Electronic structure of energy materials measured by synchrotron radia5on

Transcription

1 Electronic structure of energy materials Outline Electronic structure of energy materials measured by synchrotron radia5on Eamon McDermo,, M.Sc Candidate Solids4Fun Hearing - 18 June 2012 Outline Experimental techniques The Beamteam / Synchrotron Radia5on / X- ray Spectroscopies Photocatalyst materials Intro / GaN:ZnO / poly(triazine imide) (PTI) Other experiments Li XANES / CdZnTe XANES/XRF/Laue XRD / Novel Ferrocene XANES

2 Electronic structure of energy materials The Beamteam The Beamteam The materials research group in condensed mauer at the University of Saskatchewan, Canada We use synchrotron radia5on to study electronic structure of novel materials Biomaterials, high TC superconductors, energy materials, spintronic materials and other transi5on metal compounds

3 Electronic structure of energy materials The Beamteam The Beamteam

4 Electronic structure of energy materials Synchrotron Radiation Synchrotron Radia5on An RF cavity is synchronized with the orbit of electrons travelling around a booster ring, accelera5ng the beam to rela5vis5c speeds (CLS: 2.9 GeV, c) The beam is steered through magnets, releasing broadband radia5on Radia5on is coherent, polarized, highly tunable and extremely brilliant ( photons per second on a sub- mm spot a5er monochromater) Compact rings such as Advanced Light Source and Canadian Light Source are bright at soa X- Ray wavelengths (under 2000 ev)

5 Electronic structure of energy materials Synchrotron Radiation Synchrotron Radia5on Brilliance allows samples to be targeted with highly monochroma5c X- ray beams Rowland circle geometry discards flux in exchange for energy resolu5on For example, a good emission measurements can end up with < 5000 counts per second on a CCD Emission spectrometer configura5on Typical spherical gra5ng monochromator configura5on

6 Electronic structure of energy materials X-ray Spectroscopies X- ray Spectroscopies X- rays can be used to probe the occupied and unoccupied states that bound a semiconductor band gap Energy Vacuum Level Conduc5on Band Valence Band Core level

7 Electronic structure of energy materials X-ray Spectroscopies XPS X- rays can be used to probe the occupied and unoccupied states that bound a semiconductor band gap X- ray Photoelectron Spectroscopy (XPS) Energy e - out Vacuum Level Conduc5on Band Valence Band hνin Core level

8 Electronic structure of energy materials X-ray Spectroscopies XANES X- rays can be used to probe the occupied and unoccupied states that bound a semiconductor band gap X- ray Photoelectron Spectroscopy (XPS) X- ray Near Edge Absorp5on Spectroscopy (XANES) Energy hνin hν or e - out I(Ein) Vacuum Level Ein Conduc5on Band Valence Band Core level

9 Electronic structure of energy materials X-ray Spectroscopies XES X- rays can be used to probe the occupied and unoccupied states that bound a semiconductor band gap X- ray Photoelectron Spectroscopy (XPS) X- ray Near Edge Absorp5on Spectroscopy (XANES) X- ray Emission Spectroscopy (XES) Energy hνin hνout Vacuum Level Conduc5on Band Valence Band Eout I(Eout) Core level

10 Electronic structure of energy materials X-ray Spectroscopies X- ray Spectroscopies Selec5on rules: Δl ± 1 in dipole approxima5on: s p d Spectroscopic intensi5es scale with final state density of states (DOS) according to Fermi s golden rule: I(E) Σ <Ψi A(r) p Ψf> ² δ(e- Ef) Monochroma5c X- ray spectroscopy therefore probes DOS subject to broadening (core- hole life5me, equipment, etc) Final state rule: XANES and XPS are perturbed by a core hole XES probes ground state DOS (very few techniques do!)

11 Electronic structure of energy materials Photocatalyst Materials Photocatalyst Materials My interest is photocataly5c materials Vacuum Level Not true catalysts, but produce photocurrent in order to drive redox reac5ons Water splirng: requires 1.23 ev split + over- poten5al 4.44 ev 1.23 ev 2 ev Conduc5on Band 2H + H2 2O 2- O2 Valence Band Solar spectrum is centred at 2.5 ev, aim below this

12 Electronic structure of energy materials GaN:ZnO GaN:ZnO ZnO has a conduc5on band posi5on that can photo- reduce H + as part of a water- splirng reac5on Combining ZnO with GaN (both wurtzite structures) has been shown to decrease material s band gap from ~3.3 ev to between ev Is a new material phase forming with a reduced band gap? Proposed GaZn oxynitride superlarce structure

13 Electronic structure of energy materials GaN:ZnO GaN:ZnO Energy XES and XANES reproduce the observed band gap However XES is surprisingly featureless for a quaternary system valence band Valence Band Conduc5on Band Vacuum Level Measurements most closely resemble pure GaN and ZnO with shiaed bands

14 Electronic structure of energy materials GaN:ZnO GaN:ZnO E. J. McDermott, E. Z. Kurmaev, T. D. Boyko, L. D. Finkelstein, R. J. Green, K. Maeda, K. Domen, and A. Moewes, Structural and Band Gap Investigation of GaN:ZnO Heterojunction Solid Solution Photocatalyst Probed by Soft X-ray Spectroscopy, Journal of Physical Chemistry C, 116 (14), , (2012).

15 Electronic structure of energy materials PTI poly(triazine- imide) - PTI Planar carbon- nitride turns out to also photo- split water. A non metal catalyst. 2.7 ev band gap. Working on a related material with apparent lower band gap (PTI) Difficult material problem: Wang, X., Maeda, K., et al.. Nature materials, 8(1), (2009) Tiny sample of material (< 1g) XRD gives C- N structure Li posi5ons all equally probable No H informa5on low reflec5vity defeats UV/Vis reflectance Inequivalent N sites contribu5ng to deconvolute in XES

16 Electronic structure of energy materials PTI poly(triazine- imide) - PTI

17 Electronic structure of energy materials Other Experiments Other Experiments... Cu/Zn defect in CdZnTe single crystal Redlen technologies, Canada Comparison of Li absorp5on data (XAS vs XRS) LBNL & Stanford SLAC, USA K 1s XAS of strained ferrocene monomers in solu5on U of Saskatchewan, Canada

18 Electronic structure of energy materials Fin Acknowledgements / Ques5ons Prof. Alex Moewes and the rest of the Beam Team PTI collabora5on - Eva Wirnhier, Dr. Wolfgang Schnick (Ludwig- Maximilians- Universität, München) GaN:ZnO collabora5on - Dr. Ernst Kurmaev (Russian Academy of Sciences - Ins5tute of Metal Physics), Dr. Kazuhiko Maeda, Dr. Kazunari Domen (U. of Tokyo) FUNding - NSERC, the Canada Research Chair program and the U. of Saskatchewan Graduate Teaching Fellowship program Compu5ng Resources - Westgrid and Compute Canada Facili5es - Advanced Light Source, Canadian Light Source

19 Electronic structure of energy materials Bonus video Photocatalysis in Ac5on