Electrochemical Stability of Elemental Metal and Alloy Nanoparticles

1. Electrochemical Stability of Elemental Metal and Alloy Nanoparticles * Acknowledgement: NSF DMR

2. Elemental Metals Alloys: Dealloying behavior • Mn+ + ne- = M • MaOn + 2nH+ + 2ne- = aM + nH2O • MaOn + 2nH+ = aMn+ + nH2O • Surface dealloying Development of particle-size dependent E – pH diagrams. (B) “Bulk” dealloying/porosity formation The problem We are interested in understanding the electrochemical stability or corrosion behavior of elemental metals and alloys at the nanoscale.

3. is a length scale in the alloy connected to the alloy composition through percolation arguments. Alloys: Dealloyingbehavior Planar Surface “Bulk” dealloying/porosity formation Rugolo, Erlebacher, Sieradzki, Nature Materials, Vol., 5, 946 (2006). Single phase solid solution ApB1-p alloys Bulk dealloying results in the creation of new solid/electrolyte interface (porosity formation) of well-defined length scale. Intrinsic critical potential

4. Nano particle “Bulk” dealloying/porosity formation Aberration-corrected high angle dark field STEM of dealloyed PtCo nanoparticle showing “bulk” porosity formation, from Shao-Horn, J. Phys. Chem. C, Vol. 113, 1109 (2009).

5. Nano particle “Bulk” dealloying/porosity formation Can such morphologies form for sub-10 nm nanoparticles for which the pore size is only slightly smaller that the particle diameter? Are such structures mechanically stable or will they collapse or rumple owing to surface stress effects? STEM and HREM Image of dealloyed PdCu particles, Minhua Shao, et al. JACS, 132, 9253 (2010).

6. Alloys-Dealloying behavior:PlanarSurface Dealloying No porosity formation Wagner, Brankovic, Dimitrov, Sieradzki, J. Electrochem. Soc., Vol., 144,3545 (1997). Stripping of the more reactive component from the alloy surface without the creation of new alloy/electrolyte interface. KMC simulation of surface dealloying. J. Erlebacher.

7. PlanarSurface Dealloying Dealloying below the critical potential (K. Wagner et al., J. Electrochem. Soc., Vol. 144, pg. 3545, 1997) 340 nm 5000 s 500 s 200 s Dealloying below Ec (= 420 mV) for Ag0.80 Au0.20 : 80 mV vs Ag+/Ag in 0.001M AgClO4 + 0.1M HClO4.

8. Surface dealloying potentials (c) (a) (b) 300 K 1019 K p (AgpAu1-p) PlanarSurface Dealloying A metal/metal ion equilibrium potential can be defined for the active A component in an alloy in analogy to measuring the equilibrium vapor pressure of a component in an alloy. Thermodynamics of Ag-Au alloys. (a) The voltage difference DV between a pure Ag electrode and Ag in a solid Ag-Au alloy at T = 1019 K. Data from Wagner, Engelhardt, Z. phys. Chem.A, 159, 241-274 (1932).(b) Activity versus composition for Ag and Au in the liquid and solid alloys. Data re-plotted from Oriani, Acta Metall., 4, pp. 15-25 (1956). (c) Data from (a) for 300 K.

9. PlanarSurface Dealloying Surface dealloying occurs at the metal/metal ion equilibrium potential of the more reactive component in the alloy: For dilute AgAu alloys the equilibrium is shifted positive by ~ 100 mV What about surface dealloying in nanometer-scale solids?

10. This pressure difference between the finite size solid in equilibrium with the fluid leads to Gibbs’ result that the equilibrium condition is given by Chemical equilibrium of small crystals J.W. Gibbs, Collected Works, Vol. 1, Yale Univ. Press, New Haven, 1957, pp. 317-318. J.W. Cahn, Acta Metall., Vol. 28, pp. 1333-1338 (1980). R.C. Cammarata, Philos. Mag. 88, pp. 927-948 (2008) . For a finite-size liquid drop of radius r, in equilibrium with its vapor the pressure difference between the fluid drop and vapor, i.e., the Laplace pressure is, For a single component finite-size solid of radius r, in equilibrium with a fluid. the Laplace pressure is,

11. l; S; S; A single component solid (S) composed of n moles of component 1, , in equilibrium with a multi-component ( ) liquid (l). The Gibbs dividing surface, S, is chosen such that there is no surface excess of component 1. Chemical equilibrium of small crystals Recall Gibbs’ definition of the interfacial free energy such that the surface excess of component 1 is zero, Consider a variation involving the reversible dissolution or accretion of a layer of the solid

12. Chemical equilibrium of small crystals Equilibrium is defined by setting dU= 0 subject to the constraints . Substitution of these constraints in to dU yields,

13. 2f/r; sphere 0 gdA=2gd Vs/r; sphere Rearranging Chemical equilibrium of small crystals Inserting these “equations of condition” in to the equilibrium equation,

14. Electrochemical equilibrium of binary alloy nanoparticles Equilibrium potential of An+/A in a binary ApB1-p alloy nanoparticle

15. represents the corresponding thermodynamic potential for the bulk planar solid. Surface dealloying of small crystals All of the quantities in this equation are well defined. The most reliable numbers for the alloy surface stress and surface energy are obtained from first principles based electronic structure calculations and there is virtually no data currently available. Nevertheless, its instructive to proceed with estimates for surface dealloying in a nanoparticle systems such as CuPt and AgAu. We will estimate an upper bound for this finite size effect by considering the behavior of these components in dilute (AgAu, CuAu, and CuPt) alloy isotropic (in surface energy and surface stress nano-spheres.

16. AgAu: CuAu: CuPt: Surface dealloying of small crystals Equilibrium potential of An+/A in a binary ApB1-p alloy nanoparticle for p dilute. Assume 3 nm diameter spheres.

17. (A) Mn+ + ne- = M Electrochemical Stability of Elemental Metal Nanoparticles In the case of elemental metal stability the electrochemical and chemical equilibria that we consider are represented by the following set of equations: (A) Mn+ + ne- = M (B) MaOn + 2nH+ + 2ne- = aM + nH2O (C) MaOn + 2nH+ = aMn+ + nH2O Using the equilibrium condition we develop finite size corrections to the equilibrium reactions (A), (B) and (C).

18. Electrochemical Stability of Elemental Metal Nanoparticles (C) MaOn + 2nH+ = aMn+ + nH2O : Average molar volume of the oxide covered nanoparticle : Partial molar volume of MaOn in the nanoparticle The standard potential for the formation of the metal oxide is obtained by using equations (1) and (2) in a manner analogous to that using equations (A) and (C) to obtain equation (B); (B) MaOn + 2nH+ + 2ne- = aM + nH2O Eq. (3)

19. Data base for finite size corrections : interfacial free energy of the metal/metal-ion interface : interfacial free energy of the metal oxide/electrolyte interface : surface interfacial stress of the metal oxide : Δf related to oxide formation on the metal surface can be measured using wafer curvature. The only reliable numbers for these parameters are obtained via electronic structure calculations.

20. Data base for Pt finite size corrections

21. Data base for Cu finite size corrections

22. C Cu+ Cu2O A B Cu Particle size-dependent potential-pH diagram: Pt Pt2+ PtO C A B Pt (A) Pt = Pt2+ + 2e- (A) Cu = Cu+ + e- (B) 2Cu + H2O = Cu2O + 2H+ + 2e- (B) Pt + H2O = PtO + 2H+ + 2e- (C) 2Cu+ + H2O = Cu2O + 2H+ (C) Pt2+ + H2O = PtO + 2H+ Particle-size dependent E-pH stability diagrams showing the change in dissolution mechanism with particle size.

23. Predictions for the effect of particle size on stability of Pt nanoparticles PtO Pt2+ Pt GT predictions for the stability of Pt particles in 0.1M H2SO4 containing 10-6 M Pt2+.

24. pH = 4 Cu2O pH = 8 pH = 10 Cu+ Cu Predictions for the effect of particle size on stability of Cu nanoparticles GT predictions for the stability of Cu+ particles for various pH containing 10-6 M Cu+.

25. Experiment We examined the stability of individual unsupported Pt black particles, ~ 1 - 10 nm in diameter in aerated 0.1M H2SO4 using electrochemical STM. The Pt-black aggregates were dispersed in isopropyl alcohol by using a high-power ultrasonic wand. The final solution contained ~ 35 µg Pt nano-particles in 20mL isopropanol. Immediately after the sonification, the solution was “atomized” onto a fresh (111) oriented Au film. The mass of Pt particles was chosen in order to yield approximately 10 particles in a 100 x 100 nm field of view. A freshly prepared hydrogen loaded Pd wire served as the reference electrode which was checked against a standard reference electrode. The substrates for these experiments were ~250 nm thick {111} textured Ag0.05Au0.95 thin films prepared by ~200 nm deposition at 350 ºC.

26. Characterization of Pt-black aggregates B A C Characterization of Pt-black aggregates. (A) Transmission electron microscopy. The individual Pt particles are ellipsoidal in shape. The smallest particles visible in this image are ~ 2.5 nm in size. (B) X-ray diffraction and (C) Energy dispersive spectroscopy showing peaks only associated with Pt.

27. Particle 1, scan size 10 x 10nm. Particle 3, scan size 10 x 10nm. Electrochemical STM 1 3 4 5 2 600 mV In situ STM showing 5 Pt particles in 0.1M H2SO4. Scan size 95 x 95 nm. The stability of these particles was examined by stepping the potential in 50 mV increments and holding for ~600 s. at each potential.

28. B E F C G D 650 mV 900 mV 850 mV 750 mV 750 mV 900 mV Electrochemical STM A 1 1 1 1 3 1 3 3 4 5 5 3 2 2 2 600 mV 600 mV 1 1 In situ STM showing 5 Pt particles in 0.1M H2SO4. Potential-time sequence of particle dissolution. (A) Initial set of 5 particles at 600 mV NHE. (B) Pulse to 650 mV showing the dissolution of particles 4 and 5. Particles 1,2 and 3 were stable to 700 mV. (C) Potential pulse to 750 mV showing the dissolution of particles 2 an 3. (D) Particle 1 stability at 750 mV after 600 s at this potential. (E) Stability of particle 1 at 850 mV. (F) Particle 1 dissolution at 900 mV and (F) after 300 s. at 900 mV. Scan size 95 x 95 nm.

29. B A 4 3 2 1 D C Electrochemical STM In situ STM showing 4 Pt particles in 0.1M H2SO4. Potential-time sequence of particle dissolution. Scan size 40 x 40 nm. (A) 650 mV: The mean radii of the particles present in this image are: particle 1, rm = 2.5 nm; particle 2, rm = 1.90 nm; particle 3, rm = 3.5 nm; particle 4 rm = 2.2 nm . (B) 900 mV: Particles 2 is dissolving while 1, 3 and 4 are stable. (C) 1050 mV: all the particles are now undergoing shape change and becoming smaller (D) 1100 mV: particles 2 and 4 have disappeared while particles 1 and 3 are still present albeit smaller in size. At 1200 mV (not shown) particle 1 disappears while a remnant of particle 3 still remained.

30. PtO Pt2+ Pt Dissolution potentials and mechanism of Pt particle dissolution. Data shows that dissolution of Pt nanoparticles to D = ~ 4 nm occurs by the direct Pt = Pt2+ + 2e- pathway. Linear fit to data

31. Conclusions • Our results demonstrate that classical Gibbsian thermodynamics • accounts for the size effect on the stability of nm-scale Pt particles • to dissolution in 0.1M H2SO4. Our analysis indicates that sub 3.5 nm- • diameter Pt particles dissolve via the direct electrochemical pathway. • Pt  Pt2+ + 2e-, while a cross over in mechanism is predicted for particles • ~ 5 nm in diameter and larger. • Thermodynamics provides some guidelines for enhancing the stability of • nano-scale alloys. • Our results on elemental Pt nanoparticles hint that the behavior observed • on the planar alloy surfaces will not translate to alloy nanoparticles.

32. Electrochemistry and dissolution of Pt For polycrystalline planar electrodes, it is now generally accepted that the oxygen chemisorption process initiates with hydroxide adsorption at ~0.80 V (NHE), and that by 1.0 V the Pt surface is covered with one-half monolayer of chemisorbed oxygen. Subsequently, a place-exchange process occurs (~1.1 V) resulting in the formation of a PtO surface compound in which oxygen occupies substitutional sites in the Pt lattice. In the case of planar surfaces, it would seem that this half monolayer of adsorbed oxide sufficiently passivates the surface and inhibits the operation of Pt dissolution through reaction (A). It is not known whether a similar indirect dissolution mechanism operates for nanoparticle Pt electrodes.

33. PtO by place exchange We envision that the PtO surface layer is not confined to the top ML. Rather it is distributed near the surface and for a small particle can be viewed as a Pt-PtO “alloy” where oxygen is a “substitutional” component in the lattice. X-ray scattering, surface roughening of 3-layers Electrochemical Stability of Elemental Metals PtO; Morphology and Structure

34. Electrochemistry and dissolution of Pt Cyclic voltammetry for a Pt surface showing oxide formation and reduction in 0.5M H2SO4, 25 ºC, 0.1V s-1. Inset is the oxide formation charge as a function of potential. Angerstein-Kozlowska, Conway and Sharp, Elec. Anal. and Surf. Chem., 43, 9-36 (1973).

35. l; S; S; A single component solid (S) composed of n moles of component 1, , in equilibrium with a multi-component ( ) liquid (l). The Gibbs dividing surface, S, is chosen such that there is no surface excess of component 1. Chemical equilibrium of small crystals Recall Gibbs’ definition of the interfacial free energy such that the surface excess of component 1 is zero, Consider a variation involving the reversible dissolution or accretion of a layer of the solid

36. Chemical equilibrium of small crystals Equilibrium is defined by setting dU= 0 subject to the constraints . Substitution of these constraints in to dU yields, Since

37. 2f/r; sphere 0 gdA=2gd Vs/r; sphere Chemical equilibrium of small crystals Inserting these “equations of condition” in to the equilibrium equation, Rearranging