Patent Application
20190375653
Not applicable.
This invention provides hierarchical mesoporous decorated metal-oxide compositions and a method of synthesis of these hierarchical mesoporous decorated metal-oxide compositions.
Approaches to fabricate cellular metal-oxide structures or to decorate such ceramic scaffolds are conventionally treated as different processes and reported accordingly. The present invention produces decorated (functionalized) ceramic scaffolds in a single step avoiding such multi-step processes, reducing time and not requiring special vacuum conditions for the formation of the functionalizing materials.
The background art differs significantly from the present invention's compositions and methods of making the same, since the background art produces different output materials and uses mainly polymers that stay in the finished composition.
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The synthesis of metal-oxide decorated heterostructures is currently a time consuming and multi-step process. Typically, ceramic (metal-oxide) scaffolds are prepared first and then the decorative materials are incorporated by using multiple and repetitive infiltration routes1, or vapor-based deposition2,3. Alternative procedures decorate the primary ceramic nanoparticles first, and later form the macroscopic scaffold by pressing and sintering the decorated particles/nanomaterials in the presence or absence of a binder. Such decoration may involve UV-light photo-reduction4,5, sonochemical6 or thermal reduction processing7,8. Pre-decoration of the ceramic nanomaterials is usually performed by placing such materials in solutions containing the decorative material precursors, and centrifugation-rinsing cycles are implemented after the decorative materials are synthesized8-11, further adding steps in the processing and affecting the product yield.
With the invented method, the synthesis of hierarchical mesoporous decorated metal-oxide composites becomes a one-step process. In such step, the different composite precursors are mixed as a functional ink that can be 2D or 3D printed making the desired object/scaffold shape. Curing of this ink using UV-light or heat treatments serves two different functions: (1) to promote interconnectivity (bridging/coalescence) of the ceramic structure and (2) to induce the formation of the decorative secondary phase materials. It will be understood by those persons skilled in the art, that while the formulation of the inks requires several mixing steps, it is the synthesis of the decorated composites of this invention that is a “one step” process, since once the ink is formulated (containing all the precursors for the composites), the decorated composites are formed at once upon energy input. The synthesis of similar heterostructures relays on additional decorative materials deposition/infiltration and washing cycles.
The composites' of the present invention surface area properties, decorative material loading, and cellular configuration can be tuned via composition, by changing the relative amounts of precursors composing the multiphase systems. Because the decorated metal-oxide composites are made at a single mixing step, no centrifugation is necessary and the yield of the product is 100%, concomitantly avoiding the use of solvents and their release to the environment. Also, the precursors used are generally non-toxic and biocompatible, which makes them inherently safe and well suited for large scale materials processing, useful in a wide range of products, and facilitates their recycling and disposal.
These heterostructures may be used in applications such as H2 generation1,12-16, supercapacitors17, small molecule detection18, sensor active layers19, catalysis1,13,20-29 photovoltaics30-34, biomedicine35,36, antibacterial coatings37 and scaffolds for tissue engineering38.
The compositions and methods of the present invention provide many advantages over the background art compositions, including but not limited to, a one step process for producing the compositions, low-cost, 100% yield, environment-friendly, a homogeneous dispersion, controlled positioning of the decorative materials, and transferability to other decorative materials/ceramic composite systems.
In one embodiment of this invention a composition comprising a hierarchical cellular mesoporous metal-oxide is provided. Preferably, this composition comprises wherein the hierarchical cellular mesoporous metal oxide is a metal organic/metal oxide.
In another embodiment of this invention, a composition comprising a hierarchical cellular mesoporous metal-oxide that is a metal organic/metal oxide that is TALH:TiO2 is provided.
In another embodiment of this invention, a composition comprising a hierarchical cellular mesoporous metal-oxide is that is an Ag—TiO2 xanthan gum based film is provided. Preferably, this composition comprises said hierarchical cellular mesoporous metal-oxide that is an Ag—TiO2 oil based foam.
In another embodiment of this invention, a composition is provided comprising a hierarchical cellular mesoporous metal-oxide is the form of a three dimensional printed hierarchical based structure. Preferably, this composition comprises wherein said three dimensional structure is on a ITO/glass substrate. More preferably, this composition comprising a hierarchical cellular mesoporous metal-oxide is in the form of a film, spanning structure, or a hollow structure.
In another embodiment of this invention, a composition comprising a hierarchical cellular mesoporous metal-oxide is provided wherein said hierarchical cellular mesoporous metal-oxide is in the form of a planar hierarchical structure is provided. Preferably, this composition comprises wherein said planar hierarchical structure is on a ITO/glass substrate. More preferably, this composition comprising a hierarchical cellular mesoporous metal-oxide is in the form of a film, spanning structure, or a hollow structure.
Another embodiment of this invention provides a method of making a hierarchical cellular mesoporous metal oxide composition comprising providing a metal-organic/metal oxide aqueous phase, providing a silver-ion rich oil phase, emulsifying said metal-organic/metal oxide aqueous phase and said silver ion rich oil phase to form an emulsified component, and incorporating gas bubbles into said emulsified component by subjecting said emulsified component to frothing to form a hierarchical cellular mesoporous metal oxide composition. Preferably this method comprises wherein said metal-organic/metal oxide aqueous phase is TALH:TiO2.
In another embodiment of this invention, a method of making a hierarchical cellular mesoporous metal oxide composition is provided comprising providing a metal-organic/metal oxide aqueous phase, providing a silver acetate ethanol solution, adding triethanolamine to said silver acetate ethanol solution to form a triethanolamine silver acetate ethanol mixture, providing an oil phase, adding said triethanolamine silver acetate ethanol mixture to said oil phase to form an triethanolamine silver acetate ethanol oil phase component, evaporating said ethanol rom said triethanolamine silver acetate ethanol oil phase to form an ethanolamine silver acetate oil phase, and adding said metal-organic/metal oxide aqueous phase to said ethanolamine silver acetate oil phase to form an metal-organic/metal oxide phase dispersed in said oil phase to form a homogenized mixture of said metal-organic/metal oxide aqueous phase and said oil phase. Preferably, this method comprises wherein the metal-organic/metal oxide aqueous phase is TALH:TiO2.
In another embodiment of this invention, a method of making a hierarchical cellular mesoporous metal oxide composition is provided comprising providing a metal-organic/metal oxide aqueous phase, providing a silver precursor component solubilized in ethanol solution, mixing said silver precursor component solubilized in ethanol in a polyacrylic acid-xanthum gum solution to form a silver polyacrylic acid-xanthum gum mixture, and mixing said silver polyacrylic acid-xanthum gum mixture into said metal-organic/metal oxide aqueous phase to form said hierarchical cellular mesoporous metal oxide composition. Preferably, this method comprises wherein said metal-organic/metal oxide is TALH:TiO2.
In another embodiment of this invention, a method of making a hierarchical cellular mesoporous metal oxide composition is provided comprising providing a metal-organic/metal oxide aqueous phase, providing a silver precursor component, mixing said silver precursor component in a polyacrylic acid-xanthum gum solution to form a silver polyacrylic-acid-xanthum gum mixture, and mixing said silver polyacrylic-xanthum gum mixture into said metal-organic/metal oxide aqueous phase to form a hierarchical cellular mesoporous oxide composition.
In another embodiment of this invention, a method of making a hierarchical cellular mesoporous metal oxide composition is provided comprising providing a metal-organic/metal oxide aqueous phase, providing a silver precursor component solubilized in ethanol solution using ammonium hydroxide, mixing said silver precursor component solubilized in ethanol in a polyacrylic acid-xanthum gum solution to form a silver polyacrylic-acid-xanthum gum mixture, and mixing said silver polyacrylic-xanthum gum mixture into said metal-organic/metal oxide aqueous phase to form a hierarchical cellular mesoporous oxide composition.
In another embodiment of this invention, a method of making an oil based hierarchical cellular mesoporous metal oxide composition is provided comprising providing an oil phase composition comprising at least one of stearic acid, polyoxoethylene sorbitan monostearate, and lanolin, providing a silver precursor component solubilized in ethanol solution, adding at least one of ethanolamine or triethanolamine, or both, to said silver precursor component solubilized in ethanol to form an ethanolamine or triethanolamine or ethanolamine/triethanolamine and silver precursor component ethanol mixture, evaporating said ethanol from said ethanolamine or triethanolamine or ethanolamine/triethanolamine and silver precursor component ethanol mixture to form an ethanolamine or triethanolamine or ethanolamine/triethanolamine and silver precursor component mixture, providing a TiO2 and TAHL aqueous solution, adding polyacrylic acid to said TiO2 and TAHL aqueous solution to form a polyacrylic acid TiO2 and TAHL mixture, adding said polyacrylic acid TiO2 and TAHL mixture to said oil phase composition at a temperature of about 70 degrees centigrade to produce a homogenized mixture, and incorporating gas bubbles into said homogenized mixture to form an oil based hierarchical cellular mesoporous metal oxide composition.
In another embodiment of this invention, a method of making an oil-free hierarchical cellular mesoporous metal oxide composition is provided comprising providing a TiO2 and TAHL and deionized water aqueous solution, providing a polyacrylic acid and a xanthan gum aqueous solution, adding said polyacrylic acid and a xanthan gum aqueous solution to said TiO2 and TAHL and deionized water aqueous solution to form a polyacrylic acid and a xanthan gum TiO2 and TAHL and deionized water aqueous solution, and incorporating gas bubbles into said polyacrylic acid and xanthan gum TiO2 and TAHL and deionized water aqueous solution to form an oil free based hierarchical cellular mesoporous metal oxide composition oil free hierarchical cellular mesoporous metal oxide composition.
In another embodiment of this invention, a method of making an oil-free silver decorated foam hierarchical cellular mesoporous metal oxide composition is provided comprising providing a TiO2 and TAHL and deionized water aqueous solution, providing a polyacrylic acid and a xanthan gum aqueous solution, adding said polyacrylic acid and a xanthan gum aqueous solution to said TiO2 and TAHL and deionized water aqueous solution to form a polyacrylic acid and a xanthan gum TiO2 and TAHL and deionized water aqueous solution, providing a silver acetate solution, adding ethanol to said silver acetate solution by solubilizing said ethanol and silver acetate solution with the addition of ammonium hydroxide aqueous solution to form an ethanol silver acetate solution, and wherein the silver acetate:ammonium hydroxide ratio is 1:9 mol, adding said ethanol silver acetate solution to said polyacrylic acid and xanthum gum aqueous solution to form a homogenized ethanol silver acetate polyacrylic acid xanthum gum aqueous solution, adding said homogenized ethanol silver acetate polyacrylic acid xanthum gum aqueous solution to said TiO2 and TAHL and deionized water aqueous solution to form an ethanol silver acetate polyacrylic acid xanthum gum TiO2 and TAHL and deionized water aqueous solution, and incorporating gas bubbles into said ethanol silver acetate polyacrylic acid xanthum gum TiO2 and TAHL and deionized water aqueous solution to form an oil free silver decorated foam hierarchical cellular mesoporous metal oxide composition.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. As the color drawings are being filed electronically via EFS-Web, only one set of the drawings is submitted.
) oil-based foam-ink of this invention.
) oil-based foam-ink, Xanthan Gum baseline (▪), and Xanthan Gum-based Ag-decorated TiO2 (□) precursor inks.
Multiphase Materials Systems: Encapsulating Materials and Controlling the Nucleation of Ag on TiO2 Hierarchical Mesoporous Structures
The previously studied Ti-organic/TiO2 foam emulsion system1, can be further used for the synthesis and selective positioning of a secondary phase material on the ceramic scaffold, to realize a hierarchically ordered mesoporous composite material such as a metal-ceramic composite, as depicted in
The technology of the present invention is the process to make composite materials based on a metal-oxide and a decorating material, where the latter is encapsulated in a liquid phase complementary (different) to that metal-oxide (ceramic). The referenced material1 M. A. Torres Arango, et al, ACS Sustain. Chem. Eng., 2017 acssuschemeng. 7b02450, serves as a preliminary study and foundation for the making the ceramic on its own (without any decorative/functionalizing features). In other words, the referenced material1, discloses a method to make plain ceramic foams. In contrast, the present invention discloses the simultaneous synthesis of the decorative materials (of variable nature: metallic, metal-oxide, organic molecules, etc.) and the scaffold (metal-oxide) in the composites. Such a process of the present invention did not exist previously. Appendix I sets forth the background art technology of reference1.
In one embodiment of the method of the present invention, the oil phase of an oil-in-water emulsion system is used to encapsulate a secondary phase material or their precursors, and induce their formation by using an external energy source or by using a chemical catalyst. Alternatively, in order to reduce the thermal budget associated to the composites' transformation from the emulsion state to the final functionalized ceramic, the use of hydrocolloids as foam stabilizers2,3, and as secondary phase materials dispersion medium is disclosed. Food hydrocolloids, being widely studied and abundant rheology control compounds4,5, highly compatible with living organisms, are great substitutional candidates for the realization of environment-friendly materials. Remarkably, the stabilization role of chitosan polysaccharide hydrocolloids has been reported previously for the fabrication of cellular Si-based soft materials6.
In the multiphase system of this invention, diverse composite groups are provided: metal-oxide/metal, metal-oxide/organic-molecule materials, metal-oxide/semi-metallic materials, and metal-oxide-I/metal-oxide-II composites. The former (i.e. metal-oxide/metal composites) are widely used for enhanced light harvesting applications including H2 generation7-12, small molecule detection13, catalysis7,9,14-22 and photovoltaics23-26 as well as for biomedical applications27,28 and antibacterial coatings29. Metal-oxide/organic-molecule composites on the other side, are commonly used in optoelectronic systems such as dye-sensitized solar cells30 and biocompatible systems such as scaffolds for tissue engineering31, and catalysts32. Other composites such as metal-oxide/semi-metallic materials find applications as catalysts and as sensor active layers33, whereas multi metal-oxide composites find applications such as supercapacitors34 and other energy related due to the important interface effects35.
To illustrate an embodiment of the method of the present invention, metal-oxide/metal composites (i.e. Ag-decorated TiO2 foams) are synthesized. In the described method, the Ag decorations could be included as prepared particles dispersed within the phase complementary to the titania suspension, or they can be formed within such complementary phase from a solution including a Ag-precursor. The complementary phase (oil-based or hydrocolloid-based) serves as controlling agent of the secondary phase (decorative) material nucleation sites (i.e. the Ag particles will nucleate where the encapsulating phase reaches onto the metal-oxide structure), and to control the nucleation and growth rate (depending on the viscosity, pK□ properties, and thermal decomposition of such complementary phase). Nucleation of inorganic nanoparticles in oil-in-water emulsions has been demonstrated previously36. However, its simultaneous synthesis, with that of mesostructured metal-oxide cellular architectures is herein provided for the first time. This method produces more complex ceramic composites.
For the synthesis of Ag-decorated TiO2 foam, the previously studied metal-organic/metal-oxide (TALH:TiO2) suspension37 and a Ag ion-rich oil phase are emulsified, and frothed to incorporate the gas bubbles that lead to macropore features. The only deviation from the plain TALH:TiO2 foams synthesis procedure1, is the triethanolamine (TEA) addition to the Ag acetate ethanol solution to induce the Ag-precursor solvation, instead of at the last mixing step before frothing. Once the Ag precursor is solubilized in ethanol, it is added to the oil phase. The ethanol solution mixes readily with the oil phase constituents at ˜70° C., and the blend remains clear and does not change its color. The Ag oil phase is stirred continuously to evaporate the ethanol, and once no volumetric changes are observed, the TiO2 suspension is added and allowed to homogenize by continuous stirring while closed. Once the oil-phase and the titania suspension are visibly blended, frothing with a wisk-like mechanical mixer is performed. The viscosity of the decorated foam systems is comparable with the plain (non-decorated) foams as indicated by the dynamic viscosity measurements (see
For the hydrocolloid-based complementary phase, xanthan gum (XG) is used to reduce the amount of organic species used in the synthesis of Ag—TiO2 structures, compared to the oil-based formulations. This formulation uses the same TALH:TiO2 suspension as base, but instead of dispersing the Ag precursor in an oil-phase, it is mixed in the PAA-XG (polyacrylic acid-xanthum gum) solution and incorporated to the titania mixture. Since very small amounts of XG are required to significantly increase the viscosity of aqueous solutions38-40, a more efficient transformation of TALH into TiO2 and the nucleation of the decorative secondary phase (Ag(0)) could be expected upon energy treatments such as UV or thermal-annealing. For this formulation, the Ag precursor was first solubilized in ethanol. A minimum amount of ammonium hydroxide was used to ensure its solvation. This solution was completely clear before mixing with the XG-PAA solution, upon mixing with such solution it became slightly turbid which is indicative of sedimentation/reduction of Ag in similar solutions41. Particularly, this reaction is characteristic of metallic Ag nucleation from a Tollens' reaction in contact with a polysaccharide compound like xanthan gum42. Then, the solution was immediately added to the TiO2-TALH suspension and frothed as customary. The processing was performed in the dark to avoid uncontrolled light-induced changes of the ink.
To prepare the oil-based Ag-decorated foams, the oil phase constituents (stearic acid (SA), polyoxoethylene sorbitan monostearate (P60), and lanolin) are mixed and stirred until homogeneous at ˜70° C. (Centigrade). The Ag-acetate solution (Ag-acetate in ethanol, and the corresponding ethanolamine or triethanolamine) is added to this mixture and allowed to homogenize, afterwards the solvent is allowed to evaporate while stirring. In parallel, the TiO2 particles and TAHL aqueous solution are mixed and sonicated, and the PAA is dropwise added to the TiO2 mixture and further mixed and sonicated until becoming homogeneous. The latter, is dropwise added to the oil phase Ag-rich solution and complete homogenization is allowed at ˜70° C. Once homogeneous, the mixture is cooled down by stopping the heat while magnetically stirring. The air bubbles are then introduced with the aid of an electric wisk-like frother for ˜6-8 min. The Ag precursor added, is calculated to yield 1.2 wt % of metallic Ag from the total TiO2—Ag final composite (i.e. excluding organic molecules).
For the synthesis of the oil-free Ag-decorated foams, xanthan gum is used as viscosity enhancer. To prepare the inks, appropriate amounts of TiO2 nanoparticles are mixed with TALH and DI water. The word “appropriate” amounts will be understood by those skilled in the art as a fitting description of the amount because depending on the ratios of TALH:TiO2 foams with different surface area properties and pore interconnectivity may be realized as described in reference 1 of the reference section. For the specific example of the hydrocolloid-based route, the exact relative amounts used are disclosed in the concentration column of Table 2 in mol. (i.e. for this example for each 1 mol of TALH, 12 mol of TiO2 are used). In the case of the oil based, the amounts are disclosed similarly in Table 1 (but in this case in wt %)—this because the lanolin does not have a standardized molecular weight that would allow us to make the calculations to relate everything in mol. Below is the complete composition of the oil-based ink in the example (aqueous and oil phases). The L:S:O (liquid-solid-oil) ratios and TiO2:TALH ratios have important effects on the surface area properties and porosity as has been explained in reference 1 of the reference section. Their mention in the technical document intends to highlight the versatility of the composition in controlling these properties as has been demonstrated in such reference.
The mixture of TiO2 nanoparticles, TALH and DI water is stirred for 15 min (minutes), and sonicated in a water/ice bath for 15 min to ensure thorough dispersion (while occasionally stirring to prevent sedimentation). In parallel a PAA-Xanthan gum (XG) aqueous solution is prepared and added dropwise to the titania mixture while stirring. The resulting mixture is sonicated for 15 minutes and set to stir for 15 more minutes before frothing. For the encapsulation of the Ag ions, Ag-acetate is mixed in ethanol and solubilized by adding ammonium hydroxide aqueous solution. The Ag-acetate:NH4OH ratio is (1:9) in mol. This Ag-rich solution is added to the PAA-XG solution and allowed to homogenize before being added to the titania mixture. Aluminum foil is wrapped around all vials containing Ag to prevent light induced reactions or degradation. The inks are frothed for ˜8-20 min using a wisk-like attachment and a mechanical mixer to incorporate air bubbles. The Ag content is kept identical, and equivalent to 1.2 wt % of the final TiO2—Ag composite.
The difference in the morphology of the Ag-decorated composites (oil-based) and (XG-based) is apparent from the SEM and optical microscope images, where the foams stabilized using hydrocolloids rapidly allowed the solvent (water) to evaporate, resulting in the collapse of the gas macropores and the re-arrangement of the structure as films. See
Ambient light exposure of the films results in their staining, indicative of reactive systems. The dried films are observed to be more stable to light exposure (against staining), than when exposed while wet; with the oil based foam system exhibiting the highest stability as observed from the optical microscope images. Such staining is observed to be blocked at the films' surface, as the collapsed pores of the XG-based Ag—TiO2 non-sheared sample do not exhibit such staining, having collapsed while in the dark, after initial sample surface exposure to light (
SEM images of the Ag-decorated TiO2 foams are shown in
For the XG-based Ag—TiO2 film, mud-cracking is observed (see
Both XG-stabilized inks (Ag-decorated and baseline) result in similar film morphologies as observed from
XRD patterns for the Ag-decorated TiO2 oil-based foam (see
The XRD patterns for the XG-based inks (see
EDS information for the Ag—TiO2 composite systems were collected (see Appendix A, infra) and show gradual removal of the organic species as more energy is supplied in the post-processing treatments. Table 3 summarizes the different C:Ti and Ag:Ti atomic % ratios for the samples treated under such conditions. It is observed that almost no signal from Ag is obtained for the oil based samples, and may be explained by the small volume of sample (thin and porous); and the large amount of organics present, hindering the detection of Ag (in minimum concentration compared to the other elements detected). Nevertheless, Ag is detected for the 300° C.-20 min treatment, which significantly removes the organic species associated to the oil phase of the foam. The latter is evident from the C:Ti ratio, that decreases as more energy is provided. In contrast, the lower organics content in the XG Ag-decorated TiO2 composites shows readily the Ag content even for the lower energy treatments, and exhibits a less pronounced decrease of the C:Ti ratio.
EDS mapping of the XG- and oil-based Ag—TiO2 composites (see
Despite the challenges encountered to obtain quantitative information about the Ag decorations from EDS and XRD (especially for the oil-based foam), the Ag—TiO2 composites exhibit interesting coloration changes suggesting the presence of nanoparticles on the surface of TiO244, see
The optical bandgap of the studied samples, as calculated using the Tauc's relationship (eq. 1)45, is observed to vary slightly according to the energy treatment administered, see Table 4. All the obtained values are characteristic of TiO2 (3.1 eV-rutile; 3.3 eV anatase)46. Tauc plots and UV-Vis spectra of the different Ag—TiO2 oil-based and XG-based TiO2 films are presented in
In the Tauc plots (
(αhv)2=A(hν−Eg)n (1)
Where n=½ for direct band gap semiconductors, A is a constant, hv is the photon energy, and □ is the absorption coefficient of the semiconductor. The latter in turn, can be calculated from Eq. 2, where k is the absorbance as measured using UV-Vis spectroscopy.
The conduction band energy (Ecb) of the decorating Ag nanoparticles has been calculated based on the UV-Vis information using the Einstein photon energy equation (Eq. 3) and is also included in Table 4.
Where h is the Plank constant, c is the speed of light, and λmax the wavelength of maximum absorbance for the Ag peaks form the UV-Vis spectra. Our results for the Ecb of these nanoparticles are comparable with those obtained for spherical Ag nanoparticles of similar diameter47. The Ag nanoparticles sizes for the representative Ag—TiO2 composite systems have been determined from TEM as is discussed later in this application.
It is therefore of great interest to understand the nature of such changes in the Eg, since they may originate from localized effects of the Ag nanoparticles on the TiO2 surface (also known as band edge bending)48; as well as from the effect of the Ag nanoparticles on the crystallization of TiO2 favoring the rutile phase formation at remarkably low temperatures49,50. In these multiphase systems, where some of the TiO2 is crystallizing from TAHL, the Ag nanoparticles may induce such crystallization to take the rutile phase instead of the anatase polymorph, as has been reported for TiO2—Ag nanocomposites prepared from TiCl4 using photodeposition and annealed at 600° C.49. Typically, for the temperature ranges explored in this work (up to 500° C.), the crystallization of TALH into TiO2 from suspension formulations without Ag, occurs in the anatase phase37. However, the influence of the Ag nanoparticles (nucleating and growing in the proposed composite systems), may cause a deviation of such behavior. Such influence is expected to be significantly higher when using the XG-based Ag—TiO2 formulations, than when using the oil-based Ag—TiO2 route. The reason being, that in the oil-based Ag—TiO2 foams, the nucleation and growth of the Ag nanoparticles is expected to be generally smaller than in the oil-free XG systems, for equivalent energy treatments, due to the lower viscosity of the XG-based formulations. Also, since less energy is required to decompose the added XG, than the oil phase used as gas stabilizer of the TiO2-TALH aqueous suspension, the available energy is expected to nucleate the Ag nanoparticles more efficiently in the XG case. Additionally, since the Ag precursor is solubilized in an aqueous solution (for the XG case), the TiO2 surfaces are readily available for the Ag ions to nucleate. In contrast, during the synthesis of the Ag—TiO2 foams from the oil-based formulations, a significant amount of energy is used for the removal of the organics composing such phase (in which the Ag-precursor is solubilized). Therefore, for the Ag nanoparticles to induce the anatase to rutile phase transition, such particles would need to reach the sites where TALH is transforming. For such system (oil-based), the possible Ag nucleation sites are within the oil phase (where enough Ag+ ions cluster), and/or at the boundaries between the oil phase and the aqueous titania suspension. The latter being the preferred case as TiO2 surfaces in the suspension would provide ordered nucleation sites, as well as photoreduction centers51 leading to Ag nuclei, in the case of the samples subjected to UV light treatments.
In order to confirm the nucleation of Ag nanoparticles from the investigated systems, representative samples, with presumably larger Ag nanoparticles were imaged using TEM.
In the TEM images of the Ag—TiO2 composites, large TiO2 formations can be distinguished. Scattered around such formations, smaller Ag nanoparticles are observed as highlighted with the dashed ovals and arrows. In contrast, the images taken from the primary TiO2 particles, exhibit overall smaller and more uniform sized TiO2 particles that aggregate forming clusters. The difference in particle aggregation for the latter, contrasting the Ag—TiO2 systems can clearly be observed from
The formed Ag particles, exhibit spherical shape with diameter sizes ranging from ˜2-4 nm for the 500° C. treated oil-based Ag—TiO2 foams; and from ˜2-3 nm and ˜3-10 nm, for the XG-based Ag—TiO2 composites treated at 300° C. and 500° C., respectively. Inset
The coloration of the samples, is indicative of different nanoparticle sizes. Also, the Ecb of metal nanoparticles is dependent on their size47,52,53; thus, a correlation between the Ag-decorating nanofeatures and the calculated Ecb can be established.
X-ray photoelectron spectroscopy (XPS) was used to investigate the oxidation state of the nucleated Ag nanoparticles and confirm their metallic state, as well as to assess the transformation of the samples with the respective energy treatments. For the oil-based Ag-decorated TiO2 foam composites, the intensity of the Ag3d, Ti2p and O1s peaks is observed to increase as more energy is supplied, see
The XPS data collected for the XG-based Ag-decorated TiO2 composites displays a general decrease of the C1s peak, which is associated to the organics removal with increasing energy (see
Additionally, no significant shift is observed in the binding energy of the Ti2p peak, when comparing it to the spectra for the TiO2:TALH system as shown in the XPS results of the previously studied TALH:TiO2 system37.
Due to the incomplete removal of the organic species from the foams/inks studied when using low energy treatments (UV and 150° C.), it may be argued that the nucleated secondary phase materials correspond to mixtures of the Ag(0) with AgO, Ag2O; and possibly Ag2CO3. Calculation of the modified Auger parameters (AP)54-56 from the XPS data, indicate values ˜726 eV (see Table 55), which are characteristic of the Ag(0), i.e. metallic Ag57,58. Additionally, the non-significant change observed in the binding energy for the Ag3d5/2 peak among different energy treatments, can be taken as strong suggestion of the metallic character of the particles forming within the system.
Quantitative information on the XPS composition of the investigated samples is included in Table 6. The O:Ti ratio obtained is in all cases higher than 2 (the stoichiometric value for TiO2); this ratio is observed to decrease as more energy is supplied to the oil-based Ag—TiO2 foam systems, whereas it is kept relatively constant for the XG-based Ag—TiO2 films. The Ag content is generally found to be higher than that measured from EDS (see Table 3). Because of the higher vacuum conditions met by the XPS equipment compared to the vacuum in the EDS instrument, the XPS values could be considered more accurate.
XPS survey and detailed scans for the different Ag—TiO2 systems and the primary TiO2 particles are included in Appendix A set forth below.
In another embodiment of this invention, the method, as described herein, includes, for example, but not limited to, other metal-oxide/organic complex and decorative materials, and their combinations. For example, ZnO—Ag composites can be prepared, using zinc acetate in place of TALH(metal-organic precursor), ZnO nanoparticles in place of TiO2 nanoparticles (metal-oxide). In the same way, for example, but not limited to, the decorative material may consist of gold, or other metallic or semi-metallic nanoparticles.
Those persons skilled in the art understand that the present invention using the proposed and investigated multiphase emulsion material systems, secondary phase functionalizing features can be produced within the emulsion system, by dispersing/encapsulating their precursors in a complimentary phase of the emulsion and inducing their nucleation using energy treatments such as UV-light exposure or sintering. This method enables a novel, sustainable and relatively simple route, for the fabrication of hierarchically ordered cellular mesoporous ceramics with embedded functional nanofeatures. A distinction between the oil-based Ag—TiO2 foams and the xanthan gum-based Ag—TiO2 composites should be made, since the latter do not yield cellular macropore structures for the XG concentrations used in this invention. The xanthan gum concentrations utilized are relatively low thus not providing enough stabilization for the macropores (i.e. gas bubbles), since these are observed to collapse upon drying of the film and/or applied shearing stress. Characterization of the Ag—TiO2 composites subjected to the different energy treatments was successfully done through XRD, EDS, SEM, XPS, UV-Vis spectroscopy and TEM. The nucleated nanoparticles are found to be in the metallic state as highlighted from the XPS results, and exhibit spherical shape with sizes below 10 nm. The distribution of the Ag nanoparticles is observed to be rather uniform on the TiO2 surface. This investigation using Ag on TiO2 structures, provides an alternative approach for the nucleation of secondary phase materials on metal-oxide structures with controllable microstructures, useful across multiple applications from energy to biomedical, including H2 production though enhanced light-harvesting devices, photovoltaics, small molecule detection, catalyst, water cleaning and regeneration systems, and bio-compatible materials.
In one embodiment of this invention a composition comprising a hierarchical cellular mesoporous metal-oxide is provided. Preferably, this composition comprises wherein the hierarchical cellular mesoporous metal oxide is a metal organic/metal oxide.
In another embodiment of this invention, a composition comprising a hierarchical cellular mesoporous metal-oxide that is a metal organic/metal oxide that is TALH:TiO2 is provided.
In another embodiment of this invention, a composition comprising a hierarchical cellular mesoporous metal-oxide is that is an Ag—TiO2 xanthan gum based film is provided. Preferably, this composition comprises said hierarchical cellular mesoporous metal-oxide that is an Ag—TiO2 oil based foam.
In another embodiment of this invention, a composition is provided comprising a hierarchical cellular mesoporous metal-oxide is the form of a three dimensional printed hierarchical based structure. Preferably, this composition comprises wherein said three dimensional structure is on a ITO/glass substrate. More preferably, this composition comprising a hierarchical cellular mesoporous metal-oxide is in the form of a film, spanning structure, or a hollow structure.
In another embodiment of this invention, a composition comprising a hierarchical cellular mesoporous metal-oxide is provided wherein said hierarchical cellular mesoporous metal-oxide is in the form of a planar hierarchical structure is provided. Preferably, this composition comprises wherein said planar hierarchical structure is on a ITO/glass substrate. More preferably, this composition comprising a hierarchical cellular mesoporous metal-oxide is in the form of a film, spanning structure, or a hollow structure.
Another embodiment of this invention provides a method of making a hierarchical cellular mesoporous metal oxide composition comprising providing a metal-organic/metal oxide aqueous phase, providing a silver-ion rich oil phase, emulsifying said metal-organic/metal oxide aqueous phase and said silver ion rich oil phase to form an emulsified component, and incorporating gas bubbles into said emulsified component by subjecting said emulsified component to frothing to form a hierarchical cellular mesoporous metal oxide composition. Preferably this method comprises wherein said metal-organic/metal oxide aqueous phase is TALH:TiO2.
In another embodiment of this invention, a method of making a hierarchical cellular mesoporous metal oxide composition is provided comprising providing a metal-organic/metal oxide aqueous phase, providing a silver acetate ethanol solution, adding triethanolamine to said silver acetate ethanol solution to form a triethanolamine silver acetate ethanol mixture, providing an oil phase, adding said triethanolamine silver acetate ethanol mixture to said oil phase to form an triethanolamine silver acetate ethanol oil phase component, evaporating said ethanol rom said triethanolamine silver acetate ethanol oil phase to form an ethanolamine silver acetate oil phase, and adding said metal-organic/metal oxide aqueous phase to said ethanolamine silver acetate oil phase to form an metal-organic/metal oxide phase dispersed in said oil phase to form a homogenized mixture of said metal-organic/metal oxide aqueous phase and said oil phase. Preferably, this method comprises wherein the metal-organic/metal oxide aqueous phase is TALH:TiO2.
In another embodiment of this invention, a method of making a hierarchical cellular mesoporous metal oxide composition is provided comprising providing a metal-organic/metal oxide aqueous phase, providing a silver precursor component solubilized in ethanol solution, mixing said silver precursor component solubilized in ethanol in a polyacrylic acid-xanthum gum solution to form a silver polyacrylic acid-xanthum gum mixture, and mixing said silver polyacrylic acid-xanthum gum mixture into said metal-organic/metal oxide aqueous phase to form said hierarchical cellular mesoporous metal oxide composition. Preferably, this method comprises wherein said metal-organic/metal oxide is TALH:TiO2.
In another embodiment of this invention, a method of making a hierarchical cellular mesoporous metal oxide composition is provided comprising providing a metal-organic/metal oxide aqueous phase, providing a silver precursor component, mixing said silver precursor component in a polyacrylic acid-xanthum gum solution to form a silver polyacrylic-acid-xanthum gum mixture, and mixing said silver polyacrylic-xanthum gum mixture into said metal-organic/metal oxide aqueous phase to form a hierarchical cellular mesoporous oxide composition.
In another embodiment of this invention, a method of making a hierarchical cellular mesoporous metal oxide composition is provided comprising providing a metal-organic/metal oxide aqueous phase, providing a silver precursor component solubilized in ethanol solution using ammonium hydroxide, mixing said silver precursor component solubilized in ethanol in a polyacrylic acid-xanthum gum solution to form a silver polyacrylic-acid-xanthum gum mixture, and mixing said silver polyacrylic-xanthum gum mixture into said metal-organic/metal oxide aqueous phase to form a hierarchical cellular mesoporous oxide composition.
In another embodiment of this invention, a method of making an oil based hierarchical cellular mesoporous metal oxide composition is provided comprising providing an oil phase composition comprising at least one of stearic acid, polyoxoethylene sorbitan monostearate, and lanolin, providing a silver precursor component solubilized in ethanol solution, adding at least one of ethanolamine or triethanolamine, or both, to said silver precursor component solubilized in ethanol to form an ethanolamine or triethanolamine or ethanolamine/triethanolamine and silver precursor component ethanol mixture, evaporating said ethanol from said ethanolamine or triethanolamine or ethanolamine/triethanolamine and silver precursor component ethanol mixture to form an ethanolamine or triethanolamine or ethanolamine/triethanolamine and silver precursor component mixture, providing a TiO2 and TAHL aqueous solution, adding polyacrylic acid to said TiO2 and TAHL aqueous solution to form a polyacrylic acid TiO2 and TAHL mixture, adding said polyacrylic acid TiO2 and TAHL mixture to said oil phase composition at a temperature of about 70 degrees centigrade to produce a homogenized mixture, and incorporating gas bubbles into said homogenized mixture to form an oil based hierarchical cellular mesoporous metal oxide composition.
In another embodiment of this invention, a method of making an oil-free hierarchical cellular mesoporous metal oxide composition is provided comprising providing a TiO2 and TAHL and deionized water aqueous solution, providing a polyacrylic acid and a xanthan gum aqueous solution, adding said polyacrylic acid and a xanthan gum aqueous solution to said TiO2 and TAHL and deionized water aqueous solution to form a polyacrylic acid and a xanthan gum TiO2 and TAHL and deionized water aqueous solution, and incorporating gas bubbles into said polyacrylic acid and xanthan gum TiO2 and TAHL and deionized water aqueous solution to form an oil free based hierarchical cellular mesoporous metal oxide composition oil free hierarchical cellular mesoporous metal oxide composition.
In another embodiment of this invention, a method of making an oil-free silver decorated foam hierarchical cellular mesoporous metal oxide composition is provided comprising providing a TiO2 and TAHL and deionized water aqueous solution, providing a polyacrylic acid and a xanthan gum aqueous solution, adding said polyacrylic acid and a xanthan gum aqueous solution to said TiO2 and TAHL and deionized water aqueous solution to form a polyacrylic acid and a xanthan gum TiO2 and TAHL and deionized water aqueous solution, providing a silver acetate solution, adding ethanol to said silver acetate solution by solubilizing said ethanol and silver acetate solution with the addition of ammonium hydroxide aqueous solution to form an ethanol silver acetate solution, and wherein the silver acetate:ammonium hydroxide ratio is 1:9 mol, adding said ethanol silver acetate solution to said polyacrylic acid and xanthum gum aqueous solution to form a homogenized ethanol silver acetate polyacrylic acid xanthum gum aqueous solution, adding said homogenized ethanol silver acetate polyacrylic acid xanthum gum aqueous solution to said TiO2 and TAHL and deionized water aqueous solution to form an ethanol silver acetate polyacrylic acid xanthum gum TiO2 and TAHL and deionized water aqueous solution, and incorporating gas bubbles into said ethanol silver acetate polyacrylic acid xanthum gum TiO2 and TAHL and deionized water aqueous solution to form an oil free silver decorated foam hierarchical cellular mesoporous metal oxide composition.
3-D printing of hierarchically ordered cellular materials with tunable microstructures is a major challenge from both synthesis and scalable manufacturing perspectives. A simple, environment-friendly, and scalable concept to realize morphologically and microstructurally engineered cellular ceramics is herein demonstrated by combining direct foam writing with colloidal processing. These cellular structures are widely applicable across multiple technological fields including energy harvesting, waste management/water purification, and biomedical. Our concept marries sacrificial templating with direct foaming to synthesize multiscale porous TiO2 foams that can be 3D printed into planar, free-standing, and spanning hierarchical structures. The latter, being reported for the first time. We show how by varying the foam-inks' composition and frothing conditions, the rheological properties and foam configurations (i.e. open- or closed-cell) are tuned. Furthermore, our printing studies indicate a synergy between intermediate extrusion pressures and low speeds for realizing spanning features. Additionally, the dimensional changes associated to the post-processing of the different foam configurations are discussed. We investigate the effects of the foams' composition on their microstructure and surface area properties. Additionally, the foams' photocatalytic performance is correlated with their microstructure, improving for open-cell architectures. The proposed synthesis and scalable manufacturing method can be extended to fabricate similar structures from alternative ceramic foam systems, where control of the porosity and surface properties is crucial; demonstrating the great potential of our synthesis approach.
Ceramic based foams are highly desirable material systems because of their ability to mimic hierarchical organization widely existing in biological organisms59. Such organization is beneficial in numerous applications from catalysis60 to energy harvesting61 and storage62, to biomedical63. Developments on the colloidal processing of such ceramics are of great interest because of the versatility that colloidal science brings to manufacturing. Currently, there is a burgeoning interest in the additive fabrication of foam-based hierarchical mesoporous structures. Despite the early studies demonstration of advantageous mechanical properties of Al2O3 structures64,65, the employed synthesis methods utilize relatively large amounts of acid reagents to stabilize the particles and concomitantly gas bubbles forming the pores of the system, which represents challenges for their safe manipulation and large-scale implementation. Furthermore, such studies report exclusively closed-cell foam architectures, limiting the range of synthesized materials. Direct foaming is considered the most promising fabrication route for foam 3D printing (among the established synthesis methods: replica, sacrificial template and direct-foaming66), because of the ability to control viscosity and prepare the foam as a patternable extrudate, while producing different microstructure and porosity configurations67.
The present invention, emphasizes on the development of sustainable and relatively simple synthesis methods and formulations, to produce hierarchically ordered mesoporous cellular ceramics with tunable cell configurations (i.e. closed-/open-cell ceramic foams) and surface area properties. We investigate the design, synthesis, direct writing and post-processing of multi-phase TiO2-based wet-foams, controlling their morphology, microstructure and photocatalytic response. In particular our work addresses 8 of the 12 principles of green chemistry68 for the developed materials and synthesis approach. Waste prevention is accomplished by incorporating additive manufacturing (i.e. 3D printing), since all the prepared foam batches can be printed in the exact amounts, geometries and substrate locations. The atom economy (AE) is estimated from our ink design calculations and thermogravimetric analysis (TGA). This synthesis method and resulting materials implement the less hazardous chemical synthesis principle, by involving non-toxic, renewable and bio-compatible ink precursors. The oil phase of the foams consists of fatty acids compounds commonly found in the cosmetic industry69. Similarly, the use of ethanolamine and triethanolamine as emulsifiers is kept to minimum amounts, also comparable to those encountered in cosmetic products70. In addition, the utilization of TALH as Ti-organic precursor, allows the formulation of aqueous based systems, exhibiting very slow hydrolyzation rates in neutral pH conditions71, hence avoiding the need to use organic solvents, and allowing ample time for their printing in ambient conditions. These considerations, make our ink system inherently safe and therefore transferable to industry. Finally, the synthesized foams could be recycled/regenerated72, and are generally safer than the primary TiO2 nanoparticles for applications such as water purification, being larger in size for easier recovery in case of accidental release to the environment.
We use TiO2 as a model system, having important applications due to its interesting semiconducting properties, tunable band gap, photocatalytic properties, bio-compatibility and abundance. Our approach represents clear progress in the fabrication of TiO2 foams, traditionally fabricated using laborious multi-step methods73-77. Some of the challenges of the conventional synthesis methods include the repetitive impregnation (or incorporation) and calcination removal of organic templates76, or the time sensitive handling of rapid-hydrolyzing of liquid-liquid73,75, and gas-liquid74 emulsions systems. More recent methods, include the decomposition reaction of TiCl4 within aqueous-organic solvent mixtures, in which the pores are generated by HCl toxic fumes as a by-product of the hydrolysis of the Ti-precursor77. Thus, our synthesis approach using abundant, water-compatible, non-toxic material precursors signifies a pivotal advance in the realization of hierarchically ordered mesoporous structures with applications ranging from photocatalysis76,78,79 to biomedical (as bone-scaffold structures80) to opto-electronics81,82 and hydrogen production77.
Foam Ink Synthesis and Characterization. The foam is prepared by mixing separately the aqueous and oil phases, then combining them and frothing the resulting emulsion to incorporate the air bubbles. For the aqueous phase, appropriate amounts of Ti(IV) bis(ammonium lactato) dihydroxide (TALH) (50 wt % in H2O)—Sigma Aldrich, deionized (DI) water, and TiO2 nanoparticles Aeroxide® P25 (70% Anatase, 30% Rutile)—Sigma Aldrich, were mixed and stirred for ˜15 min in a closed container. The mixture was then set in a sonication water/ice bath for 15 min to ensure thorough dispersion of the TiO2 particles while occasionally stirring to prevent sedimentation. After this, a polyacrylic acid (PAA)—Product #323667, Sigma Aldrich—solution in water (mixed beforehand) was dropwise added while stirring, and set for 15 min of sonication. Such solution consists of a DI water to PAA mole ratio of 20. The mixture was set to stir for at least 1 h (hour) before further mixing. The TALH:TiO2 mole ratio was kept to 1:12 for all formulations (unless otherwise noted); the total TALH concentration was 0.15 and 0.3 M, for the L75-S3-O22 and L75-S5.5-O19.5 formulations, respectively. Also, the TALH:PAA ratio was kept equal to 1 for all experiments.
In parallel, stearic acid 97% (SA)—Acros Organics, polyoxoethylene sorbitan monostearate (P60)-Alfa Aesar, and lanolin-Sigma Aldrich where mixed in a closed container while heating at 80° C. constituting the oil phase. Once homogeneous, the aqueous phase was added to this oily mixture (dropwise while stirring), and rapidly closed to prevent solvent evaporation. The mixture was stirred at 350 rpm and allowed to homogenize. Then, ethanolamine (MEA)-Fisher Scientific, or triethanolamine 98% (TEA)-Alfa Aesar, was added and stirred for ˜30 seconds (enough time to be visibly blended). Finally, this mixture (oil phase+aqueous phase+MEA/TEA) was frothed with a wisk-like mechanical mixer HS583R Ovente, for 8 min. The ratio of the oil phase constituents (including the MEA or TEA) in weight % was (SA 33.44: P60 33.39: Lanolin 27.84: MEA/TEA 8.34); and the mole ratio of SA to TALH was 2 and 1 for the L75-S3-O22 and L75-S5.5-O19.5 formulations, respectively. The MEA or TEA were included as part of the oil phase, for the final liquid-solid-oil (L-S-O) volume ratio calculation purposes.
Apparent viscosity measurements of the foams were taken for foams frothed after 4 and 8 min, using a Brookfield DV-II+Pro rotational viscometer.
Foam Printing and Sintering. The foams were loaded into syringe-type cartridges and printed on ITO/glass substrates using a Nordson JR2300N robotic arm, equipped with a Performus V pneumatic pressure ink dispenser system. Different codes producing planar or 3-dimensional (3D) paths were used to fabricate films and spanning structures, and hollow column-like structures, respectively. The printing speed and extrusion pressure were varied from 2 to 15 mm/s, and from 2 to 30 kPa, respectively to study the printing space for these foam systems. The parameters chosen for printing the films used for photocatalysis were 8.2 kPa and 5 mm/s, covering an area of 2 cm2. For all the prints, SmoothFlow™ tapered plastic nozzles of 580 μm inner diameter were used at ˜500 μm dispensing height. After drying for approximately 30 min, the “green” foam printed structures were sintered at 500° C. for 30 min with a heating rate of 5° C./min and a cooling rate of 1° C./min, in a muffle furnace equipped with a SMART-3 fuzzy logic temperature controller Temp, Inc.®. Thermogravimetric analysis (TGA) of the foams was performed utilizing a TA Q500 analyzer, in air using a heating rate of 10° C./min.
The hydrophobic sol-gel coating was prepared by using a variation of the method developed by Banerjee et al.83 Briefly, tetraethoxysilane (TEOS), and perfluoropolyether-alkoxysilane (PFPE) in a mole ratio of 1:0.005 TEOS:PFPE were mixed until homogeneous. In parallel, a HCl aqueous solution (1:0.025 water:HCl mole ratio) was added dropwise to the TEOS-PFPE mixture while stirring. The solution was left under stirring for 24 h, after which ethanol was added to match a mole ratio of 1:3.75 water:ethanol. The substrates were dip-coated using a KSV Instruments dip-coater with a withdrawal speed of 50 mm/min and allowed to dry for approximately 30 min. Finally, the coated substrates were cured at 200° C. for 4 h with heating and cooling rates of 5° C./min.
Sintered Foam Characterization. X-ray diffraction (XRD) was performed with a PANalytical X'Pert Pro X-ray diffractometer with power settings of 45 kV and 40 mA. The data were analyzed with the aid of the X-Pert Highscore Plus PANalytical software. Optical microscope images were taken with a Dino Edge-Digital programmable optical microscope. Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4700 SEM machine at 10 kV accelerating voltage and 15 mm working distance. Thickness measurements for the directly written foam films and 3D structures before and after sintering, were obtained though image analysis of optical micrographs using Image? software (NIH). Microporosity analysis (by monitoring nitrogen gas absorption) was performed with an ASAP2020 accelerated surface area and porosity system Micromeritics®; degassing of samples for microporosity measurements was performed using a Micromeritics vac-prep system for 24 hours (no heat) and 3 h at 110° C.
Photocatalytic Characterization. Heterogeneous photocatalysis degradation of a 10 μM methylene blue hydrate (MB)—Acros Organics—aqueous solution, was performed by placing the printed foam-films in identical beakers containing 20 ml of solution. A control solution was placed in identical conditions and labeled as “Blank”. The samples were left to stabilize for 30 min in the dark to allow for dye adsorption on the TiO2 prior to being exposed to UV light. UV irradiation at 254 nm wavelength was performed in a SpectroLINKER™ XL-1500 Spectroline® UV crosslinker chamber. The samples were positioned at ˜9.5 cm distance from the bulbs, with an average intensity of 6000 μW/cm2.
Light absorbance of MB solutions was measured through UV-Visible spectroscopy using a JAZ UV-Vis spectrometer, OceanOptics. Solution aliquots were placed in polystyrene disposable cuvettes and analyzed every 20 min of continuous UV exposure. The spectra were recorded from 300 to 850 nm wavelength.
This embodiment of this invention is a liquid-solid-gas foam-ink formulation, which is a hybrid synergistic strategy that combines the direct foaming and sacrificial template mechanisms, to produce porous structures with features in the micro-, meso- and macro pore range, using environment—friendly and abundant material precursors. The foam consists of crystalline TiO2 nanoparticles in suspension with TALH as an organic Ti-precursor. The macro-pores of the foam structure are generated by frothing of the hybrid TALH/TiO2 mixture, while stabilization of the gas phase is attained using an oil phase and surfactants. In these systems, additional stabilizing effects can be attributed to the TiO2 particles, helping to maintain the trapped air bubbles, as is characteristic of multiple emulsion systems84,85. Once the ink-foam is prepared, it can be shaped into the desired dimensions using continuous-flow direct writing and allowed to dry. The deposition of these foams through additive manufacturing, virtually eliminates any waste sources by printing the required structures' amount of material at specific substrate locations. Next, upon heat post-processing treatments, the TALH (in contact with the TiO2 primary particles) is transformed into TiO2 aiding the bridging between neighboring particles86,87 and providing mechanical and chemical stability to structures. Also, during such treatments, the organics, used as the emulsion surfactants and stabilization agents of the wet-foams, are removed leaving additional pores.
When preparing the foams, their viscosity is observed to increase by one order of magnitude when the frothing time is increased from 4 to 8 minutes. In The apparent viscosity measured for foams with different liquid-solid-oil (L-S-O) phase ratios in vol %, indicates enhanced stability as the amount of oil phase is increased. These foams allow for more retention of gas (air) in the colloidal emulsion; in turn, resulting in larger viscosity values. The stabilization role of the oil phase, is attributed to the reduction of the gas-slurry surface tension and to the modification of their viscoelastic properties88 (i.e. it provides the emulsion with yield-stress fluid properties). Similarly, the use of different surfactants (MEA vs. TEA), and TALH:TiO2 ratio is also observed to modify the viscous properties of the foams.
Viscosity of the TALH:TiO2 1:12 foams after 4 min and after 8 min frothing time, from different liquid-solid-oil content (vol %) formulations were obtained For paste-like fluids such as the ones obtained with our method, the Herschell-Bulkley model (see Equation 1), indicates shear-thinning flow regimes. Analysis of the shear stress—shear rate relationships of the foams gives Herschell-Bulkley coefficients as indicated in Table 1. The values of the flow index n, are characteristic of shear-thinning fluids being between 0 and 1. τ is the total shear stress and τy the fluid yield-stress; k denotes the fluid consistency89. The shear stress dependence on shear rate is included as
τ=τy±kγ.n (1)
Foam flow exhibits both solid-like and liquid-like behavior. In particular, one observes a yield stress τy; the value of the yield stress is also related to the foam's morphology, especially for dry foams90. The value of the yield stress typically decreases as the liquid volume fraction increases. For our foams systems, this can be observed in the lower shear-stress values for the foams with lower TiO2 primary particles content. However, it is observed that the type of surfactant used (i.e., MEA or TEA) also affects the foams' rheology. Other aspects of solid-like behavior are a finite shear modulus and slip at solid surfaces. Liquid-like aspects are a shear-thinning viscosity and time-dependent properties. Unless stabilized, the bubbles can collapse, reducing the volume of the initially frothed system as time elapses90. The compressibility of the foams is also an important factor that influences its rheology and is mostly responsible for the foam's elastic properties upon compression stress application—such as that exerted during CDW. Theories of foam flow predict that, in shear flow, foam viscosity η is generally given by:
η=τγ_/γ.+ηp (2)
In which γ. is the shear rate and ηp is the plastic viscosity. The latter quantity arises due to dissipation in the liquid film, and its value is proportional to the viscosity of the liquid phase. This term is significant only for wet foams or those formulated with extremely viscous liquids. The first term on the right of the above equation represents the elastic component, and its value increases as the gas volume fraction is increased90.
Using our foam-ink system in combination with continuous-flow direct writing (CDW), the fabrication of planar and 3D free-standing and spanning foam structures is investigated. CDW employs a PC-controlled translation stage that moves relative to a device (i.e. a dispensing nozzle) to pattern the ink91. This method has been introduced a few years ago92 and has recently re-emerged due to its inherent ability to extrude a wide range of viscosities93 thus enabling significant ink design freedom and fabrication of planar and 3D film/pattern architectures94 with resolution comparable to inkjet printing. This TiO2-water compatible system is desirable since it restricts the use of organic solvents thus promoting sustainable and scalable manufacturing. Moreover, from our preliminary studies we observe that the rapid hydrolysis of emulsions using non-aqueous Ti n-alkoxides as the organic precursor, typically used in TiO2 nanomaterials synthesis95 inhibit the printing of emulsions due to clogging of the dispensing nozzles.
We carried out printing of planar and 3D TiO2 foams, including spanning structures for a distance up to 5 mm. These structures are observed to retain their shape, while they slightly adhere to the hydrophilic glass edges. Printing of spanning structures on hydrophilic and pre-treated substrates with hydrophobic coatings show a difference on their substrate-wetting, being greater for hydrophilic substrates. This difference is expected because of the large amount of aqueous phase in the foam formulation, and proves useful for controlling the ink/substrate interactions. Despite of the surface treatment, adhesion of the foams to the substrates was favorable, which we attribute to the amphiphilic nature of the foam. Generally, for CDW, the ink formulations should display shear-thinning behavior that allows for the printed structures to be formed, and retain their shape once printed. The extrapolated yield stress for the investigated foam systems ranging from ˜40 to 100 Pa, allows for their extrusion and shaping at relatively low pressures. Our values (summarized Table 1), are comparable to those obtained for systems used to fabricate mullite spanning structures96 and lay around the lower limit of those reported as necessary for inks to withstand their own weight across a spanning distance without collapsing, typically ranging from 1 to 1000 Pa.196-99. The values reported here are viewed with respect to the differences between the foam system used in our work and those reported for gel systems98 as well as the foam nature of our ink system being significantly different from a degassed slurry. In addition, we 3D printed hollow columns and investigated their dimensional changes before and after sintering.
We observed the printing space for the investigated TiO2 foam systems using 580 μm nozzles inner diameter at 500 μm dispensing height. We printed 3-dimensional layered hollow columns, 3-dimensional layered, and spanning foam structures, at 9.6 kPa and 5 mm/s on hydrophobically treated glass. Scale bars at 1 mm long.
When using CDW to print these foam systems, we find that the main printing parameters that affect the printing fidelity are speed and pressure (for a given dispensing nozzle size and dispensing height). In the printing space, we observe three main regions, including overflow at relatively high dispensing pressures and lower speeds, due to the high volumetric flow rate of the foams. The second regime, the printable region, is confined between ˜6 kPa and ˜20 to 28 kPa of dispensing pressure. Finally, a poor-edge definition region is found at pressures below ˜6 kPa, where the fidelity of the printing is compromised yielding wavy-shape features due to inconsistent flow and dimensional filament changes. In the printable region, at lower speeds (i.e. ˜1-7 mm/s), spanning structures can be fabricated while maintaining the dispensing pressure at intermediate levels. The latter may be attributed to the extrudate being structurally coherent and stressed to a relatively small extent at such speed range. At higher speeds, the extrudate is elongated enough for the gas and oil phases to significantly deform and eventually collapse. However, foam printing of planar structures can be performed for a significantly wider range of printing conditions. In addition, it is found that the printing resolution (i.e. line width) generally increases as speed is increasing and pressure is decreasing within the planar printing regime. The resolution of the printed features is typically improved up to a nozzle diameter in width.
We have identified different means of control of the foams' pore size, configuration (as open- or closed-cell), and surface area properties; via emulsions' composition. The amounts of solids (TiO2 primary particles) and solvent in the foams affect their viscosity, and therefore the mobility of the emulsion's liquid phase as the frothing progresses. This is yielding open-cell foam structures as the solvent amount is increased with respect to the amount of TiO2 primary particles, i.e. larger L:S ratio; and conversely closed-cell foams as this ratio is decreased. Similarly, the use of MEA or TEA as surfactants, with different viscosities and pKa properties, affects the foam's viscosity and mobility, resulting in slightly different foam microstructures. For the same L:S ratio, when using MEA, slightly larger macropores can be distinguished when compared to TEA. We investigate both TEA and MEA, since TEA is aiding towards higher ink viscosities, which are desired for 3D printing. In addition, it is observed that the larger macro-pores—corresponding to trapped air bubbles—are larger in size for the foams with lower TiO2 primary solids, suggesting that such formation is strongly influenced by the particle aggregation behavior, observed in our previous studies of the aqueous phase of the foam86. Specifically, it is postulated that the TiO2 nanoparticles tend to aggregate at the periphery of the gas bubbles, and when high in concentration, lead to macro-pore size suppression as the solvent is evacuated. Similar relationships between viscosity and foam pore size distribution have been reported for pH-particle stabilized Al2O3 foam systems8, 100, 101. The foam from the (1:6) TALH:TiO2 ratio aqueous suspension, is leading also rather open-cell configurations, strengthening the latter observation since there is more Ti-organic with respect to TiO2 primary particles, and therefore the inter-particle interactions are affected.
We performed low-magnification optical microscope and SEM images of the open-cell foam of this invention, and low-magnification optical microscope and SEM images of closed-cell foams of this invention. Scale bars corresponded to 200 μm for optical microscope images, and to 100 μm for SEM images. Thermo-gravimetric analysis profiles for the different foam formulation were performed as discussed infra.
The formation of a crust in the printed foams, common to all foams prepared, exhibits smaller pore sizes when compared to the inner regions of the foams. Such crust is characteristic of solvent drainage within the foam, coupled with rapid solvent evaporation at the printed structures' outer surface. A slight elongation of the pores along the printing direction can be distinguished at the crust surface for the foams with higher TiO2 content, which is attributed to the effects of the shear stress exerted to the foams while printing. This is ultimately providing further means to induce some ordering in these mesoporous structures, as it is known that paste-like systems exhibit plastic memory when subjected to externally applied stresses102. In particular, it has been reported that the alignment of dispersed nanomaterials using CDW is likely to occur when their characteristic size is comparable to the dispensing nozzle diameter103, thus, the present invention's foams, being colloidal suspensions, can be considered as similar systems, where the “nanomaterials” are replaced by gas bubbles or micellar formations of the oil-aqueous/TiO2 mixture. However, at the inner regions of the foams, the drained solvent contributes to the relaxation and coalescence of inner gas bubbles, after the shear stress is applied during printing. Scanning electron microscopy (SEM) observation of the inner surface of the foams' pores show similar TiO2 particle assembly characteristics irrespectively of the foam morphology, i.e. open- or closed-cell. However, slightly rougher inner macro-pore surfaces are observed in the case of L75-S5.5-O19.5 MEA foam, which correlates with the difference in the measured BET surface area (Table 2—Appendix I section). This can be used to further control the microstructure of these foams systems based on the choice of emulsifiers.
The effects of tuning the microstructure and morphology of the foams can be observed in their post-processing (i.e. sintering), surface properties and functional performance (e.g. heterogeneous photocatalysis). TGA (thermogravimetric analysis) curves of the different foams were performed and exhibited inflection points at ˜150° C., 350° C. and 450° C. These correspond in order to: end of solvent evaporation, organics decomposition (i.e. TALH to TiO2 transformation and oil phase decomposition), and amorphous TiO2 to anatase phase transformation, in agreement with previous observations for similar TiO2-TALH systems86. The studied formulations yield different amounts of TiO2 depending on their constituents. The AE, defined as the % mass ratio between the target compound (material) and its precursors104, 105 can be estimated from our materials design and the TGA results. Thus, the foams formulated with higher amounts of primary TiO2 particles (5.5 vol %)—corresponding to 18.17% of the total weight of the initial wet-foams, yield ˜21.95% of TiO2 after sintering as measured using TGA, (19.68 wt % theoretical, also AE). Similarly, the foams with smaller amounts of primary TiO2 particles (3 vol %), corresponding to 9.91 wt % of the initial wet-foams, yield ˜13.31 wt % of TiO2 solids after sintering (TGA measured); with a theoretical AE of 10.74 wt %. The increase in TiO2 after sintering is expected because the TALH (Ti-organic complex), is also transformed into TiO2. Here, the choice of TAHL:TiO2 ratio significantly affects the yield of TiO2 solids, increasing to 23.13% (TGA measured) when using 1:6 TALH:TiO2 mol ratio, theoretical AE of 24.23%. The sintered foams correspond to 92.3 wt % TiO2 primary particles and 7.7 wt % TiO2 from TALH for the 1:12 TAHL:TiO2 mol ratio formulations; and to 75 wt % TiO2 primary particles and 25 wt % TiO2 from TALH, for the 1:6 TALH:TiO2 mol ratio formulations, respectively. Therefore, it is found that the TALH:TiO2 ratio has a great effect in changing the foams' surface properties as highlighted from the BET measurements (see Table 2—Appendix I section). Additional sources of variability between the expected TiO2 solids yield and the experimentally obtained, may be attributed to the loss of weight due to solvent volatilization during frothing.
Dimensional changes associated with sintering of ceramics are usually expected to occur and still need further understanding on a fundamental level59. In our case, we postulate that by dispersing the oil phase to an already continuous aqueous phase—containing the TiO2 primary particles and TALH—the aqueous network remains coherent. During frothing, the oil droplets become smaller and aid in the stabilization of the gas bubbles; which in turn may serve as pressure reservoirs influencing the volatilization of the solvent in the foam cell walls. Then, as the solvent is evacuated, the oil-stabilized primary particles tend to agglomerate and remain suspended in the predominantly oil scaffold. The melting temperature of the oil phase may play an additional role by solidifying and providing a stronger scaffold as it increases. Here the cells—when open—serve as solvent evacuation pathways, which can diffuse towards the outer surface of the printed object (crust), or towards the inner cell cavities. The extra surface for solvent evaporation, provided by the open-cell microstructures, helps to equilibrate rates of solvent diffusion (within the colloidal suspension towards the evaporation front), and the evaporation rate at the liquid-gas interface, which has been reported as a key mechanism to prevent cracking of ceramic colloidal films106. Next, upon sintering, further shrinkage occurs when the oil phase is eliminated. However, the aggregation forces between the particles prevent the collapse of the foam structure by self-locking the primary particles thus limiting their relative displacement as sintering progresses. Printed hollow open-cell column shrinkage is found to lay around 16% in all directions; whereas closed-cell foam configurations exhibited a greater shrinkage in the z-direction of ˜25%. In the case of planar structures, anisotropic shrinkage is observed due to the constrained x and y bonding of the printed layer to the substrate, this change is approximately 25% in the z-direction for open-cell structures and 37% for closed-cell structures, respectively. The greater dimensional change experienced by the foams with closed-cell structures is attributed to the effects of the inter-particle forces, being larger for higher particle concentration formulations. These are weaker for open-cell architectures, as opposed to closed-cell foams, where the TiO2 primary content is higher, since the compressive yield-stress associated to volumetric changes is known to be strongly dependent on the particle-particle interactions106. Additionally, it is postulated that the inner surfaces of the open-cell foams, being larger in relation to the TiO2 wall thickness, as observed using SEM, provide larger stress-release surfaces. Finally, the correlation of the volumetric contraction and the weight decrease after sintering is also indicative of the strong particle-particle interactions influence in the microstructural evolution of these foams. Since it is observed that such contraction is minimum for those foams experiencing larger weight decrease with sintering (i.e. L75-S3-O22 foams), as observed from TGA.
Photocatalysis results indicate that the foam films with open-cell structures perform better than their closed-cell counterparts in degrading MB, see Table 2-Appendix I section; where the apparent first-order degradation rate constant kapp are greater (with respect to the respective solids content and surfactant type). This result can be attributed to the inherent ability of open-cell structures to facilitate better circulation of the MB solution, and their availability for decomposition at TiO2 photoactive surfaces. The heterogeneous photocatalytic degradation of MB over time was observed. In the case of closed-cell structures, the TiO2 network is more compact, thus inhibiting the diffusion of UV light through inner regions of the film. On the other hand, the open-cell structures allow a greater depth of UV light diffusion. In addition, it is observed that the films with closed-cell structures (i.e. L75-S5.5-O19.5 MEA and TEA), exhibit similar performance, suggesting no significant effects from the use of MEA or TEA as surfactants, but rather stronger dependence on the macropore size—light interactions. A slight improvement is noticed for the 1:6 TALH:TiO2 TEA formulation with respect to its 1:12 counterpart; again suggesting a stronger photocatalytic activity dependence on the circulation properties than on the amount of measured Brunauer-Emmet-Teller (BET) surface area. Specifically, a correlation between the photocatalytic performance and cell configuration can be established, since such performance is maximized for open-cell architectures, despite exhibiting generally lower surface area.
Additionally, the roughening or smoothening the surface of the TiO2 primary particles (and consequently foam walls), can be induced based on the nucleation and crystallization of secondary TiO2 from TALH86. Porosimetry measurements shown in
XRD studies confirm the primary particles' properties to drive the microstructural characteristics of the foams, where the TiO2 crystallite size show no significant coarsening even after the 500° C. sintering process. The broadening of the XRD characteristic peaks with respect to those of the primary TiO2 particles, is attributed to the formation of nanocrystallites from the TALH transformation into TiO2 which lowers their average size, as reported in our previous work86. The change in crystallite size for the 1:6 TALH:TiO2 foam formulation, shows an increase of about 11.4 and 3.4% for the rutile and anatase crystallites, respectively. The latter result, contrasts those for the TALH:TiO2 1:12 systems which exhibited a general decrease up to ˜6.8%. Furthermore, the calculated lattice parameters, with a standard deviation below 0.0118 Å with respect to those reported in literature108 display a general decrease—with respect to that calculated for the primary TiO2 particles—that accentuates more as the TALH amount is increased. Such decrease is maximum for the 1:6 formulation and it is attributed to the larger amount of N available (from TALH). This excess amount of N may result in interstitial or substitutional doping109 of the TiO2 in place of O, due to their similar atomic size and electronegativity.
As can be observed through our analysis of the proposed foam systems, the different compounds utilized for their synthesis are bio-compatible, non-toxic and water-stable. This signifies that the foams' processing does not require special humidity or vacuum conditions, and that there is ample time for these materials' manipulation. Moreover, because of the excellent bio-compatibility of all of the foam precursors (besides TiO2), the synthesized cellular structures are natural candidates for their bio-functionalization or implementation as biocomponents. We have shown how, our ceramic foam synthesis approach offers fine tailoring of the morphology, surface area and pore-size distributions; and mass/light transport interactions (photocatalytic performance) based on the careful selection of the TALH:TiO2 ratio, L:S:O ratio, and primary TiO2 particle properties. Moreover, our method enables the 3D printing of these hierarchically ordered mesoporous structures as free-standing and spanning architectures by tuning the rheological properties of the yield-stress fluid formulations.
In need of pioneering approaches for responsibly engineered materials and manufacturing, we demonstrate a practical and relatively simple route to synthesize TiO2 foams (that could be extended to other ceramic materials systems), using colloidal emulsions consisting of a Ti-organic complex—TiO2 particle's suspension as the aqueous phase, and controlling their morphology as printable open- or closed-cell foam architectures. We investigate their printing space using continuous-flow direct writing, and show that spanning structures can be fabricated at relatively low writing speeds and intermediate dispensing pressures due to minimum-to-moderate stretching of the printed foam filament. In addition, we postulate a potential mechanism for the dimensional change, as the printed foam-inks are transformed into sintered solids, when using heat treatments up to 500° C. We illustrate the differences in photocatalytic performance between open- and closed-cell printed foams, arising from the electron-photon interactions and macropore size dependence, i.e., from the more efficient circulation of the dye solution to the photocatalytically-active sites of the foam, and the improved diffusion of light within the larger macropore open-cell foam structures. The use of inexpensive, innocuous, and bio-compatible materials implemented in these foams, may hold the key for their implementation and scalable 3D printing, enabling a plethora of advanced technological applications.
3D, 3-dimensional; AE, atom economy; TALH, titanium (IV) bis(ammonium lactacto) dihydroxide; DI, deionized; PAA, polyacrylic acid; SA, stearic acid; P60, polyoxoethylene sorbitan monostearate; MEA, eltanolamine; TEA, triethanolamine; L-S-O, liquid-solid-oil volume ratio; TEOS, tetraethoxysilane; PFPE, perfluoropolyether-alkoxysilane; XRD, X-ray diffraction; SEM, scanning electron microscopy; TGA, thermo-gravimetric analysis; MB, methylene blue; UV, ultra-violet; CDW, continuous-flow direct writing; BET, Brunauer-Emmet-Teller; BJH, Barrett-Joyner-Halenda.
It will be appreciated by those persons skilled in the art that changes could be made to embodiments of the present invention described herein without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited by any particular embodiments disclosed, but is intended to cover the modifications that are within the spirit and scope of the invention, as defined by the appended claims.
This patent application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 62/681,327, filed on Jun. 6, 2018. The entire contents of U.S. Provisional Patent Application Ser. No. 62/681,327, is incorporated by reference into this patent application as if fully written herein.
| Number | Date | Country | |
|---|---|---|---|
| 62681327 | Jun 2018 | US |
