open access

https://doi.org/10.14943/doctoral.k14463

https://hdl.handle.net/2115/84524

表面/界面構造制御によるヘマタイト系光電極の効率的な水の酸化反応に関する研究

英語

2021-03-25

Hokkaido University

Ⅲ, 133p

Photoelectrochemical (PEC) water splitting is a promising approach for direct conversion of solar energy to hydrogen. Among various semiconductors, hematite (α Fe2O3) has emerged as an excellent photoanode material due to its significant light absorption, chemical stability in aqueous solutions, and earth abundant property. However, its performance has been crucially limited by poor optoelectronic properties and sluggish reaction kinetics for water oxidation. Two essential criteria, including sufficient targeted reaction sites and efficient interfacial charge transfer, should be considered to enhance the performance of hematite-based photoanodes. Thus, this thesis focused on rationally designing efficient co-catalysts with modulated active sites as well as interface engineering by inserting hole transfer mediators/constructing direct chemical interaction between α-Fe2O3 and co-catalysts. In chapter 1, a general background about photoelectrochemistry and a simple overview of α-Fe2O3 photoanodes is introduced. Then, the recent development of modulation strategies to promote the PEC performance of α-Fe2O3 is summarized. In chapter 2, an ultrathin cobalt-manganese (Co-Mn) nanosheet, consisting of amorphous Co(OH)x layers and ultrasmall Mn3O4 nanocrystals, is designed as an efficient co-catalyst on α-Fe2O3 film for PEC water oxidation. The uniformly distributed Co-Mn nanosheets lead to a remarkable 2.6-fold enhancement on the photocurrent density at 1.23 V vs. reversible hydrogen electrode (RHE) and an impressive cathodic shift (~200 mV) of onset potential compared with bare α-Fe2O3 film. Furthermore, the decorated photoanode exhibits a prominent resistance against photo-corrosion with an excellent stability for over 10 h. Detailed mechanism investigation manifests that incorporation of Mn sites in the nanosheets could create electron donation to Co sites and facilitate the activation of OH group, which drastically increases the catalytic activities for water oxidation. These findings provide valuable guidance for designing high-performance co catalysts for PEC applications and open new avenues towards controlled fabrication of mixed metallic composites. In chapter 3, in order to reinforce the interfacial interaction at the α-Fe2O3/co-catalyst interface, a novel charge transfer system for PEC water oxidation is designed by inserting MXene nanosheets (MNs) between α-Fe2O3 and co-catalyst. In this system, MNs act as the hole transfer mediators to efficiently suppress the interfacial charge recombination owing to the high hole mobility of MNs and the formation of built-in electric field at the MNs/α-Fe2O3 junction. Meanwhile, the co-catalyst layers, in turn, can protect the MNs from oxidation to achieve a prominent stability. The optimized photoanode of Co Pi/MNs/α-Fe2O3 can achieve a remarkable photocurrent density, up to 3.20 mA cm-2 at 1.23 V vs. reversible hydrogen electrode (RHE) under AM 1.5 G illumination. An impressive cathodic onset potential shift of ~250 mV is obtained with the synergistic effect of MNs and co-catalyst (Co-Pi). Furthermore, this strategy is also applicable to other photoanode materials, such as BiVO4, WO3 and ZnO, verifying the versatility by utilizing the MNs as hole transfer mediators for efficient photogenerated charge separation to enhance the PEC water oxidation. In chapter 4, direct chemical interaction is constructed at the interface of α-Fe2O3 and carbon nanosheets with single-nickel sites (Ni-NC) to accelerate the reaction kinetics by providing additional charge transport channels and abundant active sites. The interfacial carrier path induced by the chemical coupling and the efficient single-nickel sites work collaboratively, achieving an impressive photocurrent density of 1.85 mA cm-2at 1.23 V vs. RHE, up to 2.2 times higher than that of pure α-Fe2O3. These findings shed light on an interface modulation strategy and provide an alternative towards utilizing unique single active sites for efficient photoelectrochemical water splitting. In chapter 5, an overall summary of this dissertation work was presented. This thesis carried out a systematic study on the surface modification of hematite-based photoanodes for efficient photoelectrochemical water oxidation. In α-Fe2O3-based PEC water oxidation system, co-catalysts decoration has been demonstrated to be the most efficient way to lower the reaction barrier and promote charge injection to the reactants. And the delicate modification of the interface between the α-Fe2O3 and the co-catalysts is critical for promoting charge transfer from the bulk of α-Fe2O3 to the co-catalysts, which can directly influence the surface catalysis. The relevant findings in this study deepen the understanding of α-Fe2O3-based PEC water oxidation system and highlight the importance of semiconductor/co-catalyst interface modulation for the overall photoelectrocatalytic processes.

2021-03-25

甲第14463号

博士(理学)

(主査) 教授 村越 敬, 教授 上野 貢生, 客員教授 打越 哲郎, 客員教授 葉 金花, 客員教授 白幡 直人

総合化学院（総合化学専攻）

博士論文

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