Electronic band structure of photoanode heterojunctions dedicated to water splitting

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1 Electronic band structure of photoanode heterojunctions dedicated to water splitting M. Rioult 1, H. Magnan 1, D. Stanescu 1, P. Le Fèvre 2, A. Barbier 1, 1 : CEA Saclay / DSM / IRAMIS / SPEC / LISO, F Gif sur Yvette Cedex, France 2: Synchrotron SOLEIL, Saint Aubin, France Maxime Rioult October 16 th, 2014

2 Presentation outline I. Hydrogen as an energy carrier II. III. Solar water splitting with photoelectrodes 1. Physics 2. Our approach Thin films deposition and characterization 1. RHEED 2. XPS 3. Photoelectrochemistry CEA 10 AVRIL 2012 IV. Resonant PhotoEmission Spectroscopy (RPES) 1. Introduction to the technique 2. Results at the Fe L 3 edge 3. Results at the Ti L 3 edge PAGE 2 3 NOVEMBRE 2014

3 I. Hydrogen as an energy carrier Solution Various small scale applications: cameras, USB, smartphone charge Electric generators House powering Power up to 500 W Larger scale applications: Transit BC Fuel Cell Fleet (Buses for 2010 Vancouver Olympics) Toyota FCV: 100 kw 136 ch, 500 km battery life, ~ 50 k$ Solar energy storage Maxime Rioult October 16 th, 2014 Page 3

4 I. Hydrogen as an energy carrier Steam reforming: Hydrogen production: Network-assisted water electrolysis: PV-assisted water electrolysis: Credit: storedsolar.com Greenhouse gas emission Energy intensive Expensive & heavy going devices Solution: direct solar water splitting Energy storage O 2 H 2 Maxime Rioult Credit: SunCatalytix October 16 th, 2014 Page 4 Credit: EPFL / LPI / Alain Herzog

II.1. Solar water splitting using semiconducting photo electrodes

, 2007 V Photoanode : water oxidation oxygen production

production Need of external bias From Pinaud et al.

absorption Appropriate band structures with respect to water

5 II.1. Solar water splitting using semiconducting photo electrodes V E e- Metal cathode From von de Krol et al., 2007 V Photoanode : water oxidation oxygen production Photocathode Cathode : water : water reduction hydrogen production Need of external bias From Pinaud et al., 2014 Criteria for a good photo anode: Good solar spectrum absorption Appropriate band structures with respect to water energy levels Environment friendly Economy friendly Our choice : α Fe 2 O 3 (hematite) Maxime Rioult October 16 th, 2014 Page 5

II.1. Solar water splitting using a heterojunction photo anode Hematite (α Fe 2 O 3 ): good stability & absorption, but poor transport/transfer properties Ti doping greatly improves performances!

6 II.1. Solar water splitting using a heterojunction photo anode Hematite (α Fe 2 O 3 ): good stability & absorption, but poor transport/transfer properties Ti doping greatly improves performances! TiO 2 : better transport properties, but low absorption What about a TiO 2 /Ti doped Fe 2 O 3 heterostructure? Magnan et al., Appl. Phys. Lett , Rioult et al., J. Phys. Chem. C , Maxime Rioult October 16 th, 2014 Page 6

II.2. Our approach Atomic Oxygen Plasma Assisted Molecular Beam Epitaxy:

characterizations Thicknesses between 5 and 50 nm Epitaxy at ca.

characterizations: Lab + synchrotron techniques Cristallographic and

Individual cells Atomic oxygensource Samples chosen: Constant total

7 II.2. Our approach Atomic Oxygen Plasma Assisted Molecular Beam Epitaxy: Model thin films elaboration technique UHV (10 10 mbar) + in situ characterizations Thicknesses between 5 and 50 nm Epitaxy at ca. 450 C Easy control of growth parameters Solid physics approach advanced characterizations: Lab + synchrotron techniques Cristallographic and electronic structures Photoelectrolysis properties. Individual cells Atomic oxygensource Samples chosen: Constant total thickness of 20 nm (same absorbant volume) Comparison with single layers reference films. Substrate: Heterojunctions: 5 nm 15 nm 10 nm 10 nm Single layers: 20 nm Multi disciplinary studies of model samples. Thin film deposition + characterization Maxime Rioult October 16 th, 2014 Page 7

Screen Reciprocal space 20 nm Typical patterns 20 nm Ideal 3D growth 2D growth

8 III.1. Epitaxial TiO 2 /Ti doped Fe 2 O 3 photo anodes : RHEED Incident e Real space Sample Screen Reciprocal space 20 nm Typical patterns 20 nm Ideal 3D growth 2D growth Polycrystal 10 nm 10 nm 2D (0001) Ti:α Fe 2 O 3 /Pt(111) epitaxy 3D rutile (100) TiO 2 epitaxy Monocristalline growth of heterostructures by OPA MBE Maxime Rioult October 16 th, 2014 Page 8

5 nm 15 nm 10 nm 10 nm 20 nm 20 nm Same valence state for Ti : Ti 4+ (Ti2p 3/2 @ 459.

9 III.2. Epitaxial TiO 2 /Ti doped Fe 2 O 3 photo anodes : XPS Ti2p core level Fe2p core level 20 nm 5 nm 15 nm 10 nm 10 nm 20 nm 20 nm Same valence state for Ti : Ti 4+ (Ti2p ev) Same valence state for Fe : Fe 3+ (Fe2p ev and Fe 3+ satellite) Fe doped TiO 2 : presence of a satellite, but hard to conclude Maxime Rioult October 16 th, 2014 Page 9

1M (ph = 13) Photocurrent = I light I dark Direct image of water splitting

10 III.3. Epitaxial TiO 2 /Ti doped Fe 2 O 3 photo anodes : photocurrent Electrolyte : NaOH 0.1M (ph = 13) Photocurrent = I light I dark Direct image of water splitting Photocurrent decrease with TiO 2 thickness increase Slight decrease for the onset voltage. Loss of photocurrent : band structure, interface? Maxime Rioult October 16 th, 2014 Page 10

IV.1. Resonant PhotoEmission Spectroscopy of the valence band Coherent interferences between these two waves : resonant phenomena enhanced signal Photoelectric effect Auger relaxation (also called

11 IV.1. Resonant PhotoEmission Spectroscopy of the valence band Coherent interferences between these two waves : resonant phenomena enhanced signal Photoelectric effect Auger relaxation (also called auto ionization) If hν is around the Fe L 3 edge increasing the signal of photoelectrons coming from Fe If hν is around the Ti L 3 edge increasing the signal of photoelectrons coming from Ti Valence band From Yamamoto and Matsuda, Journal of Phys. Soc. of Jap , «Selective» photoemission enhanced by resonance Characterization of multi layers (interfaces, bottom layers) Characterization of dopants Need of ultra thin films (cf. electrons mean free path) Need of synchrotron radiation (high flux + tunable photon energy) Maxime Rioult Page 11 October 16 th, 2014

IV.1. RPES : 2D valence band mapping @ a L 3 edge Fe L 3 edge

units) Signal gain 2 Signal gain 7 Each line is a valence band

12 IV.1. RPES : 2D valence band a L 3 edge Fe L 3 edge Before edge Resonance at Fe L 3 edge After edge Counts/s (arb. units) Signal gain 2 Signal gain 7 Each line is a valence band spectra at a particular photon energy Maxime Rioult Page 12 October 16 th, 2014

thickness Calculations from Magnan et al.

13 IV.2. RPES : Fe L 3 edge Calculations: Different Fe absorption edge shapes Increasing of the Fe 2+ contribution when increasing TiO 2 thickness Calculations from Magnan et al., Phys. Rev. B 2010, 81, Maxime Rioult Page 13 October 16 th, 2014

IV.2. RPES : valence band spectra @ Fe L 3 edge hν = 710.9 709.

oxide (except for Fe doped TiO 2 ) Along edge : appearance of defect states in

14 IV.2. RPES : valence band Fe L 3 edge hν = ev Calculations: Before edge : gap of upper layer Along edge : gap of iron oxide (except for Fe doped TiO 2 ) Along edge : appearance of defect states in the gap for heterojunctions and Fe doped TiO 2 (likely Fe 2+ ) Maxime Rioult Page 14 October 16 th, 2014

15 IV.2. RPES : 2D valence band Fe L 3 edge Calculations: Before edge : gap of upper layer Along edge : gap of iron oxide (except for TiO 2 ) Along edge : appearance of defect states in the gap for heterojunctions (coloured zone on pictures) Maxime Rioult Page 15 October 16 th, 2014

IV.2. RPES : results @ Fe L 3 edge Characterization of the bottom layer thanks to small thickness or roughness of the upper layer; RPES revealed iron defects within the band gap for heterojunctions

16 IV.2. RPES : Fe L 3 edge Characterization of the bottom layer thanks to small thickness or roughness of the upper layer; RPES revealed iron defects within the band gap for heterojunctions (likely Fe 2+ defects); The defect concentration is higher when increasing the TiO 2 thickness in heterojunctions; The type of defects is likely the same in Fe doped TiO 2 inter diffusion at the interface These defects may lower the photocurrent by inducing the recombination rate. Maxime Rioult Page 16 October 16 th, 2014

IV.3. RPES : XAS @ Ti L 3 edge Calculations: Slight differences between single layers and

contribution when increasing thickness in heterojunctions.

17 IV.3. RPES : Ti L 3 edge Calculations: Slight differences between single layers and heterojunctions : Anatase like shape for our single TiO 2 films Increasing of the rutile contribution when increasing thickness in heterojunctions. Mixed structure for Ti doped Fe 2 O 3 Laskowski and Blaha, Phys. Rev. B 2010, 82, Maxime Rioult Page 17 October 16 th, 2014

18 IV.2. RPES : 2D valence band Ti L 3 edge Along edge : defects within the band gap: with TiO 2 thickness in heterojunctions with Fe doping in single layers No significantresultwith Ti doped hematite (no resonance): Maxime Rioult Page 18 October 16 th, 2014

IV.3. RPES : Results @ Ti L 3 edge X ray absorption spectra RHEED pa erns during films growth Same RHEED for single layers and heterojunctions: rutile (100) Anatase like XAS for single layers,

19 IV.3. RPES : Ti L 3 edge X ray absorption spectra RHEED pa erns during films growth Same RHEED for single layers and heterojunctions: rutile (100) Anatase like XAS for single layers, rutile/anatase mix for heterojunctions Mix of the two structures, bigger anatase component. RPES revealed defects within the band gap for single layers and heterojunctions; Heterojunctions : defect concentration when the TiO 2 thickness; Single layers : Fe doping the defect concentration (seen at Ti L 3 edge); These defects may lower the photocurrent by increasing the recombination rate. Maxime Rioult Page 19 October 16 th, 2014

Results can be easily visible with maps.

20 V. Ongoing work and perspectives About RPES: Resonant PhotoEmission Spectroscopy is a convenient technique for the investigation of thin film heterojunctions electronic structure One can have access to defects or gap variations; Results can be easily visible with maps. About our system: TiO 2 & Ti doped hematite is not an optimal system Band engineering is needed: tuning of band gap, valence & conduction bands. But beware of interfaces! Maxime Rioult Page 20 October 16 th, 2014

21 Thank you for your attention RPES & XAS measurements: Work support: 3 NOVEMBRE 2014 Commissariat à l énergie atomique et aux énergies alternatives Centre de Saclay Gif-sur-Yvette Cedex T. +33 (0) Etablissement public à caractère industriel et commercial RCS Paris B

12% yield, 1000 W/m² incident flux Mixing system (1 cm² surface): 2.40 ml H 2 / h Water splitting with our material : ~ 2.5 ma/cm² = 1.

22 Appendix. Solar water splitting or PV + electrolyzer? For hydrogen PRODUCTION (used as fuel) Classic electrolyzer : 1 m 3 H 2 / h = 1000 L H 2 /h with 5 kw supply Cellule solaire commerciale : 12% yield, 1000 W/m² incident flux Mixing system (1 cm² surface): 2.40 ml H 2 / h Water splitting with our material : ~ 2.5 ma/cm² = 1.05 ml H 2 / h Does not take into account: Technologic simplicity of water splitting Financial cost Electrolyte optimization Need for external bias PV & electrolyzers technology maturity Competitive! Maxime Rioult Page 22 Oct 16 th, 2014

23 Appendix. All heterojunctions 20 nm 5 nm 15 nm 10 nm 10 nm 10 nm 10 nm 20 nm Maxime Rioult Page 23 Oct 16 th, 2014

24 Appendix. All heterojunctions Small bias : Large bias : Maxime Rioult Page 24 Oct 16 th, 2014

25 Appendix. RPES : valence band Ti L 3 edge hν = ev Calculations: Before edge : heterojunctions single layer Along edge : defects within the band gap Along edge : Increase of defects concentration for the anatase structure Maxime Rioult Page 25 October 16 th, 2014

26 Appendix. Fe doped TiO 2 photoelectrochemistry 20 nm 20 nm Maxime Rioult Page 26 Oct 16 th, 2014

Schottky Tunnel Contacts for Efficient Coupling of Photovoltaics and Catalysts

Schottky Tunnel Contacts for Efficient Coupling of Photovoltaics and Catalysts Christopher E. D. Chidsey Department of Chemistry Stanford University Collaborators: Paul C. McIntyre, Y.W. Chen, J.D. Prange,