ELECTROCHEMISTRY

Tunable intrinsic strainin two-dimensional transitionmetal electrocatalystsLei Wang1*, Zhenhua Zeng2*†, Wenpei Gao3, Tristan Maxson2, David Raciti1,Michael Giroux1, Xiaoqing Pan3,4, Chao Wang1†, Jeffrey Greeley2†

Tuning surface strain is a powerful strategy for tailoring the reactivity of metal catalysts.Traditionally, surface strain is imposed by external stress from a heterogeneous substrate,but the effect is often obscured by interfacial reconstructions and nanocatalystgeometries. Here, we report on a strategy to resolve these problems by exploiting intrinsicsurface stresses in two-dimensional transition metal nanosheets. Density functional theorycalculations indicate that attractive interactions between surface atoms lead to tensilesurface stresses that exert a pressure on the order of 105 atmospheres on the surfaceatoms and impart up to 10% compressive strain, with the exact magnitude inverselyproportional to the nanosheet thickness. Atomic-level control of thickness thus enablesgeneration and fine-tuning of intrinsic strain to optimize catalytic reactivity, which wasconfirmed experimentally on Pd(110) nanosheets for the oxygen reduction and hydrogenevolution reactions, with activity enhancements that were more than an order ofmagnitude greater than those of their nanoparticle counterparts.

Recent advances in controlled growth ofnanomaterials (1–6), combined with eluci-dation of governing principles of catalysisfrom fundamental research on well-definedmodel systems, have led to a blossoming

of the field of rational catalyst design (7, 8).One particularly successful strategy has been totune surface strain (9–11) and thereby modulatethe surface electronic structure and energeticsof reaction intermediates, leading to a funda-mental understanding and identification of cat-alysts with improved activity (12–23). For thewell-known example of the oxygen reductionreaction (ORR) on platinum, previous studiesindicated that a 1% compressive strain couldimprove the activity by more than 300%(13 , 15 , 19, 20).Although strain tuning of catalysts has shown

promise for catalytic enhancement, its applica-tion has been limited by both practical andfundamental considerations. First, the effect ofstrain on high–surface area nanoparticle cata-lysts is often obscured by nanoparticle shapeand by the presence of undercoordinated defects(13, 19, 21, 24, 25), whereas model single-crystaland polycrystalline catalysts, for which highspecific activities are achievable by strain tuning(15, 16, 19, 20), have prohibitively small massactivities and are not of practical interest. It is

therefore desirable to develop strain tuningstrategies for nanomaterials that preserve theintrinsic activity of single crystals and also pos-sess surface areas and mass activities that ex-ceed those of nanoparticles. Second, existingmethods for strain tuning of catalysts oftenrely on overlaying a catalyst layer on a sub-strate, using one of a large number of tech-niques that have been developed, yieldingoverlayers where strain may be tuned eithercontinuously or discretely (10, 13, 15–22, 26–28).These approaches result in the formation ofoverlayer-substrate interfaces that can be subjectto mechanical instabilities and strain relaxation(13, 17, 20, 21). Additionally, during prolongedusage, dissolution may occur around the in-terfaces (15, 19), and for catalysis in alkaline,base metals from substrates may form surface(hydroxy)oxide species that block active sites(29). Further, the presence of heterointerfacesconvolutes multiple physical factors, such assubstrate ligand effects and production of sur-face alloys, that may complicate the optimiza-tion of catalyst properties by strain tuning.Finally, if the substrates are composed of pre-cious metals, they may limit the reactivity of thecatalyst per mass of precious metal, which isundesirable for cost-effectiveness. These con-siderations point to the importance of develop-ing strain tuning strategies that do not rely onmanipulation of a catalyst-substrate interface.Here, we report on the utilization of surface

stress to drive intrinsic strain in freestandingmetal nanosheets. These two-dimensional (2D)nanostructures do not contain catalyst-substrateinterfaces, and they integrate the high specificactivity of single-crystal catalysts with the highsurface areas of traditional nanoparticles. First,we determined the relationship between the sur-

face stress, the resulting strain, and the nanosheetthickness through density functional theory(DFT) analysis. We demonstrated that nano-sheets with thicknesses of 1 to 12 monolayers(ML) exhibit tunable strains that are up to 10%and are inversely proportional to the thickness,with the exact magnitudes also depending onthe elemental identity of the transition metaland the surface orientation.We then validated thepredicted intrinsic strain levels by synthesizingultrathin Pd nanosheets with atomic-level con-trol over the thickness and by using aberration-corrected high-resolution transmission electronmicroscopy (HRTEM) imaging to characterizethe lattice spacing. We next exploited the strainto enhance the rates of the electrochemical ORRand hydrogen evolution reaction (HER). Wedemonstrated that in both alkaline and acidicenvironments Pd nanosheets can enhance re-action rates by more than an order of magnitudeabove rates with Pd and Pt nanoparticles, sug-gesting that generating and tuning intrinsicstrain in 2D nanosheets can be a powerful strat-egy for the design and development of advancedcatalytic materials.Cleavage of bulk metal atoms to create a sur-

face often leads to charge redistribution andattractive interactions between surface atoms.These attractive interactions are manifested asa tensile surface stress that, in turn, dependslinearly on the bulk modulus of the metal, withnotable exceptions for Pt and Au (Fig. 1). Forplatinum group metals and coinage metals, in-trinsic tensile surface stresses generally induce apressure on the order of 105 atm on the surfaceatoms (Fig. 1C and figs. S1 to S4), which providesa strong driving force for surface contraction andreduction of the surface energy. Although thissurface pressure has little impact on the lattice ofsingle crystals with macroscopic thicknesses, itcan markedly affect the structure of 2D ultrathinfilms and lead to compressive in-plane strain(Fig. 1, A and D), accompanied by a correspond-ing change in the distance between metal layers,which can be approximated by the Poisson ratiofor the metals (fig. S5). The compressive strainstrongly depends on the slab thickness and onthe nature of the transition metal, as shown inFig. 1, E to G, and figs. S6 and S7 for freestandingtransition metal surfaces with face-centered cubic(111) [fcc(111)], hexagonally close-packed (0001)[hcp(0001)], fcc(100), and fcc(110) symmetriesand thicknesses varying from 1 ML (~0.2 nm)to 12 ML (~2.5 nm).For a given element, the overall strain gen-

erally increases with decreasing slab thickness,and the magnitude is inversely proportional tothe slab thickness, except for the thinnest slabs(~1 to 2 ML), for which quantum size effectsare important. For a given thickness, differencesby a factor between two and five result fromchanges in the transition metal element. Forexample, for hcp(0001) and fcc(111) surfaces(Fig. 1E), slabs with thicknesses of 2 nm (9 to10 ML), 1 nm (4 to 5 ML), and 0.5 nm (2 ML)exhibit compressive strains with magnitudesof 0.3 to 1.4%, 0.8 to 3.1%, and 2.2 to 5.7%,

RESEARCH

Wang et al., Science 363, 870–874 (2019) 22 February 2019 1 of 5

1Department of Chemical and Biomolecular Engineering,Johns Hopkins University, Baltimore, MD 21218, USA.2Davidson School of Chemical Engineering, Purdue University,West Lafayette, IN 47907, USA. 3Department of ChemicalEngineering and Materials Science, University of California,Irvine, CA 92697, USA. 4Department of Physics andAstronomy, University of California, Irvine, CA 92697, USA.*These authors contributed equally to this work.†Corresponding author. Email: zeng46@purdue.edu (Z.Z.);chaowang@jhu.edu (C.W.); jgreeley@purdue.edu (J.G.)

on August 24, 2020

http://science.sciencem

ag.org/D

ownloaded from

respectively. For (100) surfaces, the corre-sponding magnitudes are 0.5 to 2.0%, 1.0 to3.8%, and 2.7 to 6.3%, respectively (Fig. 1F).Finally, for unreconstructed (110) surfaces, com-pressive strain magnitudes of 0.1 to 1.4%, 0.4 to2.6%, and 0.4 to 5.1% were determined in the½�110� direction, which is parallel to step edges(Fig. 1G), with similar results for (110) surfaceswith missing row reconstructions (fig. S7).In general, Pt and Au exhibit the largest

strains, whereas the values for Pd, which waschosen for further experimental study, are closeto the average. The elemental dependence canbe explained by considering the ratio betweenthe intrinsic surface pressure and the corre-sponding bulk modulus (Fig. 1C). On close-packed surfaces, Pt and Au have an averageratio of 8.3%, leading to high surface strains,whereas the rest of the metals have an averagevalue of 5.5%. The larger ratios for Pt and Auare, in turn, related to their stronger relativisticeffects than observed with lighter elements(fig. S8) (30, 31). Due to increased orbital con-traction (fig. S8), these effects have a larger im-pact on undercoordinated surface atoms thanon bulk atoms. Similar trends in the dependenceof the strain on elemental composition andrelativistic effects were also found for (100)and (110) surfaces (Fig. 1, E to G, and fig. S7).To verify the predicted intrinsic strain tuning

in 2D metal nanostructures, Pd nanosheets wereexperimentally synthesized by using CO as boththe reducing agent and stabilizing ligand (2, 32).By varying the CO sources and the synthesistemperature, nanosheets were synthesized withthicknesses of 3 ± 1 ML, 5 ± 1 ML, and 8 ± 1 ML

(± standard deviations; these sheet thicknessesare denoted 3 ML, 5 ML, and 8 ML below) andwith corresponding edge lengths of 120 to 260 nm,50 to 150 nm, and ~20 nm, respectively (seeFig. 2D and the supplementary materials for ad-ditional details). Aberration-corrected HRTEMimaging with spatial resolution of 0.7 Å wasused to analyze the in-plane strain in thesenanosheets (Fig. 2, E to K). Fourier transformanalysis of the image reveals that the nanosheetsadopt a (110) basal plane (Fig. 2, E to G, and fig.S18), distinguishing these nanosheets from pre-viously reported structures that were grown usingstrong stabilizing ligands such as tetrabutylam-monium bromide (6, 33), which is difficult to re-move and can be detrimental to electrocatalyticperformance (2, 32). A 2D template-matchingmethod (figs. S20 to S26) further permitted de-termination of the lattice constants and the lateralstrain (34, 35). The lattice parameters in the 3-MLPd nanosheets, shown in Fig. 2I, were determinedto be a = 0.270 nm and b = 0.235 nm, with anangle of 124.7° between them, which correspondto the unit vectors along the norm vector of ð�110Þand ð1�12Þ planes projected along the (110) orien-tation of the fcc Pd lattice. The lattice parameterswere measured to be a = 0.272 nm and b =0.235 nm for 5-ML nanosheets (Fig. 2J) and a =0.274 nm and b = 0.237 nm for 8-ML sheets(Fig. 2K).Compared to the bulk values (a = 0.275, b =

0.238), the 8-, 5-, and 3-ML nanosheets pos-sess average compressive strains of 0.3, 1.2,and 1.5%, respectively (Fig. 2L). These resultsclearly show that the extent of compressivestrain increases as the thickness of the 2D nano-

sheets decreases. Moreover, the trend of the mea-sured strain values matches the predicted valuesfrom calculations for Pd, for which the averagestrains are 0.35%, 0.92%, and 2.50% for 8-ML,5-ML, and 3-ML slabs, respectively (Fig. 1G andfig. S6). We emphasize that these Pd nanosheetshave high surface areas [for example, ~74 m2/gfor 5-ML nanosheets (table S3)] but neverthelessexpose extended, low–Miller index surfaces,as is the case for single crystals. The demon-strated tuning of intrinsic strain thus representsa promising approach toward modulation ofsurface reactivity to produce active, practicalcatalytic materials.It has been well established that strain mod-

ifies the electronic structure and reactivity ofcatalyst surfaces (9, 10, 13, 15, 17, 19, 20, 22, 28, 36).To probe these effects in 2D nanosheets, thed-band center and the surface energy of thestrained slabs were calculated and comparedwith corresponding unstrained slabs. There isgenerally a downshift of the d-band center anda decrease of the surface energy for thinnerslabs with larger compressive in-plane strain,regardless of the surface structure [fcc(111),hcp(0001), fcc(110), or fcc(100)] or elemental iden-tity of the metal (figs. S27 to S29). These resultssuggest, in turn, that the surfaces interact lessstrongly with adsorbates as the thickness de-creases and the in-plane compression increases(9, 10). This prediction is consistent with cal-culated weakening of the adsorption energy ofatomic hydrogen and oxygen (Fig. 3A and fig.S30), which are widely used as descriptors forhydrogen and oxygen redox reactions, respec-tively (37, 38). For example, for O adsorption

Wang et al., Science 363, 870–874 (2019) 22 February 2019 2 of 5

Fig. 1. The relationship between bulk modulus (B), surface stress (t),and lattice strain (e) of platinum group and coinage metal slabs.(A) Mechanism of the generation of intrinsic strain in 2D transition metalnanosheets. h is the height of an atomic layer. (B) Surface stress offcc(111) and hcp(111) surfaces compared to the bulk modulus. (C) Ratio ofthe pressure (p) of close-packed surfaces, induced by surface stress,

and the bulk modulus for 11 transition metals. (D) Potential energy profileof strained versus unstrained Pd(111) slabs with thicknesses of 8 ML,4 ML, and 2 ML. (E to G) Intrinsic in-plane strain of fcc(111), fcc(100),and the ½�110� direction of unreconstructed fcc(110) slabs, with thicknessesfrom 1 ML to 12 ML. See the supplementary materials for results for thereconstructed fcc(110) surface.

RESEARCH | REPORTon A

ugust 24, 2020

http://science.sciencemag.org/

Dow

nloaded from

on fcc(111) and hcp(0001) surfaces, the averageweakening is 0.05, 0.10, and 0.25 eV per oxygenatom for slabs with thicknesses of around 2, 1,and 0.5 nm, respectively (Fig. 3A); for a fewmetals (in particular, Pt and Au), notable ad-sorption energy oscillations due to quantumsize effects can also be seen for slabs of fewerthan ~6 ML (39). As with the trends in surfacestrain, the weakening of adsorption is depen-dent on the nature of the substrates, with theeffect on Pt and Au being generally three tosix times as large as that on the other tran-sition metals and the effect on Pd being closeto the average. The trend in the weakeningof the adsorption therefore holds for a broad

spectrum of metals and different surfaces. Thecontinuously tunable strain and adsorptionproperties thus allow for the design of optimalcatalytic surfaces for given reactions based onthe freestanding 2D nanosheets.According to the Sabatier principle, the in-

teractions between an optimal catalyst and re-action intermediates should be neither too strongnor too weak, suggesting the presence of a maxi-mum in catalytic activity at some point in thethickness-strain continuum. Because transitionmetal surfaces often bind too strongly to reactionintermediates for common electrocatalytic reac-tions, such asHER andORR (13, 15, 19, 21, 22, 40, 41),weakening the adsorbate interactions with the

surface via tuning of strain is a useful strategyto improve the activity of transition metals forthese reactions. To illustrate these relationshipsfor the strained 2D nanosheets, we have per-formed both experimental and theoretical in-vestigations of Pd nanosheets as alkaline ORRelectrocatalysts, because Pd is of particular in-terest for anion exchange membrane fuel cells,where it is considerably more stable than inacidic environments (42).Experimentally, Pd nanosheets with (110)

terminations are found to be considerably moreactive than Pd/C (palladium nanoparticles ona carbon support) and Pt/C (Pd/C serves as apractical, high–surface area reference material

Wang et al., Science 363, 870–874 (2019) 22 February 2019 3 of 5

01

2345

12345 0 1234

5

123450

12345

12345

A B C

E F G

I J K

D

H

L

0 1 2 3

3 ML Pd NSs 0.2095 nm

5 ML Pd NSs 0.2201 nm

8 ML Pd NSs 0.2223 nm

).u .a( ytisnetnI

Distance (nm)

Pd/C 0.2235 nm

2 4 6 8

-4

-3

-2

-1

0

Calculation results [001] [110]

Experimental results [001] [110]

)%( niart

S

Thickness (ML)10

0 2 4 6 80

100

200

300

8 ML Pd NSs

5 ML Pd NSs

)mn( htgneL egd

E

Thickness (nm)

3 ML Pd NSs

10

10

10

Fig. 2. Structural characterization of Pd nanosheets. (A to C) TEMimages of as-prepared Pd nanosheets (NSs) with average thicknesses of3 ML (A), 5 ML (B), and 8 ML (C), with insets depicting typical structures.(D) Size and thickness distribution of Pd nanosheets (see figs. S15 toS17 for more details). Error bars indicate SD. Before electrochemicaltesting and lattice measurements, nanosheets are dispersed on high–surface area carbon, followed by low-temperature annealing to removeorganic ligands, which leads to loss of stacked structures (fig. S14). Panels (E)to (L) show results of the dispersed nanosheets. (E to G) Aberration-corrected HRTEM images of Pd nanosheets with average thicknesses of3 ML (E), 5 ML (F), and 8 ML (G) on a carbon support. (H) The intensityprofile and calculated average d-spacing of (111) planes from the nano-sheets in (E) to (G) and Pd nanoparticles (fig. S19). The area and thedirection of the measurement are indicated by yellow, green, magenta, andred lines in (E), (F), and (G) and fig. S19, respectively. Two adjacent (111)

planes, from which the first two peaks in the intensity profile are obtained,are indicated by two parallel lines. a. u., arbitrary units. (I to K) Illustrationof the template-matching method used in the measurement of thelattice strain, which is effected by superimposing the Pd bulk lattice(yellow dots) on the HRTEM images of Pd nanosheets of 3 ML (I), 5 ML(J), and 8 ML (K). The yellow circles, in turn, are reference points thatindicate the measured atomic positions along ½�110� and ½1�12� directionson the nanosheets. These points also superimpose directly on thebulk lattice points. The adjacent nanosheet atoms are indicatedwith blue circles. The mismatch between the blue circles and the yellowdots shows the degree of lattice contraction. (L) The measured strainof Pd nanosheets using the template-matching method on the HRTEMimages in (I) to (K). To enable a direct comparison to be made withthe DFTcalculations, the strain in the ½1�12� direction is projected onto the[001] direction. Error bars indicate SD.

RESEARCH | REPORTon A

ugust 24, 2020

http://science.sciencemag.org/

Dow

nloaded from

for the catalytic performance of nanosheets),with the ORR activity exhibiting a classic vol-cano relationship as the thickness decreases(Fig. 4, tables S3 to S7, and figs. S41 to S47).From the polarization curves (Fig. 4A), the half-wave potential is determined to be 0.90, 0.95,and 0.93 V for the 8-, 5-, and 3-ML nanosheets,respectively, compared to 0.86 V for Pd/C. Themost active 5-ML nanosheets achieve a specificactivity of 0.70 mA/cm2 at 0.95 V [10.91 mA/cm2

at 0.9 V (table S3)], representing an improve-ment factor of >18, versus Pd/C with a specificactivity of 0.04 mA/cm2 at 0.95 V (0.41 mA/cm2

at 0.9 V) (Fig. 4B and tables S3 and S5). The3-ML nanosheets are the next most active, with0.60 mA/cm2 at 0.95 V (10.42 mA/cm2 at 0.9 V),whereas the 8-ML nanosheets have consid-erably lower specific activities of 0.07 mA/cm2

at 0.95 V (1.07 mA/cm2 at 0.9 V). Similar trendswere also observed in mass activity, with en-hancement factors of up to 26 and 47 for 5-ML

nanosheets at 0.95 V (0.52 mA/mg of Pd) and0.9 V (8.02 mA/mg of Pd), respectively, com-pared to Pd/C (0.02 A/mg of Pd at 0.95 V and0.17 A/mg of Pd at 0.9 V).The relationship between the thickness of

Pd nanosheets and the ORR activity is furtherrevealed by calculation of the free-energy bar-riers of the ORR on (110)-terminated Pd nano-sheets with thicknesses of 3 to 8 ML. As discussedabove, each such surface has a different extentof compressive strain, with the thinner slabsexperiencing the greatest compression (fig. S7).The unstrained Pd surface binds to oxygen toostrongly, which limits the ORR kinetics dueto the slow oxidative desorption of hydroxide(OHad); this result is very similar to the casewith Pt surfaces (41–44). As the O* binding isweakened on the thinner, more compressed Pdnanosheets, the predicted activity gradually in-creases with decreasing slab thickness andincreasing compressive strain, with maximal re-

activity predicted for nanosheets with thick-nesses of ~4 ML (Fig. 3B). These predictionsprovide a compact explanation for the enhancedalkaline ORR activities observed in the electro-catalytic studies (Fig. 4). Further, as also pre-dicted by the DFT analyses, we expect thatthese structure-property relationships will beapplicable to 2D nanosheets of other metalsand with different lattice orientations.In addition to alkaline electrolytes, we have

conducted measurements of the ORR on thePd nanosheets in acid. We note that, althoughPd is insufficiently stable in acid and may havelimited potential for practical use in protonexchange membrane fuel cells, the predictionssuggest that similar activity enhancements willbe observed in acidic media. The measured en-hancements of specific activity in 0.1 M HClO4

at 0.95 V are 14, 10, and 5 for 5-ML, 3-ML, and8-ML nanosheets (Fig. 4C and table S7), whereasthe corresponding improvement factors in 0.1 M

Wang et al., Science 363, 870–874 (2019) 22 February 2019 4 of 5

Fig. 3. The dependence of adsorption ener-gies and ORR activity on the thicknessof nanosheets with intrinsic strain.(A) Adsorption energy shift (DEad) ofatomic oxygen on strained fcc(111) andhcp(0001) slabs, compared with adsorptionenergies on corresponding close-packedsingle-crystal surfaces. (B) PredictedORR overpotential of Pd(110) single-crystaland strained nanosheets with variousthicknesses. The dashed line in (B) is fitbased on a strained single-crystal surface(see the supplementary materials for details). 12 10 8 6 4 2

0.0

0.1

0.2

0.3

0.4 Co Ni Cu Ru Rh Pd Ag Os Ir Pt Au

ΔEad

)Ve( )

O(

Slab thickness (ML)

A B

-0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.060.39

0.38

0.37

0.36

0.35

)V( laitnetoprev

O

dEad

(OH) (eV)

Single-crystal 8ML 7ML 6ML 5ML 4ML 3ML

Fig. 4. Electrochemical activity ofPd nanosheet catalysts on car-bon. (A) ORR polarizationcurves of Pd nanoparticles, as wellas Pd nanosheets with averagethickness of 3 ML, 5 ML, and 8 ML in0.1 M KOH (inset shows the half-wave potential). RHE, reversiblehydrogen electrode. (B and C) Spe-cific activity and mass activity ofORR at 0.95 V (versus RHE) in0.1 M KOH and the correspondingimprovement factors compared tothose for Pd nanoparticles.(D) HER overpotential ofPd nanoparticles, as well asPd nanosheets with average thick-ness of 3 ML, 5 ML,and 8 ML at 5 mA/cm2 in0.1 M KOH or 0.1 M HClO4.(E and F) Specific activityand mass activity of ORR at0.95 V (versus RHE) in0.1 M HClO4 (E) and thecorresponding improvement factorscompared to those forPd nanoparticles (F).

RESEARCH | REPORTon A

ugust 24, 2020

http://science.sciencemag.org/

Dow

nloaded from

KOH are 18, 15, and 2 (table S5), confirming thattrends in ORR activities are consistent for Pdnanosheets in acid and in base. We further notethat similar trends in activity improvementshould be observable for other electrocatalyticreactions that are sensitive to surface strain ef-fects. As confirmation of this principle, we foundsimilar enhancements in reactivity trends forHER in both acid and base, with the activityfollowing the order 5-ML nanosheets > 3-MLnanosheets > 8-ML nanosheets > nanoparticles(Fig. 4, figs. S48 and S49, and table S8).The considerations discussed above demon-

strate that metal nanosheet catalytic propertiesmay be tuned via manipulation of intrinsicstrain for multiple electrocatalytic reactionsin both alkaline and acidicmedia. These results,in turn, suggest that generating and tuning ofintrinsic surface strain represent a powerfuland general strategy to engineer nanocatalystswith simultaneously enhanced specific activityand mass activity for a spectrum of electro-catalytic processes.

REFERENCES AND NOTES

1. M. Li et al., Science 354, 1414–1419 (2016).2. L. Bu et al., Science 354, 1410–1414 (2016).3. L. Zhang et al., Science 349, 412–416 (2015).4. C. Chen et al., Science 343, 1339–1343 (2014).5. L. Gan et al., Science 346, 1502–1506 (2014).6. X. Huang et al., Nat. Nanotechnol. 6, 28–32 (2011).7. V. R. Stamenkovic, D. Strmcnik, P. P. Lopes, N. M. Markovic,

Nat. Mater. 16, 57–69 (2016).8. Z. W. Seh et al., Science 355, eaad4998 (2017).9. M. Mavrikakis, B. Hammer, J. K. Nørskov, Phys. Rev. Lett. 81,

2819–2822 (1998).10. J. R. Kitchin, J. K. Nørskov, M. A. Barteau, J. G. Chen,

Phys. Rev. Lett. 93, 156801 (2004).11. M. Luo, S. Guo, Nat. Rev. Mater. 2, 17059 (2017).

12. S. Alayoglu, A. U. Nilekar, M. Mavrikakis, B. Eichhorn,Nat. Mater. 7, 333–338 (2008).

13. P. Strasser et al., Nat. Chem. 2, 454–460 (2010).14. K. Tedsree et al., Nat. Nanotechnol. 6, 302–307 (2011).15. M. Escudero-Escribano et al., J. Am. Chem. Soc. 134,

16476–16479 (2012).16. S. Brimaud, A. K. Engstfeld, O. B. Alves, H. E. Hoster,

R. J. Behm, Top. Catal. 57, 222–235 (2014).17. M. Du, L. Cui, Y. Cao, A. J. Bard, J. Am. Chem. Soc. 137,

7397–7403 (2015).18. N. Todoroki, H. Watanabe, T. Kondo, S. Kaneko, T. Wadayama,

Electrochim. Acta 222, 1616–1621 (2016).19. M. Escudero-Escribano et al., Science 352, 73–76 (2016).20. M. Asano, R. Kawamura, R. Sasakawa, N. Todoroki,

T. Wadayama, ACS Catal. 6, 5285–5289 (2016).21. H. Wang et al., Science 354, 1031–1036 (2016).22. K. Yan et al., Angew. Chem. Int. Ed. 55, 6175–6181 (2016).23. R. G. Mariano, K. McKelvey, H. S. White, M. W. Kanan, Science

358, 1187–1192 (2017).24. T. Yu, D. Y. Kim, H. Zhang, Y. Xia, Angew. Chem. Int. Ed. 50,

2773–2777 (2011).25. F. Calle-Vallejo et al., Science 350, 185–189 (2015).26. V. R. Stamenkovic et al., Science 315, 493–497 (2007).27. H. E. Hoster, O. B. Alves, M. T. M. Koper, ChemPhysChem 11,

1518–1524 (2010).28. T. Adit Maark, A. A. Peterson, J. Phys. Chem. C 118, 4275–4281

(2014).29. K. J. J. Mayrhofer, K. Hartl, V. Juhart, M. Arenz, J. Am.

Chem. Soc. 131, 16348–16349 (2009).30. P. Pyykko, Chem. Rev. 88, 563–594 (1988).31. D. J. Gorin, F. D. Toste, Nature 446, 395–403 (2007).32. D. Li et al., ACS Catal. 2, 1358–1362 (2012).33. H. Li et al., Angew. Chem. Int. Ed. 52, 8368–8372 (2013).34. J.-M. Zuo et al., Ultramicroscopy 136, 50–60 (2014).35. W. Gao et al., Sci. Rep. 7, 17243 (2017).36. S. Kattel, G. Wang, J. Chem. Phys. 141, 124713 (2014).37. B. Hammer, J. K. Nørskov, Nature 376, 238–240 (1995).38. B. Hammer, J. K. Nørskov, in Advances in Catalysis, vol. 45

(Academic Press, 2000), pp. 71–129.39. P. Jiang et al., J. Am. Chem. Soc. 130, 7790–7791 (2008).40. J. K. Nørskov et al., J. Catal. 209, 275–278 (2002).41. J. K. Nørskov et al., J. Phys. Chem. B 108, 17886–17892

(2004).42. M. Shao, in Electrocatalysis in Fuel Cells: A Non- and Low-

Platinum Approach, M. Shao, Ed. (Springer, 2013), pp. 513–531.

43. S. Kondo, M. Nakamura, N. Maki, N. Hoshi, J. Phys. Chem. C113, 12625–12628 (2009).

44. F. H. B. Lima et al., J. Phys. Chem. C 111, 404–410(2007).

ACKNOWLEDGMENTS

Funding: Work at Purdue was supported through the Officeof Science, Office of Basic Energy Sciences, Chemical, Biological,and Geosciences Division under DE-SC0010379 (J.G.) and bythe U.S. Department of Energy, Office of Energy Efficiency andRenewable Energy, under DE-EE0007270 (Z.Z.). Z.Z. andJ.G. also gratefully acknowledge the computing resources providedby the Center for Nanoscale Materials, which is supportedby the U.S. Department of Energy, Office of Science, Office ofBasic Energy Sciences, under E-AC02-06CH11357, as wellas the computational resources through the National EnergyResearch Scientific Computing Center (NERSC). C.W.acknowledges support from the National Science Foundation(CBET-1437219) and the JHU Catalyst Award. W.G. andX.P. are supported by the National Science Foundationunder grant numbers CBET 1159240, DMR-1420620, andDMR-1506535. TEM work was conducted using the facilities in theIrvine Materials Research Institute (IMRI) at the Universityof California, Irvine. Author contributions: Z.Z. and J.G.developed the strain tuning strategy. L.W. and C.W. conceivedthe idea of Pd nanosheet synthesis. Z.Z. performed strainand strain-property calculations. T.M. performed relativisticeffect calculations. L.W., D.R., and M.G. performed the synthesisand electrochemical tests. W.G. and X.P. performed TEMmeasurements. Z.Z. and J.G. wrote the manuscript with inputfrom L.W., W.G., and C.W. Competing interests: None declared.Data and materials availability: The data presented in thispaper are available in the supplementary materials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/363/6429/870/suppl/DC1Materials and MethodsFigs. S1 to S49Tables S1 to S9References (45–62)

6 April 2018; resubmitted 12 September 2018Accepted 23 January 201910.1126/science.aat8051

Wang et al., Science 363, 870–874 (2019) 22 February 2019 5 of 5

RESEARCH | REPORTon A

ugust 24, 2020

http://science.sciencemag.org/

Dow

nloaded from

Tunable intrinsic strain in two-dimensional transition metal electrocatalysts

GreeleyLei Wang, Zhenhua Zeng, Wenpei Gao, Tristan Maxson, David Raciti, Michael Giroux, Xiaoqing Pan, Chao Wang and Jeffrey

DOI: 10.1126/science.aat8051 (6429), 870-874.363Science 

, this issue p. 870Scienceevolution reactions under alkaline conditions compared with nanoparticles.form with an internal compressive strain of 1 to 2% and can be much more active for both the oxygen and hydrogen

show that freestanding palladium nanosheets (three to five monolayers thick)et al.releases the induced strain. Wang often induced with overlayers (adsorbates or heteroatoms) that can undergo reconstruction during operation thatactivity. For practical catalysts, nanomaterials with high surface areas are needed. However, for nanoparticles, strain is

Strain can modify the electronic properties of a metal and has provided a method for enhancing electrocatalyticHarnessing self-tuned strain

ARTICLE TOOLS http://science.sciencemag.org/content/363/6429/870

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2019/02/20/363.6429.870.DC1

REFERENCES

http://science.sciencemag.org/content/363/6429/870#BIBLThis article cites 60 articles, 13 of which you can access for free

PERMISSIONS http://www.sciencemag.org/help/reprints-and-permissions

Terms of ServiceUse of this article is subject to the

is a registered trademark of AAAS.ScienceScience, 1200 New York Avenue NW, Washington, DC 20005. The title (print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement ofScience

Science. No claim to original U.S. Government WorksCopyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of

on August 24, 2020

http://science.sciencem

ag.org/D

ownloaded from