, and l signifies the length of the hydrophobic chain.The reaction parameters, for example, concentration of surfactants, pH, and temperature, could effectively affect the transitions between different micelle structures and as-resulted NM nanostructures by changing their chemical parameters.The most critical issue in surfactant-templated synthesis of NM electrocatalysts is rational choice of an amphiphilic surfactant that can not only self-assemble into well-defined structures but also confine in-the-plane growth of NM nanocrystals along the surfactant.Following this design principle, some NM nanostructures, including nanowires (NWs), [14] nanodendrites, [15] nanoplates, [16] and nanospheres, [17] have been successfully prepared with commonly used surfactant (cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB)).However, the nanostructures development of NM nanocrystals have been notably slower than expected due to the limited structural functions of these surfactants.
In 2016, our group found that dioctadecyldimethylammonium chloride (DODAC) which was composed of two hydrophobic tails chemically bonded to quaternary head could behave as the functional surfactant template for the synthesis of ultrathin NWs. [18]ince then, our group continued the careful investigations of DODAC-templated synthesis of NM nanostructures for electrocatalysis.In this review, we offer a comprehensive overview of the recent advancements in DODAC-templated synthesis of NM electrocatalysts with controllable structures and compositions, with a specific focus on the findings in our group (Table 1).Three kinds of important nanostructures synthesized with DODAC as surfactant template, including one-dimensional (1D) ultrathin NWs, mesoporous nanospheres (MSs) with penetrated channels, and sophisticated (asymmetric) hollow MSs (HMSs/AHMSs), are presented in detail.Meanwhile, their design principles and corresponding formation mechanisms are discussed thoroughly.Based on above NM nanostructures, electrocatalytic performance, including activity, selectivity, stability, and corresponding promotion mechanisms are also demonstrated.Finally, we offer further developments in this research area.We expect, this review can provide insights into rational design and applications of NM electrocatalysts with amphiphilic surfactant as the template.

Chemical Structure and Role of DODAC in Synthesis of NM Electrocatalysts
As a class of cationic surfactant, DODAC chemically composes of two C 18 alkyl chains bonded with quaternary ammonium group and chloride counter ion (Figure 1).Compared with traditional surfactants (e.g., CTAC), DODAC has comparatively lower solubility, stronger hydrophobicity, and lower saturation concentration (4.6 Â 10 À6 M).The double chains of DODAC render DODAC with packing parameters in the range of 1/3 and 1, which are conducive to form vesicles and/or cylinder micelles in a well-designed condition. [19]More importantly, because of substantial volume of hydrophobic part, other self-assembled structures, including spherical and hexagonal micelles, could be accessible below or around the critical micelle concentration, or by changing the reaction conditions, for example, the type of counterions, pH, temperature, solvent, and so on. [20]Therefore, diverse and flexibly adjustable micellar structures of DODAC potentially facilitate the construction of NM nanocrystals with various structures.When templated the synthesis of NM electrocatalysts, there are two significant roles of DODAC, including: 1) precisely controllable micellar structures that enable to template universal synthesis of NM electrocatalysts with multiple nanostructures and 2) stronger hydrophobic interactions that provide a strong confinement effect to stabilize micellar structures and nucleation growth and thus allow robust formation of anisotropic NM electrocatalysts with controllable NM PdPt NWs HER [54]   PdCuP NWs EOR [35]   PdAg NWs EOR [53]   multicomponent Pd-based alloy NWs EOR [52]   RhCuAgPd/Pd EGOR [32]   heterostructure NWs RhCo NWs HER [34]   MSs PdCu MSs NITRR [57]   PdAgCu MSs EOR [39]   Au@PdAuCu core-shell MSs EOR [40]   PdB MSs EOR [42]   PdBP MSs EOR, ORR [43]   PdCuBP MSs EOR [17b] PdBS MSs HER [44]   HMSs/AHMSs PdAg HMSs EOR [48]   PdCuAg HMSs EOR [49]   PdCuAg AHMSs AOR [46]   Pt AHMSs HER [50]   PdPtCu@nitrogen-functionalized graphene AHMSs MOR [60]  elements.Therefore, under the optimal conditions, DODAC can be utilized as functional surfactant template to facilitate the targeted synthesis of NM nanocrystals with precisely controlled morphologies/structures and compositions.

NM Electrocatalysts with Different Structures
In this section, we will discuss the structures of NM electrocatalysts synthesized by DODAC template in detail.Meanwhile, their design principles and formation mechanisms will also be presented clearly.

1D Ultrathin NWs
1D nanostructures, defined as nanomaterials whose sizes are less than or equal to 100 nm in one dimension, consist of NWs, nanotubes, nanofibers, nanorods, etc.Over the past few years, 1D NWs, for example, Pt, Pd, Cu, Rh, and Ir, have demonstrated their high efficiency in the field of electrocatalysis. [21]14b,22] Till now, the synthetic strategies for NM NWs can be categorized as follows: 1) top-down approach that aims to downsize 1D microstructure directly [23] ; 2) template-directed growth, in which 1D structures are strongly dependent on structures of soft and hard templates [22b,24] ; 3) capping agent-assisted growth that is precisely controlled by kinetics and thermodynamics of nanocrystal growth [25] ; and 4) self-assembly of 0D nanoparticles that directs anisotropic self-assembly of 0D nanoparticles into 1D NWs. [26]Compared with other synthesis routes, the soft (surfactant) template possesses intrinsic advantages and allows the synthesis of 1D NM NWs with tailored structures, compositions, crystal phases, and sizes.21c,30] As a novel surfactant, DODAC has been widely utilized as the surfactant template to synthesize 1D ultrathin NM NWs.Generally, DODAC first self-assembles into rod-like micelles and further into ordered hexagonal micelles in an aqueous solution.After the addition of metal precursors, electrostatic/coordination interactions between metal precursors and quaternary ammonium of DODAC assist to stabilize the organic-inorganic hybrids.After in situ reduction of metal precursors, 1D ultrathin NWs can be prepared accordingly along the hybrid micelles.Finally, DODAC templates are thoroughly removed by being washed several times with ethanol/water (Figure 2a).
In 2016, our group reported the first example of precise synthesis of ultrathin Pd NWs with DODAC as the surfactant template. [18]By directly mixing DODAC and PdCl 4 2À in an aqueous solution, metal precursors were stabilized within quaternary ammonium group of DODAC to form hexagonal liquid phase (Figure 2b).Thanks to strong hydrophobic group of alkyl chains, in situ reduction and growth of Pd nanocrystals along rod-like interface by ascorbic acid (AA) resulted in the synthesis of 1D Pd NWs.Transmission electron microscopy (TEM) image clearly revealed that the products were highly dispersed with uniform width of nearly 3.5 nm and length of several hundred nanometers (Figure 2c).Not only monometallic Pd but also bimetallic PdM alloys can be prepared by the DODAC-templated synthesis of 1D NWs.With AA as the reducing agent, Pd and Pt or Ag with similar reducing potentials had successfully alloyed into 1D PdPt and PdAg NWs. [31]This further confirmed the key importance of DODAC in confined templated synthesis of 1D alloy NWs.Highangle annular dark-field scanning TEM (HAADF-STEM) image disclosed that PdAg NWs were highly uniform with same 1D nanostructure to monometallic Pd NWs (Figure 2d).Meanwhile, Pd and Ag were homogeneously dispersed within 1D NWs, confirming they were compositionally alloyed rather than phase-separated counterparts (Figure 2e).More interestingly, benefiting from the stronger confinement effect of DODAC and controlled reaction kinetics of AA, resultant products were single-crystalline PdAg NWs, suggesting they were prepared followed by an epitaxial growth process (Figure 2f ).Single-crystalline phase was definitely confirmed by high-resolution TEM images and corresponding selected-area electron diffraction profiles (Figure 2g).Meanwhile, 1D Pd NWs can be also utilized as the template for heterostructured NWs.By easily injecting Rh, Cu, Ag, and/or Pd precursors into the solution for synthesizing Pd NWs, heterostructured Pd-based NWs, including Rh/Pd, RhCu/Pd, RhCuAg/Pd, and RhCuAgPd/Pd, were obtained accordingly (Figure 2h). [32]The products retained the ultrathin NW structure with some heterostructured protuberances (2.7-4 nm) (Figure 2i), highlighting the potential of 1D NWs for synthesizing other nanostructures.
3d transition metals alloyed 1D NWs, for example, PdFe, PdCo, and PdNi, have recently received more attention in electrocatalysis because 3d transition metals not only remarkably optimize electronic structure of Pd but also ensure bifunctional effect that enhances electrocatalytic performance. [33]However, the huge differences in reduction potentials of Pd and 3d transition metals make the synthesis of PdM alloy NWs more challenging.Generally, to prepare PdM alloys, extremely strong reduction agents, for example, N 2 H 4 , are needed to drive concurrent nucleation into multicomponent alloys.It is synthetically inconsistent to the thermodynamically unfavorable structure of 1D NWs.Strong confinement effect of DODAC provides an opportunity to balance the equilibrium of in-the-columnar growth of 1D PdM NWs with N 2 H 4 as the reducing agent.As a typical example, bimetallic PdNi alloy NWs were successfully prepared through DODAC-templated synthesis.As-resultant NWs were ultrathin (2.6 nm) and ultralong (hundreds of nanometers) (Figure 2j).Compared to 1D NWs synthesized by AA, these NWs prepared by N 2 H 4 possessed more convex and concave kinks.Meanwhile, this route can be easily extended to synthesize multimetallic Pdbased alloy NWs, including 12 kinds of binary alloys, 8 kinds of ternary alloys, and 5 kinds of quaternary alloys (Figure 2k).Moreover, bimetallic RhM (M = Fe, Co, Ni, Cu, and Zn) alloy NWs were also prepared with DODAC as surfactant template and NaBH 4 as reducing agent. [34]More importantly, this synthetic protocol also allowed the alloys of amorphous P into 1D Pd and PdCu NWs through DODAC-templated disproportionation reaction (Figure 2l). [35]Uniform NM-metal-nonmetal ternary PdCuP ultrathin NWs further optimized electronic structure of active Pd and thus boosted their electrocatalytic performance (Figure 2m).

MSs with Penetrated Channels
MSs, whose spherical nanoparticles are surrounded by penetrated mesopores ranging from 2 to 50 nm, have drawn great attention due to their large specific surface, excellent structural stability, versatile pore size and structure, and controllable framework composition. [36]NM MSs are thus widely used in the fields related to energy conversion and storage, especially in electrocatalysis.In comparison to traditional silica and oxide-related MSs, metal MSs are synthetically more difficult, mostly because of their faster reduction kinetics that do not involve sol-gel process in the template-metal precursor hybrids. [37]Soft template synthesis, which uses surfactants and block copolymers as the mesopore-forming template, has received its potential in preparing metal MSs.For example, Yamauchi group developed a general method for synthesizing metal MSs with block copolymers as the template. [38]However, spherical micelles assembled by block copolymers possibly destroyed the connectivity of mesopores and thus decreased the exposed active metal sites for electrocatalysis.Therefore, it is highly desirable to develop new methods to prepare functional metal MSs with highly penetrated mesoporous channels.
Benefiting from flexibly adjustable micellar structure and relatively stronger hydrophobic nature, DODAC is potentially utilized as the mesopore-forming surfactant to synthesize NM MSs with penetrated mesoporous channels.In 2019, our group found that, by adjusting assembled structure of DODAC into rod-like micelle, in situ reduction of DODAC-metal precursor hybrids by AA achieved precise synthesis of PdAgCu MSs with cylindrically penetrated mesoporous channels (Figure 3a). [39]The resultant NM MSs were highly uniform with continuous metal framework and highly penetrated mesoporous channel (Figure 3b,c).Meanwhile, homogeneous reduction and nucleation resulted in uniform elemental distributions and thus formed well-alloyed MSs (Figure 3d).More importantly, this method allowed precise controllability in its reduction kinetics by synthetic conditions, affording robust synthesis of NM MSs with adjusted sizes.For example, when pH of reaction solution varied from 5.76 to 10.61, the sizes of PdAgCu MSs correspondingly changed from 21.3 to 103.8 nm (Figure 3e).More interestingly, when choosing metals with huge difference in their reduction potentials, sophisticated NM MSs can be prepared potentially by DODAC template synthesis.As a typical example, our group found that, during the formation of AuPdCu alloys, AuCl 4 À was first reduced into Au nanoparticles by AA due to its higher reduction potential.Then, remaining metal precursors in the hybrid micelles were further reduced and grown on the surface of parent Au nanoparticles because of their crystallographic compatibility. [40]Structural characterizations disclosed that resultant products were highly uniform and homogeneous with a perfect core-shell structure (Figure 3f-h).Meanwhile, both the size of interior Au shell and the thickness of external PdAuCu mesoporous framework were precisely adjusted by easily changing the amounts of metal precursors added.The results clearly confirmed the high priority of DODAC-templated route for synthesizing NM MSs with controlled structures and penetrated mesopores.
Similar to 1D NWs, the DODAC-templated synthesis strategy can be also extended to prepare NM-metal-metalloid/nonmetal MSs.Here, metalloid (B) and nonmetals (P and S) were demonstrated to optimize the electronic state of active NMs due to their negative electronegativity and smaller atomic radius. [41]or NM-metalloid MSs, the synthesis was carried out with dimethylamine borane (DMAB) and boric acid (H 3 BO 3 ) as both reducing agents and boron sources instead of traditional AA.On the basis of high hydrophobicity and nanoconfinement effect of DODAC, binary PdB MSs were successfully prepared in one step. [42]Different from traditional MSs with cylinder mesochannels, PdB MSs were structurally dendritic and center-radial that electrocatalytically exposed more active sites (Figure 3i).Meanwhile, B atoms were interstitially inserted into Pd-Pd interatomic locations and expanded their lattice distance (Figure 3j).Following this strategy, nonmetal was also alloyed into PdB MSs by adding sodium hypophosphite (NaH 2 PO 2 ) into above synthetic solution.As a result, ternary PdBP MSs with 3D dendritic mesoporous channels were prepared with DODAC as the mesopore-forming template and DMAB/NaH 2 PO 2 as B/P source and reducing agent (Figure 3k,l). [43]Meanwhile, dendritic generation (or size) of ternary PdBP MSs can be precisely controlled in the range of 25-105 nm by easily changing synthetic parameters (NH 3 •H 2 O addition amounts) (Figure 3m-o).17b,44] These results further highlighted the role of DODAC as the mesopore-forming template to prepare library of NM MSs with highly penetrated channels and uniform structures, which thus provided a new opportunity for their wide application in electrocatalysis.

Hollow and Asymmetric MSs
NM MSs have exposed abundant active sites and thus exhibited high performance in electrocatalysis.However, their long mesoporous channels also decelerated the transport of reactants and products, which possibly resulted in a huge waste of interior metal active sites for some reactions.Therefore, further engineering nanostructures of metal MSs that kinetically accelerated their mass transfer properties potentially overcame above challenge and thus boosted their performance.Of various nanostructures available, HMSs and their AHMSs have received special attention in electrocatalysis due to their add-in structural advantages. [45]Compared to traditional MSs, NM HMSs and AHMSs have obviously accelerated the transports of reactants and products and thus optimized the utilization of active metal sites.10c,46,47] However, these strategies generally required sacrificial sources that were necessary for forming hollow cavities and breaking structural asymmetries.By contrast, amphiphilic surfactants that potentially selfassembled into spherical and cylinder micelles in a same solution can be utilized as cotemplates to synthesize NM HMSs and AHMSs directly.No sacrificial sources were needed and the reaction happened in one step.Therefore, surfactanttemplated synthesis provided new opportunities in preparing metal HMSs and AHMSs in a greener and easier manner.
In 2018, our group reported the first example to synthesize NM HMSs with DODAC as dual-surfactant template in one step. [48]Under optimal conditions, DODAC can first selfassembled into vesicle and cylinder micelles simultaneously.Strong hydrophobicity of DODAC stabilized "dual" micelle templates in aqueous solution.Then, metal precursors bonded with quaternary ammonium group of DODAC and further stabilized "dual" micelle templates.After injecting AA, in situ reduction of metal precursors with cylinder micelles on a vesicle micelle formed hollow mesoporous structures (Figure 4a).Taking PdAg HMSs as an example, the products were structurally homogeneous that composed of uniform interior hollow cavity and exterior mesoporous framework (Figure 4b).Similar to NM MSs, mesoporous structure of PdAg HMSs was also cylindrically penetrated, which would further accelerate the mass transport during electrocatalysis.Meanwhile, STEM elemental mapping images showed uniform Pd and Ag within HMSs, confirming they were compositionally alloyed (Figure 4c).Mechanism studies revealed that the formation of PdAg HMSs was ascribed to precise engineering of reduction and nucleation kinetics along dual templates.Furthermore, both size and thickness of PdAg HMSs were adjusted precisely.For example, with the increase of Ag contents, the faster reduction rate of Ag þ /Ag resulted in a decrease of HMSs from 82 to 63 nm, although their sizes of hollow cavities still were kept at 30 nm (Figure 4d).Moreover, "dual" template strategy formed by DODAC has extended to prepare NM HMSs with more functional metal compositions, for example, PdCuAg (Figure 4e), that further boosted their electrocatalytic performance. [49]omogeneous nucleation of NM MSs on a vesicle micelle produced uniform HMSs with controllable compositional functions through the DODAC "dual"-template route.We thus expected that asymmetric structure of NM MSs can be obtained on a vesicle micelle by further tuning their nucleation kinetics.Interestingly, when controlling epitaxial growth of mesoporous NMs on vesicle micelle, different asymmetric degrees of  [46] Copyright 2019, the American Chemical Society.b) HAADF-STEM and c) corresponding STEM mapping images of PdAg HMSs.d) TEM images of PdAg HMSs with different Ag contents.(b-d) Reproduced with permission. [48]Copyright 2019, the American Chemical Society.e) STEM mapping images of PdAgCu HMSs.Reproduced with permission. [49]Copyright 2018, the American Chemical Society.f ) HAADF-STEM and g) corresponding STEM mapping images of PdAgCu AHMSs.STEM mapping of h) PdAgPt AHMSs, i) PdAgCuFe AHMSs, and j) PdAgPtCu AHMSs, respectively.k) TEM images and corresponding structural render-AHMSs were precisely controlled.By easily changing synthetic parameters (DODAC concentrations, precursor concentration, pH, and temperature), perfect metal AHMSs were obtained accordingly (Figure 4f ). [46]The products showed high uniformity with perfectly asymmetric and hollow structure that were distinct from traditional HMSs.Meanwhile, functional compositions of metal AHMSs were not specific; they can be generally extended from trimetallic PdAgCu and PdPtCu to multimetallic PdAgCuFe and PdPtAgCu (Figure 4g-j).Moreover, NM AHMSs were precisely controlled with the asymmetric degrees in the range of AHMSs-7/8 and AHMSs-1/8 (Figure 4k), further highlighting the generality of our DODAC "dual" template strategy in synthesizing AHMSs with different asymmetric structures.Besides, this method has been applied to prepare Pt AHMSs and their assemblies (chains) due to the slower reduction kinetics of PtCl 4 2À . [50]ere, three nanostructures were well established by means of the unique merits of DODAC.On the one hand, the larger hydrophobic volume and longer hydrophobic chain stabilize the micelles in the solvent and thus provide a stronger confinement environment for both precursors and tiny nanocrystals.On the other hand, controllable micellar structures induced by wide range of P value enable precise engineering of NM nanocrystals with various structure/morphology.

Electrocatalytic Applications
With DODAC as the surfactant template, different nanostructures of NM electrocatalysts have been prepared accordingly and displayed enhanced electrocatalytic performance (Table 2).
Considering their functional structural features, in this section, we will discuss their electrocatalytic applications in several important reactions systematically.

1D Ultrathin NWs
1D ultrathin NM NWs have featured multiple structural advantages and thus exhibited their potential in electrocatalysis. [51]On the one hand, high curvature of ultrathin NWs not only enlarged the utilization efficiency of NMs but also enriched unsaturated coordination sites, which thus boosted their mass activities in electrocatalysis.On the other hand, anisotropic structure of 1D NWs effectively inhibited the aggregation and Ostwald ripening process, which thus enhanced their electrocatalytic stability.Especially, multimetallic ultrathin NWs highlighted multiple compositional and structural synergies and thus increased their activity in electrocatalysis.Here, we discussed electrocatalytic performance of 1D ultrathin NM NWs synthesized with DODAC as the surfactant template (Figure 5a).
As a case study, we first investigated electrocatalytic performance of multimetallic Pd-based ultrathin NWs synthesized with DODAC template in ethanol oxidation reaction (EOR, the anodic reaction in direct ethanol fuel cell (DEFC)). [52]emarkably, ultrathin NWs exhibited much higher electrochemically active surface area (ECSA) than commercial Pd/C.Especially, tetrametallic PdAuCuNi NWs hold the highest ECSA of 56.3 m 2 g Pd

À1
, far passing that of its counterparts and commercial Pd/C, confirming that both multimetallic composition and anisotropic structure enlarged active NM sites for electrocatalysis (Figure 5b).When being explored for alkaline Pd NWs [18] FAOR 0.5 M HCOOH þ 0.5 M H 2 SO 4 758 mA mg À1 RhCuAgPd/Pd NWs [32] EGOR 1.0 M KOH þ 1 M EG 6.63 A mg Pd

À1
, which reached 1.3-3.6 times higher than its counterpart electrocatalysts (Figure 5c).Meanwhile, PdAuCuNi NWs were significantly stable in EOR electrocatalysis (Figure 5d), highlighting its high potential as high-performance electrocatalyst for DEFC.In addition, mechanism studies revealed that PdAuCuNi NWs possessed better CO antipoisoning ability, faster EOR kinetics, and lower activation energy.All of above results clarified the optimized 1D NW structure and optimized elemental composition not only exposed more active sites, but also synergistically boosted electrocatalytic EOR activity and stability.Similarly, single-crystalline PdAg NWs behaved well in alkaline EOR electrocatalysis. [53]ther electrochemical reactions have also been demonstrated to highlight the structural advantages of ultrathin NM NWs synthesized with DODAC template.In another case, electrocatalytic HER, the side reaction of water splitting, was investigated with ultrathin PdPt NWs synthesized by DODAC template as the electrocatalyst. [54]As imaged, ultrathin PdPt NWs exhibited abundant edge active atoms and single-crystalline structure.Remarkably lower overpotentials were obtained for PdPt NWs in both acid and alkaline solutions, indicating higher HER performance than Pd NWs and Pd/Pt NPs (Figure 5e,f ).HER mechanism investigations revealed that ultrathin PdPt NWs showed the smallest charge transfer resistance and the faster proton reduction kinetics (Figure 5g).Therefore, the high HER performance was ascribed to structural and compositional synergies that kinetically accelerated the adsorption of hydrogen and corresponding the recombination into molecular H 2 and thus promoted the Volmer-Tafel process during HER electrocatalysis.Considering high controllability in both structures and compositions, ultrathin NWs were expected to pave a path to enhance electrocatalytic performance for multiple reactions.-d) Reproduced with permission. [52]Copyright 2019, Elsevier.LSV curves in e) 0.5 M H 2 SO 4 , f ) 1 M KOH, and g) electrochemical impedance spectra of Pd NPs, Pt NPs, ultrathin Pd, and bimetallic PdPt NWs at a scan rate of 5 mV s À1 .(e-f ) Reproduced with permission. [54]Copyright 2018, Elsevier.

MSs with Penetrated Channels
36b,55] More interestingly, their mesoporous channels ensured the confinement microenvironment of intermediates and thus offered new opportunity to optimize electrocatalytic selectivity. [56]In this section, we described the application of MSs prepared with DODAC template in electrocatalysis to highlight their structural features.
We presented NM-metalloid PdB MSs synthesized with DODAC template as the first example to investigate its high activity and stability in alkaline EOR electrocatalysis. [42]Compared to other counterpart electrocatalysts, binary PdB MSs with cylindrically opened mesochannels adopted the largest ECSA value, indicating its highest NM active sites for electrocatalysis (Figure 6a).When being performed as electrocatalyst for EOR, PdB MSs hold the highest mass activity, which reached 1.47, 1.50, and 3.78 times higher than commercial Pd MNSs, PdB NPs, and Pd/C, respectively (Figure 6b).More importantly, PdB MSs exhibited much better EOR stability.After stability tests for 5000 s, the residual current of PdB MSs was about 15 higher than that of commercial Pd/C (Figure 6c).Mechanism studies revealed that PdB MSs possessed structural and compositional synergies to accelerate the oxidation removal of poisoning CO-based intermediates (the rate-determining step) and enhance EOR activity and stability.Following the strategy, NMmetalloid-nonmetal PdBP MSs exhibited superior electrochemical activity and stability in both ORR and EOR. [43]n consistence with high electrocatalytic activity and stability, high selectivity was also achieved over MS electrocatalysts synthesized with DODAC template.Here, electrochemical NITRR was  -c) Reproduced with permission. [42]Copyright 2019, the Royal Society of Chemistry.d) NH 3 Faradaic efficiencies (FE NH 3 ), e) NH 3 yield rates consecutive, and f ) recycling tests of PdCu MSs, PdCu NPs, Pd MSs, and Pd NPs collected in 0.10 M KOH and 10 mM NO 3 À .g) Finite-element method simulation for computed concentration and distribution of intermediate (NO 2 investigated in water as a model reaction, because of its multiple products, including N 2 and NH 3 , as well as competitive HER. [57]dCu MSs synthesized with DODAC template were more active and selective for NH 3 electrosynthesis from NITRR in 0.10 M KOH containing 10 mM NO 3 À .In comparison to PdCu NPs and Pd MSs, PdCu MSs disclosed remarkably high NH 3 Faradaic efficiency (FE NH 3 ) of 85% and yield rate of 3.06 mg h À1 mg À1 (Figure 6d,e).Meanwhile, structure stability of MSs ensured high electrocatalytic stability, retaining suppressive FE NH 3 of 77% after being tested for 10 cycles (Figure 6f ).Mechanism studies indicated that high active sites of NM MSs facilitated the chemisorption of NO 3 À to enhance the reactivity of NITRR electrocatalysis.Meanwhile, penetrated mesoporous channels of NM MSs endowed a strong confinement environment for key intermediates (NO 2 * and H*) (Figure 6g), which thus facilitated the deeper electroreduction of NO 3 À into NH 3 through an eight-electron reaction pathway (Figure 6h).In sharp comparison, nonporous PdCu NPs disfavored the chemisorption of key intermediates and thus decreased its selectivity for NH 3 electrosynthesis.Similarly, the stronger confinement of key intermediates has been demonstrated for selective NH 3 electrosynthesis from NITRR by PdN MSs [56c] and PdCu mesoporous nanotubes [58] that have achieved superior FE NH3 of >90%.

HMSs and AHMSs
As presented above, solid MSs have exhibited high NM sites and thus disclosed remarkable performance in electrocatalysis.
However, longer mesoporous channels of MSs would decelerate the transport of reactants, especially for inner sites, and thus partially deactivate the electrocatalysts.Alternatively, functional HMSs and AHMSs that are composed of interior hollow cavities remarkably shortened the length of mesoporous channels and accelerated transports of reactants and products, thereby maximizing the NM active sites and boosting their electrocatalytic performance (Figure 7a). [59]Here, we discussed high performance of NM HMSs and AHMSs with high functionality in electrocatalysis.
Bimetallic PdAg HMSs synthesized with DODAC template were presented and further compared with its counterparts to highlight high performance of HMSs in EOR electrocatalysis. [48]nder the same test conditions, PdAg HMSs were more electrocatalytically active and achieved high EOR activity of 4.61 A mg À1 (Figure 7b).In comparison, solid PdAg MSs had a lower mass activity of 2.89 A mg À1 , confirming structural advantages of HMSs in EOR electrocatalysis.Kinetic transport studies revealed that the diffusion process of EOR electrocatalysis was obviously accelerated over PdAg HMSs (Figure 7c).Meanwhile, interior hollow cavity of PdAg HMSs also facilitated the oxidation removal of poisoning CO-based intermediates (the ratedetermining step of EOR electrocatalysis) and accelerated the kinetic process remarkably (Figure 7d).Besides, PdAg HMSs exhibited the increased mass activity, accelerated catalytic kinetics, and stronger anti-CO poisoning capability, compared with commercial Pd/C.These structural functions and compositional effects thus synergistically boosted EOR electrocatalysis and achieved high activity.Furthermore, electrocatalytic activity  -d) Reproduced with permission. [48]opyright 2019, the American Chemical Society.e) CV curves of PdAgCu AHMSs, HMSs, and MSs, PdAg AHMSs, and Pd/C collected in 1.0 M KOH and 1.0 M ethanol.f ) Summarized mass activities of PdAgCu AHMSs, HMSs, MSs, PdAg AHMSs, and Pd/C in methanol, glycerol, and formic acid electrooxidations.(e-f ) Reproduced with permission. [46]Copyright 2019, the American Chemical Society.
was enhanced by enhancing structural asymmetry of HMSs that accelerated the oxidation removal of CO-based intermediates during EOR electrocatalysis.As expected, trimetallic PdAgCu AHMSs disclosed superior EOR activity of 6.36 A mg À1 , which was 1.22 and 2.01 times higher than that of HMSs and solid MSs (Figure 7e). [46]More interestingly, PdAgCu AHMSs were highly active in electrochemical oxidation of other fuels, including methanol, glycerol, and formic acid, further highlighting its structural advantages in electrocatalysis (Figure 7f ).Besides, high antipoisoning ability also stabilized the HMSs and AHMSs, holding high electrocatalytic stability in EOR electrocatalysis.

Conclusion and Perspective
In conclusion, this review summarized recent advances, including synthetic strategies and application, of NM electrocatalysts synthesized with amphiphilic DODAC as the case template.Three NM nanostructures, including 1D ultrathin NWs, MSs with penetrated mesoporous channels, and HMSs/AHMSs, were discussed in detail to highlight the key role of DODAC template in their formation processes.Meanwhile, their applications in electrocatalysis and corresponding promotion mechanisms (activity, selectivity, and stability) were also presented based on their functional structural functions.We expect that this review can afford some insights into designing and synthesizing highperformance NM electrocatalysts with amphiphilic surfactants as the structural templates.
Despite important studies, surfactant-templated synthesis of NM electrocatalysts is still notably slower than expected.First, there are some other functional surfactants that can selfassemble into various ordered nanostructures and have been explored to synthesize other nanomaterials.However, they are rarely reported to prepare NM electrocatalysts due to their technique challenges in balancing self-assembly of amphiphilic surfactants and in situ nucleation of NM nanocrystals.Meanwhile, functional group of surfactants was mostly quaternary ammonia that weakened the chemical interactions between surfactants and metal precursors and thus disabled the synthesis of NM nanocrystals with desired nanostructures.Therefore, it is highly desirable to develop new functional surfactants and further engineer their self-assembly and nucleation of NM electrocatalysts with various structures.Second, there are some other sophisticated NM nanostructures that had been synthesized by other methods.For example, amphiphilic surfactants have rarely been utilized to prepare other nanostructures, for example 2D nanosheets or more sophisticated mesoporous nanosheets.Considering multiple assembled nanostructures, amphiphilic surfactants are highly expected to prepare above sophisticated NM nanostructures.Third, there are some important applications of NM nanocrystals.On the basis of structural functions of NM nanocrystals with surfactant templates, other applications, for example, photocatalysis, photoelectrocatalysis, and biomedical applications, were urgently expected.Last but not least, the removal of surfactants was the essential step for the surfactanttemplated method to expose active sites.Currently, some physical or chemical methods have been proposed and applied, such as ultraviolet-ozone treatment strategy, thermal-treatment strategy, acid treatment, and plasma etching treatment.Even though, there was not given sufficient consideration on the degree of removal of surfactants and changes in the surface structure of materials, because excessive treatment will lead to the formation of defects or cause morphology damage, or composition loss, which is unfavorable for performance.It will be highly preferred to perform in situ detection and characterization technology to coordinate the removal of surfactants on the surface of nanomaterials.Overall, we believe that there could be some opportunities to enlarge NM nanostructures synthesized with surfactant templates for future applications.

Figure 1 .
Figure 1.A schematic illustration of the self-assembly structure of DODAC.

Figure 5 .
Figure 5. a) Schematic illustration of the electrocatalytic mechanism of Pd-based NWs for EOR and HER.b) Cyclic voltammograms (CVs) of PdAuCuNi NWs, PdAuNi NWs, PdNi NWs, Pd NWs, and Pd/C collected in 1.0 M KOH.c) CVs curves and d) i-t chronoamperometric curves of PdAuCuNi NWs, PdAuNi NWs, PdNi NWs, Pd NWs, and Pd/C collected in 1.0 M KOH and 1.0 M ethanol.(b-d) Reproduced with permission.[52]Copyright 2019, Elsevier.LSV curves in e) 0.5 M H 2 SO 4 , f ) 1 M KOH, and g) electrochemical impedance spectra of Pd NPs, Pt NPs, ultrathin Pd, and bimetallic PdPt NWs at a scan rate of 5 mV s À1 .(e-f ) Reproduced with permission.[54]Copyright 2018, Elsevier.

Figure 6 .
Figure 6.a) CV curves of binary PdB MSs, Pd MSs, PdB NPs, and commercial Pd NPs collected in 1.0 M KOH.b) CV curves and c) i-t curves of binary PdB MSs, Pd MSs, Pd-B NPs, and commercial Pd NPs collected in 1.0 M KOH and 1.0 M ethanol.(a-c) Reproduced with permission.[42]Copyright 2019, the Royal Society of Chemistry.d) NH 3 Faradaic efficiencies (FE NH 3 ), e) NH 3 yield rates consecutive, and f ) recycling tests of PdCu MSs, PdCu NPs, Pd MSs, and Pd NPs collected in 0.10 M KOH and 10 mM NO 3

Table 1 .
NM electrocatalysts synthesized by DODAC template and their applications in electrocatalysis.

Table 2 .
Performance summary of electrocatalysts synthesized by DODAC and others for electrocatalytic reactions in different electrolytes.