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Enabling unassisted solar water splitting by iron oxide and silicon.

Jang JW, Du C, Ye Y, Lin Y, Yao X, Thorne J, Liu E, McMahon G, Zhu J, Javey A, Guo J, Wang D - Nat Commun (2015)

Bottom Line: Here we show that, by using the prototypical photoanode material of haematite as a study tool, structural disorders on or near the surfaces are important causes of the low photovoltages.We develop a facile re-growth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45 V (versus reversible hydrogen electrode) is achieved.This result permits us to construct a photoelectrochemical device with a haematite photoanode and Si photocathode to split water at an overall efficiency of 0.91%, with NiFeOx and TiO2/Pt overlayers, respectively.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon St, Chestnut Hill, Massachusetts 02467, USA.

ABSTRACT
Photoelectrochemical (PEC) water splitting promises a solution to the problem of large-scale solar energy storage. However, its development has been impeded by the poor performance of photoanodes, particularly in their capability for photovoltage generation. Many examples employing photovoltaic modules to correct the deficiency for unassisted solar water splitting have been reported to-date. Here we show that, by using the prototypical photoanode material of haematite as a study tool, structural disorders on or near the surfaces are important causes of the low photovoltages. We develop a facile re-growth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45 V (versus reversible hydrogen electrode) is achieved. This result permits us to construct a photoelectrochemical device with a haematite photoanode and Si photocathode to split water at an overall efficiency of 0.91%, with NiFeOx and TiO2/Pt overlayers, respectively.

No MeSH data available.


X-ray diffraction, Raman and X-ray absorption analysis.(a) X-ray diffraction patterns, (b) Raman shift spectra and (c) Oxygen K-edge X-ray absorption spectra of aH, aH 800 (ALD-grown haematite annealed at 800 °C in air), sdH, rgH I, rgH II and rgH III. The details of sample IDs can be found in the captions for Fig. 1.
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f2: X-ray diffraction, Raman and X-ray absorption analysis.(a) X-ray diffraction patterns, (b) Raman shift spectra and (c) Oxygen K-edge X-ray absorption spectra of aH, aH 800 (ALD-grown haematite annealed at 800 °C in air), sdH, rgH I, rgH II and rgH III. The details of sample IDs can be found in the captions for Fig. 1.

Mentions: Our focus for the present work is to understand what underpins the strikingly high photovoltages (up to 0.80 V). It is seen from the simplified band diagram (Fig. 1b) that maximized photovoltages require the Fermi level to be as negative as possible relative to the water oxidation potential. Effects within the body (for example, low carrier concentration) and on the surface (for example, partial band edge unpinning) of the photoelectrode may move the Fermi level towards the positive direction. Our first task was to use the re-growth treatments to counteract these negative influences. To observe the effectiveness of the regrowth strategy, we probed the Fermi levels under equilibrium and quasi-equilibrium (that is, open-circuit) conditions. The results are compared in Fig. 1c. The data under intense light report on potentials close to the ‘true' flatband potential (Supplementary Fig. 4), whereas the potentials under 1-sun condition offer a reference point for us to understand the PEC behaviours as shown in Fig. 1a, which were taken under 1-sun illumination. The potentials in dark can be used to inspect whether there are undesired surface Fermi level pinning effects. Examinations of Fig. 1c revealed that ∼0.13 V potential is harvested from the Fermi level shift due to the switch of synthesis methods (Vf=0.62 (±0.01) V for ALD haematite, denoted as aH in Fig. 1c; Vf=0.49 (±0.01) V for solution derived haematite, denoted as sdH). The difference in Vf is ascribed to the difference in the detailed structures of haematite prepared by various methods, as evidenced by the X-ray diffraction patterns (Fig. 2a).


Enabling unassisted solar water splitting by iron oxide and silicon.

Jang JW, Du C, Ye Y, Lin Y, Yao X, Thorne J, Liu E, McMahon G, Zhu J, Javey A, Guo J, Wang D - Nat Commun (2015)

X-ray diffraction, Raman and X-ray absorption analysis.(a) X-ray diffraction patterns, (b) Raman shift spectra and (c) Oxygen K-edge X-ray absorption spectra of aH, aH 800 (ALD-grown haematite annealed at 800 °C in air), sdH, rgH I, rgH II and rgH III. The details of sample IDs can be found in the captions for Fig. 1.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4490416&req=5

f2: X-ray diffraction, Raman and X-ray absorption analysis.(a) X-ray diffraction patterns, (b) Raman shift spectra and (c) Oxygen K-edge X-ray absorption spectra of aH, aH 800 (ALD-grown haematite annealed at 800 °C in air), sdH, rgH I, rgH II and rgH III. The details of sample IDs can be found in the captions for Fig. 1.
Mentions: Our focus for the present work is to understand what underpins the strikingly high photovoltages (up to 0.80 V). It is seen from the simplified band diagram (Fig. 1b) that maximized photovoltages require the Fermi level to be as negative as possible relative to the water oxidation potential. Effects within the body (for example, low carrier concentration) and on the surface (for example, partial band edge unpinning) of the photoelectrode may move the Fermi level towards the positive direction. Our first task was to use the re-growth treatments to counteract these negative influences. To observe the effectiveness of the regrowth strategy, we probed the Fermi levels under equilibrium and quasi-equilibrium (that is, open-circuit) conditions. The results are compared in Fig. 1c. The data under intense light report on potentials close to the ‘true' flatband potential (Supplementary Fig. 4), whereas the potentials under 1-sun condition offer a reference point for us to understand the PEC behaviours as shown in Fig. 1a, which were taken under 1-sun illumination. The potentials in dark can be used to inspect whether there are undesired surface Fermi level pinning effects. Examinations of Fig. 1c revealed that ∼0.13 V potential is harvested from the Fermi level shift due to the switch of synthesis methods (Vf=0.62 (±0.01) V for ALD haematite, denoted as aH in Fig. 1c; Vf=0.49 (±0.01) V for solution derived haematite, denoted as sdH). The difference in Vf is ascribed to the difference in the detailed structures of haematite prepared by various methods, as evidenced by the X-ray diffraction patterns (Fig. 2a).

Bottom Line: Here we show that, by using the prototypical photoanode material of haematite as a study tool, structural disorders on or near the surfaces are important causes of the low photovoltages.We develop a facile re-growth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45 V (versus reversible hydrogen electrode) is achieved.This result permits us to construct a photoelectrochemical device with a haematite photoanode and Si photocathode to split water at an overall efficiency of 0.91%, with NiFeOx and TiO2/Pt overlayers, respectively.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon St, Chestnut Hill, Massachusetts 02467, USA.

ABSTRACT
Photoelectrochemical (PEC) water splitting promises a solution to the problem of large-scale solar energy storage. However, its development has been impeded by the poor performance of photoanodes, particularly in their capability for photovoltage generation. Many examples employing photovoltaic modules to correct the deficiency for unassisted solar water splitting have been reported to-date. Here we show that, by using the prototypical photoanode material of haematite as a study tool, structural disorders on or near the surfaces are important causes of the low photovoltages. We develop a facile re-growth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45 V (versus reversible hydrogen electrode) is achieved. This result permits us to construct a photoelectrochemical device with a haematite photoanode and Si photocathode to split water at an overall efficiency of 0.91%, with NiFeOx and TiO2/Pt overlayers, respectively.

No MeSH data available.