The Role of Surface States in the Oxygen Evolution Reaction on Hematite

Iandolo, Beniamino; Hellman, Anders

doi:10.1002/anie.201406800

Cited by 136 publications

(126 citation statements)

References 39 publications

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“…R trap and C sc account for bulk charge trapping resistance and space-charge layer capacitance inside the sample, respectively. [34] R ct and C ss relate to charge transfer resistance and surface capacitance, respectively. [34b] The data of these parameters are shown in Table S2 (Supporting Information).…”

Section: Resultsmentioning

confidence: 99%

Enhancing Charge Separation in Metallic Photocatalysts: A Case Study of the Conducting Molybdenum Dioxide

Liu

Chen

et al. 2016

Adv Funct Materials

169

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A new visible‐light responsive metallic photocatalyst, nanostructured MoO2, has been discovered. The metallic nature of MoO2 is confirmed by valance X‐ray photoelectron spectroscopy spectrum and theoretical calculations. However, MoO2 itself shows only moderate activity due to the serious charge recombination, a general disadvantage of metallic photocatalysts. The findings suggest that its effective charge diffusion length Lp is smaller than 1.0 nm while the separation efficiency ηsep is less than 10%. Therefore, only the periphery of the metallic MoO2 can effectively contribute to photocatalysis. This limitation is overcome by integrating MoO2 in a hydrothermal carbonation carbon (HTCC) matrix (mainly contains semiconductive polyfuran). This simple chemical modification brings two advantages: (i) an internal electric field is formed at the interface between MoO2 and HTCC due to their appropriate band alignment; (ii) the nanostructured MoO2 and the HTCC matrix are intertwined with each other intimately. Their small size and large contact area promote charge transfer, especially under the internal electric field. Therefore, the separation rate of photoexcited charge carrier in MoO2 is greatly enhanced. The activity increases by 2.4, 16.8, and 4.0 times in photocatalytic oxygen evolution, dyes degradation, and photoelectrochemicl cell, respectively. The new approach is helpful for further development of metallic photocatalysts.

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Section: Resultsmentioning

confidence: 99%

Enhancing Charge Separation in Metallic Photocatalysts: A Case Study of the Conducting Molybdenum Dioxide

Liu

Chen

et al. 2016

Adv Funct Materials

169

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“…However, as the Sun does not shine continuously everywhere on Earth, developing efficient and economically viable methods to concentrate and store the harvested radiation is required. 7,8 This is thanks to the following properties of hematite: (i) it has an optical bandgap energy, EG, between 1.9 and 2.2 eV (depending on the fabrication route), appropriate to absorb a wide range of solar photon energies; (ii) it is stable under operation in neutral and alkaline conditions 9 ; and (iii) its constituents are earth-abundant and non-toxic elements. 1,2 A key factor for a commercially successful introduction of PEC water splitting is the development of photoelectrodes that meet strict requirements in terms of efficiency, durability, and availability.…”

Section: Introductionmentioning

confidence: 99%

Correlating flat band and onset potentials for solar water splitting on model hematite photoanodes

et al. 2015

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Hematite (α-Fe2O3) is a very promising material for solar water splitting that requires a high anodic potential to initiate the oxygen evolution reaction (OER). In this work, we explore the correlation between the downshift in flat band potential of hematite, Vfb, and in onset potential of OER, Vonset, caused by prolonged annealing. We observed a cathodic shift (i.e., towards lower potentials) of 200 mV of Vonset on model photoanodes consisting of ultra-thin hematite films, upon increasing the oxidation time during fabrication and without any further modifications. Detailed physical, electrochemical impedance spectroscopy, and Mott-Schottky analysis revealed a quantitative correlation between the cathodic shift of Vonset and a lowering of Vfb. We identified a reduction in concentration of grain boundaries with increasing oxidation time, as the mechanism behind the observed shift of the Vfb. The approach presented here can be seen as a complementary strategy to co-catalysts and other post-fabrication treatments to lower the OER onset potential, Vonset. Moreover, it is generically applicable to photoelectrodes used to carry out oxidation and reduction half-cell reactions. 23Graphical abstract. Increasing oxidation time during fabrication of hematite (Fe2O3) films reduces the amount of grain boundaries, resulting in lower flat band potential and onset potential for water oxidation.

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“…Under this condition, the holes captured by the surface states with energy within the band gap cannot drive the water oxidation reaction effectively; the surface states would then become the recombination sites for electrons and hole. At the same time, due to the thin space charge layer, the conduction band electrons may be extracted readily by the photo-oxidized intermediates and the O 2 product [68][69][70]. Therefore, because of the presence of surface states, the thin space charge layer contributes to both the surface electron-hole recombination on surface states and the back reaction over the photoelectrodes.…”

Section: Other Methodsmentioning

confidence: 99%