Nanoparticle Self-Assembly Mechanisms in the Colloidal Synthesis of

Research Article pubs.acs.org/journal/ascecg

Nanoparticle Self-Assembly Mechanisms in the Colloidal Synthesis of Iron Titanate Nanocomposite Photocatalysts for Environmental Applications Alexander V. Agafonov,† Dmitrii A. Afanasyev,† Tatiana V. Gerasimova,† Anton S. Krayev,† Maxim A. Kashirin,‡ Vladimir V. Vinogradov,§ Alexandr V. Vinogradov,§ and Vadim G. Kessler*,∥ †

G.A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo 153045, Russian Federation Voronezh State Technical University, Voronezh 394026, Russian Federation § ITMO University, St. Petersburg 197101, Russian Federation ∥ Department of Chemistry and Biotechnology, BioCenter, Swedish University of Agricultural Sciences, Box 7015, 75007 Uppsala, Sweden ‡

ABSTRACT: Colloidal synthesis of iron titanate-based nanocomposites exploiting the interaction of solutions of binary oxide nanoparticles, Fe2O3 and TiO2, was investigated with respect to the pH of the reaction medium and the conditions used for the synthesis of the reactants. It has been demonstrated that while the phase composition of the products is rather analogous, involving the formation of iron titanate phases on the grain boundaries of the binary oxide particles, the morphology of the resulting aggregates can be a matter of pH control. The self-assembly mechanisms are guided by the surface charge of the particles, offering nanorod regular colloid crystal structures of altering particles with opposite initial charges at neutral pH and globular aggregates with random distribution of uniformly charged particles at low pH as revealed by DLS and highresolution TEM studies. The produced materials demonstrated enhanced photocatalytic activity compared to the iron titanates produced by conventional techniques. Magnetic characteristics have also been investigated disclosing the possibility of magnetic separation for the Fe2TiO5 material, making it an attractive candidate for application in the sustainable remediation of wastewaters. KEYWORDS: Nanocomposite structure, Colloidal synthesis, Aggregation driving forces, pH control, Photocatalytic activity

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INTRODUCTION Colloid synthesis is an extremely advantageous approach to the synthesis of nanostructures and nanocomposite materials.1−3 Aggregation of uniform nanoparticles permits through their dense packing to produce structures with uniform porosity and many attractive physical and chemical characteristics. Even the assembly of polydisperse nanoparticles, when driven by strong interaction forces, can result in formation of uniform macroparticles, forming in turn films and 3D assemblies with rather even porous structures.4 The applications of the colloid crystal-type structures were sought initially in the area of luminescent materials3 but were recently most frequently addressed in preparation of materials for photovoltaics and photocatalysis.5 The most addressed photocatalytic material is, of course, titanium dioxide.5−10 Its main advantages include high resistance to photocorrosion,11 thermal and chemical stability,12 biocompatibility, and affordability. Its ability to convert only 5% of the total solar spectrum and the difficulties caused by a subsequent separation from the reactors of photocatalytic © 2016 American Chemical Society

purification of sewage water make it, however, inefficient for practical remediation purposes.12 Many approaches to increase its activity, including doping, morphology and surface shape control, sensibilization,8 etc., have been reported.12,13 Further development in the field turned the attention also to other phases potentially interesting for photocatalytic applications and also to high-throughput methods for their synthesis.12,14 Typical examples are binary compounds, such as oxides (ZnO)15 and chalcogenides (quantum dots, for example, CdS),16 as well as more structurally and characteristically complex mixed oxides, such as bismuth ferrite,17 K3Ta3B2O12,18 cobalt-based systems,19,20 etc. However, none of these alternative photocatalysts is as biocompatible as titanium dioxide.8 In the view that it is the treatment of sewage water that is the major desirable application for larger scale photocatalysts, this constitutes an apparent obstacle. After all, Received: February 16, 2016 Revised: March 18, 2016 Published: March 29, 2016 2814

DOI: 10.1021/acssuschemeng.6b00317 ACS Sustainable Chem. Eng. 2016, 4, 2814−2821

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ACS Sustainable Chemistry & Engineering

of both titania and the Fe2TiO5 phase.12,14,27,29 The latter was obtained both when iron salts28 and iron oxide colloid29 were set in reaction with a titania colloid.29 In the latter case, apart from Fe2TiO5, the synthesis products contained a significant amount of crystalline phases of iron oxides and titania.30 It was found that these complex materials behaved much more efficiently in photodegradation of dyes, compared to the Degussa P25 standard. The aim of the present work was to obtain deeper understanding of the mechanisms leading to formation of open porous nanostructures and to identify the approach to materials with optimized accessibility of the grain boundaries granting the high activity of photocatalytic degradation, optimally, in combination with the possibility of magnetic retraction of the catalyst.

the efficiency of a photocatalyst is determined not only by the quantum yield and rate of photodestruction but also by the need for a post-treatment of processed water to be able to remove the catalyst and its decomposition products. The possibilities of magnetic separation and also biocompatibility (similar to that of titanium dioxide) are obviously the most relevant requirements in the search for new photocatalytic systems.13 Doping the crystal lattice of titanium dioxide with iron ions appeared as one of the possible routes to enhance its sunlight conversion efficiency.12 Both the surface adsorption and volume doping of titanium dioxide with Fe(III) ions have been demonstrated to increase the rate of photochemical reactions and to modulate the bandgap of TiO2.21,22 However, the effect in application of such materials for photodegradation often does not differ from that for pure titanium dioxide. First of all, this is due to a more facile recombination of the generated electron−hole pairs on the Fe3+ impurity centers before reaching the surface of the particle. As a result, the number of hydroxo radicals formed in aqueous solutions is on a par with that for titanium dioxide crystalline phases.21 The use of iron titanates with the total formula Fe2O3·nTiO2 as products of interaction between the components has some significant advantages: (1) high spectral activity in the red region, (2) high ferromagnetic response, and most importantly, (3) an improved mechanism for the generation of electron− hole pairs and their mobility to the hydroxyl groups on the particle surface. First, this is due to the fact that such systems have a very low Fermi level (about 0.37 eV), and the transition from the valence band to the conduction band is accompanied by internal interband transitions within the band gap to the impurity centers.22 This makes materials based on iron titanates more efficient in terms of generation and transport of charge carriers. Moreover, they are completely biocompatible, as evidenced by the standards of iron titanate contents in drinking water.23 The efficiency of using these materials, taking Fe2TiO5 and Fe2Ti2O7 as examples, was demonstrated recently, also involving the secondary important postphotosynthesis reaction, nonenzymatic conversion of molecular nitrogen to ammonia in moist air and under light.24 This brings additional confirmation for the prospects of using these materials in photocatalytic reactions. However, energy-efficient methods of producing iron titanates have until recently been unavailable. Traditionally, Fe2Ti2O7 and Fe2TiO5 were prepared either by conventional ceramic technology with annealing at 1200 °C or using a sol− gel method followed by an annealing stage at 700 °C.24−26 These conditions rule out the possibility to create a highly porous structure for the photocatalysts, rendering their efficiency. It should be noted that in both the ceramic technology and the sol−gel process the yields of the desired products (Fe2Ti2O7 and Fe2TiO5) in the composition of a material are limited, possibly due to kinetic limitations of hightemperature synthesis. It has been recently demonstrated, however, that hightemperature treatment is not indispensable for the formation of highly crystalline oxides.27,28 Nucleation of metal oxides in sol− gel synthesis occurs generally in the form of oxometallate species with ordered (crystalline) cores and amorphous shells, bearing residual organic ligands. Removal of these ligands by either evaporation or just by lowering the pH in media in combination with moderate heating in solution are often enough for their transformation into highly crystalline oxide nanoparticles. This approach was found efficient in production

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EXPERIMENTAL SECTION

X-ray diffraction analysis was performed using a D8 Advance (Bruker) multifunction X-ray diffractometer with a thermostated cell. Measurements of specific surface area by the BET method and pore size distribution were made using low-temperature nitrogen adsorption− desorption on Quantachrome Nova 1200 equipment. All samples were previously degassed at 90 °C for 7 h. Spectral analysis of nanocomposite films on the surface of glass substrates was carried out using a PG Instruments T70 + UV/vis spectrophotometer in the wavelength range of 200−850 nm. To study the samples using scanning electron microscopy (SEM), a silicon wafer was coated with the composite and, after complete drying in a vacuum desiccator, was investigated without additional sputtering using an ultrahigh resolution electron microscope Magellan 400L (Field Emission Inc.).The samples for high-resolution transmission electron microscopy (HRTEM) were prepared by dispersing small amounts of samples in ethanol to form a homogeneous suspension. A drop of the suspension was deposited on a carbon-coated copper grid for HRTEM observation (FEI TECNAI G2 F20, operated at 200 kV). The size and zeta potentials of the sol particles in an aqueous solution were characterized by dynamic light scattering data (DLSD) using Malvern ZetaSizer nano equipment at 20 °C with an He−Ne 10 mV laser at a wavelength of 633 nm. Photocatalytic activity of the powders was studied spectrophotometrically by irradiating a solution of methyl orange dye in a suspension of 0.1 g of the photocatalyst powder in 100 mL of a thermostated quartz cell with visible light simulated by luminescence of a UV lamp (250 W). The installation is described in detail in ref 31. The magnetic properties of the composites were measured using a VSM technique. All samples were measured as a powder. Arrangement of the experiment and a description of the model used for calculating the dipole−dipole interaction are given in ref 32. All chemicals were purchased from Sigma-Aldrich and used without additional treatment or purification. Preparation of Titania sol. Twelve milliliters of isopropanol was mixed for 2 h with 16 mL of titanium isopropoxide (98%) using a magnetic stirrer, and an aqueous solution of HNO3 was then added dropwise to the mixture to reach a concentration of 0.1 M. The white amorphous precipitate formed was peptized at 80 °C to give a transparent opalescent sol used subsequently to produce titanium dioxide powder by drying or for the synthesis of nanocomposites. Preparation of Hematite Sol, α-Fe2O3. Iron(III) hydroxide was precipitated from an aqueous solution of FeCl3 (0.22 mol/L) by an ammonia solution (∼1.5 M) at 70 °C. As a result, a brown suspension was obtained. The precipitate was separated from the mother liquor by centrifugation and washed first with distilled water to remove the NH4+ and Cl− ions, then with isopropyl alcohol, and dried. To form a stable hydrosol, hematite powder was added to an 0.045 M aqueous solution of HNO3. The mixture was heated to 80 °C and subjected to ultrasonic treatment (22 kHz) for 5 min. As a result, a dark-red transparent opalescent sol of iron hydroxide was obtained, which was then used for the synthesis of nanocomposites. A similar technique was employed to obtain a hematite sol in water at a neutral pH, but the 2815


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ACS Sustainable Chemistry & Engineering duration of hot (80 °C) peptization and ultrasonic treatment was increased. Synthesis of Fe2TiO5 and Fe2Ti2O7 Nanocomposites. Synthesis of Fe2TiO5 was performed by mixing stoichiometric amounts of substances contained in the acidic sols of hematite and titania. The mixture was kept under stirring at 80 °C for 1 h and then was sonicated for 15 min. The formed black-green precipitate was separated by centrifugation, washed with water, and dried at 60 °C (sample Fe2TiO5A). Synthesis of Fe2Ti2O7 was performed by mixing a hematite sol (2 g of Fe2O3 in 100 mL of 0.046 M HNO3) with a solution containing 3.69 mL of titanium tetraisopropoxide and 2.71 mL of anhydrous isopropyl alcohol under vigorous stirring at 80 °C for 1 h, followed by subjecting the mixture to ultrasonic treatment for 15 min. The formed brown precipitate was separated by centrifugation, washed with water, and dried at 60 °C (sample Fe2Ti2O7B). Synthesis of Fe2Ti2O7 and Fe2TiO5 was performed as a result of the interactions of titania nanoparticles obtained by peptization in a 0.1 M nitric acid solution (see above) and then dried and resuspended in water by sonication with a hematite sol in water at 80 °C in an ultrasonic bath. While mixing the sols, stoichiometry was maintained by matching total concentrations of corresponding ions (Fe/Ti = 1:1 and 2:1). The resulting formation of black and dark-brown precipitates yielded stable sols after vigorous stirring with simultaneous periodic sonication (5 min) for 4 h at this temperature. The obtained precipitates were separated from the mother liquor by centrifugation and dried at a temperature of 60 °C (samples Fe2TiO5C and Fe2Ti2O7D).

nanorods for Fe2Ti2O7D (Figure 1). It is worth noting that on the nanolevel all these types of materials are constructed in a rather analogous way, consisting of small, a few nanometers in size, apparently well-crystallized nanoparticles (Figure 1). The small size of the constituent particles and their primary aggregates was confirmed also by DLS data (Table 1). The DLS data have also provided a clue to the different morphology of the products. Both titania and iron oxide when produced are positively charged due apparently to the highly acidic conditions of their synthesis. This is quite common for the solution-produced hematite at both low and neutral pH33 in the absence of electrolytes and surface-capping ligands. The positive surface charge is, however, quite unusual for titania. If produced by spray pyrolysis, the TiO2 nanoparticles (Degussa P25) usually have a highly negative charge at pH ≈ 7, which decreases slowly on storage in water for over 1 week (even hematite Fe2O3 nanoparticles when produced by the same technique have initially negative zeta potential but relax to almost zero potential within the same time).34 The zeta potential of sol−gel titania equilibrated in time in Milli-Q water was found to be about −11 mV.35 It can thus be concluded that when positively charged TiO2 and Fe2O3 nanoparticles react at low pH, the driving force in their aggregation is just minimization of the surface energy, acting in a chemically nonspecific way and resulting in spherical aggregates with random placement of chemically different particles. Keeping nanoparticles of TiO2 and Fe2O3 in a neutral solution at elevated temperature results apparently in differentiation of their charges: the hematite ones remain positive, while the titania becomes negatively charged. Interaction of particles of opposite charges leads to well-defined colloid crystals of tetragonal habitus, inherited supposedly from slightly anisotropic tetragonal TiO2 in the case of Fe2TiO5. It is worth noting that this material after synthesis has a positive zero potential resembling that of the applied Fe2O3, indicating that interaction may be finished with an iron oxide layer. The same reaction pathway is effectuated even at the beginning of the reaction when a different stoichiometry with the excess of TiO2 is applied. Thus, in the synthesis of the Fe2Ti2O7D sample, the initial product consists of rod-shaped material of the same kind as the Fe2TiO5C sample, which is then coated by the excess titania, providing it with a negative potential quite close to that of the pure TiO2 equilibrated in water. Formation of well-structured colloid crystals in the electrostatically driven assembly of nanoparticles has been well documented in the literature (see, for example, ref 36), but the highly anisotropic morphology of the colloid crystals of the Fe2TiO5C sample makes it especially interesting for photochemical applications. The structures of the crystalline phases were examined by XRD analysis (Figure 2) versus temperature. According to XRD, all powders of materials extracted immediately after mild synthesis procedures and dried at 60 °C consist of a mixture of oxides with different degrees of crystallinity, yielding ample reflections due to the small size and high degree of hydroxylation of the particles. Reflections from Fe2Ti2O7 (2θ = 54.0°−55.5°) and Fe2TiO5 (2θ = 33.0°−35.7°) are present on the X-ray diffractograms of all nonannealed samples. The diffraction peaks at about 25.411 (101), 37.911 (004), 48.011 (200), 54.011 (105), 54.911 (211), and 62.811 (204) can be also indexed to anatase TiO2 phase.30 Reflections from the hematite phase in the diffraction patterns of the annealed samples could not be seen. It is interesting to note that

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RESULTS AND DISCUSSION The expectations for running the synthesis at two different pH regimes was that these conditions will strongly influence the morphology and intrinsic structure of the produced nanocomposites, permitting us to trace the mechanistic aspects of the colloid synthesis approach in this case. Very much in line with our anticipations, the resulting materials turned quite different in their morphology (Figure 1). Carrying out the synthesis at low pH produced, according to SEM data, small spherical aggregates packed in a uniform xerogel structure (samples Fe2TiO5A and Fe2Ti2O7B). The products of reaction at neutral pH are in contrast with either highly uniform separate nanorods in the case of Fe2TiO5C or fused aggregated

Figure 1. Visualization of the formation of composites and SEM and TEM images of as-synthesized products. 2816


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Table 1. Structural Characteristics of Materials Synthesized Using Soft Chemistry at Near Room Temperature as Obtained from Results of Low-Temperature Nitrogen Adsorption−Desorption and Dynamic Light Scattering in Conjunction with Kinetic Characteristics of Photodecomposition of Methyl Orange in an Aqueous Solution at Room Temperature sample Fe2O3 TiO2 Fe2Ti2O7 (low pH) Fe2Ti2O7 (neutral pH) Fe2TiO5 (low pH) Fe2TiO5 (neutral pH)

surface area, SBET, m2/g

pore diameter, DBJH, nm

pore volume, VBJH, cm3/g

Rayleigh scattering, average radius, RDLS, nm

Eg, eV

zeta potential, ζ, mV

Scherrer crystallite size, nm

231 163 143

4.0 3.2 3.6

0.122 0.005 0.044

− 96 69

− 3.25 3.20

+14.5 +35.9 (as prepared),−11.7 (equilibrated) 37.2

4.1 (maghemite) 6.8 (anatase) 7.8 (Landauite)

111

4.0

0.022

68

2.78

−11.4

10.5 (Landauite)

77

4.0

0.027

46

2.33

33.3

140

3.6

0.058

129

2.10

33.5

10.1 (pseudobrokite) 10.7 (pseudobrokite)

Figure 2. XRD pattern of as-synthesized materials prepared at (A,B) neutral pH and (C,D) low pH.

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Figure 3. Proposed mechanism of methyl orange photodestruction in our composite.

according to the results of ref 29 a “soft” synthesis of Fe2TiO5 via interaction of magnetite and titania sols peptized in nitric acid leads to a mixture of oxides, in which magnetite and anatase phases are present as well, along with Fe2TiO5. Obviously, the reactivity for hematite in the formation of Fe2TiO5 under the same conditions is higher than that for peptized magnetite. Interestingly, the increased content of the titania phase increases the degree of crystallinity of the material. This is essentially due to the higher crystallization rate of anatase under the conditions of a protonating particle surface and also increased rate of diffusion of the ions.11 It should be noted that crystallinity of the nanocomposite lattice is quite essential for improving the semiconductor characteristics of the photocatalyst upon activation by sunlight energy.28,37 As is evident from Figure 2, materials obtained using the soft synthesis contain both Fe2Ti2O7 and Fe2TiO5 phases regardless of the stoichiometry of the initial components at the synthesis stage. This may be due to a nonuniform distribution of titania and hematite nanoparticles upon mixing of sols in the soft synthesis and the formation of local concentration gradients, leading to changes in the stoichiometry of the resulting compounds. The specific surface area of the synthesized powders of ironcontaining nanocomposites and pure TiO2 appeared to be lower than those of hematite and anatase used for synthesis (Table 1). Analysis of the thermal evolution of the phase composition of the material after a multistage annealing at 400 and 700 °C for 1 h in air showed that increasing temperature promotes the growth of the degree of crystallinity of powders while simultaneously increasing the concentrations of the target products (Fe2Ti2O7 and Fe2TiO5) in the composition of materials. At 700 °C, along with the anatase phase, the rutile phase emerges in the composition of the material with a reflection at 2θ = 27.4°. It should be noted that the data presented in refs 24−26 and 38 indicate that the formation of Fe2Ti2O7 and Fe2TiO5 phases visible in XRD patterns in the products of sol−gel synthesis occurs usually upon annealing at 700 °C. The synthesis in boiling water with removal of the organic ligands by evaporation offered, however, the Fe2TiO5 phase distinguishable in the XRD pattern directly, without any additional thermal treatment.27 The produced oxide nanocomposite materials were investigated with respect to their photochemical activity. Quite enhanced characteristics could be expected in this case in the view that the separation of a photoexcited electron−hole pair is much more efficient since both anatase (crystalline phase of titanium dioxide) and hematite are n-type semiconductors,

which facilitates transport of electrons and further oxidation of hydroxo groups and oxygen molecules on their surface, whereas the holes interact with the water molecules adsorbed at the surface of titanates. The summary of the expected photochemical transformations is presented in Figure 3. Hematite (αFe2O3) is an n-type semiconductor characterized by an extremely short hole diffusion length of 2−4 nm.39 Therefore, only holes generated in close vicinity to the semiconductor− electrolyte interface will contribute to water splitting. So, the presence of the heterophase material comprising iron titanate and titanium dioxide phases may contribute to the efficient separation of electron−hole pairs with increasing content of TiO2. Such unusual activity of iron titanates (particularly Fe2Ti2O7) was demonstrated previously in photocatalytic processes of nonenzymatic binding of nitrogen molecules to form ammonia. In our work, we decided to examine the photocatalytic activity of the samples with greater material composition diversity using a classical approach for dye decomposition. In particular, the photocatalytic properties were studied by a method of destructing the model dye methyl orange.40,41 Selection of materials is due to the fact that sulfurcontaining dyes are among the most common contaminants of sewage water,40 for which the classical scheme of photocatalysis in heterogeneous media is implemented (Figure 3). The kinetics of photodegradation upon irradiating suspensions with a UV lamp are shown in Figure 4. For samples 2 and 4, the course of photocatalytic reactions corresponding to the

Figure 4. Dependence of the degree of decomposition of methyl orange dye in 0.1% suspensions of iron-containing titania-based nanocomposites at room temperature vs UV lamp exposure time. Designations correspond to samples: 1 − (Fe2TiO5 D), 2 − (Fe2TiO5 C), 3 − (Fe2Ti2O7 A), 4 − (Fe2Ti2O7 B). 2818


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reaction model regardless of the synthesis conditions. Therefore, the photophysical activity of these components is at an essentially higher level. Due to the fact that the disposal of solid particles of the photocatalyst after use in a flow reactor is a no less important task, the magnetic properties of the samples were investigated for determination of magnetic separation possibilities (Figure 6). According to the magnetization curves, the Fe2Ti2O7 samples yielded no magnetic response, so their behavior is not shown in Figure 6. The lack of magnetic response for the Fe2Ti2O7 systems is related to the increased content of titanium dioxide compared with the phase Fe2TiO5-based samples. In addition, the magnetic properties of these compounds are not detected at room temperature. For the Fe2TiO5-based samples, the magnetization curves exhibit superparamagnetic behavior. According to the data obtained, the residual magnetization of the samples, depending on the conditions for their preparation, showed the same order, at the level of 5−6 emu/g. At the same time, the coercive force of the samples showed significant differences. Thus, for a sample obtained at neutral pH, Hk = 70 Oe is observed, and for the sample prepared at low pH, Hk = 50 Oe is observed. This means that the domain organization of the samples obtained at neutral pH has more parameters than analogous materials produced at low pH. According to the dynamic light scattering (Table 1), similar structural changes were also found in the SEM images (Figure 1). In this regard, the domain−domain particle interactions using the magnetostatic processing model were analyzed32 (Figure 6B). Mathematical calculation yields a maximum interaction energy at 20 kT depending on particle size. In fact, once a particle is in the vicinity of the other particles and their moments can interact, they participate in the formation of a joint magnetization. Thus, in contrast to paramagnetic particles, one requires very low field strength to start the process of forming a chain. In this study, a relatively weak magnetic field of 10 G was applied to initiate formation of chains (Figure 6A). Moving on to organized clusters elongated along one axis and possessing a magnetic moment, as in the present case, we can expect an intensification of these effects along the planes of distribution (Figure 6B).

zeroth-order model was observed. Following the general rules, it could be noted that since the rate of diffusion to the interface for the reactants is less than the rate of their chemical conversion, the activity of the synthesized samples is at a very high level, which is also confirmed by comparative data for known photocatalysts. On the other hand, samples 1 and 3 demonstrate the classical pseudo-first-order reaction graph. Obviously, such behavior of the synthesized photocatalysts is determined by their phase composition associated with different contents of titania phase. The absorption spectra for films deposited on quartz plates from sols may serve as a confirmation (Figure 5).

Figure 5. UV−visible spectra of as-synthesized composite films, prepared at near room temperature.

Complete absorption in the region of less than 320 nm is due to the presence of the wide-gap titania. The increasing Fe(III) content in the titanate lattice leads to the spectrum shift in the long-wave red region. The most interesting fact is the increased adsorption Fe2TiO5 obtained at neutral pH (red line). A significant shift in the visible region in comparison with Fe2TiO5 (low pH) is probably due to the behavior of the rodlike structure Fe2TiO5 (neutral pH) as nanoantennas with enhanced light absorbance. Summarizing the results of methyl orange decomposition and absorption spectra, it can be concluded that the behavior of the synthesized nanocomposite was considerably different compared to the activities of pure titania and iron oxides. High photocatalytic and spectral activity of materials obtained by the low-temperature sol−gel approach allows using this approach in the future for covering of the surface of materials, which are thermally unstable materials such as polymers. This feature allows for obtaining flexible photoactive surfaces, which are used, for example, for photocatalytic filters. This is primarily due to the fact that the mechanism for the photocatalytic action of Fe2Ti2O7 in each case corresponds to the zeroth-order

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CONCLUSIONS In this work, we investigated the interaction of binary oxide nanoparticles, Fe2O3 and TiO2, in order to identify ways of increasing the photocatalytic activity of heterostructures. The

Figure 6. (A) Magnetization for an Fe2TiO5 nanocomposite prepared at different pH values. (B) Dependence of magnetic component of interaction energy on the relative position of magnetic dipoles of particles. 2819


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(8) Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results. Chem. Rev. 1995, 95, 735. (9) Wu, C. H.; Chang, C. L. Decolorization of Reactive Red 2 by advanced oxidation processes: Comparative studies of homogeneous and heterogeneous systems. J. Hazard. Mater. 2006, 128, 265. (10) Hanaor, D. A. H.; Sorrell, C. C. Supported Mixed-Phase TiO2 Photocatalysts for Water Decontamination Applications. Adv. Eng. Mater. 2014, 16, 248. (11) Yao, Y. Y.; Ciston, G. L. S.; Lueptow, R. M.; Gray, K. A.; Li, G. Photoreactive TiO2/carbon nanotube composites: synthesis and reactivity. Environ. Sci. Technol. 2008, 42, 4952. (12) Vinogradov, A. V.; Vinogradov, V. V. Low-temperature sol−gel synthesis of crystalline materials. RSC Adv. 2014, 4, 45903. (13) Ma, Y.; Wang, X.; Chen, X.; Han, H.; Li, C.; Jia, Y. Titanium dioxide-based nanomaterials for photocatalytic fuel generations. Chem. Rev. 2014, 114, 9987. (14) Vinogradov, A. V.; Agafonov, A. V.; Vinogradov, V. V.; Gerasimova, T. V. Low-temperature sol−gel synthesis of crystalline CoTiO3 coatings without annealing. J. Alloys Compd. 2012, 543, 172. (15) Vinogradov, A. V.; Agafonov, A. V.; Vinogradov, V. V. Lowtemperature sol−gel synthesis photochromic Cu/TiO2 films. J. Alloys Compd. 2012, 515, 1. (16) Zhang, B.; Yao, W.; Huang, C.; Xu, Q.; Wu, Q. Shape effects of CdS photocatalysts on hydrogen production. Int. J. Hydrogen Energy 2013, 38, 7224. (17) Guo, R.; Fang, L.; Dong, W.; Zheng, F.; Shen, M. Enhanced photocatalytic activity and ferromagnetism in Gd doped BiFeO3 nanoparticles. J. Phys. Chem. C 2010, 114, 21390. (18) Ikeda, T.; Fujiyoshi, S.; Kato, H.; Kudo, A.; Onishi, H. Timeresolved infrared spectroscopy of K3Ta3B2O12 photocatalysts for water splitting. J. Phys. Chem. B 2006, 110, 7883. (19) Mukherjee, A.; Kokhan, O.; Huang, J.; Niklas, J.; Chen, L. X.; Tiede, D. M.; Mulfort, K. L. Detection of a charge-separated catalyst precursor state in a linked photosensitizer-catalyst assembly. Phys. Chem. Chem. Phys. 2013, 15, 21070. (20) Liu, M.; Qiu, X.; Miyauchi, M.; Hashimoto, K. Energy-level matching of Fe (III) ions grafted at surface and doped in bulk for efficient visible-light photocatalysts. J. Am. Chem. Soc. 2013, 135, 10064. (21) George, S.; Pokhrel, S.; Ji, Z.; Henderson, B. L.; Xia, T. A.; Li, L. J.; Zink, J. I.; Nel, A. E.; Mädler, L. Role of Fe doping in tuning the band gap of TiO2 for the photo-oxidation-induced cytotoxicity paradigm. J. Am. Chem. Soc. 2011, 133, 11270. (22) Zhu, J.; Ren, J.; Huo, Y.; Bian, Z.; Li, H. Nanocrystalline Fe/ TiO2 visible photocatalyst with a mesoporous structure prepared via a nonhydrolytic sol-gel route. J. Phys. Chem. C 2007, 111, 18965. (23) Hono, T.; Inoue, N.; Morimoto, M.; Suzuki, Y. Reactive sintering and microstructure of uniform, openly porous Al2TiO5. J. Asian Ceram. Soc. 2013, 1, 178. (24) Kisch, H. Semiconductor photocatalysismechanistic and synthetic aspects. Angew. Chem., Int. Ed. 2013, 52, 812. (25) Smirnova, N.; Eremenko, A.; Rusina, O.; Hopp, W.; Spanhel, L. Synthesis and characterization of photocatalytic porous Fe3+/TiO2 layers on glass. J. Sol-Gel Sci. Technol. 2001, 22, 109. (26) Rusina, O.; Linnik, O.; Eremenko, A.; Kisch, H. Nitrogen photofixation on nanostructured iron titanate films. Chem. - Eur. J. 2003, 9, 561. (27) Seisenbaeva, G. A.; Daniel, G.; Nedelec, J. M.; Gun’ko, Y. K.; Kessler, V. G. High surface area ordered mesoporous nano-titania by a rapid surfactant-free approach. J. Mater. Chem. 2012, 22, 20374− 20380. (28) Seisenbaeva, G. A.; Kessler, V. G. Precursor directed synthesis−“molecular” mechanisms in the Soft Chemistry approaches and their use for template-free synthesis of metal, metal oxide and metal chalcogenide nanoparticles and nanostructures. Nanoscale 2014, 6, 6229−6244. (29) Vinogradov, A. V.; Vinogradov, V. V.; Gerasimova, T. V.; Agafonov, A. V. Low-temperature sol−gel synthesis of nanosized

self-assembly mechanisms are guided by the surface charge of the particles, offering nanorod regular colloid crystal structures of altering particles with opposite initial charges at neutral pH and globular aggregates with random distribution of uniformly charged particles at low pH as revealed by DLS and high resolution TEM studies. Having determined the relationship of the structure and properties of the materials, as well as the dependence on the preparation conditions, we have achieved enhanced photocatalytic activity for the decomposition of organic dye, which renders irreparable environmental risk to countries developing textile industries. The produced materials demonstrated photocatalytic activity superior to iron titanates produced by conventional techniques with strong magnetic separation properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

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

This work was supported by the Russian Government, Ministry of Education (research was made possible due to financing provided to the customer from the federal budget aimed at maximizing teh customer’s competitive advantage among the world’s leading educational centers) and by the RFBR, Research Project Nos.14-03-31046 and 14-03-00502. The support from the Swedish Research Council (Vetenskapsrådet), Grant No. 2014-3938, is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors (V.K.) expresses his gratitude to the Institute of Solution Chemistry, RAS, for hosting of the Guest Professorship in 2013−2014.

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REFERENCES

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