High-Temperature Ammonolysis of Thin Film Ta2O5 Photoanodes: Evolution of Structural, Optical, and Photoelectrochemical Properties
High-Temperature Ammonolysis of Thin Film Ta2O5 Photoanodes: Evolution of Structural, Optical, and Photoelectrochemical Properties
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DOI：
10.1021/cm503215p
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发表时间：
2015-02-10
影响因子：
8.6
通讯作者：
van de Krol, Roel
中科院分区：
文献类型：
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作者：
Dabirian, Ali;van de Krol, Roel
氮化钽（Ta 3 N5）和氮氧化钽（TaON）由于接近理想的带隙和带边位置而成为光电化学（PEC）水裂解的有前途的材料。然而，通常用于制造这些材料的高温氨解过程敏感地取决于退火系统的工艺参数和特定设计，并且从其他实验室报告的配方中再现高效的（氧）氮化物光阳极已被证明是具有挑战性的。为了更详细地了解和监测氮化过程，我们采用了光学吸收光谱技术，使我们能够在高达800摄氏度的温度下原位跟踪Ta 2 O 5薄膜的转变。我们的研究结果表明，在干燥的氨气氛中的氮的掺入开始于575摄氏度，并伴随着逐渐红移的Ta 2 O 5的吸收边和晶格的膨胀，由于较大的离子半径的N-3相对于O-2。尽管材料因N-2 p而着色？Ta-3d跃迁容易发生，薄膜不显示任何可见光PEC活动，直到氮浓度足够高，形成一个连续的N-2 p杂质带。Ta 3 N5被发现是在575和800摄氏度之间唯一的热稳定相，没有TaON的痕迹。较长的氮化时间导致较低的缺陷浓度、较大的晶粒尺寸和改进的PEC性能。结晶良好的薄膜的光电流受到缓慢的水氧化动力学的限制。这可以通过沉积IrO 2纳米颗粒作为水氧化助催化剂来有效地补救，这导致高达45%的外量子效率。在较长波长下PEC性能的较小增强揭示了Ta 3 N5中的空穴传输限制了IrO 2催化的Ta 3 N5光阳极的水裂解性能。
Tantalum nitride (Ta3N5) and oxynitride (TaON) are promising materials for photoelectrochemical (PEC) water splitting due to near-ideal band gaps and band edge positions. However, the high-temperature ammonolysis process that is usually used to make these materials depends sensitively on the process parameters and specific design of the annealing system, and reproducing highly efficient (oxy)nitride photoanodes from recipes reported by other laboratories has proven to be challenging. To understand and monitor the nitridation process in more detail, we employ an optical absorption spectroscopy technique that allows us to follow the transformation of thin Ta2O5 films in situ at temperatures up to 800 degrees C. Our results show that the incorporation of nitrogen in a dry ammonia atmosphere starts at 575 degrees C and is accompanied by a gradual red-shift of the Ta2O5 absorption edge and an expansion of the lattice due to the larger ionic radius of N-3 relative to O-2. Although coloration of the material due to an N-2p ? Ta-3d transition occurs readily, the films do not show any visible-light PEC activity until the nitrogen concentration is high enough to form a continuous N-2p impurity band. Ta3N5 is found to be the only thermodynamically stable phase between 575 and 800 degrees C, with no traces of TaON. Longer nitridation times result in lower defect concentrations, larger grain sizes, and improved PEC performance. The photocurrent of well-crystallized films is limited by slow water oxidation kinetics. This can be effectively remedied by depositing IrO2 nanoparticles as a water oxidation cocatalyst, which results in external quantum efficiencies of up to 45%. The smaller enhancement of the PEC performance at longer wavelengths reveals that hole transport in Ta3N5 limits the water splitting performance of IrO2-catalyzed Ta3N5 photoanodes.