Formation of an electron hole doped film in the alpha-Fe2O3 photoanode upon electrochemical oxidation

Solar hydrogen generation by water splitting in photoelectrochemical cells (PEC) is an appealing technology for a future hydrogen economy. Hematite is a prospective photoanode material in this respect because of its visible light conjugated band gap, its corrosion stability, its environmentally benign nature and its low cost. Its bulk and surface electronic structure has been under scrutiny for many decades and is considered critical for improvement of efficiency. In the present study, hematite films of nominally 500 nm thickness were obtained by dip-coating on fluorine doped tin oxide (FTO) glass slides and then anodised in 1 molar KOH at 500, 600, and 700 mV for 1, 10, 120 and 1440 minutes under dark conditions. X-ray photoelectron spectra recorded at the Fe 3p resonant absorption threshold show that the e(g) transition before the Fermi energy, which is well developed in the pristine hematite film, becomes depleted upon anodisation. The spectral weight of the e(g) peak decreases with the square-root of the anodisation time, pointing to a diffusion controlled process. The speed of this process increases with the anodisation potential, pointing to Arrhenius behaviour. Concomitantly, the weakly developed t(2g) peak intensity becomes enhanced in the same manner. This suggests that the surface of the photoanode contains Fe2+ species which become oxidized toward Fe3+ during anodisation. The kinetic behaviour derived from the experimental data suggests that the anodisation forms an electron hole doped film on and below the hematite surface.

Cu-based p-type Oxide Photocathodes for Solar Hydrogen Production

Adriana Paracchino

The actualization of a hydrogen economy requires cost-effective and environmentally benign solutions to hydrogen production. Chemical energy in the form of hydrogen is more interesting than electricity to satisfy our ever-increasing energy demand because it can be stored more easily. Photoelectrochemical (PEC) water splitting is a carbon-neutral process which converts solar energy into chemical energy (hydrogen) using a light-absorbing semiconducting material to generate the necessary photovoltage to split water into hydrogen and oxygen. So far, record efficiencies in solar-to-hydrogen conversion have been achieved using p-type semiconductors, but only with very expensive materials. However, the spread of this technology relies on the development of semiconductor electrodes made of cheap and abundant elements that are stable in water for a reasonable long time. The semiconductors investigated in this thesis are p-type copper based oxides. Hydrogen production is measured by the photocurrent of water reduction at the semiconductor surface. In the first chapter a roundup of the basics of photoelectrochemical hydrogen generation is presented. In Chapter 2, ternary oxides presenting the delafossite crystal structure are explored, a class of materials that is expected to be stable in water under reducing conditions and that has never been studied for PEC photocathodes. The main achievements of this thesis are presented in Chapters 3-6. It is shown that CU2O, a semiconductor notoriously unstable in aqueous solutions yet a very efficient light absorber, can be stabilized against photocathodic degradation and thus used for water reduction. A preliminary optimization study is carried out to understand the synthetic parameters that control the photoactivity of cuprous oxide obtained by electrodeposition. The deposition current density, temperature and bath pH are changed to find the conditions where photoactivity is optimized. They most photoactive films are then characterized in terms of their optical and electrical properties, which are relevant to the photon-to-electron conversion efficiency (Chapter 3). The surface protection technique consists in the growth of ultrathin oxide layers (about 10-20 nm) by atomic layer deposition (ALD) on the CU2O electrodes (Chapter 4). This powerful technique enables a uniform and continuous coverage of a substrate and a control of the thickness at the atomic scale. Therefore, the passivation technique developed in this thesis can be applied to CU2O morphologies much more complex than those employed in this study and most probably to other semiconductors as well. Photocurrents up to 7.6 mA cm-2 are achieved using CU2O electrodes protected with amorphous Al-doped ZnO and TiCh. and decorated with a Pt catalyst. This photocurrent equals half of the maximum photocurrent possible for C112O and is the highest photocurrent ever measured for an oxide-based photoelectrode for PEC water splitting under AM 1.5 illumination. Further improvements to extend the performance over longer timescales and improve the overall photocathode stability are presented in Chapter 5. The concept of surface passivation against reductive decomposition of a semiconductor is proved for water splitting and important progresses are made in optimizing and understanding the key parameters for preserving the photoactivity of the material under operation. Particularly, the studies presented in Chapter 5 focus on the protective layers, exploring the relationship between the electrical properties of the ALD layers and their chemical stability in different electrolytes and after different temperature treatments. Although some decay in the hydrogen evolution over time is still observed, the last results on the protected electrodes show conversion efficiencies of 60% of the initial value after 10 hours of testing at the standard potential of hydrogen evolution, while for the unprotected electrodes the efficiency drops to 0% after a few minutes.

EPFL2012

Understanding the Role of Underlayers and Overlayers in Thin Film Hematite Photoanodes

Michael Graetzel, Ludmilla Steier, Stanley David Tilley

Recent research on photoanodes for photoelectrochemical water splitting has introduced the concept of under-and overlayers for the activation of ultrathin hematite films. Their effects on the photocatalytic behavior were clearly shown; however, the mechanism is thus far not fully understood. Herein, the contribution of each layer is analyzed by means of electrochemical impedance spectroscopy, with the aim of obtaining a general understanding of surface and interface modifications and their influence on the hematite photoanode performance. This study shows that doping of the hematite from the underlayer and surface passivation from annealing treatments and an overlayer are key parameters to consider for the design of more efficient iron oxide electrodes. Understanding the contribution of these layers, a new design for ultrathin hematite films employing a combination of a gallium oxide overlayer with thin niobium oxide and silicon oxide underlayers is shown to achieve a photocurrent onset potential for the photoelectrochemical oxidation of water more negative than 750 mV versus the reversible hydrogen electrode (RHE) at pH 13.6, utilizing Co-Pi as a water oxidation catalyst. It is demonstrated that multilayer hematite thin film photoanodes are a strategy to reduce the overpotential for this material, thereby facilitating more efficient tandem cells.

Wiley-V C H Verlag Gmbh2014

Molecular Origin and Electrochemical Influence of Capacitive Surface States on Iron Oxide Photoanodes

Florent Alexandre Boudoire, Artur Braun, Michael Graetzel, Yelin Hu

The origin, the nature, and the electronic structure of surface defects causing surface states on metal oxides and their role in solar water splitting have been under scrutiny for several decades. In the present study, the surface of hematite films is treated with an oxygen plasma and then subject to a detailed investigation with electroanalytical methods and element orbital specific X-ray spectroscopy. We observe a systemic variation of photoelectrochemical properties with oxygen treatment time. Fe 2p and O is core level X-ray photoelectron spectra and resonant valence band photoemission at the Fe 3p edge reveal the filling of prevalent oxygen vacancies with concomitant oxidation of Fe2+ to Fe3+ upon the oxygen treatment. The dc bias dependent impedance spectra confirm how a prevalent capacitive surface state, which evolves parallel with the photocurrent onset potential, becomes diminished upon oxygen treatment. Surface states of iron induce higher reactivity toward water oxidation than oxygen surface states. correlation between oxygen vacancy filling, concentration of surface states, and photocurrent density in the course of treatment confirms that the surface defects are of a capacitive nature and that the onset of water splitting can be considered as a result of dielectric breakdown in an interfacial hydroxide layer between photoanode and water.

American Chemical Society2016

Cu-based p-type Oxide Photocathodes for Solar Hydrogen Production

Adriana Paracchino

EPFL2012