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ヘマタイトメソ結晶光アノードの開発に関する研究

Zhang, Zhujun 神戸大学

2020.03.25

概要

Photoelectrochemical (PEC) water splitting offers an attractive path to efficiently convert solar energy to clean and renewable energy sources such as hydrogen gas. The development of efficient, economically viable and longtime recyclable semiconductors for PEC water oxidation is still one of the main issues that hinder the practical application. Hematite (α-Fe203) is a promising candidate semiconductor for photoanode owing to its suitable bandgap (-2.1 eV) for sunlight absorption, low cost, and long-term stability in alkaline aqueous solution. During the past decade, great attention has been devoted to develop efficient hematite photoanodes, achieving a large photocurrent with a sufficient cathodic onset potential, however, remains big challenging. One limiting PEC performance of hematite is the short minority carrier lifetime (picosecond time scale) and diffusion length (ID) (2-4 nm), which lead to significant recombination in the bulk or at the surface. The nanostructuring of semiconductor is the predominant route employed to suppress such recombination by shortening the carrier difiusion length required for water splitting. While this approach has resulted effective performance enhancements in nanostructured thin films, it does not address the fundamental issue of conflict between charge separation (short diffusion length) and light harvesting. In addition, the sluggish kinetics for the multi-hole catalysis of water oxidation is recognized as an another main problem that hinders the PEC performance improvement of hematite-based photoanodes. As revealed by transient absorption, operando- spectroelectrochemistry, and density functional theory (DFT) studies, hematite follows a first order reaction at low accumulated surface hole density, but can realize a third order reaction kinetics when the surface hole density is sufficient enough to oxidize the nearest neighbor metal atoms to form the Fe(OH)-O-Fe(OH) sites which can equilibrate with three surface holes. Inspired from these theory, an increase of the surface hole density by improving the hole collection efficiency would be a possible path towards an enhanced surface water oxidation kinetics.

Metal oxide mesocrystals (MCs) are superstructures constructed by highly ordered nanoparticle subunits with tunable optical, electronic, and magnetic properties, and show great prospects in applications ranging from catalysis to optoelectronics. Contributed by the highly ordered structures with intimated interfaces between the attached nanocrystals inside the MC particle, the crystal to crystal charge transfer efficiency is much higher than the system of the randomly aggregated nanocrystals. The atomic reconstruction at the interfaces between the attached nanoparticles, such as formation of interfacial oxygen vacancy (Vo), was rarely reported before. Unlike Vo at the surface, which might play as recombination sites, the interfacial Vo located at the bulk of hematite would act as donor centers and thus significantly improve the bulk carrier density. The engineering of the location and concentration of Vo, therefore, would greatly affect the PEC behavior of hematite. To the best of our knowledge, very limited attentions have been paid to modify the PEC performance of metal oxide semiconductors by engineering the location and concentration of Vo. The development of the synthesis routes to control the location and concentration of Vo in hematite is thus very important to understand the Vo-correlated charge transfer dynamics.

In this dissertation, the basic knowledge of PEC water splitting by using hematite as the photoanode semiconductor is introduced in Chapter 1,mainly focused on the problems that limit its practical performance. And the possible paths to improve the PEC performance of hematite-based photoanodes is proposed based on highly ordered hematite mesocrystal-based hieratical structures with abundant interfacial Vo to suppress the significant recombination. Then, the properties of MC superstructures and role of interfacial Vo in hematite are carefully discussed. Finally, the goal of this study is summarized. In Chapter 2, the main characteristic technologies and theory utilized in this dissertation are presented, including the PEC water splitting cells, scanning transmission electron microscopy-electron energy-loss spectroscopy, electrochemical impedance spectroscopy, and time-resolved fluorescence microscopy.

In Chapter 3,1 present a simple additive-free solvothermal method for the synthesis of hematite MCs. The MC-photoanodes with controlled film thickness were fabricated via a multiple spin-coating technique. High temperature (700 °C) annealing treatment can not only activate the hematite MCs and strengthen the attachments between fluorine doped tin oxide (FTO) and hematite, but also play a significant role in modifying the local structures and charge transfer properties. I prove here that a novel thick Ti-modified hematite MC films (〜1500 nm) can realize the unprecedented highly efficient PEC water oxidation under back illumination. During the high temperature annealing process, abundant interfacial Vo can be created due to the partial fusion of interface between the highly ordered nanocrystals within the MC. As a result, significantly increased carrier density was realized in MC-based photoanode for efficient charge separation and transfer. In addition, the high temperature annealing also induces the formation of unique inner mesoporous inside the MC, and forms an extremely high proportion of depletion region and short depletion width at the hematite/electrolyte interfaces, and thus would greatly improve the charge separation and collection efficiencies. Moreover, time-resolved photoluminescence (PL) measurements showed that longer-lived holes can be generated by excitation with shorter wavelength light to drive a considerable number of delocalized holes (〜14%) to the distance far away from the excited regions (900 nm) to the unexcited region (1500 nm) along the intimately connected MCs. The study presented in Chapter 3 indicates that the interfacial Vo in the bulk of hematite plays a significant role in determining the carrier density, bulk conductivity, as well as the charge transfer dynamics. Inspired by this discovery, I further propose that the engineering the concentration of interfacial Vo in the bulk of hematite would be an effective way to improve the PEC performance of hematite-based photoanodes. As I found that the interfacial V。would be created by sintering the interface between the attached highly ordered nanocrystal subunits inside the MC particle, the concentration and distribution of Vo, therefore, would be engineered by modifying the morphology and size of the MC structures. Considering that, the MCs assembled with smaller nanocrystal subunits would provide more interfaces than that of the one assembled with larger nanocrystal subunits, and thus might be utilized to create a higher concentration of interfacial Vo and achieve higher photocurrent. For this purpose, I canied out the study presented in Chapter 4.

In Chapter 4, I have successfully synthesized FeiO3(Ti) MCs assembled with tiny nanoparticle subunits (〜5 nm) via a surfactant-free solvothermal method. I demonstrate that decreasing the sizes of nanoparticles inside MC is an effective route towards increasing the concentration of bulk Vo. Exceedingly high carrier density (Mi) of 4.1 χ 1021 cm-3 was created due to the formation of a high concentration of interfacial Vo in the bulk, thus resulting in the ultra-high bulk conductivity. The high carrier density contributes to the formation of a large proportion of ultra-narrow depletion layer (く1 nm) in both the outer surface and inner pore surfaces of MC particle, which effectively improves the hole collection efficiency by decreasing the hole migration distance and increasing the hole collection pathways. The superior charge transfer efficiency realized by the higher concentration of interfacial V。was further proved by temperature-dependent PEC measurements, which indicates an ultra-low activation energy for water oxidation; and photoconductive atomic force microscope (AFM) observation the PEC behavior of single particles, which suggest the high concentration of interfacial Vo for a superior electron transfer with Ohmic contact. As a result, the optimized photoanode exhibited the highest photocurrent density (4.3 mA cm-2 at 1.23 V vs. RHE) and long-time durability (over 100 h) for hematite-based photoanodes under back illumination.

In this dissertation, I have proved that the highly ordered and closely packed hematite MC hierarchical structures can promote the charge separation and long range charge transfer, which might also work for other (photo)catalysis processes or solar energy conversion systems. Therefore, the concept of hieratical assembly of highly ordered and closely packed semiconductor MCs might further promote their solar to energy conversion efficiency in the future. In addition, I proved that the high temperature annealing in air condition can successfully create interfacial Vo in the bulk of hematite MCs, which can significantly improve the conductivity of hematite. Based on this finding, I rationally controlled the concentration and location of Vo by adjusting the size and shape of MC structures according to the results presented in Chapter 3 and 4. Finally, I found that element diffusion occurred in Ti-modified hematite MCs synthesized by different methods (as presented in Chapter 3 and 4) after the high temperature annealing. Despite the detailed mechanism of charge carrier dynamics are still not clear, the methodology presented here can help us to fabricate core-shell single-crystalline MC superstructure with proper properties in other applications.

Several problems are still remained to be solved in the future. First of all, the details of charge transfer dynamics related to the interfacial Vo are still not clear. Despite I have obtained time-resolved PL data of single aggregated particle under the microscope. However, I cannot fully explain new emission band observed for the MCs that is highly correlated to the Vo. More works including theoretical calculations might help to understand the mechanism of charge transfer dynamics. Second, although significant enhancement of photocurrent was realized by the hematite MC-based photoanodes, their onset potentials for water oxidation however are still high, due to the inevitable recombination at the surface by the surface states. To solve this problem, surface treatment or active co-catalyst might help to improve the hole transfer from the surface of hematite to electrolyte and thus further lower the onset potentials and increase the photocurrent. The third is the lack of effective way to detect the concentration of Vo in the bulk of hematite. It is still hard for us to quantitatively measure the concentration of interfacial Vo, which might play a significant role in enhancement of the carrier density and charge mobility. Finally, it is still not known how the Ti ions segregated from the bulk to the surface and edge of the pores during the high temperature annealing. In situ atomic resolution scanning transmission electron microscope measurements might help us to monitor the evolution of local structures during the high temperature annealing.

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