Photocatalytic and Magnetic Porous Cellulose-Based Nanocomposite

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9858-9868

pubs.acs.org/journal/ascecg

Photocatalytic and Magnetic Porous Cellulose-Based Nanocomposite Films Prepared by a Green Method Alexandra S. M. Wittmar,*,†,‡ Qian Fu,† and Mathias Ulbricht†,‡ †

Lehrstuhl für Technische Chemie II, Universität Duisburg-Essen,Universitätsstraße 7, 45141 Essen, Germany NETZ − Nano Energie Technik Zentrum, CENIDE − Center for Nanointegration Duisburg-Essen, Carl-Benz Straße 199, 47057 Duisburg, Germany

‡

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S Supporting Information *

ABSTRACT: The present work expands our previous studies related to cellulose processing with room-temperature ionic liquids and simultaneous integration of functional nanoparticles toward photocatalytically active and easily recyclable nanocomposite porous films based on a renewable matrix material. Porosity can be tuned by the selection of phase separation conditions for the films obtained from the casting solutions of cellulose in ionic liquids or their mixture with an organic cosolvent. TiO2 nanoparticles confer to the nanocomposite photocatalytic activity, while Fe3O4 nanoparticles make it magnetically active. The photocatalytic activity of the cellulose film containing 10 mg of TiO2 was 1 order of magnitude lower than that of the same amount of pure TiO2 nanopowder, due to the reduction of the active catalytic surface which can be reached by UV irradiation after embedment in the polymer matrix. However, this fixation in a solid polymer support allows facile recovery of the catalyst after use. The rate constant when using the cellulose nanocomposite doped with TiO2 and Fe3O4 (k ≈ 0.0019 min−1) is very close to that for the corresponding composite containing only TiO2 (k ≈ 0.0017 min−1), suggesting that co-doping with Fe3O4 nanoparticles did not diminish the photocatalytic activity of the final composite, which can be easily separated from solution with a magnet. Additionally, by Fe3O4 doping, the composite material’s temperature can be homogeneously increased by ∼12 K via exposure to a high-frequency alternating magnetic field (AMF) for 5 min. For an optimal thermal response to AMF, the magnetite nanoparticles have to be homogeneously dispersed within the polymer matrix. The preparation method for the casting solution has been found to play an essential role for the one-step fabrication of multifunctional cellulose-based nanocomposite materials. KEYWORDS: titania nanoparticles, magnetite nanoparticles, cellulose nanocomposite, ionic liquid, non-solvent induced phase separation, photocatalysis

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INTRODUCTION Known since ancient times, cellulose has remained one of the most relevant polymeric resources, and because of its structure and properties, it has found use in a wide range of applications. The continuously growing interest in cellulose is strongly connected with its inexhaustibility, biocompatibility and biodegradability in the context of an increased demand for environmentally friendlier materials.1,2 Cellulose possesses very good mechanical strength due to the extended network of intraand intermolecular hydrogen bonds, which are also responsible for the partial crystallization of the material.3 The existence of numerous hydrogen bonds within the cellulose prevents its solubilization in water or common organic solvents as well as its melting; therefore, facile cellulose processing is hampered. The conventional methods for cellulose processing, which require the polymer dissolution, are done using aqueous solutions of transition metal complexes with amines, like cupriethylenediamine, or aqueous alkali base-containing solvents, e.g., 10% © 2017 American Chemical Society

NaOH. In organic polar solvents like dimethylacetamide or Nmethyl-2-pyrrolidone, cellulose has limited solubility in the presence of LiCl.4 More recently, new technologies, considered environmentally less hazardous, have been developed based on the use of N-methylmorpholine-N-oxide5 or acetone-based solvents.6 Due to their specific properties like low vapor pressure, inflammability, and good thermal and chemical stability, a special class or organic saltsroom-temperature ionic liquids (ILs)have also been amply discussed as potential environmentally friendlier solvents for cellulose solubilization and derivatization.7,8 The use of IL-based cellulose solutions as media for the preparation of cellulose-based nancomposite membranes has also gained importance in recent years.9−11 An Received: June 7, 2017 Revised: September 8, 2017 Published: September 10, 2017 9858

DOI: 10.1021/acssuschemeng.7b01830 ACS Sustainable Chem. Eng. 2017, 5, 9858−9868

Research Article

ACS Sustainable Chemistry & Engineering

nanoparticles in the solvent mixture followed by cellulose dissolution in the formed nanoparticle dispersion is used for the porous film preparation leads to the most homogeneous materials. By replacing in the nanocomposite half of the catalytically active species (TiO2) with magnetite, as expected, a certain decrease of the catalytic activity was observed, but the newly formed material could increase its temperature by ∼12 K after an exposure time of 5 min to a high-frequency AMF. The response of the multifunctional nanocomposite to the two stimuli (UV radiation and AMF) is schematically described in Figure 1. In addition, the nanocomposite can easily be separated from solutions with a simple magnet.

important downside of the use of the ILs as cellulose solvents is their relatively high viscosity: for example, 1-butyl-3-methylimidazolium chloride is a solid at room temperature and melts at ∼70 °C. As consequence, the dissolution of relevant fractions of cellulose in ILs is hindered by mass transport. The addition of a small fraction of an organic polar solvent like dimethyl sulfoxide (DMSO) to the IL improves both mass transport and cellulose solubility by partially breaking down the ionic association within the IL.12,13 Titania is one of the most important and widely studied inorganic materials, with a broad range of applications such as pigmenting,14 gas sensors,15 corrosion protection,16 optical coatings,17 solar cells,18 and photocatalysis.19,20 TiO2 can be prepared as films, rods, or nanoparticles as required for the desired applications. For the preparation of catalytically active TiO2 nanoparticles at a larger scale, many methods have been developed, but the only ones of industrial importance are flame pyrolysis and chemical vapor synthesis.21,22 Cellulose-based TiO2 nanocomposite materials, combining the advantages of a green support with the photocatalytic properties of the TiO2, are currently intensely studied for applications in photocatalysis,23−27 water purification,28,29 or self-cleaning materials.30 Magnetite (Fe3O4) in the nanoparticulate form, with good biocompatibility and superparamagnetic properties, has also been extensively studied in the past decades and has found applications in various fields of high importance like biomedicine,31,32 ferrofluids,33 metal removal from wastewater,34,35 lithium ion battery anodes,36 or recyclable catalysts.37 It is also known that, upon excitation with a highfrequency alternating magnetic field (AMF), Fe3O4 nanoparticles emit heat, a feature which has been considered, for example, for cancer therapy.38 Recently, the integration of Fe3O4 “nanoheaters” with a temperature-responsive polymer in a porous matrix had been demonstrated to be very versatile to fabricate a stimuli-responsive nanocomposite ultrafiltration membrane.39 In the past several years, Fe3O4 nanoparticles in combination with cellulose have been studied for applications as protein delivery systems,40 as new recyclable nanocatalysts,41 as superparamagnetic fabrics and membranes,42 or as material for water purification.43 In very recent studies, some authors have tried to combine the photocatalytic activity of TiO2 with the superparamagnetic properties of Fe3O4 along with a cheap cellulose support to develop a new generation of easily recyclable, photocatalytically active materials.44 In 2015, Duang and collaborators reported magnetic cellulose−TiO2 microspheres for highly selective enrichment of phosphopeptides.45 One year later, Li et al. reported the preparation of iron- and titania-doped cellulose fibers which they used as precursors for the preparation of Fe3O4−TiO2−carbon anode material for lithium ion batteries.46 In our previous works, we have reported the successful preparation of porous cellulose materials (as films or spheres) by phase separation processes from IL-based solutions in the presence or in the absence of a polar organic solvent.47,48 In the present paper, we describe in detail our studies on the preparation and characterization of porous cellulose-based porous films with catalytic and magnetic activity, obtained by the simultaneous embedding of TiO2 and Fe3O4 nanoparticles during the processing step. Special attention has been given to the method used for the dopant incorporation, and three different processing routines have been taken into account. It had been found that the method in which dispersion of the

Figure 1. Working principle of the photocatalytic and magnetic components in a photocatalytically and magnetically active porous cellulose-based film.

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MATERIALS AND METHODS

Materials. For the preparation of Fe3O4 by co-precipitation method, ferric chloride anhydrous (reagent grade 97%) and ferrous chloride tetrahydrate (puriss. p.a. ≥99.0%) from Sigma-Aldrich have been used as precursors. NH4OH 25%, obtained from Sigma-Aldrich has been used as precipitating agent. Oleic acid (analytical grade) from Fluka Analytical and triethylene glycol (purity of 99%) from Alfa Aesar have been used in some experiments for magnetite surface modification. Ethanol (reagent grade ≥99.5%) from Bernd Kraft has been used as washing agent for the removal of the excess of the coating agent. Cellulose Avicel PH 101 (Avicel) from Sigma-Aldrich, with DP ≈ 180,49 has been used as precursor for the preparation of the casting solutions. The room-temperature ionic liquids 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) and 1-butyl-3-methylimidazolium acetate ([Bmim][OAc]), both in BASF quality (≤95%), have been purchased from Sigma-Aldrich. The ILs used and the cellulose are both hygroscopic: [Emim][OAc] and [Bmim][OAc] water content was ≤0.5% according to supplier specification. All materials have been used “as received” because small amounts of water do not hinder cellulose solubilization.48 Dimethyl sulfoxide (DMSO) (analytical reagent, ≥99.5%) from VWR International was used as co-solvent for cellulose dissolution. The precipitation of the porous films was performed in distilled water. For the preparation of nanoparticledoped porous films “homemade” Fe3O4 and TiO2 Aeroxide P90 (kindly offered by Evonik Industries) were used. For the catalytic activity tests Rhodamine B (96%) from Alfa Aesar was used as model substance. Preparation of Magnetite Nanoparticles. Different types of magnetite nanoparticles have been prepared by chemical coprecipitation methods from water solutions at a molar ratio of ferric to ferrous ions of 2.45. The preparation methods were inspired by the available specialized literature.50−52 Some of the syntheses were conducted under argon and the rest under normal atmosphere at room temperature or at temperatures up to 80 °C. Oleic acid and triethylene 9859


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nonorganic; hence the metal nanoparticles can be considered to be unaffected by the GCIB bombardment. The analysis beam consisted of Bi1+ ions from a liquid metal ion-gun (LMIG) with an acceleration voltage of 30 kV. We applied the delayed extraction method, where the extraction pulse is delayed relative to the impact of the primary beam pulse, to achieve both high spectral and spatial resolution. The result of this measurement is shown as a RGB overlay. Ti-containing particles are marked in green, Fe-containing particles are marked in blue. Red signifies the surrounding cellulose matrix. The purity of the prepared magnetite nanopowders was evaluated with the help of ATR FT-IR spectroscopy using a Varian 3100 Excalibur Series spectrometer with an angle of incidence of 45° and a diamond crystal. The 4000−650 cm−1 spectral range was measured with an average of 32 scans per sample and with a resolution of 4 cm−1. The rheology of the nanoparticle-doped cellulose solutions prepared by the three methods mentioned above has been studied using an Anton Paar Physica MCR301 instrument with a plate and plate geometry with the diameter of 5.5 cm. During the viscosity vs shear rate measurements, in order to eliminate any previous shear histories and to allow the samples to achieve their equilibrium structures, a steady pre-shear was applied at a shear rate of 1 s−1 for 60 s followed by a 120 s rest period before the dynamic viscosity measurement. Temperature versus viscosity measurements have been performed for the 25−80 °C interval, at a constant rotation speed of 5 s−1 and a heating rate of 10 °C/min. The heating efficiency of the magnetic nanoparticles was studied by calorimetric method using a TruHeat HF series 5010 magnetic field generator with sandwich inductor coil made from copper tube, from TRUMPF-Hüttinger (Germany) (cf. ref 39). A homemade 1 mL vial with a vacuum mantel was filled with 1 mL magnetite dispersion in DMSO (5 mg of Fe3O4/1 g of DMSO). The sample vial was placed between the inductor coils and thus excited with an alternating magnet field at I = 15.6 A and f = 746 kHz. The temperature in the magnetite dispersions and in the background solvent was measured with a contact thermometer before and after 3 min exposure to the magnetic field. The specific loss power (SLP) was calculated with the formula53

glycol were used in some of the cases as coating agents for the nanoparticles. A detailed description of the synthesis protocols is summarized in the Supporting Information, Table S1. All prepared samples were washed first with ethanol, then with distilled water, and separated with the help of a strong magnet, and then dried at room temperature for 48 h. Before use the powders were dry milled in an agate mortar. Preparation of Cellulose-Based Magnetic and Catalytic Porous Films. Three routines have been studied and compared for the preparation of the nanoparticle-doped porous films: • Method 1: dispersion of the nanoparticles in the IL/co-solvent mixture → cellulose dissolution in the nanoparticles dispersion → membrane casting → non-solvent-induced phase separation (NIPS). • Method 2: dispersion of the nanoparticle in co-solvent in parallel with cellulose dissolution in IL → mixing of the cellulose solution with the nanoparticle dispersion → membrane casting → NIPS. • Method 3: dispersion of the nanoparticles in IL → dissolution of cellulose in nanoparticle dispersion → dilution with cosolvent → membrane casting → NIPS. In all cases polymer solutions with a 8 wt% cellulose content and a solvent ratio of IL:DMSO = 3:1 were prepared. The nanoparticle dispersions in the IL, co-solvent, or IL/co-solvent mixture have been prepared by ultrasound treatment for 2 h in an ultrasonic bath (BANDELIN, Sonorex Digitec) at a power of 80 W. For the cellulose dissolution, the Avicel powder and the corresponding amount of solvent have been added to a mortar and grinded together for several minutes until the cellulose was homogeneously dispersed within the solvent. Then the samples were transferred to a snap-cap vial and heated ∼5 h at 70 °C until the complete dissolution of cellulose and a partial degassing of the solution took place. Composite membranes with 5 and 10 wt% nanoparticles (with respect to the cellulose) have been prepared by NIPS from the solutions, i.e., by casting on a glass plate (thickness: 300 μm) and immediate immersion in a coagulation bath containing distilled water. The films were kept in water for 24 h for the completion of the phase separation, and then washed several times with water. The films were finally dried by freeze-drying method. Characterization Methods. Dispersions (Fe3O4 and/or TiO2) with 0.66 wt% nanoparticles in water, DMSO, IL and IL/DMSO mixtures were prepared by ultrasound treatment in an ultrasonic bath (BANDELIN, Sonorex Digitec) by the same procedure as described above. The aggregate sizes in dispersion were determined by dynamic light scattering (DLS) method using a Particle Metrix Stabisizer heterodyne backscattering equipment at a laser wavelength of 500 nm and a laser power of 6 mW. Each sample was measured for 3 times over a period of 60 s for each run and the result is the average of the three measurements. The DLS results were plotted as distribution by number and distribution by intensity for a better identification of differences. Scanning electron micrographs of the nanoparticles and of the porous films at different magnifications were taken with a FEI ESEM Quanta FEG instrument. The nanoparticle samples were prepared from dispersions in ethanol, spin-coated on silicon wafer. For mixtures of TiO2 and Fe3O4 the samples were investigated in normal mode, backscattering mode and combined. For the cross-section measurements of the porous films the samples were broken in liquid nitrogen. The samples were sputtered with the help of a K550 sputter coater from Emitech Ltd. with Au/Pd (80/20) at 0.1 mbar and 30 mA for 30 s until a layer of 2−3 nm was achieved. The distribution of the TiO2 and Fe3O4 nanoparticles within the polymer matrix has been studied by mapping with time-of-flight secondary ion mass spectrometry (ToF-SIMS) using a TOF.SIMS 5 device from Ion TOF. Before the actual measurement, the surface of the sample was bombarded for several minutes with a gas cluster ion beam (GCIB) of Ar1900+ clusters, at 10 kV acceleration voltage, in order to clean the surface and to remove the surface layer of cellulose. Heavy clusters sputter organic material much more efficiently than

SLP =

ΔT mpc p + mscs Δt mp

(1)

where mp and ms are the masses of particles and solution in which the particles are dispersed, cp and cs are the specific heats of particles and solution, and ΔT is the increase of the temperature in the sample due to the magnetite. The heating efficiency of the magnetite-doped porous films was evaluated using the same magnetic field generator. A porous film sample with the size of ∼1 cm × 2.5 cm was glued on the homemade glass vial with a double sided TESA tape and placed between the coils of the magnetic field generator. Digital pictures and thermal images were taken before and after 5 min exposure to the AMF with I = 15.6 A and f = 746 kHz. The thermal images were taken with an IR thermal imaging camera 885-2 from Testo AG. The bandgaps of selected photocatalysts used in this study have been determined from the Tauc plots of the studied materials. For the construction of the Tauc plots the UV−vis spectra of the samples have been recorded in reflectance mode in the 200−900 nm spectral range with a Cary 100 UV−vis spectrophotometer from Agilent. For each membrane two pieces have been measured. In order to assess the photodegradation efficiency in a heterogeneously catalyzed water treatment process, pieces of membrane (0.88 g of wet membranes or 0.11 g of dry membranes, containing ∼0.01 g of metal oxide), were inserted in 30 mL of aqueous Rhodamine B solution (5 × 10−5 M). The mixtures were first stirred in dark conditions until the adsorption−desorption equilibrium had been reached, then the samples were exposed for a certain amount of time to UV light (365 nm) in a TLC CN-15 viewing cabinet from Vilber Lourmat GmbH equipped with two 15 W lamps and yielding an UV intensity at location of the sample of 1050 μW/cm2. Small samples of dye solution were taken periodically by a syringe and the catalyst was 9860


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ACS Sustainable Chemistry & Engineering filtered off with a 0.2 μm cellulose acetate microfiltration membrane (Sartorius) prewashed with the parent dye solution. The variation of dye concentration with irradiation time was followed by UV spectroscopy using an UviLine 9400 spectrometer from Schott Instruments. The measurements were performed in the 400−800 nm spectral range with a resolution of 1 nm. For comparison purposes, the photodegradation of 30 mL of aqueous dye solution (5 × 10−5 M) with 0.01 g of pure metal oxide powders (TiO2, Fe3O4 and TiO2/ Fe3O4 mixtures) were analyzed in analogous manner. For a more convenient recovery of the photocatalyst for the reusability tests we have used cellulose:TiO2 photocatalytic spheres prepared by a similar method and with similar porous structure.42 Due to the fact that the active surface/weight ratio for spheres is considerably lower as for the membranes, samples with a higher TiO2 content (C:TiO2 = 1:0.6) have been used in this test. The amount of the composite spheres in wet state (equivalent of 0.06 g of TiO2) was immersed in the Rhodamine B solution and stirred until the equilibrium was achieved. Subsequently the photocatalyst was allowed to sediment at the bottom of the vessel and the Rhodamine B solution was decanted out and replaced with fresh one. The solution with the photocatalytic spheres was exposed for a certain amount of time to UV light (365 nm) in a TLC CN-15 viewing cabinet from Vilber Lourmat GmbH equipped with two 15 W lamps and yielding an UV intensity at location of the sample of 1050 μW/cm2. Samples were taken periodically and analyzed in a similar manner as for the membrane and powders tests. At the end of the test the remaining Rhodamine B was decanted out and the photocatalyst was reused after the addition of the fresh dye solution. The spheres were used in three photocatalytic cycles.

Table 1. Temperature Changes in the Magnetite Dispersions in DMSO and Specific Loss Power after 3 min exposure to AMF (I = 1.56 A and f = 746 kHz) sample DMSO Magnetite Magnetite Magnetite Magnetite Magnetite

1 2 3 4 5

ΔT [K]

SLP [W/g]

4.4 25 30.8 11.1 10.3 16.7

− 49.7 63.7 16.2 14.2 29.7

From the comparison of the results from Table 1 and the SEM micrographs a correlation of the specific loss power and the crystallite size can be observed; the materials composed of very small crystallites are able to generate less heat. We report in this work for the first time specific loss power data calculated from the measurements performed in DMSO and not in water. DMSO was used here due to its co-solvent role in porous film preparation in this study. However, this data offer a direct comparison between the heating activities of the five different magnetite nanopowders. Due to better dispersibility both in water and in DMSO the Magnetite 4 material was chosen for further dispersibility tests with ILs and IL/DMSO mixtures. From Figures 2 and 3 it can be observed that the magnetite disperses better in pure ILs as well as in pure DMSO. Therefore, one can conclude that the mixing of the ILs with DMSO is responsible for a decrease of the dispersions quality. In our previous studies54 we have shown that a key factor influencing the dispersibility of the metal oxide nanoparticles in ILs is the three-dimensional mesoscopic structure of the IL which allows the embedment of the particles in the structure and by this their subsequent stabilization. A possible explanation for the poorer dispersibility of the nanoparticles in the IL-DMSO mixtures may be the breaking of the anion− cation bonding within the ILs by DMSO as well as a disruption in the 3D self-organization of the IL molecules. The pure DMSO, a highly polar aprotic solvent, gave very good and very stable dispersions of the unmodified magnetite nanoparticles. Based on our previous results54 for the dispersions prepared in pure ILs, a better stability in [Bmim][OAc] was expected. For the systems diluted with DMSO it was observed that the dispersions prepared in the [Emim][OAc]/DMSO system are more stable than the equivalent ones prepared in the [Bmim][OAc]/DMSO system (Figures 2 and 3). However, the agglomerate sizes are slightly smaller in the [Bmim][OAc]based solvents. Influence of the Preparation Methods and Precursors on the Structural Properties of the Nanocomposite Membranes. Previous studies have shown that the hydrophilicity of the room-temperature ILs is increasing with the decrease of the cation alkyl chain length,55 meaning that pure [Emim][OAc] is more hydrophilic than pure [Bmim][OAc]; therefore is to be expected that the [Emim][OAc]-based solvents should be more miscible with water. Viscosity studies (see Supporting Information, Figure S4) have shown that the relative viscosities of the Avicel solutions in [Emim][OAc] are higher than the corresponding ones in [Bmim][OAc], indicating the [Emim][OAc] as the better solvent for cellulose. This suggests that using [Emim][OAc]-based solvents, due to a better thermodynamic stability of the formed polymeric solutions, a lower precipitation rate of the polymer is expected

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RESULTS AND DISCUSSION Characterization of Nanoparticles and Nanoparticle Dispersions. All five preparation methods yielded magnetite nanoparticles with sizes between 10 and 30 nm. The SEM analyses (see Supporting Information, Figure S1) revealed that the Magnetite 1 and 2 nanoparticles are constituted from welldefined nanoparticles between 20 and 30 nm. In the sample Magnetite 3, well-separated agglomerates of about 100 nm can be observed which are constituted of very small nanoparticles. The sample Magnetite 4 is constituted of small nanoparticles around 10 nm, agglomerated but well distinguishable. The Magnetite 5 nanopowder is constituted of well-defined nanoparticles with sizes between 10 and 20 nm. The differently prepared magnetite samples have very different dispersibilities in water and DMSO (see Supporting Information, Figure S2 and Table S2). As expected, the oleic acid coated materials disperse better in DMSO and poorer in water. The material modified with triethylene glycol, as expected, disperses well in water and poorly in DMSO. The Magnetite 4 disperses very well both in water and in DMSO. The differences in the nanoparticles surfaces are responsible for their different dispersibility and have been confirmed by FT-IR spectra (see Supporting Information, Figure S3). From the comparison between the particle size distributions by number and the particle size distributions by intensity one may observe that in some of the materials even if most of the sample is well dispersed (only small agglomerates are measured in distribution by number), the existence of a few large agglomerates cannot be completely excluded. The magnetic properties of the prepared Fe3O4 were evaluated by measuring the heat generated by the nanoparticles in dispersion under the exposure to an AMF. The dispersions were prepared in DMSO because the DMSO dispersions were later used for the casing solutions preparation. The results are presented in Table 1. 9861


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Figure 2. Agglomerate size distributions of Magnetite 4 in [Rmim][OAc]-DMSO solvents (R = ethyl, butyl).

led to the least homogeneous porous films, containing the most agglomerated nanoparticulate dopant. Even though the magnetite could be very efficiently dispersed in the pure IL (cf. Figure 2), during the early stages of cellulose dissolution the cellulose swells and we assume that the small agglomerates cannot penetrate the swollen cellulose and therefore tend to form larger agglomerates between the swollen polymer coils (see Figure 5, bottom). When Method 1 was used, related to a lower viscosity of the solvent caused by the dilution with DMSO, the above-mentioned phenomena were clearly less pronounced due to a better mobility of the magnetite agglomerates (see Figure 5, top). The most homogeneous material seems to be obtained from the Method 2, by diluting the cellulose solution with the nanoparticle dispersion in cosolvent (see Figure 5, middle). Similar results (not presented here) have been obtained also when the polymer matrix was doped with commercial TiO2 nanoparticles only or with Fe3O4. In order to complement the information provided by scanning electron microscopy, the mapping of the TiO2 and Fe3O4 nanoparticles within the cellulose matrix was performed by ToF-SIMS (Figure 6). The total nanoparticles (marked with black in the top left image) distributed in the film (cellulose marked with red) are composed from TiO2 (marked with green in the other images), Fe3O4 (marked with blue in the other images), and mixtures of them. Both types of nanoparticles are relatively homogeneously dispersed within the polymeric matrix. However, in the case of the TiO2, besides the smaller agglomerates also few larger agglomerates are present. For Fe3O4 the number of larger agglomerates is considerably lower, therefore we can conclude that the magnetite is better dispersed within the matrix than the TiO2. This result is in good agreement with the DLS data which indicate lower agglomerate

Figure 3. Dispersion stability of Magnetite 4 in [Rmim][OAc]-DMSO solvents: [Rmim][OAc] (1); [Rmim][OAc]:DMSO = 4:1 (2), and [Rmim][OAc]:DMSO = 3:1 (3) (R = ethyl, butyl).

in this case (cf. ref 56). These expectations have been confirmed by the reduction of the tendency for macrovoid formation and by a denser structure for the polymer film regenerated from [Emim][OAc]-based casting solutions (Figure 4). Three distinct methods for the preparation of the membrane casting solution containing the dispersed nanomaterial have been studied in detail (see Materials and Methods), and the results are presented in Figure 5. Clearly, the third method has 9862


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Figure 4. SEM micrographs of the Fe3O4-doped (5 wt% Magnetite 4) porous films prepared from 8 wt % Avicel solutions in [Emim][OAc]:DMSO = 3:1 (top) and [Bmim][OAc]:DMSO = 3:1 (bottom): inf luence of the ionic liquid used.

Figure 5. SEM micrographs of the TiO2- and Fe3O4-doped (5 wt% TiO2 + 5 wt% Magnetite 4) porous films prepared from 8 wt% Avicel solutions in [Bmim][OAc]:DMSO = 3:1: inf luence of the preparation method.

membrane containing 10 wt% TiO2 and of a cellulose membrane containing 5 wt% TiO2 and 5 wt% Fe3O4 are presented in Figure 8. The determined bandgap value of the pure TiO2 P90 in powder form as well as of the same material embedded in the polymer matrix is ∼3.33 eV, i.e., very close to the value corresponding to TiO2 anatase.57 The Tauc plot of the membrane containing 5 wt% TiO2 and 5 wt% Fe3O4 is more difficult to interpret due to the low amount of semiconductor in the sample, although the sharp cutoff at ∼3.33 eV may be also due to the presence of TiO2. The photocatalytic activity of the TiO2 nanoparticles, pure or mixed with Fe3O4, has been evaluated during the degradation of an organic model dye: Rhodamine B (Figure 9, left, and Table 2). Examples of Rhodamine B absorption spectra recorded during the photocatalytic tests performed with mixed TiO2/

sizes for Fe3O4 than for TiO2 (see Supporting Information, Figure S5). Overview images with lower magnification have proven that this presented area is representative for the entire sample. Functional Properties of the Nanocomposite Membranes. One may observe that the addition of Fe3 O 4 nanoparticles at a concentration of 5 wt% relative to cellulose confers to the porous film magnetic properties which are very valuable for practical applications. The presence of the magnetic component will allow the facile separation of the nanocomposite from solutions with help of a simple magnet (Figure 7). The bandgaps of selected photocatalysts used in this study have been determined from the respective Tauc plots. The Tauc plots of the TiO2 P90 nanopowder, of a cellulose 9863


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powder form as embedded in the porous cellulose film. The catalytic activity of the nanocomposite is 1 order of magnitude lower than of the “naked” nanoparticles. Comparing the nanocomposite catalysts prepared by the three methods, the highest catalytic activity was obtained for the composite films obtained by Method 3. This is most probably because the larger agglomerates obtained by this method had the tendency to migrate at the film surface (Figure 9 right and Table 2). By increasing the temperature at which the photocatalytic process was conducted, no significant changes of the reaction rate have been recorded (cf. Supporting Information, Figure S7). Surprisingly, the porous film containing 5 mg TiO2 and 5 mg of Fe3O4 has shown approximately the same photocatalytic activity as a similar film containing 10 mg of TiO2 Further studies will be necessary to explain this result indicating same catalytic activity at half catalyst loading. Reusability tests for the nanoparticle-doped cellulose nanocomposites have been carried out with materials prepared by the same process but having spherical form. The spherical form has been preferred for this test due to an easier recovery of the catalyst after use. The TiO2-doped porous spheres could be used successfully in at least three consecutive cycles. During the third cycle a part of the spheres broke down into smaller pieces, this phenomenon being responsible for a slight improvement of the photocatalytic activity by the creation of additional active surface (cf. Supporting Information, Figure S8). The heat generation in the porous film under the exposure to an alternative magnetic field has been studied by thermography with an IR thermal imaging camera. A porous cellulose sample prepared by Method 1 and doped with 5 wt% magnetite nanoparticles has increased its temperature by 11.7 K after an exposure of 5 min to an AMF of I = 15.6 A and f = 746 kHz (Figure 10). These results are only preliminary data which prove that the proposed concept is feasible. In future studies, it will be evaluated in depth which roles are played by the type of magnetite nanoparticles and by their amount, the duration and intensity of AMF exposure as well as of the porosity of the nanocomposite to the heat generation by AMF stimulation. The Importance of the Present Work in the Context of Other Similar Published Papers. The preparation methods proposed in the present manuscript have used an IL/DMSO platform for the preparation of the re-precipitated cellulose materials. The complete cellulose dissolution in these solvent mixtures could be achieved relatively easily by simply heating the cellulose dispersion in solvent mixture at 70 °C for 2−5 h,47 depending on the cellulose concentration. Most of the preparation routines for TiO2-doped cellulose membranes presented in the previously published literature involve dissolving of the cellulose in an alkaline solution, and for the complete process long pre-cooling (24 h) at temperatures between −8 and −18 °C are necessary, along with an acidic water bath required for cellulose re-precipitation.24−26 Besides the advantages given by the shorter solubilization times, working at temperature above 0 °C and the avoidance of strong base or acid solvents, the ILs offer the important advantage of facile recyclability from water after cellulose reprecipitation. The big downside of the IL route is the cost of these solvents, but this could be easily compensated by the adequate recycling. Our current studies regarding the recyclability of the ILs used during the preparation of porous cellulose nanocomposites will be the subject of a future publication.

Figure 6. Mapping of the TiO2 and Fe3O4 nanoparticles within a TiO2- and Fe3O4-doped (5 wt% TiO2 + 5 wt% Magnetite 4) cellulose porou films prepared by Method 1 from 8 wt% Avicel solutions in [Bmim][OAc]:DMSO = 3:1 (red = surrounding cellulose matrix; green = TiO2; and blue = Fe3O4). The imaged area for all four images is 75 μm × 75 μm.

Figure 7. Avicel doped with Magnetite 4 membrane prepared from a 8 wt% polymer solution in [Emim][OAc]:DMSO = 3:1 solvent mixture. The membrane can easily be separated from solution with a simple magnet.

Figure 8. Tauc plots of selected photocatalysts used in the work

Fe3O4 powder and with a TiO2/Fe3O4 nanoparticle-doped membrane are presented in the Supporting Information, Figure S6. One may observe, as expected, that higher amounts of photocatalyst decompose the model dye faster. There is no photocatalytic efficiency loss for the TiO2 when it is mixed with the Fe3O4. By embedment in the porous film, the photocatalytic nanoparticles loose partially their activity, probably mainly due to the loss of the active specific surface which can be accessed and excited by the UV irradiation. This fact is confirmed by the same bandgap value of the TiO2 in the 9864


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Figure 9. Photodegradation of 5 × 10−5 M Rhodamine B with nanoparticles, nanoparticle mixtures, and nanoparticle-doped porous cellulose films: treatment with membrane in the dark; photolysis (i.e., UV irradiation of dye solution) and adsorption (i.e., just exposure of dye solution to porous cellulose without nanoparticles) as control experiments.

Table 2. Rate Constant k for the Degradation of Rhodamine B (5 × 10−5 M in Water) with Different Nanoparticulate Mixtures and with Porous Nanoparticle-Doped Cellulose Films sample

amount of photocatalytically active substance [mg]

A direct comparison of the catalytic activities of the materials reported in the present work with the catalytic activities of TiO2-doped cellulose nanocomposites reported in previously published works is difficult due to the fact that various model organic pollutants have been used in very different concentrations and the level of doping with TiO2 was also in a very broad range. For example, Mohamed and collaborators25 prepared a TiO2 P25-doped regenerated cellulose membrane (RC/P25) which after 90 min degraded ∼25% of the dye in the solution. Our C:TiO2 = 10:1 (M3) membrane also degraded ∼25% of the dye in the solution after 90 min exposure to the UV irradiation. Further improvements of the catalytic activities of our membranes may be obtained after the optimization of the TiO2 and Fe3O4 doping levels and/or by using modified TiO2 nanoparticles in order to extend the catalytic activity also to the visible light range. A very positive result of the present work is the fact that the catalytic activity of the membrane doped with both TiO2 and Fe3O4 seems to be comparable with the one containing TiO2 only. Further in depth studies related to the distribution of the two nanoparticulate dopants within the membranes as a function of nanocomposite preparation conditions as well as regarding the optimal cellulose:TiO2:Fe3O4 ratio will be performed. The potential advantages due to the co-doping with magnetite were not fully explored yet and further studies are required. One obvious advantage of the magnetite integration is the easy retrieval of the catalyst from solution with a magnet. Preparation of Fe3O4- and TiO2-containing cellulose-based nanocatalysts with other geometries (e.g., spherical, monoliths) should be also explored. For photocatalytic processes which may be enhanced by temperature, the development of filter setups based on nanocomposites containing Fe3O4 and catalyst in the cellulose matrix and which can be simultaneously

k [min−1]

5 mg of TiO2 10 mg of TiO2 7.5 mg of TiO2 + 2.5 mg of Fe3O4 5 mg of TiO2 + 5 mg of Fe3O4 2.5 mg of TiO2 + 7.5 mg of Fe3O4 10 mg of Fe3O4

5 10 7.5

0.0076 ± 0.0012 0.0225 ± 0.0036 0.0108 ± 0.0013

5

0.0073 ± 0.0012

2.5

0.0062 ± 0.0009

0

C:TiO2 = 10:1 (M1) C:TiO2 = 10:1 (M2) C:TiO2 = 10:1 (M3) C:TiO2:Fe3O4 = 10:0.5:0.5 (M1)

10 10 10 5

below detection limit 0.0017 ± 0.0005 0.0016 ± 0.0002 0.0032 ± 0.0004 0.0019 ± 0.0003

Another advantage of the current method is connected with the use of the commercial dopant nanoparticles which are already available at large scale. By this, the typically used in situ generation of nanoparticles,23 which requires supplementary production steps and costly precursors, may be avoided. By comparison with the in situ generation of TiO2 nanoparticles at the surface of cellulose fibers, the methods proposed in this work leads to at least a partial embedment of the nanoparticles in the polymer matrix and, thus, to a partial loss of the catalyst active surface only. On the other hand, by partial embedment of the nanoparticles within the matrix we may expect a lower leaching rate and a better recyclability (higher number of cycles) of the photocatalyst. The feasibility has been demonstrated in this work.

Figure 10. Thermal images before (a) and after (b) exposure to AMF and digital image of the sample containing a C:Fe3O4 = 10:0.5 porous cellulose film (c). 9865



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irradiated with UV light and be exposed to an AMF may be envisaged. Additionally, as suggested in other works (e.g., ref 46), the TiO2 and Fe3O4 co-doped cellulose composites may be suited as precursors for the preparation of Fe3O4−TiO2−carbon anode materials for lithium ion batteries after the appropriate carbonization.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49-201-183 3147. E-mail: alexandra.wittmar@uni-due. de. ORCID

Alexandra S. M. Wittmar: 0000-0003-2125-3181 Funding

The work is supported by the Deutsche Forschungsgemeinschaft (DFG), grant number WI 4325/2-1.

CONCLUSIONS

In the present paper, we demonstrated that porous cellulosebased films with dual function (photocatalytic and magnetic) can successfully be prepared by non-solvent induced phase separation from cellulose solutions in ionic liquid/co-solvent mixtures doped with TiO2 and Fe3O4 using water as coagulation medium. The optimal choice of the casting solution preparation method plays a very important role for obtaining a porous nanocomposite with functional nanoparticles homogeneously dispersed within the polymer matrix. Dispersion of the functional nanoparticles either in the cosolvent or in the IL/co-solvent mixture seems to lead to their homogeneous dispersion within the final hybrid material. By embedment of photocatalytic TiO2 nanoparticles in the cellulose matrix, photocatalytically active porous films could be successfully be prepared. Magnetite co-doping to a TiO2doped porous polymer film has no negative impact on the photocatalytic activity of the final composite. The addition of only 5 wt% Fe3O4 nanoparticles to the porous cellulose material conferred the nanocomposite magnetic properties which can be used either for facile removal of the material by a simple static magnet of for its heating via embedded “nanoheaters” triggered by remote control, i.e., exposure to an alternating magnetic field. Further studies related to the optimization of the TiO2:Fe3O4 ratio as well as on the cellulose:nanoparticle ratio within the nanocomposite will be required in order to obtain materials in which the two functional nanoparticles exhibit maximal activity. Further on, by increasing the size of the magnetic nanoparticles or by targeted TiO2 concentration at the composite surface supplementary improvements can be envisaged. In relation with the targeted applications, other shapes of the nanocomposite (e.g., fibers or spheres) may also be taken into consideration.

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Research Article

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the collaboration with Mr. Smail Boukercha (SEM characterization) and Mr. Sebastian Kohsakowski (bandgap calculations) at the University of DuisburgEssen. We also thank Dr. Ulrich Hagemann from the Interdisciplinary Center for Analytics on the Nanoscale (ICAN), Nano Energie Technik Zentrum (NETZ), at the University of Duisburg-Essen, for the ToF-SIMS measurements and interpretation.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01830. Synthesis protocols for the Fe3O4 coprecipitation; SEM micrographs of the Fe3O4 nanoparticles; particle size distribution (DLS) for Fe3O4 nanoparticles in water and DMSO; FT-IR spectra of the Fe3O4 nanoparticles; relative viscosity of Avicel solutions in [Emim][OAc] and [Bmim][OAc]; particle size distribution (DLS) for TiO2 and Fe3O4 in [Bmim][OAc]:DMSO = 3:1; examples of Rhodamine B UV−vis absorption spectra during the photocatalytic tests; Rhodamine B photodegradation with a TiO2-doped cellulose film at 25 and 50 °C; recyclability tests for an TiO2-doped cellulosebased photocatalyst (PDF) 9866


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