Stabilization of Monodisperse, Phase-Pure MgFe2O

Article pubs.acs.org/JPCC

Stabilization of Monodisperse, Phase-Pure MgFe2O4 Nanoparticles in Aqueous and Nonaqueous Media and Their Photocatalytic Behavior Kristin Kirchberg, Anna Becker, André Bloesser, Tobias Weller, Jana Timm, Christian Suchomski, and Roland Marschall* Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany

J. Phys. Chem. C 2017.121:27126-27138. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/22/18. For personal use only.

S Supporting Information *

ABSTRACT: Monodisperse, monocrystalline magnesium ferrite (MgFe2O4) nanoparticles were synthesized phase purely by fast nonaqueous microwave-assisted solution-phase synthesis. Colloidal stabilization of the nanocrystals in nonaqueous media was realized either in-situ during synthesis or postsynthetically by surface capping with oleylamine and oleic acid. Phase transfer to aqueous media was performed employing citric acid and betaine hydrochloride, resulting in agglomerate-free dispersions of citrate- or betaine-functionalized MgFe2O4 nanocrystals. Furthermore, a one-step synthesis of highly stable, water-dispersible colloids of MgFe2O4 was achieved using polyvinylpyrrolidone as stabilizer. Characterization of the as-synthesized and functionalized nanoparticles was performed employing X-ray diffraction, UV−vis and infrared spectroscopy, thermogravimetry, dynamic light scattering, and transmission electron microscopy. Special focus was laid on phase purity, which was thoroughly monitored using Raman microscopy/spectroscopy. Photocatalytic reactions were performed to evaluate the use of such highly stable ferrite colloids for solar energy conversion.

1. INTRODUCTION Magnetic nanoparticles have gained significant attention in the fields of catalysis, magnetic resonance imaging, biomedicine, and data storage.1−5 In recent years, iron oxide-based materials have proven to be interesting due to their low toxicity, high magnetic susceptibility, and large variety of compositions. Recently, cubic spinel-structured iron oxide nanoparticles (MFe2O4), also called ferrite nanoparticles, have attracted attention due to their magnetic and optical properties, which make them suitable for biomedical applications,1,6 sensing,7−10 heterogeneous catalysis and photocatalysis,11−14 battery technologies,15,16 electrical devices,17 and high-density information storage.18 In particular, magnetite (Fe3O4) has been investigated intensively, but also magnesium ferrite (MgFe2O4) has gained a lot of attention due to its narrow band gap and band position reported, which make them attractive for application in photocatalytic processes such as hydrogen production.7,19,20 The synthesis of ferrite nanoparticles has been reported employing numerous different techniques, e.g., sol−gel,21−23 mechanochemical,24−26 solvothermal,27−33 coprecipitation,34,35 microwave-assisted,24,36,37 and high-temperature38−40 routes, in some cases under inert gas atmosphere. However, only a few reports take account of minor impurities of iron oxide byphases within their samples using Raman spectroscopy as a highly sensitive tool in addition to powder X-ray diffraction (PXRD) analysis. The stabilization of magnetic nanoparticles in nonaqueous and aqueous solution can improve their handling due to lower risk of exposure to fine dusts and even make them suitable for © 2017 American Chemical Society

all liquid applications. Ferrofluids have proven to be interesting for application in exclusion seals, sensors, dampers, and shock absorbers, for magnetic resonance imaging (MRI) contrast enhancement, and as cell-labeling agent because of their superparamagnetic behavior and easy liquid handling.41−45 For iron oxide nanoparticles there is a range of possible stabilizing methods in nonaqueous and aqueous media reported for various nanoparticle shapes and sizes.46−50 There are a number of reports about hydrophilically surfacemodified fer rite nanoparticles, mostly in the context of biomedical applications, but the colloidal stability of the surface-modified nanoparticles is usually not discussed.6,43,51−53 In this context, surface-modified MFe2O4 (M = Zn, Mn, Co, Mg) were reported by Naseri et al. using poly(vinylpyrrolidone) (PVP) and poly(vinyl alcohol) (PVA) as capping agents, which led to controlled growth, decreased particle agglomeration during calcination, and uniform size distribution of the ferrite nanoparticles. However, a drawback of this aqueous approach is the formation of different oxide impurities such as hematite (α-Fe2O3), ZnO, or MgO, respectively.52,53 Chandradass et al. reported the surface functionalization of MgFe2O4 nanoparticles with Igepal CO-520 (branched polyoxyethylene nonylphenylether) during reversed micelle synthesis. Variation of the water-to-surfactant ratio resulted in different particle sizes of 20−30 nm with a small size Received: September 4, 2017 Revised: November 8, 2017 Published: November 11, 2017 27126

DOI: 10.1021/acs.jpcc.7b08780 J. Phys. Chem. C 2017, 121, 27126−27138

Article

The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION 2.1. Synthesis of MgFe2O4 Nanoparticles. For microwave-assisted synthesis, a Monowave 300 (f = 2.45 GHz, Anton Paar Germany GmbH) equipped with a hydraulic pressure sensor was employed. The temperature was monitored using a ruby thermometer, which was placed inside a borosilicate glass vessel (30 mL, Anton Paar Germany GmbH) sealed with a screw cap. All reaction solutions were magnetically stirred at 300 rpm during microwave heating. 2.1.1. Microwave-Assisted Synthesis of Nonstabilized, Monocrystalline MgFe2O4 Nanoparticles. Microwave-assisted synthesis of nonstabilized, monocrystalline MgFe2O4 nanoparticles was performed using a modified procedure reported elsewhere.37 Briefly, a mixture of 0.5 mmol of magnesium acetyl-acetonate (Mg(acac)2, 98%, Acros) was dissolved in 15 mL of dry rac-1-phenyl-ethanol (>99%, Sigma-Aldrich) using ultrasonication. Then, 1 mmol of iron(III)−acetylacetonate (Fe(acac)3, >99%, Sigma-Aldrich) was added to the solution and sonicated for 5 min before being transferred into a 30 mL borosilicate glass vessel. The obtained solution was heated to maximum temperatures of 200, 250, 275, and 300 °C for 30 min using microwave irradiation under constant stirring (300 rpm). Next, the reaction mixture was quenched with compressed air to 55 °C. The nanoparticles were precipitated with trichloromethane and washed with distilled water and diethyl ether. Finally, the obtained powder was allowed to dry at room temperature, resulting in a fine powder with brown color. The pH of an 8 mg/mL suspension of these nanoparticles was 9.8. Calcination of as-prepared MgFe2O4 nanoparticles in air was performed in a muffle furnace. Nanoparticles were placed inside a ceramic crucible and heated to different temperatures between 400 and 1000 °C with a heating rate of 10 °C/min. The maximum temperature was maintained for 60 min before cooling the samples to room temperature. 2.1.2. Microwave-Assisted Solution-Phase Synthesis of Oleylamine-/Oleic Acid-Functionalized MgFe2O4 Nanoparticles. Microwave-assisted solution-phase synthesis of oleylamine-/oleic acid-functionalized MgFe2O4 nanoparticles was performed modifying a synthesis procedure reported by Sun et al.38 The synthesis was carried out using Monowave 300 instead of the batch setup reported in the literature. Briefly, a mixture of 5 mmol of 1,2-dodecanediol, 0.33 mmol of Mg(acac)2, 0.66 mmol of Fe(acac)3, 3 mL of oleylamine (OLA, 70 %, Sigma-Aldrich), 3 mL of oleic acid (OA, 90 %, abcr), and 10 mL of dibenzyl ether (>98%, Fluka) was sonicated until all solid components were fully dissolved. Then the magnetically stirred mixture was heated up to 275 °C under microwave irradiation in a 30 mL borosilicate glass vessel. After 30 min, the reaction was quenched using compressed air. The nanoparticles were precipitated with dry ethanol (99.86 %, VWR). Redispersion in a mixture of n-hexane (99 %, abcr) with 0.25 mL of OA and 0.25 mL of OLA was followed by centrifugation at 6000 rpm to separate nondispersible components. The stable supernatant was then washed three times with ethanol before being redispered in toluene (99 %, abcr; 25 mg/mL). 2.1.3. Microwave-Assisted Synthesis of Oleylamine-/Oleic Acid-Functionalized MgFe2O4 Nanoparticles. Microwaveassisted synthesis of oleylamine-/oleic acid-functionalized MgFe2O4 nanoparticles was performed by adding 4.5 mL of OA and 4.5 mL of OLA to a reaction mixture described in section 2.1.1. The work up after microwave heating was identical with that described under section 2.1.2.

distribution. With a high saturation magnetization of 40 emu/g, application of the prepared MgFe2O4 nanoparticles in hyperthermia treatments was suggested.51 Thermal decomposition of organic metal precursors (e.g., oleates, carbonyls, and acetylacetonates) in the presence of oleic acid (OA) in high-boiling solvents in combination with other ligands like trioctylphosphine oxide (TOPO) or oleylamine (OLA) is a common method to synthesize stable nonpolar colloids of MFe2O4 nanoparticles (M = Fe, Mn, Co, Zn, Ca, Mg, Cu).38,40,54−57 In contrast, the phase transfer into polar solvents or the direct synthesis of hydrophilic ferrite colloids is still a subject of current research. Munjal et al. successfully performed the phase transfer of OA-coated CoFe2O4 nanoparticles, which were synthesized by the hydrothermal method. Chemical treatment with citric acid in a solvent mixture of toluene and dimethyl sulfoxide (DMSO) led to the precipitation of water-dispersible, citric-acid-coated, polycrystalline CoFe2O4 nanoparticles of 13 ± 2 nm.31 Regarding applications, Singh et al. investigated the adsorptive removal of crystal violet cationic dye by sodium dodecyl sulfate (SDS)-modified CoFe2O4 nanoparticles. It was observed that the nanoparticle and dye concentration, the pH value, and the contact time influence the efficiency of the dye adsorption and therefore its removal from solution.58 Zhao et al. produced citrate-stabilized ZnFe2O4 nanoparticles by thermal decomposition of metal acteylacetonate precursors with sodium citrate in triethylene glycol (TREG) at reflux temperature. They investigated the influence of the surfactantto-precursor ratio, finding that high surfactant amounts lead to low aggregated nanoparticle dispersions with good stability and small size distribution.59 For most of the reported synthesis procedures for bare and functionalized MgFe2O4 and ZnFe2O4 colloidal solutions, no Raman spectroscopy data were provided to confirm the results from X-ray diffraction phase analysis. As ferrites, being an iron oxide modification, tend to form various by-phases, e.g., maghemite (γ-Fe2O3) or the thermodynamically stable modification hematite (α-Fe2O3), the investigation of phase purity using Raman spectroscopy is helpful to detect small amounts of impurity phases, as this technique is very sensitive in differentiation between various iron oxide species in nanocrystalline solids.60−62 Due to similar cell parameters and crystal structures of the different iron-containing materials, typical reflections in X-ray diffraction differ only slightly. In combination with a large width of the reflections because of small coherent scattering domains in nanoparticles, sole X-ray diffraction phase analysis is not sufficient to detect minor impurities. Here, we developed a one-pot synthesis for aqueous MgFe2O4 colloids by microwave-assisted nonaqueous synthesis. The production of stable nonaqueous colloids of phase-pure MgFe 2O 4 nanoparticles was achieved by postsynthetic functionalization with long-chain organic ligands. Phase transfer of the highly stable hydrophobic colloids into aqueous medium was performed employing citric acid and betaine hydrochloride, resulting in agglomerate-free dispersions of citrate- or betainefunctionalized MgFe2O4 nanocrystals. Furthermore, first results on the photocatalytic activity of stabilized MgFe2O4 nanoparticles and the advantage of stabilization toward degradation of rhodamine B (RhB) are presented. 27127


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The Journal of Physical Chemistry C 2.1.4. Microwave-Assisted Solution-Phase Synthesis of Water-Dispersible, Monocrystalline MgFe2O4 Nanoparticles. Microwave-assisted solution-phase synthesis of water-dispersible, monocrystalline MgFe2O4 nanoparticles was performed adapting a procedure by Abdulwahab et al. for microwaveassisted synthesis.39 Here, 0.05 mmol of Mg(acac)2 was dissolved in 12 mL of triethylene glycol (TREG, 99%, SigmaAldrich). Then 0.1 mmol of Fe(acac)3, 150 mg of poly(vinylpyrrolidone) (PVP, MW 10 000, Alfa-Aesar), and 3 mL of TREG were added, and the mixture was sonicated until all solid components were completely dissolved. Microwave heating to 275 °C for 45 min was followed by precipitation with a mixture of diethyl ether and acetone (1:2). The precipitate was washed with diethyl ether and dried at room temperature. The obtained nanoparticles were dispersible in polar solvents, e.g., water (15 mg/mL). 2.1.5. Microwave-Assisted Synthesis of PVP-Modified Monocrystalline MgFe2O4 Nanoparticles. Microwave-assisted synthesis of PVP-modified monocrystalline MgFe2O4 nanoparticles was performed for comparison by adding 1.5 g of PVP to the microwave-assisted synthesis presented in section 2.1.1, following reaction for 45 min at 275 °C. Particles were precipitated in a mixture of acetone and diethyl ether. The precipitate was washed with diethyl ether and then dried at room temperature. The nanoparticles were dispersible in polar solvents, e.g., water (15 mg/mL). The pH of an 8 mg/mL colloidal solution of these nanoparticles was 7.5. 2.2. Postsynthetic Treatment of MgFe2O4 Nanoparticles. Several methods were used for surface functionalization on the one hand to obtain stable colloids in polar and nonpolar solvents, and ligand stripping on the other hand to obtain bare MgFe2O4 nanoparticles. Furthermore, the nanoparticle samples prepared after section 2.1.1 were calcined at different temperatures. 2.2.1. Postsynthetic Stabilization of MgFe2O4 To Form Stable Nonpolar Colloids. Postsynthetic stabilization of MgFe2O4 to form stable nonpolar colloids was performed for nanoparticles obtained from microwave-assisted synthesis described in section 2.1.1. An approach described by Patil et al.63 was therefore modified. Briefly, 20 mg of MgFe2O4 nanoparticles was added to a mixture of 1 mL of OA and 1 mL OLA in 10 mL toluene. The solution was refluxed for 48 hrs. Afterward, the nanoparticles were precipitated with methanol (>99.9 %, Sigma-Aldrich) and dried at room temperature before being dispersed in nonpolar solvents such as toluene. Here, nanoparticle concentrations up to 25 mg/mL were possible. 2.2.2. Phase Transfer of Nonpolar Colloids into Polar Solutions. Phase transfer of nonpolar colloids into polar solutions was performed using two different phase transfer agents. First, an approach by Patil et al.63 was adapted. To obtain stable aqueous colloids, 8 mg of MgFe2O4 nanoparticles was stirred in a betaine hydrochloride solution of 2 wt% (>98%, TCI) for 6 hrs. Precipitation with acetone was followed by drying at room temperature. Nanoparticles functionalized with betaine hydrochloride were afterward dispersible in polar solvents, e.g., water (8 mg/mL, pH = 4.6). In a second approach after Lattuada et al.,46 30 mg of OLA/ OA-functionalized MgFe2O4 nanoparticles was added to a solution of 1.9 mL of N,N-dimethylformamide (99%, Acros), 1.9 mL of 1,2-dichlorobenzene (>99%, Merck), and 25 mg of citric acid monohydrate (>99.5%, Alfa-Aesar). The mixture was kept at 100 °C for 24 hrs. Then the nanoparticles were

precipitated using diethyl ether and washed with acetone. After drying at room temperature, the particles were dispersible in water (8 mg/mL, pH = 8.4). 2.2.3. Ligand Stripping of MgFe2O4 Nanoparticle Surfaces. Ligand stripping of MgFe2O4 nanoparticle surfaces was performed using a modified procedure reported by Dong et al.64 Here, a solution of 5 mg of nanoparticles per milliliter of toluene was mixed with 5 mL of triethyloxonium tetrafluoroborate solution (10 mM Et3OBF4 (>95%, TCI)) per liter of acetonitrile (99.9 %, Acros) and stirred for 2 hrs. Then distilled water was added, which led to the transfer of MgFe2O4 nanoparticles into the aqueous phase. The particles were washed with distilled water and dried at room temperature. The pH of an 8 mg/mL suspension of these nanoparticles was 5.7. 2.3. Materials Characterization. UV−vis absorption spectra were collected from 800 to 200 nm on a PerkinElmer Lambda 750 UV/vis−NIR spectrometer equipped with PrayingMantis mirror arrangement. The step width was 1 nm. Data evaluation was performed using UV Winlab software (PerkinElmer). For degradation experiments, 3 mL of the observed solutions was placed inside a 1 cm quartz glass cuvette (Hellma). Thermogravimetric analysis (TGA) was performed on a STA409PC thermos scale (Netzsch) in combination with a QMG421 quadrupole mass spectrometer (MS) from Balzers with a heating rate of 5 °C/min in synthetic air (80 % N2, 20 % O2) or argon. The ionization energy was set to 70 eV, and a temperature range between 30 and 1000 °C was analyzed. All samples were dried at 80 °C for several hours prior to TG-MS analysis. Powder X-ray diffraction (PXRD) measurements were performed on a PANalytical Empyrean powder diffractometer with a θ−2θ geometry equipped with a PIXcel3D detector using Cu Kα radiation (λ = 1.540598 Å). Diffractograms were recorded between 25° and 65° 2θ in a step scan mode with an emission current of 40 mA and an acceleration voltage of 40 kV. In-situ PXRD measurements were performed in a melted off glass capillary at the I15 beamline at the Diamond Light Source, Didcot with an acceleration voltage of 60 kV (λ = 0.4246 Å). The capillary was placed in the beam focus and stirred in a stable upfront position. The heating process was inducted by a Gas blower GSB 1300 from FMB Oxford, which heated the sample as fast as possible (heating rate approximately 1 °C/ min). The data was collected on a mar345 image plate detector. Crystallite sizes were calculated using the Scherrer equation for (400) and (440) reflections of MgFe2O4 (JCPDS card no. 360398). For nitrogen physisorption measurements at 77 K, an Autosorb-1-MP (Quantachrome) setup was used. The nanoparticles we degassed in vacuum at 120 °C for at least 20 hrs prior to the measurement. Infrared (IR) spectra were collected on a Bruker IFS25 FTIR spectrometer in a frequency range between 4000 and 400 cm−1 using KBr pellets. Dynamic light scattering (DLS) and zeta-potential measurements were performed on a Zetasizer Nano ZS (Malvern) in high dilution. For DLS measurements a refractive index of 2.39 for MgFe2O4 was assumed.65 For analysis of aqueous solutions polystyrol cuvettes (d = 1 cm) were used, whereas quartz glass cuvettes (d = 1 cm, Hellma) were chosen for nonaqueous solutions. Three consecutive measurements were performed for each sample, and the average number distribution was used for particle size distribution evaluation. Zeta potentials were 27128


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The Journal of Physical Chemistry C measured for aqueous solutions applying the Smoluchowski equation. Every sample was measured six times using polycarbonate DTS capillaries (Malvern). All DLS and zetapotential measurements were performed at 25 °C. Transmission electron microscopy (TEM) images were collected on a CM 30-ST electron microscope (Philips) equipped with a LaB6 cathode (300 kV). Samples were suspended in either water or toluene and drop casted on carbon-sputtered copper grids (Plano GmbH) at room temperature. After drying, the grids were loaded. Raman spectroscopy was performed using a Senterra Raman spectrometer (Bruker) with OPUS 7.5 software. A Nd:YAG laser (λ = 532 nm) with a laser power of 0.2 mW and a magnification of 100 was chosen. Measurements were performed with a spectral resolution of 3−5 cm−1, 250 coaddition, and 3 s integration time. The nanoparticle samples were dried in a drying oven at 90 °C prior to measurements. Then the powders were spread on a microscope slide. 2.4. Testing the Photocatalytic Activity of MgFe2O4 Nanoparticles. A 10 mg amount of MgFe2O4 nanoparticles was mixed with 20 mL of rhodamine B solution (10 μmol L−1, resulting nanoparticle concentration 0.5 mg mL−1). Constant homogenization was performed using low-power ultrasonication (ultrasonic bath). Before irradiation, the reaction mixture was sonicated for 30 min in the dark to reach adsorption− desorption equilibrium of RhB with the particles. To avoid temperature increase, a glass coil flushed with temperaturecontrolled water was immersed into the ultrasonic bath keeping the reaction temperature constant at 27 ± 1 °C. In intervals of 30 min, 3 mL of the reaction solution was taken and the particles were separated with a magnet. The clear solution was measured with absorption spectroscopy in the range of 200 to 800 nm. Irradiation was performed with a 150 W solar simulator (Newport Sol1A) equipped with an AM 1.5G filter.

Figure 1. PXRD patterns with assigned reflections (JCPDS card no. 36-0398) and Raman data with assigned vibrational bands66 of assynthesized MgFe2O4 nanoparticles after microwave-assisted synthesis at different reaction temperatures.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization and Thermal Stability of MgFe2O4 Nanoparticles. Microwave-assisted synthesis of nonstabilized MgFe2O4 nanoparticles resulted in crystalline powder materials directly after the synthesis procedure without any additional heat treatment. Figure 1 shows the powder X-ray diffraction (PXRD) patterns after synthesis at different temperatures. All major reflections attributable to spinel magnesium ferrite (JCPDS card no. 36-0398) are detected after 200 °C with a crystallite size of ca. 4.9 nm. With increasing synthesis temperature, the reflections become slightly narrower with an estimated crystallite size of 6.3 nm at 250 °C, 6.7 nm at 275 °C, and 9.3 nm at 300 °C. However, at 300 °C additional reflections were observed, which derive from impurities out of the PTFE-coated silicone septum of the microwave vial; small amounts can be already seen at 275 °C. Those additional reflections do not derive from iron oxide by-phases, which can be determined via Raman spectroscopy of the as-synthesized samples after the different synthesis temperatures, also shown in Figure 1. Five Raman-active modes for cubic MgFe2O4 (space group Fd3m) were observed at ≥250 °C, as expected.66 The sample prepared at 200 °C shows only very weak Raman signals due to lower crystallinity. No Raman bands of α-Fe2O3, other iron oxides, or MgO were observed, which demonstrates that the as-synthesized MgFe2O4 nanoparticles do not contain these by-phases.61,67 However, an additional shoulder band of the A1g mode can be seen at roughly 660 cm−1, indicating a defective inverse spinel structure in which both Fe3+ and Mg2+

ions occupy tetrahedral sites, which is in good agreement with literature results.68,69 In a defect-free magnesioferrite spinel structure, Mg2+ ions are solely located on octahedral sites. The direct band gap for all as-prepared nanoparticles in the given temperature range can be determined from absorption spectroscopy to be 2.43 eV (exemplarily shown in Figure S1, Supporting Information SI), which is in good agreement with the direct band gap of MgFe2O4 nanoparticles reported in the literature and slightly higher than the band gap of bulk MgFe2O4.7,70 The observed blue shift is likely due to a quantum size effect increasing the band gap caused by the small crystallite sizes below 10 nm.71 FTIR spectra of the assynthesized nanoparticles indicate residual carboxylate species on the surface of the nanoparticles (Figure S2 SI) and an increasing number of OH groups with increasing synthesis temperature. At 300 °C, additional bands from residues of the corroded Teflon foil (i.e., PTFE-coated silicone septum) were observed. Therefore, 275 °C was used as the standard synthesis temperature for the stabilization experiments. The nanoparticles were found to be monocrystalline as proven by high-resolution TEM analysis (HR-TEM) (Figure S3 SI). In order to investigate the thermal stability of the nanoparticles, the as-synthesized sample prepared at 275 °C was calcined at different temperatures (for details please refer to the Experimental Section). The respective PXRD patterns and Raman results are shown in Figure 2. As can be seen, the reflections in the PXRD patterns become much narrower after calcination, indicating crystallite growth. Moreover, at temper27129


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small amounts of γ-Fe2O3 (= (Fe3+)8[Fe3+ 5/61/6)]16O32, maghemite) in the presence of oxygen.72 The latter compound is stable up to 500−600 °C and has again very similar PXRD reflections and Raman bands like MgFe2O4. Maghemite (γFe2O3, JCPDS card no. 04-0755) transforms into α-Fe2O373 (JCPDS card no. 33-0664) above 500−600 °C, as confirmed by PXRD and Raman data, which confirms indirectly the initial existence of Fe2+ and the transformation mechanism described. Even these results underline the necessity and importance of Raman investigations regarding the phase purity of the synthesized and thermal-treated nanoparticles. Calcination of the as-synthesized MgFe2O4 nanoparticles in an oxygen-free environment should lead to no formation of maghemite and accordingly to no formation of hematite. Figure S4 shows PXRD patterns and Raman data of MgFe2O4 nanoparticles (microwave-assisted synthesis at 275 °C) after calcination in oxygen-free conditions (sealed capillary) to 650 °C. As a result of the oxygen-free calcination, no hematite formation can be observed. However, since IR spectra (Figure S2) showed residual carboxylate species on the surface of the nanoparticles, those could act reductively on the spinel lattice. In order to investigate the thermal stability of the MgFe2O4 nanoparticles in more detail, in-situ PXRD with hard X-ray irradiation at the Diamond Light source, Beamline I15, in the temperature range of 20−640 °C (PXRD pattern measured every 20 °C in a closed capillary) was performed. All reflections of the PXRD pattern of the as-synthesized sample (bottom pattern in Figure 3) could be assigned to MgFe2O4 (JCPDS card no. 36-0398). Additionally, the full width at half-maximum (FWHM) of all reflections deriving from MgFe2O4 nanoparticles is decreasing with increasing temperature, indicating that the coherent scattering domains grow with increasing temperature. Moreover, the intensities of the reflections increase with increasing temperature, both indicating higher crystallinity of the samples after heat treatment. These trends confirm the ex-situ measurements (Figure 2 and Table S1, Supporting Information). At 600 °C, in-situ synchrotron measurements show additional reflections (between (311) and (222) reflection) and shoulders (at (400) and (440) reflection) verifying the presence of MgO (marked with a hashtag in the pattern), which could not be observed with the lab instrument (Figure S4). Finally, in the PXRD pattern which was collected after cooling the heated sample down to room temperature, reflections of MgO as a by-phase are clearly visible; no αFe2O3 formation is detected. The results confirm the inhibited formation of maghemite under oxygen-free conditions, resulting in no α-Fe2O3 formation. MgO is however formed due to reducing conditions present in the sealed capillary due to residual precursors on the nanoparticle surface, clearly visible with synchrotron radiation. 3.2. In-Situ Stabilization of Microwave-Derived MgFe2O4 Nanoparticles. In order to disperse the nanoparticles without agglomeration in nonpolar environments, the addition of stabilizing agents to the microwave synthesis was investigated. As mentioned in the Experimental Section, the high-temperature solution-phase synthesis of Sun et al. was adapted to the microwave approach. Additionally, common surfactants OA and OLA were added to the presented rac-1phenylethanol-based microwave synthesis. In both cases, well-dispersible nanoparticles with a size of 7.2−8.2 nm were achieved. PXRD patterns indicate phase-pure MgFe2O4 nanoparticles after both syntheses (Figure 4).

Figure 2. PXRD patterns with assigned reflections (JCPDS card no. 36-0398) and Raman spectra with assigned vibrational bands66 of MgFe2O4 nanoparticles (microwave-assisted synthesis at 275 °C) calcined in air at indicated temperatures; impurity phases observed after heat treatment are marked: (*) α-Fe2O3, (#) MgO.

atures above 700 °C additional reflections are found, belonging to α-Fe2O3 (indicated with an asterisk in Figure 2) and MgO (indicated with a hashtag in Figure 2). However, Raman data indicate the formation of α-Fe2O3 already at 600 °C, showing that Raman spectroscopy is a very powerful tool to analyze the phase purity of ferrite spinels, especially in the case of controlling α-Fe2O3 by-phase formation.61,62 The crystallite size of the calcined MgFe2O4 samples increases up to 41 nm at 1000 °C, with the band gap decreasing to a bulk value of 2.2 eV (Table S1, SI). The occurrence of minor amounts of α-Fe2O3 cannot be prevented at elevated temperatures: A very small fraction of Fe2+ is formed during the microwave-assisted synthesis by partial oxidation of 1-phenylethanol with Fe3+ above 200 °C. Small quantities of acetophenone were found in the reaction solution evidenced by gas chromatography connected with mass spectrometry (data not shown). For that reason, besides MgFe2O4, small amounts of Fe3O4 (magnetite) or even Fe2+2+ 3+ containing MgFe2O4 (as (Mg2+ 1−xFex )A[Fe2 )]BO4−δ) could be present. Fe3O4 has an inverse spinel structure with one-third of the Fe2+ cations occupying octahedral sites in the ccp lattice formed by oxygen atoms. It has very similar PXRD reflections and Raman bands to MgFe2O4. Thus, its existence cannot unambiguously be proven due to the reflection broadening caused by the small nanoparticles. Structural changes induced by thermal treatment are expected at temperatures ≥ 300 °C, accompanied by oxidation of Fe2+ to Fe3+ and formation of a mixture of MgFe2O4 and 27130


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Figure 4. PXRD patterns with assigned reflections (JCPDS card no. 36-0398) (top) and Raman spectra with assigned vibrational bands61,66 (bottom) of microwave-derived MgFe2O4 nanoparticles synthesized at 275 °C according to section 2.1.1 (A), adapting a synthesis procedure reported by Abdulwahab et al. according to section 2.1.4 (B), by PVP addition to the 275 °C synthesis according to section 2.1.5 (C), by OLA/OA addition to the 275 °C synthesis according to section 2.1.3 (D), and adapting a synthesis procedure reported by Sun et al. according to section 2.1.2 (E).

Figure 3. Temperature-dependent in-situ PXRD patterns of assynthesized MgFe2O4 (275 °C, olive) prepared by microwave-assisted synthesis measured at the Diamond Light Source with hard X-ray irradiation (λ = 0.4246 Å). Further diffractograms between room temperature and 500 °C are given in Figure S5.

dispersible MgFe2O4 nanoparticles in a one-step synthesis indicated by the similar Raman pattern compared to the synthesis of bare MgFe2O4 (section 2.1.1). The crystallite size was determined to be 5.5 and 6.2 nm. TEM images of the resulting nanoparticles are given in Figure S8. Moreover, both approaches result in stable dispersions of PVP-stabilized MgFe2O4 nanocrystals in water (Figure S9). The size distribution is, however, broader and shifted to larger hydrodynamic diameters compared to the OA/OLA-stabilized impure MgFe2O4 nanocrystals (compare Figure S7), which might be due to particle aggregation or PVP layer(s) formed around the particles. Physisorption measurements revealed a BET surface area of only 50 m2/g compared to 140 m2/g for nonfunctionalized nanoparticles. This confirms the assumption of PVP-coated nanoparticle agglomerates. Nevertheless, stable colloidal dispersions of MgFe2O4 nanocrystals in water were successfully prepared in a one-step approach. More details about the stabilization agents are given in the next section. Detailed characteristics (e.g., zeta potentials, isoelectric points) of nonstabilized and stabilized MgFe2O4 nanoparticles can be found in Table S2. The BET surface area of ligand-stripped nanoparticles was found to be 40 m2/g. This indicates an agglomeration of the nanoparticles, which goes along with the

Moreover, TEM (Figure S6) and DLS measurements (Figure S7) show a very narrow size distribution and a low degree of agglomeration. However, Raman spectroscopy revealed that the resulting nanoparticles are not phase pure or even MgFe2O4 but rather a mixture of iron oxides with some amounts of MgFe2O4 (Figure 4).61,62 Although the PXRD patterns indicate phase-pure samples, it has to be noted that feroxyhyte (δFeOOH, JCPDS card no. 98-003-8299) and maghemite (γFe2O3, JCPDS card no. 04-0755) exhibit a very similar crystal structure and therefore similar XRD peak positions as MgFe2O4 and thus cannot be distinguished at crystallite sizes below 10 nm due to peak broadening. Therefore, Raman spectroscopy is used as a suitable tool to investigate the phase composition in detail. For PVP functionalization, adapting the procedure by Abdulwahab et al.,39 Raman spectroscopy (Figure 4) analysis also suggests the formation of iron oxide by-phases indicated by strong shoulders above 700 cm−1, which do not refer to the partly inverted spinel structure of MgFe2O4 as discussed before.68,69 By adding PVP to the presented microwave-assisted synthesis it was possible to prepare phase-pure, water27131


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narrower size distribution with a maximum of 20 nm in hydrodynamic diameter. A slight tailing was determined up to 40−50 nm, indicating the formation of a few small agglomerates. In order to obtain agglomerate-free dispersions of stabilized MgFe2O4 nanoparticles in water for photocatalytic applications, nanoparticles were transferred by two approaches from nonpolar to polar media. On one hand, the OA/OLA ligands were exchanged with citric acid, while in a second approach the nonfunctionalized, as-synthesized nanoparticles were modified with betaine hydrochloride. The surface area citric acid-treated nanoparticles (5.1 m2/g) is quite low, as the electrostatic stabilization of the colloidal solution breaks down during drying, which leads to heavy agglomeration. Both approaches resulted in highly dispersible, nearly agglomerate-free MgFe2O4 nanoparticles in water as proven by DLS. The zeta potential for betaine hydrochloridemodified nanoparticles (8 mg/mL) was 37 mV, indicating the presence of positively charged surface groups (betaine cations), and the isoelectric point was found to be at pH = 7.9 (see Table S2). Figure 5 gives the size distribution for citric acid- and betaine hydrochloride-modified MgFe2O4 nanoparticles dispersions in water. In both cases, the size distribution is much narrower compared to OA/OLA-MgFe2O4 dispersions in toluene. Moreover, the peak hydrodynamic diameters are only slightly larger than the determined crystallite sizes of the microwave-derived MgFe2O4 nanoparticles prepared at 275 °C, which is due to the ligand and the hydration shell. These results indicate agglomerate-free dispersions. In particular, the citric acid-modified MgFe2O4 nanoparticle dispersion shows a very narrow size distribution, with the peak of the hydrodynamic diameter at 6−7 nm, a zeta potential of −20 mV, and an isoelectric point at pH = 10.6 (see Table S2). Figure S10 shows the long time stability test for the latter dispersion and for betaine-stabilized MgFe2O4 nanoparticles. Even after 4 weeks virtually no changes in the hydrodynamic diameters were observed. Scheme 1 summarizes the different possibilities to achieve stable dispersions of MgFe2O4 nanoparticles.

full removal of surface groups. Also, the isoelectric point compared to nonfunctionalized nanoparticles increases. 3.3. Postsynthetic Stabilization in Nonaqueous Media and Phase Transfer. To obtain stable dispersions of phasepure MgFe2O4 nanocrystals in nonpolar media, the nonstabilized nanocrystals were postsynthetically functionalized with OA/OLA. As a result, stable dispersions in common nonpolar solvents, e.g., trichloromethane or toluene, were achieved. Figure 5 shows DLS results after postsynthetic OLA/

Figure 5. DLS results of stabilized MgFe2O4 nanocrystal dispersions in different media. OLA/OA: postsynthetically functionalized nanoparticles with OLA/OA dispersed in toluene. PVP: in-situ PVP-coated nanoparticles dispersed in water. BETA: betaine hydrochloride stabilized nanoparticles dispersed in water. CIT: citrate-stabilized nanoparticles dispersed in water.

OA functionalization of MgFe2O4 nanocrystals in toluene. The particles showed a BET surface area of 4.7 m2/g due to organic ligands strongly forcing agglomeration during the drying of the colloidal solution. The FTIR spectra of surface-functionalized MgFe2O4 nanocrystals are given in Figure 6. For comparison, the DLS data for the in-situ PVPfunctionalized MgFe2O4 nanoparticles (PVP addition to standard synthesis, C in Figure 4) are shown. The OA/OLAfunctionalized MgFe2O4 nanoparticles in toluene show a much

Scheme 1. Pathways To Stable MgFe2O4 Nanoparticle Colloids Developed in This Study

The changes in the ligand sphere upon phase transfer of the MgFe2O4 nanoparticles can be followed in the FTIR spectra in Figure 6. The FTIR spectrum of the unwashed particles (“UW” in Figure 6) shows a broad resonance band between 3650 and 3110 cm−1 attributed to −O−H stretching vibrations resulting from the rac-1-phenylethanol bond to the particle surface.74 Also, residual Mg(acac)2 and Fe(acac)3 on the particle surface can be identified due to the −C−H stretching vibration bands between 3005 and 2825 cm−1.75 Moreover, the strong band at 1580 cm−1 indicates −CO stretching vibrations of the

Figure 6. FTIR spectra of as-synthesized, functionalized, and ligandstripped MgFe2O4 nanocrystals. STR: ligand-stripped. NF: nonfunctionalized (washed). UW: unwashed. OLA/OA: postsynthetically functionalized with OLA/OA. PVP: in-situ PVP-coated. BETA: betaine hydrochloride stabilized. CIT: citrate stabilized. 27132


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group are very strong.59 At 1069 cm−1 −C−O stretching vibration signals appear.59 Also, weak signals are visible in the region between 2900 and 2500 cm−1 due to the −C−H stretching and deformation vibrations, respectively. Bands at 907 and 850 cm−1 indicate the deformation vibrations of the carboxylate group. Also, the vibration bands for solid MgFe2O4 are detectable with maxima at 598 cm−1 for Fe−O stretching vibration in the tetrahedral site and 434 cm−1 for Fe−O stretching vibration in the octahedral site.21 The FTIR spectrum of PVP-coated MgFe2O4 nanoparticles (“PVP” in Figure 6) shows typical bands for aliphatic −C−H stretching vibrations appearing between 3000 and 2820 cm−1. The −CO stretching vibration of the lactam carboxylic group appears at 1659 and 1597 cm−1.81 Bands for aliphatic −C−H bending vibrations from PVP can be detected at 1424 and 1351 cm−1.81 The−C−N stretching vibrations of the lactam ring manifest in a band at 1287 cm−1.81 In addition, the characteristic bands for solid MgFe2O4 described earlier are detected in all surface-modified samples too.21 Thermogravimetric investigations reveal the amounts and chemical nature of surface capping by different moieties (Figure 7). In the case of the as-synthesized nanoparticles, without washing the mass loss is around 30 %, indicating a lot of unreacted precursors (which is in agreement with FTIR data from Figure 6) and adsorbed solvent molecules on the surface.

carboxyl groups are present in the precursor molecules.75 Additionally, the deformation vibration of the aliphatic −C−H band is detectable at 1420 cm−1.75 The broad vibration band having its peak at 586 cm−1 as well as the vibration band at 431 cm−1 can be assigned to solid MgFe2O4 as the shape and wavenumber region match well with values for Fe−O stretching vibrations on tetrahedral and octahedral sites, respectively, reported for magnesium ferrite in the literature.5,14,21,70 A similar vibration pattern is obtained for washed but not functionalized MgFe2O4 nanoparticles (“NF” in Figure 6). Here, the bands resulting from acetylacetonates are less intense, indicating partial removal of the adsorbed precursor species. Additionally, no aromatic bands of rac-1-phenylethanol are detected, which underlines the removal of residual reaction solvent. Instead, the −OH stretching vibration in the region between 3600 and 3050 cm−1 as well as aliphatic −C−H stretching vibrations between 3000 and 2800 cm−1 can be ascribed to adsorbed ethanol, which was used during the workup. A −C−O stretching vibration at 1100 cm−1 is also due to adsorbed ethanol. The shape and intensity of these bands matches well with reference data of ethanol.76 Ligand stripping of OLA/OA-MgFe2O4 nanocrystals with Et3OBF4 can remove all of the surface ligands, e.g., OLA/OA or carboxylate residues from the synthesis. The respective IR spectrum (STR in Figure 6) shows a broad band for −O−H stretching vibrations in the region between 3600 and 3050 cm−1 as well as −C−H stretching vibrations between 3000 and 2800 cm−1, which can be ascribed to ethanol adsorbed on the particle surface. A weak peak at 1079 cm−1 can be assigned to BF4− anion from the stripping agent.77,78 Furthermore, all bands reported for mixedmagnesium ferrites in the literature are observable.21 While for the unwashed and washed as-synthesized MgFe2O4 nanoparticles the bands for remaining unreacted precursor molecules (carboxylate vibrations) are visible, those nearly disappear after OA/OLA functionalization (see “OLA/OA” in Figure 6). In contrast, bands for asymmetric and symmetric −C−H bond stretching vibrations of aliphatic and olefinic bonds become apparent with peaks at 2919 and 2850 cm−1 due to the long alkyl chains of OA and OLA.79 Moreover, bands for stretching vibrations of −O−H groups of oleic acid and adsorbed water around 3420 cm−1 can be detected.79 At 1556 and 1416 cm−1 two bands for asymmetric and symmetric stretching vibrations of −COO− are observable indicating an absorption of deprotonated oleic acid on the nanoparticle surface.79 The asymmetric deformation vibration of the −NH3+ group of oleylamine appears at 1620 cm−1.79 After phase transfer with betaine hydrochloride, the vibration bands assigned to −C−H bonds nearly disappear, which is a clear indication for the ligand exchange (see “BETA” in Figure 6). A small signal remains for the methyl groups of betaine at 2940 cm−1.63 Moreover, a strong signal for the −CO stretching vibration of the carboxylic acid group of betaine is apparent at 1623 cm−1. The −C−N bending vibrations can be observed at 1396 cm−1.63 The citrate-stabilized nanoparticles show an FTIR vibration pattern matching well with reference data of iron(III) citrate.80 This indicates that not citric acid but citrate is bound directly to iron surface ions due to alkaline conditions during functionalization. This would be in agreement with the zeta-potential value of −19.5 mV, indicating negatively charged surface groups (citrate anions). Furthermore, the vibration band at 3426 cm−1 for −O−H stretching vibrations results from adsorbed water. Vibrations at 1617 and 1388 cm−1 for −COOM stretching vibrations of the carboxylate

Figure 7. Thermogravimetric analysis for different stabilized and nonstabilized MgFe2O4 nanoparticles. STR: ligand-stripped. NF: nonfunctionalized (washed). UW: unwashed. OLA/OA: postsynthetically functionalized with OLA/OA. PVP: in-situ PVP coated. BETA: betaine hydrochloride functionalization. CIT: citrate stabilized. 27133


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The Journal of Physical Chemistry C After washing the mass loss is reduced to 17 %, which is still too much for adsorbed solvent molecules only. In agreement with Figure 6, unreacted precursors indicated by residual carboxylic acid groups (from the acetylacetonate precursors) are adsorbed on the surface of the nanocrystals, decomposing during temperature treatment at around 300 °C according to TG data. MS traces for common decomposition products are given in Figure S11. For the unwashed nanocrystals, fragments typical for H2O+ (m/z = 18), CO2+ (m/z = 44), and CH3C = O+ (m/z = 43) can be detected during decomposition of surface precursors.70 Furthermore, MS trace of HF+ (m/z = 20) derives from decomposition products of the PTFE sealing cap. The same signals are detectable for washed, nonfunctionalized nanoparticles, too, with remarkably less intensity, indicating partial removal of the species adsorbed to the particles’ surface (Figure S11). After stripping, TG mass loss is only 8−9 %, and MS traces indicate hardly any HF+, CO2+, and CH3CO+ fragments. The mass loss derives from water/solvent desorption and Et3OBF4 decomposition, which is underlined by TG-MS data (Figure S11). In addition to MS traces for H2O+, CO2+, and CH3C O+ fragments, postsynthetically OA/OLA-modified MgFe2O4 nanocrystals also show MS traces of NO+ (m/z = 30) and NO2+ (m/z = 46) due to decomposition of the amine groups of OLA (Figure S11). Overall, in the latter case the mass loss is only about 14%, which is less than that for the washed assynthesized MgFe2O4 nanocrystals, indicating that the OA/ OLA functionalization also removes some residual precursors during OA/OLA capping. This conclusion is also supported by the FTIR spectra (Figure 6) showing a reduced intensity for carboxylic groups. Similar traces can be observed for the PVPcoated nanocrystals showing a mass loss of around 22 % and typical nitrogen-indicating fragments of NO2+ (m/z = 46) and NO+ (m/z = 30) from the decomposition of the lactone groups of PVP between 300 and 500 °C (Figure S11). After phase transfer with betaine hydrochloride TG analysis gives a mass loss of 20 % due to the relatively large amount of heteroatoms and the heavy chlorine fragments. TG-MS data presented in Figure S11 show decomposition fragments for H2O+ (m/z = 18), NO+ (m/z = 30), Cl− (m/z = 35), CH3C O+ (m/z = 43), CO2+ (m/z = 44), and NO2+ (m/z = 46), which match the functional groups being present in betaine hydrochloride. Citrate-stabilized MgFe2O4 nanocrystals show an even larger mass loss of 41%. This is because of the large amount of oxygen from the surface capping due to the oxygento-carbon ratio of 7:6 in citric acid being removed at elevated temperatures. MS traces of H2O+ (m/z = 18), CH3CO+ (m/ z = 43), CO2+ (m/z = 44), and NO2+ (m/z = 46) were detected. Figure 8 shows transmission electron microscopy (TEM) images of MgFe2O4 nanoparticles synthesized after standard protocol (section 2.1.1) and betaine hydrochloride-functionalized and citrate-functionalized nanoparticles. It is apparent that neither the shape nor the size of the phase-pure nanoparticles is affected by postsynthetic treatment. Further investigation using selected electron diffraction (SAED) is in good agreement with PXRD data reported previously and literature values.24 Further SAED images of MgFe2O4 nanoparticle samples described in this article can be found in Figure S12. A high-resolution TEM image of betaine hydrochloridestabilized MgFe2O4 nanocrystals is given in Figure S13, indicating that the samples are still single crystals after postsynthetic functionalization.

Figure 8. Bright-field TEM and corresponding SAED images of nonfunctionalized (A−C), betaine hydrochloride-functionalized (D and E), and citrate-functionalized (G−I) nanoparticles.

3.4. Investigation of the Photocatalytic Properties of MgFe2O4 Nanocrystals. Photocatalytic experiments with MgFe2O4 nanocrystals (as-synthesized, citrate-functionalized, betaine-functionalized, ligand-stripped) have been performed for the degradation of model pollutant Rhodamine B (RhB). Figure 9 gives the concentration of RhB over time under irradiation of MgFe2O4/RhB dispersions in water. Nonfunctionalized and ligand-stripped MgFe2O4 nanocrystals show only little photocatalytic activity. 24 % degradation after 2 h can be achieved with nonfunctionalized MgFe2 O 4 nanocrystals. In contrast, only 10 % degradation can be achieved with ligand-stripped nanocrystals. On the other hand, betaine- and citrate-functionalized MgFe2O4 nanocrystals show much better photocatalytic performance. 62 % degradation can be achieved after 1.5 h with citrate-stabilized MgFe2O4. UV−vis spectra of the as-prepared reaction solution, the dark equilibration adsorption, as well as the time-dependent absorption spectra of the irradiated MgFe2O4/RhB dispersions are given in Figure S14. Maximum values after equilibration in the dark were normed to be crel = 1, and all following spectra were evaluated relative to this. The quantitative investigations of the reaction kinetics were done by fitting the experimental data with a first-order kinetics model. The resulting kinetic constants and degradation rates are given in Table S3. The kinetic constant for nonfunctionalized MgFe2O4 nanoparticles is already 1.5 times larger than that for ligand-stripped nanoparticles, which might be due to increased agglomeration of ligand-stripped nanoparticles in aqueous solution. Therefore, the particles’ surface is less accessible for the dye molecules, resulting in inhibited degradation kinetics. Surface functionalization with betaine hydrochloride increases the reaction constant by factor 2.5. Surface treatment with citric acid leads to a 5 times higher kinetic constant compared to nonfunctionalized nanoparticles. This underlines the positive influence of deagglomeration by surface functionalization. Furthermore, kinetic constants for surface-function27134


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been reported by Liu et al.85 The authors found considerably smaller rate constants for single-phase photocatalysts g-C3N4 (k = 0.459 h−1) and α-Fe2O3 (k = 0.020 h−1). Li and co-workers reported similar kinetic constant for α-Fe2O3 (k = 0.044 h−1).86 These values for α-Fe2O3 are already by factors of 1.9−4.1 smaller than our calculated kinetic constant for ligand-stripped nanoparticles. Thus, even higher rate constants for MgFe2O4 hybrid systems might be possible. More photocatalytic experiments for stabilized MgFe2O4 nanoparticles will be performed in the future, including other model compound degradation, as well as photocatalytic hydrogen evolution and water splitting experiments.

4. CONCLUSIONS Monocrystalline MgFe2O4 nanoparticles were prepared in a microwave-assisted water-free sol−gel process. The nanoparticles are phase pure directly after synthesis in 30 min. Postsynthetic calcination results in by-phase formation above 600 °C, which has been identified thoroughly. Dispersions of MgFe2O4 nanoparticles in nonpolar or polar solvents can be tailored by stabilization approaches. Stable MgFe2O4 nanoparticle dispersions in water can be achieved in-situ by PVP addition to the microwave-assisted synthesis. For stable dispersions in toluene, postsynthetic surface functionalization with OA and OLA can be performed. Subsequent phase transfer with betaine hydrochloride or citric acid leads to highly stable and agglomerate-free MgFe2O4 nanocrystal dispersions in water with a narrow size distribution below 10 nm. Photocatalytic experiments are presented, indicating that surface stabilization can positively influence compound adsorption and enhance pollutant degradation.

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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/acs.jpcc.7b08780. Tauc plots for band-gap determination of MgFe2O4, FTIR spectra, crystallite sizes and corresponding band gaps, in-situ PXRD patterns, TEM images, additional DLS measurements, pH values, isoelectric points, BET surface areas, MS traces for TG-MS investigations, RhB absorption spectra, SAED images, calculated kinetic constants for RhB degradation (PDF)

Figure 9. Photocatalytic activities for the degradation of RhB under solar light irradiation (AM 1.5G) in the presence of MgFe2O4 nanocrystals (top), and first-order kinetic evaluation to calculate rate constants (bottom). STR: ligand-stripped. NF: nonfunctionalized (washed). BETA: betaine hydrochloride stabilized. CIT: citrate stabilized.

alized nanoparticles are in the range or even exceed the value reported for MgFe2O4 nanoparticles with a crystallite size of 50−80 nm (k = 0.372 h−1) reported by Yuan et al.82 The reason for the improved degradation rates of the stabilized nanocrystals is the enhanced dispersion of the nanocrystals in water providing improved access of the model pollutant to the nanoparticle surface. Moreover, citrate stabilization seems to help adsorbing RhB electrostatically to the surface of MgFe2O4, better compared to the betaine stabilization, resulting in improved photocatalytic degradation. It was already reported in the literature that citrate ions can improve the adsorption of cations at a high concentration of surface-bond citrate ions.83 The maximum reaction constant for RhB degradation in the presence of citrate-stabilized nanoparticles (0.638 h−1) is already a promising value for a single-compound system. Ye et al. reported a maximum kinetic constant of 2.16 h−1 under visible light for a α-Fe2O3/g-C3N4 photocatalyst system with an improvement of factor 1.8 in comparison to pure g-C3N4.84 The higher values were attributed to improved charge carrier separation in the system due to heterojunction formation as a synergistic effect in the hybrid system. A comparable value of 1.26 h−1 for α-Fe2O3/g-C3N4 nanocomposite photocatalyst has

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Roland Marschall: 0000-0002-1057-0459 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Hubert Woerner for TG-MS measurements, Brigitte Weinl-Boulakhrouf for IR measurements, Rüdiger Ellinghaus for N2 physisorption measurements, and Prof. Bernd M. Smarsly for his support (all Justus-Liebig-University Giessen). 27135


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Further thanks go to Dr. Annette Kleppe and Dr. Michael Wharmby for their help during the measurements at I15, Diamond Light Source, Didcot. K.K. and R.M. acknowledge funding by the German Research Foundation DFG under the priority program SPP 1613, project MA 5392/5-1. R.M. and T.W. gratefully acknowledge funding in the Emmy-Noether program (MA 5392/3-1) of the German Research Foundation DFG. C.S. acknowledges financial support within the SIGNO project (03SHWB073) by the German Federal Ministry for Economic Affairs and Energy (BMWi). R.M., J.T., and A.B. gratefully acknowledge financial support from the AiF within the program for promoting the Industrial Collective Research (IGF) of the German Federal Ministry of Economic Affairs and Energy (BMWi), based on a resolution of the German Parliament (project “QuinoLight”, 18904N1-3).

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