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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.


Haematite with radically improved turn-on characteristics.(a) Steady-state current density-potential behaviours of various haematite photoelectrodes. The current densities of Si photocathode placed behind the haematite photoanode are shown to illustrate the meeting points. (b) Band diagram of unmodified haematite (grey) and NiFeOx-decorated haematite after re-growth treatments (red) under flat-band, quasi-equilibrium conditions. The Fermi level shift (denoted as 1) is a direct result of the re-growth treatment. The hole quasi-equilibrium potential shift (denoted as 2) is due to the application of NiFeOx. (c) Open circuit potential measurements of aH, sdH, rgH I, rgH II, rgH III and NiFeOx-decorated rgH II under 8-sun (red, triangle), 1-sun (blue, square) and dark (black, circle) conditions. Throughout this manuscript, sdH refers to solution-derived haematite; rgH I, rgH II, and rgH III denote haematite samples subjected to the regrowth treatments one, two and three times, respectively. Haematite prepared by atomic layer deposition (ALD) and then annealed at 500 °C and 800 °C are labelled aH and aH 800, respectively. NiFeOx/rgH II represent rgH II haematite decorated with amorphous NiFeOx catalysts. The error bars were obtained by taking s.d. values of measurements on at least three different samples for each data point.
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f1: Haematite with radically improved turn-on characteristics.(a) Steady-state current density-potential behaviours of various haematite photoelectrodes. The current densities of Si photocathode placed behind the haematite photoanode are shown to illustrate the meeting points. (b) Band diagram of unmodified haematite (grey) and NiFeOx-decorated haematite after re-growth treatments (red) under flat-band, quasi-equilibrium conditions. The Fermi level shift (denoted as 1) is a direct result of the re-growth treatment. The hole quasi-equilibrium potential shift (denoted as 2) is due to the application of NiFeOx. (c) Open circuit potential measurements of aH, sdH, rgH I, rgH II, rgH III and NiFeOx-decorated rgH II under 8-sun (red, triangle), 1-sun (blue, square) and dark (black, circle) conditions. Throughout this manuscript, sdH refers to solution-derived haematite; rgH I, rgH II, and rgH III denote haematite samples subjected to the regrowth treatments one, two and three times, respectively. Haematite prepared by atomic layer deposition (ALD) and then annealed at 500 °C and 800 °C are labelled aH and aH 800, respectively. NiFeOx/rgH II represent rgH II haematite decorated with amorphous NiFeOx catalysts. The error bars were obtained by taking s.d. values of measurements on at least three different samples for each data point.

Mentions: As previous results suggest defects on or near the surface of haematite may be an important reason for the late turn-on characteristics, we hypothesize that chemistries that alter the surfaces will have a direct impact on the resulting material's PEC behaviours. A solution-based re-growth technique is employed to test the hypothesis, where pre-formed haematite is subjected to acidic solutions under which condition dissolution (of Fe2O3) and deposition (of FeOOH) occur concurrently (Supplementary Fig. 1). A brief (∼5 min) post-growth annealing at 800 °C converts FeOOH to haematite. The striking effect on the PEC behaviours is evident in Fig. 1a. The turn-on voltages of haematite subjected to the re-growth chemistry at 0.67 (±0.01) V are significantly lower than those of atomic layer deposition (ALD) grown haematite without the re-growth treatments (∼1.05 (±0.02) V). The difference of 0.38 V is well beyond sample variations (Supplementary Fig. 2). It is noted that, although similar turn-on voltages have been obtained by Li and colleagues27, and Hamann and Zandi28, separately, the overall performance of the photoelectrodes reported by them trail what we report here by a large margin (Supplementary Fig. 3).


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)

Haematite with radically improved turn-on characteristics.(a) Steady-state current density-potential behaviours of various haematite photoelectrodes. The current densities of Si photocathode placed behind the haematite photoanode are shown to illustrate the meeting points. (b) Band diagram of unmodified haematite (grey) and NiFeOx-decorated haematite after re-growth treatments (red) under flat-band, quasi-equilibrium conditions. The Fermi level shift (denoted as 1) is a direct result of the re-growth treatment. The hole quasi-equilibrium potential shift (denoted as 2) is due to the application of NiFeOx. (c) Open circuit potential measurements of aH, sdH, rgH I, rgH II, rgH III and NiFeOx-decorated rgH II under 8-sun (red, triangle), 1-sun (blue, square) and dark (black, circle) conditions. Throughout this manuscript, sdH refers to solution-derived haematite; rgH I, rgH II, and rgH III denote haematite samples subjected to the regrowth treatments one, two and three times, respectively. Haematite prepared by atomic layer deposition (ALD) and then annealed at 500 °C and 800 °C are labelled aH and aH 800, respectively. NiFeOx/rgH II represent rgH II haematite decorated with amorphous NiFeOx catalysts. The error bars were obtained by taking s.d. values of measurements on at least three different samples for each data point.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Haematite with radically improved turn-on characteristics.(a) Steady-state current density-potential behaviours of various haematite photoelectrodes. The current densities of Si photocathode placed behind the haematite photoanode are shown to illustrate the meeting points. (b) Band diagram of unmodified haematite (grey) and NiFeOx-decorated haematite after re-growth treatments (red) under flat-band, quasi-equilibrium conditions. The Fermi level shift (denoted as 1) is a direct result of the re-growth treatment. The hole quasi-equilibrium potential shift (denoted as 2) is due to the application of NiFeOx. (c) Open circuit potential measurements of aH, sdH, rgH I, rgH II, rgH III and NiFeOx-decorated rgH II under 8-sun (red, triangle), 1-sun (blue, square) and dark (black, circle) conditions. Throughout this manuscript, sdH refers to solution-derived haematite; rgH I, rgH II, and rgH III denote haematite samples subjected to the regrowth treatments one, two and three times, respectively. Haematite prepared by atomic layer deposition (ALD) and then annealed at 500 °C and 800 °C are labelled aH and aH 800, respectively. NiFeOx/rgH II represent rgH II haematite decorated with amorphous NiFeOx catalysts. The error bars were obtained by taking s.d. values of measurements on at least three different samples for each data point.
Mentions: As previous results suggest defects on or near the surface of haematite may be an important reason for the late turn-on characteristics, we hypothesize that chemistries that alter the surfaces will have a direct impact on the resulting material's PEC behaviours. A solution-based re-growth technique is employed to test the hypothesis, where pre-formed haematite is subjected to acidic solutions under which condition dissolution (of Fe2O3) and deposition (of FeOOH) occur concurrently (Supplementary Fig. 1). A brief (∼5 min) post-growth annealing at 800 °C converts FeOOH to haematite. The striking effect on the PEC behaviours is evident in Fig. 1a. The turn-on voltages of haematite subjected to the re-growth chemistry at 0.67 (±0.01) V are significantly lower than those of atomic layer deposition (ALD) grown haematite without the re-growth treatments (∼1.05 (±0.02) V). The difference of 0.38 V is well beyond sample variations (Supplementary Fig. 2). It is noted that, although similar turn-on voltages have been obtained by Li and colleagues27, and Hamann and Zandi28, separately, the overall performance of the photoelectrodes reported by them trail what we report here by a large margin (Supplementary Fig. 3).

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.