Rutile Samples Research Articles

Structure and strength characteristics of rutile ceramics obtained with the approach of oxidative constructing

As a result of applying the oxidative constructing approach, compact samples of rutile ceramics were obtained at 875°C for 160 days. An analysis of the distribution of microhardness over the thickness of a rutile sample obtained at 875°C at different times of synthesis indicates an inhomogeneous structure of rutile ceramics, which indicates that the material is defective due to its porosity. The regularities of changes in the nature of the layering of the structure of ceramics are established depending on the portion of the kinetic curve of the direct oxidation of titanium. It is shown that the presence of a defect in the form of a gap between the layers formed during the oxidation of titanium blanks at the stage of completion of the exponential growth of the kinetic curve is due to a natural barrier – a massive rutile ceramic layer, which limits the free access of oxygen to the TiO2/Ti interface.

The abnormal multiple dielectric relaxation responses of Al3+ and Nb5+ co-doped rutile TiO2 ceramics

Al3+ and Nb5+ co-doped rutile TiO2 ceramic samples were prepared by solid state reaction. Raman spectra, element mapping and EDX confirmed that (Al + Nb) dissolved into grain. The samples showed colossal permittivity (ε′≈3.8 × 104) with low dielectric loss (under 0.08) over frequency (20 Hz–100 kHz) and temperature (80 K–240 K) ranges. Abnormal multiple dielectric relaxation properties were obtained around 5 K. Using universal relaxation law and thermal polaron hopping, we concluded that high dielectric constant of (Al1/2Nb1/2)0.05Ti0.95O2 was due to thermal activation of electrons in defect-dipoles, polaron hopping polarization and interfacial polarization. This investigation could increase our understanding models of multiple dielectric relaxation and inhomogeneous composition in co-doped rutile TiO2.

Impact of H2O2 on the Lactic and Formic Acid Degradation in Presence of TiO2 Rutile and Anatase Phases under UV and Visible Light

The degradation rates of formic acid and lactic acid in the presence and absence of H2O2 were studied, utilizing several TiO2 catalysts: PC105 (100% anatase), MPT 625 (100% rutile), and P25 (80% anatase/20% rutile), and the results were discussed with regards to the current literature. The impact of hydrogen peroxide on the photocatalytic efficiency of eleven TiO2 samples was then determined, using commercial anatase structures (PC105, PC500, UV100), commercial mixed anatase/rutile (P25 and P90), and six rutile (two commercial samples: MPT 625 and C-R160, and four home-made rutile samples were synthesized by TiCl4 hydrolysis). The effect of catalyst surface area and TiO2 phase on the degradation rate of lactic acid (LA) and the decomposition of H2O2 was studied and discussed in regard to the active species generated. The intermediate products formed in the absence and presence of H2O2 were also an important factor in the comparison. Finally, the efficiency of the degradation of LA and formic acid (FA) in the presence of rutile and H2O2 was determined under visible light, and their reactivity was compared. The intermediate products formed in the degradation of LA were identified and quantified and compared to those obtained under UV (Ultra-Violet).

Comparative Analysis of Physicochemical Properties of Rutile TiO2 with Hierarchical 3D Architecture Prepared by Liquid Hydrolysis of TiCl4 and Hydrothermal Method

TiO2 (rutile) samples with a hierarchical 3D nanostructure of the particles were synthesized by two methods: liquid hydrolysis of TiCl4 at 90 °С and atmospheric pressure; hydrothermal synthesis from TiCl4 at 160 °С and different [H2O]/[Ti] ratios. The effect exerted by conditions of the synthesis and post-treatments on the crystallite size, morphology, electronic properties and pore structure of the rutile samples was investigated. It was shown that severe hydrothermal conditions with the ratio [H2O]/[Ti] = 20 provide the formation of a more perfect crystal structure of rutile with a smaller band gap energy (3.00 eV against 3.06 eV for the rutile obtained by liquid hydrolysis at atmospheric pressure). The study revealed the stabilizing effect of cerium cations on the pore structure of rutile, which changes upon thermal treatment.

High Cr(VI) adsorption capacity of rutile titania prepared by hydrolysis of TiCl4 with AlCl3 addition

Rutile titania (TiO2) was successfully prepared via hydrolysis of TiCl4 in the presence of AlCl3. The powders were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface area analysis. In the present system, AlCl3 functions as a nucleating agent and induces the formation of rutile TiO2. The influences of HCl and isopropanol concentrations on the purity and morphology of the rutile TiO2 were investigated. The purity of the rutile TiO2 increased with increasing concentration of HCl. Evenly dispersed rutile TiO2 particles with a spherical morphology were obtained when the HCl and isopropanol concentrations were 0.5 and 1 mol·L−1, respectively. Furthermore, the prepared TiO2 powders were used in adsorption tests of the heavy metal pollutant Cr(VI). Rutile TiO2 sample S-9 demonstrated greater adsorption performance and a removal efficiency that was greater than 99.95% after 60 min of adsorption when the Cr(VI) concentration was 200 mg·L−1 The maximum adsorption capacity on rutile TiO2 was 28.9 mg·g−1. This work provides an easy path to prepare a high-performance rutile TiO2 adsorbent with potential applications in water pollution treatment.

Photoluminescence study of anatase and rutile structures of Fe-doped TiO2 nanoparticles at different dopant concentrations

In this work, dependence of the surface and lattice defects of Fe-doped TiO2 at different iron concentrations and annealing temperatures was monitored by utilizing the photoluminescence (PL) spectroscopy. To achieve the maximum spectral resolution, PL spectroscopy was performed in vacuum and at different temperature of 25–300 K. It has been found that the photoluminescene properties were significantly influenced by different crystalline structure. The PL spectrum of the rutile 1% and 5% Fe:TiO2 anatase show similar PL behavior while the emission intensity of the rutile sample was lower than the anatase. This result, which can be explained by the presence of iron ions at the surface for the anatase and the increased oxygen vacancies due to the diffusion of iron into the lattice in rutile, as a result of annealing at high temperature proposes the rutile phase for the applications of environmental photocatalysis.

Study of Nanostructured TiO₂ Rutile with Hierarchical 3D-Architecture. Effect of the Synthesis and Calcinations Temperature.

The effect of TiCl₄ hydrolysis temperature on the structural, textural and morphological properties of the resulting rutile and on the changes of these properties upon calcination was studied. The XRD, Raman spectroscopy, mercury porosimetry, BET, SEM and TEM studies have revealed that TiO₂ rutile has a hierarchical 3D-architecture. The obtained nanostructured rutile had a cauliflowerlike/ spherical morphology composed of fan-shaped nanofibers. Rutile samples were shown to have a heterogeneous pore structure including micro-, meso- and macropores with a BET surface area of 110-140 m₂/g. According to the mercury porosimetry, among mesopores and macropores the latter dominate in the samples. Elevation of the synthesis temperature from 50-70 to 80-90 °C decreased the fraction of macropores from 95 to 70%. The BET method showed that the samples synthesized at low temperatures (50-70 °C) contain 30-44% of micropores in the total amount of mesopores and micropores. The fraction of micropores decreases to 25-18% with a subsequent increase in the fraction of mesopores as the synthesis temperature is raised to 80-90 °C. As shown by a study of the samples upon calcination in the temperature range of 100-1000 °C, temperature is the key factor that produces changes in the crystallites size, nanofiber length and packing density, and 3D particle shape at different levels of the hierarchical system and determines features of the porous structure and morphological properties of nanostructured rutile. The assessment of photocatalytic activity in the oxidation of acetone vapor demonstrated that, regardless of the hydrolysis temperature, the synthesized samples of nanostructured rutile are able to oxidize acetone vapor to carbon dioxide and water. In the process, activity of the samples is comparable with that of commercial photocatalysts under UV light and is superior to the activity of commercial photocatalysts P25 (2-4 times) and TiO₂ KRONOS vlp 7000 (1.2-2 times) under visible light in dependence on the synthesis temperature.

Highly textured boron/nitrogen co-doped TiO2 with honeycomb structure showing enhanced visible-light photoelectrocatalytic activity

In this work, we report a novel photocatalyst based on boron and nitrogen co-doped TiO2 rutile (1 1 0) honeycomb structures. The photocatalyst has been prepared by simultaneously oxidizing and doping a Ti-foil substrate at 750 °C. The unit cell volume and the crystallite size of grown TiO2 films were measured by Rietveld refinement analysis. The co-doping by boron and nitrogen was achieved simultaneously with the oxidation of the titanium, resulting in a rutile (1 1 0) textured TiO2 film. X-ray photoelectron spectroscopy analysis revealed the presence of Ti-O-N and Ti-O-B-N bonds, and the presence of crystal defects in the lattice was detected and displayed by Raman spectroscopy. The water photo-oxidation properties have been measured as well, and the photocurrent of the B/N co-doped rutile sample prepared reached 270 µA/cm2 at 1 V under visible light and was stable in time. The efficient visible light absorption properties of the fabricated nanomaterial were attributed to the presence of oxygen vacancies and the introduction of impurity levels, as well as to synergistic effects between the introduced boron and nitrogen elements. The results presented demonstrate a new route for the preparation of TiO2 based catalysts, and open prospects for the photocatalysis community.

Titania surface chemistry and its influence on supported metal catalysts

This work characterized the surface chemistry of a number of different titania samples including four commercial anatase samples, an anatase sample that we synthesized, the pyrogenic titania samples P25 and P90, and a commercial rutile sample. X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), were used to identified surface species that might interfere with the acid/base properties of the surface hydroxyls. All commercial anatase samples were contaminated by sulfur, which diminished their effectiveness as metal oxide supports for heterogeneous catalysis and has implications for their utility as photocatalysts. Hydrogen-bonded surface hydroxyls remained after calcination up to 400 °C for all anatase samples, in contrast to rutile and the pyrogenic titania materials P25 and P90, in which they were eliminated. Ru(0) catalysts on titania without hydrogen-bonded surface hydroxyls showed enhanced CO hydrogenolysis selectivity in the presence of water while Ru(0) catalysts on titania with hydrogen-bonded surface hydroxyls showed diminished selectivity in water, suggesting that surface hydrophilicity is important for this reaction. Heteroepitaxy between rutile RuO2 and rutile TiO2 is not essential for the creation of small evenly-spaced supported Ru(0) nanoparticles, which are important in many catalytic reactions.

Effect of particle size on the photocatalytic activity of modified rutile sand (TiO2) for the discoloration of methylene blue in water

The advanced oxidation process (AOP) has been identified as a promising technology for pollutant degradation. To maximize the performance of this process, new materials need to be found or existing materials need to be modified. In this work, the effect of the particle size of modified rutile materials on the photocatalytic discoloration of methylene blue was investigated. Rutile samples with different particle sizes were obtained by milling, and the photocatalytic reaction was carried out in a continuous magnetic stirring photoreactor under UV–vis illumination. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), laser diffraction for particle size distribution (PSD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), N2 adsorption (SBET) and total X-ray fluorescence (TXRF). The results showed an efficiency increase in the photocatalytic activity of the materials for the discoloration of methylene blue in water; the rutile hematite sand had an efficiency of 8%, while the sample that underwent the highest energy milling had an efficiency of 64%. The improved efficiency of the photocatalytic activity occurred as the particle size decreased and the optical band gap shifted from the visible to the UV region of the light spectrum. By eliminating the natural iron impurities (hematite, according to the DRS and XPS results) from the surface of the rutile particles through milling, enhanced behavior was achieved. This was explained because the smaller particle size promoted an increase in the surface area, leading to a higher number of active sites on the nanoparticle surface, which allowed an increase in the removal of water pollutants in the UV–vis range of the solar spectrum.

Impact of rutile and anatase phase on the photocatalytic decomposition of lactic acid

Rutile TiO2 photocatalysts with high surface area were synthesized by hydrolysis of TiCl4 in aqueous solution during 2 h and 48 h and either treated at 300 °C or not. These samples of home-made rutile TiO2 were characterized by X-ray diffraction (XRD), electron microscopy (ESEM and TEM), BET surface area analysis, TG-DTA-MS and UV–vis diffuse reflectance spectroscopy. All four home-made (HM) TiO2 photocatalysts are nanorods of pure rutile TiO2 with surface areas between 92 and 173 m2/g. The photocatalytic efficiencies of HM rutile samples were compared with those of commercial rutile, anatase and mixed anatase/rutile towards the degradation of lactic acid (LA), and discussed in regards to the adsorption sites. Our results showed (i) it is possible to obtain a rutile TiO2 more efficient than TiO2 P25 and (ii) the time of hydrolysis and calcination have an important impact on photocatalytic efficiency. The analysis of organic intermediates present in aqueous and gaseous was also performed. At similar LA conversion, high pyruvic acid selectivity was observed only in the case of the two commercial rutile TiO2 samples.

Hydrothermal Synthesis of Rare-Earth Modified Titania: Influence on Phase Composition, Optical Properties, and Photocatalytic Activity.

In order to expand the use of titania indoor as well as to increase its overall performance, narrowing the band gap is one of the possibilities to achieve this. Modifying with rare earths (REs) has been relatively unexplored, especially the modification of rutile with rare earth cations. The aim of this study was to find the influence of the modification of TiO2 with rare earths on its structural, optical, morphological, and photocatalytic properties. Titania was synthesized using TiOSO4 as the source of titanium via hydrothermal synthesis procedure at low temperature (200 °C) and modified with selected rare earth elements, namely, Ce, La, and Gd. Structural properties of samples were determined by X-ray powder diffraction (XRD), and the phase ratio was calculated using the Rietveld method. Optical properties were analyzed by ultraviolet and visible light (UV-Vis) spectroscopy. Field emission scanning electron microscope (FE-SEM) was used to determine the morphological properties of samples and to estimate the size of primary crystals. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical bonding properties of samples. Photocatalytic activity of the prepared photocatalysts as well as the titania available on the market (P25) was measured in three different setups, assessing volatile organic compound (VOC) degradation, NOx abatement, and water purification. It was found out that modification with rare earth elements slows down the transformation of anatase and brookite to rutile. Whereas the unmodified sample was composed of only rutile, La- and Gd-modified samples contained anatase and rutile, and Ce-modified samples consisted of anatase, brookite, and rutile. Modification with rare earth metals has turned out to be detrimental to photocatalytic activity. In all cases, pure TiO2 outperformed the modified samples. Cerium-modified TiO2 was the least active sample, despite having a light absorption tail up to 585 nm wavelength. La- and Gd-modified samples did not show a significant shift in light absorption when compared to the pure TiO2 sample. The reason for the lower activity of modified samples was attributed to a greater Ti3+/Ti4+ ratio and a large amount of hydroxyl oxygen found in pure TiO2. All the modified samples had a smaller Ti3+/Ti4+ ratio and less hydroxyl oxygen.

Effect of tungsten doping on structural and optical properties of rutile TiO2 and band gap narrowing

Pure and W doped nanocrystalline rutile TiO2 samples were synthesized using high energy ball milling process. Rietveld refinement results of X-ray diffraction data confirmed that pure sample exhibited rutile TiO2 phase whereas doped samples contained both rutile TiO2 and a secondary Ti0.54W0.46O2 phase. Different models of Williamson–Hall method were employed to evaluate crystallite size and strain in the samples. The crystallite size was found to decrease from 50 to 47 nm with increase in the dopant concentration. The pure TiO2 exhibited tensile microstrain which became compressive and increased upon doping. A blue shift in A1g Raman mode with doping of W also indicated the increase in the compressive strain. The HR-TEM images also confirmed the presence of higher strain in doped samples compared to un-doped sample. The observed decrease in band gap from 3 to 2.83 eV with dopant concentration, as calculated from UV–vis spectroscopy data, may be attributed to the increased strain. The decrease in the intensity of photoluminescence emission indicated the increase in number of defects and oxygen vacancies with increasing dopant concentration. This is further, supported by the rise in Urbach energy, a signature of increased number of defects in doped samples. This study shows that the dopant induced strain plays significant role in band gap narrowing.

Improvement of Low-grade rutile using chemical and physical processes

This paper describe improvement of Low-grade rutile Rutile is very important mineral for several industrial applications . So, in this work we will discuss the physical and chemical methods which are used to beneficiation of low grade rutile sample (10 Kg ) was collected from Abu Khashaba beach area at the Mediterranean cost ( Fig. 1 ) . The leach process whereby iron coatings around rutile particles is removed by the addition of sulphuric acid at 20 wt% to mineral at 160°C .Then the sample is sent for the physical processes which include up-current classifiers, gravimetric separation, hydraulic separation and magnetic separation . Stereo microscope and XRF analyses were used to study heavy and light mineral fraction .Rutile identification is established and the results at optimum conditions gave a high grade of rutile product which analyzed 95.8 % Ti with 0.8 % Fe, 1.8 % Zr and 0.80 % Nb.

Characterisation and photocatalytic assessment of TiO2 nano-polymorphs: Influence of crystallite size and influence of thermal treatment on paint coatings and dye fading kinetics

A study on the thermal effects on TiO2 rutile and anatase nano-powders was undertaken and displayed some unusual photoactivity and crystal structure properties. Rutile nano-particles with different crystallite sizes were characterised and the possible effect on activity were investigated. One of the rutile samples appeared to have trace amounts of anatase and was annealed at high temperatures at 1172 K and 1272 K to highlight the thermodynamic stability phenomenon of titania. Parallel to this study, anatase nano-particles were investigated before and after being annealed up to 1022 K. For all the samples used in this work, characterisation was undertaken using micro-Raman microscopy/XRD and Scanning Electron Microscopy (SEM) while photoactivity assessment was made by measuring and monitoring the photodegradation of a mixture of dye methyl-orange (MeO) and nano-powders under UV-light for 3h30 min in suspension. The study revealed that rutile nano-powder samples were thermodynamically stable even at very high temperatures and poorly active but with an unusual photoactive feature. Concerning the anatase samples; SEM investigation revealed a questioning size growth as the samples showed a different particle size depending on the temperature of thermal treatment. It revealed that annealing at 672 K seemed to be a key temperature as the particles change from a polyhedral structure to a two-dimensional structure showing a platelet like shape. The photocatalytic studies of the anatase nano-particles showed a very high activity especially before annealing. This highlighted the fact that the anatase phase can subsist at high temperatures such as 1022 K and exhibit a persistence in photoactivity even though it has decreased significantly after 672 K. SEM analysis was in accordance with the photoactivity investigation. Nevertheless, the most interesting feature of the results emanates from the reaction order study and rate constant analysis taken from the kinetic shape of the graph of the degradation of MeO as a function of the irradiation time for the different particle sized rutile nanoparticles. Here a zero-order reaction was determined and as a consequence raised questions about the theory of the mechanism of the activities of titania in terms of surface chemistry, surface area dependence and photoactivity. For example, for the nano-rutiles the sample with a 25 nm crystallite size was the most active and the sample with the smallest crystallite size (15 nm) was the least active and yet was found to contain trace levels of nano-anatase. This effect was also substantiated by UV absorption and weathering studies on doped isocyanate-acrylic paint films. UV analysis clearly shows that the absorptivity of the nanoparticles plays a role and correlates with the photoactivity. The 15 nm particles have decreased absorptivity in the near UV and hence decreased activity.

Experimental and first principle study of room temperature ferromagnetism in carbon-doped rutile TiO2

Room temperature ferromagnetism has been observed experimentally by implanting 1.2 MeV carbon on rutile TiO2. Density functional theory has been employed to study the magnetic properties of different possible carbon-related defects in TiO2. Theoretical calculation suggests carbon-doped at oxygen site (CO) can induce a significant magnetic moment in rutile TiO2. The 2p orbital electrons of the carbon atom are the primary source of the induced magnetic moment in TiO2. The magnetic measurements using a SQUID magnetometer, at room temperature, confirmed the ferromagnetic behavior of carbon doped rutile TiO2 sample.

Size and shape effect on the photocatalytic efficiency of TiO2 brookite

Thanks to aqueous sol–gel chemistry, it is now possible to prepare several phase pure TiO2 brookite colloidal systems that significantly differ on nanoparticles size and shape. This TiO2 polymorph is more difficult to be obtained as phase pure material than anatase or rutile. Here we have prepared a set of four different sol–gel brookite syntheses with particles size ranging from 10 to 500 nm and significantly different morphologies as demonstrated by X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. We have studied their photocatalytic activities in aqueous solution on phenol and formic acid. The brookite sample with higher specific surface displays better activity for both pollutants abatement than anatase and rutile reference samples and very close to the TiO2 P25 commercial reference. Additional experimental characterization of photogenerated charge carriers and their lifetime is performed using time-resolved microwave conductivity. We could then explain why another efficient brookite material is able to compensate a significantly lower specific surface with a higher photon conversion rate. This study involving a broad set of pure phase brookite samples brings back that phase into the TiO2 polymorphs race for light-enhanced applications. It confirms that size/shape–activity correlation already observed for the anatase polymorph is also valid for the brookite phase.

Determination of the crystallinity of TiO2 photocatalysts

This study reports a new simple method to determine the crystallinity of anatase and rutile TiO2 photocatalysts. The crystallinity degree of various anatase and rutile samples was estimated by XRD analysis from the ratio among the full width at half maximum intensities of the main diffraction peaks of anatase or rutile and the (111) peak of CaF2 as internal standard. The photocatalytic activity of selected powders was tested employing the photodegradation of 4-nitrophenol in aqueous solution as probe reaction. The results showed that the photoactivity of the investigated samples increased with increasing crystallinity and crystallite size.

Effects of Fe and V States on the Fenton Catalytic Activity of Natural Rutile

AbstractAs a common mineral phase on Earth and Martian regolith, natural rutile was reported as a potential candidate for use as a Fenton catalyst in this study. The influences of Fe and V in various chemical states on the generation of reactive oxygen species (ROSs) and the catalytic activity of rutile were examined. A series of rutile samples with various surface and bulk states of Fe and V were obtained initially by hydrogen annealing of natural rutile at ~773–1173 K. X-ray diffraction, electron paramagnetic resonance spectra, and X-ray photoelectron spectroscopy demonstrated that the atomic fractions of Fe(III) and V(V) decreased sharply with increasing temperature, along with the accumulation of surface Fe(II) and bulk V(III). All as-prepared materials showed enhanced Fenton degradation efficiency on methylene blue (MB) compared with P25-TiO2, and the treated samples exhibited up to 3.5-fold improvement in efficiency at pH 3 compared to the untreated sample. The improved efficiency was attributed mainly to Fenton catalysis involving Fe(II) and V(III). The dissolved Fe2+ played a crucial role in the homogeneous Fenton reaction, while the bound V(III) favored adsorption primarily and may have facilitated heterogeneous Fenton reaction and the regeneration of Fe2+. The pH regulated the reaction mechanism among homogeneous (pH = 3) and heterogeneous (pH = 3.7) Fenton catalysis and physical adsorption (pH = 5, 6). The aim of the present study was to improve the understanding of the potential role of natural rutile with advanced oxidation functions in Earth systems and even on Mars, which also provide an inspiration for screening natural rutile and any other similar, Earth-abundant, low-cost minerals for environmental application.

(Invited) Singularity in Chemistry: Digitally Controlled Kinetics of Titania-Photocatalyzed Oxygen Evolution

Oxygen evolution (EO) from water in heterogeneous photocatalyses is one of the most significant chemical reactions. It has been believed that number of electrons in this multielectron-transfer (MET) process is four with standard electrode potential (SEP) 1.23 V (vs SHE), but there seems to be no effective method to determine the number for any OE reactions, not limited to photocatalysis, except for electrolytic OE in which its electrode potential might suggest n et. Here we present the light intensity-dependent kinetics of titanium(IV) oxide (titania)-photocatalyzed OE and show, for the first time for non-electrochemical METs, n et depending on the titania photocatalysts and reaction conditions. This also suggests that MET kinetics depends digitally on its thermodynamics, i.e. possible SEPs, as a novel concept, to result in the appearance of "singularity" in chemical kinetics. Digitally controlled kinetics is important to clarify the true mechanism and to design, for example, highly efficient photocatalytic artificial photosynthetic systems. Highly intense 365-nm UV-LEDs (NS Lighting ULEDN-101 (NSL) and Hamamatsu Photonics L11921-400 (HMP)) were used in this study. The maximum power of focused NSL light beams and that of unfocused HMP light beams from those LEDs measured by a Hioki 3664 power meter with a 9742 optical sensor through a 1 cm-square aperture in front of a rectangular quartz cell were ca. 340 mW and 500 mW, respectively. Light intensities were adjusted by changing the LED-cell distance or source power. Those maximum intensities were more than one-order of magnitude higher than those of UV light from ordinary mercury or xenon arc lamps (ca. several tens of mW cm-2). Commercial titania powder (30 mg) was suspended in aqueous 0.05 mol-L–1 sodium iodate (pH 10) or iron(III) chloride (pH ca. 2) solution (3.0 mL) in a quartz cell. Air was purged off from the head space by argon, and the cell was tightly sealed with a rubber septum. UV irradiation on such titania suspensions containing strong electron acceptors (EAs), as photoexcited-electron scavengers, induced OE, and the observed OE rate (R a/µmol h–1) was measured by gas chromatography. Figure 1 shows the I L-dependent OE rate (r) for suspensions of two sets of anatase and rutile titania samples, including small anatase ("SA"; 4 nm), small rutile ("SR"; 13 nm), large anatase ("LA"; 170 nm) and large rutile ("LR"; 360 nm), using iodate (IO3 –) as an EA. Although all of the samples except for LR showed an almost linear correlation, i.e., r was almost proportional to I L, at the middle I L region (0.5 < I L < 2), plots for small particles (SA and SR) at the lower I L region seemed to be deviated downward from the straight lines. From the simulation curves of double logarithmic LID, the order of LID for SA and SR can be presumed to be 2 at the lower limit of I L and the order was gradually decreased to 1. In addition to this bimodal "spoon-like" behavior, r for SA showed a sudden increase, i.e., singularity, at ca. 2.3 W cm-2, and the order seemed to be suddenly changed from 1 to 4, i.e., "singularity" was observed . To the best of the authors’ knowledge, this is the first report of such higher-order LIDs for photoreaction induced by ordinary (non laser) continuous-light irradiation. The former second-order LIDs were also observed when iron(III) (Fe3+) was used as an EA with threshold I L (I thr), at which the LID order is changed from 2 to 1, higher than that for IO3 –, though fourth-order LID could not be observed even for SR (One of the possible reasons for SR not showing a fourth-order LID even at higher I L is shorter lifetime of one positive-hole bearing particle (t 1) which is presumed from the I thr of SR being higher than that of SA for IO3 – EA as discussed later. Therefore, at further higher I L, fourth-order LID may be observed.). The reactions of those EAs are assumed to be one-electron and six-electron processes as: Fe3+ + e– = Fe2+ and IO3 – + 3H2O + 6e– = 6OH– + I–, but bimodal (second-to-first order) LIDs were commonly observed, suggesting that the hole-transfer (oxidation) step, i.e, OE from water (See SI.), is attributable to the bimodal LIDs. The observed multimodal (unimodal) LIDs can be interpreted using a model in which the number of accumulated positive holes in each photocatalyst particle is counted, taking the individual action of each particle into account. The detailed mechanism and effect of particle size in this OE reactions are discussed. Figure 1