Oxygen deficient α-Fe<sub>2</sub>O<sub>3</sub> photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses

Forster, Mark; Potter, Richard J.; Ling, Yichuan; Yang, Yi; Klug, David R.; Li, Yat; Cowan, Alexander J.

doi:10.1039/c5sc00423c

Cited by 95 publications

(91 citation statements)

References 53 publications

(150 reference statements)

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“…Only a small fraction of the photogenerated holes are being transferred to the cocatalyst, likely due to the higher rate of recombination of electron−hole pairs in the hematite film close to the surface. 22,24,25 This is consistent with the TPC results described later on, which indicate there is no difference in the amount of charge available at the surface for the air-treated films before and after treatment with the cocatalyst. The rate of recombination within the hematite structure is therefore the limiting factor in these functionalized films.…”

Section: ■ Results and Discussionsupporting

confidence: 92%

Kinetics versus Charge Separation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst

et al. 2016

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Oxygen-deficient iron oxide thin films, which have recently been shown to be highly active for photoelectrochemical water oxidation, were surface-functionalized with a monolayer of a molecular iridium water oxidation cocatalyst. The iridium catalyst was found to dramatically improve the kinetics of the water oxidation reaction at both stoichiometric and nonstoichiometric α-Fe 2 O 3-x surfaces. This was found to be the case in both the dark and in the light as evidenced by cyclic voltammetry, Tafel analysis, and electrochemical impedance spectroscopy (EIS). Oxygen evolution measurements under working conditions confirmed high Faradaic efficiencies of 69−100% and good stability over 22 h of operation for the functionalized electrodes. The resulting ∼200−300 mV shift in onset potential for the iridiumfunctionalized sample was attributed to improved interfacial charge transfer and oxygen evolution kinetics. Mott−Schottky plots revealed that there was no shift in flat-band potential or change in donor density following functionalization with the catalyst. The effect of the catalyst on thermodynamics and Fermi level pinning was also found to be negligible, as evidenced by opencircuit potential measurements. Finally, transient photocurrent measurements revealed that the tethered molecular catalyst did improve charge separation and increase charge density at the surface of the photoanodes, but only at high applied biases and only for the nonstoichiometric oxygen-deficient iron oxide films. These results demonstrate how molecular catalysts can be integrated with semiconductors to yield cooperative effects for photoelectrochemical water oxidation.

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Section: ■ Results and Discussionsupporting

confidence: 92%

Kinetics versus Charge Separation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst

et al. 2016

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“…Transient absorption (TA) spectroscopy has been used extensively to understand the processes that govern the relaxation dynamics of photoexcited electrons and holes and has been successfully applied to the study of midgap states in metal oxides (21,22,26,27,(30)(31)(32)(33)(34). By studying highly defected materials and modeling the TA decay as a function of defect concentration, time, and excitation wavelength, a more detailed understanding of the dominant charge carrier relaxation processes can be obtained (22)(23)(24).…”

Section: Significancementioning

confidence: 99%

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO ₂ reduction

Hoch

Szymanski

Ghuman

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

101

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In 2 O 3-x (OH) y nanoparticles have been shown to function as an effective gas-phase photocatalyst for the reduction of CO 2 to CO via the reverse water-gas shift reaction. Their photocatalytic activity is strongly correlated to the number of oxygen vacancy and hydroxide defects present in the system. To better understand how such defects interact with photogenerated electrons and holes in these materials, we have studied the relaxation dynamics of In 2 O 3-x (OH) y nanoparticles with varying concentration of defects using two different excitation energies corresponding to above-band-gap (318-nm) and near-band-gap (405-nm) excitations. Our results demonstrate that defects play a significant role in the excited-state, charge relaxation pathways. Higher defect concentrations result in longer excited-state lifetimes, which are attributed to improved charge separation. This correlates well with the observed trends in the photocatalytic activity. These results are further supported by density-functional theory calculations, which confirm the positions of oxygen vacancy and hydroxide defect states within the optical band gap of indium oxide. This enhanced understanding of the role these defects play in determining the optoelectronic properties and charge carrier dynamics can provide valuable insight toward the rational development of more efficient photocatalytic materials for CO 2 reduction.indium oxide | solar fuels | CO 2 hydrogenation | transient absorption | surface defects C oncerns over climate change and the projected rise in global energy demand have motivated researchers to develop alternative, more sustainable ways to generate energy from naturally abundant and renewable sources (1-3). An important challenge associated with using renewable energy sources, such as solar, is their inherent intermittent nature (4-6). The emerging field of solar fuels seeks to address this issue by storing radiant solar energy in chemical bonds, which can then be released on demand and act as a drop-in replacement for traditional fossil fuels (7-11). By using the greenhouse gas, CO 2 , currently regarded as a waste product, as a feedstock and converting it into valuable products such as solar fuels or platform chemicals, we could simultaneously address concerns over climate change and energy security while creating significant economic benefits (12)(13)(14). However, despite recent advances in the development of active materials that can drive the photocatalytic reduction of CO 2 into useful chemical species, much remains unknown about the fundamental physical properties that control the activity of a photocatalyst. To facilitate the rational development and improvement of photocatalytic materials, a detailed understanding of the complex interplay between chemical, optical, and electronic processes is needed.Indium oxide has many favorable optical, electronic, and surface properties that make it a compelling choice as a photocatalyst for CO 2 reduction. It has a relatively high conduction band, and common defects such as oxyge...

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“…We focus on understanding the influence of the reduction process on the activation of the hematite/FTO diffused junction and elucidating the mechanism of charge separation/transport in the bulk and at the underlayer interface. Previously, transient absorption spectroscopy (TAS), 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 solution and 16 mL DI water were transferred into a 20 mL PTFE lined stainless steel autoclave, where a FTO substrate was put at the bottom with the conductive side facing …”

mentioning

confidence: 99%

“…26 interface. 27 In parallel, to address the "dead layer" effect at the hematite/substrate interface, [28][29] ultrathin Ga 2 O 3 , 30 SiO x , 31 TiO 2 ,32 or Nb 2 O 5 33 buffer layer has been employed as growth directing agent 28 or interface passivation layer for hematite 34 . But the diffusion of high valence metal 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 atoms from the buffer layer or substrate into hematite lattice is possible, 16, 35 which would form a diffused layer/junction.…”

mentioning

confidence: 99%

Dual Influence of Reduction Annealing on Diffused Hematite/FTO Junction for Enhanced Photoelectrochemical Water Oxidation

Yang

Yan

et al. 2016

ACS Appl. Mater. Interfaces

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Band structure engineering of the interface between the semiconductor and the conductive substrate may profoundly influence charge separation and transport for photovoltaic and photoelectrochemical devices. In this work, we found that a reduction-annealing treatment resulted in a diffused junction through enhanced interdiffusion of hematite/FTO at the interface. The activated hematite exhibited higher nanoelectric conductivity that was probed by a PeakForce TUNA AFM method. Furthermore, charge accumulation and recombination via surface states at the interface were dramatically reduced after the reduction-annealing activation, which was confirmed by transient surface photovoltage measurements. The diffused hematite junction promises improved photoelectrochemical performance without the need for a buffer layer.

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Oxygen deficient α-Fe₂O₃ photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses

Abstract: Intrinsic doping of hematite through the inclusion of oxygen vacancies (VO) is being increasingly explored as a simple, low temperature route to preparing active water splitting α-Fe2O3–x photoelectrodes.

Cited by 95 publications

References 53 publications

Kinetics versus Charge Separation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst

Kinetics versus Charge Separation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO ₂ reduction

Dual Influence of Reduction Annealing on Diffused Hematite/FTO Junction for Enhanced Photoelectrochemical Water Oxidation

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Oxygen deficient α-Fe2O3 photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses

Abstract: Intrinsic doping of hematite through the inclusion of oxygen vacancies (VO) is being increasingly explored as a simple, low temperature route to preparing active water splitting α-Fe2O3–x photoelectrodes.

Cited by 95 publications

References 53 publications

Kinetics versus Charge Separation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst

Kinetics versus Charge Separation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO 2 reduction

Dual Influence of Reduction Annealing on Diffused Hematite/FTO Junction for Enhanced Photoelectrochemical Water Oxidation

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Product

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About

Oxygen deficient α-Fe₂O₃ photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO ₂ reduction