Enhanced water oxidation efficiency of hematite thin films by oxygen-deficient atmosphere

Freitas, André L. M.; Carvalho, Waldemir M.; Souza, Flávio L.

doi:10.1557/jmr.2015.353

Cited by 19 publications

(18 citation statements)

References 54 publications

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“…The use of tin as an overlayer on the hematite electrode is known not to affect the electronic properties, [8] althoughw hen added as ad opant it can enhancet he conductivity up to two orders of magnitudes, accordingt ot he doping level. [24] Moreover,t he V fb values of pure and tin-modified hematite electrodes were estimated to be between 0.25-0.33 and 0.24-0.5 Vv ersus RHE, respectively.T he V fb values obtained for all hematite electrodes were smallert han those previously reported, [25] except for the values obtained for the pure and tin-modified hematite electrodes synthesized for 10 h, which were higher.T his is an unexpected result once the enhancement in flat-bandp otential values has been reported,…”

Section: Resultscontrasting

confidence: 74%

“…Moreover, the V fb values of pure and tin‐modified hematite electrodes were estimated to be between 0.25–0.33 and 0.24–0.5 V versus RHE, respectively. The V fb values obtained for all hematite electrodes were smaller than those previously reported, except for the values obtained for the pure and tin‐modified hematite electrodes synthesized for 10 h, which were higher. This is an unexpected result once the enhancement in flat‐band potential values has been reported, and suggests that the photocurrent increase caused by the deposition of Sn 4+ contributes to more efficient charge separation.…”

Section: Resultsmentioning

confidence: 99%

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Hematite Surface Activation by Chemical Addition of Tin Oxide Layer

Carvalho

Souza

2016

ChemPhysChem

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In this study, the effect of tin (Sn(4+) ) modification on the surface of hematite electrodes synthesized by an aqueous solution route at different times (2, 5, 10, 18, and 24 h) is investigated. As confirmed from X-ray diffraction results, the as-synthesized electrode exhibits an oxyhydroxide phase, which is converted into a pure hematite phase after being subjected to additional thermal treatment at 750 °C for 30 min. The tin-modified hematite electrode is prepared by depositing a solution of Sn(4+) precursor on the as-synthesized electrode, followed by thermal treatment under the same abovementioned conditions. This modification results in an enhancement of the photocurrent response for all hematite electrodes investigated and attains the highest values of around 1.62 and 2.3 mA cm(-2) at 1.23 and 1.4 V versus RHE, respectively, for electrodes obtained in short synthesis times (2 h). Contact angle measurements suggest that the deposition of Sn(4+) on the hematite electrode provides a more hydrophilic surface, which favors a chemical reaction at the interface between the electrode and electrolyte. This result generates new perspectives for understanding the deposition of Sn(4+) on the hematite electrode surface, which is in contrast with several studies previously reported; these studies state that the enhancement in photocurrent density is related to either the induction of an increased donor charge density or shift in the flat-band potential, which favors charge separation.

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Section: Resultscontrasting

confidence: 74%

Section: Resultsmentioning

confidence: 99%

Hematite Surface Activation by Chemical Addition of Tin Oxide Layer

Carvalho

Souza

2016

ChemPhysChem

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“…The Fe 2 O 3 nanorods were grown onto commercial conductive glass substrate FTO (F:SnO 2 / fluorine-doped tin oxide) under hydrothermal conditions, following the modified procedure from the original purpose-built method. 9,[51][52][53] For that, a precursor solution was prepared by dissolving iron chloride (FeCl 3 •6H 2 O) and sodium nitrate (NaNO 3 ) in deionized water (ρ > 17 MΩ cm −1 ). The substrate and the precursor solution were added into a 30-mL Teflon-lined stainless steel autoclave, transferred a conventional oven, and heated it up to 95°C for 4 hours, allowing it to cool down at room temperature.…”

Section: Nanoceramic Hematite and Snhematite Photoelectrode Preparamentioning

confidence: 99%

Interface engineering of nanoceramic hematite photoelectrode for solar energy conversion

et al. 2020

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This work addresses the role of different modifiers on the overall photocurrent response, which allowed a dual material insertion, increasing the charge separation without compromise the surface catalysis. Sn-addition onto nanoceramic hematite photoelectrodes clearly increased flat band potential, promoting a good charge separation, and shifting the onset to a higher potential, attributed to the surface-trapping state created by this modification. Notoriously, Sn-hematite photoelectrodes loaded with NiFeO x exhibited the highest photocurrent density, suggesting a partially recovered surface-trapping states created during the electrode designing. The well-known cocatalyst acted in the overall photoelectrocatalytic response with no significant effect on the turn-on voltage, in other words, with minor effect related to catalytic efficiency. The dual modification contributes to understand the role of different modifiers allowing to satisfactorily improve charge separation while maintaining the conductivity attributed to IV-group ions.

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“…26,38,41 As shown in Figure 6, for all electrodes with appreciable photoactivity the onset of photocurrent shifted negative 400 mV, to 0.5-0.6 V, and its rise was more immediate, as expected from the more rapid H 2 O 2 oxidation kinetics.…”

Section: Resultsmentioning

confidence: 67%

“…Others have reported similar findings for hematite electrodes prepared by hydrothermal synthesis from the hematite surface by effectively increasing the rate of hole transfer to the electrolyte, while also noting a decrease in charge carrier density as electrodes are exposed to a higher O 2 concentration during annealing. 38 Li and co-workers also found that oxygen vacancies increase donor densities, but propose an alternate explanation whereby the primary effect is to reduce recombination of bulk electrons with holes that have accumulated at the surface. 39, 40 Yet neither of these mechanisms alone accounts for our observations here.…”

Section: Resultsmentioning

confidence: 99%

Enhancement of Water Oxidation Photocurrent for Hematite Thin Films Electrodeposited with Polyvinylpyrrolidone

Ishaq

Sikora

Scheidler

et al. 2016

J. Electrochem. Soc.

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Hematite photoanodes were prepared by anodic electrodeposition in the presence of polyvinylpyrrolidone (PVP). PVP has been shown to alter hematite nanoparticle morphology, but has not been used before for anodic electrodeposition of thin films. Significant improvement in the water oxidation photocurrent was observed with PVP, with greater charge-collection efficiency due to a finer nanostructured morphology, and higher quantum yields for photons absorbed throughout the thickness of the hematite film from its increased porosity. Different post-deposition treatments showed distinct effects with and without PVP. Compared to electrodes dried 48 hours before annealing under N 2 , electrode performance was significantly reduced upon annealing in air, or annealing immediately after deposition when PVP was not used. A distinct wavelength dependence for front-versus back-side illumination, and a ∼100-fold decrease in dopant density, was attributed to formation of a dead layer on the top of those electrodes with reduced photoactivity, and was avoided using PVP. The stability, abundance, and environmental compatibility of hematite (α-Fe 2 O 3 ) make it an attractive candidate for realizing the goal of directly generating hydrogen gas from water and sunlight with high efficiency and at low cost. Hematite is stable in all but acidic solutions, can absorb ∼40% of the solar spectrum, and has suitable band edge positions for water oxidation. However, its performance in photoelectrochemical (PEC) cells is plagued by its short holediffusion lengths. To overcome this, electrodes with nanostructured morphologies are commonly prepared to optimize photon absorption and charge-carrier collection.1-3 High aspect-ratio nanorods or other nanostructures allow for the orthogonalization of the absorption depth and charge-carrier-diffusion length, which is on the order of 2-10 nm for hematite. 4,5 In addition, modification of the surface with catalysts has been shown to reduce overpotentials by improving water oxidation kinetics and/or reducing surface recombination rates.1,6-10 Other studies have focused on doping hematite to alter charge-carrier densities, increase conductivity, and/or increase charge separation, transport, and collection efficiency. 2,3,[11][12][13][14][15] Although these efforts have led to impressive gains in performance, conversion efficiencies remain well below the theoretically predicted maximum value and many questions remain about how to maximize charge-carrier generation and collection, and how to optimize interfacial electron-transfer kinetics for the sluggish oxygen evolution reaction.Hematite thin films can be grown by a variety of methods, including atomic layer deposition, 7,15-17 hydrothermal precipitation, 3,18 spray pyrolysis, 19,20 chemical vapor deposition, 1,6,21 and electrodeposition, 22-26 among others. Electrodeposition provides a relatively simple, flexible, low-cost and easily scalable approach to synthesizing hematite. In this report, we focus on anodic electrodeposition to fabricate hematite phot...

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Enhanced water oxidation efficiency of hematite thin films by oxygen-deficient atmosphere

Abstract: Abstract

Cited by 19 publications

References 54 publications

Hematite Surface Activation by Chemical Addition of Tin Oxide Layer

Hematite Surface Activation by Chemical Addition of Tin Oxide Layer

Interface engineering of nanoceramic hematite photoelectrode for solar energy conversion

Enhancement of Water Oxidation Photocurrent for Hematite Thin Films Electrodeposited with Polyvinylpyrrolidone

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