Daiki Atarashi

Enhanced Photoactivity with Nanocluster-Grafted Titanium Dioxide Photocatalysts

Titanium dioxide (TiO2), as an excellent photocatalyst, has been intensively investigated and widely used in environmental purification. However, the wide band gap of TiO2 and rapid recombination of photogenerated charge carriers significantly limit its overall photocatalytic efficiency. Here, efficient visible-light-active photocatalysts were developed on the basis of TiO2 modified with two ubiquitous nanoclusters. In this photocatalytic system, amorphous Ti(IV) oxide nanoclusters were demonstrated to act as hole-trapping centers on the surface of TiO2 to efficiently oxidize organic contaminants, while amorphous Fe(III) or Cu(II) oxide nanoclusters mediate the reduction of oxygen molecules. Ti(IV) and Fe(III) nanoclusters-modified TiO2 exhibited the highest quantum efficiency (QE = 92.2%) and reaction rate (0.69 μmol/h) for 2-propanol decomposition among previously reported photocatalysts, even under visible-light irradiation (420-530 nm). The desirable properties of efficient photocatalytic performance with high stability under visible light with safe and ubiquitous elements composition enable these catalysts feasible for large-scale practical applications.

Photocatalytic carbon dioxide reduction by copper oxide nanocluster-grafted niobate nanosheets.

Amorphous copper oxide (Cu(II)) nanoclusters function as efficient electrocatalysts for the reduction of carbon dioxide (CO2) to carbon monoxide (CO). In addition to promoting electrocatalytic activity, Cu(II) nanoclusters act as efficient cocatalyts for CO2 photoreduction when grafted onto the surface of a semiconductor (light harvester), such as niobate (Nb3O8(-)) nanosheets. Here, the photocatalytic activity and reaction pathway of Cu(II)-grafted Nb3O8(-) nanosheets was investigated using electron spin resonance (ESR) analysis and isotope-labeled molecules (H2(18)O and (13)CO2). The results of the labeling experiments demonstrated that under UV irradiation, electrons are extracted from water to produce oxygen ((18)O2) and then reduce CO2 to produce (13)CO. ESR analysis confirmed that excited holes in the valence band of Nb3O8(-) nanosheets react with water, and that excited electrons in the conduction band of Nb3O8(-) nanosheets are injected into the Cu(II) nanoclusters through the interface and are involved in the reduction of CO2 into CO. The Cu(II) nanocluster-grafted Nb3O8(-) nanosheets are composed of nontoxic and abundant elements and can be facilely synthesized by a wet chemical method. The nanocluster grafting technique described here can be applied for the surface activation of various semiconductor light harvesters, such as metal oxide and/or metal chalcogenides, and is expected to aid in the development of efficient CO2 photoreduction systems.

Selective Growth of n-Type Nanoparticles on p-Type Semiconductors for Z-Scheme Photocatalysis

Nanoparticles of an n-type WO3 semiconductor were segregated on the surface of p-type CaFe2O4 particles by a heterogeneous nucleation process under controlled hydrothermal conditions. By use of this approach, WO3 nanoparticles were selectively deposited on the surface of CaFe2O4, resulting in a significant increase in the photocatalytic reaction rate of the WO3/CaFe2O4 composite for the decomposition of gaseous acetaldehyde under visible-light irradiation. The high visible-light activity of the WO3/CaFe2O4 composite was due to efficient charge recombination through the junctions that formed between the two semiconductors.

Ti(IV) nanoclusters as a promoter on semiconductor photocatalysts for the oxidation of organic compounds

The surface modification of semiconductors is a potential approach for the development of visible-light-sensitive photocatalysts. Here, we report that amorphous Ti(IV) nanoclusters grafted onto metal oxide photocatalysts function as efficient promoters for the oxidation of organic contaminants, including acetaldehyde and 2-propanol. Ti(IV) nanoclusters were facilely grafted onto metal oxides like titanium dioxide (TiO2) and tungsten trioxide (WO3) by using a simple impregnation method. The photocatalytic activity of Ti(IV) nanocluster-grafted TiO2 under UV-light irradiation was much higher than that of bare TiO2, even though the surface area and photon absorption of these two materials were identical. The improved photocatalytic activity was attributable to hole trapping by the Ti(IV) nanoclusters, which form a unique electronic structure, resulting in efficient charge separation. Kelvin probe force microscopy analysis revealed that the highest occupied molecular orbital (HOMO) of the Ti(IV) nanoclusters has a more negative potential than the valence band of bulk rutile TiO2, which allows hole transfer from bulk TiO2 to the Ti(IV) nanoclusters. The grafting of Ti(IV) nanoclusters was also shown to increase the photocatalytic activity of WO3. WO3 also requires reduction reaction promoters, such as Cu(II) nanoclusters, because of the low energy of its conduction band (CB). The grafting of both Ti(IV) and Cu(II) nanoclusters onto WO3 resulted in the highest reaction rate reported to date for the decomposition of gaseous 2-propanol under visible-light irradiation.

Examination of interfacial charge transfer in photocatalysis using patterned CuO thin film deposited on TiO2

We examined the interfacial charge transfer effect on photocatalysts using a patterned CuO thin film deposited on a rutile TiO2 (110) substrate. Photocatalytic activity was visualized based on the formation of metal Ag particles resulting from the photoreduction of Ag+ ions under visible-light illumination. Ag particles were selectively deposited near the edge of CuO film for several nanometer thick CuO film, indicating that interfacial excitation from the valence band maximum of TiO2 to the conduction band minimum of CuO plays a key role in efficient photocatalytic activity of CuO nanocluster-grafted TiO2 systems with visible-light sensitivity.

Tungstate nanosheet ink as a photonless and electroless chromic device

Cesium tungstate (Cs4W11O362−) nanosheets coated on an aluminum (Al) substrate turned blue in an aqueous solution, acting as an ink, and rapidly decolorized under air exposure, acting as an eraser. The decoloration rate of Cs4W11O362− nanosheets was several orders of magnitude more efficient than that of conventional tungsten trioxide (WO3) particles, owing to efficient electron transfer in the nanosheet structure.

Copper Sulfide Catalyzed Porous Fluorine-Doped Tin Oxide Counter Electrode for Quantum Dot-Sensitized Solar Cells with High Fill Factor

The performance of quantum dot-sensitized solar cell (QDSSC) is mainly limited by chemical reactions at the interface of the counter electrode. Generally, the fill factor (FF) of QDSSCs is very low because of large charge transfer resistance at the interface between the counter electrode and electrolyte solution containing redox couples. In the present research, we demonstrate the improvement of the resistance by optimization of surface area and amount of catalyst of the counter electrode. A facile chemical synthesis was used to fabricate a composite counter electrode consisting of fluorine-doped tin oxide (FTO) powder and CuS nanoparticles. The introduction of a sputtered gold layer at the interface of the porous-FTO layer and underlying glass substrate also markedly reduced the resistance of the counter electrode. As a result, we could reduce the charge transfer resistance and the series resistance, which were 2.5 [Ω] and 6.0 [Ω], respectively. This solar cell device, which was fabricated with the presently designed porous-FTO counter electrode as the cathode and a PbS-modified electrode as the photoanode, exhibited a FF of 58%, which is the highest among PbS-based QDSSCs reported to date.

INFLUENCE OF SODIUM GLUCONATE ON THE HYDRATION OF CEMENT IN SLUDGE WATER

This paper investigated the influence of sodium gluconate on the hydration of cement in sludge water. And the interaction between sodium gluconate and calcium aluminate was also investigated. The rate of heat liberation of ordinary Portland cement paste with sodium gluconate was measured by using conduction calorimeter. The concentration of residual sodium gluconate in liquid phase was calculated by carbon analysis using a total organic carbon analyzer (TOC). And the adsorbed amount of sodium gluconate was calculated from the concentration of residual sodium gluconate. The hydration rate of alite was retarded by addition of sodium gluconate. And the hydration of calcium aluminate in cement was also retarded by adding of sodium gluconate. The adsorption behavior of sodium gluconate onto calcium aluminate did not obey the Langmuir adsorption. There was the linear relation between dosage and adsorbed amount of sodium gluconate. And the interaction between calcium aluminate and sodium gluconate was very strong. So it is considered that late-addition of sodium gluconate for cement sludge is effective to control the hydration of alite in cement sludge. And the hydration of cement with comb-type superplasticizer was retarded by addition of sodium gluconate. So the authors can consider that sodium gluconate is also effective to control the hydration of alite with superplasticizer in sludge water.本研究ではC3Aとグルコン酸ナトリウムの相互作用およびポリカルボン酸系分散剤を添加したセメントペーストにグルコン酸ナトリウムを添加した場合の水和発熱特性について検討を加え、スラッジ水中のセメントへのグルコン酸ナトリウムの作用機構の解明を目的とした。C3Aとグルコン酸ナトリウムの相互作用は非常に強く、セメント中のエーライトの水和を効果的に制御するためにはC3Aの反応が進行した後にグルコン酸ナトリウムを後添加することが有効であると考えられた。ポリカルボン酸系分散剤を添加したセメントにおいてもグルコン酸ナトリウムは遅延作用を示し、スラッジ水中のセメントのように分散剤が吸着した後でも有効に作用すると考えられた。

CHARACTERIZATION OF ULTRA-FINE BLAST FURNACE SLAG POWDER AND THE EFFECT ON PACKING FRACTION AND FLUIDITY OF HIGH VOLUME BLAST FURNACE SLAG CEMENT PASTE

Reduction of environmental loading from CO₂ is a global issue. In Japan, high volume blast furnace slag cement, which contains over 60％ of blast furnace slag（BFS）was developed. However, in order to enhance the durability and strength of this high volume blast furnace slag cement, it is necessary to reduce mixing water while preserving the fluidity at appropriate level. For the purpose of improving the fluidity of cement at low water to powder ratio, it is very efficient to increase free water by enhancing packing fraction of particles in aggregation. In this research, the authors found that fine BFS includes ultra-fine BFS particles and the particle size of them is 200-500nm；as similar as silica fume（SF）. Considering such ultra-fine BFS, the geometrical simulation for packing fraction of particles suggested that the packing fraction of whole high volume blast furnace slag cement would be increased. At the water-powder (W/P) ratio of 0.16, the apparent viscosity of high volume blast furnace slag cement paste including ultra-fine BFS and the viscosity of paste without ultra-fine BFS were 1.19Pa・s and 1.69Pa・s respectively. Furthermore, in actual measurement, the packing fractions of particles in aggregation of these high volume blast furnace slag cement pastes had good agreement with the simulation. These results show that the ultra-fine BFS particles improve the fluidity of paste by enhancing the packing fraction of particles and this ultra-fine BFS is thought to be used as an alternative material of SF.CO₂排出量の削減はセメント産業にとって大きな課題の一つである。クリンカー成分の60％を高炉スラグ（BFS）で置換し、CO₂排出量を大幅に削減した高炉スラグ高含有セメントが開発されているが、この高炉スラグ高含有セメントを高強度・高耐久コンクリート用セメントとして利用するためには、低水比での作業性を確保することが必要である。本研究では粉末度の高いBFS中に粒径200-500nmのBFS超微粉末が存在することを見出した。またその超微粉末が高炉スラグ高含有セメントの充てん性を向上させることにより、ペーストの流動性が改善されることを明らかにし、SF代替材料としての利用可能性を示した。

USE OF HIGH-ALITE CLINKER IN BLENDED CEMENT

From the point of view of an increase in the waste utilization and the amount of CO₂ reduction, the role of blended cement with fly ash or blast furnace slag (BFS) is important. However, it is pointed out that blended cement has lower initial strength compared with ordinary Portland cement (OPC). It is possible to improve the initial strength by using the high alite base cement. Trial A, B and C were prepared as cement samples. According to the different of phase content, calculated by Bogue’s equation, Trial A cement was assumed to be OPC. Trial B and C were high alite cement which had alite amount larger in order. According to the result of heat liberation of blended cement at 1, 3 and 7 days, high alite blended cement with 40% BFS obtained the total heat liberation which is comparable to OPC. Since total heat liberation is related to the hydration reaction in cement, which could related to strength of hardened sample, it was suggested that this high alite blended cement might have the same level of strength as OPC. further result showed that blended high alite cement replacing with 40% BFS has porous structure of the hardened body and initial heat liberation characteristic equivalent to those of OPC.従来の普通ポルトランドセメントに対し、混合セメントとすることで、クリンカーの焼成量を減少し、二酸化炭素排出量削減が実現できる。しかし、混合セメントは従来のセメントと比較して初期強度や中性化抵抗性が劣る。これに対して、混合セメント中のクリンカーのエーライト含有量を高めることで、初期強度発現性に優れる混合セメントの実現が可能であると考えられる。本研究ではセメント原料にカルシウムを多く配合したクリンカーを用いることで、高炉スラグを40％程度置換した高炉セメントが普通ポルトランドセメントと同等の初期水和特性と硬化体の空隙構造を有することを明らかにした。