Tuning strain effect and surface composition in PdAu hollow nanospheres as highly efficient ORR electrocatalysts and SERS substrates

Journal Pre-proof Tuning Strain Effect and Surface Composition in PdAu hollow nanospheres as Highly Efficient ORR Electrocatalysts and SERS Substrates Wenling Jiao, Chen Chen, Wenbin You, Guanyu Chen, Shuyan Xue, Jie Zhang, Jiwei Liu, Yuzhang Feng, Peng Wang, Yizhe Wang, Huijuan Wen, Renchao Che

PII:

S0926-3373(19)31044-6

DOI:

https://doi.org/10.1016/j.apcatb.2019.118298

Reference:

APCATB 118298

To appear in:

Applied Catalysis B: Environmental

Received Date:

6 August 2019

Revised Date:

7 October 2019

Accepted Date:

12 October 2019

Please cite this article as: Jiao W, Chen C, You W, Chen G, Xue S, Zhang J, Liu J, Feng Y, Wang P, Wang Y, Wen H, Che R, Tuning Strain Effect and Surface Composition in PdAu hollow nanospheres as Highly Efficient ORR Electrocatalysts and SERS Substrates, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118298

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Tuning Strain Effect and Surface Composition in PdAu hollow nanospheres as Highly Efficient ORR Electrocatalysts and SERS Substrates

Wenling Jiao1, Chen Chen2, Wenbin You1, Guanyu Chen1, Shuyan Xue1, Jie Zhang1, Jiwei Liu3,

1

of

Yuzhang Feng4, Peng Wang4, Yizhe Wang5, Huijuan Wen2 and Renchao Che 1*

Laboratory of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan

ro

University, Shanghai 200433, China

Department of Macromolecular Science, Fudan University, Shanghai 200433, China

3

Department of Materials Science and Engineering, Changzhou University, Jiangsu 213164, China

4

National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Collaborative

-p

2

University, Nanjing 210093, China

Materials Genome Institute, International Centre of Quantum and Molecular Structures, and Physics Department,

lP

5

re

Innovation Center of Advanced Microstructures and Center for the Microstructures of Quantum Materials, Nanjing

Shanghai University, Shanghai, 200444 China

Jo

ur na

Graphical abstract

of ro -p

Highlights

re

PdAu hollow nanospheres (PdAu HSs) with ultrathin shells composed of Au‐rich interlayers and  tunable skins have been synthesized by a simple one‐pot strategy. By decreasing Pd‐to‐Au ratio, 

lP

Pd‐skin  transformed  into  PdAu‐skin  and  then  yielding  more  Au  atoms  on  the  surfaces.  Surface  composition  and  strain  can  be  tuned  by  Pd‐to‐Au  ratio,  which  makes  it  to  be  a  paradigm  for 

ur na

exploring the most optimal bimetallic PdAu catalyst structure.    Such bimetallic nanocrystals are multifunctional nanocomposites with the integration of ORR  and SERS performances, arising from Pd and Au, respectively. The optimal ORR performance can  deliver the half‐wave potential of 0.878 V (vs. RHE) even superior to commercial Pt/C, which can  be attributed to the following reasons:  PdAu‐skin possesses greater strain core density than Pd‐skin illustrated by geometry phase 

Jo



analysis (GPA) and sufficient strain effect can boost ORR property, 



off‐axis  electron  holography  revealed  that  the  electron  polarization  located  at  PdAu 

interfaces  could  give  rise  to  local  micro‐electric  field,  which  is  beneficial  to  electron  transfer in redox reactions,   



the introduction of Au atoms in moderation on the surface can weaken the adsorption  strength of oxygen compared with Pd‐skin, confirmed by density functional thoery (DFT) 

calculations.  

Additionally,  SERS  performances  further  supported  the  variation  trend  of  surface  composition and the signals were degraded owing to the decrease in the active Au atom  density. Au HSs with maximum enhancement factor of 3.8×105 resulted from plasmon‐ induced electronic coupling. 

Abstract

of

Although palladium-gold (PdAu) bimetals have been extensively investigated as oxygen reduction reaction (ORR) electrocatalysts, the comprehensive mechanism

ro

studies on the strain and electronic effect are in their infancy. Herein, the strain effect and surface composition of PdAu hollow nanospheres (PdAu HSs) can be tuned by

-p

Pd-to-Au ratio. The optimal ORR performance can deliver the half-wave potential of 0.866 V versus reversible hydrogen electrode (vs. RHE) even superior to commercial

re

Pt/C, which can be attributed to sufficient strain effect, local micro-electric field and the adsorption strength of oxygen-containing species in moderation. In addition,

lP

surface-enhanced Raman scattering (SERS) performances further supported the variation trend of surface composition. The enhancement factor (EF) of Au HSs can reach 3.8×105, further indicating its great potential as efficient SERS substrates for

ur na

analysis and detection applications.

Key words: hollow structure, lattice strain, surface composition, ORR, SERS, off-

Jo

axis electron holography

1. Introduction The oxygen reduction reaction (ORR) has received considerable interest as a

pivotal half-reaction in fuel cells and metal-air batteries [1, 2]. Concerns over the sluggish kinetics of ORR bring urgent demands to develop high efficient ORR electrocatalysts.[3, 4] Recently, palladium (Pd) has been demonstrated to be a promising candidate due to its appreciable ORR activity in an alkaline solution and

relatively low cost.[1, 3, 5, 6] Meanwhile, bimetallic catalysts have attracted significant attention, whereby their catalytic activity and stability are superior to single metal-based materials.[7] Researchers have combined Pd with foreign metals (e.g. Au[8, 9], W[10], Fe[11, 12], Cu[13, 14] and Rh[15]), which all exhibited improvement of ORR activity.[16-18] However, catalysts based on transition group of metal lack sufficient durability during the operation of both fuel cells and metal-air batteries due to the irreversible oxidation. Au-based substances have prominent

of

electrochemical self-stability, because of its extreme chemical inertness.[18] To date, extensive efforts to establish PdAu bimetallic ORR electrocatalyst system have been

ro

dedicated. Among them, the designed morphologies of PdAu alloys/reduced graphene oxide,[8] PdAu flowerlike nanochains,[19] AuPd nanoporous clusters,11 Au@Pd

-p

nanothorns,[20] especially hollow nanostructures including nanocages[21],

nanospheres[22] and nanoframes[23], have a significant influence on ORR activity

re

due to the enlarged electrochemical active surface area (ECSA) and exposed more active sites. Such a desired morphology can increase the atomic utilization efficiency

lP

of noble metal electrocatalysts and reduce the cost. Besides, high-index facets (HIFs) exhibit much higher catalytic activities than low-index facets due to high densities of atomic steps, ledges and kinks serving as active sites.[24, 25] It has been supported

ur na

that HIFs can enhance oxygen catalysis effectively.[25] Moreover, lattice strain effect has become a widely used and efficient strategy to tune ORR activity,[26] which could influence the overlap of the outer electron orbitals among the Pd atoms.[18, 27, 28] Lots of the core-shell architectures have been fabricated, such as Pd coated Au nanoparticles,[29] Au@Pd,[18, 27, 28] Au@Pd nanoflowers /reduced graphene

Jo

oxide,[30] Au@Pd nanothorns[20] and AuPd@Pd nanocrystals13. Such a tensile strain effect can be tuned by Pd-to-Au ratio, resulting in a volcano relationship between ORR activity and Pd-to-Au ratio.[31] Nevertheless, characterizations of strain effect on bimetallic nanocomposites still remains a great challenge. More importantly, the heterometallic bonding interactions with the bimetallic surfaces can modify the electronic structure of Pd metal to tune the oxygen adsorption energy (electronic or ligand effect).[16, 19, 32] Inevitably, there occurs lattice strain induced by structural

misfit in the bimetallic catalysts at the same time (strain effect).[19] Likewise, Hong et al. have revealed that the formic acid oxidation electrocatalytic activity of PdAu alloy nanocrystals was superior to that of Au@Pd core-shell ones.[33, 34] However, comprehensive mechanism studies about the strain effect and electronic effect on the ORR performance have been rarely reported yet. Apart from being ORR electrocatalysts, PdAu bimetals are also used as surfaceenhanced Raman scattering (SERS) substrates. As we all known, Au-based substances

of

can exhibit outstanding SERS performances due to their fantastic plasmonic properties, which is extremely sensitive to surface composition and morphology.[35,

ro

36] Feng et al. investigated that the synergistic effects of composition and

morphology in Au-Pd nanobrambles played roles in SERS performances.[37] Sun et

-p

al. found that the flower-shaped Au-Pd nanoparticles showed stronger SERS signals than spherical ones, attributed to their abundant tips and polycrystalline structure.[38]

re

Thus, SERS not only detects the target molecules precisely, but also is a promising tool for providing surface structural information of SERS substrates.

lP

Herein, palladium-gold hollow nanospheres (PdAu HSs) with ultrathin shells composed of intermetallic interlayers and tunable skins have been synthesized by a simple one-pot strategy using cobalt nanoparticles as templates. Interestingly, this

ur na

decrease in Pd-to-Au ratio is accompanied by surface composition changes from Pdskin to PdAu-skin, which was derived from the lower reduction rate of Pd precursor than that of Au precursor. Such bimetallic nanocrystals are multifunctional nanocomposites with the integration of ORR and SERS performances, arising from Pd and Au, respectively. The study of these hierarchical nanostructures identified

Jo

strain and electronic effects as levers to tune ORR performance. Apparently, different Pd-to-Au ratio caused the disparity in the Pd lattice fluctuations, yielding variable dislocation dipoles illustrated by geometry phase analysis (GPA). Secondly, electron polarization between Pd and Au can result in local micro-electric field suggested by off-axis electron holography, which is crucial to electron transfer in redox reactions. Thirdly, oxygen adsorption strength of Pd-skin and PdAu-skin were calculated by density functional theory (DFT) method, which makes it available for understanding

the promotional effect of Au on the surface. In addition, SERS signal is sensitive to Pd-to-Au ratio and it can reflect the surface structural information of HSs. Notably, HSs were regarded as excellent SERS substrates due to strong localized surface plasmon resonance (LSPR) effect and rough surfaces. This work shed new lights on the comprehensive mechanism studies on the strain and electronic effect, beneficial to highly efficient ORR electrocatalysts and SERS substrates design.

2. Results and Discussion

lP

re

-p

ro

of

2.1. Morphologies and Structures for As-Prepared PdAu Catalysts.

ur na

Scheme 1. A schematic illustration for the synthetic process of PdAu HSs.

HSs under various Pd/Au precursor feed ratios (0:1, 2:1, 3:1, 4:1, 6:1 and 1:0) were fabricated in aqueous solution by a simple one-pot cobalt (Co) sacrificial template approach (Scheme 1). The reduction potential of Co2+/Co (-0.28V) is lower than that

Jo

of AuCl4-/Au (1.002 V) and PdCl42-/Pd (0.591 V), making cobalt templates react spontaneously with AuCl4- and PdCl42-.[39] Previous literatures have revealed that the reaction activity of AuCl4- was higher than that of PdCl42- when the same reductant

was used. In this case, PdAu bimetallic nanostructures usually possess Pd-rich surfaces [29, 33, 40, 41], which is coincident with our results. Pd-to-Au ratio near the surface were larger than bulk value (Figure S1 and Table S1), indicating that Pd atoms tend to grow on the surface leading to a novel Pd-enriched surface.[41] Owing to the

higher reactivity of AuCl4-, HSs was initiated by nucleation of Au atoms outside the Co nanoparticals, forming the Au-rich intermetallic interlayers. Then, Co nanoparticals disappeared and epitaxial growth of Pd-skin or PdAu-skin on the inside and outside surfaces of intermetallic interlayers appeared, as shown in Scheme 1. Such sandwich construction imposed strong strain effect on the PdAu HSs, which plays remarkable roles in ORR activity.[18] A series of HSs were synthesized by employing various Pd/Au precursor feed ratio under otherwise identical experimental

of

conditions. As the Pd/Au precursor feed ratio decreasing, Pd-skin would transform into PdAu-skin and then increasing Au content on the surface.[33] Specially, cyclic

ro

voltammogram (CV) curve of Pd6Au/C existed exclusively of Pd oxides peaks (0.80 V versus reversible hydrogen electrode, 0.80 V vs. RHE), whereas CVs of Pd4Au/C,

-p

Pd3Au/C and Pd2Au/C had Pd and Au oxides peaks (1.10 V vs. RHE) simultaneously (Figure S2).[18, 32] It proved that Pd6Au HSs possessed Pd-skin. Furthermore, gold

re

element was almost invisible on the surface of Pd6Au HSs (Figure S3). Moreover, electrochemical impedance spectroscopy (EIS) analysis (Figure S4, Table S2)

lP

exhibited that the increase in Au precursor was accompanied by increasing impedance. The incorporation of Au atoms caused the formation of tensile strain and associated local micro-electric fields to reduce the electrical conductivity. These

ur na

phenomenon supported the successful synthesis of PdAu HSs with different Pd-to-Au

Jo

ratio.

of ro -p re lP

Figure 1. a) SEM images of Pd3Au HSs. b) High-resolution TEM image of Pd3Au HSs. HRTEM

ur na

image and the corresponding FFT pattern in the region of the red box. c) High-resolution TEM image of Pd HSs. d) Low-magnification HAADF-STEM image of Pd3Au HSs. e) EDS of Pd3Au HSs. f, g, h) HAADF-STEM image of Pd3Au HSs and the corresponding Pd and Au elemental

Jo

mapping images.

Transmission electron microscopy (TEM) and scanning electron microscopy

(SEM) were employed for characterizing morphologies and nanostructures.[42, 43] All HSs exhibit well-defined morphology with a monodisperse diameter distribution of 33±4 nm and the shell thickness of 4±1 nm due to the similar template method (Figure 1a-d and Figure S5). HSs possessed rough surfaces (Figure 1d and f) with numerous tips, corners and edges serving as “hot-spots” for SERS. Moreover,

randomly arranged lattice patterns inside a single nanosphere (Figure 1b-c) demonstrate that the polycrystalline architecture was achieved by the aggregation of metal clusters or nanoparticles.[41] The grain boundary-rich structure can cause a large number of micropores (Figure 1b-c, red arrows), which increases the ECSA and benefits the mass transport. Besides, previous studies have proved that abundant crystal defects gave rise to higher ORR activity and SERS performance.[18, 38, 44, 45] The lattice spacing is 0.23 nm in Figure 1e, slightly larger than that of face-

of

centered cubic (fcc) Pd (111) plane because of alloying with Au. The high-resolution transmission electron microscopy (HRTEM) images taken from an individual

ro

nanospheres further confirm the edges of HSs contain abundant HIFs (Figure 1b and Figure S6). By analysis of the fast Fourier transform (FFT) pattern, the HIFs were

-p

found to be dominated by the (220) and (311) facets. HIFs own low coordination, which are capable of providing more active sites for ORR.[24] In addition, lattice

re

fringe spacings of 0.22 nm and 0.20 nm corresponded to the (111) and (200) crystal planes of fcc Pd in Figure 1f, which indicates that Pd HSs are enclosed by (111)

lP

facets. Furthermore, high angular annular dark-field scanning transmission electron microscopy (HAADF-STEM) mapping (Figure 1f-h) revealed that Pd and Au elements were distributed along the whole nanospheres, indicating the formation of

ur na

Pd-Au bimetallic nanostructures in accord with energy dispersive X-ray spectroscopy (EDS) results (Figure 1e). The Co signals of HAADF-STEM mapping images (Figure S7), EDS results and electron probe micro analyzer (EPMA) spectra (Figure S8) were almost invisible. Moreover, the Co content in alloys was calculated to be 0.17% by inductively coupled plasma-optical emission spectrometry (ICP-OES) measurement

Jo

(Figure S9), which further confirmed that there existed little cobalt in alloys.

of ro

-p

Figure 2. a) The XRD results of PdAu HSs. b) Pd 3d and c) Au 4f XPS spectra of PdAu HSs.

X-ray diffraction (XRD) analysis was used to illustrate the crystalline structures of

re

these bimetallic HSs and a single alloy phase of fcc symmetry was confirmed to be formed (Figure 2a).[46] Diffraction peaks of Au HSs and Pd HSs can be well indexed

lP

to the standard Au (PDF#65-2870) and Pd (PDF#65-2867) respectively.[39] With Pdto-Au ratio decreasing, the peaks of PdAu HSs kept shifting negatively compared to the pure Pd. The surface compositions and chemical states of HSs were investigated

ur na

by X-ray photoelectron spectroscopy (XPS),[42, 46] as represented in Figure 2b-c and Figure S10. The main peaks of five elements (Pd, Au, O, C and Cu) appeared in the XPS full spectrum. C 1s signal was used as a reference of 284.6 eV and the samples were put on the copper slices when characterized by XPS. The O element may arise from the adsorbed O2 or oxygen-containing groups. The high-resolution Pd 3d

Jo

spectrum (Figure 2b) exhibits that a couple of diffraction peaks at 340.4-340.7 eV and 335.1-335.4 eV are ascribed to metallic Pd0 3d3/2 and 3d5/2 respectively, accompanied by a pair of weak peaks emerged at about 342.1 eV and 336.9 eV, corresponding to Pd2+ species[3, 47, 48]. The high-resolution Au 4f spectrum is convoluted into two strong peaks at 87.3-87.9 eV and 83.6-84.2 eV, indexed into metallic Au0 4f5/2 and 4f7/2 respectively (Figure 2c).[8] It confirmed that PdAu HSs consisted of metallic Pd0 and Au0 in principal, indicating the alloy nature. Notably, the positive shift of binding

energy take place in the both Pd 3d and Au 4f peaks of PdAu HSs as the content of Au increased. Specifically, the binding energies of Pd0 3d3/2 of Pd2Au HSs (340.7 eV) and Pd3Au HSs (340.6 eV) were increased by 0.3 eV and 0.2 eV respectively, with respect to that of Pd HSs (340.4 eV). The Au 4f7/2 binding energy of Pd6Au HSs is 83.6 eV, which is 0.6 eV lower as compared with that of Au HSs (84.2 eV). The Pd-to-Au ratio dependent binding energies can be assigned to the interplay of Pd and Au atoms and lattice strain.[49, 50] Au atoms would withdraw the electrons from Pd in the alloys

Pd 3d binding energy is related to Pd 4d electron density (Pd:

of

due to the higher electronegativity of Au (2.4) than that of Pd (2.2). Considering that

ro

1s22s2p63s2p6d104s2p6d10), 4d electrons of Pd decrease. Besides, tensile strain in the Pd lattice can decrease the overlap of the Pd 4d electron orbitals as illustrated in Figure

-p

4e, suggesting the higher binding energy and lower electron density accordingly.[41] It has been demonstrated that the shifts of high-resolution XPS binding energy could

re

reflect the similar changes of the d-band center of materials.[51, 52] The positive shift of Pd binding energy was meant to be the lower Fermi level and lower electron

lP

density, which decreases the interactions between Pd active sites and oxygencontaining species and consequently tailors the ORR properties.[53, 54] 2.2. Component-dependent ORR activity of PdAu HSs and performance

ur na

comparison with commercial Pt/C.

HSs/C were obtained by mixing the individual HSs with Vulcan XC-72 carbon in absolute ethanol by ultrasonication.[55] The component-dependent ORR performances of HSs/C were investigated and further benchmarked against commercial Pt/C (Johnson Matthey, 20 wt% Pt). The ORR began at slightly more

Jo

positive potentials for Pd3Au/C and Pd2Au/C followed very closely by Pt/C. The onset potential (Eonset) is regarded as the potential with a current density of -0.05

mA·cm-2 in the linear sweep voltammogram (LSV). The Eonset of Pd3Au/C (0.988 V vs. RHE) is more positive than those of the Pd/C (0.92 V vs. RHE) and Pt/C catalyst (0.96 V vs. RHE). And the ORR half-wave potential (E1/2) is defined as the critical potential when the reduction current density reaches half of the diffusion-limited current density at 0.5 V vs. RHE in the LSV. Indeed, the E1/2 were more positive in the

following order: Pd3Au/C > Pd2Au/C > Pt/C > Pd4Au/C > Pd6Au/C > Pd/C >> Au/C (Figure 3a). Compared with Au/C, the half-wave potentials of Pd/C varied significantly with large positive shift obviously, demonstrating that Pd atoms were highly effective active sites for ORR in HSs/C. Thus, Pd was the major component responsible for catalyzing the ORR and Au component modified the electronic structure of Pd through tensile strains and heterometallic bonding interactions. The tafel slopes of Pd/C, Pd6Au/C, Pd4Au/C, Pd3Au/C, Pd2Au/C, Au/C and commercial

of

Pt/C were 110 mV·dec-1, 93 mV·dec-1, 107 mV·dec-1, 71 mV·dec-1, 80 mV·dec-1, 163 mV·dec-1and 85 mV·dec-1 respectively (Figure 3b), which manifested that the ORR

ro

kinetics in alkaline medium of Pd3Au/C and Pd2Au/C were faster than that of

commercial Pt/C and the activities of HSs/C heavily relied on Pd-to-Au ratio.

-p

Moreover, the Pd oxide reduction peaks of HSs/C were shifted to higher potentials as the content of Au increased (Figure S2, CVs in N2-saturated 0.1 M KOH).[41, 56, 57]

re

Specifically, Pd oxide reduction potential of Pd3Au/C located at 0.80 V, whereas that of Pd/C was 0.72 V. These findings implied that the incorporation of Au into Pd HSs

lP

could improve their resistance to oxidation indicating a weaker binding strength of oxide species on the Pd3Au HSs surfaces.[39] The reduction potential peaks of Pd3Au/C and Pd2Au/C shift positively by 102 and 99 mV respectively compared with

ur na

Pd/C (Figure S11, CVs in O2-saturated 0.1 M KOH), revealing the decrease in oxygen affinity of PdAu/C.[1, 29] In addition, similar diffusion-limited current densities were observed at Pd6Au/C and Pd/C, which were higher than those of others due to their lower electron transfer impedance (Figure S4). To evaluate the correlation between Pd-to-Au ratio and ORR activity accurately, the corresponding specific activities (SA)

Jo

and mass activities (MA) of these electrocatalysts at 0.84 V vs. RHE were analyzed (Figure 3f). ECSAs for HSs/C with various Pd-to-Au ratio and commercial Pt/C were calculated by using the CVs (Figure S12).[32, 58] The current response at different scan rates arose from the charging of the double-layer. The ECSA directly proportional to the double-layer capacitance (Cdl) was estimated (Table S3).[59] As expected, the SA of Pd3Au/C catalyst could reach 0.13 mA cm-2, 2.6 times the SA of Pd/C (0.05 mA cm-2, Table S4). The MA of Pd3Au/C was 0.153 A mg-1Pd+Au, 2.28

times greater than that of Pd/C (0.067 A mg-1Pd, Table S4). It elucidated that the introduction of Au atoms into Pd HSs could optimize the electronic structure of Pd effectively for higher ORR performance. Interestingly, the ORR activities displayed a volcano type as a function of Pd-to-Au ratio. Moderate Pd-to-Au ratio played a

Jo

ur na

lP

re

-p

ro

of

positive role in ORR improvement, but excessive Au atoms reduced the activity.

Figure 3. a) ORR polarization curves for PdAu HSs/C and commercial Pt/C in O2-saturated 0.1 M KOH solution at 1600 rpm (scan rate of 10 mV s–1). b) The corresponding tafel plots. c) Polarization curves obtained at rotation speeds ranging from 400 to 2500 in 0.1 M KOH solution for Pd3Au/C (inset: K-L plots). d) RRDE data for Pd3Au/C. e) Chronoamperometric responses of

Pd3Au/C and Pt/C catalysts at 0.6 V vs. RHE. f) Comparison of specific activities and mass activities of the PdAu HSs/C and commercial Pt/C at 0.84 V vs. RHE.

Notably, E1/2 value (0.866 V vs. RHE) with optimal Pd-to-Au ratio of 3:1 was more positive (23 mV) than that of commercial Pt/C (0.846 V vs. RHE). SA and MA of Pd3Au/C were 4.81 and 1.56 times larger than that of commercial Pt/C (0.027 mA cm2

, 0.098 A mg-1Pt, Table S4) respectively. In addition, Pd3Au/C possessed preferable

of

wettability compared with Pt/C, attributed to the surface hydrophilic groups confirmed by Fourier transformation infrared spectroscopy (FT-IR) spectra (Figure

ro

S13). The improvement of wettability can increase the solid-liquid contact area,

beneficial to the desorption of OH* and increase the diffusion-limited current (Figure

-p

S14).[60] Meanwhile, Pd3Au/C showed comparative ORR activity toward the Pdbased electrocatalysts reported previously (Table S5). To obtain deeper insight into

re

the electron transfer kinetics of Pd3Au/C, LSVs at different rotating speeds were investigated (Figure 3c). The ORR electron transfer number of Pd3Au/C was

lP

calculated according to K-L equation slopes. As shown in the inset of Figure 3c, the K-L plots at 0.4, 0.5 and 0.6 V vs. RHE display good linearity and the electron transfer number was around 4.0, revealing that the ORR on Pd3Au/C complies with

ur na

the first-order kinetics and a dominant “4e−” pathway similar to commercial Pt/C. Moreover, H2O2 yield was less than 15% at 0.3-0.7 V vs. RHE, which further confirmed a “4e−” pathway (Figure 3d, rotating ring disk electrode results, RRDE results). Computed from the ring current (Ir) and disk current (Id) data, the n values at 0.3-0.7 V vs. RHE were found to be 3.72-3.90, which is close to 4.0 and consistent

Jo

with K-L plot analysis. Furthermore, the stability of Pd3Au/C and commercial Pt/C

were evaluated in the O2-saturated 0.1 M KOH electrolyte by i-t chronoamperometry at 0.6 V vs. RHE. Pd3Au/C held a high current retention of 86% after 40000 s whereas

commercial Pt/C had the current retention of only 64% (Figure 3e), which indicated that Pd3Au/C possessed higher stability in alkaline medium than commercial Pt/C. Besides, Pd3Au/C before and after the durability tests were characterized by TEM (Figure S15), which showed no noticeable change of the morphology.

2.3. Understanding the enhanced ORR activity of PdAu HSs induced by lattice strain and surface atomic composition. The strain distribution maps (Figure 4 a1-a3, b1-b3) can be obtained from HRTEM images (Figure 4a, b). The Fourier transform and inverse Fourier transform of HRTEM lattice images were performed. Then, the phase component of the resulting images can provide information about local displacements of atomic planes and strain fields (the detailed description see Supporting Information).[61-63] Since the lattice

of

parameter of Au (a=4.079 Å) is slightly larger than that of Pd (a=3.890 Å)[17], the introduction of Au caused a tensile strain into the Pd lattice, generating the distortion

ro

of Pd atomic structure. Figure S17 give the profiles of HRTEM images for the interplane spacing analyses of Pd3Au (111) and Pd (111) respectively to estimate the

-p

tensile strain. The d (111) spacings of Pd3Au HSs and Pd HSs were about 0.2369 and 0.2271 nm respectively, which are tensile by 4.3%.[64] The distortions of Pd-Pd

re

bonds generally exhibited compressing-tensile dislocation dipoles in the GPA strain maps (Figure 4 a1-a3, b1-b3), indicating that the Au atoms were inclined to become

lP

strain cores.[65] Local regions of HSs with various Pd-to-Au ratio were randomly selected for 10 times to analyze the strain distribution. Definitely, different Pd-to-Au ratio resulted in the changes of surface composition and structure, leading to disparity

Jo

ur na

in the Pd lattice fluctuations.

of ro -p re lP ur na

Figure 4. a) TEM images, corresponding dilatation strain maps along a1) Exx, a2) Exy and a3)

Jo

rotation strain map of Pd6Au HSs. b) TEM images, corresponding dilatation strain maps along b1) Exx, b2) Exy and b3) rotation strain map of Pd3Au HSs. (The compressive strain is represented by the color from green to dark blue, while the tensile strain is depicted by the color from red to bright yellow). c) and d) The strain center density (/nm2, Exx) and the ORR half-wave potentials (E1/2) of PdAu HSs. e) Schematic illustrations for the lattice deformation and the effect of tensile strain on Pd 4d orbit.

Figure 4c displayed composition-dependent surface strains, where more strain cores

corresponding to the bond movements and rotations appeared as Pd-to-Au ratio decreased. Certainly, there existed residual lattice strain in the Pd HSs imposed by the structure with abundant of grain boundary. Particularly, a visual evidence for the strain distribution of Pd6Au HSs and Pd3Au HSs verified that both tensile strain and compressive strain were more intense on PdAu-skin than Pd-skin. The ORR activities displayed a volcano type as a function of Pd-to-Au ratio, as illustrated in Figure 4d and Figure S16. Pd6Au HSs with Pd-skin provided higher performance than Pd HSs,

of

attributed to tensile strains imposed by Au-rich interlayers. Lower Pd-to-Au ratio yielded more Au atoms on the surfaces (PdAu-skin), giving rise to stronger strained

ro

shells supported by GPA results. Thus, sufficient lattice strain is beneficial for ORR enhancement. Diagrammatic drawing in Figure 4e displayed that tensile strain and

-p

compressive strain coexisted in the PdAu HSs and the displacements of Pd atoms predominated the overlap of the outer electron cloud (Pd 4d orbit).[31] On this

re

occasion, noble metal electrocatalysts with compressively strained surfaces can downshift the d-band centers to improve ORR activity.[31, 34] Tensile strains

lP

decreased the overlap of Pd 4d orbit, which could weaken the interaction between catalyst and oxygenated intermediates and activated the low-coordinated surface atoms.[26] However, when beyond the critical Pd-to-Au ratio, fewer active sites and

Jo

ur na

weaker active oxygen adsorption inhibited the ORR performance in turn.[28, 41]

of

Figure 5. a1) Electron holograms of Pd HSs. b1) Electron holograms of Pd3Au HSs. a2, b2)

ro

reconstructed phase images (unrap). a3, b3) charge density distribution maps calculated from the reconstructed phase and amplitude images. a4, b4) 2D electric field distribution maps (the insets

-p

are color wheels representing the electric field direction).

re

Polarized electronic structures derived by dislocation dipoles (strain effect) and PdAu interfaces (electronic effect) existed in the PdAu HSs, as illustrated by off-axis

lP

electron holography (Figure S19, the detailed description see Supporting Information).[44, 52] The corresponding polarized electronic distribution line profiles in HSs revealed that electronic polarization existed in all samples (Figure S19 a3-d3,

ur na

red lines). As Pd-to-Au ratio decreased, more intense electronic polarization were obtained due to more dislocation dipoles and PdAu interfaces. Furthermore, charge density distribution maps and 2D electric field distribution maps of Pd HSs and Pd3Au HSs were calculated respectively (Figure 5). Comparatively, higher density of the gathered electrons was observed in the Pd3Au HSs region (-6.5 e/nm3 ~ 6.5 e/nm3)

Jo

than that of Pd HSs region (-6 e/nm3 ~ 6 e/nm3), which demonstrated that Pd3Au HSs

region owned more and stronger electronic polarization. More electronic polarization can give rise to more abundant local micro-electric field, confirmed by 2D electric field distribution maps (Figure 5 a4, b4).[66] More importantly, local micro-electric fields were not conducive to the migration rates of electrons in a certain direction, decreasing the electrical conductivity. However, local micro-electric fields have been reported to promote the electron transfer process during the redox reaction

-p

ro

of

effectively.[1, 67]

Figure 6. a) The ORR mechanism on PdAu HSs catalysts in alkaline media. b) Free energy

re

diagram for ORR process at the equilibrium potential U =1.23 V vs. RHE and U=0 V. The optimized models of O2 adsorption on the surface of c) Pd(111) and d) Pd1.96Au(111). e) Trends in

lP

ORR activity plotted as a function of the oxygen binding energy. f) Partial density of states (PDOS) of Pd(111) and Pd1.96Au(111) after O2 adsorption.

ur na

Based on the electrochemical experimental data that the four-electron process was the dominant mechanism on the Pd3Au/C catalyst, the ORR steps in alkaline condition can be schematically depicted in Figure 6a and described as follows[68-70]: ∗

2

4

∗

→

∗

Jo

3

∗

2

∗

2

3

Overall ∶

2

∗

2

∗

3

→

→

→4

3

1 2

2

3

4 4

→4

5

To understand the mechanisms of enhanced performance of Pd3Au/C compared with Pd/C, the free energy diagrams of ORR steps were calculated at the equilibrium potential (U =1.23 V vs. RHE at pH =13) over Pd (111) and Pd1.96Au (111) models

(Figure 6b).[16, 71] The HRTEM results (Figure 1f) revealed that PdAu HSs were basically enclosed by (111) facets (Pd-to-Au ratio of Pd3Au HSs was 1.96:1 obtained by ICP-OES, Table S1). The formation of O* and OH* were exergonic, whereas the Gibbs free energies over Pd (111) were -0.33 eV and -0.62 eV respectively, reducing faster than those (-0.20 eV and -0.49 eV) of Pd1.96Au (111). The results implied that the reaction intermediates were more stable over Pd (111) than over Pd1.96Au (111), resulted from the binding strength was stronger on Pd-skin. Moreover, the formation

of

of OOH* (step 2) was associated with an increase in free energy, whereas more free energy needs to be increased about the desorption of OH* (step 6). Thus, the

ro

desorption of OH* (step 6) was concluded as a rate-determining step for the ORR and step 6 became easier on Pd1.96Au (111) than on Pd (111). The changes in Gibbs free

-p

energies may be attributed to that the adsorptions of oxygenated intermediates were reduced after the introduction of Au atoms, which can improve ORR performance.

re

The ORR activity essentially depends on the binding strength of the catalysts to the oxygen-containing species, such as O2*, O*, OH*, OOH*. In terms of Pd-skin, the

lP

binding strength was found to be too strong, in accordance with previous literatures (Figure 6e). [1, 16] More oxygen-containing species would accumulate on the catalyst surface and step 6 became the rate-determining step of ORR, making the active sites

ur na

unavailable for next cycle.[32, 70] Take oxygen adsorption as an example, preliminary DFT calculations were performed to estimate the oxygen adsorption strength on PdAu-skin and Pd-skin, which further benchmarked against Pt (111) as shown in Figure 6c, d and Table S6. In step 1, Pd-Pd bond length increases induced by the tensile strain, which makes the active oxygen inclined to be adsorbed on the

Jo

catalyst surface in this way as shown in Figure 6a, c and d. The longer Pd-O bond, shorter O=O bond and more positive △EO of Pd1.96Au (111) than that of Pd (111) were obtained, implying that the introduction of Au atoms into Pd-skin may bring about the repulsive force of oxygenated intermediates and improve ORR activity.[32] Moreover, both △EO and the bond lengths demonstrated that the oxygen adsorption

strength on Pt (111) was weaker than on Pd (111) in accordance with the trend in Figure 6e.[16] Additionally, the negative shift of partial densities of state (PDOS) of

Pd1.96Au (111) after O2 adsorption relative to Pd (111) after O2 adsorption demonstrated that the gap between ground state and Fermi energy became larger in PdAu-skin (Figure 6f). The larger gap means weaker d orbital bonding ability of noble metals, indicating the lower coordination bond strength between oxygen atoms and PdAu-skin.[72] Nonetheless, the active oxygen adsorption became too weak with excessive Au, preventing the O-O bond from breakage (step 3) and then hampering the ORR performance. Therefore, the ORR activity of Pd3Au HSs, Pd2Au HSs and

of

Pd4Au HSs were higher than that of Pd6Au HSs and Pd HSs due to their stronger lattice strain and the promotional effect of Au on the surface as illustrated in Figure 8

ro

and Figure S16, which makes it to be a paradigm for exploring systematically the

Jo

ur na

lP

re

-p

most optimal bimetallic PdAu catalyst structure.

Figure 7. a) SERS studies of HSs with different Pd-to-Au ratio using R6G as the Raman probe molecule. b) The EF of those catalysts for SERS at 1364 cm-1. c) SERS for Au HSs using 10-6 M,

10-8 M, 10-10 M and 10-12 M R6G solution. d) SERS for Pd3Au HSs using 10-6 M, 10-8 M and 10-10 M R6G solution.

2.4. SERS studies of PdAu HSs. The SERS performances were investigated on the HSs with different Pd-to-Au ratio using rhodamine 6G (R6G) as the Raman probe molecule as presented in Figure 7a. Normal Raman spectrum of pure R6G film prepared by placing 0.05 M R6G solution on the silicon wafer was employed as a reference (Figure S20). 10-5 M R6G on the SERS active substrates of HSs exhibited strong enhancements at 612 cm-1, 770 cm-1, 1132 cm-1, 1185 cm-1, 1312 cm-1, 1364 cm-1 and 1513 cm-1, in accordance with

of

previous literatures.[38] For every experiment, the SERS spectra were recorded from 10 random spots and the ISERS were averaged from the 10 spectra for final

ro

comparisons. (Figure S21-S23). The enhancement factor (EF) values were calculated considering the high intense peaks at the specific peak positions of 1364 cm-1.

-p

Distinctly, SERS signals of R6G are increased progressively with Pd-to-Au ratio decreasing, as shown in Figure 7a, b and Table S7. SERS signal enhancement can be

re

obtained by that the excitation wavelength perfectly matches the plasmonic resonances of PdAu hollow nanospheres.[73, 74] As shown in Figure S24, the LSPR

lP

peak for Au hollow nanospheres appeared at 600 nm, leading to the selection of 633 nm laser as the excitation source. Since inactive Pd component increased, the LSPR peaks disappeared. Attenuation of SERS signal was attributed to the degradation of

ur na

plasmonic properties, which was consistent with experimental data. SERS was regarded as an ultrasensitive vibrational spectroscopy, related closely to surface structural information of SERS substrates.[75, 76] Considering the identical morphology of all HSs and intrinsic LSPR effect of Au atoms, the increase of active Au atom density on the surface can amplify SERS signal.[37] Accordingly, the EF

Jo

value of Pd4Au HSs increased dramatically compared with Pd6Au HSs, attributed to that Pd6Au HSs had scarcely any Au atoms on the surface (Figure S2 and Figure S3).

The highest EF value of 3.8×105 was observed at a peak position of 1364 cm-1 for pure Au HSs. When the R6G concentration was decreased to 10-6 M or 10-8 M, the intensity of peaks was still obvious (Figure 7c). The signal noise ratio dropt visibly as the concentration was further reduced to 10-12 M and the limit of detection (LOD) for Au HSs were 10-10 M or lower in this work. The excellent SERS performance of Au

HSs can be ascribed to that abundant tips on the Au surfaces were served as hot-spots forming strong plasmon-induced electronic coupling.[35] Besides, Sun et al. have reported that crystal defects could trap the dye molecules to generate better SERS signals.[38] The SERS enhancement partly results from the polycrystalline structures of HSs. In addition, Pd3Au HSs with optimal ORR activity also delivered favorable SERS performance (Figure 8). The EF value and LOD of Pd3Au HSs can reach 1.0 ×105 and 10-8 M (Figure 7d) respectively, which is comparable to previously reported

Jo

ur na

lP

re

-p

ro

of

Au-Pd substrates for Raman enhancement of R6G.[38, 77]

Figure 8. Schematic illustrations for the mechanism of the enhanced ORR and SERS performances of Pd3Au HSs.

3. Conclusions In summary, PdAu HSs whose shells consist of Au-rich interlayers and Pd-rich skins were successfully fabricated with excellent ORR and SERS performances. The excellent ORR performance was attributed to the three reasons as follows. Firstly, Pdskin transformed into PdAu-skin and then yielding more Au atoms on the surfaces by decreasing Pd-to-Au ratio. The introduction of Au caused the distortions of Pd-Pd bonds, which were inclined to become strain cores. PdAu-skin possesses greater strain

of

core density than Pd-skin illustrated by GPA and sufficient strain effect can boost ORR property. Secondly, off-axis electron holography revealed that the electron

ro

polarization located at PdAu interfaces could give rise to local micro-electric field, which is beneficial to redox reactions. Thirdly, the introduction of Au atoms in

-p

moderation on the surface can weaken the adsorption strength of oxygen to accelerate the desorption of OH*compared with Pd-skin, confirmed by DFT calculations. As for

re

SERS studies, rough surfaces and crystal defects can served as hot-spots to generate better SERS performances. Moreover, more Au atoms on the surfaces can amplify the

lP

SERS signals, ascribed to the stronger LSPR effect. Accordingly, Au HSs exhibited optimal SERS performance with EF of 3.8×105, resulted from plasmon-induced electronic coupling. This work has instructional implications for understanding the

ur na

structure-activity relationship, beneficial to ORR electrocatalysts and SERS substrates design.

Jo

Declaration of interests  The  authors  declare  that  they  have  no  known  competing  financial  interests  or  personal  relationships that could have appeared to influence the work reported in this paper. 

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work was supported by the Ministry of Science and Technology of China (973

Project No. 2018YFA0209102) and the National Natural Science Foundation of China

Jo

ur na

lP

re

-p

ro

of

(11727807, 51725101, 51672050, and 61790581).

References [1]  Y. Zuo, D. Rao, S. Li, T. Li, G. Zhu, S. Chen, L. Song, Y. Chai, H. Han, Advanced Materials 30 (2018).  [2]  D. Tat Thang Vo, J. Wang, K.C. Poon, D.C.L. Tan, B. Khezri, R.D. Webster, H. Su, H. Sato,  Angewandte Chemie‐International Edition 55 (2016) 6842‐6847.  [3]  J. Wang, L. Liu, S. Chou, H. Liub, J. Wang, Journal of Materials Chemistry A 5 (2017) 1462‐1471.  [4]  L. Xiao, L. Zhuang, Y. Liu, J. Lu, H.D. Abruna, Journal of the American Chemical Society 131 (2009)  602‐608.  [5]  S.K. Konda, M. Amiri, A. Chen, Journal of Physical Chemistry C 120 (2016) 14467‐14473.  [6]  X.‐T. Wu, J.‐C. Li, Q.‐R. Pan, N. Li, Z.‐Q. Liu, Dalton Transactions 47 (2018) 1442‐1450.  [7]  B. Wang, Journal of Power Sources 152 (2005) 1‐15. 

of

[8]  X.‐X. Lin, X.‐F. Zhang, A.‐J. Wang, K.‐M. Fang, J. Yuan, J.‐J. Feng, Journal of Colloid and Interface  Science 499 (2017) 128‐137. 

[10]  Y. Dai, P. Yu, Q. Huang, K. Sun, Fuel Cells 16 (2016) 165‐169. 

ro

[9]  C.P. Deming, A. Zhao, Y. Song, K. Liu, M.M. Khan, V.M. Yates, S. Chen, Chemelectrochem 2 (2015)  1719‐1727. 

[11]  J. Liu, C.Q. Sun, W. Zhu, Electrochimica Acta 282 (2018) 680‐686. 

-p

[12]  K. Maiti, J. Balamurugan, S.G. Peera, N.H. Kim, J.H. Lee, ACS Appl. Mater. Interfaces 10 (2018)  18734‐18745. 

[13]  J. Liu, X. Fan, C.Q. Sun, W. Zhu, Materials (Basel, Switzerland) 11 (2017).  Morais, Langmuir 33 (2017) 2734‐2743. 

re

[14]  M.V. Castegnaro, W.J. Paschoalino, M.R. Fernandes, B. Balke, M.C.M. Alves, E.A. Ticianelli, J.  [15]  F. Tzorbatzoglou, A. Brouzgou, P. Tsiakaras, Applied Catalysis B‐Environmental 174 (2015) 203‐

lP

211. 

[16]  J.K. Norskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. Jonsson, Journal of  Physical Chemistry B 108 (2004) 17886‐17892. 

[17]  G. Wang, J. Guan, L. Xiao, B. Huang, N. Wu, J. Lu, L. Zhuang, Nano Energy 29 (2016) 268‐274. 

ur na

[18]  Q. Xue, J. Bai, C. Han, P. Chen, J.‐X. Jiang, Y. Chen, Acs Catalysis 8 (2018) 11287‐11295.  [19]  L.‐L. He, P. Song, A.‐J. Wang, J.‐N. Zheng, L.‐P. Mei, J.‐J. Feng, Journal of Materials Chemistry A 3  (2015) 5352‐5359. 

[20]  G. Fu, Z. Liu, Y. Chen, J. Lin, Y. Tang, T. Lu, Nano Research 7 (2014) 1205‐1214.  [21]  L. Zhang, L.T. Roling, X. Wang, M. Vara, M. Chi, J. Liu, S.‐I. Choi, J. Park, J.A. Herron, Z. Xie, M.  Mavrikakis, Y. Xia, Science 349 (2015) 412‐416.  [22]  Y. Chen, Z. Li, Y. Zhu, D. Sun, X. Liu, L. Xu, Y. Tang, Advanced materials (Deerfield Beach, Fla.)   

Jo

(2018) e1806312‐e1806312. 

[23]  C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H.L. Xin, J.D. Snyder, D. Li, J.A. Herron, M. Mavrikakis,  M. Chi, K.L. More, Y. Li, N.M. Markovic, G.A. Somorjai, P. Yang, V.R. Stamenkovic, Science 343 (2014) 

1339‐1343. 

[24]  Y. Zhao, L. Tao, W. Dang, L. Wang, M. Xia, B. Wang, M. Liu, F. Gao, J. Zhang, Y. Zhao, Small 

(Weinheim an der Bergstrasse, Germany) 15 (2019) e1900288‐e1900288.  [25]  Y. Sun, X. Zhang, M. Luo, X. Chen, L. Wang, Y. Li, M. Li, Y. Qin, C. Li, N. Xu, G. Lu, P. Gao, S. Guo,  Advanced Materials 30 (2018).  [26]  L. Bu, N. Zhang, S. Guo, X. Zhang, J. Li, J. Yao, T. Wu, G. Lu, J.‐Y. Ma, D. Su, X. Huang, Science 354  (2016) 1410‐1414. 

[27]  D. Chen, J. Li, P. Cui, H. Liu, J. Yang, Journal of Materials Chemistry A 4 (2016) 3813‐3821.  [28]  Z. Zong, K. Xu, D. Li, Z. Tang, W. He, Z. Liu, X. Wang, Y. Tian, Journal of Catalysis 361 (2018) 168‐ 176.  [29]  J.H. Shim, J. Kim, C. Lee, Y. Lee, Chemistry of Materials 23 (2011) 4694‐4700.  [30]  S.‐S. Li, A.‐J. Wang, Y.‐Y. Hu, K.‐M. Fang, J.‐R. Chen, J.‐J. Feng, Journal of Materials Chemistry A 2  (2014) 18177‐18183.  [31]  Y. Zhao, Y. Wu, J. Liu, F. Wang, ACS Appl. Mater. Interfaces 9 (2017) 35740‐35748.  [32]  Y. Wang, W. Huang, C. Si, J. Zhang, X. Yan, C. Jin, Y. Ding, Z. Zhang, Nano Research 9 (2016) 3781‐ 3794.  [33]  J.W. Hong, D. Kim, Y.W. Lee, M. Kim, S.W. Kang, S.W. Han, Angewandte Chemie‐International  Edition 50 (2011) 8876‐8880.  5526‐5532. 

of

[34]  C. Wang, X. Sang, J.T.L. Gamier, D.P. Chen, R.R. Unocic, S.E. Skrabalak, Nano Letters 17 (2017)  [35]  M.K. Kinnan, G. Chumanov, Journal of Physical Chemistry C 111 (2007) 18010‐18017. 

ro

[36]  M.‐H. Wang, J.‐W. Hu, Y.‐J. Li, E.S. Yeung, Nanotechnology 21 (2010). 

[37]  J.‐J. Feng, X.‐X. Lin, S.‐S. Chen, H. Huang, A.‐J. Wang, Sensors and Actuators B‐Chemical 247  (2017) 490‐497. 

-p

[38]  D. Sun, G. Zhang, X. Jiang, J. Huang, X. Jing, Y. Zheng, J. He, Q. Li, Journal of Materials Chemistry A  2 (2014) 1767‐1773. 

[39]  L.‐M. Luo, W. Zhan, R.‐H. Zhang, D. Chen, Q.‐Y. Hu, Y.‐F. Guo, X.‐W. Zhou, Journal of Power 

re

Sources 412 (2019) 142‐152. 

[40]  J.‐N. Zheng, S.‐S. Li, X. Ma, F.‐Y. Chen, A.‐J. Wang, J.‐R. Chen, J.‐J. Feng, Journal of Power Sources  262 (2014) 270‐278.  Materials 8 (2018). 

lP

[41]  S. Zhu, Q. Wang, X. Qin, M. Gu, R. Tao, B.P. Lee, L. Zhang, Y. Yao, T. Li, M. Shao, Advanced Energy  [42]  X.‐Y. Huang, A.‐J. Wang, L. Zhang, Q.‐L. Zhang, H. Huang, J.‐J. Feng, Journal of Colloid and  Interface Science 552 (2019) 51‐58. 

ur na

[43]  H.‐J. Niu, H.‐Y. Chen, G.‐L. Wen, J.‐J. Feng, Q.‐L. Zhang, A.‐J. Wang, Journal of Colloid and  Interface Science 539 (2019) 525‐532. 

[44]  F. Cheng, J. Shen, B. Peng, Y. Pan, Z. Tao, J. Chen, Nature Chemistry 3 (2011) 79‐84.  [45]  S. Liu, H. Zhang, X. Mu, C. Chen, Applied Catalysis B‐Environmental 241 (2019) 424‐429.  [46]  H.‐Y. Chen, M.‐X. Jin, L. Zhang, A.‐J. Wang, J. Yuan, Q.‐L. Zhang, J.‐J. Feng, Journal of Colloid and  Interface Science 543 (2019) 1‐8. 

[47]  P. Chandran, A. Ghosh, S. Ramaprabhu, Scientific reports 8 (2018) 3591‐3591. 

Jo

[48]  X.‐X. Lin, A.‐J. Wang, K.‐M. Fang, J. Yuan, J.‐J. Feng, Acs Sustainable Chemistry & Engineering 5  (2017) 8675‐8683.  [49]  F.H.B. Lima, J. Zhang, M.H. Shao, K. Sasaki, M.B. Vukmirovic, E.A. Ticianelli, R.R. Adzic, Journal of  Physical Chemistry C 111 (2007) 404‐410.  [50]  E. Toyoda, R. Jinnouchi, T. Hatanaka, Y. Morimoto, K. Mitsuhara, A. Visikovskiy, Y. Kido, Journal of 

Physical Chemistry C 115 (2011) 21236‐21240.  [51]  Y. Lu, S. Zhao, R. Yang, D. Xu, J. Yang, Y. Lin, N.‐E. Shi, Z. Dai, J. Bao, M. Han, ACS Appl. Mater.  Interfaces    (2018).  [52]  W. Jiao, C. Chen, W. You, J. Zhang, J. Liu, R. Che, Small 15 (2019).  [53]  J.L. Fernandez, K.P. Imaduwage, C.G. Zoskia, Electrochimica Acta 180 (2015) 460‐470. 

[54]  Y. Lu, J. Wang, Y. Peng, A. Fisher, X. Wang, Advanced Energy Materials 7 (2017).  [55]  J. Wu, L. Hu, N. Wang, Y. Li, D. Zhao, L. Li, X. Peng, Z. Cui, L.‐J. Ma, Y. Tian, X. Wang, Applied  Catalysis B: Environmental 254 (2019) 55‐65.  [56]  C. Koenigsmann, E. Sutter, R.R. Adzic, S.S. Wong, Journal of Physical Chemistry C 116 (2012)  15297‐15306.  [57]  Y. Suo, I.M. Hsing, Electrochimica Acta 56 (2011) 2174‐2183.  [58]  I.S. Amiinu, Z. Pu, X. Liu, K.A. Owusu, H.G.R. Monestel, F.O. Boakye, H. Zhang, S. Mu, Advanced  Functional Materials 27 (2017).  [59]  Z. Sadighi, J. Liu, L. Zhao, F. Ciucci, J.‐K. Kim, Nanoscale 10 (2018) 22549‐22559.  [60]  D. Ma, Q. Zhu, X. Li, H. Gao, X. Wang, X. Kang, Y. Tian, ACS Appl. Mater. Interfaces 11 (2019) 8009‐ 8017.  D. Su, G. Lu, S. Guo, Nature 574 (2019) 81‐85.  [62]  M.J. Hytch, E. Snoeck, R. Kilaas, Ultramicroscopy 74 (1998) 131‐146. 

of

[61]  M. Luo, Z. Zhao, Y. Zhang, Y. Sun, Y. Xing, F. Lv, Y. Yang, X. Zhang, S. Hwang, Y. Qin, J.‐Y. Ma, F. Lin, 

ro

[63]  H. Bi, X. Han, L. Liu, Y. Zhao, X. Zhao, G. Wang, Y. Xu, Z. Niu, Y. Shi, R. Che, ACS Appl. Mater.  Interfaces 9 (2017) 26642‐26647. 

[64]  X. Zhao, S. Takao, K. Higashi, T. Kaneko, G. Samjeske, O. Sekizawa, T. Sakata, Y. Yoshida, T. Uruga, Y. 

-p

Iwasawa, Acs Catalysis 7 (2017) 4642‐4654. 

[65]  J. Deng, C. Wang, G. Guan, H. Wu, H. Sun, L. Qiu, P. Chen, Z. Pan, H. Sun, B. Zhang, R. Che, H.  Peng, Acs Nano 11 (2017) 8464‐8470. 

re

[66]  H. Bi, Q. Sun, X. Zhao, W. You, D.W. Zhang, R. Che, Applied Physics Letters 112 (2018).  [67]  Q.‐X. Chen, Y.‐H. Liu, X.‐Z. Qi, J.‐W. Liu, H.‐J. Jiang, J.‐L. Wang, Z. He, X.‐F. Ren, Z.‐H. Hou, S.‐H. Yu,  Journal of the American Chemical Society 141 (2019) 10729‐10735.  2356. 

lP

[68]  D. Li, Y. Jia, G. Chang, J. Chen, H. Liu, J. Wang, Y. Hu, Y. Xia, D. Yang, X. Yao, Chem 4 (2018) 2345‐ [69]  G. Fazio, L. Ferrighi, D. Perilli, C. Di Valentin, International Journal Of Quantum Chemistry 116  (2016) 1623‐1640. 

ur na

[70]  J. Li, H. Zhou, H. Zhuo, Z. Wei, G. Zhuang, X. Zhong, S. Deng, X. Li, J. Wang, Journal of Materials  Chemistry A 6 (2018) 2264‐2272. 

[71]  Z. Yang, B. Chen, W. Chen, Y. Qu, F. Zhou, C. Zhao, Q. Xu, Q. Zhang, X. Duan, Y. Wu, Nature  Communications 10 (2019). 

[72]  E. Schulte, G. Belletti, M. Arce, P. Quaino, Applied Surface Science 441 (2018) 663‐669.  [73]  R. Zhang, Y. Hong, B.M. Reinhard, P. Liu, R. Wang, L. Dal Negro, ACS Appl. Mater. Interfaces 10  (2018) 27928‐27935. 

Jo

[74]  Q. Zhang, K. Dong, C. Wang, Y. Cheng, Crystengcomm 17 (2015) 7469‐7472.  [75]  J. Wei, Y.‐J. Zhang, S.‐N. Qin, W.‐M. Yang, H. Zhang, Z.‐L. Yang, Z.‐Q. Tian, J.‐F. Li, Chemical  communications (Cambridge, England)    (2019). 

[76]  W.‐S. Huang, I.W. Sun, C.‐C. Huang, Journal of Materials Chemistry A 6 (2018) 13041‐13049. 

[77]  H. Liu, Q. Yang, Journal of Materials Chemistry 21 (2011) 11961‐11967.