1. HEMATITE NANOWIRES FOR SOLAR WATER SPLITTING: DEVELOPMENT AND STRUCTURE OPTIMIZATION J. Azevedo 1,2, C.T. Sousa 1, M.P. Fernandez-García 1, A. Apolinário 1, J. M. Teixeira 1, A.M. Mendes 2 and J.P. Araújo 1 U NIVERSITY OF PORTO MAP-F IS P H D R ESEARCH C ONFERENCE Porto, January 20, 2011 1 IN-IFIMUP and Dep. Física, Rua do Campo Alegre 687, 4169-007 Porto, Portugal 2 LEPAE – Dep. de Engenharia Quıímica, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal.

2. Outline Introduction Fabrication Methods Results Conclusions and Future work 2

3. Fossil fuels are rapidly being depleted; Solar energy is the most available renewable energy source; Covering 0.16% of the surface of the earth with 10% efficient solar cells would satisfy our present energy requirements; Solar cells only generate electricity during daytime; Hydrogen Economy powered by photoelectrochemical cells 3

4. Photoanode Counterelectrode 4 + + + + + OH - H2H2 O2O2 Photoelectrochemical Cell 1) Absorption of light near the surface of the semiconductor creates electron-hole pairs. 2) Electrons (majority carriers) are conducted to a metal electrode (typically Pt) where they combine with H + ions in the electrolyte solution to make H 2 : 3) Holes (minority carriers) drift to the surface of the semiconductor (the photo anode) where they react with water to produce oxygen: 4) Transport of H + from the anode to the cathode through the electrolyte completes the electrochemical circuit. 1.23 eV http:/newenergyandfuel/com/2011/05/11/

5. Hematite (α-Fe 2 O 3 ) as photoanode Advantages High chemical stability Low cost Adequate band-gap of 2.2 eV High theoretical efficiencies Disadvantages Low absorption coefficient Red shifted band-gap Rapid electron- hole recombination 5 Nano structuring Larger diffusion coefficients More efficient charge collection Bang gap blue shift Independence on light absorption coefficient Nanowires

6. Objectives 6 Create a highly oriented alumina template Fill the template with Iron nanowires Anneal the iron into hematite

7. ELECTRODEPOSITION 7

8. Pulsed Electrodeposition Pulsed Mode Alumina Aluminium Barrier layer thinning Pulsed deposition 8

9. Perfis de Deposição 9 I, IIIIIIV V VII, IIIII IVVVIVII VI V IV III I, II a) b) c)

10. 10

11. Pore modulation 11

12. 12 10 µm 1 µm

13. OXIDATION 13

14. Atmosphere dependence on oxidation Comparison of oxidation state between different atmospheres: a) left in ambient conditions for 2 months, b) and c) annealing for 6 h at 600 o C in air and oxygen, respectively. The α-Fe 2 O 3 Bragg reflections are shown with their respective Miller indices. 14

15. Temperature dependence on oxidation Annealing temperature study on samples with 60 μm thickness. Between 400 o C - 600 o C the samples were annealed together with the Al substrate. For annealing’s above 600 o C, the substrate was removed prior to oxidation, due to the Al melting point 15

16. Reference samples X-ray absorption spectroscopy measurements at the Fe k-edge in transmission 16

17. Comparison with prepared samples 17 > Temperature > Time O2 atmosphere < Temperature < Time Air atmosphere Higher Oxidation Lower Oxidation

18. (a), (c) and (d) SEM images of annealed NWs; (b) EDS profile of annealed NWs. 18

19. Conclusions  Fabrication of highly organized alumina templates;  Optimization of an industrially viable Fe nanowires deposition method;  Fabrication of Fe nanowires with high degree of organization with lengths from 1 μm to 10 μm up to 99 % of pore filling; 19

20. Conclusions  Enlarged nanowire surface area through pore modulation;  Oxidation studies indicate the presence of hematite after an annealing. 20 1 µm

21. Future Work  Expose only a fraction of the nanowires by a partial removal of the alumina template;  Test solar water splitting efficiencies;  Reproduce results in TiO 2 templates. 21

22. Acknowledgments 22

23. Thank you for your attention João Carlos Azevedo azevedo.jcam@alunos.fc.up.pt https://sites.google.com/site/azevedojcam/ 23

24. INTRODUCTION SUPPORT 24

25. Photoelectrochemical Scheme 25 Potentiostat Photoanode Counterelectrode + + + + + H2H2 O2O2 H+H+ H+H+ H+H+

26. 26 1)Absorption of light near the surface of the semiconductor creates electron-hole pairs. 2)Holes (minority carriers) drift to the surface of the semiconductor (the photo anode) where they react with water to produce oxygen: 3)Electrons (majority carriers) are conducted to a metal electrode (typically Pt) where they combine with H + ions in the electrolyte solution to make H 2 : 4)Transport of H + from the anode to the cathode through the electrolyte completes the electrochemical circuit. The overall reaction :

27. Photoelectrolysis 27 Electric Current Solar Cell Electric Current Electrolysis Photoelectrolysis

28. Nernst Equation 28 For an oxidation/reduction reaction we have: Where F is the Faraday constant and n is the number of necessary electrons (in this case two). Energy losses

29. Theoretical efficiencies 29 The overall solar energy conversion efficiency can be written as the product of the efficiencies of the cell in performing these processes:

30. Quais as dificuldades? Adapted from M. Grätzel, Nature 414, 388 (2001) 30 Óxidos Quimicamente estáveis mas baixa eficiência (baixa condutividade) Não óxidos Boa condutividade mas fraca estabilidade química

31. Maximum efficiency possible Depending upon semiconductor bandgap, under xenon arc lamp and AM1.5 solar illuminations. 31

32. Armazenamento de Hidrogénio 32 http://en.wikipedia.org/wiki/File:XASEdges.svg Compressed hydrogen Liquid hydrogen Chemical storage Physical storage Carbon nanotubes

33. PAA SUPPORT 33

34. First Anodization The four major stages of nanoporous alumina template formation: 1)2)3)4) 1)oxide barrier formation; 2)pore initial nucleation; 3)pore initial growth; 4)pore continuous growth; 34 Al

35. Two Step Anodization 1 µm Aluminium Pattern formed Better organization! 1 µm No organization Alumina 1 st Anodization Dissolution of Oxide Layer 2 nd Anodization SEM surface 35

36. Ordered triangular lattices 36

37. Ordered triangular lattices 37

38. ELECTRODEPOSITON SUPPORT 38

39. Experimental parameters 39 2º Anodization 240min at 40V Dendrite Formation 8V Electrodeposition P.andodization (8V; 2ms) P.deposition (70mA/cm 2 ; 8ms) P.rest (700ms)

40. General Concepts 40

41. Different methods 41 Electrodeposition different methods

42. Simulação numérica da influência do pulso de repouso na deposição 42

43. Influência do tamanho de poro na qualidade da deposição Amostras de 10μm de espessura, preparadas a 20 o C, 0.43M e 14mA/cm 2 43

44. CHARACTERIZATION SUPPORT 44

45. Estrutura Cristalina 45 Os eletrões emitidos pelo cátodo de uma ampola onde foi previamente realizado vácuo são acelerados por um potencial elevado aplicado ao longo dela, dirigindo-se a alta velocidade em direção a uma placa metálica (alvo) utilizada como ânodo. Quando os eletrões chocam com o alvo dá-se a emissão de raios-X. O espectro emitido é composto por radiação-X cujo comprimento de onda varia continuamente, ao qual se sobrepõe uma série de riscas muito estreitas e em posições discretas.

46. Estrutura Cristalina 46 Fatores que contribuem para o alargamento dos picos medidos experimentalmente: tensões mecânicas não homogéneas variações de composição ao longo da amostra a sua espessura as larguras e alturas das fendas de colimação do feixe (instrumento) falta de monocromatismo do feixe incidente (instrumento) o o tamanho médio das cristalites que compõem a amostra (policristalina) A relação entre o tamanho L e o alargamento é dada pela fórmula de Scherrer, que se escreve do seguinte modo:

47. Taxamento da deposição 47

48. Magnetic Characterization Coercive field: H C // (~ 1550 Oe) >> H C  (~ 385 Oe) Saturation field: H S // (~ 4 kOe) << H S  (~ 15 kOe) 48 // 

49. OXIDATION SUPPORT 49

50. FC and ZFC measurements ZFC and FC measurements in a 100 Oe field. The annealing temperature was 800 o C. (C. H. Kim et al, “Magnetic anisotropy of vertically aligned alpha-fe2o3 nanowire array”, Ap. Phys. Let., vol. 89.) 50

51. Spectra of loose Fe oxide NWs, annealed at 800 o C. The α-Fe 2 O 3 Bragg reflections are identified. 51

52. Synchrotron radiation 52 Synchrotron radiation is produced from the electromagnetic radiation emitted when charged particles are accelerated radially.

53. Synchrotron radiation 53 Properties of synchrotron radiation: Broad Spectrum (which covers from microwaves to hard X-rays); High Flux of energy; High Brilliance (highly collimated photon beam); High Stability (submicron source stability); Polarization (both linear and circular); Pulsed Time Structure (pulsed length down to tens of picoseconds allows the resolution of process on the same time scale).

54. X-ray absorption spectroscopy X-ray absorption spectroscopy (XAS) is a widely-used technique for determining the local geometric and/or electronic structure of matter. XAS data are obtained by tuning the photon energy using a crystalline monochromator to a range where core electrons can be excited. 54 http://en.wikipedia.org/wiki/File:XASEdges.svg

55. X-ray absorption spectroscopy There are two main regions found on a spectrum generated by XAS data 55 http://en.wikipedia.org/wiki/File:XASEdges.svg

56. XANES X-ray Absorption Near Edge Structure (XANES), also known as Near edge X-ray absorption fine structure (NEXAFS) is the absorption of an x-ray photon by a core level of an atom in a solid and the consequent emission of a photoelectron. The resulting core hole is filled either via an Auger process or by capture of an electron from another shell followed by emission of a fluorescent photon. 56 http://en.wikipedia.org/wiki/File:XASEdges.svg

57. XANES The great power of XANES derives from its elemental specificity. Because the various elements have different core level energies, XANES permits extraction of the signal from a surface monolayer or even a single buried layer in the presence of a huge background signal. 57 NEXAFS can also determine the chemical state of elements which are present in bulk in minute quantities