WO2015028529A1 - Visible light photoactive nanoparticles and methods for the preparation thereof - Google Patents

Visible light photoactive nanoparticles and methods for the preparation thereof Download PDF

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Publication number
WO2015028529A1
WO2015028529A1 PCT/EP2014/068212 EP2014068212W WO2015028529A1 WO 2015028529 A1 WO2015028529 A1 WO 2015028529A1 EP 2014068212 W EP2014068212 W EP 2014068212W WO 2015028529 A1 WO2015028529 A1 WO 2015028529A1
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particle
titanium
core
range
metal
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Sotiris Pratsinis
Yiannis Deligiannakis
Kakeru Fujiwara
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Eidgenoessische Technische Hochschule Zurich ETHZ
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Eidgenoessische Technische Hochschule Zurich ETHZ
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Definitions

  • Visible light photoactive nanoparticles and methods for the preparation thereof
  • the present invention relates to nanoparticles with specific architecture so they have the capability to efficiently photogenerate holes and electrons under UV-irradiation but also under visible light irradiation. Therefore they are photoactive and in particular act as a catalyst not only under UV-irradiation but also under visible light irradiation. Furthermore the invention relates to methods for making such particles in an efficient and if needed easily scalable and reliable process.
  • a so-called “black Ti0 2" consisting of a Ti0 2 core and a distorted TiO x (x ⁇ 2) shell (titanium suboxide) with remarkable photoactivity under visible-light irradiation has been proposed for efficient solar energy utilization.
  • the photoactivity preferentially occurs on the surface of these particles.
  • a titanium suboxide shell on a core Ti0 2 particle can be produced by laser irradiation or by reduction with H 2 , organic solvents or CaH 2 .
  • binary Ti0 2 -Ti 2 0 3 has been formed under extensive UV irradiation of sol-gel- made Ti0 2 .
  • the improved visible-light photoresponse of Ti suboxides involves the modulation of the energy levels of the semiconductor: Ti 3+ defects enhance visible-light photoactivity by introducing band-gap energy states.
  • the distorted titanium suboxide shell creates an additional "tail" from the valence band into the band gap.
  • the engineering of a suboxide phase preferably shell-type or as point Ti 3+ reduced states on crystalline semiconducting Ti0 2 , opens new opportunities for visible-light-photoactive Ti0 2 .
  • Visible light photoactive materials are most attractive in solar energy utilization, sensors, photo voltaics and efficient photodegradation of pollutants.
  • a narrowing of its band gap is most promising. This is attained by creation of oxygen defects (e.g. Ti 3+ ) or the formation of titanium suboxide (TiO x ) phases.
  • oxygen defects e.g. Ti 3+
  • TiO x titanium suboxide
  • this is proposed by continuous, one-step, flame aerosol synthesis of layers of titanium suboxides (T1 4 O 7 and T1 3 O 5 ) onto nanosilver supported on nanostructured Ti0 2 particles that are characterized by electron microscopy, Raman, N 2 adsorption, Diffuse reflectance and X- ray diffraction.
  • the proposed core-shell nanoparticles exhibit strong photo catalytic activity toward e.g. Cr 6+ and methylene blue (half life of 15 - 55 minutes) under visible light irradiation ( ⁇ > 400 nm, 450 W).
  • Ti0 2 and specific metals exhibit an abnormal interaction, the so-called strong metal-support interaction (SMSI).
  • SMSI strong metal-support interaction
  • the Ti0 2 surface is reduced and the so-formed Ti 3+ can either interact with the metal phase, thereby forming weak covalent bonds, or receive electrons from it.
  • the so-formed Ti 3+ can diffuse subsequently to the surface of noble metal or Ti0 2 and surround them, thereby forming a shell of titanium suboxide.
  • the presence of Ti 3+ on the Pt-Ti0 2 surface with SMSI can be observed by X-ray photoelectron spectroscopy.
  • Such a distorted titanium suboxide shell can also be observed also by transmission electron microscopy (TEM) in Pd-Ti0 2 .
  • TEM transmission electron microscopy
  • the chemical structure of the titanium suboxide depends on the reducing conditions. Above 600°C in H 2 , T1 4 O 7 can be formed, whereas further reduction to T1 3 O 5 may require a radical reorganization of the structure.
  • the present invention proposes a new type of titanium dioxide nanoparticles (typical average primary particle - as determined by nitrogen adsorption or microscopy - or crystallite - as determined by X-ray diffraction - diameter in the range of 3-300 nm preferably in the range of 3 - 90 nm or 5-40 nm) on the surface of which there are attached metal nanoparticles (typical average diameter in the range of 2-90 nm, as determined by X-ray diffraction or microscopy) preferably in the range of 2 - 50 nm or 2- 10 nm), namely in only a partial coverage by a metal, preferably a noble metal such as silver.
  • a metal typically a noble metal such as silver.
  • this is 0.2 -50% or 0.2 - 30%, preferably in the range of 1-27 % of the surface area of the titanium dioxide nano-particle is covered by individual separated dots of elementary (noble) metal, in the form of metal nanoparticle deposits.
  • elementary noble metal nanoparticles are on the outer side at least partially, preferably completely, covered by a layer of crystalline titanium sub-oxide of variable thickness.
  • Ti0 2 nanoparticles Apart from the particle as such and its applications as a visible light catalyst, a continuous gas-phase and proven, scalable method to kg/h, flame spray pyrolysis is proposed, for synthesis of black Ti0 2 nanoparticles by simultaneous formation of Ti0 2 , titanium suboxide and nanosilver in one-step: Titanium and silver precursors in flammable solvents are sprayed and dispersed by 0 2 in fine mists that are combusted resulting in layered (or shell) TiO x onto nanosilver and Ti0 2 that is immobilized or supported onto core, predominantly anatase Ti0 2 Nano particles.
  • the present invention relates to a particle or powder, suspension or the like of such particles with catalytic activity under irradiation in the visible range of the electromagnetic spectrum comprising a core-particle of titanium dioxide with an average diameter in the range of 10-40 nm, on the surface of said core-particle a layer or at least one noble metal nano-deposit partially covering said surface, on the surface of said layer or of said at least one noble metal nano deposit a layer of titanium-suboxide.
  • the layer of titanium-suboxide is contacting free surface of the core-particle of titanium dioxide.
  • the titanium-suboxide can be selected from the group Ti x 0 2x -i consisting of: crystalline like T1 3 O 5 , T1 4 O 7 , T1 5 O 9 , or a mixture and/or aggregate thereof, wherein preferably it is selected from the group consisting of: crystalline T1 3 O 5 , T1 4 O 7 and where the crystalline mass ratio of Ti30 5 ,to T1 4 O 7 is preferably chosen to be in the range of 1 :6 to 6: 1.
  • the weight ratio of the titanium dioxide of the core-particle to the metal is in the range of 29: 1 - 3: 1, preferably around 12:1 -4: 1.
  • the noble metal is preferably selected from the group consisting of: silver, gold, nickel, cobalt, palladium, rhodium, iridium, platinum, copper or a combination thereof, wherein it is preferably selected to be silver.
  • the present invention furthermore relates to a method for making such a particle or powder of such particles.
  • a method for making such a particle or powder of such particles preferably in a flame spray pyrolysis process titanium and silver precursors in flammable solvents are sprayed and dispersed by oxygen and combusted in a flame.
  • titanium dioxide and silver nanoparticles are separately formed and the nanosilver deposited on the titanium dioxide nanoparticles because of the fast nucleation of titanium dioxide.
  • Titanium suboxides are formed in the by strong metal (Ag) support (Ti0 2 ) interactions (SMSIs).
  • the weight ratio of the titanium and the silver precursors in the flammable solvent is preferably adapted to be in the range of 3-90 %and preferably 7-14%..
  • the present method is a continuous gas-phase and scalable method to kg/h, flame spray pyrolysis (FSP), for synthesis of black Ti0 2 nanoparticles (NPs) in one-step.
  • FSP flame spray pyrolysis
  • the present invention relates to uses of such particles, preferably made according to a method as outlined above as a visible light photo catalyst, or in solar energy utilisation.
  • the particles can be used for oxidation reactions and/or photodegradation reactions, in particular of pollutants.
  • Fig. 1 shows in (A) commercial Ti0 2 (P25) and FSP-made Ti0 2 loaded with Ag
  • Electron paramagnetic resonance (EPR) spectra for the 20Ag/TiO 2 NPs prepared at X/Y 8/5 and recorded at 77 K in the dark (bottom; prior to illumination), after illumination with UV-Vis radiation (middle; ⁇ > 240 nm), or Vis radiation (top; ⁇ > 400 nm);
  • Photocatalytic reduction of (A) Cr 6+ ions and (B) Methylene blue by pure Ti0 2 (commercial P25, black circles) and 20Ag/TiO 2 prepared by flame spray pyro lysis at X/Y 3/5 (orange triangles) or 8/5 (green squares) under visible-light irradiation ( ⁇ > 400 nm, 450 W); the presence of TiO x layers containing nano crystalline Ti 4 0 7 and Ti 3 0 5 onto nanosilver supported on nano Ti0 2 facilitate the attainment if very short photodegradation half-life, 25-55 min for Cr 6+ and 15-25 min for methylene blue;
  • Diffuse reflectance (DR) absorption spectra of commercial Ti0 2 ; "black" Ti0 2 NPs loaded with Ag (10 and 20 wt.%) prepared at FSP feed ratios of X/Y 3/5 and 8/5 in the absence or presence of 0 2 sheath gas (20 L/min) and mechanically mixed 20 wt.% of 95Ag/Si0 2 and 80 wt.% of Ti0 2 (P25, black broken line); the 20Ag/TiO 2 particles show significant absorption from 360 to 800 nm, which is the origin of their characteristic black color (Fig.
  • the mechanically mixed Ti0 2 -Ag° (supported on Si0 2 ) particles do not show any visible absorption, the improved absorbance at visible wavelengths (400 - 800 nm) of the present FSP-made Ag/Ti0 2 over commercial nano-Ti0 2 (P25) with or without equivalent nanosilver mass is therefore attributed to the presence of the titanium suboxides;
  • Diffuse reflectance absorption spectra of 20Ag/TiO 2 NPs "as prepared" at FSP feed ratio X/Y 8/5 and annealed at 350 or 550°C in air for 2 h.
  • Raman spectra of black Ti0 2 NPs loaded with Ag (10 or 20 wt.%) and prepared at X/Y 3/5 or 8/5; for comparison, the Raman spectra of commercial Ti0 2 (P25, Degussa) and FSP-made Ag (95 wt.%) supported on Si0 2 NPs (95Ag/Si0 2 ) are also included; commercial Ti0 2 (black solid line) gives the well-known four peaks at 199, 397, 514, and 639 crrf 1 that are typical for anatase; notably, no Raman peak for silver (95Ag/Si0 2 , dashed line) was detected; thus, the Raman peaks detected for the Ag/Ti0 2 NPs correspond to pure Ti0 2 or titanium suboxide phases; when the Ag loading is increased from 10 to 20 wt.% and/or the flame conditions becomes more reducing (by increasing the precursor/fuel stream for 3 to 8 mL/min), the anatase peaks at 5
  • Axial flame temperatures during FSP synthesis 20Ag/TiO 2 at feed rate X/Y 8/5 with (triangles) and without (circles) 0 2 sheath; the temperature was measured by non-intrusive Fourier transform infrared (FITR) emission and transmission spectroscopy; the FTIR spectra ranged from 500 ⁇ 8000 crrf 1 with 32 cm 1 resolution and were obtained by an FTIR spectrometer (Bomen MB155S); path correction spectra for the emission measurements were taken by a blackbody cavity (PYROcal LAB, TRANS-METRA GmbH); the flame temperature is an integral along the IR beam of ⁇ 4 mm in diameter through the flame centerline at different heights above the FSP nozzle tip; the 0 2 sheath does not affect the maximum flame temperature (-2700 K); downstream from the maximum, the temperature decreases with cooling by entrainment flow; the cooling rate in the presence of sheath 0 2 is slightly slower downstream from the maximum than that in its absence, increasing the high temperature
  • Core-Ti0 2 /shell-titanium suboxide (“black" Ti0 2 ) NPs loaded with 10 or 20 wt.% of Ag (10Ag/TiO 2 or 20Ag/TiO 2 ), the percentages always being calculated with respect to the total weight of the particles, were prepared in a single step by FSP and were collected on glass-fiber filters. Silver acetate (Aldrich, purity > 99%) and titanium (IV) isopropoxide (TTIP, Aldrich, purity > 97%) were used as silver and titanium precursors, respectively.
  • TTIP titanium isopropoxide
  • the total precursor (TTIP and silver acetate) concentration in a 1 : 1 mixture of 2-ethylhexanoic acid (Aldrich, purity > 99%) and acetonitrile (Aldrich, purity > 99.5%) was 0.16 M.
  • High-resolution TEM was performed with a Tecnai F30-ST microscope (FEI, operated at 300 kV, point resolution ⁇ 2 A). As-prepared particles were dispersed in ethanol and deposited onto a perforated carbon foil supported on a copper grid.
  • EPR spectra were recorded with a Bruker ER200D spectrometer at liquid-N 2 temperatures, equipped with an Agilent 531 OA frequency counter. For each experiment, 10 mg of NPs are placed inside a quartz EPR tube (Suprasil, Willmad Glass) with an outer diameter of 3 mm. The EPR signals presented herein were recorded at 9.49 GHz, using a modulation amplitude of 4 G, a field modulation of 100 kHz, and a microwave power of 4 mW. Adequate signal-to-noise ratios were obtained with 10 scans.
  • a solar- light-simulating Xe light source (Oriel model 66929, 450 W) equipped with a water IR filter was used to investigate photogeneration of electrons and holes. Visible-light illumination ( ⁇ > 400 nm) was performed using a longpass 400 nm filter (Edmund Optics 85754). The same light source was used for photocatalytic experiments.
  • Photocatalytic Cr 6+ and methylene blue (MB) degradation was preformed in a Pyrex glass container.
  • K 2 Cr 2 0 7 Aldrich
  • Methylene blue Aldrich
  • MB suspension was centrifuged for 15 min at 4000 rpm using a Hettich Universal 16A centrifuge for further analysis.
  • the MB decolorization was determined by monitoring the change in optical absorption at 660 nm using a Perkin Elmer Lambda 35 double-beam UV-Vis spectrophotometer.
  • the Cr 6+ ions in each solution were determined by the diphenylcarbazide method using the UV-Vis spectrophotometer.
  • a high Ag loading e.g. 20 wt.%) and a high FSP feed ratio (e.g. 8/5) resulted in the darkest particles (Fig. 1A).
  • the dark particle colors are similar to those of H 2 -reduced Ti0 2 . This dark color is not due to incomplete combustion products i.e.
  • Figures 1C,D A closer inspection of Figures 1C,D, however, reveals a distorted layer onto the darker Ag NPs (indicated by broken red lines). Similar morphologies have been observed on strongly interacting Co and Pt nanostructures on Ti0 2 produced by H 2 reduction at high temperatures. This morphology indicates SMSIs that result in coating of noble metals by titanium suboxides.
  • Figure IE shows schematically such distorted suboxide layers encapsulating nanosilver in Figures 1C,D (indicated by arrows). This coating hinders nanosilver oxidation and leaching or release of Ag + ions in aqueous solutions. This suppression of Ag + ion release is attributed to the Ti- suboxide formation.
  • the Ag + release (or leaching) from the present Ag/Ti0 2 particles is 61-87 % less than that by similar, flame-made nanosilver supported on Si0 2 and suspended in water (see Table 1).
  • the crystal sizes of Ag were obtained by Rietveld analysis (TOPAS 4.0) of the XRD spectra at 44° (Ag°).
  • the Ag + ion concentration in de-ionized water containing 100 mg/L of nanosilver was measured with an ion-selective electrode and an ion meter (both Metrohm).
  • the suppression of Ag + ion release was calculated by the difference between the calculated Ag + ion release on Si0 2 and the observed one on Ti0 2 here divided by the former.
  • Increasing the X/Y ratio increases particle mass concentration and residence time at high temperatures that lead to larger particles by enhanced coagulation and sintering.
  • the peaks at 25.5° and 48° correspond to anatase Ti0 2 (diamonds) that are dominant in all particles made here.
  • Only particles prepared at X/Y 8/5 contain a small peak around 27.5° corresponding to rutile (circles).
  • the Ag° peaks (squares) resolved at 44° show a distorted shape compared with flame-made Ag° NPs supported on Si0 2 indicating the SMSI between Ag and Ti0 2 here.
  • titanium suboxide peaks were resolved in the XRD patterns: T1 3 O 5 (down triangle) at 33° and Ti 4 0 7 (up triangles) at 28.5° and 29.5° in Fig. 2A.
  • Analogous features have been reported for T1 3 O 5 and Ti 4 0 7 phases formed by SMSI.
  • the suboxide peaks are broad, indicating small crystals and/or low crystallinity consistent with their layered formation onto nanosilver (Fig. 1C, D).
  • the peak intensities increased with Ag loading and FSP feed ratio (reducing flame conditions) corresponding to the darkening of these particles (Fig. 1 A).
  • the 20Ag/TiO 2 NPs were black whereas the 10Ag/TiO 2 NPs were gray or light-brown (Fig. 1 A). This color difference is due to enhanced light absorbance at visible wavelengths corresponding to the formation of titanium suboxides (Fig. 6).
  • external mixtures of commercial Ti0 2 (P25) and flame-made nanosilver equivalent to that in 20Ag/TiO 2 show no visible light absorption (black broken line spectrum in Fig. 6).
  • the diffraction peak for Ag° is no longer distorted but narrower and stronger upon annealing at 550°C :
  • the Ag° crystal size from 5.0 to 50.8 nm, exhibiting the plasmonic absorption of Ag around 550 nm and a stronger visible spectrum (Fig. 7).
  • the preparation of the present "black” Ti0 2 is based on a different physical mechanism than in wet-made “black” Ti0 2 : SMSIs between Ag° and Ti0 2 result in a crystalline suboxide layer (i.e. T1 4 O 7 and T1 3 O 5 ) that forms a shell around the silver NPs, which are strongly attached to a Ti0 2 support nanoparticle. Distortion of the reduced phases could be responsible for the distribution of the density of states creating the so-called energy tails that extend into the band-gap energies of the semiconductor. This phenomenon is responsible for the absorbance of light at visible wavelengths, rendering the black color of the as prepared Ag/Ti0 2 NPs.
  • the reduced distorted phase of TiO x is attirubbed to a thin layer containing crystalline T1 4 O 7 and T1 3 O 5 as detected by XRD.
  • the Ti 3+ centers belong to lattice oxygen vacancies of the type Ti 3+ -V 0 -Ti 4+ .
  • the Ti 3+ EPR signals are broad indicating strain/distortion of the local environment around the Ti 3+ centers.
  • FIG. 3 shows the photogenerated e and h + signals.
  • Fig. 6 shows the enhanced UV-Vis absorbance (Fig. 6), which can be attributed to the generation of sub-band-gap energy tails from the distorted suboxide shell.

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Abstract

A particle (1) is proposed with photoactivity under irradiation in the visible range of the electromagnetic spectrum comprising a core-particle (2) of titanium dioxide with an average diameter in the range of 3-300 nm; on the surface of said core-particle (2) nanoparticles with an average diameter in the range of 2-90 nm forming at least one metal nano-deposit (3) partially covering said surface; on the surface of said nanoparticles of at least one noble metal nano-deposit a layer containing crystalline titanium-suboxides (4) that preferably contact said core-particle (2) surface.

Description

TITLE
Visible light photoactive nanoparticles and methods for the preparation thereof
TECHNICAL FIELD
The present invention relates to nanoparticles with specific architecture so they have the capability to efficiently photogenerate holes and electrons under UV-irradiation but also under visible light irradiation. Therefore they are photoactive and in particular act as a catalyst not only under UV-irradiation but also under visible light irradiation. Furthermore the invention relates to methods for making such particles in an efficient and if needed easily scalable and reliable process.
PRIOR ART
Recently, a so-called "black Ti02" consisting of a Ti02 core and a distorted TiOx (x < 2) shell (titanium suboxide) with remarkable photoactivity under visible-light irradiation has been proposed for efficient solar energy utilization. The photoactivity preferentially occurs on the surface of these particles. A titanium suboxide shell on a core Ti02 particle can be produced by laser irradiation or by reduction with H2, organic solvents or CaH2. In addition, binary Ti02-Ti203 has been formed under extensive UV irradiation of sol-gel- made Ti02.
The improved visible-light photoresponse of Ti suboxides involves the modulation of the energy levels of the semiconductor: Ti3+ defects enhance visible-light photoactivity by introducing band-gap energy states. On the other hand, the distorted titanium suboxide shell creates an additional "tail" from the valence band into the band gap. Thus, the engineering of a suboxide phase, preferably shell-type or as point Ti3+ reduced states on crystalline semiconducting Ti02, opens new opportunities for visible-light-photoactive Ti02.
Unfortunately, these techniques require extreme processing conditions such as high pressures, H2 (or CO and NO) reduction at high temperatures, laser or UV irradiation for a prolonged period (several hours/days). Therefore, such techniques provide limited control of the phase formation so their applicability to mass production is not at least questionable.
SUMMARY OF THE INVENTION
Visible light photoactive materials are most attractive in solar energy utilization, sensors, photo voltaics and efficient photodegradation of pollutants. For strong visible-light activity by Ti02, a narrowing of its band gap is most promising. This is attained by creation of oxygen defects (e.g. Ti3+) or the formation of titanium suboxide (TiOx) phases. Here this is proposed by continuous, one-step, flame aerosol synthesis of layers of titanium suboxides (T14O7 and T13O5) onto nanosilver supported on nanostructured Ti02 particles that are characterized by electron microscopy, Raman, N2 adsorption, Diffuse reflectance and X- ray diffraction. These as-prepared crystalline suboxides are stable upon annealing, at least, up to 350 C in air for two hours. The proposed core-shell nanoparticles exhibit strong photo catalytic activity toward e.g. Cr6+ and methylene blue (half life of 15 - 55 minutes) under visible light irradiation (λ > 400 nm, 450 W).
Ti02 and specific metals (e.g. Pt, Pd, Ni or Ag) exhibit an abnormal interaction, the so- called strong metal-support interaction (SMSI). Under H2 reduction at high temperatures, the Ti02 surface is reduced and the so-formed Ti3+ can either interact with the metal phase, thereby forming weak covalent bonds, or receive electrons from it. The so-formed Ti3+ can diffuse subsequently to the surface of noble metal or Ti02 and surround them, thereby forming a shell of titanium suboxide. Indeed, the presence of Ti3+ on the Pt-Ti02 surface with SMSI can be observed by X-ray photoelectron spectroscopy. Such a distorted titanium suboxide shell can also be observed also by transmission electron microscopy (TEM) in Pd-Ti02. The chemical structure of the titanium suboxide depends on the reducing conditions. Above 600°C in H2, T14O7 can be formed, whereas further reduction to T13O5 may require a radical reorganization of the structure.
Correspondingly therefore the present invention proposes a new type of titanium dioxide nanoparticles (typical average primary particle - as determined by nitrogen adsorption or microscopy - or crystallite - as determined by X-ray diffraction - diameter in the range of 3-300 nm preferably in the range of 3 - 90 nm or 5-40 nm) on the surface of which there are attached metal nanoparticles (typical average diameter in the range of 2-90 nm, as determined by X-ray diffraction or microscopy) preferably in the range of 2 - 50 nm or 2- 10 nm), namely in only a partial coverage by a metal, preferably a noble metal such as silver. In case of partial coverage this is 0.2 -50% or 0.2 - 30%, preferably in the range of 1-27 % of the surface area of the titanium dioxide nano-particle is covered by individual separated dots of elementary (noble) metal, in the form of metal nanoparticle deposits. These elementary noble metal nanoparticles, are on the outer side at least partially, preferably completely, covered by a layer of crystalline titanium sub-oxide of variable thickness.
Apart from the particle as such and its applications as a visible light catalyst, a continuous gas-phase and proven, scalable method to kg/h, flame spray pyrolysis is proposed, for synthesis of black Ti02 nanoparticles by simultaneous formation of Ti02, titanium suboxide and nanosilver in one-step: Titanium and silver precursors in flammable solvents are sprayed and dispersed by 02 in fine mists that are combusted resulting in layered (or shell) TiOx onto nanosilver and Ti02 that is immobilized or supported onto core, predominantly anatase Ti02 Nano particles. By controlling the ratio of the Ti/Ag precursor solution and dispersion 02 flows to the FSP unit, appropriate flame conditions for particle synthesis can be selected. Silver does normally not form independently SMSIs because of its inability to dissociate molecular hydrogen to generate Ti3+ states; however, Ag can form SMSIs in the presence of Pt, which facilitates hydrogen dissociation. Here, abundant intermediate combustion products that can reduce Ti02 in a manner similar to dissociated hydrogen are present in the flame that can induce SMSIs on Ti-Ag. The structural profiles of the present flame-made core-Ti02/shell-TiOx NPs were investigated by TEM and X-ray diffraction (XRD). Their photoactivity was monitored by electron paramagnetic resonance (EPR) spectroscopy. Finally, the photocatalytic performance of the NPs with respect to Cr6+ and methylene blue (MB) degradation were examined under visible light (λ > 400 nm).
Specifically, the present invention relates to a particle or powder, suspension or the like of such particles with catalytic activity under irradiation in the visible range of the electromagnetic spectrum comprising a core-particle of titanium dioxide with an average diameter in the range of 10-40 nm, on the surface of said core-particle a layer or at least one noble metal nano-deposit partially covering said surface, on the surface of said layer or of said at least one noble metal nano deposit a layer of titanium-suboxide.
It was found that these particles while also showing catalytic activity under UV irradiation are showing a much higher catalytic activity under visible irradiation only than any other previously known particles.
According to a preferred embodiment of such a particle, on the surface of said core-particle at least one metal (preferably noble metal) nano deposit is located, and wherein the coverage of the total surface of said core-particle by said metal nano deposits is in the range of 0.2-50%, preferably in the range of 1-27%. According to yet another preferred embodiment of such a particle or group of particles, the layer of titanium-suboxide is contacting free surface of the core-particle of titanium dioxide.
The titanium-suboxide can be selected from the group Tix02x-i consisting of: crystalline like T13O5, T14O7, T15O9, or a mixture and/or aggregate thereof, wherein preferably it is selected from the group consisting of: crystalline T13O5, T14O7 and where the crystalline mass ratio of Ti305,to T14O7 is preferably chosen to be in the range of 1 :6 to 6: 1.
The weight ratio of the titanium dioxide of the core-particle to the metal (preferably noble metal) is in the range of 29: 1 - 3: 1, preferably around 12:1 -4: 1. The noble metal is preferably selected from the group consisting of: silver, gold, nickel, cobalt, palladium, rhodium, iridium, platinum, copper or a combination thereof, wherein it is preferably selected to be silver.
The present invention furthermore relates to a method for making such a particle or powder of such particles. According to this method preferably in a flame spray pyrolysis process titanium and silver precursors in flammable solvents are sprayed and dispersed by oxygen and combusted in a flame. During the process, titanium dioxide and silver nanoparticles are separately formed and the nanosilver deposited on the titanium dioxide nanoparticles because of the fast nucleation of titanium dioxide. Titanium suboxides are formed in the by strong metal (Ag) support (Ti02) interactions (SMSIs).
The weight ratio of the titanium and the silver precursors in the flammable solvent is preferably adapted to be in the range of 3-90 %and preferably 7-14%..
The present method is a continuous gas-phase and scalable method to kg/h, flame spray pyrolysis (FSP), for synthesis of black Ti02 nanoparticles (NPs) in one-step.
Furthermore the present invention relates to uses of such particles, preferably made according to a method as outlined above as a visible light photo catalyst, or in solar energy utilisation. In the context of such a use the particles can be used for oxidation reactions and/or photodegradation reactions, in particular of pollutants.
Further embodiments of the invention are laid down in the dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,
Fig. 1 shows in (A) commercial Ti02 (P25) and FSP-made Ti02 loaded with Ag
(10 or 20 wt.%) prepared at low (X/Y = 3/5) and high (8/5) temperature and Ti concentration flames; (B) TEM image of 20Ag/TiO2 made at X/Y = 8/5 with selected (C, D) high-resolution images; the arrows and broken lines indicate a distorted titanium oxide layer on nanosilver and the Ti02 support; (E) Schematic of present titanium suboxide-encapsulated nanosilver on Ti02 induced by strong metal support interactions (SMSI);
shows XRD patterns of (A)10Ag/TiO2 or 20Ag/TiO2 prepared at FSP feed X/Y = 3/5 or 8/5 and 20Ag/TiO2 prepared at X/Y = 8/5 with 20 L/min of sheath 02, and (B) 20Ag/TiO2 prepared at X/Y = 8/5 (as in Fig. 2A) and annealed at 350 or 550°C in air for 2 h; Ti02 Diffraction patterns are marked for Ti02 anatase (diamonds) and rutile (circles) as well as for Ag° (squares), Ti407 (up triangles) and Ti305 (down triangle);
Electron paramagnetic resonance (EPR) spectra for the 20Ag/TiO2 NPs prepared at X/Y = 8/5 and recorded at 77 K in the dark (bottom; prior to illumination), after illumination with UV-Vis radiation (middle; λ > 240 nm), or Vis radiation (top; λ > 400 nm);
Photocatalytic reduction of (A) Cr6+ ions and (B) Methylene blue by pure Ti02 (commercial P25, black circles) and 20Ag/TiO2 prepared by flame spray pyro lysis at X/Y = 3/5 (orange triangles) or 8/5 (green squares) under visible-light irradiation (λ > 400 nm, 450 W); the presence of TiOx layers containing nano crystalline Ti407 and Ti305 onto nanosilver supported on nano Ti02 facilitate the attainment if very short photodegradation half-life, 25-55 min for Cr6+ and 15-25 min for methylene blue;
XRD-mass fractions of anatase (diamonds), rutile (circles), Ag° (squares), Ti305 (down triangles), and Ti407 (up triangles) phases for all FSP-made samples; a background signal corresponding to amorphous crystals was corrected in the spectra by software (EVA); the fractions were calculated from the integral intensities of each XRD peak as obtained by software (OriginPro, 8.6); as the peaks for Ti407 at 28.5° and 29.5° overlap, the integral intensity for Ti407 was assumed to be half of the area for the peak at 29.5°; the anatase mass fraction decreased by the formation of rutile and titanium suboxides (T13O5 and Ti407); the Ag° mass fractions for all particles were slightly lower than the nominal Ag atom loading (10 or 20 wt.%) and decreased by the formation of titanium suboxide phases (Ti305 and T14O7); the decrease in Ag mass fraction might be attributed to the interaction of isolated Ag atoms with Ti3+. Such Ag atoms can be formed by electron transfer from Ag to titanium;
Diffuse reflectance (DR) absorption spectra of commercial Ti02; "black" Ti02 NPs loaded with Ag (10 and 20 wt.%) prepared at FSP feed ratios of X/Y = 3/5 and 8/5 in the absence or presence of 02 sheath gas (20 L/min) and mechanically mixed 20 wt.% of 95Ag/Si02 and 80 wt.% of Ti02 (P25, black broken line); the 20Ag/TiO2 particles show significant absorption from 360 to 800 nm, which is the origin of their characteristic black color (Fig. 1A); the mechanically mixed Ti02-Ag° (supported on Si02) particles do not show any visible absorption, the improved absorbance at visible wavelengths (400 - 800 nm) of the present FSP-made Ag/Ti02 over commercial nano-Ti02 (P25) with or without equivalent nanosilver mass is therefore attributed to the presence of the titanium suboxides;
Diffuse reflectance absorption spectra of 20Ag/TiO2 NPs "as prepared" at FSP feed ratio X/Y = 8/5 and annealed at 350 or 550°C in air for 2 h. At 350°C , UV absorption of 20Ag/TiO2 increases but in the visible decreases; this indicates surface oxidation of suboxide layers, which is analogous with the 20Ag/TiO2 NPs prepared at X/Y = 8/5 in the presence of sheath 02 (Fig 6); by annealing at 550°C , the UV absorption further increases, compared to that at 350°C as further conversion of suboxides and anatase to rutile took place (Fig. 2B); furthermore, the plasmonic absorption of Ag around 550 nm was revealed by removal of the Ti-suboxide layers with Ag° sintering and crystal growth (Fig. 2B);
Raman spectra of black Ti02 NPs loaded with Ag (10 or 20 wt.%) and prepared at X/Y = 3/5 or 8/5; for comparison, the Raman spectra of commercial Ti02 (P25, Degussa) and FSP-made Ag (95 wt.%) supported on Si02 NPs (95Ag/Si02) are also included; commercial Ti02 (black solid line) gives the well-known four peaks at 199, 397, 514, and 639 crrf1 that are typical for anatase; notably, no Raman peak for silver (95Ag/Si02, dashed line) was detected; thus, the Raman peaks detected for the Ag/Ti02 NPs correspond to pure Ti02 or titanium suboxide phases; when the Ag loading is increased from 10 to 20 wt.% and/or the flame conditions becomes more reducing (by increasing the precursor/fuel stream for 3 to 8 mL/min), the anatase peaks at 514 and 639 cm 1 are broadened, and the peak intensity at
199 cm 1 is increased; most importantly, then the new peaks at 246 and, to some extent at 361 cm 1 become stronger; analogous features have been observed in the Raman spectra of shell-type, H2-reduced titanium suboxides (1, 2);
Axial flame temperatures during FSP synthesis 20Ag/TiO2 at feed rate X/Y = 8/5 with (triangles) and without (circles) 02 sheath; the temperature was measured by non-intrusive Fourier transform infrared (FITR) emission and transmission spectroscopy; the FTIR spectra ranged from 500 ~ 8000 crrf 1 with 32 cm 1 resolution and were obtained by an FTIR spectrometer (Bomen MB155S); path correction spectra for the emission measurements were taken by a blackbody cavity (PYROcal LAB, TRANS-METRA GmbH); the flame temperature is an integral along the IR beam of ~4 mm in diameter through the flame centerline at different heights above the FSP nozzle tip; the 02 sheath does not affect the maximum flame temperature (-2700 K); downstream from the maximum, the temperature decreases with cooling by entrainment flow; the cooling rate in the presence of sheath 02 is slightly slower downstream from the maximum than that in its absence, increasing the high temperature particle residence time; this might explain the formation of T13O5 (with sheath 02 flow, Fig. 5) that requires higher energy.
DESCRIPTION OF PREFERRED EMBODIMENTS
Methods: Core-Ti02/shell-titanium suboxide ("black" Ti02) NPs loaded with 10 or 20 wt.% of Ag (10Ag/TiO2 or 20Ag/TiO2), the percentages always being calculated with respect to the total weight of the particles, were prepared in a single step by FSP and were collected on glass-fiber filters. Silver acetate (Aldrich, purity > 99%) and titanium (IV) isopropoxide (TTIP, Aldrich, purity > 97%) were used as silver and titanium precursors, respectively. The total precursor (TTIP and silver acetate) concentration in a 1 : 1 mixture of 2-ethylhexanoic acid (Aldrich, purity > 99%) and acetonitrile (Aldrich, purity > 99.5%) was 0.16 M. The precursor solution was fed through the FSP capillary nozzle at X = 3 or 8 mL/min, dispersed to a fine spray by Y = 5 L/min oxygen flow (Pan Gas, purity > 99%) (hereafter referred to as the FSP X/Y feed ratio) and combusted to produce high-purity NPs. Additionally, 20Ag/TiO2 NPs was prepared at X/Y = 8/5 with a sheath of 02 flowing at 20 L/min through an annulus surrounding the 02 dispersion.
High-resolution TEM was performed with a Tecnai F30-ST microscope (FEI, operated at 300 kV, point resolution ~2 A). As-prepared particles were dispersed in ethanol and deposited onto a perforated carbon foil supported on a copper grid. XRD patterns were obtained using a Bruker AXS D8 Advance diffractometer (Cu Ka, 40 kV, 40 mA) at 2Θ = 20°-50° with a step size of 0.03°. The exposure time for each step was set to 2.5 s to obtain high-quality XRD patterns.
EPR spectra were recorded with a Bruker ER200D spectrometer at liquid-N2 temperatures, equipped with an Agilent 531 OA frequency counter. For each experiment, 10 mg of NPs are placed inside a quartz EPR tube (Suprasil, Willmad Glass) with an outer diameter of 3 mm. The EPR signals presented herein were recorded at 9.49 GHz, using a modulation amplitude of 4 G, a field modulation of 100 kHz, and a microwave power of 4 mW. Adequate signal-to-noise ratios were obtained with 10 scans. A solar- light-simulating Xe light source (Oriel model 66929, 450 W) equipped with a water IR filter was used to investigate photogeneration of electrons and holes. Visible-light illumination (λ > 400 nm) was performed using a longpass 400 nm filter (Edmund Optics 85754). The same light source was used for photocatalytic experiments.
Photocatalytic Cr6+ and methylene blue (MB) degradation was preformed in a Pyrex glass container. For Cr6+ degradation, 9 mg of NPs was dispersed in 90 mL of Milli-Q water (Ccataiyst = 100 mg/L) in a water-bath sonicator. The solution pH was adjusted to 3.5 with HNO3. K2Cr207 (Aldrich) was added at an initial concentration of Ccr = 0.5 mM. Salicylic acid (SA) (Aldrich) was added at a concentration of CSA = 5 mM to optimize the photocatalytic kinetics. For MB degradation, 22.5 mg of NPs were dispersed in 90 mL of Milli-Q water (Ccataiyst = 250 mg/L) in a water-bath sonicator. Methylene blue (Aldrich) was added at an initial concentration of CMB = 0.01 mM. The solution pH was adjusted to 9.0 with NaOH. The solutions were mixed with the appropriate amount of photocatalyst and magnetically stirred before and during the illumination. The suspensions were kept in the dark for 30 min prior to illumination to reach adsorption equilibrium on the semiconductor surface. As the reaction progressed, samples were withdrawn from the reactor at specific time intervals. MB suspension was centrifuged for 15 min at 4000 rpm using a Hettich Universal 16A centrifuge for further analysis. The MB decolorization was determined by monitoring the change in optical absorption at 660 nm using a Perkin Elmer Lambda 35 double-beam UV-Vis spectrophotometer. The Cr6+ ions in each solution were determined by the diphenylcarbazide method using the UV-Vis spectrophotometer.
Results: Figure 1 A compares images of P25 (commercial Ti02) and titanium oxide NPs loaded with Ag (10 or 20 wt.% of composite Ag/Ti02 particles) prepared at FSP X/Y feed ratio (the ratio between the feed rates of the Ag/Ti precursor solution, X mL/min, and dispersion 02, Y L/min) = 3/5 (low precursor concentration) or 8/5 (high precursor concentration). A high Ag loading (e.g. 20 wt.%) and a high FSP feed ratio (e.g. 8/5) resulted in the darkest particles (Fig. 1A). The dark particle colors are similar to those of H2-reduced Ti02. This dark color is not due to incomplete combustion products i.e. carbon, remaining on particle surface as confirmed by thermogravimetric analysis, which shows < 2% of organic loading, and EPR which shows zero carbon-based radicals. However, the typical plasmonic absorption band of Ag° (supported on Ti02) around 550 nm was not observed (see Figure 7). The absence of this band is attributed to a strong electron interaction between Ag-Ti induced by SMSI. Therefore, the color changes correspond to the formation of titanium suboxide, which exhibits an absorption band in the visible range. Figure IB shows a TEM image of 20Ag/TiO2 NPs prepared at X/Y = 8/5 with its higher resolution and magnifications at 1C and ID. The lighter and darker parts correspond to titanium oxide and Ag°, respectively. A closer inspection of Figures 1C,D, however, reveals a distorted layer onto the darker Ag NPs (indicated by broken red lines). Similar morphologies have been observed on strongly interacting Co and Pt nanostructures on Ti02 produced by H2 reduction at high temperatures. This morphology indicates SMSIs that result in coating of noble metals by titanium suboxides. Figure IE shows schematically such distorted suboxide layers encapsulating nanosilver in Figures 1C,D (indicated by arrows). This coating hinders nanosilver oxidation and leaching or release of Ag+ ions in aqueous solutions. This suppression of Ag+ ion release is attributed to the Ti- suboxide formation. So the Ag+ release (or leaching) from the present Ag/Ti02 particles is 61-87 % less than that by similar, flame-made nanosilver supported on Si02 and suspended in water (see Table 1). For example, Ag NPs (dp = 4.5 nm for 10Ag/TiO2 made at X/Y = 8/5) release 20.9 % of their mass as Ag+ ions, whereas such Ag NPs on Si02 release 62 % of their mass as Ag+ ions. This 66 % inhibition of Ag+ ion leaching is consistent with that other FSP-made Ti02 NPs loaded with 5 wt.% of Ag that reduced the Ag+ ion release by 72 % compared to that from Ag supported on Si02. The suppression of the Ag+ ion release by the Ti suboxide coating is analogous to the hindering of chemisorption on noble metals during SMSI.
Table 1. Ag+ ion release from FSP-made Ag/Ti02 NPs prepared at different FSP feed ratios
Figure imgf000012_0001
The crystal sizes of Ag were obtained by Rietveld analysis (TOPAS 4.0) of the XRD spectra at 44° (Ag°). The Ag+ ion concentration in de-ionized water containing 100 mg/L of nanosilver was measured with an ion-selective electrode and an ion meter (both Metrohm). The suppression of Ag+ ion release was calculated by the difference between the calculated Ag+ ion release on Si02 and the observed one on Ti02 here divided by the former.
Figure 2 A shows the XRD patterns of the 10 and 20Ag/TiO2 NPs prepared at X/Y = 3/5 or 8/5, respectively. A higher FSP feed ratio (e.g. X/Y = 8/5) results in stronger and sharper peaks indicating larger crystals. Increasing the X/Y ratio increases particle mass concentration and residence time at high temperatures that lead to larger particles by enhanced coagulation and sintering. The peaks at 25.5° and 48° correspond to anatase Ti02 (diamonds) that are dominant in all particles made here. Only particles prepared at X/Y = 8/5 contain a small peak around 27.5° corresponding to rutile (circles). The Ag° peaks (squares) resolved at 44° show a distorted shape compared with flame-made Ag° NPs supported on Si02 indicating the SMSI between Ag and Ti02 here.
Most interestingly, titanium suboxide peaks were resolved in the XRD patterns: T13O5 (down triangle) at 33° and Ti407 (up triangles) at 28.5° and 29.5° in Fig. 2A. Analogous features have been reported for T13O5 and Ti407 phases formed by SMSI. Here, the suboxide peaks are broad, indicating small crystals and/or low crystallinity consistent with their layered formation onto nanosilver (Fig. 1C, D). The peak intensities increased with Ag loading and FSP feed ratio (reducing flame conditions) corresponding to the darkening of these particles (Fig. 1 A).
The use of additional 02 as a sheath gas in the FSP unit, which makes the flame more oxidative, affected the suboxide phase formation, as shown in Fig. 2A (purple spectrum). The mass fraction of Ti407 decreased whereas that of T13O5 slightly increased compared to that in the absence of sheath 02 as shown quantitatively in Fig. 5. Highly reducing conditions result in SMSIs of Pd and Pt with Ti02 that lead to formation of Ti407. For further reduction to T13O5, a radical reorganization of the structure may be required. The use of 02 sheath during FSP increases the oxidation rate of the precursor solution and subsequently the high temperature residence time of the particles (Fig. 9). Therefore, T13O5 conversion from Ti407 (or directly from Ti02), which requires a large amount of energy, seems to be facilitated by sheath 02 during FSP synthesis of these core-shell nanostructures.
The 20Ag/TiO2 NPs were black whereas the 10Ag/TiO2 NPs were gray or light-brown (Fig. 1 A). This color difference is due to enhanced light absorbance at visible wavelengths corresponding to the formation of titanium suboxides (Fig. 6). Interestingly, "black" Ti02 prepared at high Ag loading (20Ag/TiO2) and precursor concentration (X/Y = 8/5, green spectrum) exhibits greater absorption intensity in the visible range but lower in the UV than black Ti02 prepared at more oxidative conditions, either at lower fuel concentration (X/Y = 3/5, orange spectrum) or with sheath 02 (X/Y = 8/5, purple spectrum). Therefore, formation of this titanium suboxide is accelerated under reducing flame conditions. Worth noting is that external mixtures of commercial Ti02 (P25) and flame-made nanosilver equivalent to that in 20Ag/TiO2 show no visible light absorption (black broken line spectrum in Fig. 6).
Figure 2B shows the XRD patterns of 20Ag/TiO2 NPs prepared at X/Y = 8/5 (green spectrum) and annealed at 350°C and 550°C in air for 2 h. These patterns indicate that the titanium suboxide phases are stable at 350°C but are transformed to rutile at 550°C with remnant T13O5 and T14O7. The UV absorption of 20Ag/TiO2 NPs prepared at X/Y = 8/5 increased by annealing temperature, indicating oxidation of the suboxides (Fig. 7). Moreover, during this annealing, Ag° particles were exposed by the decomposition of the suboxide phases, resulting in Ag° sintering and crystal growth. The diffraction peak for Ag° is no longer distorted but narrower and stronger upon annealing at 550°C : The Ag° crystal size from 5.0 to 50.8 nm, exhibiting the plasmonic absorption of Ag around 550 nm and a stronger visible spectrum (Fig. 7).
Raman spectra of all FSP-made Ag/Ti02 show new peaks at 246 and 361 cm-1 and a shorter increase of the peak at 199 cm"1 compared to that of commercial, flame-made P25. (Fig. 8). Analogous features have been observed in the Raman spectra of "black" Ti02. However, some peaks (294, 690, 765, 849 and 938 cm J) detected for a non-crystalline Ti- suboxide shell are not detected here. Here, a new form of "black" Ti02 was prepared by the FSP process.
The preparation of the present "black" Ti02 is based on a different physical mechanism than in wet-made "black" Ti02: SMSIs between Ag° and Ti02 result in a crystalline suboxide layer (i.e. T14O7 and T13O5) that forms a shell around the silver NPs, which are strongly attached to a Ti02 support nanoparticle. Distortion of the reduced phases could be responsible for the distribution of the density of states creating the so-called energy tails that extend into the band-gap energies of the semiconductor. This phenomenon is responsible for the absorbance of light at visible wavelengths, rendering the black color of the as prepared Ag/Ti02 NPs. Here the reduced distorted phase of TiOx is attirbuted to a thin layer containing crystalline T14O7 and T13O5 as detected by XRD.
Figure 3 presents the low-temperature EPR spectra (77 K) of 20Ag/TiO2 NPs prepared at X/Y = 8/5. No signal for carbon radicals was observed indicating that there is minimal, if any, incomplete combustion products on the NPs. In the dark, the NPs contain two types of paramagnetic centers: (I) a sharp axial signal with g = 1.998 assigned to lattice electrons in the anatase phase (27) and (II) a broad signal extending from 3400 to 4000 G corresponding to the surface of Ti3+ electron trapping sites. Titanium suboxides are known to be rich in Ti3+ electron trapping sites which are easily detected by EPR spectroscopy (29). In pure T14O7 or T13O5 crystals, the Ti3+ centers belong to lattice oxygen vacancies of the type Ti3+-V0-Ti4+. Here, the Ti3+ EPR signals are broad indicating strain/distortion of the local environment around the Ti3+ centers.
Irradiation under visible-light wavelengths (Fig. 3: λ > 400 nm) rapidly photoinduces (< 30 s) three signals: A strong narrow axial signal appears at g = 1.980 is attributed to lattice electron (Ti3+) trapping sites located the interior of the crystal phase. The broader troughlike signal at g = 1.913 is a characteristic of superficial electron-trapping Ti3+ sites. This signal is significantly narrower than that typically produced by any surface-Ti3+ sites in P25. Its narrower linewidth indicates restricted mobility of the trapped Ti3+ electrons; therefore, it is assigned to Ti3+ states localized at the interface of Ti02. In parallel with the Ti3+ signals at g < 2, photoinduced h+ centers are detected under illumination at g = 2.0016-2.0024.
Most importantly, the photogenerated e and h+ signals can be induced at high yield (> 80%) under visible-light irradiation at λ > 400 nm (Fig. 3). This high visible-light photoactivity is consistent with the enhanced UV-Vis absorbance (Fig. 6), which can be attributed to the generation of sub-band-gap energy tails from the distorted suboxide shell. Figure 4A shows the visible-light photocatalytic activity of commercial Ti02 P25 (black circles) and present composite 20Ag/TiO2 particles made at X/Y = 3/5 (orange triangles) and 8/5 (green squares) for reduction of Cr6+, a priority toxic pollutant. The half lives of Cr+6 were 30 and 55 min by 20Ag/TiO2 NPs made at X/Y = 3/5 and 8/5, respectively. Furthermore, both photocatalysts are highly active also for Methylene blue degradation under visible light irradiation with even shorter half lives of 15 and 25 min the same NPs respectively (Fig. 4B). The faster kinetics of the NPs made at X/Y = 3/5 are consistent with their higher specific surface area (SSA) of 208 m2/g (Table 1), which is almost twice that (113 m2/g) of the NPs made at X/Y = 8/5. In comparison, the P25 (reference Ti02) exhibited the well-known minor catalytic efficiency under visible light. Even though nanosilver on Ti02 might slightly enhance the visible light photodegradation of phenol and Alizarin Red S, the present core-shell Ag/Ti02-TiOx nanostructures exhibit a much stronger photoactivity due to the suboxide presence. LIST OF REFERENCE SIGNS nano-particle 8 10 Ag/ Ti02 (8/5) core-particle of titanium 9 20 Ag/ Ti02 (3/5) dioxide 10 20 Ag/ Ti02 (8/5) metal-nano-deposits on the 11 20 Ag/ Ti02 (8/5) with 02 surface of 2 sheath
crystalline titanium-suboxide 12 TiO2(80%) + Ag (19%) + layer 4 Si02 (1%)
free surface of 2 13 as prepared
P25 14 annealed at 350°C
10 Ag/ Ti02 (3/5) 15 annealed at 550°C

Claims

1. Particle (1) with photoactivity under irradiation in the visible range of the electromagnetic spectrum comprising a nanoparticle architecture of
a support or core-particle (2) of titanium dioxide with an average primary particle (as determined by nitrogen adsorption or microscopy) or crystallite (as determined by X-ray diffraction) diameter in the range of 3-300 nm;
on the surface of said core-particle (2) and attached thereto nanoparticles with an average diameter in the range of 2-90 nm (as determined by X-ray diffraction or microscopy) of at least one metal forming at least one nano-deposit (3) only partially covering said surface;
on the surface of said nano-deposit (3) a layer of titanium-suboxides (4) containing at least one crystalline titanium-suboxide component.
2. Particle (1) according to claim 1, wherein on the surface of said core-particle (2) at least one metal nano-deposit (3) is located, and wherein preferably the coverage of the total surface of said core-particle (2) by said metal nano deposits (3) is in the range of 0.2-50%.
3. Particle (1) according to claim 2, wherein preferably the coverage of the total surface of said core-particle (2) by said metal nano deposits (3) is in the range of 1-27%.
4. Particle (2) according to claim 2 or 3, wherein the layer of titanium-suboxides (4) containing at least one crystalline titanium suboxide component is at least partly contacting the free surface (5) of the core-particle (2) of titanium dioxide.
5. Particle (2) according to any of the preceding claims, wherein the titanium- suboxide (4) is selected from the group consisting of: crystalline T13O5, crystalline T14O7, crystalline T15O9, or a mixture and/or aggregate thereof, wherein preferably it is selected from the group consisting of: crystalline T13O5, crystalline T14O7.
6. Particle (2) according to claim 5, wherein the crystalline mass ratio of Ti305,to T14O7 is chosen to be in the range of 1 :6 to 6: 1.
7. Particle (2) according to any of the preceding claims, wherein the weight ratio of the titanium dioxide of the core-particle (2) to the metal of the nano-deposits is in the range of 29: 1 - 3: 1.
8. Particle (2) according to any of the preceding claims, wherein the weight ratio of the titanium dioxide of the core-particle (2) to the metal of the nano-deposits is in the range of 12: 1 -4: 1.
9. Particle (2) according to any of the preceding claims, wherein the metal is selected from the group consisting of: silver, gold, nickel, cobalt, palladium, rhodium, iridium, platinum, copper or an alloy of a combination thereof.
10. Particle (2) according to any of the preceding claims, wherein the metal is selected to be silver or an alloy thereof.
11. Method for making a particle according to any of the preceding claims, wherein in a flame spray pyrolysis process titanium and metal precursors, preferably silver precursors, in flammable solvents are sprayed and dispersed by oxygen and combusted in a flame.
12. Method according to claim 11, wherein the weight ratio of the titanium and the metal precursors as present in the flammable solvent is adapted to be in the range of 29: 1 - 3: 1, preferably around 12: 1 -4: 1.
13. Method according to any of the preceding claims, where additional oxygen as sheath is provided surrounding the flame spray pyrolysis process and facilitating partial oxidation of the product particle (2), (3) and (4).
14. Use of a particle according to any of the preceding claims 110, preferably made according to a method according to one of the claims 11-13, as a visible light photoactive material to be used as photo catalyst, or sensor or in solar energy utilisation or on photovoltaics.
Use according to claim 14, wherein it is used for oxidation reactions and/or photodegradation reactions, in particular of pollutants.
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