Highly active mixed-metal catalysts made by pulsed-laser ablation in liquids

Abstract

The invention is directed to mixed-metal nanocatalysts, particularly nano-dimensioned layered double-hydroxide nanostacks, methods of making nanocatalysts using laser ablation techniques, and the electrochemical devices comprising and using these nanocatalysts, for example in the electrochemical oxidation of water oxidation.

What is claimed: 
     
         1 . A layered double hydroxide nanostack comprising a plurality of nanosheets represented by the general formula: [M (1-x) M′ x  (OH) 2 ] x+ , wherein: M is a metal cation in a formal +2 oxidation state; M′ a metal cation in a formal +3 oxidation state; wherein
 the nanosheets are associated with or intercalate [A m−   x/m ], where A is a displaceable anion; 
 m is an integer; 
 x is a positive number less than 1; 
 the nanostack being optionally hydrated with a stoichiometric amount or a non-stoichiometric amount of water; and 
 the nanosheet having at least one lateral edge dimension in a range of from about 5 nm to about 100 nm. 
 
     
     
         2 . The layered double hydroxide nanostack of  claim 1 , wherein M is at least one of Ba 2+ , Be 2+ , Ca 2+ , Cd 2+ , Cu 2+ , Co 2+ , Fe 2+ , Mg 2+ , Mn 2+ , Ni 2+ , Sr 2+ , and Zn 2+ , and M′ is at least of Al 3+ , Ce 3+ , Co 3+ , Cr 3+ , Fe 3+ , Ga 3+ , In 3+ , La 3+ , Mn 3+ , V 3+ , Y 3+ , Ce 3+ . 
     
     
         3 . The layered double hydroxide nanostack of  claim 1 , wherein A comprises an organic or inorganic anion, or a combination thereof. 
     
     
         4 . The layered double hydroxide nanostack of  claim 3 , wherein A comprises F − , Cl − , Br − , I − , BF 4   − , PF 6   − , CO 3   2− , HCO 3   − , CrO 4   2− , NO 2 , NO 3 , ONO 2   − , ClO 2   − , ClO 3   − , ClO 4 . − , H 2 PO 4   − , HPO 4   2− , PO 4   3- , IO 3   − , OH − , S 2− , SO 3   2− , S 2 O 3   2− , SO 4   2− , WO 4   2− , or a combination thereof. 
     
     
         5 . The layered double hydroxide nanostack of  claim 3 , wherein A comprises acetate, propionate, lactate, terephthalate, adipate, succinate, dodecyl sulfonate, p-hydroxybenzoate, and benzoate, or a combination thereof. 
     
     
         6 . The layered double hydroxide nanostack of  claim 3 , wherein A comprises a complex anion comprising a transition metal compound. 
     
     
         7 . The layered double hydroxide nanostack of  claim 1 , wherein M is or comprises Ni 2+ . 
     
     
         8 . The layered double hydroxide nanostack of  claim 1 , wherein M′ is or comprises Fe 3+ . 
     
     
         9 . The layered double hydroxide nanostack of  claim 1 , wherein x is in a range of from 0.05 to 0.95. 
     
     
         10 . The layered double hydroxide nanostack of  claim 1 , wherein A is or comprises hydroxide. 
     
     
         11 . The layered double hydroxide nanostack of  claim 1 , wherein the nanosheet has at least one lateral edge dimension in a range of from about 7 nm to about 22 nm. 
     
     
         12 . The layered double hydroxide nanostack of  claim 1 , wherein at least one of the nanosheets is further doped with at least one Lewis acid ion of a transition metal, lanthanide, or actinide metal ion. 
     
     
         13 . The layered double hydroxide nanostack of  claim 1 , wherein at least one of the nanosheets is further doped with titanium or lanthanum. 
     
     
         14 . A method of preparing a layered double hydroxide nanostack of  claim 1 , comprising subjecting a solid ablation target to an energy source, the solid ablation target comprising a first metal capable of oxidizing to a positive oxidation state, the energy source impinging on the first metal in the presence of an aqueous ablation solution containing a second metal ion in a positive oxidation state for a time and energy sufficient to ionize at least a portion of the first metal to a positive oxidation state. 
     
     
         15 . The method of  claim 14 , wherein the aqueous ablation solution is substantially free of surfactants. 
     
     
         16 . The method of  claim 14 , wherein the first metal is capable of achieving a +2 oxidation state, and the second metal is in a +3 oxidation state. 
     
     
         17 . The method of  claim 14 , wherein the first metal is capable of achieving a +3 oxidation state, and the second metal is in a +2 oxidation state. 
     
     
         18 . The method of  claim 16 , wherein the first metal comprises beryllium, calcium, cadmium, copper, cobalt, iron, magnesium, manganese, nickel, strontium, zinc, or an alloy or mixture thereof and the second metal ion comprises at least one of Al 3+ , Ce 3+ , Co 3+ , Cr 3+ , Fe 3+ , Ga 3+ , In 3+ , La 3+ , Mn 3+ , V 3+ , and Y 3+ . 
     
     
         19 . The method of  claim 17 , wherein the first metal comprises aluminum, cerium, cobalt, chromium, iron, gallium, indium, lanthanum, manganese, vanadium, yttrium, or an alloy or mixture thereof, and the second metal comprises at least one of Be 2+ , Ca 2+ , Cd 2+ , Cu 2+ , Co 2+ , Fe 2+ , Mg 2+ , Mn 2+ , Ni 2+ , Sr 2+ , and Zn 2+ . 
     
     
         20 . The method of  claim 14 , wherein the energy is a laser. 
     
     
         21 . The method of  claim 20 , wherein the laser energy is provided in pulses 
     
     
         22 . The method of  claim 21 , wherein the laser pulse delivers an energy in a range of from about 90 mJ/pulse to about 210 mJ/pulse. 
     
     
         23 . The method of  claim 14 , wherein the displaceable ion A of the layered double hydroxide nanostack comprises a counterion associated with the second metal ion in the aqueous ablation solution. 
     
     
         24 . The method of  claim 23 , further comprising exchanging the displaceable ion A of the layered double hydroxide nanostack with a different anion. 
     
     
         25 . The method of  claim 14 , wherein the initially-formed layered double hydroxide nanostack is further sintered. 
     
     
         26 . An electrode comprising a coating comprising the layered double hydroxide nanostack of  claim 1 . 
     
     
         27 . The electrode of  claim 26 , wherein the electrode comprises gold, nickel, platinum, or an allotrope of carbon. 
     
     
         28 . The electrode of  claim 26 , wherein the electrode comprises graphite, graphene, glassy (or vitreous) carbon, diamond, or a combination thereof. 
     
     
         29 . The electrode of  claim 27 , that exhibits an overpotential for the oxidation of water to oxygen of less than 300 mV at 10 mA/cm 2  on a flat supporting electrode. 
     
     
         30 . An electrochemical cell comprising an electrode of  claim 26 . 
     
     
         31 . A method for oxidizing water comprising applying a potential to an electrode of  claim 26 , and passing sufficient current to oxidize water to form oxygen. 
     
     
         32 . The method of  claim 31 , wherein the potential is within 300 mV of the thermodynamically determined potential for the oxidation of water to oxygen at 10 mA/cm 2 .