製備高品質氮化鉭光陽極於光電化學之應用

Abstract

光電化學 (PEC) 水分解利用半導體材料將太陽能用以分解水轉換為氫氣，是一個有前景的能量儲存方式。在過渡金屬氮化物中，五氮化三鉭 (Ta3N5) 因其比過度金屬氧化物 (Transition metal oxides, TMOs) 好的載子傳輸性質、約2.1電子伏特的能隙及合適進行水氧化反應之能帶位置而成為有前景的光陽極候選材料，其理論太陽能轉氫氣之效率 (solar-to-hydrogen efficiency, STH) 可以達到15.9%。近期的研究中，雖然Ta3N5光陽極系統的效率也已經趨近理論值所預期，但其缺點是因大量使用鉭金屬所導致的較差的材料使用效率。因此，本研究中利用矽基板作為導電層以磁控反應濺鍍法 (reactive magnetron sputtering) 合成介穩態三氮化二鉭 (Ta2N3) 作為前驅物，並將前驅物以氨氣燒結合成Ta3N5光陽極。首先，本研究利用氨氣燒結之溫度調控表面缺陷含量，特別是在Ta3N5能帶結構中做為電子電洞複合中心的晶格中的氮空缺 (NV) 及低價態的鉭陽離子 (Ta3+) 可在燒結溫度820°C 並持溫三小時的情況下在表面消除。除此之外，使用Ta2N3而非Ta2O5作為Ta3N5製備之前驅物優點便在於，以Ta2N3作為前驅物時會在薄膜塊材之相組成中引進低價態的氮化鉭族群如Ta2N及Ta5N6，這些低價態的氮化鉭族群可以有效的增加Ta3N5光陽極的載子傳輸性質。在黃血鹽 (K4[FeCN6]) 作為電洞捕捉劑 (hole scavenger) 存在於電解液中時，沉積在高度砷參雜之n型矽 (111) 基材上生長之Ta3N5光陽極有著最好的光電化學活性，其光電流密度可達3.86 mA/cm2，並表現出對可逆氫電極0.48伏 (VRHE) 之起始電位，這樣的表現優於使用其他參雜及方向之矽基材系統。這樣的差異最主要來自於使用不同參雜及不同方向性之矽基材時其與Ta3N5之異質接面的差異造成，並可用光電化學阻抗譜 (photoelectrochemical impedance spectroscopy, PEIS) 分析驗證其造成之光電化學活性差異。沉積在磷參雜之n型矽 (100) 基材上之Ta3N5光陽極會因兩個半導體材料相接形成之n-n異質接面 (n-n heterojunction)，而產生光生電子電洞堆積在n-n異質接面之間，產生大量的漏電流消耗光生載子並降低可獲得的光電流密度。本研究之結果近一步驗證了藉由更改氮化前驅物及使用矽基板作為導電層可以完整地建立一個更有效利用鉭金屬的光陽極合成平台，並可以做為之後以矽基板作為導電層時進一步改善光電化學活性的基礎。

Photoelectrochemical (PEC) energy conversion is a promising pathway to store excess energy in the form of chemical fuels. Ta3N5 represents a promising photoanode candidate owing to its better charge transport properties compared to transition metal oxides (TMOs), theoretical its ~ 2.1 eV band gap, suitable band energetics for oxygen evolution reaction (OER), and a theoretical solar-to-hydrogen (STH) efficiency of 15.9%. As far, as the Ta3N5-based photoanode system has almost achieved its theoretical limitation but with the outcome of inefficient material usage caused by the massive usage of tantalum metal. Herein, Ta3N5 thin films were fabricated on n+-Si(111) substrate through nitridation of the metastable Ta2N3 films synthesized via reactive magnetron sputtering. To begin with, the surface composition, especially, the deep-trap sates in the Ta3N5 system, namely nitrogen-vacancy (VN), and low-valence Ta cations (Ta3+) can be effectively tuned by precisely controlling annealing temperature. A defect-free surface composition can be achieved by annealing at 820°C for 3 hours. Aside from the surface composition, the incorporation of Ta2N3 as a nitridation precursor can effectively introduce low-valence Ta2N and Ta5N6 content in the bulk composition that facilitates the carrier transport of Ta3N5 photoanode system compared to Ta3N5 fabricated from Ta2O5. In the presence of K4[Fe(CN)6] as a hole scavenger, the Ta3N5/n+-Si(111) photoanode demonstrated an onset potential at 0.48 VRHE and 3.86 mA/cm2 photocurrent density at 1.23 VRHE, outperforming comparable films grown on the other two substrates. Such differences in PEC performance were attributed to the heterojunction between Ta3N5 and Si and verified by photoelectrochemical impedance spectroscopy (PEIS). This heterojunction dominated the carrier dynamics as the leakage current across the heterojunction would consume the photogenerated holes and consequently diminish the photocurrent density. The present study reveals the importance of constructing a platform for efficient material usage of the Ta3N5-based photoanode system via the incorporation of silicon as a conductive substrate and the metastable Ta2N3 as nitridation precursor films, which can serve as a foundation for further improvement of the PEC performance of silicon-based Ta3N5 photoanode system.