Carrier dynamics of a visible-light-responsive Ta<sub>3</sub>N<sub>5</sub>photoanode for water oxidation

Ziani, Ahmed; Nurlaela, Ela; Dhawale, Dattatray S.; Silva, Diego Alves; Alarousu, Erkki; Mohammed, Omar F.; Takanabe, Kazuhiro

doi:10.1039/c4cp05616g

Cited by 86 publications

(116 citation statements)

References 47 publications

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“…1,[5][6][7][8][9][10][11][12][13][14][15][16] Our recent work on Ta 3 N 5 thin films for the photoelectrochemical oxygen evolution reaction (OER) noted that despite having suitable absorption in the visible-light range, Ta 3 N 5 suffered from very low transport properties and fast carrier recombination (<10 ps) without any surface decoration. 17 These detrimental intrinsic semiconducting properties might explain why overall water splitting has not been achieved at a detectable efficiency using Ta 3 N 5 as the main photocatalyst to date.…”

Section: Introductionmentioning

confidence: 99%

“…Indeed, changes in the structural, electronic, optical, and morphological properties of the Ta 3 N 5 have been observed to have beneficial effects for improving its photocatalytic activity. [15][16][17][18][19][20][21][22][23] Among these studies, our recent work reported that not only the bulk properties but also the surface properties greatly affected the photocatalytic activity of Ta 3 N 5 . 24 A thin TaN layer on the surface (~ 2 nm), which formed depending on the synthesis method, was observed to change the energetic profile on the Ta 3 N 5 -electrolyte interface, thus changing the photocatalytic activity.…”

Section: Introductionmentioning

confidence: 99%

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Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta₃N₅ Photocatalyst

et al. 2015

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ABSTRACT:In a photocatalytic suspension system with a powder semiconductor, the interface between the photocatalyst semiconductor and catalyst should be constructed to minimize resistance for charge transfer of excited carriers. This study demonstrates an in-depth understanding of pretreatment effects on the photocatalytic O 2 evolution reaction (OER) activity of visible-lightresponsive Ta 3 N 5 decorated with CoO x nanoparticles. The CoO x /Ta 3 N 5 sample was synthesized by impregnation followed by sequential heat treatments under NH 3 flow and air flow at various temperatures. Various characterization techniques, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS), were used to clarify the state and role of cobalt. No improvement in photocatalytic activity for OER over the bare Ta 3 N 5 was observed for the as-impregnated CoO x /Ta 3 N 5 , likely because of insufficient contact between CoO x and Ta 3 N 5 . When the sample was treated in NH 3 at high temperature, a substantial improvement in the photocatalytic activity was observed. After NH 3 treatment at 700 °C, the Co 0 -CoO x core-shell agglomerated cobalt structure was identified by XAS and STEM. No metallic cobalt species was evident after the photocatalytic OER, indicating that the metallic cobalt itself is not essential for the reaction. Accordingly, mild oxidation (200 °C) of the NH 3 -treated CoO x /Ta 3 N 5 sample enhanced photocatalytic OER activity. Oxidation at higher temperatures drastically eliminated the photocatalytic activity, most likely because of unfavorable Ta 3 N 5 oxidation. These results suggest that the intimate contact between cobalt species and Ta 3 N 5 facilitated at high temperature is beneficial to enhancing hole transport and that the cobalt oxide provides electrocatalytic sites for OER. IntroductionWater splitting using powder photocatalyst systems has attracted significant interest for the production of renewable hydrogen generation using abundant solar energy because of its simplicity, scalability, and low capital cost.1,2 To effectively convert solar energy, substantial utilization of photon energy in the visible-light range is essential. Using the AM 1.5G standard spectrum 3 , a photocatalyst with absorption from UV to 600 nm with a quantum efficiency of 30% accounts for ~5% solar to hydrogen efficiency, making the technology commercially feasible.1,4 Among the photocatalysts investigated to date, tantalum(V) nitride (Ta 3 N 5 ) has emerged as one of the most promising candidates and has been extensively investigated for more than a decade. [4][5][6][7][8][9][10][11][12][13][14][15] Ta 3 N 5 has a visible-light response (approximately 600 nm, ~2.1-eV band gap) and is capable of producing hydrogen or oxygen in the presence of appropriate sacrificial reagents and surface modifications under visible-light irradiation. 1,[5][6][7][8][9][10][11][12][13][14][15][16] Our recent work on Ta 3 N 5 thin films for the photo...

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta₃N₅ Photocatalyst

et al. 2015

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“…3 However, recent experiments have reported low photocurrent and poor overall performance of Ta 3 N 5 photoanodes for water oxidation, 4,5 which is likely caused by poor charge transport properties [6][7][8] as well as by rapid photodegradation of the samples in water.…”

Section: Introductionmentioning

confidence: 99%

Charge transport properties of bulkTa3N5from first principles

Morbec

Galli

2016

Phys. Rev. B

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Tantalum nitride is considered a promising material for photoelectrochemical water splitting, however its charge transport properties remain poorly understood. We investigated polaronic and band transport in Ta 3 N 5 using first-principles calculations. We first studied the formation of small polarons using density-functional theory (DFT) including DFT+U and hybrid functionals. We found that electron small polarons may occur but hole polarons are not energetically favorable.The estimated polaronic mobility for electrons is at least three orders of magnitude smaller than that measured in Ta 3 N 5 films, suggesting that the main transport mechanism for both electrons and holes is band-like. Since band transport is strongly affected by the carrier effective masses, and Ta 3 N 5 in known to have large electron and hole effective masses, we also investigated whether substitutional impurities or strain may help lower the effective masses. We found significant reduction in both electron and hole effective masses (up to 17% for electron and 39% for hole) under applied strain, which may lead to substantial improvement (up to 30% for electron and 15% for hole) in the carrier mobilities.

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“…[1][2][3][4][5][6] The main challenge lies in identifying materials suitable for efficient photocatalysis, i.e., with an optical response in the visible light region, and energy levels compatible with chemical reduction and/or oxidation. These properties converge in Ta3N5, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] which can theoretically achieve a solar to hydrogen efficiency of approximately 17% using a single semiconductor; nevertheless, non-biased, overall efficient water splitting using this material has not been achieved. For efficient photocatalysis with the powder semiconductor, a solid-electrolyte interface is effectively utilized for charge separation, therein directly inducing surface chemical redox reactions.…”

Section: Introductionmentioning

confidence: 99%

Enhanced Kinetics of Hole Transfer and Electrocatalysis during Photocatalytic Oxygen Evolution by Cocatalyst Tuning

et al. 2016

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Understanding photophysical and electrocatalytic processes during photocatalysis in a powder suspension system is crucial for developing efficient solar energy conversion systems. We report a substantial enhancement by a factor of 3 in photocatalytic efficiency for the oxygen evolution reaction (OER) by adding trace amounts (~0.05 wt%) of noble metals (Rh or Ru) to a 2 wt% cobalt oxide-modified Ta3N5 photocatalyst particulate. The optimized system exhibited high quantum efficiencies (QEs) of up to 28 and 8.4% at 500 and 600 nm in 0.1 M Na2S2O8 at pH 14. By isolating the electrochemical components to generate doped cobalt oxide electrodes, the electrocatalytic activity of cobalt oxide when doped with Ru or Rh was improved compared with cobalt oxide, as evidenced by the onset shift for electrochemical OER. Density functional theory (DFT) calculation shows that the effects of a second metal addition perturbs the electronic structure and redox properties in such a way that both hole transfer kinetics and electrocatalytic rates improve. Time resolved terahertz spectroscopy (TRTS) measurement provides evidence of long-lived electron populations (>1 ns; with mobilities μe ~0.1-3 cm 2 V -1 s -1 ), which are not perturbed by the addition of CoOx-related phases. Furthermore, we find that Ta3N5 phases alone suffer ultrafast hole trapping (within 10 ps); the CoOx and M-CoOx decorations most likely induce a kinetic competition between hole transfer toward the CoOx-related phases and trapping in the Ta3N5 phase, which is consistent with the improved OER rates. The present work not only provides a novel way to improve electrocatalytic and photocatalytic performance but also gives additional tools and insight to understand the characteristics of photocatalysts that can be used in a suspension system.

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Carrier dynamics of a visible-light-responsive Ta₃N₅photoanode for water oxidation

Cited by 86 publications

References 47 publications

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta₃N₅ Photocatalyst

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta₃N₅ Photocatalyst

Charge transport properties of bulkTa3N5from first principles

Enhanced Kinetics of Hole Transfer and Electrocatalysis during Photocatalytic Oxygen Evolution by Cocatalyst Tuning

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Carrier dynamics of a visible-light-responsive Ta3N5photoanode for water oxidation

Cited by 86 publications

References 47 publications

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta3N5 Photocatalyst

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta3N5 Photocatalyst

Charge transport properties of bulkTa3N5from first principles

Enhanced Kinetics of Hole Transfer and Electrocatalysis during Photocatalytic Oxygen Evolution by Cocatalyst Tuning

Contact Info

Product

Resources

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Carrier dynamics of a visible-light-responsive Ta₃N₅photoanode for water oxidation

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta₃N₅ Photocatalyst

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta₃N₅ Photocatalyst