The reactive hypersaline route: A one-pot synthesis of porous

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The reactive hypersaline route: A one-pot synthesis of porous photoactive nanocomposites Roberto Nisticò, Silvia Tabasso, Giuliana Magnacca, Thomas Jordan, Menny Shalom, and Nina Fechler Langmuir, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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The reactive hypersaline route: A one-pot synthesis of porous photoactive nanocomposites Roberto Nisticò*†‡, Silvia Tabasso‡, Giuliana Magnacca‡§, Thomas Jordan||, Menny Shalom||#, and Nina Fechler*||. ‡

University of Torino, Department of Chemistry, Via P. Giuria 7, 10125 Torino (Italy).

§

NIS Centre, Via P. Giuria 7, 10125 Torino (Italy).

||

Max Planck Institute of Colloids and Interfaces, Department of Colloids Chemistry, Am Mühlenberg 1, 14476 Potsdam-Golm (Germany). KEYWORDS: Composites; Functional nanoparticles; Hypersaline medium; Photoactivity; Titania.

ABSTRACT: Herein, porous photoactive nanocomposites are prepared by a simple one-pot synthesis approach using a salt and aqueous media. Within this “reactive hypersaline route”, the salt not only serves for the structuring of the composite but also becomes an integral active part of it. Here, the addition of sodium thiocyanate to a titania precursor guides on the one hand the formation of needle-shaped nanoparticles and on the other hand forms the yellow compound isoperthiocyanic acid which is homogeneously incorporated into the porous nanocomposite. Compared to a pure titania reference this material reveals a 7-fold increased photodegradation rate of Rhodamine B as a model compound. This reveals the reactive hypersaline route to be a promising and facile synthesis route towards photoactive porous materials.

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INTRODUCTION For the synthesis of porous solids, various different strategies have been established.1 Even though soft- and hardtemplating approaches are the most widely used ones, both techniques still present inherent disadvantages.2-6 In particular, the main critical step for both routes is the template removal with the risk of collapse7 or deformation7-8 of the entire nanostructure. Moreover, the softtemplating route usually requires the removal of templating agents by washing with organic solvents or thermal treatment, which is not sustainable and may lead to the formation of carbonaceous residues, respectively,9-10 while usually producing amorphous materials.3,11 The hardtemplating issues are due to structural and compatibility restrictions between the rigid solid template and the precursor.5-6 Additionally, hard-template removal is rather inconvenient: the procedure is usually quite timeconsuming, requires several steps and the use of hazardous chemicals in order to completely dissolve the templating species.12 Very recently a procedure denoted as “salt-templating” has been developed and successfully applied for the synthesis of porous materials with different morphologies by using inorganic salts as porogen or stabilizing agent.13 The use of inorganic water-soluble salts allows for simple aqueous template removal and thus makes this route more safe, convenient and sustainable. Furthermore, due to the utilization of simple salts which in principle can also be reused, the entire process becomes rather cheap.13-

The salt-templating procedure has been used for the synthesis of different kinds of porous materials such as carbons and composites.13,15-17 Recently, we have further varied this approach and extended it to hypersaline conditions which now also allows for the synthesis of porous oxide materials, e.g. silica and titania.14,18,19 In general, for the hypersaline route, the sol-gel process is conducted in the presence of inorganic salts which alter the hydrolysis behavior and thus leads to nanoporous materials. The possibility of pore tuning is related to the Hofmeister series, i.e. different abilities of ions to stabilize the colloidal system. Ions which cause salting-in effects lead to the formation of small particles whereas ions with salting-out character rather act as coalescent agent producing bigger spheres.20-22 In the case of silica or titania it was found that the porosity can be altered by the nature of the ions, their concentration and also the solvent used.14,18 In our previous studies, we only used inorganic nonreactive salts. However, there exists a wide variety of different salt types which are claimed to offer much more possibilities. For instance, polymer-derived carbon aerogels can be obtained by direct polymerization of monomers under hypersaline conditions using inorganic salts.23 Here, cyanate and thiocyanate ions can be described as borderline species bridging inorganic and organic chemistry. It is known, that the thiocyanate ion SCN- is the anion with the strongest salting-in capacity in the Hofmeisterseries.20-22 The thiocyanate group can be considered as a

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pseudo-halogen, with the negative charge equally shared between the sulfur and the nitrogen atoms.24 -

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[S=C=N  S-C≡N] (Eq. 1) According to the Hard and Soft Acids and Bases (HSAB) theory,25 the thiocyanate ion generates preferentially Nbonded species in the presence of hard acids, i.e. highly charged and weakly polarizable metals with a small atomic radius. However, S-bonded species occur with soft acids, i.e. low charged and strongly polarizable metals with a large atomic radius. Cyanates and thiocyanates are typically involved in different reactions: dissociation with salt formation and/or addition to the C≡N triple bond or even polymerization.24 Previous studies revealed thiocyanatecontaining salts to generate a yellow isoperthiocyanic compound (IPTC) in the presence of strong acids such as hydrochloric acid, which was used in the past for dyeing cotton fibers.26 As for modern materials chemistry, it is known that the photoactive properties of titania can be enhanced by the combination with further components.2731 However, the composite synthesis usually requires several steps while already the shaping of titania represents a challenge. Herein, we use the reactive salt sodium thiocyanate within our hypersaline synthesis approach in order to produce porous nanocomposite materials of titania and IPTC. Different post-synthesis treatments were also performed in order to investigate the best removal condition of the IPTC component, thus leaving a fully-templated porous oxide. Lastly, to exemplarily demonstrate the optical properties of the nanocomposite obtained, the photodegradation of the standard dye Rhodamine B was investigated revealing the reactive hypersaline synthesis (RHS) route to be a promising, facile and sustainable one-pot approach towards photoactive titania nanocomposites. EXPERIMENTAL SECTION Materials Titanium-(IV)-Tetra-Iso-Propoxide (TTIP, CAS 546-68-9, assay = 97%, Aldrich) was selected as titania precursor and sodium thiocyanate (NaSCN, CAS 540-72-7, assay ≥ 98%, Fluka) as inorganic salt. Rhodamin B (RhB, CAS 509-34-2, assay = 95%, Aldrich) was used for photodegradation testing. Hydrochloric acid 1M solution (HCl, CAS 7647-01-0, Merck) was used as acidic catalyst for the synthesis of the oxide powders. THF (CAS 109-99-9, assay ≥ 99.5%, Acros) was selected as organic solvent for the extraction of the IPTC component from the composites. Methanol (MeOH, CAS 67-56-1, assay ≥ 99.9 %, Aldrich) and isopropanol (i-PrOH, CAS 67-63-0, assy ≥ 99.5%, Aldrich) were used as hole- and radical-quencher for the photodegradation testing. All chemicals were used without further purification. DMSO-d6+0.03% TMS v/v (CAS 2206-27-1, Euriso-Top) was used as solvent for the NMR measurements. (Reactive) hypersaline synthesis of titania powders Nanocomposite synthesis: In a typical synthesis of oxide powders, sodium thiocyanate (0.5, 1, 2, 3 or 4 g) was dis-

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solved at RT in 1M HCl (15 ml) and then mixed with 1 g of the titania precursor (TTIP).17 The resulting mixtures were placed into an open glass vial and put into an oil bath at 50°C under magnetic stirring for 10 days. Under these hypersaline conditions, the salt starts to react in the presence of titania and thus the mode is shifted to the reactive hypersaline route, i.e. the IPTC compound was formed by polymerization of the thiocyanate ions in the acidic environment. Afterwards, samples were heated at 60°C for 4 hours. In order to remove all the residual ionic species, materials were grinded, washed in water for 24 hours and finally filtrated and dried in vacuum. Post-synthesis treatments: To the above described approach, the nanocomposite material produced in presence of sodium thiocyanate (4 g) was further treated and three different post-synthesis procedures were investigated in order to remove the IPTC compound: i) pyrolysis in a nitrogen oven (heating from RT up to 550°C, with a heating rate of 2°C min-1, followed by 4 hours of an isothermal step at 550°C), ii) calcination in an air furnace (heating from RT up to 550°C, with a heating rate of 2°C min-1, followed by 4 hours of an isothermal step at 550°C), and iii) solvent extraction by stirring in THF for one day, followed by filtration. The yellow compound, extracted in THF, was recovered by rotary evaporation. Reference oxide materials were prepared following the same procedure without porogen, whereas the organic IPTC reference was synthesized by mixing 4 g of inorganic salt and 15 ml of 1M HCl without alkoxides. Samples were denoted as SaTi-X-Y-Z, with SaTi indicating Salt/Titania, X the type of anion used as porogen (SCN or 0 for the reference materials), Y indicating the salt/alkoxide mass ratio and Z for the removal procedure used (none (A), thermal treatment in air at 550°C (B), thermal treatment in nitrogen at 550°C (C) and THF extraction (D)). The IPTC compound extracted by THF was denoted as E, whereas the reference sample obtained without the metal oxide precursor was denoted as SaSCN-0-E. Characterization X-ray diffraction (XRD) was performed with a Bruker D8 Advance instrument using Cu-Kα-radiation. TEM micrographs were obtained using a Zeiss EM 912Ω instrument with an accelerator voltage of 120 kV. The diffraction patterns of the samples were obtained by Selected Area Electron Diffraction (SAED) with the TEM. SEM images were obtained on a LEO 1550-Gemini instrument after sputtering with platinum. Nitrogen sorption measurements were carried out with N2 at 77K after outgassing the samples at 150°C under vacuum for 20 hours using a Quantachrome Quadrasorb SI porosimeter. The specific surface area was calculated by applying the Brunauer-Emmett-Teller (BET) model32 to the isotherm data points of the adsorption branch in the relative pressure range 0.06 < p/p° < 0.3. The pore size distribution was calculated from N2 sorption data using the Non-Local Density Functional Theory (NLDFT) mod-

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el33 for cylindrical pores provided by Quantachrome data reduction software QuadraWin Version 5.11. Fourier transformed infrared (FTIR) spectra were recorded in transmission mode using the sample dispersed in KBr (1:20 weight ratio) on a Bruker Vector 22 spectrophotometer equipped with Globar source, DTGS detector, performing 128 scans at 4 cm-1 of resolution in the range 4000-400 cm-1. NMR spectra were recorded on a Bruker Avance 300 (300 MHz and 75 MHz for 1H and 13C, respectively) at 25°C. Chemical shifts were calibrated to the residual solvent proton and carbon resonances (DMSO-d6). UV-Vis absorption spectra were recorded on a Varian Cary 50Conc UV-Visible spectrometer, using neoLab semi-micro polystyrene cuvettes. UV-Vis diffusive reflectance absorbance spectra were measured using a Varian spectrophotometer equipped with an integrating sphere. The emission spectra were recorded on a LS-50B PerkinElmer instrument. The elemental composition of the solid was determined using a Thermo FlashEA 1112 CHNS–O analyzer. Photodegradation testing The testing for the photodegradation of RhB was carried out in aqueous solution, with 20 ml of a solution of RhB (20 mg l-1) and the dispersed sample (SaTi-SCN-4A or SaTi-0-0-A as reference sample) of 1 mg ml-1. To allow complete adsorption of the RhB on the surface of the catalyst, the RhB/catalyst-dispersion was stirred in the dark for 2 h prior to the testing. For the testing, the RhB/catalyst-dispersion was illuminated with a white light LED (OSA Opto Lights, 50 W light output, λ > 410 nm. Distance between lamp and solution: 7 cm) under vigorous stirring and aliquots were taken out every 10 minutes. Each aliquot was centrifuged (8000 rpm, 5 min.) and the optical absorption of the aliquot solution measured by UV-Vis spectroscopy.

were added to the reaction solution straight before starting the illumination. RESULTS AND DISCUSSION The (reactive) hypersaline synthesis of porous titania and composite materials For the synthesis of mesoporous titania powders by the hypersaline approach, sodium thiocyanate was selected as porogen salt and dissolved in 1M HCl. To this solution, Titanium-(IV)-Tetra-Iso-Propoxide (TTIP) titania was added as oxide precursor as described previously.18 After addition of TTIP, the solutions turn into lightyellow up to dark-orange and red colored liquids, depending on the thiocyanate amount used (Figure 1). However, upon heating the coloration diminishes again. The initial color changes can be attributed to the different complexes formed between Ti(IV) ions and the IPTC species, which are sensitive to hydrolysis and pH-changes.34 It is important to note that, depending on the salt amount used, the result of the hypersaline synthesis changes: at lower concentrations the oxide formation can be mainly regarded to proceed within the previously described mode of salting-in and salting-out effects resulting in porous oxides. Instead, at higher concentrations the salt here starts to react and becomes part of a composite. This mode is here denoted as reactive hypersaline synthesis which allows for the synthesis of titania nanocomposites. After the sol-gel process, all materials were further treated using different methods: i) thermal heating in a furnace (either in air or in nitrogen atmosphere) or ii) solvent extraction by stirring in THF. In particular, the solvent extraction procedure allows for the dissolution and recovery of the organic compound.

The relative concentration c/c0 of the RhB (with c0 being the initial RhB-concentration before illumination) was calculated by dividing the optical absorption a of a certain aliquot at 554 nm (absorption maximum of RhB) by the initial optical absorption a0 of the RhB at 554 nm before illumination. A comparison test for the photodegradation of RhB without the catalyst was carried out as described above, but without adding the catalyst SaTi-SCN-4A or SaTi-0-0-A to the RhB-solution. To test the photostability and the stability of the photodegradation performace, three repetitive cycles of the RhB-degradation experiments were carried out. After each completed degradation, the catalyst was recovered from the reaction solution by centrifugation (10 min at 8000 min-1), washing with water (three times, followed by centrifugation) and drying (60°C, vacuum oven).

Figure 1. Photographs of the samples in solution after the addition of TTIP (top row): SaTi-0-0-A, SaTi-SCN-0.5-A, SaTi-SCN-1-A, SaTi-SCN-2-A, SaTi-SCN-3-A, SaTi-SCN-4-A (from left to right), and as powders after further treatments (bottom row): SaTi-SCN-4-A, SaTi-SCN-4-B, SaTi-SCN-4-C and SaTi-SCN-4-D (from left to right).

To determine the primary active species in the RhBdegradation, MeOH or i-PrOH were added as hole- or radical-quencher. Here, 100 μl of the respective alcohol

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Figure 2. TEM (left) and SEM pictures (right) of titania powders obtained by the hypersaline route after varying post-treatment: reference SaTi-0-0 (A, A’), untreated sample SaTi-SCN-4-A (B,B’), after thermal treatment in air SaTi-SCN-4-B (C,C’), after thermal treatment in nitrogen SaTi-SCN-4-C (D,D’) and after washing in THF SaTi-SCN-4-D (E,E’). Panels A and A’ reprinted with 18 permission from ref. .

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Macroscopically, the titania samples change from a yellow hard powder (SaTi-SCN-4-A) to ivory soft powders for both the heat treatment in air (SaTi-SCN-4-B) and solvent extracted (SaTi-SCN-4-D) samples, to a dark-grey/black, hard powder (SaTi-SCN-4-C) for the material treated in nitrogen oven (Figure 1). Based on the elemental analysis, the sample SaTi-SCN-4-A reveals the presence of 7.1 wt.% of N, 6.2 wt.% of C, 1.2 wt.% of H and 19.7 wt.% of S, whereas the thermal treatment in air (SaTi-SCN-4-B) causes an almost complete combustion of the IPTC compound giving a complete white residue (almost pure titania). In contrast, the heat treatment carried out under nitrogen atmosphere (i.e., pyrolysis) causes the formation of dark-black powders probably due to the presence of carbon residues also evidenced by the elemental analysis (2.3 wt.% of N, 1.2 wt.% of C, 0.9 wt.% of H and 1.9 wt.% of S) formed by the decomposition of the organic compound (see Table S1, whereas the thermal stability of SaTi-SCN4-A under nitrogen atmosphere was reported in Figure S1). Finally solvent extraction (SaTi-SCN-4-D) does remove most of the IPTC compound (Table S1). The morphology of bare sample (SaTi-0-0) and the samples from hypersaline conditions using thiocyanate salt before and after post-treatment is shown in Figure 2.

from the organic component (confirming the data in Table S1).

All titania samples reveal rod-shaped particles of high aspect ratio. As already reported in the literature18, the material prepared without salt (Sa-Ti-0-0-A) also reveals a rather dense structure formed by rod-like particles, but shorter, thicker and aggregated compared to materials prepared in the presence of a SCN salt (shown in Figure 2B). Upon post-treatment, the thickness and size of these hypersaline structures changes. In particular, SaTi-SCN-4D is constituted by thin, acicular-like particles, with minor differences to the untreated composite material SaTiSCN-4-A, whereas the thermal treatment, both in air and in nitrogen, resulted in the formation of thicker nanostructures of approx. 50 nm of length. This agglomeration is further visible in the SEM images, revealing fine needles especially in the case of SaTi-SCN-4-A. This is attributed to a certain degree of sintering of the starting structures upon heating.35 X-ray diffraction patterns of all titania-based samples are shown in Figure 3A and 3B, whereas the patterns of the pure IPTC compound (Sa-SCN-0-E) and of NaSCN are shown in Figure 3C. In general, it is found that all titaniabased products show the crystalline diffraction patterns typical of rutile, with signals corresponding to the (110), (101), (111), (210) and (220) planes,36 confirming that the hypersaline medium favors the formation of rutile crystalline phase.17 No signals due to NaSCN are found in the titania samples, but additional diffraction signals become visible at 2θ = 26° which can be attributed to the IPTC compound (see Figure 3C). The absence of these signals in both samples, thermally treated in air and washed with THF, shows that the organic compound can be efficiently removed from the titania. Contrary, the thermal treatment in nitrogen causes very weak reflections at 2θ = 31°, 44° and 45° ascribed to carbonization byproducts formed

Figure 3. XRD patterns of titania samples (A,B), and the references NaSCN and Sa-SCN-0-E (C). (A): From bottom to top: increasing amount of porogen used (SaTi-0-0-A black, SaTi-SCN-0.5-A grey, SaTi-SCN-1-A orange, SaTi-SCN-2-A red, SaTi-SCN-3-A magenta, SaTi-SCN-4-A violet). (B): from bottom to top: varying post-treatment (SaTi-SCN-4-A violet, SaTi-SCN-4-B light green, SaTi-SCN-4-C cyan, SaTi-SCN-4D, brown). (C): NaSCN (blue), Sa-SCN-0-E (dark green). The main diffraction signal of the IPTC-compound is marked with the dark dot.

In order to analyze the influence of salt addition on the porosity, nitrogen sorption measurements were performed with all samples (Figure 4, Table 1). Compared to the titania reference prepared without salt (SaTi-0-0-A 79 m2 g-1, 0.11 cm3 g-1), the addition of thiocyanate salt results in an increase of surface area and pore volume (SaTi-SCN-0.5-A 182 m2 g-1, 0.26 cm3 g-1). Additionally, the shape of the isotherms changes from a featureless one for the reference to a type IV one indicating the structure development of rod shaped particles where the interstitial space contributes to mesopore formation.

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Figure 4. Nitrogen sorption isotherms (left) and cumulative pore volume (right) of titania obtained by the reactive hypersaline route with thiocyanate salt. (A,B): increasing the amount of porogen used (SaTi-0-0 black, SaTi-SCN-0.5-A grey, SaTi-SCN-1-A orange, SaTi-SCN-2-A red, SaTi-SCN-3-A magenta, SaTi-SCN-4-A violet). (C,D): varying the post- treatment (SaTi-SCN-4-A violet, SaTi-SCN-4-B light green, SaTi-SCN-4-C cyan, SaTi-SCN-4-D, brown). Table 1. BET specific surface areas and NLDFT total pore volumes Samples

SaTi-0-0-A SaTi-SCN-0.5-A SaTi-SCN-1-A SaTi-SCN-2-A SaTi-SCN-3-A SaTi-SCN-4-A SaTi-SCN-4-B SaTi-SCN-4-C SaTi-SCN-4-D

BET surface 2 -1 area [m g ]

Pore volume 3 -1 [cm g ]

79 182 124 144 91 133 85 70 209

0.11 0.26 0.29 0.21 0.24 0.31 0.29 0.24 0.41

However, higher salt amounts lead to decreased surface areas without mesopore features pointing to surface adsorption processes (Figure 4A). This is attributed to the additional formation of the organic compound IPTC on top of the titania which eventually covers the surface and thus fills the interstitial space between the rods. Post-treatment of the composites both in air and in nitrogen leads to a decrease in specific surface area and porosity (Figure 4C, 4D), which can be explained by sintering of the particles during the thermal treatment as revealed by

SEM. Additionally, upon heat treatment the organic component is carbonized and elemental analysis reveals substantial amounts of carbon, nitrogen and sulfur (Table S1). In contrast, the washing of the composite in THF successfully dissolves and removes the organic compound thus freeing the surface of the titania and eventually increasing both the specific surface area (209 m2 g-1) and porosity (0.41 cm3 g-1), compared to the unwashed as well as the reference material. In all cases the pore size distribution curves (not reported for the sake of brevity) are quite broad distributed, thus indicating a very high dispersity of pores in both micro- and mesoporosity range, without any relationship with the initial porogen concentration. The chemistry of SCN in acidic aqueous environment The IPTC species formed in the acidic environment during the reactive hypersaline synthesis of titania using thiocyanate salt was extracted from the oxide particles using THF. It is found that it is equivalent to the reference IPTC compound (Sa-SCN-0-E), which will be discussed in the following. It is well-known that SCN groups can polymerize in the presence of strong acids like HCl, forming the yellow compound canarine. Canarine was first described by Prokoroff, Miller and by Liebig, who proposed the empirical formula C3HN3S3 for this compound.37 Offord38 reported that thiocyanate ions can be oxidized by a mixture of chlorates and chlorides in an oxygen-rich acidic environ-

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ment, forming first thiocyanic acid (HSCN) as intermediate, which subsequently polymerizes to a mixture of yellowish oxidation products. The products were analyzed as a mixture of canarine (in this case, the empirical formula C8H6N8S7O was proposed), pseudo-thiocyanic acid (C3HN3S3), isoperthiocyanic acid (C2H2N2S3) and hydropseudothiocyanic acid (C3H3N3S2O). In 1892, Völckel described a yellow substance identical to that obtained by the treatment of aqueous thiocyanate solutions with strong mineral acids.39 This yellow substance was found to be isoperthiocyanic acid, a weak acid with the empirical formula C2H2N2S3 and the same decompositionpattern already observed by Wöhler for the decomposition of anhydrous thiocyanic acid. Based on the data present in the literature, it is summarized that the substance “canarine” comprises several different compounds with different empirical formulas. Recently, a detailed analysis of the group of such organo-sulfur-nitrogen heterocycles was reported by Bowman and co-workers.40 Eventually, we use these data for the investigation of our compound. The infrared absorption spectrum of the sample Sa-SCN-0-E is shown in Figure 5. The most important signals are related to the fivemembered ring structure of isoperthiocyanic acid, also known as xanthane hydride, and its tautomeric form 5imino-1,2,4-dithiazolidine-3-thione.41 In particular, the broad absorptions in the range between 3300-2900 cm-1 are due to the N-H stretchings with two main absorption maxima, whereas the sharp triplet signals at 1707, 1630 and 1520 cm-1 are associated to the C=N stretching and the NH2 bending vibrational mode, respectively. The signals at 1325 cm-1 are assigned to a ringstretching mode, whereas the signals at 1150 and 1082 cm-1 are assigned to a ring-skeletal mode and the NH2 rocking mode, respectively. Additionally, the very intense and sharp peak at 1003 cm-1 and the two shoulders at 1508 and 1307 cm-1 are due to the C=S stretching mode. The broad bands in the range between 600-700 cm-1 and the peak at 750 cm-1 are assigned to ring-deformation, C-S stretching and NH2 wagging vibrational modes.42-43 Figure S2 shows the NMR spectra in DMSO-d6, confirming the structure of isoperthiocyanic acid. The two broad singlets at 9.62 and 9.87 ppm in the 1H-NMR spectrum (Figure S2A) are related to the equilibrium between the two tautomers I and II (Scheme S1A), as confirmed by the broad singlet at 3.88 ppm (NH in position 4, II). This structure is also confirmed by 13C NMR (two quaternary carbon atoms at 209.80 and 184.61 ppm, corresponding to C5 and C3 respectively). Isoperthiocyanic acid forms from thiocyanate ions and hydrochloric acid.44 As shown in Scheme S1B, formation of (I) occurs via a tautomer depending on the protonation of the intermediate (III). On the other hand, the intermediate is formed only in the presence of SCNion. For this reason, the acidity of the medium is crucial for the synthesis of (I); it should be acidic enough to protonate the intermediate, but not too acidic in order to allow for the formation of the intermediate (III).45 Elemental analysis performed on Sa-SCN-0-E revealed the following composition: 15.8 wt.% of N, 23.8 wt.% of C, 2.7

wt.% of H and 49.1 wt.% of S, whereas the residual amount not determined (8.6 wt.%) can be due to incomplete combustion of the sample (see Table S1).

-1

Figure 5. FTIR spectrum in the 4000-500 cm range of SaSCN-0-E.

Photo-activity of the composite towards the degradation of Rhodamine B As revealed by the data presented above, the sample SaTiSCN-4A is a composite material consisting of titania and IPTC. Due to the advantageous properties of titania and the organic compound, the composite presents a promising material for photocatalysis. To proof this, we tested the isoperthiocyanate containing material SaTi-SCN-4A as a catalyst for the photodegradation of the organic dye Rhodamin B (RhB) in aqueous solution. To compare the photoactivity of this sample with reference materials, the RhB-degradation experiments were also carried out with the pure titania sample SaTi-0-0-A and pure RhB-solution (Figure 6). When adding the composite material SaTi-SCN-4A to a solution of RhB, under illumination with a white light LED the dye degrades completely as evidenced by UV-Vis absorption spectroscopy (Figure S3). Calculated from the optical absorption maxima of the RhB at 554 nm, the decrease of the dye concentration after a certain time of illumination is shown in Figure 6A. Complete degradation is reached in 50 minutes. By plotting the logarithmic concentration of the RhB in solution, the obtained linear slope reveals pseudo-first-order kinetics for the RhB degradation (Figure 6B).46 The rate constants k of the photodegradation reaction can be calculated from the slope of the linear fit and are given in Figure 6B. It shows that the photocatalyst SaTi-SCN-4A leads to a nearly 7-fold increase in photoactivity (k = 0.072 min-1) when compared to the titania reference sample prepared without the SCN-compound (SaTi-0-0-A, k = 0.011 min-1). Without the addition of any catalyst, no significant photodegradation can be observed at all (Figure 6B).

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Figure 6. Photodegradation of RhB dye with the sample SaTi-SCN-4A as photocatalyst. (A): Decrease of the relative RhBconcentration c/c0 under illumination with the catalyst SaTi-SCN-4A (violet), the reference sample SaTi-0-0-A (green) and pure RhB-solution (brown). (B): The relative RhB-concentration c0/c displayed on a logarithmic scale, with the slope of the linear fit nd rd given in brackets. (C): Repetitive cycles of the RhB-degradation, first degradation cycle shown in black, the 2 and 3 cycles shown in grey and orange, respectively. (D): UV-Vis diffusive reflectance absorbance spectra and emission spectra (inset) of the samples SaTi-SCN-4A (violet) and SaTi-0-0-A (green). Dotted lines indicate estimated band gap position by extending the linear range of the absorption edge.

The photodegradation of RhB can occur via two pathways, by a step-by-step N-deethylation and by a cleavage of the whole chromophore structure (cycloreversion).47 The N-deethylation goes along with a gradual shift of the absorption maximum from 554 nm to 499 nm (absorption maximum of the completely N-deethylated product Rhodamine), whereas for the cycloreversion no shift of the absorption maximum but only a decrease in intensity can be observed. This described shift of the absorption maximum can be observed for both samples, which indicates that the RhB degradation occurs via the N-deethylation pathway. While the photocatalyst SaTi-SCN-4A leads to a complete N-deethylation of the RhB to Rhodamine in 50 min., the reference material SaTi-0-0-A even after 5 h only leads to the partially N-deethylated compound Nethylrhodamine (λmax = 522 nm).48 To further determine the active species in the degradation process, methanol as a hole scavenger and isopropanol as a quencher for hydroxyl radicals (•OH) were added during the photodegradation experiments.49-53 For both photocatalysts, methanol shows an inhibiting effect, indicating

that the primary active species in the degradation process are photogenerated holes (Figure S4). However, this effect is clearly more pronounced for the catalyst SaTi-SCN-4A. Furthermore, only for the sample SaTi-SCN-4A also isopropanol reveals an inhibiting effect, which indicates that also •OH-radicals are generated during the photodegradation process.47 As those radicals are created by the reaction between H2O molecules and electrons/holes on the surface of the photoexcited semiconductor, the occurrence of those radicals and the higher photodegradation rate for the catalyst SaTi-SCN-4A are attributed to a more efficient electron/hole separation. This assumption can be further determined by the optical absorption and emission spectra of the samples SaTiSCN-4A and SaTi-0-0-A (Figure 6D). The pure titania reference sample (SaTi-0-0-A) reveals an absorption edge around 420 nm, corresponding to the electron transition at the titania band gap. For the isoperthiocyanate titania composite (SaTi-SCN-4A), the absorption edge is clearly shifted towards the visible region (around 480 nm), also an additional absorption tail with low optical density is

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observed. Upon excitation at 410 nm, both samples reveal a broad emission peak around 585 nm. Compared to the pure titania sample SaTi-0-0-A, the emission of the isoperthiocyanic containing sample SaTi-SCN-4A is significantly quenched. The additional absorption tail can be attributed to the creation of additional electronic states in the band gap, which are formed due to the composite formation between titania and the IPTC coating. The lowered PL emission intensity for the composite sample indicates a decrease in the rate of recombination via radiative paths, thanks to a better charge separation process under illumination.54 The more efficient electron/hole separation can be explained by a transfer of the photogenerated charge carries towards additional electronic states in the composite and will be a major contribution towards the superior photocatalytic activity of the composite sample SaTi-SCN-4A. Furthermore, an additional sensitization effect of the yellow organic IPTC compound, which leads to an increased light-harvesting for the composite, is also expected to contribute to the increased photocatalytic performance of the SaTi-SCN-4A composite. Here it should be also noted, that the dye degradation experiments were carried out with a light source emitting in the visible part of the spectrum (> 410 nm). As pure titania absorbs mostly in the UV region, the shifted optical absorption for the composite will contribute to the increased photocatalytic performance when compared to the pure titania reference sample. As a minor contribution, also the slightly increased porosity of the composite sample should be taken into account. Besides photoexcitation of the titania, also RhB molecules adsorbed at the photocatalyst surface can be photoexcited. Since the conduction band level of titania (-0.37 V vs. NHE for rutile)55 is lower than the E0 for RhB*/RhB•+ (-1.09 V),56 photoexcited electrons can be transferred from RhB to the titania conduction band. After migrating to the photocatalyst surface, these electrons can lead to the formation of •OHradicals, which then in turn can lead to the degradation of RhB (self-photosensitization).57 To proof the stability of the performance of the catalyst SaTi-SCN-4A, repetitive degradation cycles were performed. As shown in Figure 6C, the repeated photodegradation-cycles show comparable performances, and even in the 3rd cycle the RhB was completely decomposed after 50 minutes. XRD measurements of the photocatalyst SaTi-SCN-4A after the third degradation cycle shows no differences compared to the XRD patterns of this sample before the photodegradation reaction, which proves the materials photostability (Figure S5). Eventually, all data taken together reveal the reactive hypersaline route to be very promising for the synthesis of porous titania composites. Furthermore, these materials can be applied for photocatalytic applications such as the removal of organic pollutants in water. CONCLUSIONS In conclusion, photoactive porous nanocomposites have been prepared by the “reactive hypersaline route”. By the simple addition of the salt sodium thiocyanate to an

aqueous solution of a titania precursor, porous composites of titania and the yellow compound isoperthiocyanic acid could be obtained in a facile one-pot approach. Here, the salt not only guides the structure formation but at the same time also serves as integral precursor for the porous composite. This material was then tested for the photodegradation of Rhodamine B and revealed a 7-fold increased activity compared to pure titania. This shows the reactive hypersaline route to be a facile alternative for the synthesis of photoactive composites with controlled shape and porosity which are promising candidates for environmental applications like the removal of sea water pollutants or organic waste treatment.

ASSOCIATED CONTENT Supporting Information. Elemental compositions, BET specific surface areas and NLDFT pore volumes, NMR analysis, reaction mechanism of formation of IPTC, UV-Vis spectra, effect of methanol and isopropanol addition, and XRD pattern after photo-catalytic tests. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author(s) * E-mail: [email protected]; Tel.: (+39)011-0904745 (R.N.). * E-mail: [email protected]; Tel.: (+49)331–5679568 (N.F.).

Present Addresses † Now at: Polytechnic of Torino, Department of Applied Science and Technology DISAT, C.so Duca degli Abruzzi 24, 10129 Torino (Italy). # Now at: Ben-Gurion University of the Negev, Department of Chemistry, Mailbox 653, 84105 Beer-Sheva (Israel).

Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT This work was supported by the Max Planck Society and University of Torino. Compagnia di San Paolo and University of Torino are gratefully acknowledged for funding Project Torino_call2014_L2_126 through ‘‘Bando per il finanziamento di progetti di ricerca di Ateneo – anno 2014’’ (Project acronym: Microbusters) and European Commission is acknowledged for the MSC Research and Innovation Staff Exchange project funded (H2020-MSCA-RISE-2014, project number 645551 Mat4Treat). Authors would like to acknowledge Dr. Paola Benzi (University of Torino) for the elemental analyses.

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