Anatase Particles Research Articles

In Vivo Interactions of Nanosized Titania Anatase and Rutile Particles Following Oral Administration

In Vivo Interactions of Nanosized Titania Anatase and Rutile Particles Following Oral Administration

TiO2-based photocatalysts for controllable hydrogen evolution from ammonia borane

Results on the photocatalytic hydrolysis and hydrothermolysis of NH3BH3 in the presence of the titania-based photocatalysts (TiO2 Hombifine N, samples obtained by its hydrothermal treatment in KOH and after modification by colloidal silver) are presented. It was shown that in the hydrolysis of 0.3 M solution of NH3BH3 at 40 °C under UV irradiation (λ = 365 nm) the highest activity has been achieved with the photocatalyst synthesized at 120 °C in 10 M КOH solution. In this sample, the nanodispersed particles of the unreacted anatase get in contact with titanate nanorods. Modification of the titania-based photocatalysts by colloidal silver allowed the quantum efficiency under UV irradiation to be increased and the hydrogen formation became possible under visible light (λ = 450 nm). It was established that the use of low concentration of the supported silver (0.06 wt.%) makes possible to control the H2 generation by the use of irradiation. Photocatalytic hydrothermolysis of NH3BH3 under heating (90 °C) and UV irradiation of the reaction mixture (NH3BH3+H2O + Ag/TiO2) with the molar ratio H2O/NH3BH3 = 1.24 has been carried out for the first time. High values of the rate of generation and the yield of hydrogen have been achieved.

A Pure Titania Photocatalyst: Preparation, Characterization and Photocatalytic Activity of Octahedral-Shaped Anatase Particles

Different from organic molecules and metal complexes, which can be identified by chemical composition and structural characteristics such as NMR spectra, inorganic solid materials especially metal oxides cannot be identified, i.e., their purity cannot be evaluated in a strict scientific sense, because at least three structural parameters, bulk structure, surface structure and bulk/surface size, must be determined. However, in the other words, solid metal-oxide samples can be assigned to be "pure", if their three parameters are determined, by macroscopic measurements, to be uniform. Here, attempts to prepare octahedral-shaped anatase titanium(IV) oxide (titania) particles (OAPs) of uniform bulk structure (anatase with negligible non-crystalline components), surface structure (only exposing {101} facets) and particle size are reported as the first and sole example of a "pure" metal-oxide powder. In this study, OAP samples were prepared by sonication-hydrothermal (HT) reaction of potassium-titanate nanowires. Characterization of the samples by Rietveld analysis of XRD patterns, reversed double-beam photoacoustic spectroscopy and SEM observations revealed the homogeneousness of crystalline bulk, surface and particle size, respectively, to indicate the samples "pure".

Seawater activated TiO2 photocatalyst for degradation of organic compounds

The world's oceans are polluted by a continuous inflow of plastic. Plastic fragments finally into microplastic, which can be taken up, for example by plankton, and subsequently by the entire ocean food web. An approach to reduce plastic pollution constitutes the accelerated microplastic degradation in marine environments. TiO2 (anatase) is commonly used as an oxidative photocatalyst and well known to catalyze the degradation of organic compounds upon UV irradiation.In this study, a selective activation of TiO2 (anatase) particles encapsulated by Ca- or Sr-polyphosphate is presented. The TiO2 polyphosphate core-shell particles are envisaged as additives in plastic products. The highly concentrated cations from seawater, viz. Na+ and Mg2+, displace the Ca2+ or Sr2+ cations from the polyphosphate shell. As a result, the polyphosphate coating dissolves and thus the photocatalytically active TiO2 core is released. The stability of the TiO2 polyphosphate particles in potable water and the seawater activated disintegration of methylene blue, methyl methacrylate, terephtalic acid, and poly(vinyl alcohol) was shown. It has been demonstrated, that the sweetwater stable polyphosphate coating degrades in the presence of seawater, which could be monitored by the activation of the TiO2 (anatase) photocatalyst.

Tailored Hydrothermal Synthesis of Specific Facet-Dominated TiO2 Nanocrystals from Lepidocrocite-Type Layered Titanate Nanosheets: Systematical Investigation and Enhanced Photocatalytic Performance

A series of samples including leaf-like and rod-like rutile TiO2 nanoparticles with various facets exposed on the surface, parallelepiped-shaped anatase nanoparticles with [111] vertical facet exposed on the surface, irregular anatase nanoparticles, microsized six-point star-like anatase aggregates, and almond-like brookite aggregates had been hydrothermally synthesized from lepidocrocite-type layered titanate nanosheets. A systematical investigation was established to uncover the phase transition and morphological evolution from nanosheets to TiO2 polymorphs, and a phase diagram was determined by adjusting the synthesis parameters of the pH value and temperature. Two kinds of mechanisms composed of the dissolution-deposition process following Ostwald's ripening mechanism and the in situ topochemical conversion process following Ostwald's step rule had been proposed based on the time-dependent hydrothermal experiments. Briefly, the formation of the single-crystalline rutile phase appeared only at high temperatures with very low pH values, and similarly, the brookite phase strictly formed at high temperatures with a very high pH value. Nevertheless, the anatase phase could moderately appear in a wide range of temperatures and pH values. In addition, the single-crystalline rutile adopted a leaf-like morphology at low temperatures with high pH values and a rod-like morphology at high temperatures with low pH values, while the morphological evolution of anatase particles proceeded from irregular to parallelepiped-shaped and finally to six-point star-like morphology, and the crystal size was reduced from 1000 to 5 nm with decreasing pH values. Meanwhile, with the prolongation of the hydrothermal time, the layered titanate nanosheets first dissolved into the amorphous state and further converted into small anatase nanoparticles and finally to rutile or anatase nanoparticles based on the dissolution-deposition process, or the {010}-faceted layered titanate structure first converted into the [111]-vertical faceted anatase nanosheets by the topochemical transformation reaction and then split into the [111]-vertical faceted anatase nanoparticles. More importantly, the mesoporous [111]-vertical faceted anatase nanoparticles exhibited enhanced photocatalytic performance compared to that of Degussa P25, which was ascribed to its superior electronic band structure and effective charge separation. The systematical investigation in this work would be significant for consummating the preparation of the TiO2 polymorphs from layered titanate nanosheets and provided some reference values and guide schemes for the preparation of TiO2 nanoparticles with outstanding photocatalytic performance.

Octahedral Anatase Titania as Efficient Photocatalyst: Influence of Preparation Conditions

Octahedral anatase particles (OAPs) belonging to faceted titania photocatalysts exhibit one of the highest photocatalytic activities among various titania photocatalysts, probably due to hindered recombination of charge carriers resulting from low content of defects (recombination centers). In this study, OAPs have been examined for oxidative decomposition of acetic acid, dehydrogenation of methanol and bacteria inactivation. OAPs were prepared from partly protonated potassium nanowires by hydrothermal reaction (HT) at various experimental conditions, i.e., the influences of HT temperature and content of reaction reagents were studied in detail. The morphology, composition and crystal structure of the products were characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS) and time-resolved microwave conductivity (TRMC) method. It was found that all parameters during HT influenced simultaneously the morphology of obtained products, and even slight change in one of them could result in significant change in the properties, and thus photocatalytic activities. Moreover, it was shown that titania morphology was a key-factor for photocatalytic activity for decomposition of both organic compounds and microorganisms. Although, an imperfection in octahedral shape (semi-OAPs) did not influence the efficiently for decomposition of organic compounds, it had tremendous negative impact on antimicrobial performance, probably due to hindered adsorption of bacterial cells on the photocatalyst surface (preferential on {101} facets).

Charge Transfer in Mixed-Phase TiO2 Photoelectrodes for Perovskite Solar Cells

In mesoscopic perovskite solar cells (PSCs) the recombination processes within the TiO2 photoelectrode and at the TiO2/perovskite interface limit power conversion efficiency. To overcome this challenge, we investigated the effect of TiO2 phase composition on the electronic structure of TiO2 photoelectrodes, as well as on PSCs performance. For this, a set of PSCs based on TiO2 thin films with different content of anatase and rutile particles was fabricated under ambient conditions. X-ray diffraction, optical spectroscopy and scanning electron microscopy were used to study the structural, morphological and optical characteristics of TiO2 powders and TiO2-based thin films. X-ray photoelectron spectroscopy (XPS) analysis of anatase revealed a cliff conduction band alignment of 0.2 eV with respect to the rutile. Energy band alignment at the anatase/rutile/perovskite interfaces deduced from the XPS data provides the possibility for interparticle electron transport from the rutile to anatase phase and the efficient blocking of electron recombination at the TiO2/perovskite interface, leading to efficient electron-hole separation in PSCs based on mixed-phase TiO2 photoelectrodes. PSCs based on TiO2 layers with 60/40 anatase/rutile ratio were characterized by optimized charge extraction and low level of recombination at the perovskite/TiO2 interface and showed the best energy conversion efficiency of 13.4% among the studied PSCs. Obtained results provide a simple and effective approach towards the development of the next generation high efficiency PSCs.

Preparation and photocatalytic properties of rutile TiO2 with a unique morphology and SrTiO3–TiO2 composites obtained by acid treatment of SrTiO3

In this work, rutile, anatase, and SrTiO3–TiO2 composites were prepared by acid treatment of SrTiO3 using various concentrations of HNO3 at 90 °C. The product of the acid treatment depended on the acid concentration: fine TiO2 (anatase) particles were obtained for 0.5 M, a mixture of anatase- and rutile-phase TiO2 was obtained for 1 M, and large dumbbell-like TiO2 (rutile) particles were obtained for 2, 3, and 4 M HNO3. Furthermore, SrTiO3–TiO2 (anatase) composites in which fine TiO2 (anatase) particles formed on SrTiO3 cores were produced by a short-duration acid treatment. The composite exhibited a higher photocatalytic activity than the SrTiO3, TiO2 (anatase), TiO2 (rutile), and mixture of anatase- and rutile-phase TiO2 in methylene blue decomposition. The hybridization of SrTiO3 and anatase with a large contact area was considered to prevent rapid electron–hole charge recombination and to enhance the photocatalytic activity of the composites.

Phase transformations among TiO2 polymorphs.

Polymorphs widely exist in nature and synthetic systems and are well known to determine material properties. Understanding phase transformation mechanisms among polymorphs enables the design of structures and tuning of phases to tailor material properties. However, current understanding is limited due to the lack of direct observations of the structural evolution at the atomic scale. Here, integrating (semi) in situ transmission electron microscopy and density functional theory, we report atomic structural evolutions of phase transformation from anatase (A) to rutile (R), brookite (B), R-phase, and TiO. Besides the consistent paths with previous reports, we discover several unreported paths, including a [001] direction and (020) plane of anatase to [100]R and (01[combining macron]1)R of rutile, respectively, ([001]A||[100]R, (020)A||(01[combining macron]1)R) and [001]A||[001]B, (020)A||(220)B. Density functional theory analysis elucidates atomic structural evolution during the processes and over 16% of Ti-O bonds break and reform during the processes with energy barriers of ∼0.7-1.0 eV per TiO2 formula unit. Under electron-beam irradiation, anatase particles transform into TiO2-R phase or TiO at high or room temperature, respectively. We also reveal the anisotropic nature of the electron-beam effect, which is seldom discussed: dependence of crystallographic orientation with respect to electron-beam irradiation direction. Understanding the atomic structural evolution sheds light on interpreting and controlling TiO2 polymorphs and intermediate structures for various applications. The revealed electron-beam effects in our work provide guidance for in situ transmission electron microscopy studies.

High-Pressure Phase Transitions in Densely Packed Nanocrystallites of TiO2-II

Phase behaviors of nanocrystalline TiO2-II have not been well studied due to the difficulty in its preparations at ambient pressure. In this work, we prepared nanocrystallites of TiO2-II with average sizes of ∼10–20 nm via high-pressure processing of anatase particles in a large volume press. X-ray diffraction (XRD) and electron microscopy examinations show that the TiO2-II nanocrystallites formed from cracking and phase transformation of large anatase particles under compression. The high-pressure phase behaviors of the TiO2-II nanocrystallites were investigated using in situ synchrotron XRD and Raman spectroscopy. Results show that, under compression up to ∼35–40 GPa, the phase transitions in nano TiO2-II proceed via two parallel routes of TiO2-II (∼9–15 GPa) → baddeleyite (∼25 GPa) → TiO2-OI and TiO2-II (∼19 GPa) → TiO2-OI. At different TiO2-II sizes, the TiO2-II-to-baddeleyite transition occurs at pressures from ∼9 to 15 GPa, while the TiO2-II-to-TiO2-OI transition occurs at ∼19 GPa. This difference originates from the different interfacial energies of the involved titania phases, which control their relative thermodynamic phase stabilities at nano sizes. This work enriches our understanding of the complex phase behaviors of titania and provides fundamental knowledge for developing applications of the less-explored titania nanophase of TiO2-II.

Sol-Gel Synthesis of TiO2 Nanocrystalline Particles with Enhanced Surface Area through the Reverse Micelle Approach

Titania nanocrystalline particles were synthesized by hydrolysis-condensation of titanium tetraisopropoxide in water-in-oil micellar solutions of water/cyclohexane/Triton X-100 system, and the effects of reflux time and water-to-surfactant molar ratio on the particle uniformity, crystallinity, and surface area were studied. Several characterization techniques including TEM and SEM, as well as X-ray diffraction and FT-IR spectroscopy, helium pycnometry, and nitrogen physisorption, were employed to evaluate the particle density and dimensions, crystallite size, surface area value, and the porosity features in the as-prepared condition and also after thermal treatment at 500°C. The results show that all treated samples are dense nanocrystalline anatase particles with BET surface area values over 100 m2·g−1 and primary particle size of 10–15 nm. However, for the as-prepared samples, as the reflux time increases, a better purification of particles from the synthesis environment is resulted, leading to denser and more crystalline powders with smaller particle size and higher BET surface area values culminating in 179 m2·g−1 for 24 hours of refluxing. Moreover, decreasing the water-to-surfactant molar ratio from 10 to 5 and 2 increases the particles surface area to 239 and 224 m2·g−1, respectively, at the expense of slight density and crystallinity degradation and considerable prolongation of surfactant removal step. Supportively, the comparison between photocatalytic activities of as-prepared samples also evidences the effectiveness of reflux time extension on improving the sample features and enhancing their functionality. This study can highlight how the earlier synthesis steps can influence the evolution of the structure of the final products.

Effect of the carbon loading on the structural and photocatalytic properties of reduced graphene oxide-TiO2 nanocomposites prepared by hydrothermal synthesis

This work deals with the preparation of reduced graphene oxide (RGO)-TiO2 composites by a one-step hydrothermal treatment. The effect of the RGO loading on both the structural properties and photocatalytic behavior of RGO-TiO2 is deeply addressed herein. The hydrothermal treatment promoted the reduction of graphene oxide, crystallization of TiO2 into anatase, and anchoring of TiO2 nanoparticles on RGO sheets. It was observed that the prepared anatase particles showed sizes below 10nm, whereas the RGO sheets displayed thicknesses smaller than 1nm. The use of RGO at concentrations up to 15wt% greatly increased the specific surface area of RGO-TiO2. It was demonstrated that the combination of RGO and TiO2 gives rise to materials with improved photocatalytic properties and tailored structural properties. The composite with the highest photoactivity was the one containing an RGO loading of 1wt%; this composite displayed a photocatalytic rate constant about 9.5 times higher than that evaluated for pure TiO2. This behavior may be related to the stacking of RGO nanosheets when its concentration is above 1wt%. Moreover, the addition of RGO in excess may prevent the activation of the TiO2 surface by UV light and also decrease the lifetime of the photogenerated electron-hole pairs. Therefore, it appears that 1wt% is the optimal loading of RGO to obtain a close interfacial contact between RGO and TiO2, leading to both an effective activation of TiO2 by UV radiation and an enhanced charge transfer between RGO and TiO2.

Biochemical and histopathological impacts of rutile and anatase (TiO2 forms) in Mytilus galloprovincialis

Titanium dioxide (TiO2) particles have been widely used in various industrial applications and consumer products. Due to their large production and use, they will eventually enter into aquatic environments. Once in the aquatic environment TiO2 particles may interact with the organisms and induce toxic effects. Since the most common crystallographic forms of TiO2 are rutile and anatase, the present study evaluated the effect of these two forms of TiO2 particles in Mytilus galloprovincialis. For this, mussels were exposed to different concentrations of rutile and anatase particles (0, 5, 50, 100 µg/L) for twenty-eight days. Ti concentrations, histopathological alterations and biochemical effects were evaluated. Similar Ti concentrations were found in mussels exposed to rutile and anatase, with the highest values in mussels exposed to the highest exposure concentration. Histopathological results demonstrated that both forms of TiO2 induced alterations on gills and digestive glands along the increasing exposure gradient. Biochemical markers showed that mussels exposed to rutile maintained their metabolic capacity (assessed by the activity of the Electron Transport System, ETS), while anatase increased the metabolism of mussels. Mussels exposed to rutile increased their detoxifying defences which, due to the low tested concentrations, were sufficient to avoid cellular damage. On the other hand, mussels exposed to anatase suffered cellular damages despite the increase of the antioxidant defences which may be related to the high ETS activity. Both rutile and anatase particles were toxic to M. galloprovincialis, being the highest oxidative stress exerted by the crystalline form anatase.

Geological controls on enrichment of Mn, Nb (Ta), Zr (Hf), and REY within the Early Permian coals of the Jimunai Depression, Xinjiang Province, NW China

Previous investigations on major and trace elements within the Early Permian coals from Xinjiang Province are very scarce. This paper reports on the geochemistry of the Early Permian coals and associated non-coal rocks in the Jimunai depression, Xinjiang Province, northwestern China. Samples collected from coal exploration boreholes ZK2603, ZK3003, and ZKH1102 as well as the Haerjiao underground coal mine (Nos. HRJ-7 and HRJ-9 Coals) in the Jimunai Depression were analyzed using SEM-EDS, ICP-AES, and ICP-MS. These Early Permian coals reach the rank of semi-anthracite to anthracite and have low-sulfur contents (0.30% on average). In comparison with the average element concentrations of world coals, Zr, Hf, and rare earth elements and Y (REY) are slightly enriched (concentration coefficient (CC) = 2–5, as the ratio of element content in sample vs. that in world coals) and Mn is enriched (CC = 5–10) in the ZK2603, ZK3003, and ZKH1102 coals. Niobium and Ta are highly enriched in the ZK2603 and ZK3003 coals (CC = 10–100). The HRJ-7 and HRJ-9 coals are slightly enriched in Mn (CC = 2–5).The coals have a sediment source region mainly composed of intermediate-felsic compositions from the Saur Volcanic Arc to the south of the Jimunai Depression, which may have provided relatively high geochemical background concentrations of Zr, Hf, Nb, and Ta. Manganese is hosted mainly in ankerite that was derived from epigenetic precipitation from Mn-bearing hydrothermal fluids. Zirconium and Hf in the ZK2603, ZK3003, and ZKH1102 coals are controlled primarily by detrital matter or hydrothermal fluids, the latter of which results in the higher Zr/Hf ratio in the ZK2603 and ZK3003 coals due to the re-distribution of Zr and Hf. Niobium and Ta in the ZK2603 and ZK3003 coals are concentrated chiefly in zircon, and hydrothermal fluid activity caused the release of Nb and Ta from previously-existing inorganic constituents and subsequent incorporation into zircon, resulting in the lower Nb/Ta ratio due to Ta being more easily incorporated into zircon than Nb. REY are hosted primarily within multiple mineral phases (e.g., zircon, phosphates, anatase, clay minerals) and their enrichment results from a combination of detrital matter input and hydrothermal fluid influx, the former of which is evidenced by the occurrence of REY-bearing detrital zircon and anatase, and the latter of which is indicated by the existence of REY-bearing authigenic rhabdophane and anatase and corroded anatase particles, as well as by the positive Gd anomalies and MREY-type or HREY-type REY distribution patterns.

Influence of Semiconductor Morphology on Photocatalytic Activity of Plasmonic Photocatalysts: Titanate Nanowires and Octahedral Anatase Nanoparticles.

Octahedral anatase particles (OAP) with eight exposed and thermodynamically most stable (101) facets were prepared by an ultrasonication-hydrothermal (US-HT) reaction from potassium titanate nanowires (TNW). The precursor (TNW) and the product (OAP) of US-HT reaction were modified with nanoparticles of noble metals (Au, Ag or Pt) by photodeposition. Samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), scanning transmission electron microscopy (STEM) and time-resolved microwave conductivity (TRMC). The photocatalytic activity was investigated in three reaction systems, i.e., anaerobic dehydrogenation of methanol and oxidative decomposition of acetic acid under UV/vis irradiation, and oxidation of 2-propanol under vis irradiation. It was found that hydrogen liberation correlated with work function of metals, and thus the most active were platinum-modified samples. Photocatalytic activities of bare and modified OAP samples were much higher than those of TNW samples, probably due to anatase presence, higher crystallinity and electron mobility in faceted NPs. Interestingly, noble metals showed different influence on the activity depending on the semiconductor support, i.e., gold-modified TNW and platinum-modified OAP exhibited the highest activity for acetic acid decomposition, whereas silver- and gold-modified samples were the most active under vis irradiation, respectively. It is proposed that the form of noble metal (metallic vs. oxidized) as well as the morphology (well-organized vs. uncontrolled) have a critical effect on the overall photocatalytic performance. TRMC analysis confirmed that fast electron transfer to noble metal is a key factor for UV activity. It is proposed that the efficiency of plasmonic photocatalysis (under vis irradiation) depends on the oxidation form of metal (zero-valent preferable), photoabsorption properties (broad localized surface plasmon resonance (LSPR)), kind of metal (silver) and counteraction of “hot” electrons back transfer to noble metal NPs (by controlled morphology and high crystallinity).

Free energy of adhesion of lipid bilayers on titania surfaces.

The adhesion strength between a flexible membrane and a solid substrate (formally the free energy of adhesion per unit area) is difficult to determine experimentally, yet is a key parameter in determining the extent of the wrapping of a particle by the membrane. Here, we present molecular dynamics simulations designed to estimate this quantity between dimyristoylphosphatidylcholine (DMPC) bilayers and a range of low-energy titanium dioxide cleavage planes for both anatase and rutile polymorphs. The average adhesion strength across the cleavage planes for rutile and anatase is relatively weak ∼-2.0 ± 0.4 mN m-1. However, rutile has two surfaces (100 and 101) displaying relatively strong adhesion (-4 mN m-1), while anatase has only one (110). This suggests a slightly greater tendency for bilayers to wrap rutile particles compared to anatase particles but both would wrap less than amorphous silica. We also estimate the adsorption free energies of isolated DMPC lipids and find that only the rutile 101 surface shows significant adsorption. In addition, we estimate the adhesion enthalpies and infer that the entropic contribution to the adhesion free energy drives adhesion on the rutile surfaces and opposes adhesion on the anatase surfaces.

Influence of precursor characteristics on properties of porous calcium phosphate-titanium dioxide composite bioceramics

Highly porous (macroporosity 76–90%) bioceramics containing interconnected pores (>100 μm) with compressive strength between 0.54 and 0.32 MPa were prepared by polyurethane foam replica method. Effect of following variables, i.e., calcium phosphate/anatase ratio (30/70, 50/50, 70/30 wt%) in the ceramic slurry, anatase particle size (15 nm, 180 nm), Ca/P molar ratio of calcium phosphate (1.67 and 1.50 for hydroxyapatite and apatitic-tricalcium phosphate (ap-TCP), respectively), on the bioceramics properties was investigated. Bioceramics prepared using anatase and hydroxyapatite consisted of three high-temperature crystalline phases - β-tricalcium phosphate (β-TCP), rutile and CaTiO3. In case of anatase and ap-TCP, two phases (β-TCP and rutile) were obtained. Interaction of anatase and hydroxyapatite during sintering caused formation of CaTiO3 at β-TCP and rutile grain boundaries thus contributing to a denser grain packing. Combination of ap-TCP and nanosized anatase facilitated decrease of grain sizes. Correlation was found between compressive strength and calcium phosphate precursor in the ceramic slurry.

Unraveling the strength interaction in a TiO2-Graphene photocatalytic nanocomposite synthesized by the microwave hydrothermal method

Graphene oxide (GO) based composites were synthesized using a hydrothermal process, by these means, it was possible to tailor the loading of titanate nanotubes (TNTs) and TiO2 anatase particles in order to get insights into the photoactivity of the as-prepared photocatalyst. GO was prepared by the modified Hummers' method. The interactions stuck between titanium nanostructures and GO were studied by XRD, FESEM, FTIR, UV–vis and N2 physisorption along with computational studies using the DFT formalism on a graphene-TiO2 model. The photocatalytic performance was evaluated in the degradation of 4-clorophenol (4-CP) under UV light. The coexistence of anatase and titanate crystalline phases grown on the GO defects gave adsorptive and photocatalytic properties to the GO-based nanocomposites (TGO) and the favorable adsorption energy calculated by the DFT computational method showed that formyl acid surface groups on the tip area featured the main interaction of the TiO2-based photocatalyst. As a result, a photocatalyst with high surface area and functional sites was obtained, thus boosting the photoactivity and mass transfer properties to remove 4-CP from wastewater. Cycle voltammetry and electrochemical impedance measurements of the optimal TGO opened the possibility for using this material as a capacitor.

(Invited) Effect of Particle Size on Photocatalytic Reaction Kinetics Including Multielectron-Transfer Processes

Oxidation of water into molecular oxygen (O2 + 4H+ + 4e– → 2H2O) must be the largest-scale chemical reaction occurring on the earth as a part of photosynthesis by plants, and the corresponding backward half-cell reaction, reduction of oxygen into water (O2 + 4H+ + 4e– → 2H2O) might proceed in the same scale by the activity of plants and animals to get energy by combustion of chemicals keeping the concentration of oxygen on the earth. It is well known that the forward and backward reactions are counter oxidation and reduction reactions, respectively, of photocatalytic water splitting/carbon-dioxide fixation and photocatalytic decomposition of organic compounds by semiconductor particles. Standard electrode potential (SEP) of this four electron-transfer redox (O2 + 4H+ + 4e– = 2H2O) is 1.23 V (versus SHE = standard hydrogen electrode potential (0 V) in definition). Almost all the papers reporting oxygen evolution use this SEP, while those of photocatalytic organics decomposition tend to use SEP for one-electron transfer, O2 + e– = O2•– (–0.28 V), for discussion. One of the possible reasons for such difference is that it is convenient to presume the production of superoxide anion radical which may produce "reactive oxygen species (ROS)" to decompose organic compounds. Anyway, it can be stated that those proposed number of transferred electrons seems speculative without any experimental evidence. We have been studying light-intensity dependence (LID) of photocatalytic reaction rates using highly intense UV-LEDs with intensity maximally ca. 3 W cm-2. For oxygen evolution from aqueous titania suspensions in the presence of electron acceptors such as iodate (IO3 –) or iron(III) ions under deaerated conditions, small anatase (8 nm) and rutile (13 nm) particles show bimodal, at first second and then first order, LID in the lower and middle intensity range while large anatase and rutile particles show simple first-order LID in the whole intensity range. This is attributable to two-electron (positive hole) transfer process in the initial step. An interesting feature is that forth-order LID was observed with small anatase particles at high (2.5 W cm-2) intensity to suggest shift of the kinetics to four-electron transfer process [1]. For oxygen reduction, LID analyses for the rate of oxidative decomposition of acetic acid by a bismuth tungstate photocatalyst suggested that two-electron oxygen reduction predominantly proceeds [2]. On the basis of these results, "effective particle size" defined as size of particle (or particle aggregates) in which multiple electrons (positive holes) are accumulated to induce multielectron processes is proposed. This "effective particle size" may govern the effect of size on photocatalytic reaction rates. [1] Takeuchi, S.; Takashima, M.; Takase, M.; Ohtani, B. Chem. Lett. 2018, 47, 373-376. [2] Hori, H.; Takashima, M.; Takase, M.; Ohtani, B. Catal. Today 2018, 313, 218-223.

Low-Temperature Sol–Gel Synthesis and Photoactivity of Nanocrystalline TiO2 with the Anatase/Brookite Structure and an Amorphous Component

Nanodispersed TiO2 consisting of anatase and brookite phases in a ratio of 3 : 1 is obtained by the sol–gel method from titanium(IV) butoxide at a temperature below 100°C. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy are used to show that the size of anatase particles is about 6 nm, and brookite particles consist of a crystal core, 4 nm in diameter, surrounded by an amorphous shell. Compared with the commercial Degussa P25 photocatalyst, which consists of the anatase and rutile phases in the same 3 : 1 ratio, the resulting TiO2 exhibits an increased photoactivity in hydroquinone oxidation. Quantum chemical modeling of anatase and brookite surfaces as well as their interface does not reveal the specific features of their electronic structure. This fact is indicative of the amorphous phase as an active participant in the transfer of charge carriers in the photocatalytic process.