Solution Phase Routes to Transition Metal Doped TiO 2

Transcription

1 Solution Phase Routes to Transition Metal Doped TiO 2 Spintronics Symposium J. Daniel Bryan Gamelin Research Group July 10 th 2003

2 The Challenge Cobalt enrichment & Cluster formation Chambers, S. et al, App Phys Lett (2003), 82, 8, 1257

3 Why Alternate Routes to Diluted Magnetic Semiconductors (DMSs( DMSs)? Wet chemical control of oxdiation states Ability to template novel architectures using solution phase precursors Study magnetic exchange at various length scales: nano, meso, micro Low cost

4 Nanocrystal DMS Length Scales nano meso micro DMS nanocrystal < 10nm electronically coupled cluster of nanocrystal continuous film of DMS nanocrystals nm > 00 nm Colloids Clusters Films

5 Sol-Gel Basics Sol A stable suspension of colloidal particles within a liquid (2 200 nm) Gel A porous 3-dimensional interconnected solid network that expands in a stable fashion throughout a liquid medium and is limited by the size of the container. nanoparticle systems via Sol- Gel route means hydrolysis & condensation mechanism We totally avoid the Gel step. Ti + hydrolysis condensation TiO 2

6 Solution Phase Routes to TiO 2 Nanocrystals Non-hydrolytic lyothermal (Colvin et al.) elimination of surface Ti-OH (traps, less strongly reducing) elimination of surface bound H 2 O Catalyzed Sol-Gel (hydrolytic) Acid/Base/Water (many authors) Inverse micellular Sol-Gel (Lin et al., Bryan et al.) Pseudo-nonhydrolytic Controlled hydrolysis

7 Lyothermal Non-Hydrolytic Route TiX (l) + Ti(OR) (l) X etherolysis X M X X TOPO, airfree 300 C, 5 min δ + M(OR) z-1 O 2TiO 2 (s) + RX X X -M-O M (OR) z-1 + RX X R 5 nm particles pros: highly crystalline, rapid cons: difficult to dope, transition metal precursors difficult

8 1) Hydrolysis H H δ- O δ + + M(OR) O M(OR) δ + z z-1 H 2) Condensation oxolation How to Create a Ti δ+ -O-Co δ+ Bond Part I: Partial Charge Model H -M-O M proton δ + + O rearrangement R -M-O M + HOR H leaving group O R δ + M(OR) z-1 O H + ROH Species δ(m) N N-z Si(Oet) Ti(Oet) δ (M) partial charge is related to the Pauling electronegativity greatly influences hydrolysis rates, Ti is much faster than Si, can t get a mixed Si/Ti species by normal Sol-Gel in excess H 2 O.

9 AOT How to Create a Ti δ+ -O-Co δ+ Bond sodium bis(2-ethylhexyl) sulfosuccinate Part II: Limiting the Water W 0 inverse Micelles have: surfactant, water, non-polar phase ( moles H O) ( moles surfact) = 2 at low W 0, little or no free unbound H 2 O. at high W o precipitation occurs limited hydrolytic route to TiO 2 W 0 =.5, r w = 1. nm, r h =2.3 nm* *Maitra, A. J Phys Chem, (198), 88, 5122 Co 2+

10 Inverse Micelle Preparation Doping During Hydrolysis stir Inverse Micelle + Cobalt air-free transfer Ti precursor/ co-surfactant light rose to light purple exposed to air teflon lined Autoclave ambient atmospheric overhead (25 C) ~ C 2 hours light purple to light blue colloidal doped nanocrystals

11 Spectroscopic Characterization of Synthesis Absorbance (a.u.) x Energy (10 3 cm -1 )

12 Surface Chemistry - Cobalt Speciation Internal & Surface TOPO ppt O=P ligand exchange Trioctyl Phosphine Oxide TOPO O=P = TiO 2 stearyl phosphate* -O O=P-O -O as made pre-treated TOPO/SP treated % (by mol) dopant (% mol) (% mol) Db2_107 Co:TiO 2 7 ~5 7 3

13 Absorption Spectroscopy 2.0 Absorbance (a.u.) :TiO 2 TiO Wavelength (nm)

14 Nanocrystal Size by TEM and X-RayX nm

15 Summary Controlling both the type of water available and the hydrolysis rate can force an interaction between both metals and give a suitable in-situ mixed-metal precursor Autoclave treatments crystallize in solution Surface cleaning of nanocrystals improve speciation homogeneity. Other metals can be included: Ni 2+, Cr 3+ & Mn 2+.

16 absorbance (a.u.) magnetization (a.u.) K abs x : TiO 2 : ZnO energy (10 3 cm -1 ) 5 K 6 T energy x10 3 (cm -1 ) MCD of TOPO treated Ti 0.97 O 2 Site Symmetry Investigation Co 0.03 Ti magnetization (a.u.) 0.17 M TiO 2 e= 0.2 Co Ti 0.97 O 2 e = 1.2 Co(H 2 O) cm cm BH/2kT

17 Interstitial Sites in the Anatase Lattice Anatase with interstitials two interstitial sites in anatase from experimental Cu + /Al 3+ doping of anatase 8d b

18 b Interstitial Site in Anatase A 1.93 A Symmetry of b interstitial site is -m2 or D 2d. Elongated axial ligands, appears more like coord. Distorted Square planar C 2 Ionic Radii Cu pm Ti + 75 pm 89 pm evidence from Cu 2+ implanted anatase from EPR I.V. Grebenshikov, et al. Phys. Stat. Sol. b, , (1976).

19 8d Interstitial Site Site Symmetry 2/m or C 2h A A Compression of axial ligands and elongation of equitorial ligands. No experimental evidence observed

20 Terms and Orbitals for D 2d geometry F A 2g T 2g T 1g O h B 1g E 1g B 1g B 2g E 1g D h simulated A cm -1 E 8226 cm -1 B B E D 2d 723 cm cm -1 0 e g t 2g O h D h x 2 -y 2 z 2 xy (zy, xz) x 2 -y 2 z 2 xy (zy, xz) D 2d a b b e increasing distortion high symmetry octahedral low symmetry anatase site

21 Cobalt Speciation XANES / EXAFS XANES EXAFS normalized absorption nc film Co metal from analysis Co O 2.08 C.N. = 5.1 Oxygens E-E 0 (ev) 0 collaboration with Dr. Steve Heald Argonne National Lab - APS

22 Nanocrsytal DMS Length Scales nano meso micro DMS nanocrystal < 10nm electronically coupled cluster of nanocrystal continuous film of DMS nanocrystals nm > 00 nm Colloids Clusters Films

23 Co:TiO 2 Nanocrystal Magnetism TiO 2 nanoscale DMS nanocrystal < 10nm 5 nm 2.0 Absorbance (a.u.) TiO Co: TiO Wavelength (nm) 800 magnetization (10-3 emu/g) K Field (koe) single isolated colloids (capped) paramagnetic

24 Co:TiO 2 Nanocrystal Crystalline Sizes meso scale 22 nm 600 C 30 min electronically coupled cluster of nanocrystals Intensity (a.u.) 5 nm degrees 2θ 60

25 Co:TiO 2 Magnetism at the Mesoscale µ Β /Co x K 22 nm 5 nm 300K 22 nm 5 nm µ B /Co x Field (koe) Field (koe) -8 Ferromagnetism increasing with annealing

26 Co:TiO 2 Nanocrystal Microscale spin coating/annealing nanocrystal precursor micro continuous film of DMS nanocrystals µ B / K polycrystalline sapphire substrate Field (koe) K µ B / Field (koe)

27 Summary Controlling both the type of water available and the hydrolysis rate can force an interaction between both metals and give a suitable in-situ mixed-metal precursor Autoclave treatments crystallize in solution Surface cleaning of nanocrystals improve speciation homogeneity. Magnetism can be turned on using varied length scale assemblies Thermal treatment may be adding carriers via the creation of defect/vacancies Thin film greater degree of ferromagnetism substrate influence