WO2002074431A1

WO2002074431A1 - Hollow spheres from layered precursor deposition on sacrificial colloidal core particles

Info

Publication number: WO2002074431A1
Application number: PCT/EP2002/003127
Authority: WO
Inventors: Frank Caruso
Original assignee: Max-Planck-Gesellschaft Zur Förderung Der Wissenschaften
Priority date: 2001-03-21
Filing date: 2002-03-20
Publication date: 2002-09-26

Abstract

The preparation of monodisperse hollow titania spheres with defined diameter, wall thickness and crystal phase is reported. The hollow spheres have been produced by the layered deposition of a water-soluble titania precursor, e.g. titanium(IV) bis (ammonium lactato) dihydroxide (TALH), in alternation with poly(diallydimethylammonium chloride) (PDADMAC) onto submicrometer-sized template particles e.g. polystyrene (PS) particles, followed by calcination at elevated temperatures. the layer-by-layer growth of the coating on the colloid particles was observed by microelectrophoresis and transmission electron microscopy (TEM). Calcination of the TALH/PDADMAC-coated particles resulted in intact, hollow titania spheres, as confirmed by scanning electron microscopy (SEM) and TEM. Calcining the coated particles at 450 or 950° C resulted in hollow sphere consisting of titania in anatase or rutile form, respectively. Nanometer-level control over the sphere wall thickness was achieved by varying the number of layers deposited on the PS particles. The hollow titania spheres produced can be used in photonic applications, where hollow spheres of high refractive index materials are desired, and in catalysis.

Description

Hollow Spheres from Layered Precursor Deposition on Sacrificial Colloidal Core Particles

Description

The invention relates to hollow inorganic spheres and in particular to hollow titania spheres from layered precursor deposition on sacrificial colloidal core particles.

The preparation of monodisperse hollow inorganic spheres, in particular hollow titania spheres with defined diameter, wall thickness and crystal phase is reported. The hollow spheres have been produced by the layered deposition of a water-soluble titania precursor, e.g. titanium(IV) bis (ammonium lactato) dihydroxide (TALH), in alternation with po l y ( d i a l lyld i methyl a mmo n i u m ch l o ri d e ) ( PD A D M AC) o nto submicrometer-sized template particles, e.g. polystyrene (PS) particles, followed by calcination at elevated temperatures. The layer-by-layer growth of the coating on the colloid particles was observed by microelectrophoresis and transmission electron microscopy (TEM) . Calcination of the TALH/PDADMAC-coated particles resulted in intact, hollow titania spheres, as confirmed by scanning electron microscopy (SEM) and TEM. Calcining the coated particles at 450 or 950°C resulted in hollow spheres consisting of titania in anatase or rutile form, respectively. Nanometer-level control over the sphere wall thickness was achieved by varying the number of layers deposited on the PS particles. The hollow titania spheres produced can be used in photonic applications, where hollow spheres of high refractive index materials are desired, and in catalysis.

Hollow spheres have attracted attention in recent years because of their broad range of applications. For example, they are used as delivery vehicles for the controlled release of substances such as drugs, cosmetics, dyes, and inks, for the protection of biologically active macromolecules, as fillers, and in catalysis and waste removal [1 ,2\. A variety of chemical and physicochemical methods have been employed to produce hollow spheres comprised of polymer, glass, metal and ceramic materials, including nozzle reactor approaches (spray drying or pyrolysis) [3], emulsion/phase separation techniques (often combined with sol-gel processing) [4] emulsion/interfacial polymerization strategies [5], and self-assembly processes [6]. Templating colloid particles has recently become an attractive alternative means for produeing hollow spheres. For the preparation of hollow inorganic spheres, a common approach has been to coat particles in solution by either controlled surface precipitation of inorganic (silica, titania etc.) molecular precursors or by direct surface reactions utilizing specific functional groups on the cores to induce coating, followed by removal of the templated core by calcination or solvent etching [7]. These methods have generated micron-sized hollow colloids of yttrium compounds [7a] and silica [7b], as well as monodisperse hollow silica nanoparticles [7c, d]. However, precise optimization of the reaction conditions is required to obtain uniform coatings and to avoid the occurrence of aggregation of the coated particles prior to core removal [8]. Ordered polystyrene colloidal sphere assemblies have also been templated with sol-gel precursor solutions [9]. Subsequent dissolution of the core by toluene yields hollow (amorphous) ceramic spheres, provided that the sol-gel reaction is fast enough to form a gel network around the particles prior to solvent evaporation [9f]. Precursors with slower sol-gel reactions (e.g. silica) yield three-dimensional ceramic porous structures [9f].

Recently, we have introduced a colloid templating strategy that involves the layer-by-layer (LbL) self-assembly of polyelectrolyte and nanoparticles on colloids for the fabrication of hollow inorganic spheres [2, 10]. The process utilizes the electrostatic interactions between the oppositely charged species that are deposited on the particles for film growth. Multilayers of inorganic nanoparticles, bridged by interlayers of polyelectrolyte, have been formed on colloidal spheres. Upon calcination, the core template and bridging polymer are removed and the nanoparticles sinter, thereby imparting structural stability to the hollow spheres. A primary consideration for the creation of hollow spheres using this strategy is the close packing of the nanoparticles on the template spheres, as the sintering process occurs between neighboring nanoparticles. Uniform-sized hollow spheres of various diameters and wall thickness have been generated for a variety of inorganic materials, including silica, iron oxide, titania, zeolite, clay, and inorganic-heterocomposites [1 0-1 2]. The advantage of the LbL-colloid templating approach over other methods is the nanoscale control that can be exerted over the wall thickness of the hollow spheres by varying the number of deposition cycles, while the shell size and shape are determined by the morphology of the templating colloid, and the composition of the hollow spheres by the nanoparticles used [10- 1 2].

Although the LbL manipulation of preformed nanoparticles onto submicron- and micron-sized spheres has been demonstrated for the creation of hollow inorganic spheres [10-1 2], the use of inorganic molecular precursors deposited in alternation with polyelectrolyte multilayers on colloidal templates has not been examined. Here we report a versatile LbL-colloid templating method based on a water-soluble and stable inorganic precursor, titanium(IV) bis(ammonium lactato) dihydroxide (TALH) (chemical formula: [CH₃CH(O-)CO₂NH₄]₂Ti(OH)₂) that allows the generation of monodisperse hollow titania spheres with defined diameter and wall thickness. This approach offers the advantage of simplicity and does not require the separate preparation of nanoparticles prior to adsorption.

The invention therefore relates to a process for preparing coated particles comprising the steps: (a) providing template particles and (b) coating said template particles with a multilayer comprising

(i) alternating layers of oppositely charged inorganic molecular precursors, in particular titania precursors, and polyelectrolytes and/or (ii) alternating layers of oppositely charged inorganic molecular precursors, in particular titania precursors.

Inorganic molecular precursors are in particular molecules which can be converted to inorganic materials, such as inorganic ceramics, metal oxides or metals. Preferred materials, which are formed from the precursors by e.g. calcination and/or hydrolysis/condensation reactions are titania, alumina, zirconia, silica, manganese oxides, magnesium oxides, vanadium oxides, cerium oxides or combinations thereof. Suitable inorganic molecular precursors, which can be used in the process according to the inventon include, but are not limited to alkoxides (e.g. methoxides, ethoxides, isopropoxides, etc.), halides (e.g. chlorides, fluorides, iodides, etc.), sulfates, nitrates, oxides or hydroxides of metals, such as Si, Al, Zr, Mn, Mg, V, Ce or Ti. Particularly preferred are titania precursors.

Titania precursors are in particular molecules, which can be converted to titania, e.g. by calcination and/or hydrolysis/condensation reactions. Preferably, the titania precursors are water soluble and water stable, i.e. no or very little decomposition of the titania precursor molecules takes place in water. A particularly preferred titania precursor is titanium(IV) bis(amminum lactato) dihydroxide (TALH) .

In a further preferred embodiment the inorganic molecular precursors are metal precursors which can be converted to the respective metals. Such a conversion can e.g. be performed by reduction to produce metallic coatings and upon calcination hollow metal spheres. Suitable metal precursors are, e.g. metal salts and in particular salts of noble metals, such as Ag, Au, Pd, Pt, Rh or Ir. Particularly preferred as metal precursor is gold tetrachloride. Further suitable inorganic precursors, which can be used as template particles according to the invention include semiconductor nanocrystals, which are also called quantum dots, in particular semiconductor nanocrystals of the ll-IV and lll-V type according to the periodic table. Semiconductors useful in the present invention are e.g. described in WOOO/1 7656. Preferred examples include ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgTe, HgSe, HgS, InAs, InP, GaAs, GaP, AIAs and AIP.

Further suitable inorganic precursors are rare earth nanoparticles, particularly those containing Ln^{3 +}, Ce^{3 +} or Tb^{3 +} and suitable counter ions, such as VO₄ ^3', MoO₄ ^2" or PO₄ ^3", e.g. LnPO₄. By using aqua regia (75 vol% HCI : 25 vol% HNO₃) semiconductor nanocrystals and rare earth nanocrystals can be dissolved or decomposed.

Also gold nanoparticles can be used as templates according to the invention. They typically have a size of at least 2 nanometers, more preferably at least 5 nanometers, most preferably at least 1 5 nanometers but also more than 50 nanometers. The gold nanoparticle templates can easily be removed with a potassium cyanide solution, thus leaving hollow spheres.

Thus, in one aspect the invention relates to layer-by-layer (LbL) self- assembly of inorganic molecular precursors and oppositely charged polyelectrolytes.

Besides the layer-by-layer method described above, the spheres can also be made by exposing template particles coated with polymer or/and polyelectrolyte to the precursor and heating, in particular calcining in a second step. In this embodiment of the invention the inorganic molecular precursors, e.g. titania precursor molecules are impregnated into the polymer or polyelectrolyte coating around the template particles. The template particles used preferably have a diameter of at least 1 nm, in particular at least 5 nm, more preferably at least 10 nm and most preferably at least 20 nm and up to 1 0 μm, more preferably up to 5 μm. Suitable template particles may be selected from organic particles, inorganic particles or combinations thereof. In a preferred embodiment the template particles are selected from organic polymer latices, such as polystyrene or styrene copolymer latices. On the other hand, also partially cross-linked melanine-formaldehyde-template particles can be used. Preferably a template particle is used, which desintegrates under conditions of elevated temperature, e.g. at temperatures above 100°C, preferably above 400°C, or upon exposure to solvents.

Upon treatment under elevated temperature, e.g. calcination, preferably core and bridging polymers (e.g. polyelectrolyte molecules, which are present in the shell and were inserted during layer-by-layer formation of the shells) are removed, wherein hollow spheres are obtained, the shells of which exclusively consist of inorganic materials.

If the core is disintegrated by a solvent, preferably only the core is removed by the solvent and the polymer in the shell (e.g. polyelectrolytes) can remain, thus giving inorganic-organic hollow spheres. It is also possible, to select the solvent so as both the core polymer and the polymer present in the shell is removed, wherein only inorganic hollow spheres are obtained.

In a preferred embodiment the process according to the present invention comprises coating the template particles with alternating coatings of polyelectrolyte molecules and inorganic molecular precursors, particular titania precursor molecules. The polyelectrolytes are usually polymers having ionically dissociable groups, which may be a component or substituent of the polymer chain. Dependent on the type of dissociable group, polyelectrolytes are subdivided into polyacids and polybases. Upon dissociation polyacids separate protons to give polyanions. Examples of polyacids are polyphosphoric acid, polyvinyl or polystyrene sulfuric acid, polyvinyl or polystyrene sulfonic acid, polyvinyl or polystyrene phosphonic acid and polyacrylic acid.

Polybases contain groups which are capable of accepting protons, e.g. by reacting with acids to give salt. Examples of polybases are polyamines, such as polyethylene amine, polyvinyl amine and polyvinyl pyridine or poly(amonium salts) such as poly(diallyl dimethyl amonium chloride). Polybases form polycations by accepting protons. Particularly preferred is poly(diallyl dimethyl amonium chloride) (PDADMAC) or poly(allylamine hydrochloride) is used as the polyelectrolyte.

For the preparation of the coated particles according to the present application, preferably an aqueous dispersion of template particles of suitable sizes is provided. Alternating layers of oppositely charged components, i.e. polyelectrolyte molecules and titania precursor molecules or combinations thereof are then deposited on said template particles. The thickness of the coating can be adjusted by the number of layers applied and is preferably from 10 to 1 000 nm, with 4 to 40, particularly 5 to 20 coatings being applied. Suitably each layer may be comprised of a single species of polyelectrolyte or inorganic molecular precursor or of a mixture comprising at least two different species. After application of each layer, the excess molecules (e.g. polyelectrolyte or precursor) which have not contributed to forming the layer are preferably separated off before the next layer is applied. Such separation can be done according to any known method, particularly centrifugation, filtration or/and dialysis.

In a preferred embodiment of the invention the template particles are first coated with several layers of oppositely charged cationic and anionic polyelectrolytes before the alternating layers of inorganic precursor molecules and polyelectrolyte molecules are applied. Preferably, the template particles are coated with at least 2 and up to 6 layers of oppositely charged cationic and anionic polyelectrolytes, e.g. with 3 layers as precoating.

Preferably the coated template particles are further subjected to a calcination step at elevated temperature. In this calcination step the precursor molecules are converted to an inorganic material, e.g. to an inorganic ceramic, metal oxide or metal. Titania precursor molecules are e.g. converted to titania spheres. Preferably at the same time the template core particles and polyelectrolyte molecules present in the shell are disintegrated by the application of heat. Thus, hollow shells or spheres, e.g. hollow titania spheres are obtained. The spheres obtained are remarkably smooth, which makes them useful for many applications. Further, according to the invention it is possible to determine the crystal structure of the titania of the hollow titania spheres by controlling the calcination temperature. It was observed that by calcining in a temperature range from 400°C to 600°C, in particular from 430°C to 480°C, spheres consisting of titania in anatase form are obtained. By adjusting the calcination temperature to 800°C to 1 200°C, in particular to 900°C to 1 000°C rutile-phase titania spheres can be formed. These are of particular interest in photonics and catalysis. For the first time a method is presented, which allows for the formation of intact spheres consisting of titania in rutile form.

The conversion of the molecular precursor into the desired inorganic material can be carried out in any arbitrary way, e.g. by hydrolysis of the inorganic molecular precursors via chemical reactions. Preferably chemical hydrolysis and calcination are performed at the same time or subsequently in order to remove the core. The core can also be removed by solvent decomposition. In this case it is also possible to obtain inorganic-organic composite hollow spheres. A further aspect of the invention is a coated particle having a core which is a template particle and a multilayer shell comprising alternating layer of (i) oppositely charged inorganic molecular precursors, in particular molecular titania precursor molecules and polyelectrolyte or (ii) oppositely charged inorganic molecular precursors, in particular titania precursor molecules, as well as a hollow shell or hollow sphere obtainable by disintegrating the template particles of the coated particles as described above. Hollow titania shells or hollow titania spheres are preferred. However, according to the invention the shells can be formed of any inorganic material, in particular of inorganic ceramics, metal oxides or/and metals.

The hollow shells can be used for any application in which capsules are applied, such as medicine, pharmaceutics, catalysis, optics, magnetics, separation and sensing methods. The hollow shells according to the invention are particularly useful for photonics and catalysis applications.

Figure 1 illustrates the procedure used to prepare hollow titania spheres. The first stage involved the construction of polyelectrolyte/TALH multilayers on colloid particles. This comprised adding a solution of poly(diallyldimethylammonium chloride) (PDADMAC) to a colloidal suspension of negatively charged polystyrene (PS) particles, and allowing the polyelectrolyte to adsorb onto the particle surface via electrostatic interactions. Following removal of the unadsorbed polyelectrolyte after centrifugation and water washing of the particles (three times), the PDADMAC-coated PS particles were exposed to an aqueous solution of poly(styrenesulfonate) (PSS), which resulted in PSS adsorption. An additional PDADMAC layer was then deposited in identical fashion. (This precursor three-layer film provides a uniformly charged and smooth polyelectrolyte film that facilitates subsequent deposition of various species [10]) TALH, a chelating compound [1 3-1 5], was then added to the suspension of polyelectrolyte-coated PS particles. Excess TALH was removed and subsequent stepwise adsorption of PDADMAC and TALH resulted in the growth of TALH/PDADMAC multilayers. Calcination of the TALH/PDADMAC-coated particles resulted in the formation of hollow titania spheres. The success of this method for forming hollow spheres relies on the use of stable inorganic molecular precursors that can be assembled in alternation with polyelectrolytes. While titanium alkoxides hydrolyze rapidly in the presence of water, TALH is stable and hydrolyzes only slowly at ambient temperature in neutral solution [ 1 3-1 5] . This prevents significant hydrolysis and condensation reactions, which would otherwise lead to precipitation of titania in solution and possible aggregation of the coated particles. Therefore, the water-soluble and stable TALH is an ideal candidate for use in LbL deposition as it can be deposited and hydrolyzed when desired. A further requirement of this method is that the precursor loading must be sufficiently high to permit the formation of a continuous titania network upon calcination.

Qualitative evidence for the LbL growth of the TALH/PDADMAC multilayers on the PS particles was provided by microelectrophoresis measurements. The sign and magnitude of the -potential for the coated particles depended on whether PDADMAC (ca. + 40 mV) or TALH (ca. -20 mV) formed the outermost layer. Regular and reproducible alternating ^-potentials (from + 40 to -20 mV) were observed for the coated particles with additional deposition of PDADMAC and TALH, indicating that TALH can be successfully used as an anionic chelating compound in the LbL process. (TALH has been confirmed to bind to PDADMAC, as TALH/PSS multilayers did not form.) Such alternating behavior in the -potentials is indicative of multilayer growth of charged species on particles [10-1 2].

Figure 2 shows transmission electron microscopy (TEM) images of 640 nm PS colloidal spheres coated with five TALH/PDADMAC multilayers. The LbL deposition process gives uniformly coated and discrete particles. (The spheres are contacting in image (a) as a result of their drying on the TEM grid.) Both the uniformity of the multilayer coating and the surface roughness can be clearly seen on the surface of the spheres (magnified in image (b)) . Uncoated PS spheres appear smooth at these magnifications [10] . Energy dispersive X-ray analysis of the coated particles revealed the presence of Ti, confirming the deposition of TALH. Further evidence for the successful deposition of the TALH/PDADMAC multilayers is provided by the regular diameter increase that occurs with deposition of each TALH/PDADMAC layer pair. Evaluation of the data for PS particles coated with one to five TALH/PDADMAC multilayers yielded an average diameter increment of 8-1 1 nm for each TALH/PDADMAC layer pair, corresponding to an average thickness of about 4-5 nm. From previous studies on the deposition of oppositely charged polyelectrolytes on colloid particles under similar conditions to those used in this study, the thickness of each polyelectrolyte layer was determined to be approximately 2 nm [1 6]. Assuming such a thickness for the polyelectrolyte layer, each deposition cycle of TALH causes a thickness increment of ca. 2-3 nm, implying that multilayers of the precursor molecules (i.e. oligomeric clusters) are deposited with each step. This can be explained by partial hydrolysis of TALH in solution prior to deposition. It is also possible that hydrolysis might occur upon drying prior to TEM measurements. Infiltration of TALH into the polyelectrolyte layers and resultant swelling may, in part, also be responsible for such a large layer thickness observed. Nonetheless, the above data show that the thickness of the multilayer coatings can be tailored by the number of TALH/PDADMAC layers deposited. Refluxing the coated particles at 100°C resulted in titania nanoparticle/polyelectrolyte- coated PS particles (as observed by TEM), in agreement with the conversion of TALH to anatase titania nanoparticles at this temperature [1 3].

The (non-refluxed) multilayer-coated particles were subsequently calcined in order to prepare hollow titania spheres. Calcination results in removal of the PS core, (bridging) polyelectrolyte and other organic material during heating to 500°C [10], with most of the mass loss occurring between 350 and 450°C (as observed by thermogravimetric analysis). TALH is known to hydrolyze and condense at elevated temperatures to form nanoparticles [ 1 3, 1 4]. Hence, the titanium precursor that was adsorbed within the multilayer film was expected to form a titania network comprising nanoparticles upon heating. The shape, structure, and wall thickness of the resulting hollow titania spheres were examined by TEM and scanning electron microscopy (SEM).

Figure 3 illustrates TEM micrographs of the hollow spheres produced by heating PS particles coated with seven TALH/PDADMAC multilayers to 450°C. The TEM images (a and b) show a reduced electron density of the spheres (compared to the corresponding uncalcined samples; e.g. see Figure 2), suggesting that hollow particles were obtained. A wall thickness of approximately 30-35 nm is estimated by TEM (from the dark ring around the perimeter of the hollow spheres) for the seven-layer TALH coating. The diameters of the hollow titania spheres produced are approximately 1 0-1 5% smaller than those of the corresponding core-shell particles. A similar degree of shrinkage was observed in our previous work using preformed nanoparticles to produce hollow inorganic spheres [1 0-1 2]. High resolution TEM (image d) showed that the hollow titania spheres were comprised of intimately connected crystalline titania nanoparticles of approximately 6-8 nm in diameter, with lattice fringes consistent with the anatase phase (0.352 nm). This verifies the conversion of TALH to titania under the calcination conditions used. Intact hollow titania spheres were also produced in the same manner for PS particles coated with five and ten TALH/PDADMAC multilayers, but not for two or three layer coatings. (Infiltration of the precursor three-layer polyelectrolyte film alone, followed by calcination did not yield hollow titania spheres; only a very thin and fragile film of titania was produced.) From the wall thickness data for the hollow titania spheres prepared from five (25-30 nm), seven (30-35 nm) and ten (45-50 nm) TALH/PDADMAC layer pairs, the average thickness per titania layer is ca. 5 nm, which is in close agreement with the size of the titania nanoparticles formed.

SEM analyses were performed to examine the morphology of the hollow spheres. Hollow titania spheres produced by calcining to 450°C were intact and maintained the spherical morphology of the template particle (Figure 4a), provided five or more TALH layers were deposited on the PS latices prior to heating. The hollow nature of the titania spheres was verified by SEM examination of deliberately broken spheres. The calcination treatment was also performed at 950°C for seven layer TALH/PDADMAC coated PS particles in an attempt to form hollow titania spheres comprised of rutile-phase titania. An SEM micrograph of the resulting spheres is shown in Figure 4b. In all cases, the walls of the titania spheres have a macroporous structure. Large crystals of the titania can be seen in individual spheres. Electron diffraction measurements confirmed the rutile titania phase (d_hkl = 0.32 nm {1 10}, 0.25 nm {101 }, 0.21 nm {1 1 1 } and 0.1 6 nm {21 1 }). "Rutile-titania" hollow spheres are desirable for photonic applications [17] and for catalysis [18].

In conclusion, we have demonstrated a simple and effective stepwise route to the formation of hollow spheres of titania. Using the LbL-colloid templating strategy, the inorganic molecular precursor TALH has been sequentially adsorbed in alternation with PDADMAC at a density and distribution that is amenable to forming hollow titania spheres upon calcination. These hollow spheres have well-defined diameters, largely determined by that of the template, and uniform walls with thicknesses that are controlled by the number of layers deposited. Our recent work has demonstrated that polyelectrolyte multilayers can be formed on 1 5-50 nm diameter particles by LbL deposition [1 9]. However, the strategy employed here can also be readily extended to smaller (as well as larger) template particles. This method also provides a novel approach to produce other hollow metal oxide spheres, such as silica or zirconia spheres. In direct comparison to other methods, the primary advantages of this new approach are: (i) careful optimization of the solution conditions is not required to produce uniform inorganic-organic composite coatings on colloid templates, overcoming many of the problems associated with previous titanium precursor coating methods; (ii) layer deposition is conducted from aqueous solutions, hence eliminating the need for solvents used with more reactive alkoxide precursors and making it applicable to a range of (core) colloid particles dispersed in water; (iii) it is more appropriate for coating colloids in the nanometer-regime, compared with the LbL deposition of preformed nanoparticles and (iv) it avoids the separate preparation of nanoparticles prior to adsorption. In the process according to the invention the LbL deposition can be performed in aqueous solutions, in particular in solutions consiting of more than 80% (vol/vol), more preferred of more than 90% (vol/vol), particularly preferred of more than 95% (vol/vol) and most prefrered of more than 99% (vol/vol) of water. The aqueous solutions may contain salts, or buffers or other additives and preferably contain less than 1 0%, more preferably less than 5% and most preferably less than 1 % (vol/vol) of organic solvents. Most preferably an aqueous solution being free of any organic solvent is used.

The fabrication of these colloidal entities also opens a wide range of possibilities for their application. For example, since the coatings produced are homogeneous and the hollow spheres are discrete with tailorable wall thickness (as demonstrated for the anatase spheres), they are attractive candidates for the formation of colloidal crystals for photonic applications. The hollow rutile titania spheres are of special interest as they would exhibit a high refractive index contrast. Another possibility of producing hollow spheres of titania is by the direct infiltration of TALH into preformed polyelectrolyte multilayers, followed by calcination. Studies show that when the film thickness is large enough, hollow titania spheres can be obtained. While the process of the invention was described in detail in particular with reference to titania precursors, it is not limited to this preferred embodiment. The use of other inorganic molecular precursors in the LbL process is also possible within the described process.

Examples

Experimental

Poly(diallyldimethylammonium chloride) (PDADMAC), M_w < 200000, poly(sodium 4-styrenesulfonate) (PSS), M_w 70000, and poly(ethyleneimine) (PEI), M_w 55000 were obtained from Aldrich. PSS was dialyzed against Milli-Q water (M_w cutoff 1 4000) and lyophilized before use. The titanium precursor, TALH, 50% in aqueous solution, was purchased from Aldrich and diluted to 5% with water before use. Negatively charged, sulfate stabilized polystyrene (PS) latices of diameter 640 nm were prepared as described elsewhere [20]. Sodium chloride was obtained from Merck. The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system with a resistivity higher than 1 8.2 MΩ cm.

The first step in the preparation of the hollow titania spheres involved fabrication of the composite particles. Prior to deposition of TALH/PDADMAC multilayers, a precursor polyelectrolyte multilayer film (PDADMAC/PSS/PDADMAC) was assembled onto the PS particles. 50 μl of a concentrated suspension of PS spheres (3.8 wt% solution) was diluted to 1 mL, and 1 mL of PDADMAC (1 mg mL^"\ containing 0.5 M NaCI) was added. After 1 5 min for PDADMAC adsorption, the excess PDADMAC was removed by three repeated centrifugation (3800 g, 10 min) and washing cycles. PSS ( 1 mg mL^"1 containing 0.5 M NaCI) or TALH (5%) were then deposited onto the coated PS particles using the same conditions. In all cases, the concentration of the polyelectrolytes and TALH were sufficient to give saturation coverage of the particle surface. Titania nanoparticle (anatase)/PDADMAC coatings were formed on the PS spheres by refluxing the coated PS spheres in water solution for 24 h at 1 20°C. (Crystalline nanoparticle formation was verified by high resolution TEM.) Hollow titania spheres were produced by drying the coated PS spheres on quartz slides at room temperature, and then calcining (heating rate 5 K min^'1) at either 450 or 950°C under N₂ for 4 h and then for a further 8 h under O₂.

Electrophoretic mobilities of the coated PS particles were measured with a Malvern Zetasizer 4 by taking the average of 5 measurements at the stationary level, as described elsewhere [1 6]. All measurements were performed in air-equilibrated pure water (pH ~ 5.6) without added electrolyte. SEM measurements were carried out with a Zeiss DSM 940 instrument operated at an acceleration voltage of 20 kV. SEM samples (on carbon or quartz surfaces) were sputter-coated with about 5 nm of Pd or Au. TEM and electron diffraction measurements were performed with a Philips CM 1 2 microscope operated at 1 20 kV, equipped with an energy dispersive X-Ray analyzer. Samples for TEM were sonicated in water for 1 min (to redisperse the hollow titania spheres), and then deposited onto a carbon grid and allowed to air-dry. A Netzsch TG 209 apparatus was used for thermogravimetric analysis.

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Figure Captions

Figure 1 . Schematic illustration of the procedure used to construct hollow titania spheres. The scheme is shown for PS spheres. The small dots represent oligomeric clusters of the TALH precursor.

Figure 2. TEM images of PS spheres coated with five TALH/PDADMAC layer pairs, (a) Uniformly coated particles, (b) Higher magnification image showing the surface of a coated sphere. The PS spheres were first primed with a precursor trilayer of PDADMAC/PSS/PDADMAC.

Figure 3. TEM images of hollow anatase titania spheres prepared by calcination of composite particles consisting of a PS core coated with seven layer pairs of TALH and PDADMAC at 450°C. (a) A number of hollow titania spheres, (b) Higher magnification showing a single hollow titania sphere, (c) Region of a sphere showing the wall structure, (d) High resolution image displaying the crystalline nature (anatase) of the titania nanoparticles that make up the hollow spheres.

Figure 4. SEM images of hollow titania spheres prepared by calcination of PS spheres coated with seven layer pairs of TALH and PDADMAC at (a) 450 or (b) 950°C. (a) Hollow anatase (refractive index, n ~ 2.5) titania spheres, showing their spherical and intact nature, (b) Macroporous titania spheres of the higher refractive index (n ~ 2.6-2.9) rutile phase.

Claims

1 . A process for preparing coated particles comprising the steps: (a) providing template particles and

(b) coating said template particles with a multilayer comprising

(i) alternating layers of oppositely charged inorganic molecular precursors and polyelectrolytes and/or (ii) alternating layers of oppositely charged inorganic molecular precursors.

2. A process for preparing coated particles comprising the steps: (a) providing template particles and (b) coating said template particles with single- or multi-layers of polymers and/or polyelectrolytes and exposing said coated template particles to an inorganic molecular precursor.

3. The process of claims 1 or 2, wherein a precursor comprising Si, Al, Zr, Mn, Mg, V, Ce, Zn, Cd, Hg, In, Ga, Ln, Tb, Au or/and Ti is used.

4. The process of any of claims 1 -3, wherein a metal precursor or/and a metal oxide precursor is used.

5. The process of any of claims 1 -4, wherein a water stable titania precursor, in particular titanium (IV) bis(ammonium lactato) dihydroxide is used.

6. The process according to any of claims 1 -5, wherein said template particles have an average diameter from 1 nm to 10 μm and are selected from organic particles, inorganic particles or any combinations thereof, in particular from organic polymeric particles, such as polystyrene.

7. The process of any of claims 1 -6 further comprising the step: (c) calcining the coated template particles and thereby converting the inorganic molecular precursor to an inorganic material, in particular to a metal oxide or/and a metal.

8. The process of any of claims 1 -7 further comprising the step: (d) hydrolyzing the inorganic molecular precursor by chemical reactions.

9. The process of any of claims 1 -8 further comprising the step: (e) solvent decomposition of the core.

10. The process of claim 7, wherein during calcination the template particles are at least partially disintegrated.

1 1 . The process of claim 7 or 10, wherein the calcination is performed at a temperature in the range of 400°C to 600°C to form hollow spheres consisting of titania in anatase form.

1 2. The process of claim 7 or 10, wherein the calcination is performed at a temperature of 800°C to 1 200°C to form hollow spheres consisting of titania in rutile form.

1 3. Coated particle having a core which is a template particle and a multilayer shell comprising alternating layers of

(i) oppositely charged inorganic molecular precursors, in particular titania precursors and polyelectrolytes or

(ii) oppositely charged inorganic molecular precursors, in particular titania precursors.

4. Hollow spheres obtainable by a process according to any of claims 5-8.

5. Hollow spheres according to claim 14 being metal or metal oxide, in particular titania spheres.

6. The hollow spheres according to claim 14 or 1 5 containing an active agent, such as a pharmaceutical, contrasting agent, herbicide, pesticide, catalyst or/and pigment.

7. Use of the hollow spheres according to any of claims 14-1 6 in photonic applications or/and catalysis.