Defect Engineering of Ta<sub>3</sub>N<sub>5</sub> Photoanodes: Enhancing Charge Transport and Photoconversion Efficiencies via Ti Doping

Wagner, Laura I.; Sirotti, Elise; Brune, Oliver; Grötzner, Gabriel; Eichhorn, Johanna; Santra, Saswati; Munnik, Frans; Olivi, Luca; Pollastri, Simone; Streibel, Verena; Sharp, Ian D.

doi:10.1002/adfm.202306539

Cited by 8 publications

(1 citation statement)

References 43 publications

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“…Such an oxygen-inductive effect has also been observed for tantalum nitrides and can generally be used to promote the formation of nitrogen-rich metal nitride compounds. 37,38 This nitrogen enrichment, along with the high oxidation state of Zr, leads to the opening of a band gap, as indicated in the transmission spectra shown in Figure 1c. As more oxygen is introduced into the reaction gas mixture, crystalline thin films of bixbyite-type Zr 2 ON 2 and fluorite-type ZrO 2 emerge.…”

Section: ■ Results and Discussionmentioning

confidence: 90%

Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting

Streibel,

Schönecker,

Wagner

et al. 2024

ACS Appl. Energy Mater.

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Transition metal oxynitrides are a promising class of functional materials for photoelectrochemical (PEC) applications. Although these compounds are most commonly synthesized via ammonolysis of oxide precursors, such synthetic routes often lead to poorly controlled oxygen-to-nitrogen anion ratios, and the harsh nitridation conditions are incompatible with many substrates, including transparent conductive oxides. Here, we report direct reactive sputter deposition of a family of zirconium oxynitride thin films and the comprehensive characterization of their tunable structural, optical, and functional PEC properties. Systematic increases of the oxygen content in the reactive sputter gas mixture enable access to different crystalline structures within the zirconium oxynitride family. Increasing oxygen contents lead to a transition from metallic to semiconducting to insulating phases. In particular, crystalline Zr 2 ON 2 -like films have band gaps in the UV−visible range and are n-type semiconductors. These properties, together with a valence band maximum position located favorably relative to the water oxidation potential, make them viable photoanode candidates. Using chopped linear sweep voltammetry, we indeed confirm that our Zr 2 ON 2 films are PEC-active for the oxygen evolution reaction in alkaline electrolytes. We further show that high-vacuum annealing boosts their PEC performance characteristics. Although the observed photocurrents are low compared to state-of-the-art photoanodes, these dense and planar thin films can offer a valuable platform for studying oxynitride photoelectrodes, as well as for future nanostructuring, band gap engineering, and defect engineering efforts.

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Section: ■ Results and Discussionmentioning

confidence: 90%

Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting

Streibel,

Schönecker,

Wagner

et al. 2024

ACS Appl. Energy Mater.

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Impact of Defects and Disorder on the Stability of Ta₃N₅ Photoanodes

Wolz,

Grötzner,

Rieth

et al. 2024

Adv Funct Materials

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The photoelectrochemical performance of Ta3N5 photoanodes is strongly impacted by the presence of shallow and deep defects within the bandgap. However, the role of such states in defining stability under operational conditions is not well understood. Here, a highly controllable synthesis approach is used to create homogenous Ta3N5 thin films with tailored defect concentrations to establish the relationship between atomic‐scale point defects and macroscale stability. Reduced oxygen contents increase long‐range structural order but lead to high concentrations of deep‐level states, while higher oxygen contents result in reduced structural order but beneficially passivate deep‐level defects. Despite the different defect properties, the synthesized photoelectrodes degrade similarly under water oxidation conditions due to the formation of a surface oxide layer that blocks interfacial hole injection and accelerates charge recombination. In contrast, under ferrocyanide oxidation conditions, it is found that Ta3N5 films with high oxygen concentrations exhibit long‐term stability, whereas those possessing lower oxygen contents and higher deep‐level defect concentrations rapidly degrade. These results indicate that deep‐level defects result in rapid trapping of photocarriers and surface oxidation but that shallow oxygen donors can be introduced into Ta3N5 to enable kinetic stabilization of the interface.

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Bixbyite‐Type Zirconium Tantalum Oxynitride Thin Films as Composition‐Tunable High Refractive Index Semiconductors

Wagner,

Canever,

Sirotti

et al. 2024

Adv Materials Inter

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Multinary nitrides and oxynitrides offer a range of tunable structural and optoelectronic properties. However, much of this vast compositional space remains to be explored due to the challenges associated with their synthesis. Here, reactive sputter deposition is used to synthesize isostructural polycrystalline zirconium tantalum oxynitride thin films with varying cation ratios and systematically explore their structural and optical properties. All films possess the cubic bixbyite‐type structure and n‐type semiconducting character, as well as composition‐tunable optical bandgaps in the visible range. Furthermore, these compounds exhibit remarkably high refractive indices that exceed a value 2.8 in the non‐absorbing sub‐bandgap region and reach 3.2 at 589 nm for Ta‐rich compositions. Photoemission spectroscopy reveals non‐uniform shifts in electron binding energies that indicate a complex interplay of structural and compositional effects on interatomic bonding. In addition to being high‐index materials, the measured band edge positions of the films align favorably with the water oxidation and reduction potentials. Thus, this tunable materials family offers prospects for diverse optoelectronics application, including for production of photonic metamaterials and for solar water splitting.

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Defect Engineering of Ta₃N₅ Photoanodes: Enhancing Charge Transport and Photoconversion Efficiencies via Ti Doping

Cited by 8 publications

References 43 publications

Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting

Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting

Impact of Defects and Disorder on the Stability of Ta₃N₅ Photoanodes

Bixbyite‐Type Zirconium Tantalum Oxynitride Thin Films as Composition‐Tunable High Refractive Index Semiconductors

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Defect Engineering of Ta3N5 Photoanodes: Enhancing Charge Transport and Photoconversion Efficiencies via Ti Doping

Cited by 8 publications

References 43 publications

Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting

Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting

Impact of Defects and Disorder on the Stability of Ta3N5 Photoanodes

Bixbyite‐Type Zirconium Tantalum Oxynitride Thin Films as Composition‐Tunable High Refractive Index Semiconductors

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Product

Resources

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Defect Engineering of Ta₃N₅ Photoanodes: Enhancing Charge Transport and Photoconversion Efficiencies via Ti Doping

Impact of Defects and Disorder on the Stability of Ta₃N₅ Photoanodes