Facile assembly of TiO2 nanospheres/SnO2 quantum dots composites with excellent photocatalyst activity for the degradation of methyl orange

In this report, SnO₂ quantum dots anchored on TiO₂ nanospheres (TiO₂/SnO₂ composites) have been synthesized by a simple one-step hydrothermal process, and then employed as photocatalyst in photodegradation system. The microstructure of TiO₂/SnO₂ composites reveals that the SnO₂ quantum dots are dispersed on the surface of TiO₂ nanospheres uniformly. The photocatalytic behavior of the as-prepared samples revealed that the composites exhibited highly efficient performance by degrading 100 mL of 10 mg/L methylene orange in 15 min completely under ultraviolet-visible light. Owning to the special structure of the composites, TiO₂/SnO₂ shows a more UV–vis light absorption than either pure TiO₂ nanospheres or pure SnO₂ quantum dots. This study offers a facile method to prepare TiO₂/SnO₂ composites, which will be a choice for greatly extending potential applications in water pollution treatment, degradation of pollutants and other related fields.     

Absorption boost of TiO2 nanotubes by doping with N and sensitization with CdS quantum dots

A process of obtaining N-doped TiO₂ nanotubes sensitized by CdS nanoparticles is presented, including detailed characterizations performed along the synthesis. Transparent TiO₂ films consisting of nanotubes, 2.5 µm long and of ~60 nm inner diameter, were obtained after anodization of a titanium film deposited onto FTO glass substrate. N-doping was achieved by annealing of TiO₂ film in ammonia. X-ray Photoelectron Spectroscopy measurements showed that nitrogen was substitutionally incorporated in the TiO₂ matrix, with the N:Ti concentration ratio of 1:100. The doping changed the optical properties of the material in such a way that the absorption edge was shifted from 380 nm to 507 nm, as observed from diffuse reflectance spectra. The influence of the microwave (MW) irradiation on the synthesized CdS quantum dots and their optical properties was investigated. It was shown that the diameter of CdS nanoparticles was increased due to releasing of S^2- ions from dimethyl sulfoxide (DMSO) as a consequence of the MW treatment. The (N)TiO₂ films were then used as substrates for matrix assisted pulsed laser deposition of the CdS quantum dots with DMSO as a matrix. The laser parameters for the deposition were optimized in order to preserve the nanotubular structure open, the latter being an important feature of this type of photoanode. The structure obtained under optimized conditions has an additional absorption edge shift, reaching 603 nm.     

A single-step method for synthesis of CuInS2 nanostructures using cyclic microwave irradiation

Pure and well-crystalline CuInS₂ nanostructures were successfully synthesized via a cyclic microwave irradiation method at low microwave power using cupric chloride dihydrate (CuCl₂·2H₂O), indium chloride tetrahydrate (InCl₃·4H₂O) and thioacetamide as starting reagents. The effects of processing cycles (2–75 cycles) and microwave heating powers (180–600 W) on purity, crystallinity, particle size and morphology of the synthesized products were investigated. The synthesized products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and UV–vis spectroscopy. Increasing processing cycles and microwave heating power improved the purity and crystallinity of the synthesized products. Using the microwave powers of 450 and 600 W with 75 cycles, pure CuInS₂ nanocrystallites were obtained. The CuInS₂ crystallites displayed nanoparticles in clusters with decreasing in the diameter sizes from 1.6 to 0.9 µm when the microwave power was increased from 180 to 600 W. The reaction mechanism of the CuInS₂ formation was also provided. A CuInS₂ solar cell, made from FTO/TiO₂/CuInS₂/CdS/Pt-FTO layers, was fabricated to explore the potential use of the CuInS₂ nanoparticles in photovoltaic devices. The photovoltaic properties of the CuInS₂ solar cell including open-circuit voltage (V_oc), short-circuit current density (J_sc) and fill factor (FF) were 281.10 mV, 0.072 mA/cm² and 0.47, respectively, with cell conversion efficiency (η) of 0.01% under 1.5 AM illumination.     

All-solid-state quantum-dot-sensitized solar cells with compact PbS quantum-dot thin films and TiO2 nanorod arrays

To improve the electron injection efficiency from PbS quantum dots to TiO₂ nanorods and prevent the direct contact of spiro-OMeTAD and TiO₂ nanorods, a compact PbS quantum-dot thin film can be successfully obtained on TiO₂ nanorod arrays 360 nm in length by repeated spin coating of Pb(Ac)₂, Na₂S and 1,2-ethanedithiol solution in a step-by-step process. The corresponding solid-state quantum-dot-sensitized solar cells are fabricated using a novel structured FTO/compact PbS quantum-dot thin film sensitized TiO₂ nanorod array/spiro-OMeTAD/Au that achieves a photoelectric conversion efficiency of 3.57% under AM 1.5 G illumination (100 mW cm⁻²), which represents a high value among all-solid-state PbS quantum-dot-sensitized TiO₂ nanorod array solar cells.     

Preparation of dye sensitized solar cell by using supercritical carbon dioxide drying

Dye-sensitized solar cells (DSSC) derived from TiO₂ aerogel film electrodes were fabricated. TiO₂ aerogels were obtained by using sol–gel method and supercritical carbon dioxide (sc-CO₂) drying. First, TiO₂ wet gels were obtained by sol-gel method. Then, the solvents in the TiO₂ wet gels were replaced by acetone. The TiO₂ aerogels were obtained by using sc-CO₂ drying from the TiO₂ wet gels. The conditions of sc-CO₂ drying were at 313, 323 K and 7.8–15.5 MPa. The electrodes with TiO₂ aerogel films were obtained by deposition of the aerogels on glass substrates. The electrodes with TiO₂ aerogel films and a commercial particle film of various thickness were obtained by repetitive coatings and calcinations. The amount of dye adsorbed on the TiO₂ films with sc-CO₂ drying was higher than that of commercial particle film. The amount of dye adsorbed on the TiO₂ films increased with increasing surface area of the TiO₂ film. DSSCs were assembled by using the TiO₂ aerogel film electrodes and their current–voltage performance was measured. The power performance of DSSC made by supercritical drying was higher than that of commercial particles. The DSSC with the film electrode made at 313 K and 15.5 MPa showed the best power performance (J_sc = 7.30 mA/cm², V_oc = 772 mV, η = 3.28%).     

Growth mechanism and photocatalytic activity of chrysanthemum-like anatase TiO2 nanostructures

Chrysanthemum-like hierarchical anatase TiO₂ nanostructures self-assembled by nanorods have been successfully fabricated by a simple solvothermal route without using template materials or structure-directing additives. The products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Raman spectrometer system (Raman), UV–vis absorption spectroscopy (UV–vis) and N₂ adsorption–desorption measurement. The results indicate that synthesized chrysanthemum-like hierarchical anatase TiO₂ nanostructures have a spherical shape with an average diameter of 1.5 μm and they are composed of nanorods with a width of about 30 nm and a length of about 300 nm. The pore distribution of the sample exhibits two kinds of pores. Such mesoporous structure of the sample might be extremely useful in photocatalysis because they possess efficient transport pathways to the interior and supplies higher specific area for more pollutant molecules to be absorbed. In addition, the synthesized TiO₂ nanostructures show enhanced photocatalytic activity compared with commercial P25 for the degradation of RhB under UV light irradiation, which can be attributed to their special hierarchical structure and high light-harvesting capacity.     

The effect of TiO2 particle size on the characteristics of Au–Pd/TiO2 catalysts

The nanocrystalline TiO₂ materials with average crystallite sizes of 9 and 15 nm were synthesized by the solvothermal method and employed as the supports for preparation of bimetallic Au/Pd/TiO₂ catalysts. The average size of Au–Pd alloy particles increased slightly from sub-nano (< 1 nm) to 2–3 nm with increasing TiO₂ crystallite size from 9 to 15 nm. The catalyst performances were evaluated in the liquid-phase selective hydrogenation of 1-heptyne under mild reaction conditions (H₂ 1 bar, 30 °C). The exertion of electronic modification of Pd by Au–Pd alloy formation depended on the TiO₂ crystallite size in which it was more pronounced for Au/Pd on the larger TiO₂ (15 nm) than on the smaller one (9 nm), resulting in higher hydrogenation activity and lower selectivity to 1-heptene on the former catalyst.     

Oriented rutile TiO2 nanorod arrays for efficient quantum dot-sensitized solar cells with extremely high open-circuit voltage

TiO₂ nanoparticles are typically employed to construct the porous films for quantum dot-sensitized solar cells (QDSCs). However, undesirable interface charge recombination at grain boundaries would hinder the efficient electron transport to the conducting substrate, giving rise to the decline of open-circuit voltage (V_oc). In this work, vertically aligned architectures of oriented one-dimensional (1D) TiO₂ nanorod arrays hydrothermally grown on substrates pave a way in designing highly efficient QDSCs with efficient radial-directional charge transport. SEM, TEM, XRD, and Raman spectroscopy were employed to characterize the as-prepared TiO₂ nanorods, showing the rutile phase with single-crystalline structure. The homogeneous deposition of CdS/CdSe QDs on the surface of TiO₂ nanorods has been achieved by in-situ grown strategies (i.e., successive ionic layer absorption and reaction, and chemical bath deposition). An extremely high V_oc value up to 0.77 V has been achieved for CdS/CdSe QDSCs based on the well-ordered 1D nanorod arrays. To the best of our knowledge, it is the highest V_oc reported for TiO₂-based QDSCs. Dependencies of photovoltaic performance, optical absorption, and interfacial charge behavior on the length of nanorods were systematically investigated. A 1.7 μm nanorod-array photoelectrode-based QDSC delivers a remarkable power conversion efficiency up to 3.57% under simulated AM 1.5 100 mW cm⁻² illumination, attributed to the balance of competition between the increase of QD loading and suppression of interfacial recombination. This work highlights the combination of QDs with high absorption coefficient 1D architectures possessing efficient charge transport for constructing high efficiency solar cells.     

Comparison of the photovoltaic efficiency on DSSC for nanometer sized TiO2 using a conventional sol–gel and solvothermal methods

When the two types of TiO₂ coatings prepared by sol–gel and solvothermal methods were applied to dye-sensitized solar cell (DSSC) in this study, the energy conversion efficiency of the solvothermal-modified TiO₂ was considerably higher than that on the sol–gel modified TiO₂; approximately 8.51 (solvothermal) and 5.93% (sol–gel) with the N719 dye under 100 mW/cm² of simulated sunlight, respectively. These results are in agreement with an electrostatic force microscopy (EFM) study showing that the electrons were transferred rapidly to the surface of the solvothermal-modified TiO₂ film, compared with that on a sol–gel modified TiO₂ film. Furthermore, FT-IR analysis of the films after N719 dye adsorption showed that the solvothermal-modified TiO₂ had a strong band at 500 cm^?1, which was assigned to metal–O, due to a new Ti–O bond between the O of COO^? and a Ti atom. This peak was considerably weaker in the sol–gel modified TiO₂.     

Polypyridyl ruthenium complexes containing anchoring nitrile groups as TiO2 sensitizers for application in solar cells

Ruthenium polypyridyl complexes of general formula [Ru(bpy)_3 − x(Mebpy-CN)_x]^2 + (x = 1,2 and 3, bpy = 2,2′-bipyridine, Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile) can be used as visible dyes in novel solar cells formed with a porous TiO₂ film (1 cm²), Pt counter-electrode and iodine/iodide as the redox mediator electrolyte dissolved in a polymeric matrix. These complexes can be anchored over the surface of nanocrystalline TiO₂ through nitrile groups, as evidenced by Raman spectra of the adsorbed species. Irradiated by a solar simulator (67 mW cm^− 2), the cells assembled with the Ru complexes with x = 2 and 3 as TiO₂ sensitizers exhibit almost identical current–potential curves, with short-circuit photocurrents of 1.25 mA cm^− 2, fill factors of 0.5 and overall efficiencies around 0.44%. The Ru complex with x = 1 and a similar Re complex did not perform as well as sensitizers. These data were consistent with results obtained from quantum efficiency curves and impedance spectra. We conclude that complexes with nitrile groups as anchoring entities are promising candidates for designing efficient DSCCs.     

Dye sensitized solar cells based on hydrothermally grown TiO2 nanostars over nanorods

A rutile titanium dioxide nanostar over nanorods is synthesized by a simple and cost-effective hydrothermal deposition method onto conducting glass substrates. In order to study the effect of precursor concentrations on the growth of TiO₂, the amount of Ti precursor is varied from 0.1 mL to 0.5 mL at the interval of 0.1 mL. These TiO₂ thin films are characterized for their morphological, structural, optical and J–V properties using various characterization techniques. SEM images showed the formation of densely packed nanostars over nanorods for 0.3 mL titanium tetraisopropoxide (TTIP). XRD patterns show the formation of polycrystalline TiO₂ with tetragonal crystal structure possessing rutile phase. Further, the TiO₂ thin films are used for dye sensitized solar cells using N3-dye.The films were photoelectrochemically active and can be viewed as a promising application in DSSC with maximum current density of 1.459 mA/cm² with enhanced photovoltage of 696 mV for the sample prepared at 0.3 mL TTIP.     

Photovoltaic efficiency on dye-sensitized solar cells (DSSC) assembled using Ga-incorporated TiO2 materials

This study examined the photoelectric conversion efficiency of DSSC (dye-sensitized solar cell) when nanometer sized Ga (0.25, 0.50, and 1.00 mol%)–TiO₂ prepared using a hydrothermal method was employed as a working electrode material. The particle sizes observed in the transmission electron microscopy images were <20 nm in all samples. However, with increasing Ga concentration, the size increased and the shapes transformed to a stick form. The absorption band was slightly blue-shifted upon the incorporation of gallium ions, but the intensity of the photoluminescence (PL) curves of the Ga-incorporated TiO₂ was significantly smaller, with the smallest case being the 0.50 mol% Ga–TiO_2, which was related to recombination between the excited electrons and holes. When Ga–TiO₂ was applied in DSSC, the energy conversion efficiency was enhanced considerably compared to that using pure TiO₂; it was approximately 4.57% with the N3 dye under 100 mW/cm² of simulated sunlight. These results are in agreement with an electrostatic force microscopy (EFM) study showing that the electrons were transferred rapidly to the surface of Ga–TiO₂ film, compared with that on a pure TiO₂ film.     

Photocatalytic and adsorptive treatment of 2,4-dinitrophenol using a TiO2 film covering activated carbon surface

Anatase titanium dioxide (TiO₂) in particle sizes of roughly 0.5–20 μm was prepared from amorphous TiO₂ in an aqueous H₂O₂ solution by heating at 90 °C for 9 h and directly deposited on a PET film. On the other hand, granular activated carbon (AC) particles in sizes of 1–2 mm in diameter were adhesively deposited on a PET film, and their surfaces were also coated with TiO₂. The resulting three preparations (TiO₂-, AC-, and TiO₂/AC-PET films) were set up in an annular-flow reactor to treat aqueous solutions of 2,4-dinitrophenol (DNP) in a batch-recirculation mode. The rate of DNP adsorption onto the TiO₂/AC-PET film without UV irradiation was almost the same as that onto the AC-PET film, indicating that the attraction of DNP to AC was not lowered in the presence of TiO₂ film. Observation of SEM photographs suggests that this result is attributed to the porous structure of the thin TiO₂ film covering AC particles. The rate of DNP removal by the TiO₂-AC PET film under UV irradiation was 2.9 times higher than that by the TiO₂-PET film under UV irradiation, and was 1.1 times higher than the rate of DNP adsorption onto the AC-PET film. The rate of DNP removal by the AC-PET film decreased by 40% after six runs, while that by the TiO₂/AC-PET film decreased by 22%. Durable experiments using the TiO₂/AC-PET and AC-PET films clarified that the lifetime of the TiO₂/AC-PET film is at least two times longer than that of the AC-PET film. This result suggests that DNP molecules are photocatalytically decomposed when passing through the porous TiO₂-PET film, which lessens a burden of DNP adsorption on AC. Moreover, the DNP treatments in the batch-recirculation flow system suggested that the TiO₂/AC-PET film saturated with DNP can be successfully regenerated at 60 °C.     

Preparation of TiO2-loaded nanocapsules and their sun protection behaviors

Titanium dioxide (TiO₂) sol (TS) or TiO₂ nanoparticles with chitosan shell were encapsulated to enhance their sun protection abilities. Nanocapsules loaded with TiO₂ were evaluated for their ultraviolet (UV) absorption and UV protection rate. The effect of eicosapentaenoic acid (EPA) incorporation into the nanocapsules was examined in relation to UV absorption. The particle size of TiO₂ crystallites in TS was below 30 nm, which was considerably smaller than that of a commercial TiO₂ (P25 TiO₂) particles. In the encapsulation of TiO₂ with chitosan, the loading efficiencies of TS were higher than those of P25 TiO₂. The sizes of the nanocapsules loaded with TiO₂ particles ranged from 30 to 80 nm. The absorption range of irradiation wavelength was enlarged by the incorporation of EPA into the nanocapsules. The TS-loaded nanocapsules exhibited a high UV protection rate of up to 95% to both UV-A and UV-B.     

Transparent and flexible films of new segmented polyurethane nanocomposites incorporated by NH2-functionalized TiO2 nanoparticles

In this work, TiO₂ nanoparticles are surface modified by NH₂-terminated organic moieties arised from 4,4′-methylene diphenyl diisocyanate (MDI). These nanoparticles are incorporated into ether-based segmented polyurethane (SPU) matrix. MDI is utilized as monomer together with poly(tetramethylene oxide) (PTMO) comonomer for preparing the final polymer as well. The NH₂-functionalized TiO₂ nanoparticles are covalently linked to the NCO terminals of the resulting SPU macromolecules during film preparation stage. Therefore, in addition to butylene glycol, these surface modified nanoparticles with enhanced organophilicity could play the role of the second chain extender of NCO-capped SPU macromolecules through formation of urea linkages. Optical and thermal behaviors of the transparent and flexible film (SPU/TiO₂–MDI) is compared with those of unmodified TiO₂ (SPU/TiO₂) and TiO₂-unloaded SPU films. Though the particle loading is only 5 wt.%, incorporation of TiO₂ and TiO₂–MDI nanoparticles into the SPU polymer enhances significantly the light absorption in UV region at 300–400 nm. SEM images of the prepared films clearly show a considerable decrease in particle aggregation for TiO₂–MDI into SPU matrix compared to that of unmodified TiO₂. TG analyses indicate a one-step decomposition pattern with onset temperatures of about 360 and 380 °C for neat SPU and SPU/TiO₂–MDI, respectively. Moreover, DTA thermograms of both nanocomposites show obviously two exothermic phase transitions in the thermal range of 330–440 °C.     

CuInS2 quantum dot-sensitized TiO2 nanorod array photoelectrodes: synthesis and performance optimization

CuInS₂ quantum dots (QDs) were deposited onto TiO₂ nanorod arrays for different cycles by using successive ionic layer adsorption and reaction (SILAR) method. The effect of SILAR cycles on the light absorption and photoelectrochemical properties of the sensitized photoelectrodes was studied. With optimization of CuInS₂ SILAR cycles and introduction of In₂S₃ buffer layer, quantum dot-sensitized solar cells assembled with 3-μm thick TiO₂ nanorod film exhibited a short-circuit current density (I_sc) of 4.51 mA cm⁻², an open-circuit voltage (V_oc) of 0.56 V, a fill factor (FF) of 0.41, and a power conversion efficiency (η) of 1.06%, respectively. This study indicates that SILAR process is a very promising strategy for preparing directly anchored semiconductor QDs on TiO₂ nanorod surface in a straightforward but controllable way without any complicated fabrication procedures and introduction of a linker molecule.     

Visible-light-induced photocatalytic activity of TiO2−xNy prepared by solvothermal process in urea–alcohol system

Nitrogen-doped anatase, rutile and brookite titania photocatalyst TiO_2−xN_y which can be excited by visible light were prepared by mixing aqueous TiCl₃ solutions with urea ((NH₂)₂CO) and various type of alcohols followed by solvothermal treatment at 190 °C. The phase composition, crystallinity, microstructure and specific surface area of titania powders greatly changed depending on the pH and type of solvents. Violet, yellowish and grayish TiO_2−xN_y with excellent visible light absorption and photocatalytic activity were prepared. The TiO_2−xN_y powders prepared in urea–methanol solution showed excellent photocatalytic ability for the oxidative destruction of nitrogen monoxide under irradiation of visible light λ > 510 nm.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

Fabrication of highly efficient TiO2/Ag/TiO2 multilayer transparent conducting electrode with N ion implantation for optoelectronic applications

Fabrication of highly conductive and transparent TiO₂/Ag/TiO₂ (referred hereafter as TAT) multilayer films with nitrogen implantation is reported. In the present work, TAT films were fabricated with a total thickness of 100 nm by sputtering on glass substrates at room temperature. The as-deposited films were implanted with 40 keV N ions for different fluences (1×10¹⁴, 5×10¹⁴, 1×10¹⁵, 5×10¹⁵ and 1×10¹⁶ ions/cm²). The objective of this study was to investigate the effect of N⁺ implantation on the optical and electrical properties of TAT multilayer films. X-ray diffraction of TAT films shows an amorphous TiO₂ film with a crystalline peak assigned to Ag (111) diffraction plane. The surface morphology studied by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM) revealed smooth and uniform top layer of the sandwich structure. The surface roughness of pristine film was 1.7 nm which increases to 2.34 nm on implantation for 1×10¹⁴ ions/cm² fluence. Beyond this fluence, the roughness decreases. The oxide/metal/oxide structure exhibits an average transmittance ~80% for pristine and ~70% for the implanted film at fluence of 1×10¹⁶ ions/cm² in the visible region. The electrical resistivity of the pristine sample was obtained as 2.04×10⁻⁴ Ω cm which is minimized to 9.62×10⁻⁵ Ω cm at highest fluence. Sheet resistance of TAT films decreased from 20.4 to 9.62 Ω/□ with an increase in fluence. Electrical and optical parameters such as carrier concentration, carrier mobility, absorption coefficient, band gap, refractive index and extinction coefficient have been calculated for the pristine and implanted films to assess the performance of films. The TAT multilayer film with fluence of 1×10¹⁶ ions/cm² showed maximum Haacke figure of merit (FOM) of 5.7×10⁻³ Ω⁻¹. X-ray photoelectron spectroscopy (XPS) analysis of N 1s and Ti 2p spectra revealed that substitutional implantation of nitrogen into the TiO₂ lattice added new electronic states just above the valence band which is responsible for the narrowing of band gap resulting in the enhancement in electrical conductivity. This study reports that fabrication of multilayer transparent conducting electrode with nitrogen implantation that exhibits superior electrical and optical properties and hence can be an alternative to indium tin oxide (ITO) for futuristic TCE applications in optoelectronic devices.     

Suspension high velocity oxy-fuel spraying of a rutile TiO2 feedstock: Microstructure,phase evolution and photocatalytic behaviour

Nano-structured TiO₂ coatings were produced by suspension high velocity oxy fuel (SHVOF) thermal spraying using water-based suspensions containing 30 wt% of submicron rutile powders (~180 nm). By changing the flame heat powers from 40 kW to 101 kW, TiO₂ coatings were obtained with distinctive microstructures, phases and photocatalytic behaviour. Spraying with low power (40 kW) resulted in a more porous microstructure with the presence of un-melted nano-particles and a lower content of the anatase phase; meanwhile, high powers (72/101 kW) resulted in denser coatings and rougher surfaces with distinctive humps but not necessarily with a higher content of anatase. Linear sweep voltammetry (LSV) was used to evaluate the photocatalytic performance. Surprisingly, coatings with the lowest anatase content (~20%) using 40 kW showed the best photocatalytic behaviour with the highest photo-conversion efficiency. It was suggested that this was partially owing to the increased specific surface area of the un-melted nano-particles. More importantly, the structural arrangement of the similarly sized TiO₂ nano-crystallites between rutile and antase phases also created catalytic “hot spots” at the rutile−anatase interface and greatly improved the photo-activity.