Site-Selective TiO2 Coating on Asymmetric Patchy Particles

Sep 12, 2017 - The successful fabrication of TiO2-faced asymmetric patchy particles consisting of polystyrene and a Fe3O4/SiO2/TiO2 part is described...
0 downloads 27 Views 4MB Size
Download PDF
Subscriber access provided by Gothenburg University Library

Article

Site-selective TiO2 Coating on Asymmetric Patchy Particles Gurudas Mane, Jeganathan Akilavasan, Emmanouil Passas-Lagos, and Frank Marlow Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02001 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

manuscript

Site-selective TiO2 Coating on Asymmetric Patchy Particles G. Mane,a J. Akilavasan,a E. Passas-Lagosa and F. Marlow*ab a

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany. E-mail: [email protected] b Center for Nanointegration Duisburg-Essen (CENIDE), University of DuisburgEssen, 47057 Duisburg, Germany *To whom correspondence should be addressed ((s)) Supporting Information (Supp. Info.) Abstract: The successful fabrication of TiO2-faced asymmetric patchy particles consisting of polystyrene and a Fe3O4/SiO2/TiO2 part is described. Such particles can be of large interest for photochemistry. The used site-selective coating approach demonstrates a modification strategy for a special patchy particle family which may have a general character. The stability of the coating has been tested under several conditions with no sign of disruption. The influence of the prepared asymmetric particles on oil/water phase mixing behavior was tested and turned out to be very diverse. Oils with low polarity (e.g. hexadecane, cyclohexane, octadecane) can form Pickering emulsions by the help of these particles; oils with high polarity (e.g. 1-octanol) form monodisperse macro droplet systems with unusual stability.

1. Introduction Since de Gennes mentioned Janus particles (JPs) in his Nobel lecture in 1992, they have attracted a lot of attention in many interesting areas of research such as self-assembly,1 biomaterials,2 handling of protein units,3 and molecular machines,4 and nanomotors.5 In catalysis, these particles can inspire to think about applications at phase boundaries or the creation of bifunctional catalysts6. Here, special catalytically active materials need to be incorporated into the JPs, e.g. transition metal oxides or noble metals. Photocatalytic investigations would require especially TiO2 and related materials to be incorporated in the JPs. Therefore, we focus on this material in this paper. Since there are a wide variety of JPs resulting from the different synthesis approaches, it is useful to specify their kind more in detail. Loget and Kuhn7 introduced a useful terminology enabling the easy distinction of similar Janus-like particles. In this terminology, “asymmetric patchy particles” (APPs) describe our products well and will be used throughout the paper. 1 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 14

The synthesis of APPs remained challenging over a long period due to the requirement of an asymmetric particle structure. However, important developments in preparation have been achieved in last years. Several methods have been reported especially towards the “bottom up” approach for the APPs synthesis using masking,8,9 microfluidics,10 biphasic electrojetting,11 twinhead electrospraying,12 seeded emulsion polymerization,13 polymerization-induced dewetting,14 or partial embedding into a polymerizing droplet.15 More techniques for the preparation of APPs are summarized in several review articles. 1, 16-18 Herein we report the successful fabrication of TiO2-faced APPs named as PS&Fe3O4/SiO2/TiO2 with an organic polystyrene (PS) part by a wet-in-wet approach. It demonstrates a modification strategy for one APP family connected with a highly defined and stable incorporation of TiO2. The APPs used in this work have been fabricated by the Feyen approach.15 This approach delivers very monodisperse and equally shaped particles with a high yield, but with a few kinds of materials only. In the published works,15,19 combinations of different iron oxides, silica, and PS have been shown. Therefore, application possibilities are a bit restricted up to now. Incorporation of specific semiconductor materials in this APP structure will extend the application possibilities, e.g. to photocatalysis. TiO2 as a well-known photocatalyst is of interest for our APPs for which we consider photocatalysis as the most important application. An example for that are oil spills, which are a big environmental problem worldwide. Biological remediation is a way for tackling with them, but suffers from certain limitations. Therefore, photocatalytic degradation of oil spills could be an attractive additional solution. For an effective degradation of oil spills, the photocatalyst should be in close vicinity of the oil/water interface and they must be located in a stable manner there. The APPs introduced here show an extreme interfacial activity and highly stable incorporation into the oil/water interface. There, they can be exposed to sunlight and can become a part of a photocatalytic reaction system. Investigations of that require however extensive tests and will therefore be a part of future works. TiO2 incorporation into APPs was also investigated with other synthesis methods. For example, Liu et al. reported the synthesis of APPs of PS and TiO2 using an emulsion-swelling approach.20 This and other works realize, however, very specialized systems and they show only restricted tuning possibilities. Having in mind that the APPs are intended to be used in elaborated multi-functional systems, it is desirable to develop methods applicable for entire particle classes. We think the proposed selective coating process has this general potential.

2. Experimental Details 2.1. Chemicals. For the particle synthesis, glycidyl methacrylate (GMA, ≥ 95%) was purchased from TCI. Iron (III) chloride hexahydrate (≥ 99%), divinyl benzene (DVB, technical grade, mixture of isomers), and tetraethyl orthosilicate (TEOS, 98.0%), titanium butoxide (TBOT), and titanium diisopropoxide bis(acetylacetonate) (Ti-AcAc) were obtained from SigmaAldrich. Hydrochloric acid (37%-38%) and ethanol for washing (99.8%) were purchased from J. T. Baker. Isopropanol (≥ 99.9%, Chromasol V), ammonium peroxodisulfate (≥ 98.0%), ammonium hydroxide solution (28%), and iron(II) chloride tetrahydrate (≥ 99.0%) were purchased from Fluka. DVB and styrene (> 99%, Fluka) were freshly distilled at 50 °C under vacuum, before use. 16-heptadecenoic acid was synthesized by one of the authors (E.P.-L.) within a former work.19 2 ACS Paragon Plus Environment

Page 3 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

For the emulsification experiments, cyclohexane (≥ 99.5%) and 1-octanol (≥ 99.9%) were purchased from Sigma-Aldrich, n-hexadecane (99%) was purchased from Alfa Aesar. 2.2. Synthesis of primary APPs and silica coating. Aqueous suspensions of stabilized FexOy nanoparticles were synthesized according to the literature.15 For the synthesis of the primary APP suspension (and similar suspensions), we have also followed the Feyen procedure as published.15 In short, 100 mg of surfactant-stabilized iron oxide particles in suspension (in the range of 2 wt%) was stirred with the monomer mixture (31.76 mmol styrene + 0.67 mmol DVB + 2.32 mmol GMA) at 50 °C for tw50 = 1 min. Then, 142 mL ammonia solution (1.3 wt%, 50 °C) was added. The resulting dispersion was heated to 70 °C during 10 min. Without any waiting time tw70 at 70°C, 0.17 mmol of (NH4)2S2O8 in 2 mL mQ water was added with vigorous stirring. This initiates the polymerization reaction. After 17 h, a gray dispersion was observed in the case of x = 3, y = 4. The dispersion was treated by centrifugation, decanting, and re-dispersing by ultrasound for about 9 min at a USC500 bath (VWR) three times with about 10 mL mQ water (“washing steps”). We refer the product as PS-D2&FexOy with the appropriate numbers for x and y in the iron oxide. The symbol “&” is used for the asymmetric patch (or part) assembly. The synthesis of PS-D20&FexOy suspensions was analogous, except a changed monomer composition (23.56 mmol styrene + 5.89 mmol DVB + 2.32 mmol GMA) and changed waiting times (tw50 = 60 min, tw70 = 20 min).15 For Fe2O3, the product was reddish-brown in color. The resulting products were characterized by SEM and turned out to be well-shaped PS nanospheres with immobilized Fe3O4 or Fe2O3 at their surface. Feyen et al.15 pointed out that the position of the oxide particles can be tuned by the stirring period. We have selected appropriate parameters (tw50, tw70) and kept them fixed for this work. For site-selective SiO2 coating, 30 mg of PS-Dz&FexOy was dispersed in an ammonia solution (12.4 mL mQ water + 0.6 mL NH3·H2O (29%) + 5 mL isopropanol) and stirred for 1 h at room temperature. Then, a concentrated ammonia solution (20 mL isopropanol + 0.81 mL NH3·H2O (29%)) was added, followed by 50 µL TEOS dissolved in 14.3 mL isopropanol. The system was stirred for 20 h at room temperature. A small part of the obtained product was washed 3 times with ethanol followed by centrifugation (14k rpm, 12 min), decanting, and redispersion in about 5 mL. The thickness of the resulting SiO2 coating was varied by the amount of silica precursor (50 or 150 µL) in some cases. The products are named as PS-Dz&FexOy/SiO2. Some small modifications of the published procedure are used in these steps, namely the exchange of the ethanol with isopropanol and the use of the still-suspended SiO2-coated product without washing or drying for the next steps. 2.3. Selective TiO2 coating. Several attempts were made to achieve a selective coating. The details of these experiments can be found in the supplementary material. Here we only describe the conditions for the most successful attempt. In this approach, the as-made suspension of PS-D2&Fe3O4/SiO2 in the NH3·H2O·isopropanol solvent was used. A solution of 15 µL Ti-AcAc in 5 mL isopropanol was added dropwise into the above described basic PS-D2&Fe3O4/SiO2 suspension (containing about 30 mg of NPs) after 20h stirring at room temperature, without being centrifuged or washed. Then, the temperature of the reaction mixture was increased to 50°C by heating in an oil bath. The mixture was stirred for 4 h. The product was centrifuged and re-dispersed with ethanol 3 times. We named this as PSD2&Fe3O4/SiO2/TiO2.

3 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 14

2.4. Emulsification tests. The influence of the fabricated APPs on liquid mixtures have been studied for different oil/water systems. In a typical experiment, 1 mg of APPs was dispersed in 0.5 mL of diluted HCl (“water phase”) using ultrasonication (1 min in a USC500 bath (VWR)). Then, 0.5 mL of a lipophilic liquid (“test oil”) was added to the particle suspension and mixed by manual shaking for 15 s. The pH was varied from 1.0 to 6.0 by adding 0.1M HCl to mQ water. The properties of the resulting systems were characterized macroscopically and microscopically by direct visual observation as well as digital photography. Images were taken either with a stereo microscope (Nikon SMZ2) by a microscope digital camera MA88 (C&A Scientific Co. Inc.) or with an iPhone4 camera. Both methods were supported by the coloration of the particles (brownish because of the iron oxide) and sometimes by enhancing scattering effects with laser radiation (Laser pointer). 2.5. Other characterizations. TEM images were made with a Hitachi H 7100 as well as with a Hitachi HF 2000 depending on the required resolution. The use of the H 7100 is noted in the text for distinction. STEM images and energy-dispersive x-ray (EDX) mappings were made with a Hitachi HD-2700 and supplemented with an EDAX Octane T Ultra W EDX detector. A solid content analysis was applied to the suspensions containing the reaction products. Typically about 0.5 mL of these suspensions was filled into a glass vial. This vial was exposed to 50°C overnight and the amount of the dried product was determined. Three parallel experiments were carried out for each sample and the average value was taken as the final result.

Figure 1. Examples of different generations of the APP family investigated in this paper. TEM images of A) the primary particles (APP1) and B) the secondary particles (APP2).

3. Results and discussion 3.1. Verification and tuning of the Feyen approach. The Feyen approach to APPs delivers highly defined mono-sized particles. It is tunable and reproducible. Although it is a multi-step synthesis, nearly the entire fabrication process can be controlled well. The only critical step is the incorporation of the FexOy into the PS surface. According to the current understanding15 it is kinetically determined, thus the exact time protocol and geometry conditions are likely of high importance. This is also what we recognized and, therefore, we have worked as near as possible to the original conditions. 4 ACS Paragon Plus Environment

Page 5 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

The chemical composition of the reaction solution can be used efficiently for fine-tuning the synthesis. We made e.g. modifications of the monomer composition similar as already published.19 The GMA is expected to enhance the polarity of the polymer spheres which is beneficial for forming stable suspensions. The DVB enhances the network interconnection and influences the iron oxide incorporation.19 We have chosen two concentrations which were appropriate for our experiments. The amount of polymer was chosen a bit higher than needed for incorporating of all FexOy particles, and therefore, only a part of the polymer particles (about 41% according to TEM) shows attachment to a FexOy particle. The lonely polymer particles are removed by washing the sample after the silica coating step. The size of the PS spheres attached to the iron oxide particle can be tuned by varying the 16-HDA concentration. Here, this was verified for two different concentrations (0.3 g and 0.2 g) in case of PS-D20&Fe2O3 resulting in PS diameters of 20 and 30 nm, respectively. We would like to call these primary asymmetric particles APP1. They are the basis of the investigated APP family. In the following step, silica is coated onto the iron oxide part and the resulting particles will be called APP2. The size of silica part can be efficiently tuned by varying the TEOS amount, for example, 150 µL resulted in 250 nm SiO2 hemispheres whereas 50 µL produced 100 nm SiO2 hemispheres (Figure 1). 50 µL of TEOS has been used as a standard for materials used in the next steps. Having a closer look, the deposition of silica onto the precursor particles resulted in surprisingly large APP parts (Figure 1). The chemical composition of the educts explains only 1/5 of the observed volume in relation to the PS-Dz half. We conclude from this discrepancy that a part of the PS-Dz-precursor particles was not involved in the reaction. Likely, they had no FexOy condensation seed and could, therefore, not be covered by silica. This would be consistent with our above observation of missing FexOy kernels in some particles. Although most of the characterizations of theses stages of the synthesis were carried out for dried samples, we used the as-made suspensions for the next step. This wet-in-wet procedure is simple and efficient. Moreover, the avoiding of drying has proved to be essential for the subsequent treatment. In fact, it is known21 that drying can change surface morphology and chemical termination of the surface. 3.2. Site-selective TiO2 coating. In order to achieve the selective TiO2 coating of the silica part of the APPs, different strategies were adopted. As already noted above, different competing processes play a role in this step, such as particle growth in the solution, coating of both sides of the APPs, destruction of the APPs, and agglomeration of the APPs. The occurrence of these competing processes can be influenced by the chosen precursor system, reaction conditions as well as by the process sequence chosen (see Scheme 1). An important process incorporated in many reaction protocols is the drying of intermediate products which has a significant influence on the final product. The process sequence is a little underestimated, but it decides on important details such as the surface type which is exposed to the reactive Ti precursor.

5 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 14

Scheme 1. Possible approaches for site-selective titania coating. Besides the chemical coating reaction, the sequence of the other processes (drying, washing, heating) influences the product.

Before we found the successful method in our experiments, we tested several textbook methods and literature procedures. In approach 1 of Scheme 1, we tested the TBOT deposition method with varying water addition (see Supp. Info., Figure S1). In addition, Joo’s method22 using hydroxypropyl-cellulose for reaction tuning as well as the use of the precursor Ti-AcAc was investigated (see Supp. Info., Figure S1). In approach 2, deposition experiments were carried out with the TBOT precursor under alkaline conditions23 (Supp. Info., Figure S2). In approach 3, W. Li’s method23 was tested especially because it requires very low temperatures only (see Supp. Info., Figure S3). All these attempts have shown one of the failures described above. The only signs for asymmetric deposition were observed within approach 3, however, with some additional TiO2 network structures (Supp. Info., Figure S3). In order to achieve a highly selective coating of TiO2 on one half of the APP2 without extra growth of TiO2 in the reaction suspension, it is important, first, to choose the right process sequence as shown in Scheme 1 and, second, to control the rate of hydrolysis of Ti alkoxide precursor. This can be done by choosing different alkoxides24 and by chelating ligands. E.g. TiAcAc is known25 to have slow hydrolysis rate under alkaline conditions in comparison with other Ti precursors such as TBOT. Therefore, this precursor was tested especially in approach 3 and showed the positive result. The TEM images in Figure 2 clearly show that additional material was deposited on the silica half of the APP2 leaving the PS half free from coating. In the low magnification image (Figure 2A), no extra growth of TiO2 or agglomeration of the primary particles can be detected. Several batches confirmed the reproducibility.

6 ACS Paragon Plus Environment

Page 7 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. The selective coating step as visible in TEM images. The APPs (PS&Fe3O4/SiO2/TiO2) have been prepared by approach 3 using different amounts of Ti-AcAc. A+B) 15 µL, C) 30 µL, D) TiO2 shell thickness variation with Ti-AcAc amount. The inset shows the scheme of the selective TiO2 shell on the APP.

A STEM analysis of the sample allows the combination with elemental analysis by EDX (Figure 3). Figure 3B shows that the additional material contains titanium. It gives the mapping results along with the STEM-DF mode image of the same sample region. The signals for Ti were detected only on the silica part and they are absent on the PS part. This indicates the selective coating of TiO2. Therefore, TiO2 coating has been successfully used for the fabrication of another APP (number 3) within this APP family. Having a more detailed look at the TEM images, one recognizes a constant thickness of the titania coating everywhere on the silica part. A precursor amount of 15 µL resulted in a thickness of 15 nm whereas 45 µL resulted in 25 nm (see Supp. Info. Figure S4). This demonstrates an efficient tuning possibility for these complex particles. The thickness dependence as shown in Figure 2D is nonlinear, likely arising from a competing process (e.g. growth of small titania nanoparticles in the solution) leading to a loss of titania for the coating. At high precursor concentration (above 45 µL), the system seems to be saturated and growth starts on the PS surface as well.

7 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 14

Figure 3. A) STEM-DF image of APPs prepared by the wet-in-wet approach (approach 3); B) superimposed image of an EDX mapping of the Si-K (pink), C-K (yellow), and Ti-K (green) signals which can be assigned to silica, PS, and titania, respectively.

Since catalytic and photocatalytic applications are the main future purpose of the APPs, the stability of the coating is crucial. We tested the stability of TiO2 coating by three methods, (1) by aging the sample in an aqueous environment (mQ water) ultrasonicated for 5 min and then stirred for 6 days at room temperature; (2) by refluxing the sample in water at 90 °C for 24h; and (3) by exposing the sample to water vapor at 80°C for 10 h. The morphology of the samples obtained after each stability test was investigated with TEM. Test 1 has shown no change even after long aging periods. In test 2, the refluxing at this high temperature led some polystyrene spheres to melt. However, the TiO2 shell looks intact (Figure 4C). In test 3, interestingly both, the TiO2 shell and polystyrene, remain unchanged after water vapor treatment (Figure 4F). We conclude from these 3 tests that the titania coating is strongly attached to the silica sphere.

8 ACS Paragon Plus Environment

Page 9 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 4. Aging and stability of the coating. The TEM images show samples after stirring in water for 6 days (A,B), after water vapor exposure for 10 h (C,D), and after refluxing at 90°C for 24h (E,F). The figures C to F were obtained with the H 7100.

3.3. Emulsification. Emulsification tests with APPs have been carried out in several works.9 Long-term stabilization of emulsions was demonstrated9 and interfacial enrichment was discussed in this context.8 Other authors investigated the influence of APP on the microstructure of emulsions.20 The emulsification tests performed for our work were designed to test very small amounts of particles. These amounts cannot emulsify the whole o/w mixture, but either a small part of it, or they stabilize droplets with large size. Already in case of titania, we have found a very diverse mixing behavior. Sometimes, the titania remained in the water phase with a small part enriched at the sharp o/w interface. In other examples, for octanol|H2O(pH6) mixtures, a sharp o/w boundary was formed, the water phase cleared up, and a precipitate was observed after a few minutes. In a further case, droplet formation was observed at the interface (o-in-w for o=cyclohexane, see Figure 5) attached to a milky water phase in which a precipitate occurred. After some time, the milky phase started to clear up from the top. In some cases, stable droplets were observed in the range of millimeters. The fabricated APPs (PS&Fe3O4/SiO2/TiO2) have shown a significantly different behavior. Always, our emulsification tests led to thin transition regions between the oil phase and 9 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 14

the water phase. This transition region consisted of stabilized droplets. In case of cyclohexane, this region consisted of stabilized, very polydisperse oil droplets as shown in Figure 5. However, the same particles can also stabilize water droplets in octanol. In all cases, the water phase was clear and no precipitate occurred. Therefore, we conclude that the entire amount of solid was incorporated in the transition region. It is an interesting point that different emulsions have been observed with the same kind of particles. This indicates a quite complex emulsification behavior of APPs. The pickering behavior of homogeneous particles do not prefer a specific curvature. However, APPs are often considered as solid surfactants analogously to the 2-part structure of molecular surfactants. This could implicate that the preferred interface curvature could mainly be concluded from geometrical considerations. Obviously, such a line of arguments is too simple. Different curvatures can be realized by the same APPs.

Figure 5. Optical microscopy images of the o/w transition region. These regions can consist of droplets and diverse emulsions. A) Interface region stabilized by PS&Fe3O4/SiO2/TiO2 in cyclohexane/water at pH 1. B) o-in-w droplets for o = cyclohexane and w = water at pH 7 stabilized C) A macro droplet system stabilized by TiO2-P25 in with TiO2-P25. hexadecane/styrene/water(1:1:4).

In literature, Hirose et al. described the concept of “spontaneous curvature”26 which is in agreement with the above mentioned solid-surfactant consideration of APPs. They showed that equilibrium contact angle depends on the geometry of the particle, the wettability, and on the interfacial curvature. Aveyard27 raised the question of thermodynamic stability again and concluded that the magnitude of the adsorption energy can be considerably higher for APPs adsorbed at the oil-water interface than in case of homogeneous particles. When lateral interactions were taken into account, long-range electrical repulsion through the oil phase could have the significant effect on the free energy. Electrostatic effects might also influence our systems significantly. The deeper understanding of the unusual curvature behavior of the APPstabilized emulsions is however needed. The observed interface activity of the APPs can be regarded as extreme. Many solid particles such as TiO2 or SiO2 show interfacial enrichment; however in case of the APPs, seemingly all particles are always deposited at the interfaces. This is for example visible in Figure 6. Here, the phase mixing behavior is shown under various pH conditions. The pH has a strong influence on the interaction of the particles with each other and on their wettability. Therefore, it also influences the stability of Pickering systems. TiO2 shows either no visible interface activity at all or it has restricted activity windows. For the APPs the interface activity windows are much larger, or they even contain all investigated examples as in Figure 6. It is also 10 ACS Paragon Plus Environment

Page 11 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

remarkable that no precipitation was observed. This fact indicates a very favored energetic situation of the APPs at the interface, decisively more favorable than for the TiO2 particles. The stability times of the observed phase-mixed systems are also interesting to be compared. In octanol/water systems, P25 forms suspensions in the water phase at low pH, while at high pH, TiO2 precipitated. After 1 week, only precipitated systems were found for P25, while the emulsion system with APPs was stable (see Figure S5 in the Supp. Info.).We noticed that emulsions or droplet systems formed by APPs were stable over more than a month. We ascribe this different behavior to the long-term processes presuming at the o|w interface. Pickering TiO2 particles seem to have the chance to aggregate more strongly and precipitate. This amount of TiO2 is then missing for emulsion stabilization. Interestingly in our experiments we noticed the formation of monodisperse macro droplet systems (MDS) with the low amount of Pickering particles. Without mechanical stress, these “pseudo” emulsions were stable even over 1 month. But they are destroyed with the stress. Due to the small number of Pickering particles, the total interface is forced to reduce its area leading to the observed monodisperse MDS. On the other hand, Pickering systems with low particle amounts could have a large importance in environmental remediation or also in large-scale catalytic processes. The Janus particles could make these systems highly stable.

Figure 6. Distribution between the different phases after emulsification tests. The photographs of the phases were obtained from the water/octanol system (1:1) using (A) 1 mg of P-25 powder and (B) 1 mg of APPs (PS&Fe3O4/SiO2/TiO2). The pH is indicated. C and D compare of the volumes of the different phases in the two systems . The grey areas indicate the location of the solid particles. O – oil-like phase, W – water phase, P – precipitate, E – emulsion. Sometimes the emulsions contained droplets up to 1 mm.

11 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 14

4. Conclusions The Feyen approach leads the reproducible APPs useful for the synthesis of an APP family with different outer parts. Three different members of this family were investigated in this paper. The especially desired APPs with a TiO2 patch (PS&Fe3O4/SiO2/TiO2) were successfully fabricated by an efficient selective TiO2 coating method. The main obstacles for selective coating are the nucleation of TiO2 oligomers in the solution and the simultaneous growth of TiO2 on the other APP part (polystyrene). Both processes were successfully suppressed by adjusting the reaction rate of the titanium precursor by choosing a special alkoxide and an alkaline reaction medium. The TiO2 patches turned out to be strongly attached to the SiO2 part of the APPs and were stable in water even at higher temperatures (90 °C) and for long periods (1 week). This is crucial for catalytic applications. The influence of the prepared APPs on phase mixing behavior is remarkable and diverse. Oils with low polarity (e.g. hexadecane, cyclohexane, octadecane) can form Pickering emulsions by the help of these particles; oils with high polarity (e.g. 1-octanol) form monodisperse macro droplet systems. These phenomena can be ascribed to strong interfacial particle enrichment. For normal particles such as TiO2-P25, unavoidable precipitation normally leads to an extinction of the solid form the oil/water system over longer times.

Associated content ((s)) Supporting Information The supporting Information is available free of charge on the ACS Publications webpage at DOI: xxxxx. Alternative pathways for selective TiO2 coating, APP3 tuning, emulsification tests with different solvents, and detection of selective coating in TEM images. Figures S1-S7 (PDF)

Acknowledgement We thank F. Schüth for the encouragement to this work. We are also grateful for the characterization support from the MPI TEM department, especially from B. Spliethoff (HRTEM) and from A. Swertz (STEM and elemental mapping). This work was supported by the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft.

References 1. Walther, A.; Müller, A. H. E., Janus Particles: Synthesis, Self-Assembly, Physical Properties, and Applications. Chem. Rev. 2013, 113, 5194−5261. 2. Yoshida, M.; Roh, K. H.; Mandal, S.; Bhaskar, S.; Lim, D. W.; Nandivada, H.; Deng, X. P.; Lahann, J., Structurally Controlled Bio-hybrid Materials Based on unidirectional Association of Anisotropic Microparticles with Human Endothelial Cells. Adv. Mater. 2009, 21, 4920–4925. 3. Suci, P. A.; Kang, S.; Young, M.; Douglas, T., Streptavidin-Protein Cage Janus Particle for Polarized Targeting and Modular Functionalization. J. Am. Chem. Soc. 2009, 131, 9164–9165. 12 ACS Paragon Plus Environment

Page 13 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

4. Golestanian, R.; Liverpool, T. B.; Ajdari, A., Propulsion of a molecular machine by asymmetric distribution of reaction products. Phys. Rev. Lett. 2005, 94, 220801. 5. Xuan, M.; Shao, J.; Lin, X.; Dai, L.; He, Q., Self-Propelled Janus Mesoporous Silica Nanomotors with Sub-100 nm Diameters for Drug Encapsulation and Delivery. ChemPhysChem 2014, 15, 2255–2260. 6. Thomas, J. M.; Thomas, W. J., Heterogeneous Catalysis. VCH: Weinheim, 1997. 7. Loget, G.; Kuhn, A., Bulk synthesis of Janus objects and asymmetric patchy particles. J. Mater. Chem. 2012, 22, 15457. 8. Jiang, S.; Schultz, M. J.; Chen, Q.; Moore, J. S.; Granick, S., Solvent-Free Synthesis of Janus Colloidal Particles. Langmuir 2008, 24, 10073-10077. 9. Jiang, S.; Chen, Q.; Tripathy, M.; Luijten, E.; Schweizer, K. S.; Granick, S., Janus Particle Synthesis and Assembly. Advanced Materials 2010, 22, 1060–1071. 10. Lone, S.; Kim, S. H.; Nam, S. W.; Park, S.; Joo, J.; Cheong, W., Microfluidic synthesis of Janus particles by UV-directed phase separation. Chem. Commun. 2011, 47, 2634–2636. 11. Roh, K.-H.; Martin, D. C.; Lahann, J., Biphasic Janus particles with nanoscale anisotropy. Nature Materials 2005, 5, 759-764. 12. Mou, F.; Chen, C.; Guan, J.; Chen, D.-R.; Jing, H., Oppositely charged twin-head electrospray: a general strategy for building Janus particles with controlled structures. Nanoscale 2013, 5, 2055–2064. 13. Mock, E. B.; De Bruyn, H.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F., Synthesis of Anisotropic Nanoparticles by Seeded Emulsion Polymerization. Langmuir 2006, 22, 4037–4043. 14. Liu, B.; Zhang, C.; Liu, J.; Qu, X.; Yang, Z., Janus non-spherical colloids by asymmetric wet-etching. Chem. Commun. 2009, 3871–3873. 15. Feyen, M.; Weidenthaler, C.; Schüth, F.; Lu, A., Regioselectively Controlled Synthesis of Colloidal Mushroom Nanostructures and Their Hollow Derivatives. J. Am. Chem. Soc. 2010, 132, 6791. 16. Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E.; Duguet, E., Design and synthesis of Janus micro- and nanoparticles. Journal of Materials Chemistry 2005, 15, 3745-3760. 17. Lattuada, M.; Hatton, T. A., Synthesis, properties and applications of Janus nanoparticles. Nano Today 2011, 6, 286-308. 18. Wurm, F.; Kilbinger, A. F. M., Polymeric Janus Particles. Angew. Chem. Int. Ed. 2009, 48, 84128421. 19. Passas-Lagos, E.; Schüth, F., Amphiphilic Pickering Emulsifiers Based on Mushroom-Type Janus Particles. Langmuir 2015, 31, 7749−7757. 20. Liu, B.; Liu, J.; Liang, F.; Wang, Q.; Zhang, C.; Qu, X.; Li, J.; Qiu, D.; Yang, Z., Robust Anisotropic Composite Particles with Tunable Janus Balance. Macromolecules 2012, 45, 5176−5184. 21. Lv, H.; Meng, F.; Zhang, M.; Geng, Z.; Sun, Y., Effect of the drying process on the preparation of porous silica microspheres. Chemical Engineering Science 2015, 135, 285-293. 22. Joo, J. B.; Zhang, Q.; Lee, I.; Dahl, M.; Zaera, F.; Yin, Y., Mesoporous Anatase Titania Hollow Nanostructures though Silica-Protected Calcination. Adv. Funct. Mater. 2012, 22, 166−174. 23. Li, W.; Yang, J.; Wu, Z.; Wang, J.; Li, B.; Feng, S.; Deng, Y.; Zhang, F.; Zhao, D., A Versatile KineticsControlled Coating Method To Construct Uniform Porous TiO2 Shells for Multifunctional Core−Shell Structures. J. Am. Chem. Soc. 2012, 134, 11864−11867. 24. Soler-Illia, G. J. A. A.; Sanchez, C., Interactions between poly(ethylene oxide)-based surfactants and transition metal alkoxides. New J. Chem. 2000, 24, 493-499. 25. Mahoney, L.; Koodali, R. T., Versatility of EISA Method for Preparation of Mesoporous TiO2 for Energy and Environmental Applications. Materials 2014, 7, 2697-2746. 26. Hirose, Y.; Komura, S.; Nonomura, Y., Adsorption of Janus particles to curved interfaces. J. Chem. Phys. 2007, 127, 054707. 27. Aveyard, R., Can Janus particles give thermodynamically stable Pickering emulsions? Soft Matter 2012, 8, 5233-5240. 13 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 14

TOC Graphic

14 ACS Paragon Plus Environment