EP2900376A1 - Photocatalytic composites - Google Patents

Abstract

The invention provides a method for preparing a photocatalytically active composite, the method comprising the steps of (i) providing a mixture of a semiconductor particle, a metal salt and a chemical reducing agent; (ii) permitting the reducing agent to reduce the metal salt in the presence of the semiconductor material; and (iii) heating the mixture, thereby to generate a composite having a semiconductor particle decorated with one or more metal particles. Also provided is the photocatalytically active composite obtainable or obtained by the method, and the use of the composite as a photocatalyst.

Description

PHOTOCATALYTIC COMPOSITES

Related Application The present application claims the benefit and priority of GB 1217403.3 filed on 28

September 2012 (28/09/2012) the contents of which are hereby incorporated by reference in their entirety.

Field of the Invention

The present invention provides semiconductor composites having photocatalytic activity and methods for preparing the composites. The semiconductor composites are suitable for use in waste water treatment processes. Background

Nanostructured titanium dioxide, ns-Ti0₂, is a commonly used photocatalyst. However, in its pristine form, Ti0₂ can only absorb photons in the UV region, which constitute a minor component of the solar spectrum (around 5%), thus limiting the efficiency of any

photo-activated processes.

Titanium dioxide is a wide band-gap semiconductor (3.2 eV, corresponding to a wavelength of 385 nm). It absorbs light in the UV region of the spectrum and has a white colour. Its optical properties, chemical stability, availability and cost have seen it used extensively used in industry for applications ranging from paint and food pigments through to catalysts.

Advanced applications of Ti0₂ include self-cleaning and anti-fogging coatings [Fujishima 2008].

Upon illumination, UV photons (with energy greater than 3.2 eV) promote an electron in the valence band, generating an electron-hole pair. The electronic structure of the Ti0₂ surface becomes chemically active, resulting in the production of several different species, including highly oxidising radicals that can degrade organic compounds.

Current research looks to develop Ti0₂ or Ti0₂-based systems active under visible light, in order to use the available solar spectrum more efficiently. An example of this is given by the research into dye sensitised solar cells, in which Ti0₂ is bound to an optically active dye which generates an electron-hole pair with visible light photons [Hardin 2012]. Other examples include more direct modification of Ti0₂ particles by doping or deposition of metal particles. The field has been reviewed, among others, in Fan et al. [Han 2009].

Wen et al. describe the preparation and characterization of Ag-loaded Ti0₂ powders having photocatalytic properties. Here, the authors prepared Ti0₂ powder which was then dispersed in water by ultrasound. A solution of 0.05 M AgN0₃ was then added, in varying amounts depending upon the intended ratio of Ag to Ti in the product, and the resulting mixture irradiated with for 2 hours with a 15 W UV lamp having a central wavelength of 365 nm. A powder was obtained after filtration and drying of the collected solids at 90°C. The composite was analysed by XRD, Raman spectroscopy, TEM, EDS, XPS, and UV-vis spectroscopy. The composites are shown to catalyse the reaction of MO under UV irradiation.

Wen et al. use anatase nanorods and platelet-like particles, as seen in the TEM images recorded by the authors. Although the authors claim a grain size of 20 nm based on the recorded XRD data, the TEM images show highly anisotropic shapes (i.e. high aspect ratios). There is apparently no evidence that the Ti0₂ is well crystallised and the XRD data points to a low crystallinity, in view of the low S/N values. Whilst Wen et al. show that the Ag-loaded Ti0₂ powders absorb minor amounts of light in the visible region, this absorbance is not apparently capable of supporting a catalytic activity, and UV light is necessary to provide catalysis. The absorption in the range between 400 and 600 nm is wide and relatively unpronounced, for example the maximum absorption ratio is ca. 0.57 or less where the absorption at 465 nm is compared to the absorption at 300 nm.

Amin et al. describe the synthesis of a Ag-Ti0₂ composite and the use of the composite as an antibacterial agent when irradiated with UV light. The composite was prepared by treating TTIP in propanol with nitric acid at a constant pH of 1 .5 to give a Ti0₂ sol. The sol is treated with a 0.1 M AgN0₃ solution for around 30 minutes to absorb Ag^* ions onto the Ti0₂ surface. A 0.02 M ascorbic acid solution was added drop-wise to reduce the mixture to reduce the Ag ions. The product was then heated at 1 10°C to remove water. The powder collected after this procedure was then calcined at 300 or 500°C. The composite was analysed by XRD and TEM. Also described by Ingram et al. are composites of N-doped Ti0₂ together with Ag spheres or nanocubes. However, these composites are simply physical mixtures of the Ag sphere and cubes amongst the N-doped Ti0₂. The metal nanostructures are coated with a

non-conducting organic stabiliser in order to provide a separation between the metal and the semiconductor particles. The stabilizer also limits or prevents the transfer of Forster energy between the semiconductor and the metal. The mixture of components described by Ingram et al. is incorporated into a photoelectrode.

Summary of the Invention

The present invention provides a composite material comprising particles of a semiconductor material decorated with metal nanoparticles. The composites of the present invention may be obtained in a simple preparation. The composites of the invention are photocatalytically active when irradiated with visible and UV light. Indeed, the composites absorb significant amounts of light across the entire visible and UV spectrum. The composites are highly stable and may be used repeatedly without loss in photocatalytic activity. The composites of the invention are suitable for use in the photocatalytic degradation of pollutants, such as dyes, and may be usefully used to clean waste water.

The present inventors have established that composites having excellent photocatalytic activity may be obtained in a method where a semiconductor particle is decorated with metal particles simultaneously with the formation of the metal by the reduction of a precursor metal ion species. The method includes the step of heating the composite, which is shown to increase the loading of the semiconductor particle with the metal. The composites resulting from this method are photocatalytically active when incident with visible and UV light.

Without wishing to be bound by theory, the inventors believe that the method of the invention permits the development of stable metal/semiconductor interfaces, which dramatically improve the efficiency of the photocatalyst, and advantageously also increases the stability, and therefore recyclability of the photocatalyst.

In one aspect there is provided a photocatalytically active composite obtained or obtainable from a method comprising the steps of:

(i) providing a mixture of a semiconductor particle, a metal salt and a chemical reducing agent;

(ii) permitting the reducing agent to reduce the metal salt in the presence of the semiconductor material, and

(iii) heating the mixture,

thereby to generate a composite having a semiconductor particle decorated with one or more metal particles. The composite that is obtained or obtainable by the method comprise a semiconductor particle that is decorated with one or more, preferably a plurality, of metal particles, such as metal nanoparticles. The composite is catalytically active upon irradiation with visible light and is additionally catalytically active upon irradiation with UV light. The semiconductor particle may be a Ti0₂ particle, such as a Ti0₂ nanoparticle.

The metal particle may be a nanoparticle.

The metal may be Ag.

In step (iii) the mixture may be heated to a temperature in the range 50-150°C.

In step (iii) the mixture may be heated in a first step, and the resulting product ground. Here, the ground product may be heated, for example to the same temperature as the first heating step. In the composite, the ratio of the absorbance at 465 nm may be at a ratio of 0.60 or more in relation to the absorbance at 300 nm.

The present inventors have established that the composites of the invention may be used on a large scale to promote the degradation of common pollutants found in fluids such as waste water. The composite is photocatalytically active at ambient temperatures when irradiated with natural light. Complete degradation of pollutant compounds, such as dyes, is observed in as little as 3 minutes. In a second aspect, the present invention provides a method for preparing a

photocatalytically active composite according to the first aspect of the invention, the method comprising the steps of:

(i) providing a mixture of a semiconductor particle, a metal salt and a chemical reducing agent;

(ii) permitting the reducing agent to reduce the metal salt in the presence of the semiconductor material; and

(iii) heating the mixture,

thereby to generate a composite having a semiconductor particle decorated with one or more metal particles.

The method may result in the surface modification of the semiconductor particle.

In a further aspect, the present invention provides a method for treating a pollutant, the method comprising the step of exposing the pollutant to a composite as described herein in the presence of visible and/or UV light, thereby to degrade the pollutant.

In another aspect of the invention, there is provided a method of sterilizing a sample, the method comprising the step of exposing a sample comprising a microorganism to a composite as described herein in the presence of visible and /or UV light, thereby to degrade the microorganism.

In a further aspect there is provided a composite of the invention as a film. The film may consist or comprise the composite. The film may be provided on a surface. The film may be used in methods for treating a pollutant and sterilizing a sample, as described in the aspects above.

Summary of the Figures

Figure 1 is an XRD spectrum of an Ag-Ti0₂ based composite after a 2 hour reaction time (at room temperature). Figure 2 is an XRD of an Ag-Ti0₂ based composite after a 12 hour reaction time (at room temperature).

Figure 3 is an XRD of an Ag-Ti0₂ based composite after 95 min reaction time. The composite was heated in the presence of HCHO. The reaction was performed at room temperature for 95 min minutes and then was heated up to 100°C until all the solvents were evaporated (20 minutes were required for complete evaporation).

Figure 4 is a pair of UV-vis absorbance spectra for (a) unheated catalyst synthesized for 2 hours (2HWH) and 12 hours (2HWH); and (b) catalysts after heat treatment for 2 hours, up to 100°C until all the solvents were evaporated (2HAH). Both plots also show the pure anatase Ti0₂ nanoparticle signal as reference. Note that the onset of absorption for Ti0₂ is estimated to correspond to a direct band gap of 3.32 eV (375 nm). Figure 5 is a pair of graphs showing the BET isotherms for (a) a 12.5 mole % Ag-Ti0₂ composite prepared according to the present invention, and (b) pure Ti0 .

Figure 6 shows STEM images of samples at different deposition times, without heating (top row) and with heating (bottom row, highlighted in red). For each sample, two different magnifications are shown (top scale bar 50 nm, bottom scale bar 20 nm). The silver loading increases with time and, more significantly, with heating. Small silver particles (5 nm or less) are present in the heated samples, but not in the non-treated ones.

Figure 7 shows EELS spectra acquired on an area containing Ti0₂ composite aggregates of the invention. Zero-loss peak width 1 .2 eV, acquisition time 1 s (top spectrum) and 3 s (bottom spectrum). The adsorption edges of C, Ag, Ti and O are indicated by arrows.

Figure 8 is a HRTEM image of a silver particle (indicated) in the hexagonal phase, here imaged from the (1 1 1 ) zone axis. The inset reports the FFT of the particle lattice. A silver particle in the common FCC phase, here imaged from the (100) zone axis. Both images were taken on a sample after the silver deposition, before the final thermal treatment.

Figure 9 is a HRTEM of a silver particle growing from a Ti0₂ support. The (1 1 1 ) lattice planes of the Ag particle are at the correct angle to match the (101 ) planes of the titania. This matching indicates a good interface between the two phases of the composite.

Figures 10(a) to 10(c) are UV-vis spectra of (a) Methylene Blue degradation, before (blue) and after (red) photocatalysis under standard office lighting - 300-800 nm, with an estimated intensity of 60 pW/cm²; (b) Acid Red degradation, before (blue) and after (red)

photocatalysis under standard office lighting - 300-800 nm, with an estimated intensity 60 W/cm²; and (c) RhB degradation , before (blue) and after (red) photocatalysis under standard office lighting - 300 800 nm, with an estimated intensity of 60 W/cm². A catalyst having 6.93 mole % Ag was used.

Figure 1 1 shows the change in the relative concentration (In C₀/C) of methylene blue over time (minutes) in the presence of a Ti0₂ catalyst (♦), a 3 mole % Ag-Ti0₂ composite (■) and a 12.5 mole % Ag-Ti0₂ composite of the invention (A ).

Figure 12 are UV-vis spectra of methylene blue samples degraded in the presence of a Ti0₂ catalyst under UV-vis irradiation with an estimated intensity of 500 W/m². Spectra were recorded at 0, 5, 10, 15, 20 and 25 minutes irradiation.

Figure 13 are UV-vis spectra of methylene blue samples degradation in the presence of a 12.5 mole % Ag-Ti0₂ composite of the invention, under UV-vis irradiation with an estimated intensity of 500 W/m². Spectra were recorded at 0, 1 , 2, 3, 4 and 5 minutes irradiation.

Figure 14 are UV-vis spectra of methylene blue degradation in the presence of (A) a

1 mole %, (B) a 2 mole % and (C) a 12.5 mole % Ag-Ti0 composite of the invention, under UV-vis irradiation with an estimated intensity of 500 W/m². The spectra show the

absorbance of methylene blue (MB) prior to treatment with the photocatalyst. Each photocatalyst was recovered after use, and subsequently reused in a second degradation test (all catalysts) and a third degradation test (1 mole % and 2 mole % catalysts). The axes are relative absorbance and wavelength (in nm).

Figure 15 shows the results of a microorganism degradation study using a 12.5 mole % Ag-Ti0₂ composite of the invention (UV + catalyst), UV irradiation alone (UV) and a Ti0₂ anatase catalyst (Anatase). The top graph shows that the reduction in cell number increases significantly for the composition of the invention from 30 to 45 minutes after the start of irradiation. The bottom graph shows the change in microorganism number

(logio(CFU/ml_)) over time (min.) for each of the three conditions. The microorganism was E. cols^'.

Figure 16 are UV-vis spectra of methylene blue degradation in the presence of a film containing Ti0₂ (top graph), an Ag-containing film according to one embodiment of the invention (second from top graph), an Ag-containing film according to a further embodiment of the invention prepared by a Sol-Gel process (third from top), and a comparative example where methylene blue was degraded with UV-vis (i.e. without catalyst) (bottom graph).

Spectra were recorded at 0 and 120 minutes.

Detailed Description of the Invention The present invention provides a composite material for use as a photocatalyst. The composite material comprises particles, such as nanoparticles, of a semiconductor material decorated with metal particles, such as nanoparticles. The composite material includes aggregates of the composite.

The composites of the present invention may be obtained or are obtainable by a one-step synthesis that attaches the metal particles to the semiconductor material. The method is also believed to modify the surface of the semiconductor material.

Composites

The composite of the invention comprises a particle of a semiconductor material. The particle is decorated with metal nanoparticles. In one embodiment, the composite is a nanoparticle of a semiconductor material decorated with metal nanoparticles.

The term decorated refers to the surface modification of the semiconductor particle. The particle is connected to a plurality of metal particles, thereby providing at least a partial coating of the particle. The coating is generally a discontinuous coating. In one

embodiment, the semiconductor is not completely coated with particles. Thus, surfaces of the semiconductor material may be exposed.

The semiconductor material is a material may be selected from any material having a band gap. The semiconductor material may have a bang gap that is less than 4 eV, such as less than 3.5 eV, and is typically around 3 eV. The semiconductor material may comprise one or more of Ti0₂, Sn0₂, W0₃, SrTi0₃, ZnO, Nb₂0₅, Ta₂0₅, KTa0₃ and Fe₂0₃. Semiconductor materials comprising two or more metals may also be used. Examples of such materials include bismuth titanates, lithium niobates, potassium tantalates, amongst others.

Preferably, the semiconductor material is or comprises Ti0₂.

In one embodiment, the semiconducting material consists essentially of Ti0₂.

In one embodiment, the semiconducting material is doped Ti0₂.

Where Ti0₂ is present, it may be anatase. The average largest dimension of the particles of the semiconducting material may be in the range 1 nm to 50 nm, such as 10 to 30 nm, such as 20 to 30 nm. Thus, the semiconducting particles may be conveniently referred to as semiconducting nanoparticles.

In one embodiment, the average largest dimension of the particles of the semiconducting material is 1.000 nm, 500 nm, 100 nm or less, 40 nm or less, 30 nm or less, or 25 nm or less.

The semiconductor material may be substantially monodisperse.

In one embodiment, the semiconducting material is substantially crystalline. The nature of the semiconductor particle in the product is determined by the choice of semiconductor material particle used in the preparation reaction, as described herein. The inventors have observed that the preparation conditions do not substantially alter the size and shape of the semiconductor particle.

The aspect ratio of the semiconductor material, the ratio of the average largest dimension to a dimension perpendicular to the dimension axis, may be around 1 :1 , or 1 :0.9, or 1 :0.8, or 1 :0.7.

In one embodiment, the aspect ratio is 1 :x where x is at most 0.95, at most 0.9, at most 0.8 or at most 0.7.

In one embodiment, the aspect ratio is 1 :x where x is at least 0.1 , at least 0.2; at least 0.4, at least 0.5, or at least 0.6. In one embodiment, the semiconductor material is substantially spherical. In this

embodiment, the aspect ratio may be high, such as around 1 :1 , or 1 :0.9.

In one embodiment, the semiconductor material is a rod. In this embodiment, the aspect ratio may be relatively low, such as 1 :0.1 . The semiconductor material is decorated with one or more metal particles, such as nanoparticles.

Each metal particle may be an Au, Ag, Ir, Pt or Pd particle, or a mixture thereof. In one embodiment, each metal particle is an Ag particle

The average largest dimension of the metal particles may be in the range 0.1 nm to 50 nm, for example 5 to 50 nm, 20 to 30 nm, 0.1 to 20 nm, or 1 to 10 nm. Thus, the metal particles may be conveniently referred to as metal nanoparticles.

In one embodiment, the metal nanoparticles present include nanoparticles having a dimension of 5 nm or less, such as 2 nm or less, such as 1 nm or less.

In one embodiment, metal nanoparticles having a largest dimension of 20 nm or more, 30 nm or more, or 50 nm or more, are present at no more than 10%, at no more than 5%, no more than 2 %, no more than 1 %, or no more than 0.5 % of the population of the metal particles. The number may be determined from a sample selection viewed by TEM.

In one embodiment, the dimensions of the metal particles are less than the dimensions of the semiconductor material. In one embodiment, the average dimension of the

semiconductor material is at least 2 times, at least 5 times, at least 10 times, at least 20 times, or at least 50 times that of the average dimension of the metal particles.

In one embodiment, the average dimension of the semiconductor material is in the range 5 to 10 times, 2 to 10 times, or 5 to 20 times that of the average dimension of the metal particles.

In one embodiment, the dimensions of the metal particles may be substantially the same as the dimensions of the semiconductor material. The size of the metal particles is determined by the reaction conditions for the preparation of the composite, as described herein.

The semiconductor particle may be decorated with at least one metal particles. In one embodiment the semiconductor particle is decorated with 2 or more, 5 or more, 10 or more, 50 or more, or 100 or more metal particles. The number of metal nanoparticles may be established from suitable visualisation of a sample of composite material. The number may be an average. Where the composite is an aggregation of semiconductor particles decorated with metal particles, a metal particle may be shared between two or more semiconductor particles, thereby forming a bridge.

The surface modification of the semiconductor material, using metal particles, is for the purpose of modifying the absorption properties of the material so that the energy gap can be decreased to the visible light regions, or may extended from the UV light region to also include the visible light range. The advantage in using such a composite is the ability to make use of natural light to bring about the photocatalytic activity. Where the energy gap is extended to the UV and visible ranges, the composite has the ability to make use of the full natural UV-vis spectrum to bring about photocatalytic activity.

The percentage mass of the metal with respect to the total weight of the composite may be 1.5 wt % or more, 2.0 wt % or more, 5.0 wt % or more, 10 wt % or more, or 15 wt % or more. The percentage mass of the metal with respect to the total weight of the composite may be 40 wt % or less, 30 wt % or less, 25 wt % or less, or 20 wt % or less.

In one embodiment, the percentage mass is in a range selected from the upper and lower values given above. For example, the percentage mass is in the range 10 to 25 wt %.

The mole % of the metal with respect to the mole amount of all components in the composite may be 1.5 mole % or more, 2.0 mole % or more, 5.0 mole % or more, 10 mole % or more, or 15 mole %.

The mole % of the metal with respect to the mole amount of all components in the composite may be 40 mole % or less, 30 mole % or less, 25 mole % or less, or 20 mole % or less. In one embodiment, the mole % is in a range selected from the upper and lower values given above. For example, the mole % is in the range 10 to 25 mole %.

The amount of metal present in the composite may be determined by, for example, removing the metal form the composite material using acid, and determining the amount of material removed, using techniques such as ICP (inductively coupled plasma).

The amount of the metal particle present on the semiconductor material is determined by the reaction conditions for the preparation of the composite, as described herein. In one embodiment, the composite absorbs light in the visible and the UV spectrum. Visible light, as used herein may refer to light having a wavelength in the range 380 nm to 740 nm. Composites capable of acting as photocatalysts in response to irradiation with visible light are particularly attractive as they can be used directly in both natural and artificial light conditions.

In one embodiment, the composite absorbs light substantially across the entire visible range. The absorption characteristics of the composite may be determined by standard UV-vis spectroscopy techniques, such as those known to the skilled person and as described herein.

The composites of the invention may be contrasted with those described by Wen et al. The present composites have absorbances values throughout the visible region. In contrast, the composites of Wen et al. do absorb significant amounts of light in the region 600 to 700 nm, for example. The work of Amin et al. shows that the photocatalysts are inactive unless they are exposed to UV light. From the published work it can be seen that UV absorbances predominate. In the composites of the present invention the absorbance values for longer wavelengths are comparable, if not greater, than the absorbance values at shorter wavelengths.

The composite of the invention absorbs light in the UV and visible regions. Advantageously, the absorbance at longer wavelengths in the visible range, for example above 500 nm, is the same, if not higher, than the absorbance at lower wavelengths in the visible range and the absorbance at UV wavelengths. In contrast, Ti0₂ composites that lack an appropriate metal decoration do not absorb significant amounts of visible light, for example above 500 nm.

The ratio of absorbances at lower and higher visible wavelengths may be compared. The lower visible wavelength may be selected from 400, 450 or 500 nm. The higher visible wavelength may be selected from 600, 650 or 700 nm. In a composite of the invention the ratio of absorbance values of the lower wavelength to the higher wavelength may be 1 to 0.7 or more; 1 to 0.8 or more; 1 to 0.9 or more; 1 to 1.0 or more; or 1 to 1 .1 or more.

Thus, where the ratio is 1 to 1.0 or more, the absorbance at the higher wavelength may be the same or greater than the absorbance at the lower wavelength.

In one embodiment, the absorption ratio may be a comparison of the absorbance at 465 nm with the absorbance at 300 nm. In one embodiment, the ratio is greater than 0.60, greater than 0.70, greater than 0.80, greater than 0.90 or greater than 0.95.

In contrast the absorption ratio reported by Wen et al. is generally 0.57 or less.

In one embodiment, the absorbances in the range at 500 to 700 nm remain substantially constant. In one embodiment, the absorbances in the range at 550 to 600 nm remain substantially constant.

In contrast, many of the high loaded samples in Wen et al. have significantly different absorbances over the ranges described above. Only at very low metal loadings are the absorbance values substantially constant over the ranges described above. How such absorbances are very low, and the very low metal loading is not particularly useful.

In one embodiment, the composite is photocatalytically active. The composite is preferably photocatalytically active when irradiated with visible light. Such catalysts may also be active when irradiated with UV light. Thus, the catalyst may use the full range of natural light available.

The photocatalytic activity of a composite may be gauged in the photocatalytic degradation of the dye methylene blue, as described herein. A photocatalytically active composite is capable of catalysing the degradation of the dye when irradiated with UV and/or visible light. Typically, the reaction is monitored for a time in the range of 1 to 20 minutes, such as 3, 5, 10 or 20 minutes. The incident light may be of an intensity of about 60 W/cm². The amount of dye present in a test sample may be determined using, for example, UV-vis spectroscopy. The amount of dye present after the specified treatment time may be 50% or less of the starting amount, 40% or less of the starting amount, 30% or less of the starting amount, 20% or less of the starting amount, or 10% or less of the starting amount. The irradiation may include only light in the visible region, such as in the range 400 to 700 nm. Thus, the irradiation may be to test the activity of the photocatalyst under visible light conditions, without UV light present. The catalysts of the invention are active under visible light.

Additionally, the catalysts are active under UV light.

The composite of the invention may be analysed by XRD, as describe herein.

The present composites, where they include Ag as the metal particle give rise to detectable signals derived from Ag at 38° (1 1 1 ), 44° (200), 64° (220) and 77° (31 1 ). Such signals become visible only for composites having higher metal particle loadings and for those prepared over longer reaction times and/or including a heating step. It is noted that the effects of Ag, for example, in the XRD are generally small. The present inventors have observed Ag signals corresponding to 1 1 1 and 200 in the composite materials prepared according to the present invention. In contrast, the materials prepared by Wen et al., for example, do not apparently give rise to these peaks in the XRD. Thus, in one embodiment, the composite of the invention has a detectable signal at 38° and/or 44° in an XRD spectrum.

In one embodiment, the lattice plane of the nanoparticles is matched to the lattice plane of the semiconductor particles. In one embodiment, the nanoparticles have a (1 1 1 ) lattice plane and the semiconductor particles have a (101 ) plane. The inventors have found that the (1 1 1 ) lattice planes of the nanoparticles are at an appropriate angle to match the (101 ) planes of the semiconductor. The matching indicates that there is a good interface between the two phases of the composite.

The XRD values are the positions at 2Theta employing Cu-Κα radiation, as described herein.

The composite of the invention may be analysed by TEM. Analysis of the TEM images of the composites of the present invention shows that the interfaces between the metal nanoparticle and the semiconductor material are very strong. The metal nanoparticles are well distributed on most, if not all, the semiconductor particles. It is believed that the semiconductor particles, such as Ti0₂ particles, present in the composite have at least one protrusion on the surface.

The TEM images reported by Wen et al. and Amin et al. are of generally poor quality, and it is difficult to draw meaningful conclusions from the work as to the exact structures formed. However, it is clear that significant structural differences exist between the composites described herein and the composites of Wen and Amin. The present composites are shown to be active in visible light, whilst those of Wen and Amin are apparently not.

The composite of the invention may be analysed by EELS.

As described above, the semiconductor material may include dopants. The presence of the dopant is for the purpose of further modifying the band gap in the material such the energy gap can be further decreased from the UV light region to the visible light range. The dopant may therefore provide additional changes to the overall photocatalytic activity of the composite, and may be used to fine tune the photocatalytic properties. Suitable dopants are well known in the art. The dopant may be selected from one or more of C, N, P, S and H. Such are particularly suitable for Ti0₂. Nb(V), W(VI) and Ti(V) are additional or alternative dopants for use with Ti0₂.

A semiconductor material having a dopant may be purchased from commercial sources, or may be prepared from un-doped semiconductor material using methods well known in the art.

A composite of the invention may be used directly in the methods described herein as composite particles. In other embodiments, it may be beneficial to immobilise the

composite, for example on a surface. Such may be suitable for use in flow systems for the treatment of waste materials. In one example, a composite of the invention may be provided as a film deposited onto to a surface. The surface is not particularly limited and is chosen based on the intended use of the composite. An example surface material is glass. The present inventors have found that a film containing the composite retains catalytic activity, for example to catalyse the degradation of pollutants such as dye materials. A film containing the composite of the invention has superior activity to a film containing only Ti0₂. As expected, films where the metal particles are present at higher mole percentages show have better activity than those films where the metal content is low (e.g. at 1 or 2 mole %). Methods for Preparing Composites

The composites of the invention may be prepared in a method that simultaneously reduces metal ions and decorates the semiconductor material. The method includes a further step of heating the composite, thereby to increase the amount of decoration (the amount of metal present as decoration on the semiconductor material). The composite that results from this process has desirable physical and photocatalytic properties. The combination of steps provides metal particles having a useful distribution of sizes. The combination of steps also allows the lattice phase of the metal to be matched to that of the semiconductor. In one aspect there is provided a method for preparing a composite, the method comprising the steps of:

(i) providing together a semiconductor particle, a metal salt and a chemical reducing agent;

(ii) permitting the reducing agent to reduce the metal salt in the presence of the semiconductor material, thereby to generate a semiconductor particle decorated with one or more metal particles; and

(iii) heating the mixture,

thereby to generate a composite having a semiconductor particle decorated with one or more metal particles.

In step (i) the components may be provided together as an aqueous mixture. In this embodiment step (iii) may include the step of removing water from the mixture. Step (iii) may include the step of removing reducing agent from the mixture. Typically the heating step is sufficient to remove any water and any remaining reducing agent from the mixture.

The semiconductor particle is a particle of a semiconductor material selected from those materials discussed above. Thus, a semiconductor particle may be a particle comprising one or more of Ti0₂, Sn0₂, W0₃, SrTi0₃, ZnO, KTa0₃ and Fe₂0₃. In a preferred

embodiment, the semiconductor particle is a Ti0₂ particle. The Ti0 may be an anatase particle. The semiconductor particle is substantially crystalline. These crystalline particles are then modified through the decoration so the particle with a metal nanoparticle. A highly uniform, crystalline semiconductor particle is thereby provided in the product composite. In contrast, the sol-gel methods of Amin et al. generate the semiconductor particles in situ, and the calcining steps are believed to be necessary to produce crystalline anatase in the product, together with the metal nanoparticle.

The semiconductor material present in the semiconductor particle may be doped. In this embodiment, the semiconductor material is doped in order to alter the band gap, for example to fine tune the optical and electronic properties of the composite. Example dopants for use in the present invention include Nb(V) and W(VI). Where Ti0₂ is used as the semiconductor material, surface oxidation may be used to provide Ti(V) amongst the Ti(IV) of the semiconductor material. As described herein, the use of a small molecular weight reducing agent in the preparation method may provide an additional contribution to the surface modification of the

semiconductor material, where the contribution serves to provide additionally beneficial photocatalytic activity in the visible light region. The modifications here may be provided, for example, by C bonding to the semiconductor surface.

The inventors have established that the methods of the invention do not substantially change the morphology or crystallography of the semiconductor material. Thus, the size and shape of the semiconductor particle selected for use in the method will determine the size and shape of the semiconductor particle within the product composite.

Similarly, the size distribution of the semiconductor particles is not apparently altered by the method of the invention. The present inventors have found that where a narrow size distribution of semiconductor particles is used in the method, the composite similarly contains a narrow-size distribution of semiconductor particles. Thus, a substantially monodisperse collection of semiconductor particles used in step (i) will yield a composite having semiconductor particles that are similarly monodisperse.

The method of the invention comprises the step of reducing a metal salt thereby providing metal particles that adhere to the semiconductor particles. Without wishing to be bound by theory, the present inventors believe that the metal particles are at least partially embedded into the surface of the semiconductor particle surface. The manner in which the metal particles are attached to the surface is believed to contribute to the visible light activity of the photocatalyst. The metal salt comprises a metal in an oxidised form compared to the metal that forms the decoration on the semiconductor particle. The metal salts for use in the present method are those oxidised forms of the metals discussed above. Thus, a metal salt may comprise an oxidised form of Au, Ag, Ir, Pt or Pd. The oxidation state of the metal may be (I) or (II), as appropriate. Preferably, the metal salt is an Ag salt. Preferably the metal salt is an Ag (I) (Ag') salt. The metal salt may be selected for its availability, its aqueous solubility and its ease of reduction. The metal salt may be a nitrate. For example, where the metal is Ag, the metal salt may be Ag 0₃. Other suitable Ag salts include citrate and acetate salts. Soluble salts for metals such as Pt, Ag and others are well known, and include H₂PtCI₆ and NaAuCN_?, as well as metal amine chloride complexes such as Pt amine chloride complexes.

The reducing agent is a compound that is capable or reducing the metal ion present in the metal salt to a metal. Typically, the reducing agent is a small organic molecule, for example a compound having a molecular weight of 200 or less, 100 or less, or 50 or less. Generally, the metal ions in the metal salt are relatively easy to reduce, and a mild reducing agent is sufficient.

The reducing agent is believed to modify the surface of the semiconductor material by absorbing onto the surface, possibly as radical species. The absorbate alters the optical properties of the composite, and is believed to provide a contribution to the composite that allows photons to be captured in the visible range. When the composite is heated, such as described herein as step (iii) in the method of synthesis, the absorbates are further modified thereby providing absorption in the visible range, particularly at wavelengths at and above 600 nm. Suitable for use as reducing agents are formaldehyde, thiourea, formic acid, oxalic acid and other small organic carboxylic acids, and amine compounds.

In one embodiment, the reducing agent is formaldehyde.

These reducing agents are found to produce suitable composite structures for use, and provide superior results to reducing agents such as copper and sodium borohydride.

The composites of the invention may comprise semiconductor material that consists essentially of one semiconductor material. For example, the semiconductor material may consist essentially of Ti0₂.

The reducing agent is typically used in large mole excess over the mole amount of metal ion provided by the metal salt and the mole amount of semiconductor material.

In one embodiment, the mole ratio of semiconductor material to metal ion is 100:X, where X is 1 or more, such as 2 or more, such as 5 or more, such as 10 or more, such as 15 or more, such as 20 or more. In one embodiment, X is 50 or less, such as 40 or less, such as 30 or less. Thus, the mole amount of semiconductor material is in excess to the mole amount of metal ion. Composites having a higher mole % of metal decoration are obtainable from those reaction solutions having higher ratio of metal ion present with respect to

semiconductor material. The mole amount of the semiconductor material may refer to the mole amount of metal present in the semiconductor. For example, where the semiconductor is Ti0₂, the mole amount refers to mole amount of Ti present.

The reducing agent, metal salt and the semi-conductor material are mixed together and permitted to react. The reaction time is at least 1 hour, at least 2 hours, at least 4 hours, at least 8 hours, at least 10 hours, at least 12 hours, at least 18 hours or at least 24 hours. This preliminary mixing step may be conducted at room temperature, for example at a temperature in the range 10 to 25°C.

Composites having a higher mole % of metal decoration are obtainable from longer reaction times.

In one embodiment, the method of the invention is conducted, at least in part, at a temperature at most 250°C, at most 200°C, at most 150°C or at most 140°C.

In one embodiment, the method of the invention is conducted, at least in part, at a temperature at least 50°C, at least 70°c, at least 90°C, at least 100°C, at least 105°C, or at least 1 10°C.

In one embodiment, the percentage mass is in a range selected from the upper and lower values given above.

The method of the invention is conducted, at least in part, at a temperature above room temperature, for example at a temperature in the range 30 to 200°C, 30 to 150°C, for example 50 to 150°C, for example 100 to 150°C, for example 1 10 to 140°C.

The heating step is performed at moderate temperatures, in contrast to the high temperature calcining treatments described by Amin et al. It is noted that the catalysts produced by Amin et al. are not active under visible light, whilst those of the present invention are.

In one embodiment, the reaction is conducted at an elevated temperature only for the latter part of the reaction. Thus, 30% or less, 20% or less, 10% or less or 5% or less of the total reaction time may be conducted at an elevated temperature. Where a reaction time is given, the reaction time includes any heating step.

In one embodiment, the reaction mixture is heated for at least 1 minute, at least 5 minutes, at least 10 minutes, at least 20 minutes, at least 30 minutes.

In one embodiment, the reaction mixture is heated for at most 60 minutes, at most 90 minutes, at most 120 minutes, or at most 180 minutes.

It will be appreciated that at elevated temperatures, water, where present, may be removed from the reaction mixture. Where the reducing agent has a relatively low boiling point, it too may be removed from the reaction mixture, for example when the reaction is deemed complete. Higher boiling point solvents may be removed in vacuo at the temperatures specified above, for example. Longer mixing times are preferred as the metal content on the semiconductor particles is maximised with longer reaction times. In a preferred embodiment, the reaction time is at least 4 hours.

The presence of a heating step is provides a maximal loading of the metal onto the semiconductor particle. In a preferred embodiment, the reaction mixture is heated for at least 30 minutes.

Furthermore, the longer mixing times, such as those described herein, also allow the development of a more stable metal/semiconductor interface. This provides a noticeable improvement in the efficiency of the photocatalyst and the stability of the photocatalyst, as gauged by its highly recyclable nature.

The heating step has also been shown by the inventors to increase the number of particles of metal on the semiconductor surface having a dimension of 5 nm or less. This change in particle distribution is observable from the TEM images of composites prepared with and without heating.

Analysis of the product mixture by the inventors has established that the methods allow substantially all the semiconductor particles to become decorated with one or more metal particles. After the reaction is deemed complete, for example after the reaction time has passed, the composite may be separated from other components in the reaction mixture, such as the reducing agent and water, where present. Typically, the composite is produced as a solid, which may be separated from fluid components by filtration or evaporation of the other components.

The reaction can be monitored by eye or using suitable spectroscopic techniques known to those of skill in the art. The formation of the composite is associated with a colour change in the reaction mixture. After heating, the product mixture may be allowed to cool to room temperature.

In one embodiment, after the heating step, the product mixture may be ground. Suitable methods for grinding a material are well known in the art.

In one embodiment, after a grinding step, the product mixture may be heated for a second time. In this second heating step the material may be heated to the same temperature used in the first heating step. Alternatively, the material may be heated to a different temperature. In an alternative embodiment, the material is heated to a temperature above room

temperature, for example to a temperature in the range 30 to 150°C, for example 50 to 150°C. for example 100 to 150°C, for example 1 10 to 140°C.

In one embodiment, the material is heated for at least 1 minute, at least 5 minutes, at least 10 minutes, at least 20 minutes, at least 30 minute.

In one embodiment, the material is heated for at most 60 minutes, at most 90 minutes, or at most 120 minutes.

The present inventors have found that the addition of a grinding step, optionally together with a further heating step, minimises the aggregation of composite particles in the product. The introduction of a grinding step, optionally together with a further heating step, therefore increases the effective surface area of a sample of composite product. As described herein, surface area may be determined through standard BET measurements. The composites of the invention may be analysed by XRD, UV-Vis, TEM-EELS, TEM and XPS as described herein, amongst others.

The activity of the composite of the invention may be measure in relation to the ability of the composite to photocatalytically degrade a dye, such as an organic dye, as described herein, under incident visible light.

Methods and Uses

The composites of the present invention may be used as photocatalysts, for example in chemical or biological reactions.

In one embodiment of the invention a composite may be used in a method of degrading a pollutant compound, such as a water pollutant or an air pollutant. The inventors have established that the composites may be used to degrade organic water pollutants, such as organic dyes, rapidly at ambient temperature and under visible light conditions. The composite may be regarded as a heterogeneous photocatalyst. The photocatalyst is in solid form, and may be used to catalyse reactions within fluid phases, such as the liquid and gas phases. Thus, in one aspect there is provided a method of treating a pollutant, the method comprising the step of exposing the pollutant to a composite as described herein in the presence of visible light, optionally together with UV light, thereby to degrade the pollutant.

The pollutant may be present in waste water, for example waste water from an industrial process. The pollutant may be present in a water that is to be treated, e g. for subsequent release into the environment or for subsequent use in industry or for consumer consumption. Where the composite is used to reduce the amount of a pollutant present in a water sample, the method may be referred to as a water purification method.

The present method provides a useful alternative to the water purification methods known in the art. Large-scale purification methods typically make use of porous materials, such as activated carbon, to absorb pollutants. The pollutants are effectively stored on the material, and must be separately decomposed. The materials also have a saturation level, beyond which they are not effective at absorbing further pollutants. Here, the materials must be replaced or treated for reuse.

In contrast, the composite materials of the present invention may be used directly to decompose the pollutants. Where the pollutant is an organic molecule, the composite may be used to catalyse the decomposition of the compound to water and carbon dioxide, amongst others.

The use of the composites of the present invention is particularly attractive as the composites are active as photocatalysts under visible light. The composites may therefore be used to treat pollutants, such as water pollutants, at locations that have poor access to power supplies. For the composite to work, a light source, such as daylight, is all that is required.

The method reduces the amount of the particular pollutant present. In this way, a sample comprising a pollutant may be purified. The form of degradation is not particularly limited and is generally any form of photocatalytically-induced change that results in a chemical transformation of the pollutant molecule. The photocatalytically-induced change may be an oxidative change.

The presence of a pollutant in a sample for treatment, and its degradation, may be monitored by standard spectroscopic means.

A sample comprising a pollutant may be treated in a batch process. Thus, a sample comprising a pollutant may be brought together with a composite and the mixture exposed to visible and/or UV light. The sample may be held together with the composite and may be exposed to light until such time as the amount of the pollutant is reduced to a satisfactory level. Subsequently, the sample, which has reduced pollutant levels, may be separated from the composite. It may be further processed as necessary.

A sample comprising a pollutant may be treated in a flow process. Thus, a fluid sample comprising a pollutant may be passed across a composite and exposed to visible and/or UV light. In one embodiment a fluid sample is continuously passed over the composite. In one embodiment, sufficient composite material is provided to allow the pollutant to be degraded to a suitable level as it passed across the composite. A fluid sample may be repeatedly passed across the composite until the desired reduction in pollutant level is attained.

In one embodiment, the pollutant is a dye, such as an organic dye or an inorganic dye. The dye may be a water pollutant.

Examples of organic dyes include fluorone dyes, such as Rhodamine B, Rhodamine 6G, Carboxytetramethylrhodamine (TAMRA), Acid Red 87; Fluorescein and

Tetramethylrhodamine (TMR); and phenothiazinium dues, such as methylene blue.

Examples of inorganic dyes include potassium permanganate.

Dyes, such as textile dyes are a major pollutant of inland waterways. There is need to find methods and materials thaw ill allow the levels of these pollutants to be reduced within minimal energy input. The present inventors have shown that the composites of the invention may be used to significantly reduce the levels of common dye pollutants in test water samples. The degradation of the pollutants may be achieved at ambient temperature on exposure to daylight, or alternatively on exposure to artificial light, or a combination of both.

In one embodiment, the pollutant is an organic molecule.

The composite of the present invention may be used to sterilise a fluid sample. The sample may comprise one or more different disease-causing pathogens. The composite may be used to catalyse the degradation of the pathogen by exposure of that pathogen to the catalyst in combination with incident visible light, optionally in combination of UV light.

In one embodiment, the pollutant is a microorganism, prion or a virus.

Examples of microorganism include fungi, and gram positive and gram negative bacteria.

In one embodiment, the microorganism is selected from E. coll. and S. aureus

The use of alternative Ag-Ti0₂ composites to inhibit bacterial activity has been previously described.

The present invention provides a method of reducing the activity of a microorganism, the method comprising the steps of contacting a microorganism with composition of the invention in the presence of UV and/or visible light.

The method may be an in vitro method.

The present invention provides a composition for use in a method of treatment.

The present invention provides a composition for use in a method of treatment of a bacterial infection.

The method of treatment may include the step of irradiating the composite with UV and/or visible light. In the methods of the invention the photocatalyst is irradiated UV and/or visible light. Such light may be daylight or may be artificial light which is incident upon the photocatalyst. The photocatalyst may be directly exposed to daylight, or daylight may be directed onto the photocatalyst using such techniques familiar to those in the art.

In a simple embodiment of the invention, a composite may be added to a liquid sample containing a pollutant or a microorganism that is held in a tank located outside and is directly exposed to, or is exposable to, daylight. After a suitable treatment time, the fluid, which is purified, may be separated from the solid composite,

In the methods of the invention, the photocatalyst may be mixed through the fluid for treatment before and/or during the treatment process.

In the methods of the invention, the photocatalyst may be recovered for further use. The composite may be recovered by simple filtration, for example.

The present invention provides the use of a composite of the invention as a photocatalyst. The present invention provides the use of a composite of the invention as a water purifier. The present invention provides the use of a composite of the invention as a sterilising agent. Such uses are provided in combination with incident light, such as visible and/or UV light, such as daylight.

In an alternative aspect of the invention there is provided a mixture comprising a

photocatalyst of the invention and a pollutant.

In another aspect of the invention there is provided a mixture comprising a photocatalyst of the invention and a microorganism, prion or virus.

A reference to degradation of a pollutant or the sterilisation of an agent may refer to the reduction in the quantity of the pollutant or agent in a sample. Thus, in one embodiment, after photocatalytic treatment, there is at least a 20% reduction, at least a 40% reduction, at least a 50% reduction, at least a 70% reduction, or at least a 90% reduction in the amount of the pollutant or agent in a sample. The amount of pollutant or agent present may be measured after 10 mins, 20 mins, 30 mins, 1 hour, 2 hours, 12 hours, or 24 hours.

Other Preferences

Each and every compatible combination of the embodiments described above is explicitly disclosed herein, as if each and every combination was individually and explicitly recited. Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure. "and/or" where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example "A and/or B" is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein.

Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described. Certain aspects and embodiments of the invention will now be illustrated by way of example and with reference to the figures described above.

Experimental and Results

All chemicals used were purchased from Sigma Aldrich.

A composite material was obtained by combining a Ti0₂ nano powder (anatase, particle size < 25 nm) with Ag, which was generated in situ by reduction of Ag⁺ (aq.) with formaldehyde.

The starting Ag:Ti atomic ratio in the reagents was 6:100, 8:100, 10:100 and 12.5:100.

In a typical preparation, 2.0 g of Ti0₂, 10 cm³ of formaldehyde (37.7%) and 0.368 g, 0.468 g or 0.675 g AgN0₃ were combined in a Pyrex glass beaker. The components were mixed at room temperature for different times, from 30 minutes to 18 hours. In some cases, the samples were then centrifuged at 2.500 rpm, and the excess AgN0₃ rinsed off. The sample was then further washed with distilled water and the product dried in a vacuum oven at 40°C. In other cases, once the reaction time had passed, the mixture was then heated to 80 to 120°C (the so-called heating step). The mixture was typically heated until all solvents were removed. The product was then ground down, and heated for a second time, typically to 80 to 120°C, and preferably the same temperature as the initial heating step, then allowed to cool to give a dark black solid. The solid material was analysed as discussed below.

The photocatalytic characteristic of each material was tested by using the material to catalyse the decomposition of three different dyes: RhB, Methylene blue and Acid red 87. Such reactions are described in further detail below. Silver Deposition

The loading of silver on titania was measured quantitatively by the following process. Ag was removed from a sample of composite material using an aqueous solution of HN0₃; inductively coupled plasma (ICP) was used to measure the resulting Ag mass in the aqueous solution.

A slow silver deposition process was observed at room temperature, yielding a silver loading of 1.7 wt % to 3.7 wt % for times ranging from 2 to 18 hours (as shown in Table 1 ). The addition of a heating step increases the Ag loading to roughly 20 wt % (as shown in Table 2), corresponding to a deposition of the entirety of the Ag in the initial solution.

Table 1

^* as determined by ICP; the reaction time is shown to the nearest hour Table 2

Reactants Composite

Reaction Amount of Mole % of

Time

TiC-2 Ag 0₃ Ag Ag in wt % Ag in Ag Ag (hours)^*

(g) (g) (g) product^* product (Ag:Ti0₂) XRD peak

(g) deposited

2 2 0.67 0.425 nd nd Strong

4 2 0.67 0.425 0.34 17 12.5 Strong

13 2 0.67 0.425 0.38 19 14.0 Strong

6 2 0.375 0.238 0.187 9.35 6.93 ^* as determined by ICP; nd, not determined; the reaction time is shown to the nearest hour. The reaction time includes the heating step.

The loading of silver on titania was determined by the method described herein, where the Ag was removed from a composite sample using nitric acid. ICP was used to measure the resulting Ag mass in the aqueous solution.

XRD Analysis X-Ray diffraction was used to verify the crystalline phase of Ti0₂ and the deposited silver. The XRD spectra of Ti0₂ after Ag deposition are shown in Figures 1 to 3. Where Ag was deposited over a 2 hour period at room temperature, no change was observed in the product XRD spectrum (Figure 1 ) compared to that of Ti0₂ alone (not shown here). However, room temperature deposition results in a clear Ag XRD signal only at extended deposition times, for example over 12 hours 45 mins (as shown in Figure 2). The addition of a heating step to the method significantly increases the Ag content in the product composite, as observed by ICP. The four main expected Ag peaks at 2Θ = 38° (1 1 1 ), 2Θ =44° (200), 2Θ =64° (220), and 2Θ = 11° (31 1 ) are observed in the XRD spectrum after a heating step, even after only 30 minutes of reaction time and 20 minutes of heating. Figure 3 is the XRD spectrum of a composite obtained after a 105 min reaction time at room temperature, followed by ca. 20 minutes of heating, which was sufficient to remove the solvents. This composite

corresponds to the first entry in Table 2 above.

The stability of the Ti0₂ peaks confirms that silver deposition does not alter the crystal structure of the underlying titania (in the anatase phase here).

UV-vis Spectra

The change in the optical absorption of the composites was investigated using UV-Vis spectroscopy (see Figure 4).

All the composites prepared show an absorption peak in the visible region, with a maximum intensity between about 400 and 530 nm (violet to green portion of the spectrum), whereas the absorption of pure anatase Ti0₂ nanopowder is close to zero in the visible and rapidly increases below 400 nm due to direct interband transitions. The absorption spectra in

Figure 4b show the dramatic effect of post-synthesis heating on the optical properties of the composite. For those composites prepared with a heating step, the absorption is high throughout the UV-vis wavelength range, excluding a small (less than 13%) dip between 320 and 450 nm, which indicates that the specific Ag deposition route and the heating procedure are responsible for the catalyst's visible light response (as discussed in later sections). Surface Area

The surface area of nanoparticles is a very important factor in determining the efficiency of that particle as a catalyst. It is therefore useful to monitor the influence of silver deposition on the surface area of the composite. The BET surface areas of Ti0₂ and Ag/Ti0₂

(12.5 mole % from Table 2) composites were found to be 53 and 46 m²g^"1 respectively. This difference in surface area is a result of the morphological change due to the deposition of Ag particles, but also the slight aggregation of the particles upon drying. While the nucleation of small Ag particles on Ti0₂ is thought to increase the surface area, the aggregation between particles is noticeable in samples dried from solution. This was addressed by adding grinding and heating step to the method of preparation. Thus, after the initial heating step, the resulting product is ground and then heated at the same temperature as before, until for example a suitable black composite is obtained. The isotherm curve (type IV) seen in the BET analysis, with its hysteresis loop at relative pressure P/Po between 0.5 and 1 (see Figure 5), confirms the mesoporous structure for both Ag/Ti0₂ and pure Ti0₂.

Morphology

The morphology of the Ag-Ti0₂ based composites was investigated by scanning

transmission electron microscopy (STEM) (Fig. 6). Analysis showed that the Ag particles ranged in size from sub-nm to 20-30 nm. Some of the Ag particles are therefore of comparable size to the Ti0₂ particles (narrowly peaked on 25 nm) to which they are connected. However, the majority of the metal particles were found to have a largest dimension of 1 nm or less.

It is possible to identify Ag particles because they appear as bright regions (high electron density) in a STEM dark field micrograph. However, the inventors verified the elemental composition using STEM-EDX and EFTEM mapping. Almost all Ti0₂ particles are decorated by several small Ag particles; larger particles are intermixed within the Ti0₂ powder and their presence can be ascribed to coalescence of smaller particles during both the growth process and the drying step at the end of the synthesis. This morphology is very useful for photocatalysis: the broad size distribution of Ag particles results in the adsorption of different wavelengths due to surface plasmon resonance, thus providing good collection efficiency for photons across the whole visible range. Moreover, the good coverage of Ag on Ti0₂ surfaces ensures that all the Ti0₂ particles will take part in the visible light-activated steps needed for oxidative catalytic activity, such as dye degradation. The effect of the length of the room-temperature deposition (2 hours to 18 hours) and the heating stage (100 C for 30 minutes until drying of sample) was studied by STEM. An increased reaction time at room temperature resulted in slow silver loading as described at the start of this section (1.7% to 3.7 wt % Ag in 2 to 18 hours. However, a much larger silver loading 12.5 mole % was obtained with a heating step at 100 C until the sample is dry (e.g. to constant mass). The morphology of the silver particles before and after the heating step is significantly different. While both present large (from around 5 up to 30 nm) particles, only the heated sample provide Ag particles down to sub-nm size. These Ag particles extensively decorate the Ti0₂ surfaces, resulting in a configuration where each Ti0₂ particle has up to tens of small Ag particles anchored to the surface of the oxide.

TEM-EELS

The possible presence of carbon within the composites was investigated by TEM-EELS (performed in vaccum). Figure 7 reports two spectra acquired in an area of a composite containing several Ag-Ti0₂ aggregates. A small presence of carbon is observed, but the carbon signal does not possess fine structural features. Such a signal may result from the presence of carbon radicals or amorphous carbon remaining on the particles surfaces after the synthesis. The presence of carbon is also confirmed by XPS, so it is unlikely that the signals result from the deposition of carbon with the electron beam during the acquisition.

TEM

Crystal structure plays a significant role in the photocatalytic behaviour of Ti0₂, because of variations in the band gap, but also because it can determine the low-energy surfaces terminating the structure. Some surfaces present different features that have been correlated with photo-induced activity [Nature 453, 638-641 , doi:10.1038/nature06964], such as two correlated oxygen atoms. Such oxygen atoms sometimes referred to as "bridging oxygen atoms" since they connect two titanium atoms on the surface). Said bridging atoms can easily be desorbed (for example by a photo-generated exciton), creating surface defects.

Another point investigated is how the silver particles nucleate and interact with the Ti0₂ substrate. The interfaces formed between the two materials are of paramount importance in determining the injection of electrons from the Ti0₂ into the silver particles (see

photocatalytic mechanism). High resolution TEM (see figure 9) was used to study the properties of the composite material.

As expected, the titanium dioxide particles are in the anatase phase and the moderate thermal treatments (up to 200°C) during the composite synthesis do not alter the crystal phase or crystal size of the Ti0₂. The Ag particles present on the Ti0₂ surfaces have been observed in different crystal forms. Some of particles were found in the expected cubic (FCC) phase, whereas others were in a less common hexagonal phase [Taneja et al.]. This latter phase is metastable and it is thought that a mixing process, where it occurs at room temperature only and therefore at a slow rate, is gentle enough to allow such a phase to nucleate and survive long enough to produce particles up to 10 nm in size.

The hexagonal Ag form has been observed in the materials prepared without a heat treatment step. However, some hexagonal phase particles have been observed in the composites even after a thermal treatment step. Unfortunately HR-TEM does not provide enough statistically relevant data to allow the effect of the thermal treatment on the crystal phases of Ag to be measured.

The Ag particles within the composites show a high degree of crystallinity and a size that ranges from sub-nm to tens of nm. The Ag particles have been observed to grow, adjusting to the underlying Ti0₂ particles (see Figure 8), and matching the Ti0₂ lattice. This constitutes a promising interface for charge injection. Moreover, the faceting of the Ag particles presents a low contact angle on the Ti0₂ at different length scales, indicating that surface interaction between the two materials in energetically favourable.

It is noted that crystallographic characterisation could not be carried out on the smallest Ag particles (smaller than about 5 nm) due to the difficulty of obtaining digital diffraction patterns of sufficient quality to discriminate between different structures. XPS

X-Ray photoemission spectroscopy (XPS) experiments were performed to complement the TEM characterization described above. A surface-sensitive technique was used to characterise three powder specimens: pure Ti0₂ (sample A), formaldehyde treated Ti0₂ (sample B), and Ag-Ti0₂ (sample C - the composite comprising 6.9 mole % Ag from Table

2).

A monochromated Al K_a x-ray beam was focused onto the surface of each powder specimen with an elliptical spot size of 600 x 800 μπι. The signal is therefore averaged over a large area and a large number of particles. The technique, however, is sensitive to the

composition and bonding configuration of the material within about 5 nm of the surface. The positions of the main elemental peaks, C 1 s, O 1 s, Ti 2p, Ag 3d, and their relative shifts, were monitored to determine the composition of the specimen. Throughout all the samples, the position of the Ti 2p peaks is compatible with a Ti(IV) oxide, most likely anatase given the slightly higher binding energy and the separation between 2p _3/2 and 2p _½ peaks. The oxygen signal shows features compatible with O-Ti bonding and a fraction of O-C bonding. The carbon signal has a large C-C or C-H component, followed by the C-OH, and a 0-C=0 components. The silver signal is compatible with metallic silver. The fine structure is still under investigation.

To compare the three specimens, all the atomic elemental percentages were normalized to the titanium content, and the pure Ti0₂ powder was taken as the reference. A summary of the XPS analysis is shown in the table below.

Table 3 - XPS elemental quantification, normalized to Ti content (atomic ratio)

Comparing the elemental results, it is noted that:

i) all the samples contained more oxygen than expected from the stoichiometric ratio (compatible with the nanoparticle nature of the material);

ii) all the samples contain surface carbon;

iii) the formaldehyde treatment on Ti0₂ (without Ag decorating the surface) seems to remove about 50% of the surface carbon, leaving behind a higher proportion of C-OH and 0-C=0 bonds;

iv) the Ag-Ti0₂ sample contains 3% less oxygen, and 4% less carbon than pure Ti0₂, and has the least amount of C-OH and 0-C=0 bonding.

The relative intensity of the Ti 2p peaks and the fine structure of the Ti 2s peaks for the Ag-Ti0₂ sample are significantly different from those of pure Ti0₂ and formaldehyde-treated Ti0₂. These changes may reflect changes induced by the presence of Ag and are still under investigation.

The relative intensity of the Ti 2p and Ti 2s peaks for the Ag-Ti0₂ samples are significantly different from those of pure Ti0 and formaldehyde-treated Ti0₂. A significant change between pure Ti0₂ and Ag-Ti0₂ is observed at low binding energies, where the O 2p peak at 4-7 eV overlaps with the Ag 4d 5eV peak. These changes may reflect changes induced by the presence of Ag. Electron transfer could be more efficient between O-Ag states of similar energies. This may be important both for photon absorption and for photocatalysis (a photon being absorbed by the Ag, with excitation transferred to the oxygen outer shell orbitals). Photocatalytic Activity

The photocatalytic activity of a Ag-Ti0₂ composite of the invention was investigated by photocatalytic decomposition of aqueous solutions of RhB, Methylene Blue (MB), and Acid Red 87 at room temperature. The composite comprised 6.93 mole % Ag (see the relevant entry in Table 2).

The RhB solution (3 mL, 10 μΜ) was mixed with 10 mg of Ag-Ti0₂ composite in a quartz test tube. The suspension was then exposed to UV and visible light. In particular, a light source from Newport (power 150 W, Xenon ozone-free lamp equipped with filters to select different regions of the spectrum - a 420 nm cut-on filter to remove the UV part of the emission and a 340-700 nm bandpass filter. The dye concentration after treatment was measured by UV/Vis spectroscopy after filtration of the solution (to remove the composite).

Figure 10 shows the change in absorbance for a Methylene Blue dye solution irradiated with UV and visible light in the presence of pure Ti0₂ or Ag-Ti0₂. The sharp decrease in the absorbance peak for all dyes treated with Ag-Ti0₂ is an indication of the high photocatalytic decomposition efficiency of this material, compared with pure Ti0₂.

Methylene Blue shows a major absorption peak at λ = 664 nm, Acid Red at λ = 505 nm, and RhB at λ = 553 nm. The absorption of RhB was checked at different times while stirring the solution containing the dye with the Ag-Ti0₂ composite in a test tube under UV and visible light in two different runs. The decrease in absorption was proportional to the Ag loading in the sample and 99% of the dye was decomposed in 3 (UV and visible light) and 10 min (visible light) by 6 mole % Ag-Ti0₂.

Dye degradation was also tested in the same conditions under standard office lighting, corresponding to a less intense illumination, for 20 minutes, and compared to the

degradation obtained with the pristine Ti0₂ nanoparticles under the same conditions (see Figure 10).

Additional Photocatalytic Activity

The photocatalytic activity of the composites was further investigated in a series of degradation studies. A 12.5 mole % Ag-Ti0₂ composite was prepared as described above, together with a 3 mole % Ag-Ti0₂ composite and Ti0₂ particles. The three catalysts were used to photocatalytically degrade methylene blue, and the degradation of this dye was monitored over time to determine the catalytic rate constant. The change in relative methylene blue concentration over time, as evidenced by a decrease in absorbance around 650 nm, is shown in Figure 1 1 . The gradients of the lines fitted to the recorded data were 0.278 (12.5 mole % Ag-Ti0₂; R² = 0.9586), 0.0984 (3 mole % Ag-Ti0₂; R² = 0.9669) and 0.0484 (Ti0₂; R² = 0.9708).

The 3 mole % Ag-Ti0₂ composite refers to a catalyst where the expected Ag fraction is 3 mole %. The measured concentration of Ag was determined to be 2.7 mole % using the techniques described above. The Ti0₂ particles were anatase particles of less than 25 nm diameter. The light source was a standard UV-Vis lamp rated at 500 W/m².

Each catalyst was used as 2 mg of material in 3 mL of test dye solution. The test solution was 10 μΜ methylene blue in water.

The photocatalytic activity of the 12.5 mole % Ag-Ti0 composite was compared to a Ti0₂ catalyst. The degradation of methylene blue under UV-vis irradiation was studied. The change in absorbance values over time was recorded. The measured UV spectra are shown in Figure 12 and 13. The results show that the absorbance values at 650 nm change significantly with 5 minutes of irradiation in the presence of the 12.5 mole % Ag-Ti0₂ composite. In contrast, a similar reduction in the absorbance values for the Ti0₂ catalyst is achieved only after 25 minutes. The Ti0₂ particles were anatase particles of less than 25 nm diameter. The light source was a standard UV-Vis lamp rated at 500 W/m². Each catalyst was used provided as 2 mg of material in 3 mL of test solution. The test solution was 10 μΜ methylene blue in water.

The recycling activity of a 12.5 mole % Ag-Ti0₂ composite was compared to 1 mole % and 2 mole % Ag-Ti0₂ composite catalysts. The 1 mole % and 2 mole % catalysts were prepared in a similar manner to the 3 mole % catalyst described above, with a reduction in the amount of Ag used in the preparation, as appropriate (i.e. a lower AgN0₃ concentration in the preparation method). Each catalyst was used to photodegrade methylene blue under UV-vis irradiation. Once the relative absorbance of the sample dropped to around 0.2, the catalyst was recovered. The catalyst was then use to treat a second batch of methylene blue in a second run. The 12.5 mole % Ag-Ti0₂ composite retained its activity to degrade methylene blue in a second run. The 12.5 mole % Ag-Ti0₂ composite shares with the 1 mole % and 2 mole % catalysts the advantage that it can be reused in further treatment runs. The 1 mole % and 2 mole % catalysts were also used in a third run, and each catalyst retained its ability to degrade methylene blue. The results are shown in Figure 14. The light source was a standard UV-Vis lamp rated at 500 W/m². Each catalyst was used provided as 2 mg of material in 3 mL of test solution. The test solution was a 10 μΜ methylene blue solution in water.

The ability of a 12.5 mole % Ag-Ti0₂ composite to photodegrade a microorganism was studied. E. coli was used as the model organism and samples of E. coli cells were photodegraded in the presence of a 12.5 mole % Ag-Ti0₂ composite, a Ti0₂ catalyst and a sample without catalyst. As before, the light source was a standard UV-Vis lamp rated at 500 W/m².

Testing was conducted using E. coli strain DH5a in a Phosphate Buffered Saline (PBS) suspension held in a quartz test tube. The tube (with or without catalyst present, as appropriate) was placed directly in front of the light source. The suspension was mixed with 3 mm magnetic stir fleas, with the initial exposure time varying from 1 to 45 min as the conditions were optimised.

Aliquot samples of 100 mL were taken three times per test (including at t = 0 for all conditions) and diluted over several orders of magnitude in PBS before each sample was incubated on non-selective agar (approximately 24 hours at 37°C). Counts were made and CFUs (Colony Forming Units) per mL calculated.

The results are shown in Figure 15. The 12.5 mole % composite was found to significantly reduce the number of cells in the sample over a period of 45 minutes, and particularly in the period from 30 minutes to 45 minutes after initial irradiation. In contrast the Ti0₂ catalyst had essentially no impact on the degradation of E. coli and the change in microorganism number was the same as the system without a photocatalyst.

The composite of the invention is therefore suitable for use in the photocata lytic degradation of microorganisms. The composite is therefore suitable for use in methods of sterilisation.

The composite of the invention was formulated as a film on a glass slide. Such a film retains photocatalytic activity and is suitable for use in degrading pollutants, such as dyes. Films containing a 12.5 mole % Ag-Ti0₂ composite and Ti0₂ were prepared and tested against methylene blue. The degradation of methylene blue, as gauged a by a drop in absorbance at around 650 nm, was monitored over a period of 120 minutes. The results are shown in Figure 16. The 12.5 mole % Ag-Ti0₂ composite film significantly reduced the amount of methylene blue. Over the same time period the amount of methylene blue degraded by the Ti0₂ film was only moderate. The films comprising the composite of the invention was prepared by doctor blading a mixture of Ti0₂, HN0₃ and water onto the slide. The resulting film was heated for 1 hour at 450°C. The coated slide was then immersed in a mixture of water, AgN0₃ and

formaldehyde for 1 hour. The slide was heated to 90°C to dry for 5 minutes.

The light source was a standard UV-Vis lamp rated at 500 W/m². The test solution was 10 μΜ methylene blue solution in water. The film area for each test was about 10 cm² (corresponding to half a standard microscope slide). Other films displaying catalytic activity were also identified, including a film prepared via a sol-gel process.

A mixture of titanium isopropoxide (TTIP) in aqueous acid (HCI) was prepared and a glass slide was dipped into this mixture. The slide was then immersed for 1 hour in a mixture comprising 75 mL water, 75 ml_ formaldehyde and 0.1 g AgN0₃. The slide was heated to 90°C to dry for 5 minutes.

References All documents mentioned in this specification are incorporated herein by reference in their entirety.

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Claims

Claims:

1 . A method for preparing a photocatalytically active composite, the method comprising the steps of:

(i) providing a mixture of a semiconductor particle, a metal salt and a chemical reducing agent;

(ii) permitting the reducing agent to reduce the metal salt in the presence of the semiconductor material; and

(iii) heating the mixture,

thereby to generate a composite having a semiconductor particle decorated with one or more metal particles.

2. The method of claim 1 , wherein the semiconductor particle is a nanoparticle.

3. The method of claim 1 , wherein the semiconductor particle is a Ti0₂ nanoparticle.

4. The method of claim 2 or claim 3, wherein the average largest dimension of the nanoparticle is 25 nm or less.

5. The method according to any one of claims 1 to 4, wherein each metal particle is a nanoparticle.

6. The method of any one of claims 1 to 6, wherein metal is one or more of Au, Ag, Ir, Pt and Pd.

7. The method of claim 6, wherein the metal is Ag.

8. The method of any one of the preceding claims, wherein the absorbance of the composite at 465 nm is at a ratio of 0.60 or more in relation to the absorbance of the composite at 300 nm.

9. The method of any one of the preceding claims, wherein is step (iii) the mixture is heated to a temperature in the range 50 to 200 C.

10. The method of claim 9, wherein the mixture is heated for at least 5 minutes.

1 1 . The method of any one of the preceding claims, wherein in step (iii) the mixture is heated, and the resulting product is then ground.

12. The method of claim 1 1 , wherein the ground product is heated.

13. The method of claim 12, wherein the ground product is heated for at least 5 minutes.

14. The method of claim 12 or claim 13, wherein the ground product is heated to a temperature in the range 50 to 150°C.

15. A photocatalytically active composite obtainable or obtained by the method of any one claims 1 to 14.

16. The photocatalytically active composite of claim 15, wherein the composite is photocatalytically active under visible light irradiation.

17. The photocatalytically active composite of claim 15 or claim 16, wherein the metal particles decorating the semiconductor particles are present in at least 10 mole %.

18. The photocatalytically active composite of any one of claims 15 to 17, wherein the metal particles have an average largest dimension of less than 20 nm.

19. The photocatalytically active composite of claim 15, wherein the metal particles have an average largest dimension of less than 5 nm.

20. The photocatalytically active composite of any one of claims 15 to 19, wherein the composite comprises metal particles having a largest dimension of 1 nm or less.

21 . A method for treating a pollutant, the method comprising the step of exposing the pollutant to a composite as described in any one of claims 15 to 20 in the presence of incident visible light, thereby to degrade the pollutant.

22. The method of claim 21 , wherein the method is performed in the presence of incident visible and UV light.

23. The method of claim 21 or claim 22, wherein the pollutant is one or more pollutants selected from the group consisting of a dye, a microorganism, a prion and a virus.

24. The method of claim 23, wherein the pollutant is a dye.

25. The method of claim 24, wherein the dye is an organic dye.

26. The method of claim 23, wherein the pollutant is a microorganism.

27. The method of claim 26 wherein the microorganism is selected from E. coli. and S. aureus.

28. A method of sterilizing a sample, the method comprising the step of exposing a sample comprising a microorganism, prion and/or a virus to a composite as set out in any one of claims 15 to 20 in the presence of visible light, thereby to degrade the microorganism, prion and/or a virus.