Kochi University of Technology Academic Resource Repository

長野, 寛司

Japanese

FPGA を対象としたデータ駆動型プロセッサの設計自動化フローの検討

ZHENG, Yilin

English

The Strategies of Text Revision in Virtual Reality Environments

秋澤, 秀夫

レイリー散乱型光ファイバ分布センサによる VaRTM 成形の樹脂流動モニタリング

永松, 萌

三軸ループアンテナ及びインフラサウンドセンサを用いた多地点観測による雷のエネルギー推定

前川, 泰紀

Web 上で制御可能な信号処理回路の設計と製作

増田, 梓月

バンドギャップ基準電源回路のための素子バラツキの抑制を目的とした設計手法

Ayat, Jabr Salem Al-Azab

Induced Cell Death in Diverse Algal Species by the Exposure to Resin Nanoparticles

高知工科大学博士（工学）thesi

隈部, 佳孝

－1－ 
 
論 文 内 容 の 要 旨 
 
Introduction 
Porous materials having accessible pores and a large surface area, including activated carbon, silica gel, metal 
oxides, zeolites, and metal–organic frameworks (MOFs), are promising functional materials such as gas storage and 
separation, energy storage, drug delivery, catalysis, and catalyst supports. In particular, metal oxides are intriguing 
materials because of their intrinsic properties of semi-conductive behavior, catalytic activity, high temperature stability, 
durability, easiness of handling, and cost. In order to improve the performance of materials, it is extremely important to 
control the size, morphology, crystallinity, and stoichiometry according to the specific application. Therefore, 
considerable effort has been devoted to developing new methods for the synthesis of porous materials. Among them, 
hydrothermal and solvothermal methods have received much attention because of their simplicity, low cost, and ability to 
make new porous nanomaterials.1 In this thesis, solvothermal conditions for controlling the morphology and crystal 
structure of porous nanomaterials are focused. 
 
1. Porous niobia spheres with large surface area: alcothermal synthesis and controlling of their composition and 
phase transition behavior 
Niobia (Nb2O5), and Nb2O5 and titania (TiO2) composite materials have been used as photocatalysts, solid acid, 
catalyst supports, and anode materials for rechargeable batteries, and fuel cells. Several preparation methods are available 
for these materials, but simpler preparation methods should be developed for further acceleration of their practical uses. 
On the other hand, we have successfully synthesized porous spherical nanocrystal assemblies, namely MARIMOs 
(mesoporously architected roundly integrated metal oxides), by our original one-pot solvothermal method (alcothermal 
method) using alcohol as a solvent.2 This synthetic method is an extremely simple method of heating a precursor solution 
consisting of metal compounds and additives in alcohol at a high temperature for a short time in a pressurized vessel. The 
obtained MARIMOs with micro- and meso-pores have a large surface area and nano concave–convex surface structure. 
Therefore, these MARIMOs can be useful materials as catalysts and catalyst supports.3,4 In this chapter, the simple 
synthesis of Nb2O5 MARIMO, TiO2–Nb2O5 composite MARIMO, and TiNb2O7 MARIMO with large surface area by 
applying alcothermal method are described. 
Simple heating of a methanol solution of Nb(OEt)5 and formic acid at 300 °C for 10 min afforded perfectly 
monodisperse Nb2O5 MARIMO in high yield (>90%, Figure 1a). The obtained Nb2O5 MARIMO showed a high specific 
surface area (302 m2/g) and porous structure whose pore size was less than 10 nm-sized. However, the obtained Nb2O5 
－2－ 
MARIMO was not crystalline but amorphous. The crystal phases significantly affect the activity of the photocatalyst and 
solid acid. Thus, it is important to control of the crystal phase formation in the obtained Nb2O5 MARIMO. The easiest 
method to improve the crystallinity is further heating. When the heating time of the precursor solution was prolonged to 
60 min, the crystal phase dramatically changed from amorphous to the hexagonal phase of Nb2O5. This was concomitant 
with a drastic morphology change from spherical to rod-shaped (Figure 1b). Although the crystallinity was successfully 
improved, the porosity was lost due to the morphology change, and the specific surface area was greatly reduced. 
Calcination of the as-synthesized amorphous Nb2O5 MARIMO was used as an alternative method to change the crystal 
phase. Calcination of the as-synthesized amorphous Nb2O5 MARIMO yielded a hexagonal phase while maintained the 
spherical morphology at 500 °C.  
Next, TiO2–Nb2O5 composite MARIMOs were synthesized by heating of a methanol solution of Ti(OiPr)4, 
Nb(OEt)5, and formic acid at 300 °C for 10 min. The molar ratio of Ti to Nb in the product MARIMO was easily 
controlled by changing the ratio in the corresponding precursor solution. As shown in STEM/EDX mappings in Figure 
1c, Ti and Nb atoms were equally distributed at the nano-level. In addition, the TiO2–Nb2O5 composite MARIMOs were 
highly porous with specific surface areas larger than 200 m2/g. 
 
 
Finally, a two-step synthetic approach for the generation 
of TiNb2O7 porous spheres being a significant anode material for 
ion batteries was developed via an alcothermal treatment of a 1:2 
(molar ratio) mixed solution of Ti(OiPr)4 and Nb(OEt)5 in methanol followed by simple calcination. First step, a similar 
alcothermal treatment afforded a precursor composite MARIMO with tetragonal TiO2 and amorphous Nb2O5. The 
second step involved calcination of the precursor MARIMO at 700 °C for 2 h and yielded monoclinic TiNb2O7 
－3－ 
MARIMO. However, the XRD patterns showed that the calcined MARIMO contained mainly monoclinic TiNb2O7 with 
a small amount of tetragonal TiO2 and orthorhombic Nb2O5 (Figure 2). This indicates that a pure TiNb2O7 MARIMO 
was not afforded by the calcination of the as-synthesized TiO2–Nb2O5 MARIMO obtained at 300 °C. Therefore, 
amorphous TiO2–Nb2O5 composite MARIMO was selected as an alternative precursor. As shown in Figure, calcination 
of the amorphous TiO2–Nb2O5 composite MARIMO at 600 °C for 2 h produced the desired TiNb2O7 MARIMO with a 
pure monoclinic phase. The specific surface area of the obtained monoclinic TiNb2O7 MARIMO was not so large (50 
m2/g) as compared to that of before calcination (497 m2/g), however the number 50 m2/g was the largest one to our best 
knowledge.  
 
 
3. Rapid one-pot synthesis of ultrafine titania nanocrystals and their conversion into transparent mesoporous thin 
layer films 
Controlling the size and morphology of inorganic nanocrystals is an important aspect of improving certain 
intrinsic characteristics, such as electronic, magnetic, 
photophysical, and catalytic properties. Especially, 
nanocrystals less than 10 nm in size show unique 
properties. However, synthesis of such ultrafine 
nanoparticles is quite difficult, since sintering or 
agglomeration of primary particles occur easily in their 
preparation process. Herein, a simple one-pot approach 
to producing ultrafine TiO2 nanocrystals using a rapid 
heating technique in conjunction with a solvothermal 
reaction using aliphatic carboxylic acid are described. 
This rapid heating method enhances the nucleation and 
crystal growth of the nanocrystals, thus avoiding 
undesired sintering during the high temperature 
solvothermal reaction and ensuring sub-5 nm sized nanocrystals. 
Ultrafine TiO2 nanocrystals were synthesized by rapid heating solvothermal method using liquid mixtures of 
titanium alkoxide and oleic acid as precursor solutions. The important point of this method is to heat rapidly (500 
˚C/min). The obtained TiO2 nanocrystals have extremely small size and narrow distribution (3.0 ± 0.4 nm), resulting in 
the periodic nanostructures with hexagonal symmetry (Figure 3). These results show that the present method yields 
extremely fine and uniformly sized TiO2 nanocrystals. To confirm the role of the oleic acid in the formation of these TiO2 
－4－ 
nanocrystals, a detailed characterization of the products using FT-IR spectroscopy and TG analysis were performed. 
These results show that the surfaces of the nanocrystals are covered with oleate anions. The surface covering of oleate 
anion on the nanocrystals may also contribute the hexagonal periodic alignment of the nanocrystals through the lipophilic 
interaction between unsaturated aliphatic chains. 
The nanocrystals can also be re-dispersed stably in a non-polar solvents such as n-hexane and toluene. The 
n-hexane dispersion was easily converted into porous TiO2 thin layer transparent film through drop-casting and 
calcination process. The optical properties of the resultant mesoporous TiO2 film were also assessed. As shown in 
Figure4a, the film had good optical transparency even after three cycles of drop-casting and calcination. In order to 
evaluate the catalytic ability of the mesoporous TiO2 thin layer film, the transient photocurrent measurements were 
performed. The use of the mesoporous TiO2 film electrode prepared from our ultrafine TiO2 nanocrystals afforded 2.7 
times higher photocurrent than that of the commercial TiO2 nanoparticles film electrode (Figure 4b). 
 
 
 
 
4. Selective synthesis of Ti-based nanocrystals via hydrothermal method using squaric acid 
TiO2 is known as a promising material used in a wide area of applications in energy storage, photocatalysts, solar 
cells, and other optoelectronic devices. Controlling the crystal structure and exposing desired crystal face are important 
for improving performance. Three types of TiO2 crystal structures were known: rutile, anatase, and brookite. Stability of 
the TiO2 phases depends on their particle sizes. Rutile is the most stable phase for particles above 35 nm, anatase is the 
most stable phase for nanoparticles below 11 nm, and brookite is found to be the most stable for nanoparticles of 
11–35nm.5 Although anatase of TiO2 with large surface area has been well studied, few studies have been reported on the 
synthesis of rutile-type TiO2 with a specific surface area over 100 m2/g. In this chapter, hydrothermal synthesis of TiO2 
nanocrystal assemblies with rutile crystal phase was studied using squaric acid having quite strong acidity (pKa1 = 0.54) as 
an organic acid as well as unique square molecular structure with various coordination modes to metal cation. 
－5－ 
Ti(OiPr)4 was mixed with water to form a white suspension. Squaric acid was added to the suspension and the 
suspension was sonicated. Then, the white suspension changed to a homogeneous red solution. Next, the precursor 
aqueous solutions with different concentrations of squaric acid were heated at 150 °C for 3 h in a Teflon autoclave. 
Hexagonal plates (Figure 5a) and urchin-like submicron sized particles (Figure 5b) were obtained from high and low 
concentrations of squaric acid, respectively. XRD patterns of the obtained hexagonal plates were totally different from the 
know crystal phases of TiO2 (Figure 5c, red line). In addition, the TG data of the hexagonal plates demonstrate 
weight-loss of approximately 40% in the temperature range from 300 to 400 °C, indicating the inclusion of organic 
materials. These results strongly suggest that the hexagonal plate is MOF consisting of titanium ions and squarate ligands. 
On the other hand, XRD patterns of urchin-like particles demonstrated the presence of a rutile phase (Figure 5c, blue 
line). The urchin-like particles showed a high specific surface area (122 m2/g). 
 
 
 
Activity of Ru/TiO2 methanation catalysts strongly depends 
on its support structure. According to the crystallography studies, 
RuO2 possesses not only the same crystalline structure to rutile-type 
TiO2, but also has a high degree of lattice matching with rutile-type 
TiO2 owing to their close lattice parameters. Therefore, this strong interaction can promote the dispersion of RuO2 
nanoparticles on the TiO2 support to a great extent and it can prevent RuO2 nanoparticles aggregation. Ru/TiO2 catalysts 
with Ru amount of 2 wt% were prepared by the wet impregnation method using the obtained urchin-like rutile TiO2 
assemblies (Ru/r-TiO2). Actually CO2 methanation using Ru/r-TiO2 as a catalyst showed better performance than 
reference standard catalyst, Ru/JRC-TIO-16 (Figure 6). To obtain direct evidence for Ru dispersion on the TiO2 supports, 
quantitative evaluation was performed by CO pulse experiment. The Ru dispersion of Ru/rTiO2 and Ru/JRC-TIO-16 
were 16.5% and 4.7%, respectively. Thus, Ru/rTiO2 obtained by hydrothermal synthesis using squaric acid resulted in 
highly efficient dispersion of Ru and acceleration of the CO2 methanation. 
－6－ 
 
 
Summary 
Porous metal oxide nanomaterials with controlled crystal structures were prepared by changing several 
experiment parameters, including solvent, additive, reaction temperature, and heating rate, in solvothermal method. 
Nb2O5 MARIMO, TiO2–Nb2O5 composite MARIMO, and TiNb2O7 MARIMO with large surface area were 
successfully synthesized using alcothermal method at high temperature. Ultrafine TiO2 nanocrystals were obtained by a 
rapid heating technique in conjunction with a solvothermal reaction using aliphatic carboxylic acid. These nanoparticles 
were easily converted into porous TiO2 thin layer transparent film through drop-casting and calcination process, showing 
not only anti-reflection properties but also high photocatalytic activity. The rutile-type urchin-like TiO2 obtained by 
hydrothermal synthesis using squaric acid exhibited an excellent ability as a support for Ru-supported catalyst. 
 
 
Reference 
1. Lane, M. K. M.; Zimmerman, J. B. Controlling metal oxide nanoparticle size and shape with supercritical fluid 
synthesis. Green Chem. 2019, 21, 3769–3781. 
2. Wang, P.; Kobiro, K. Ultimately simple one-pot synthesis of spherical mesoporous TiO2 nanoparticles in 
supercritical methanol. Chem. Lett. 2012, 41, 264–266. 
3. Ohtani, M.; Muraoka, T.; Okimoto, Y.; Kobiro, K. Rapid one-pot solvothermal batch synthesis of porous 
nano-crystal assemblies composed of multiple transition-metal elements. Inorg. Chem. 2017, 56, 11546–11551. 
4. Duriyasart, F.; Irizawa, A.; Hayashi, K.; Ohtani, M.; Kobiro, K. Sintering-resistant metal catalysts supported on 
concave-convex surface of TiO2 nanoparticle assemblies. ChemCatChem 2018, 10, 3392–3396. 
5. Chen, S.; Xiong, F.; Huang, W. Surface chemistry and catalysis of oxide model catalysts from single crystals to 
nanocrystals. Surf. Sci. Rep. 2019, 74, 100471. 
 
 


Solvothermal Synthesis of Porous Nanomaterials with Precisely Controlled Crystal Structure

清水, 岬

１次元水素原子系における非リュードベリ・スペクトルの数値的研究

寺村, 舞童華

ガイスターにおいてプレイ履歴から駒色推定を行うアルゴリズムの研究

Kochi University of Technology

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