学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 堂免 一成, 東京大学教授 山下 晃一, 東京大学教授 S.Ted Oyama, 東京大学准教授 脇原 徹, 東京大学准教授 嶺岸 耕, 東京理科大学教授 工藤 昭彦University of Tokyo(東京大学

Liu Jingyuan

刘 婧媛

UTokyo Repository

論 文 の 内 容 の 要 旨 
 
 
 
論文題目 Development of lanthanum titanium copper oxysulfide (La5Ti2CuS5O7) based 
photocathodes for solar water splitting    
(太陽光による水分解反応を目的としたランタンチタン銅酸硫化物（La5Ti2CuS5O7）系
光カソードの開発） 
 
氏  名    刘 婧媛 （リュウ セイエン） 
 
 
 
Chapter 1: General Introduction 
Conversion of renewable solar energy into storable and transportable chemical fuels is an 
important strategy towards realization of a sustainable society. Photoelectrochemical (PEC) 
water splitting is an ideal means for direct conversion of the solar energy into hydrogen as a 
renewable chemical energy. Many semiconductors are capable of generating a high 
photocurrent attributable to hydrogen or oxygen production; however a sufficiently high applied 
voltage is often required. To solve this problem, a p/n-PEC cell based on two-step excitation of 
series-connected photocathode and photoanode has attracted much attention, because this 
system can generate a sufficient photovoltage to drive the water splitting reaction. By matching 
the photocurrents of the photocathode and the photoanode at the same electrode potential, and 
the reduction and oxidation of water occur separately on the surfaces of the respective 
photoelectrodes. The operation photocurrent of a p/n-PEC cell can be estimated from the 
intersection of steady current-potential curves of the respective photoelectrodes, and it is 
proportional to the solar-to-hydrogen energy conversion efficiency. Therefore, the onset 
potential of photocathodes should be more positive to fabricate more efficient p/n PEC cells. 
La5Ti2CuS5O7 (LTC) is an oxysulfide semiconductor material with an absorption edge 
wavelength of 650 nm. LTC consists of series of TiOxS6-x octahedra and CuS4 tetrahedra 
growing along the b-axis and forms rod-like particles. The previous studies revealed that 
LTC/Au photoelectrodes fabricated by the particle transfer method was active in PEC hydrogen 
production from water and exhibited a photocurrent onset potential of +0.8 V vs. RHE, which 
was one of the most positive potentials among ever-existing photocathodes. However, the 
photocurrent observed was lower than the calculated photocurrent of 16.8 mA cm-2 assuming 
the incident photon-to-current efficiency of unity. There is still large room to improve the 
photocurrent of LTC. 
 
Chapter 2. Photoelectrochemical activity of doped La5Ti2CuS5O7 electrodes fabricated by 
particle transfer in hydrogen evolution. 
Doping of photoelectrode materials such as BiVO4 and Fe2O3 has been widely researched 
to control the electronic properties and improve the photocurrent. In this chapter, effects of 
doping LTC with aliovalent cations were investigated.Sc3+, Ga3+, Al3+, Mg2+, Nb5+ and Ta5+were 
doped into the Ti4+ site during the synthesis process. As Sc3+ and Ga3+ have lower valence states 
than Ti4+, they are expected to provide free holes in LTC. Conversely, Nb5+ and Ta5+ have 
higher valence states than Ti4+; hence, they are expected to provide free electrons in LTC. 
The successful synthesis of doped La5Ti2CuS5O7 was confirmed by XRD, SEM and DRS. 
By particle transfer method,3 undoped and doped LTC particles are prepared into electrode and 
their PEC properties are measured. The onset potential of 1% Sc doped-La5Ti2CuS5O7 
photocathode was +0.88 V vs. RHE, which is 80 mV more positive than undoped one. In 
addition, the photocurrent density of the 1% Sc doped-La5Ti2CuS5O7 photocathode was 0.80 
mA cm-2 at 0 V vs. RHE, which was approximately eight-fold compared with the undoped 
sample. The enhanced PEC activity was also observed for La5Ti2CuS5O7 photocathodes doped 
with 1% Ga. By contrast, Ta- and Nb-doped La5Ti2CuS5O7 photocathodes showed lower PEC 
activity. These results suggest that an increase in the majority carrier concentration in 
La5Ti2CuS5O7 by doping of lower valent cations into Ti sites improved the PEC activity. The 
PEC activity of La5Ti2CuS5O7 at different doping levels of Sc was also investigated. It was 
found that the doping level of 1% was the optimum. 
 
Chapter 3. Effect of Particle Size of La5Ti2CuS5O7 on Photoelectrochemical Properties in 
Solar Hydrogen Evolution 
Density functional theory (DFT) calculations have shown that the conduction band 
minimum and valence band maximum of LTC are localized around linear chains of the Ti(O,S)6 
and CuS4 units, respectively. Therefore, photogenerated electrons and holes can move along the 
b-axis to the edge surfaces of LTC particles separately through these linear chains. The selective 
deposition of Pt particles on the edge surface of rod-like LTC particles by the PEC reduction of 
[PtCl6]2- has been demonstrated. Based on the one-dimensional carrier transport properties of 
rod-like LTC particles, it is believed that the morphology of LTC particles have a significant 
influence on the PEC performance of LTC photoelectrodes. In section 3.2, LTC particles with 
different sizes were prepared by controlling the annealing duration and through sequential 
centrifugation, and effects of particle size on the PEC properties were investigated. 
At first, LTC particles prepared with longer annealing durations were found to exhibit 
higher photocurrents, which may due to the higher ratio of large particles formed. In addition, a 
sequential centrifugation was implemented to more vigorously discuss the effect of particle size 
on the PEC performances of LTC electrodes. Results indicated that Rod-like LTC particles with 
larger sizes were found to exhibit higher photocurrents.SEM observations revealed that Pt 
nanoparticles were PEC-deposited not only on the edge surfaces but also on the lateral surface 
for small Mg-doped LTC particles, while they were selectively deposited on the edge surfaces 
for large Mg-doped LTC particles. The use of well-grown, rod-like LTC particles is 
advantageous for PEC hydrogen production. 
 
Chapter 4. Effect of surface modification of particulate La5Ti2CuS5O7 photocathodes with 
oxides on charge separation and hydrogen evolution 
The surface modification of compact thin film photocathodes composed of materials such 
as metal oxides, pnictogenides and chalcogenides with n-type semiconducting oxide layers has 
been widely researched with the aim of enhancing the charge separation and improving both the 
photocurrent and the stability. TiO2 is one of the most popular anti-corrosive, passivation and/or 
charge separation layers, owing to its high chemical stability and the suitable position of its 
band gap. Despite this, the effects of surface modifications using TiO2 and similar compounds 
on the PEC properties of particulate photocathodes with a high degree of inhomogeneity have 
rarely been studied. In Chapter 4, TiO2, Nb2O5, Ta2O5 and ZrO2 were deposited on particulate 
Mg-doped LTC (Mg-LTC) electrodes by radio-frequency (RF) reactive magnetron sputtering, 
and effects on the PEC properties were investigated.  
As a result, Mg-LTC electrodes modified with Nb2O5, Ta2O5 and TiO2 exhibited cathodic 
photocurrents that approximately doubled over the potential range of 0 to 0.9 V vs. RHE 
compared to the value obtained from an unmodified electrode. In addition, at 0.7 V vs. RHE, 
Mg-LTC photocathodes modified with these oxides maintained a photocurrent attributed to PEC 
hydrogen evolution. Increases in the population of long-lived photoexcited carriers existing on 
the microsecond timescale in these materials demonstrated the vital role of the oxide surface 
modification in obtaining effective charge separation. This work suggests opportunities for 
improving PEC hydrogen evolution on particulate photocathodes based on surface oxide 
modifications and also highlights the importance of the band gap alignment. 
 
Chapter 5: Summary and Outlooks 
Through the above studies, the photocathodic current of LTC electrodes attributed to PEC 
H2 evolution was improved by almost two orders of magnitude. As a consequence, the STH of 
p/n PEC cells based on LTC photocathodes was estimated to be upgraded by more than 20 
times. However, there is still a large gap between the observed and calculated photocurrents of 
LTC electrode, indicative of necessity of further improvements.  
Considering the one-dimensional carrier transport properties of LTC particles, the 
orientation of rod-like LTC particles on Au conductive layer should be controlled. In addition, 
my recent study showed that two-step post-annealing in sulfur vapor enhanced the photocurrent 
of LTC electrodes, suggesting room for further refinement in the properties of particulate LTC 
electrodes. The nature of LTC material and electrodes should be investigated further in 
follow-up studies.In addition, this thesis reveals the necessary of studying oxysulfide materials. 
The valence band of oxysulfides are often composed of S 3p orbitals as well as O 2p orbitals, 
hence the band gap of oxysulfides are narrow enough to absorb visible light. On the other hand, 
due to the involved of oxygen, a relatively stable performance can be expected from those 
materials.5 Until now, La5Ti2CuS5O7 is the only oxysulfide material that can be prepared and 
used as photocathode, but with property experiment design and the help from computer science 
such as chemoinformatics or density functional theory (DFT), hopefully more oxysulfide 
materials with high performance will be discovered and utilized in the future. 


太陽光による水分解反応を目的としたランタンチタン銅酸硫化物（La5Ti2CuS5O7）系光カソードの開発

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太陽光による水分解反応を目的としたランタンチタン銅酸硫化物（La5Ti2CuS5O7）系光カソードの開発

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