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Cite this: Nanoscale, 2021, 13, 3709

Received 28th December 2020,Accepted 2nd February 2021

DOI: 10.1039/d0nr09164b

rsc.li/nanoscale

Engineering porous Pd–Cu nanocrystals withtailored three-dimensional catalytic facets forhighly efficient formic acid oxidation†

Linfang Lu, *a Bing Wang,a Di Wu, a Shihui Zou *b and Baizeng Fang *c

Rational synthesis of bi- or multi-metallic nanomaterials with both dendritic and porous features is

appealing yet challenging. Herein, with the cubic Cu2O nanoparticles composed of ultrafine Cu2O nano-

crystals as a self-template, a series of Pd–Cu nanocrystals with different morphologies (e.g., aggregates,

porous nanodendrites, meshy nanochains and porous nanoboxes) are synthesized through simply regu-

lating the molar ratio of the Pd precursor to the cubic Cu2O, indicating that the galvanic replacement and

Kirkendall effect across the alloying process are well controlled. Among the as-developed various Pd–Cu

nanocrystals, the porous nanodendrites with both dendritic and hollow features show superior electro-

catalytic activity toward formic acid oxidation. Comprehensive characterizations including three-dimen-

sional simulated reconstruction of a single particle and high-resolution transmission electron microscopy

reveal that the surface steps, defects, three-dimensional architecture, and the electronic/strain effects

between Cu and Pd are responsible for the outstanding catalytic activity and excellent stability of the Pd–

Cu porous nanodendrites.

1. Introduction

The direct formic acid fuel cell (DFAFC) is regarded as apotential energy supply system in the near future due to thebroad source, low price, high energy storage and non-toxicityof formic acid.1–5 Its successful commercialization, however,is restricted by the slow reaction kinetics of the anode reac-tion, i.e., formic acid oxidation (FAO).6,7 In order to addressthis problem, extensive efforts have been devoted to engin-eering the composition and structure of the electrocatalyststo improve the efficiency of FAO.8–14 As far as the compo-sition is concerned, alloying noble metals (e.g., Pd and Pt)with transition metals (e.g., Fe, Co, Cu) to tailor their geo-metric and electronic structures is a benchmarking strategyto accelerate the kinetics of the FAO.5–17 From the structureperspective, shape regulation can selectively expose reactivecrystal planes to improve the specific activity, and the engin-eering of three-dimensional (3D) structure provides a facile

way to increase the catalytically-active surface area.18–28

During the past decades, people have successfully syn-thesized various nanocrystals with different morphologiessuch as cubes,29,30 dendrites,31,32 and hollow crystals.33,34

For example, Sun et al. adopted electrochemical method tosynthesize Pt tetrahexahedron with high-index facets such as{730}, {210} and {520}.35 These high-energy surfaces exhibi-ted considerably enhanced catalytic activity for electro-oxi-dation of small organic fuels. Lou and coworkers developedvarious strategies to prepare hollow nanostructures.36 Owingto the maximized catalytic-active surface area, these hollownanostructures have exhibited excellent performances inenergy storage/conversion, catalysis and biomedicalapplications.37–39 In principle, bi- or multi-metallic nano-materials with both highly active facets and high surfaceareas are ideal for electrocatalysis, but scalable synthesis ofsuch nanomaterials, e.g., porous nanodendrites, is stilldifficult and even challenging.

Galvanic replacement reaction (GRR) and Kirkendall effectduring the GRR were commonly utilized to synthesize noblemetal-based hollow nanocrystals by galvanic replacementbetween noble metal precursor and template (such as Cu, Ag,Mn3O4, Cu2O, etc.).

40–47 For example, Lu and coworkers pre-pared PtAg nanocrystals with different hollow morphologies bya regulated GRR between Ag nanocubes and Pt precursor.40

Linic and coworkers synthesized PtAg hollow nanocrystalsthrough a plasmonic visible light-mediated reduction.41

†Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr09164b

aCollege of Material, Chemistry and Chemical Engineering, Hangzhou Normal

University, Hangzhou 311121, China. E-mail: [email protected] Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry,

Zhejiang University, Hangzhou 310027, China. E-mail: [email protected] of Chemical and Biological Engineering, University of British Columbia,

2360 East Mall, Vancouver, BC V6 T 1Z3, Canada. E-mail: [email protected]

This journal is © The Royal Society of Chemistry 2021 Nanoscale, 2021, 13, 3709–3722 | 3709

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However, although the hollow nanocrystals can be facilely syn-thesized, the driving forces of GRR and Kirkendall effectduring the preparation scarcely attracted one’s attentionbefore. It is conjectured that by adjusting the ratio betweentemplate and noble metal precursor, we may regulate thedriving forces and accordingly synthesize materials withdifferent morphologies (more accurately, different hollowdegrees). On the other hand, if the template itself is composedof dendritic small particles, then the resulting nanocrystalsshould also have dendritic characteristics.

Herein, based on the above analysis and hypothesis, Cu2Onanocubes composed of ultra-small Cu2O nanoparticles aresynthesized as a template for a nanoscale GRR with Pd precur-sors. A simple regulation of the molar ratio of the Pd precursorto the Cu2O template provides a precise control over the GRRand Kirkendall effect during the process. As a result, variouskinds of porous PdCu nanostructures are synthesized. Whenexamined as an electrocatalyst for the FAO, the porous Pd–Cunanodendrites with both of dendritic and hollow featuresexhibit the best catalytic activity, which is ascribed to the com-bined effects from the abundant surface steps, defects, 3Dnanostructure and the electronic/strain interactions betweenCu and Pd. This is fully supported by the detailed experi-mental evidences provided by high-resolution transmissionelectron microscopy (HRTEM), 3D reconstruction of a singleparticle, X-ray photoelectron spectrum (XPS) and electro-chemical measurements.

2. Experimental section2.1. Materials

Commercial Pd/C (30 wt% Pd) and sodium tetrachloropalla-date(II) (Na2PdCl4) were purchased from Sigma-Aldrich. Nafion(5%) was purchased from Alfa Aesar. Cupric sulfate (CuSO4),sodium citrate (Na3C6H5O7·2H2O), sodium hydroxide (NaOH),formic acid (HCOOH), sulfuric acid (H2SO4) and ascorbic acid(C6H8O6) were purchased from Sinopharm Chemical ReagentCorporation.

2.2. Synthesis of cubic Cu2O composed of small particulateCu2O

Cu2O cubes were synthesized by reducing a copper–citratecomplex solution with ascorbic acid based on the previousreports with minor changes.48,49 Typically, 180 mg of CuSO4

and 110 mg of sodium citrate were dissolved in 75 mL of de-ionized water under stirring. The mixture solution was heatedto 40 °C, followed by the addition of 15 mL of NaOH solution(1.25 M). After stirring for 0.5 h, 10 mL of an ascorbicacid solution (0.5 M) was added dropwise into the above solu-tion to reduce the copper–citrate complex. The resultantorange suspension was further aged at 40 °C for 3 h toproduce cubic Cu2O. The solid Cu2O was collected by centrifu-ging and washed by deionized water for several times, anddried at 80 °C in an oven overnight for further surfacecharacterizations.

2.3. Galvanic replacement between cubic Cu2O and Pdprecursor

Typically, 100 mg of the prepared cubic Cu2O was dispersed ina round-bottom flask with 50 mL of H2O. Then, 55 mg ofsodium citrate was added into the flask under stirring. Next,10 mL of an aqueous solution of Na2PdCl4 with different Pdconcentrations (0.0116 M, 0.0232 M, 0.0464 M, 0.1392 M,0.4176 M) was added dropwise into the above solution. Themolar ratio of Pd to Cu was 1/12, 1/6, 1/3, 1/1 and 3/1, respect-ively. After reacting for 30 min, the solid product was centri-fuged, washed, and dried for further characterizations. The as-synthesized five samples are defined as Pd1/12Cu, Pd1/6Cu,Pd1/3Cu, Pd1Cu and Pd3Cu.

2.4. Synthesis of PdCu nanocrystals with differentmorphologies

PdCu nanocrystals with different morphologies were syn-thesized by acid treatment of the as-obtained Pd1/12Cu,Pd1/6Cu, Pd1/3Cu, Pd1Cu and Pd3Cu samples. Typically, 50 mgof Pd1/12Cu (Pd1/6Cu, Pd1/3Cu, Pd1Cu or Pd3Cu) was dispersedin 50 mL of deionized water. Then, 5 mL of a dilute solution ofsulfuric acid (0.2 M) was added dropwise into the above solu-tion under stirring. After reacting for 30 min, the solid productwas centrifuged and washed, and dried for further characteriz-ations. Based on the following scanning electron microscope(SEM) and transmission electron microscopy (TEM) obser-vations, the as-obtained samples are defined as PdCu aggre-gate-1 (PdCu AG-1), PdCu aggregate-2 (PdCu AG-2), PdCuporous nanodendrite (PdCu PND), PdCu meshy nanochain(PdCu MNC) and PdCu porous nanobox (PdCu PNB).

2.5. Physical characterization

A Hitachi HT-7700 microscope was used to perform TEM ana-lysis. SEM analysis was performed with a HitachiSU-8010 microscope. A JEOL JEM-2100F microscope was usedto perform high-resolution TEM (HRTEM). High-angle annulardark-field scanning TEM (HAADF-STEM) and energy dispersiveX-ray (EDX) mapping were carried out on the JEOL JEM-2100Fmicroscope equipped with a probe-corrector and an EDXdetector system. Powder X-ray powder diffraction (XRD) pat-terns were recorded on a Rigaku Ultima IV diffractometerusing Cu Kα radiation operated at 40 mA and 40 mV. X-rayphotoelectron spectrum (XPS) was conducted using Al Kα X-raysource on a Thermo Scientific ESCALAB 250Xi spectrometer.Inductively coupled plasma mass spectrometry (ICP-MS) wasperformed in an Agilent 7700 equipment. 3D reconstructionimages of single particle were acquired on a TECNAI G2 F20(FEI corporation) viewed from distinct angles. 3D model ofsingle particle was simulated by computer based on hundredsof above acquired 3D reconstruction images.

2.6. Electrochemical measurements

Electrochemical measurements were carried out with anelectrochemical analyzer (CH Instrument, Shanghai) in athree-electrode cell. A glass carbon electrode (diameter: 3 mm)

Paper Nanoscale

3710 | Nanoscale, 2021, 13, 3709–3722 This journal is © The Royal Society of Chemistry 2021

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coated with a PdCu catalyst was used as the working electrode(WE). A carbon rod was employed as the counter electrode. Asaturated calomel electrode (SCE) was served as the referenceelectrode.50,51 The measured potential versus SCE was trans-formed to that versus reversible hydrogen electrode (RHE)based on the following equation: E (vs. RHE) = E (vs. SCE) +0.2438 + 0.0592 × pH.52,53 The preparation processes of the WEare as follows. First, 4 mg of carbon supported PdCu catalyst(∼20%) was dispersed in a mixture solution (1 mL) containingNafion (5%), isopropanol and water (v/v/v = 0.05 : 0.15 : 0.8) toprepare a suspension (∼0.8 mgPdCu mL−1). Then, ∼3 μL of thecatalyst suspension was dropped on the top of the glasscarbon electrode (the actual Pd loading on electrode was con-trolled at 15 μg cm−2 based on the ICP-MS). Next, the catalystsuspension covered glass carbon electrode was dried at roomtemperature. The cyclic voltammetry (CV) experiments wereperformed in a 0.5 M H2SO4 solution (scan rate: 50 mV s−1) atroom temperature. The electrolyte solution was deaerated byhigh-purity nitrogen for 1 h prior to any CV measurement. TheFAO reaction was performed in 0.5 M H2SO4 + 0.5 M HCOOHsolution. The specific activities of different Pd-based catalystsfor FAO were calculated by normalizing the current to theactual surface area of Pd on the electrode. The mass activitieswere calculated by normalizing the current to the actual massof Pd on the electrode. The specific activities or mass activitiesfor different catalysts were compared at a potential of 0.45 V(vs. RHE). The electrochemical active surface areas (ECSAs) ofPd were determined by integrating the columbic charge ofPdO reduction in CVs operated in 0.5 M H2SO4 with a potentialrange from 0.04 to 1.45 V according to the followingequation:54

ECSAPdO ¼ QPdO=ð0:42½Pd�Þwhere QPdO (mC) is the columbic charge of the PdO reduction(0.40–0.75 V), 0.42 mC cm−2 is the charge density corres-ponding to monolayer adsorption of oxygen on Pd, and [Pd] isthe load amount of Pd on the glass carbon working electrode.

3. Results and discussion3.1. Synthesis of the Cu2O template

Fig. 1a shows the XRD pattern for the as-synthesized Cu2Osample, from which five diffraction peaks located at 2θ = 29.8°,36.6°, 42.4°, 62.5° and 73.5° are observed. They could beindexed to (110), (111), (200), (220) and (311) planes of thecubic phase Cu2O, respectively. It is clear that the synthesizedmaterial features a pure Cu2O crystal structure. SEM images(Fig. 1b and Fig. S1a, b, ESI†) and TEM images (Fig. 1c andFig. S1c, d†) reveal that the prepared Cu2O nanocrystals arewell-defined nanocubes with an average particle size of 161 ±24 nm (Fig. S2, ESI†). Selected area electron diffraction (SAED)suggests that they are single crystals (inset of Fig. 1c). Theinterplanar spacing acquired from the HRTEM image is0.246 nm (Fig. 1d), corresponding to the (100) facet of Cu2O.Besides, a key feature of the synthesized Cu2O is observed,

namely, these cubic Cu2O nanocrystals with a rough surfaceare made up of very small Cu2O nanoparticles (Fig. 1d andFig. S3, ESI†), which enables a facile galvanic replacementwith suitable metal precursors.

3.2. Adjusting the molar ratio of Pd and Cu

Considering that Pd is the best monometallic catalyst for FAOreaction, we chose sodium tetrachloropalladate(II) as the metalprecursor for the GRR. Various Pd-to-Cu molar ratios (i.e., 1/12, 1/6, 1/3, 1/1 and 3/1) were used to control the rate of theGRR, which was expected to dominate the morphology andcatalytic performance of the as-synthesized PdCu bimetallicnanocrystals. The samples with different Pd-to-Cu ratios aredefined as PdxCu for convenience, where x is the feedingmolar ratio of Pd-to-Cu. The actual Pd-to-Cu molar ratios deter-mined by EDX are given in Table S1 (ESI†). It is observed thatthere is a big difference between the feeding molar ratio andthe actual molar ratio of Pd and Cu. The actual molar ratio ofPd and Cu is far less than the feeding molar ratio becauselarge amount of the Pd2+ precursor does not react with Cu2O.The galvanic replacement reaction between Pd2+ and Cu2Oneeds sufficient Pd2+ concentration. The remaining unreactedPd2+ ions were removed by centrifugation.

Fig. S4a–c (ESI†) show the microstructure of the Pd1/12Cu.Owing to the GRR between the small Cu2O nanoparticles andPd precursors, many spherical particles emerged on the outer-most layer of the cubic Cu2O. HRTEM images (Fig. S4e–h,ESI†) indicate that the interplanar spacing of these smallnanoparticles is very different from that of the bulk Cu2O andcan be ascribed to the (111) facet of PdCu alloy. The line scanand EDX mapping (Fig. S5, ESI†) exhibit the enrichment of Pdsignals in the shell and the homogeneous distribution of Cuthroughout the sample, indicating a Cu2O@PdCu core–shellstructure.

With the increase in the Pd-to-Cu ratio to 1/6, more Pd pre-cursors were available for the GRR. The increased productionof PdCu particles (Fig. 2a and b) resulted in a more unevensurface of cubic Cu2O. Representative HRTEM images andEDX mapping of Pd1/6Cu (Fig. 2c–f ) confirm that Pd1/6Cu alsoadopts a Cu2O@PdCu core–shell structure though the PdCushell is thicker than that of the Pd1/12Cu. It is important tohighlight that once all the surface Cu2O nanoparticles are con-sumed by Pd precursors, the inner Cu2O will diffuse to thesurface. The Kirkendall effect associated with the largerdiffusion rate of Cu2O than Pd will then lead to the formationof hollow structures (red dotted circle in Fig. 2f).

As the Pd-to-Cu ratio further increased to 1/4.5, more Cu2Odiffused from the inner to the surface, leading to a strength-ened Kirkendall effect and therefore a more porous Cu2O core(Fig. S6†). The large production of small PdCu particles on thesurface of Cu2O, on the other hand, ensures a structure evol-ution of PdCu shell. As shown in Fig. S6 (ESI†), most obtainedPdCu particles assembled into flower-like dendrites. Thesesmall dendrites further developed into nanodendrties whenthe Pd-to-Cu ratio increased to 1/3 (Fig. 2g and h). HRTEMimages and EDX mapping (Fig. 2i–l and Fig. S7†) demonstrate

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that the generated nanodendrites are PdCu alloy. The detailedHRTEM (Fig. S8†) and single-particle EDX mapping (Fig. S9†)characterizations indicate that the PdCu nanodendrites areporous and all of them stick to the edge of Cu2O.

When the molar ratio of Pd-to-Cu was as large as 1, theGRR between Pd precursors and Cu2O produced a largenumber of PdCu nanoparticles on the entire surface, whichsubsequently assembled into a thick PdCu shell instead of the

Fig. 1 XRD pattern (a), SEM (b) and TEM (c) images of the pristine Cu2O cube. The inset of (b) shows the enlarged SEM image. The inset of (c) is theSAED of the pristine Cu2O cube. (d) HRTEM image of the pristine Cu2O cube. The inset of (d) displays the lattice fringe of Cu2O (100). The pinkdotted circle and blue dotted line show the rough surface of Cu2O cube.

Fig. 2 (a–f) TEM (a and b), HRTEM (c–e) images and EDX mapping (f) of Pd1/6Cu. The enlarged HRTEM images of d and e were extracted from c. (g–l)TEM (g and h), HRTEM (i–k) images and EDX mapping (l) of the as-produced Pd1/3Cu. The enlarged HRTEM images of j and k were extracted from i.

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separated nanodendrites (Fig. 3a and b). As shown inFig. S10a,† the thickness of the PdCu shell at this stage isaround 17 nm. It is observed that the face of the cubic Cu2Owas etched to form PdCu shell but the edge and corner of thecubic Cu2O were not etched (Fig. 3c and d), likely due to thelower surface energy of the (100) facet (face of cube) than thatof (111) or (110) facet (edge and corner). The Cu2O inside thePdCu shell aggregated at the corner as small crystals, resultingfrom the Kirkendall effect. Preferential etching of the face ofthe cubic Cu2O enabled the sample to maintain the cubicframework during the GRR and Cu diffusion. These structuralcharacteristics were further confirmed by the EDX mapping,line scan and HRTEM images in Fig. 3d–f and S10b–g.†

Once the molar ratio of Pd-to-Cu reached 3 : 1, the excess ofPd precursors ensured an entire GRR of Cu2O. As shown inFig. 3g–j, S11 and S12,† the TEM images, HRTEM images, EDXmapping and line scan demonstrate the formation of purePdCu nanoboxes. The wall thickness is around 40 nm(Fig. S11d†), which is larger than that of Pd1Cu (∼17 nm). Theaverage size of the formed PdCu nanoboxes (219 ± 33 nm,Fig. S13†) is ca. 60 nm larger than that of the original Cu2O,indicating the formation of PdCu nanoboxes was due to theinterplay of the GRR and Kirkendall effect. Further increasingthe Pd-to-Cu ratio to 5 : 1 did not change the morphology of

PdCu nanoboxes (Fig. S14†), indicating that the Pd precursorsof Pd3Cu was enough for a complete reaction of Cu2O nano-cubes. The disappearance of Cu2+ from the XPS spectra ofPd3Cu also confirms the thorough consumption of Cu2O(Fig. S15†). The detectable Cu0, Pd0 and Pd2+ XPS signals ofPd3Cu sample demonstrates the successful formation of PdCualloy eventually (Fig. S15†).55

3.3. The mechanism of structure evolution

The structural evolution with the increase of Pd-to-Cu molarratio was also confirmed by the XRD analyses and SEM obser-vations. Distinct morphologies with different Pd-to-Cu molarratios were revealed by SEM images (Fig. S16†). As shown inFig. 4, the peak at around 40.7° can be ascribed to (111) facetof PdCu alloy (0.6° positive shift compared to (111) facet ofpure Pd) and it becomes more pronounced with the increasedmolar ratio of Pd-to-Cu, indicating the increased force of theGRR. The disappearance of characteristic peaks of Cu2O forthe Pd3Cu sample indicates the complete reaction of Cu2Owith the Pd precursor.

Combining the XRD results with that from the TEM,HRTEM and EDX mapping shown above, we can summarizethe morphology evolution. As illustrated in Scheme 1, the GRRand Kirkendall effect gradually enhance with the increase of

Fig. 3 (a–f ) TEM images (a–c), EDX mapping (d) and line scans (e and f) of the as-produced Pd–Cu nanostructure when the molar ratio of theadded Pd/Cu was 1. (g–j) TEM images (g and h), EDX mapping (i) and HRTEM image ( j) of produced Pd–Cu nanostructure when the molar ratio ofPd/Cu was 3.

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Fig. 4 (a) XRD patterns of the as-produced Pd–Cu nanostructures when the molar ratio of the added Pd/Cu was 1/12, 1/6, 1/3, 1, 3 and the pureCu2O cube. (b) The enlarged 2θ degree from the yellow part of (a).

Scheme 1 The schematic diagram of the evolution of Pd–Cu nanostructure with the increase of Pd/Cu molar ratio. The molar ratio of Pd/Cu is 1/12, 1/6, 1/3, 1, and 3 for the stage 1 to stage 5, respectively. The samples before the acid treatment are defined as Pd1/12Cu, Pd1/6Cu, Pd1/3Cu, Pd1Cu,Pd3Cu from stage 1 to stage 5. After the acid treatment they are defined as PdCu aggregate-1 (PdCu AG-1), PdCu aggregate-2 (PdCu AG-2), PdCuporous nanodendrite (PdCu PND), PdCu meshy nanochain (PdCu MNC) and PdCu porous nanobox (PdCu PNB) for the stage 1 to stage 5,respectively.

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Pd-to-Cu molar ratio. The whole process can be divided intofive stages. At stage 1, small PdCu alloy particles first emergeon the surface of bulk cubic Cu2O. At stage 2 the number ofthe small PdCu particles increases. At stage 3, these smallPdCu particles are mobilizable and they will assemble intoPdCu nanodendrites. When the GRR further increases (stage4), the small PdCu form a shell on cubic Cu2O. Finally, thewell-defined pure PdCu nanoboxes are developed at stage 5.

Acid treatment can be used to remove the Cu2O templateand increase the active sites of catalysts. Interestingly, after theacid treatment, as shown in Scheme 1, Fig. S17, S18† andFig. 5, we successfully prepared a series of Pd–Cu bimetallicmaterials with different morphologies, namely, PdCu aggre-gate-1 (PdCu AG-1), PdCu aggregate-2 (PdCu AG-2), PdCuporous nanodendrite (PdCu PND), PdCu meshy nanochain

(PdCu MNC) and PdCu porous nanoboxes (PdCu PNB).Obviously, the morphology of nanomaterials obtained afterthe acid treatment is dependent on the morphology of nano-materials before the acid treatment. The enlarged HRTEMimages (Fig. S17d† and Fig. 5b, e, h, k) and EDX mappings(Fig. 5c, f, i and l) confirm that all of the nanomaterials afterthe acid treatment are PdCu alloy. The XRD patterns (Fig. 6a)and selected area electron diffractions (SAEDs) (Fig. S19†) alsoindicate that all the samples form Pd–Cu alloy structure andtheir crystallinities are quite good. No other peaks in the XRDpatterns and no other diffraction rings in the SAED images areobserved, suggesting the Cu2O template has been removedthoroughly.

XPS technique was applied to detect the surface propertiesof the as-obtained PdCu samples. As shown in Fig. 6b, com-

Fig. 5 (a–c) TEM (a), HRTEM (b) and EDX mapping (c) of the as-produced Pd–Cu nanostructure after the acid treatment when the molar ratio ofthe added Pd/Cu was 1/6. The sample is defined as PdCu AG-2. (d–f ) TEM (d), HRTEM (e) and EDX mapping (f ) of the as-produced Pd–Cu nano-structure after the acid treatment when the molar ratio of Pd/Cu was 1/3. The sample is defined as PdCu PND. (g–i) TEM (g), HRTEM (h) and EDXmapping (i) of the as-produced Pd–Cu nanostructure after the acid treatment when the molar ratio of Pd/Cu was 1. The sample is defined as PdCuMNC. ( j–l) TEM ( j), HRTEM (k) and EDX mapping (i) of the as-produced Pd–Cu nanostructure after the acid treatment when the molar ratio of Pd/Cu was 1/3. The sample is defined as PdCu PNB. The inset of (b, e, h and k) is enlarged from the dotted box in (b, e, h and k), respectively.

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pared to that of the commercial Pd/C, the proportions of Pd2+

of all Pd–Cu samples are dramatically decreased, implying thatthe alloying of Cu into Pd prevents the oxidation of Pd, whichmight benefit the catalytic activity. Moreover, the binding ener-gies of Pd0 of all Pd–Cu samples shift to a lower value com-pared to that of the Pd/C, indicating that Cu donates electronsto Pd and makes Pd negatively charged. These electronic andstrain effects benefited from alloying would protect the catalystfrom CO poisoning and largely improve the catalytic activityfor the FAO, which will be discussed later.

As shown in Table 1, the actual Pd-to-Cu molar ratios of thedifferent Pd–Cu samples after the acid treatment were identi-fied by XRD, EDX and XPS analyses. The values obtained fromthe XRD and EDX measurements represent the bulk ratio ofPd-to-Cu while the values acquired from the XPS analysesexhibit the surface ratio. The Cu in the PdCu alloy was partiallyremoved by the acid treatment. Taking PdCu PNB as anexample, the molar ratio of Pd-to-Cu of PdCu PNB before the

acid treatment determined by STEM-EDX is 1.8 (Table S1†).However, the value increases to 3.1 after the acid treatment(Table 1), indicating that partial Cu was removed through theacid treatment. Interestingly, for all the samples, the surfacePd-to-Cu ratio is much higher than the bulk ratio, suggestingthe acid treatments produce a dealloyed PdCu@Pd core–shellstructure, similar to the previous reports.56,57 It is important tonote that the Pd-to-Cu ratio of the PdCu PND is higher thanthose of the other samples (including bulk and surface ratios),indicating that more Cu of the PdCu PND is removed duringthe acid treatment. This result also signifies that there may bemore holes and defects inside the PdCu PND. As shown inFig. 5e and f, the HRTEM and HADDF-STEM images clearlyshow plentiful holes inside the PdCu PND. The line scan alsoconfirms the existence of micropores and mesopores(Fig. S20†). These micro/meso structures can increase the cata-lytic active sites of catalyst, enhance the mass transfer, andimprove the catalytic activity for FAO.

3.4. The FAO activity of Pd–Cu catalysts

The electrocatalytic performances of the different PdCu cata-lysts were evaluated by the FAO. Fig. S21† shows the FAO activi-ties of the PdCu catalysts before and after acid treatment. Theresults clearly indicate that the acid treatment accelerates thecatalytic performance. The detailed electrochemical activitiesof the PtCu catalysts after the acid treatment are shown inFig. 7. Fig. 7a shows the CV profiles of the PdCu samples in0.5 M H2SO4. The ECSAs evaluated from the reduction peak ofPdO are displayed in Fig. 7c. Interestingly, the peak potentials

Fig. 6 XRD (a) and XPS spectra (b) of commercial Pd/C and the as-produced Pd–Cu nanostructure after the acid treatment when the molar ratio ofthe added Pd/Cu was 1/12, 1/6, 1/3, 1, 3, which corresponds to PdCu AG-1, PdCu AG-2, PdCu PND, PdCu MNC and PdCu PNB, respectively.

Table 1 The molar ratios of Pd to Cu of the different Pd–Cu samplesafter the acid treatment determined by XRD, STEM-EDX and XPS

Samples(after acid treatment)

Pd : Cu Pd : Cu Pd : CuXRD(bulk ratio)

STEM-EDX(bulk ratio)

XPS(surface ratio)

PdCu AG-1 2.5 3.0 3.8PdCu AG-2 2.3 3.1 4.7PdCu PND 4.3 5.2 9.2PdCu MNC 2.3 2.7 4.1PdCu PNB 2.8 3.1 5.8

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of PdO reduction all shift to higher values compared to that ofthe Pd/C, demonstrating the effect from the alloy structure ofPd–Cu samples. The potential shift of PdCu PND is smallerthan those of other PdCu samples (Fig. S22†), which is consist-ent with its higher Pd-to-Cu ratio. Besides, among all thesesamples, PdCu PND shows the highest ECSA (i.e., 61.4 m2 g−1),likely due to its rich holes and defects. Fig. 7b shows the linearsweep voltammetry (LSV) curves of FAO (positive scan, 50 mVs−1) over the different catalysts in 0.5 M H2SO4 + 0.5 MHCOOH. The corresponding specific activities and mass activi-ties are summarized in Fig. 7d. The results show that all thePdCu samples exhibit better catalytic activities than the com-mercial Pd/C. The enhanced activities of PdCu samples can beascribed to two reasons. Firstly, the alloying of Cu with Pdchanges the electronic structure of Pd and further improvesthe FAO activity. As mentioned above, the addition of Cumakes the surface of Pd electron-rich. This will weaken theadsorption of oxygen species (such as CO), and thus promotethe direct oxidation of formic acid without the poisoning ofCO.32 Secondly, the porous structure of the PdCu samples afteracid treatment also enhances the FAO. It was reported that thehigh-index surfaces can boost the catalytic activity due to thelow coordinated atoms on the surface.35 The acid treatment

produces abundant pores and defects inside and outside ofthe PdCu catalysts. These pores and defects should have plentyof high-index surfaces and will contribute to improve the FAOactivity. Among the various PdCu samples, PdCu PND showsthe best specific activity and mass activity for the FAO, whichare 3.2 times and 4.6 times as high as that of the commercialPd/C. The superior activity of the PdCu PND compared to theother catalysts (Table S2†), on the other hand, implies that themorphology of the catalyst is critical to its catalytic activity,which will be discussed in details below. The Tafel slope ofPdCu PND (237 mV dec−1) is smaller than that of the Pd/C(294 mV dec−1) (Fig. S23†), indicating the faster kinetics ofPdCu PND for the FAO.

3.5. Detailed structural characteristics of PdCu porousnanodendrites

To rationalize the structure–property relationship, the atomic-ally resolved HRTEM and 3D reconstructions were performedfor the PdCu PND. As shown in Fig. 8, there are many high-index defects including point defects (Fig. 8a) and face defects(Fig. 8b) on the surface of the PdCu PND, which have beenproved to provide very high catalytic activity because of theirlow coordinated atoms.33–35 Moreover, 3D reconstruction of a

Fig. 7 Catalytic activity tests of the commercial Pd/C, and the as-produced Pd–Cu nanostructures after the acid treatment. (a) CV curves of thedifferent catalysts in 0.5 M H2SO4. (b) Specific activities of FAO of different catalysts in 0.5 M H2SO4 + 0.5 M HCOOH. (c) Calculated ECSA of thedifferent catalysts. (d) Mass activity and specific activity of FAO of the different catalysts at 0.45 V.

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single PdCu PND particle was conducted to map the mor-phology inside and outside of the particle by imaging thenanoparticle at different specimen orientations (Fig. 9).58,59

Through 3D imaging and computer simulation, it is visually

observed that there are a plenty of holes inside the PdCu PND(Fig. 9a–d and S24†), which are consistent with the HRTEMand HAADF-STEM results shown in Fig. 5e and f. The detailscan be found in Movies S1, S2 in the ESI.† The outside mor-

Fig. 8 Atomically resolved HRTEM images of the PdCu porous nanodendrites (PND). (a) High-index surface atoms. (b) Surface steps.

Fig. 9 3D reconstructions from the images of a single PdCu porous nanodendrite particle. (a) TEM image of a single PdCu porous nanodendriteparticle. (b) The cross section inside the particle shows the internal pore structure. Top view (c) and side view (d) of the cross section inside the par-ticle. (e–l) 3D reconstruction models of a single PdCu porous nanodendrite particle viewed from different angles.

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phology of PdCu PND can be visually captured by rotating theparticle with a certain angle. As shown in Fig. 9e–l and MovieS3 (ESI†), the rough surface of the PdCu PND features abun-dant defects or high-index surfaces, which are consistent withthe results shown in Fig. 8. Based on all these results, it isbelieved that the high catalytic activity of the PdCu PND forthe FAO results from the combined effects of the rich surfacesteps, defects, 3D porous structure of the PdCu PNDs as wellas the electronic/strain effects between Cu and Pd (Scheme 2).

3.6. Durability tests

The accelerated durability tests were also performed by cyclingthe catalyst electrode in the potentials range of 0.1 to 1.2 V(RHE) in the solution of 0.5 M H2SO4 + 0.5 M HCOOH at roomtemperature. The PdCu PND catalyst was elected because of itsbest FAO activity. As shown in Fig. 10a and b, the FAO activityof the commercial Pd/C electrocatalyst lost ca. 48% after 2000cycles. In contrast, the PdCu PNDs catalyst shows a loss of only

Scheme 2 Schematic illustration of the bulk structure and surface property of the PdCu porous nanodendrite.

Fig. 10 (a) LSV curves of the FAO recorded at 50 mV s−1 in 0.5 M H2SO4 + 0.5 M HCOOH for the commercial Pd/C and PdCu PNDs before and afterthe stability test. (b) The variation of the mass activity of the commercial Pd/C and PdCu PNDs before and after the stability tests. (c) The chronoam-perometry curves at 0.45 V of the commercial Pd/C and PdCu PNDs. TEM image (d), HADDF-STEM (e) and EDX mapping (f ) of the PdCu PND afterthe stability test.

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19% after 2000 cycles, suggesting that the PdCu PNDs electro-catalyst has better durability than the commercial Pd/C cata-lyst. The chronoamperometry curves of the commercial Pd/Cand PdCu PND also confirm the better stability of the PdCuPNDs for the FAO (Fig. 10c). The TEM image in Fig. 10d con-firms that the morphology of the PdCu PNDs maintains afterthe durability test. The HAADF-STEM in Fig. 10e shows thatthe holes are still inside the PdCu PNDs, which helps preserveits high catalytic activity and good cycling stability for the FAO.In literature, a hierarchical porosity composed of mesoporesconnected with macropores (micropores) favors fast masstransport and buffers the volume change, resulting inenhanced cycling performance.60–62 The obvious Cu signal inEDX mapping (Pd/Cu = 5.4) in Fig. 10f demonstrates the lossof Cu is negligible during the stability test. Taking all theseresults together, it is conclusive that the high stability of PdCuPND results from the special three-dimensional structure ofPdCu PND catalyst. The sphere/dendrite-like morphology ofPdCu PND likely prevents the loss of ECSA (Fig. S25†).

4. Conclusions

In summary, we rationally synthesize a series of porous PdCunanocrystals with tailored nanostructures through tightly con-trolled GRR and Kirkendall effects. Benefiting from the richsurface steps, defects, and 3D structure, the as-engineeredPdCu porous nanodendrites exhibit the best specific activityand mass activity towards the FAO, which are 3.2 times and 4.6times as high as that of the commercial Pd/C. This workdemonstrates the great potential and the key parameters of theGRR for facile and scalable synthesis of well-defined porousnanocrystals. The full control over the GRR and Kirkendalleffects might be further extended to synthesize other multime-tallic dendrites for diverse electrocatalysis and electrochemicalenergy storage/conversion applications.

Author contributions

L. Lu conceived the project and designed experiments. L. Lu,B. Wang and D. Wu conducted the experiments and partici-pated in interpreting and analyzing the data. L. Lu, S. Zou andB. Fang wrote and revised the manuscript. All authors havegiven approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by National Natural ScienceFoundation of China (21802122), Zhejiang Provincial NaturalScience Foundation of China (LQ20B030011) and the startup

fund of Hangzhou Normal University (2019QDL001). Thethree-dimensional (3D) reconstruction experiments were per-formed at the Center of Electron Microscopy of SoochowUniversity.

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