生物化工基础

1、会计学1生物化工基础生物化工基础23d/4d et 3d/5d Alloys Stability: High magnetic anisotropy 4d,5d elementExample: CoxRh1-x nanoparticles Sensitivity:High magnetic moments per atom 3d elements High density:Small size nanoparticles3q Cobalt : hcp (T750K)q Rhodium : fccfcchcpq Alloy: hcp (x50,5%) Disordered solid solut

2、ionW. Koster, Z. Metallkunde, 43 (1952)Bulk crystals: Co, Rh and Co1-xRhx4(fcc)Octahedron:1st and 2d neighbours distances)d2,d(mmmmABCABC (111) stackingABCATetrahedron:1st neighbours distance)d(mmClose packed structures :face centered cubic (fcc) and hexagonal compact (hcp) 5(fcc)Close packed struct

3、ures :face centered cubic (fcc) and hexagonal compact (hcp) In between two successive (111) planes (AB) or (BC) or (CA)Exist also in the other closed packed hcp structure (ABAB.) Tetrahedron and octahedron are the building blocks of close-packed structures 1st and 2d neighbours distances are charact

4、eristic of close-packed structuresTetrahedron:1st neighbours distance)d(mmOctahedron:1st and 2d neighbours distances)d2,d(mmmm6Close-packed nanoparticles Based on tetrahedronsonlynon periodic-based polytetrahedronsthe 2d neighbours distance is absent)d2(mmmmdBased on tetrahedrons and octahedronsmmd2

5、mmd1st and 2d neighbours distances are presentmmdperiodic or non periodic-based nanoparticles78Chemical techniques in mild conditions (T=300 - 400K) Co-decomposition by H2 of the two organometallic precursors in THF Size and dispersion stabilized by polymer (PVP), or ligand (HDA) Pure Co, M, Co3M1 C

6、o1M1, Co1M3 (close packed metals and alloys in cristalline state)SynthesisExperimentsStructure TEM, HREM Wide angle x-ray scattering Chemical distribution EFTEM EXAFS PVP- Co1Rh1 mean f f = 1.8 nm9CoRhCoCo3Rh1Co1Rh1Co1Rh3Rhr (nm)a. u.PVPhcpfccWAXS measurements of radial distribution functiondmmCoRur

7、 (nm)a. u.PVPhcphcp10CoRhPtRu3d4d5d678Specific non periodicStructures associated to Co andCo rich particlesPtCofcchcpa. u.CoPtPt1Co3Pt1Co1Pt3Co1Pt5Co1a. u.PtRuPtRufcchcpPt1Ru9Pt1Ru5Pt1Ru3Pt1Ru1Pt3Ru111First neighbour distance dmm as deduced fromWAXS measurements of radial distribution functionBulk f

8、ccRh concentration (at. %)nanoparticlesbulkfcchcpCoRhRu concentration (at. %)CoRu12HREM image showing a four fold symetry in the Co nanoparticle13Elemental map : cobalt rhodiumZero loss image - Co3Rh1dmm higher than in the bulk(shifted towards dmm in pure Rh) Rh-rich core ? EXAFS : oxidation at coba

9、lt edge, Rh environment for Rh EFTEM : on bigger particles (5- 6 nm) synthesized in HDA (hcp-fcc)14Co concentration (at. %)nanoparticlesbulk (B/at) (B/at) Co concentration (at. %)nanoparticlesbulk +CoRhCoRuMagnetic moment measurements151617IntroductionLevels of materials modelling10102103104105106fo

10、rce-fieldnumber of atomsaccuracytight-bindingab initio(parameter-free)quantum chemistryHartree-Fock, configuration interactiondensity functional theoryempirical potentialsmolecular dynamicsMonte-CarloCompromise between accuracy and computational tractabilityour study18ab initio DFTvStudy bonding mec

11、hanisms at the local levelvInfer general tendencies as a function of the chemical and structural ordersmall clusterslargeclusterssurfacesnowIntroductionsemi-empirical methodsvModel larger particlesvStudy properties that are not easily calculated in ab initio(CPU time)bulks19ab-initio Density Functio

12、nal Theory calculations Use of VASP code DFT-GGA (PW91) calculations : spin only, colinear magnetism PAW method (Projector Augmented Wave) advantages : very good accuracy, espec. for magnetic systemsreasonable computation time Calculation of ground state properties: cohesive energy Ecoh = E(system)

13、- E(isolated atoms) (eV/at.) geometry local / average magnetic moments (B/at.) Tests on bulks and small clusters of pure materials: OK Tests of different XC functionals20 4*3 parameters: , ij = CoCo, RhRh, CoRh rij0 : normalisation factors (first neighbours distances)rr(pjijj)rr(qijiijijijijijijeAeE

14、11220021 q N-body central force field model :v second-moment approximation of the tight-binding model: d-electron band contribution (n-body)v Born-Mayer short distance electron core-core repulsion (pair interaction)ijijqpAijij,Semi-empirical calculations21q Parameters fit: v Co-Co and Rh-Rh direct i

15、nteractions from F. Cleri, V. Rosato, (Phys. Rev. B, 48 (1993): Cohesive energy: Ec Elastic constants: Cij, Lattice parameter(s): a (c).v Co-Rh mixed interactions: Cohesive energies Lattice parametershypothetical ordered alloysat the studied compositionsofab-initioSemi-empirical calculations2223Whic

16、h structure for the pure Co particles? Polytetrahedral 100 150 at. Apparent four fold symetry axis Reproducing the WAXS curves24Polytetrahedral geometrical models105 atomsVan de Waal, Non Cris. Sol. 189ExpModel024681012r (A)137 atomsCorrespondingHREM simulationsDoy et al, Cond. Mat. 97 024681012r (A

17、)ExpModelView along the 4 fold symetry axis25Co particles (100N150)-3.9-3.85-3.8-3.75-3.7-3.6590100110120130140150160Number of atomsT = 0 K Potential energy Ep (eV/at.)MD StudyThermalisation in the liquid state1,2000,000 DM steps runSnapshots every 10,000 stepsQuenching to T = 0 K Selection of the l

18、owest energy configurations26 -3.78-3.76-3.74-3.72-3.7-3.680200400600800100012001400Cohesive energy at 0 K (eV/at.) Recovery and quenching (K) 00.20.40.60.81024681012300 K1200 Ka. u.d () 300K1200 KCo particle (105 at.)60-70 meVRadial distribution function27RELATIVE STABILITIES OF THE DIFFERENT Co MO

19、DELSPerfect octahedron50-60 meV30 meV-3.86-3.84-3.82-3.8-3.78-3.76-3.74-3.72-3.7020040060080010001200Ep (T=0K) T (recovery and quenching)105 polytetrahedron137 polytetrahedron129 octahedron137 octahedron105 octahedron147 (surface vacancy + top surface atom)Perfect polytetrahedral structures are unst

20、able relative to the defective octahedral structuresMost probable:Metastable glass like high temperaturedisordered polyhedral structures28024681012r (A)a. u.Exp105 at.129 at.137 at. High T105 at. 137 at. Polytetrahedra2930Which chemical order for the CoxRh1-x particles ? Co rich: Polytetrahedral Rh

21、rich: with octahedral sites 100 500 at. Strong non linear dmm dilation with x Reproducing the WAXS curvesMonte Carlo Metropolis study:Simulated Annealing with Atomic Relaxation and Exchanges between Co and Rh atoms.A large variety of initial states: Disordered, Ordered, Core/Shell fcc, hcp, Cuboctah

22、edral, Different initial temperatures31Co3Rh1(hcp) Co Rhdistance to centre()Co1Rh1(fcc)Co1Rh3(fcc)300K MCM Relaxation and ExchangeStructure and chemical order32Co1Rh1Co1Rh3 Co Rhdistance to centre()Co3Rh11600K TO 300 K MCM Relaxation and Exchange Simulated AnnealingStructure and chemical order33Radi

23、al Distribution Functions ExperimentalModel1600 K Simulated annealing300K Structure and chemical order34Intermetallic distance dmm(nm)Structure and chemical order0.250.260.2700.51d0 (nm)at % RhsimulationsexperimentalBulk :fcc modelfcc exp.hcp exp.353637magnetismmagnetism enhancedby geometricaldilati

24、oninteratomicdistancemagnetism enhanced by coordination reductioncoordinationGeometrical contraction at reduced coordinationz = 2z = 6z = 12Magnetism: from the single atom to the infinite cristalEcohMagnetic moment (B)Monolayer(111)Linear chainbulkcfcIsolatedatomz = 12z = 2z = 6Cobalt example3831.57

25、1.93CoCoFrom the atoms up to the bulks31.82RhRh0.00Magnetic moment per atom (B/atom)CoRhCoRhxCo = 0.52.291.601.430.40Magnetism: alloying effect 39Fcc(111) Rh surface doped with Covacuumtowards bulkS positionS-1 positionS-2 position-0,60-0,55-0,50-0,45-0,40-0,35-0,30-0,25-0,200,000,250,500,751,00SS-1

26、S-2single layer Co concentrationFormation energy (eV)subsurface position favoredMagnetism: surface effect 40Fcc(111) Rh surface with Co monolayervacuumtowards bulkThere is a surface Co segregationWith a marked preference for the subsurface positionS positionS-1 positionS-2 position-2,10-2,00-1,90-1,

27、80-1,70-1,60-1,50-1,401234567position of the Co MLmagneticnon-magneticAdsorption energy (eV/Co at)Magnetism: surface effect 411.001.502.002.503.003.504.004.500.000.200.400.600.801.00concentration en Co intracouchemoment magntique (B/atome)Moment magntique de CoCo subsurface Co preserves a high magne

28、tic moment1.001.502.002.503.003.504.004.500.000.200.400.600.801.00concentration en Co intracouchemoment magntique (B/atome)Co subsurfaceSubsurface position stable an highly favorable for magnetism: effective giant magnetic moments there are high induced moments on neibourghing RhsEffective magnetic

29、moment (Co+ induced Rh)Co subsurfacevacuumMagnetic enhancement at surface?Magnetism: surface effect 42 importance of size effectsRhCorat =1.25 1.35 1.39 PtAtomic radiusCo Core ?ConRh13-n centred icosahedral clustersdradialdsurfaceIcosahedral specific geometry : radial bonds in compression surface bo

30、nds in tensionMagnetism: size effect 43Ecoh (eV/at)Sz (B)RhColegend3,34313,7921Co Core and magnetic enhancement?Magnetism: size effect 3,87153,44213,30213,7821440.000.501.001.502.002.503.003.500.000.250.500.751.00Co concentrationaverage magnetic moment (B/atom)N=2N=3N=4N=5N=6N=7N=13bulksatomsisolate

31、d atomsdimersbulksN= 4 4N= 6 4N= 5 4N= 3 4N= 7N=13 4 Strong enhancement in nanoparticles compared to bulkCoMRhNMean magnetic momentComparaison CoMRhN et CoMPtNMagnetism: size effect 45Rh37Co Sz = 18 B = 0.47 B/atom Remarquable magnetic enhancement : eff(Co)=16 B but antiparallel moments at the surfa

32、ceVers des particules de taille exprimentaleRh38 Sz = 2 B = 0.05 B/atom (exp. : 0.160.13 B/atom for Rh34)Magnetic core poorly magneticLocal magnetic moments chart (B)Local magnetic moments chart (B)Magnetism: size effect Co Core and magnetic enhancement?46SynthesisMagnetic measurements47Intermetalli

33、c distance dmm(nm)0.10.50.90.240.260.280.3dC (nm)Co-Rh0.240.260.280.30.10.50.9d0 (nm)dC (nm)Rh-Rh0.10.50.90.240.260.280.3dC (nm)Co-CoCo1Rh1Structure and chemical order48Low THigh TExp105 at.129 at.137 at.024681012a. u.r (A)CoModel490,000,100,200,300,400,500,600,700,800123456number of Co first neighb

34、orsRh_SRh_S-1Rh_S-21012010Local Moments induced on Rhby Co in S-1 positionCo magnetic momentvs Co layer concentrationMagnetic enhancement at surface?Magnetism: surface effect 503d/4d et 3d/5d Alloys Stability: High magnetic anisotropy 4d,5d elementExample: CoxRh1-x nanoparticles Sensitivity:High mag

35、netic moments per atom 3d elements High density:Small size nanoparticles51q Cobalt : hcp (T750K)q Rhodium : fccfcchcpq Alloy: hcp (x50,5%) Disordered solid solutionW. Koster, Z. Metallkunde, 43 (1952)Bulk crystals: Co, Rh and Co1-xRhx52(fcc)Octahedron:1st and 2d neighbours distances)d2,d(mmmmABCABC (111) stackingABCATetrahedron:1st neighbours distance)d(mmClose packed structures :face centered cubic (fcc) and hexagonal compact (hcp) 53Chemical techniques in mild conditions (T=300 - 400K) Co-decomposition by H2 of the two organometallic precursors in THF Size and dispersion stabilized b