The concept of Fermi level pinning at semiconductor/liquid junctions. Consequences for energy conversion efficiency and selection of useful solution redox couples in solar devices

Bard, Allen J.; Bocarsly, Andrew Bruce; Fan, Fu Ren F.; Walton, Erick G.; Wrighton, Mark S.

doi:10.1021/ja00531a001

Cited by 514 publications

(420 citation statements)

References 6 publications

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“…10 The Helmholtz layer was set to 5x10 -8 cm and κ l was set to 4.0. 12,13 The potential distribution in the two-phase Gr/electrolyte system was modeled by the following equations:…”

Section: S9mentioning

confidence: 99%

Photoelectrochemical Behavior of n-Type Si(111) Electrodes Coated With a Single Layer of Graphene

et al. 2013

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Methods Chemicals/MaterialsAll experiments employed single-crystalline, Czochralski grown, (111)-oriented, planar, 380 μm thick, phosphorus doped, 1.1 Ω-cm resistivity (doping density, N D ≈ 5x10 15 cm -3 ) n-type silicon (University Wafer).Water was obtained from a Barnstead Nanopure system and had a resistivity ≥ 18.0 MΩ-cm. hexafluorophosphate, 98%, Sigma-Aldrich) was recrystallized from a mixture of ethanol (ACS grade, EMD) and acetonitrile (ACS grade, EMD) and dried under vacuum. Acetylferrocenium (AcFc + ) was

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“…10 The Helmholtz layer was set to 5x10 -8 cm and κ l was set to 4.0. 12,13 The potential distribution in the two-phase Gr/electrolyte system was modeled by the following equations:…”

Section: S9mentioning

confidence: 99%

Photoelectrochemical Behavior of n-Type Si(111) Electrodes Coated With a Single Layer of Graphene

et al. 2013

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“…[30][31] A direct result of this high electronic state density at the surface is a reduction in photovoltage at the semiconductor/electrolyte junction. 32 This is unfavorable since band bending is the driving force for the separation of photogenerated carriers. One possible method for decreasing surface states is to passivate the surface effectively.…”

Section: Introductionmentioning

confidence: 99%

Si/InGaN Core/Shell Hierarchical Nanowire Arrays and their Photoelectrochemical Properties

et al. 2012

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Three dimensional hierarchical nanostructures were synthesized by the halide chemical vapor deposition (HCVD) of InGaN nanowires on Si wire arrays. Single phase InGaN nanowires grew vertically on the sidewalls of Si wires and acted as a high surface area photoanode for solar water splitting. Electrochemical measurements showed that the photocurrent density with hierarchical Si/InGaN nanowire arrays increased by 5 times compared to the photocurrent density with InGaN nanowire arrays grown on planar Si (1.23 V vs RHE). High resolution transmission electron microscopy showed that InGaN nanowires are stable after 15 hours of illumination. These measurements show that Si/InGaN hierarchical nanostructures are a viable high surface area geometry for stable solar water splitting. KEYWORDS:Hierarchical nanostructure, InGaN nanowire, Si wire, photoanode, solar water splitting IntroductionPhotolysis of water with semiconductor materials has been investigated as a clean and renewable energy conversion process by storing solar energy in chemical bonds such as hydrogen. [1][2][3] Since Fujishima and Honda 4 reported the capability of water splitting with TiO 2 , metal oxide semiconductors have been studied extensively due to their stability under photo-anodic conditions. [5][6] However, the valence band of metal oxide semiconductors consists mainly of O 2p orbitals resulting in a low energy maximum (around +3 V vs NHE compared to +1.23 V vs NHE for the water oxidation reaction). This leads to a significant loss in energy from the large difference in potential between the valence band and water oxidation reaction. In addition, lowering the metal oxide bandgap energy for visible light absorption generally comes at the cost of moving the conduction band minimum (CBM) towards lower potentials than the hydrogen reduction potential which cannot perform spontaneous solar water splitting.On the other hand, metal nitrides have less positive valence band maximum (VBM) potentials than metal oxides because N 2p orbitals have smaller ionization energies than O 2p orbitals. [6][7] For example, GaN is one of the nitride semiconductors that have been studied for photocatalytic applications [8][9][10][11] because its CBM and VBM straddle the hydrogen reduction (H + /H 2 ) and water oxidation (H 2 O/O 2 ) potentials. Although GaN has a large bandgap (3.4 eV), indium alloying to form InGaN can tune the bandgap from the ultraviolet to the near infrared region 12 encompassing the entire solar spectrum. The In 0.5 Ga 0.5 N alloy has a bandgap of around 2.0 eV which is desirable for overall water splitting considering the overpotential for the water oxidation reaction. [13][14] Moses et al. 15 calculated that the VBM of InGaN alloy increases in energy almost linearly with indium composition. Therefore, the InGaN alloy was suggested to accomplish spontaneous overall water splitting with up to 50% indium incorporation since both the CBM and VBM can satisfy the energetic requirements. Experimentally, single crystalline In x Ga 1-x N nanowire...

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“…For the etched sample, a value of 2.6 Â 10 12 cm À2 for the surface state density can be estimated assuming a voltage of %1 V. This is relevant for electronic processes at the GaN surface as it is known that surface state densities of 10 13 cm À2 are sufficient to cause complete Fermi level pinning. 28,29 Financial support from the Deutsche Forschungsgemeinschaft (DFG STU 139=12 À 1), TUM.solar in the frame of the Bavarian Collaborative Research Project "Solar technologies go Hybrid" (SolTec) and the Excellence Cluster Nanosystems Initiative Munich is gratefully acknowledged.…”

Section: Q Jxmentioning

confidence: 99%

GaN surface states investigated by electrochemical studies

Winnerl

Garrido

Stutzmann

2017

Applied Physics Letters

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We present a systematic study of electrochemically active surface states on MOCVD-grown n-type GaN in aqueous electrolytes using cyclic voltammetry and impedance spectroscopy over a wide range of potentials and frequencies. In order to alter the surface states, the GaN samples are either etched or oxidized, and the influence of the surface treatment on the defect-mediated charge transfer to the electrolyte is investigated. Etching in HCl removes substoichiometric GaO x , and leads to a pronounced density of electrochemically active surface states. Oxidation effectively removes these surface states. Published by AIP Publishing. Recently, group-III nitride materials have attracted considerable interest in fields such as photo-electrochemistry and photocatalysis due to the favorable energy position of their band edges with respect to the redox levels of relevant species in liquid electrolytes. [1][2][3][4][5][6] The flexibility in doping III-nitrides in a controlled way enables the deterministic generation of either holes (for n-type doping) or electrons (for p-type doping) at the surface under illumination to selectively drive oxidation or reduction reactions, respectively. The presence of electronic defect states directly at or close to the semiconductor surface is known to influence charge transfer at semiconductor electrodes. Such electronically active surface defects, e.g., are responsible for surface band bending determining the separation of photo-excited charge carriers and laterally inhomogeneous charging of the surface due to local charge trapping. For a quantitative analysis of the influence of surface states on electrochemical processes and Schottky diodes, it is important to understand the mechanisms governing charge transfer across the GaN surface. In a previous work, 7 we have combined the contact potential difference (CPD) and photoconductivity measurements to investigate electronic processes at or close to illuminated GaN surfaces. We could show that localized surface defect states play a crucial role in the kinetics of photo-generated charges. Sachsenhauser et al. have investigated the charge transfer across the n-type SiC/electrolyte interface. 8 They performed cyclic voltammetry and impedance spectroscopy measurements over a wide range of potentials and frequencies. Adding the ferricyanide/ferrocyanide redox couple to the electrolyte made it possible to reveal the contribution of surface states to the charge transfer and determine the energy distribution of the surface states. In this work, we have investigated whether or not electrically active states located in the GaN band gap are also involved in the charge transfer to an electrolyte. To this end, we have systematically studied the n-type GaN grown by metal-organic chemical vapor deposition (MOCVD) in aqueous electrolytes using cyclic voltammetry and impedance spectroscopy. Different surface treatments are applied in order to alter the surface states and investigate the influence of these on the details of defectmediated charge transfer.(000...

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The concept of Fermi level pinning at semiconductor/liquid junctions. Consequences for energy conversion efficiency and selection of useful solution redox couples in solar devices

Cited by 514 publications

References 6 publications

Photoelectrochemical Behavior of n-Type Si(111) Electrodes Coated With a Single Layer of Graphene

Photoelectrochemical Behavior of n-Type Si(111) Electrodes Coated With a Single Layer of Graphene

Si/InGaN Core/Shell Hierarchical Nanowire Arrays and their Photoelectrochemical Properties

GaN surface states investigated by electrochemical studies

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