Overview of undergraduate masters project research (given at first WMD group meeting in Bath)

1. First principles calculations of
electron mobility in
nanocrystalline TiO2
Suzanne Wallace and Dr. Keith Mckenna
Department of Physics, University of York, Heslington, York, UK

2. DFT+U for correlated systems
 Coulomb interaction of localised electrons
 electron density NOT uniform
 Employ DFT+U (UTi = 4.2 eV)
 Correct for SI error
 U: adjustable correcting potential
 4.2 eV: produces good agreement with spectroscopic data [1]
[1] Morgan, B. J., & Watson, G. W. (2009). J Phys Chem C, 113(110), 7322–7328.

3. Finitex-dimension
∆E = Es - Eb
Eb
Es
Localising electrons
 Initialise magnetic moment at the site
 Create a potential well by dilating bonds
 Increase U-parameter (if necessary)

4. Wallace, S. K., & McKenna, K. P. (2014). Grain Boundary
Controlled Electron Mobility in Polycrystalline Titanium
Dioxide. Advanced Materials Interfaces, n/a–n/a.
doi:10.1002/admi.201400078

5. TiO2 anatase grain boundary
Deep trap at GB relative
to bulk:
-0.46 eV
(c.f. rutile GB: -0.2 eV)

6. Nanoparticle surfaces
Low index rutile surfaces: (100), (001), (101), (110), (111)
∆E = Es - Eb

8. (110) surface
subsurface trap of -0.28 eV

9. (001) surface
surface trap of -0.4 eV

10. Repulsive surfaces

11. TiO2 rutile nanoparticles
(a) (b) (c) (d)
(110)
(101)
(001)
(100)
(110)
(001)
(101)
(101)(100)
a  equilibrium nanoparticle shape b  trapping shape
c  strongly repulsive shape d  weakly repulsive shape
K. Momma and F. Izumi, "VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data," J. Appl. Crystallogr., 44, 1272-
1276 (2011).

12. Correlation between trapping energy
and on-site electrostatic potential
(001) site 9
(111) site 13

13. Surface models
(110) (101) (100) (001) (111)
4-coordinated surface sites

14. Conclusion
[2] Barnard, a., & Zapol, P. (2004). Effects of particle morphology and surface hydrogenation on the phase stability of TiO2.
Physical Review B, 70(23), 235403. doi:10.1103/PhysRevB.70.235403
[3] Dinh, C., Nguyen, T., Kleitz, F., & Do, T. (2009). Shape-Controlled Synthesis of Highly Crystalline Titania Nanocrystals. ACS
Nano, 3(11), 3737–3743.
 Nanoparticle mostly composed of (110) and (101) surfaces
 (110)  strong subsurface trapping
 (101)  weakly repulsive
Reduce area of (110) relative to (101) for
improved electron transport
e.g. by surface hydrogenation [2]
or capping [3]