Research Article | Open Access
CoFe2O4-TiO2 Hybrid Nanomaterials: Synthesis Approaches Based on the Oil-in-Water Microemulsion Reaction Method
CoFe2O4 nanoparticles decorated and wrapped with TiO2 nanoparticles have been prepared by mixing well-dispersed CoFe2O4 with amorphous TiO2 (impregnation approach) and growing amorphous TiO2 over the magnetic core (seed approach), respectively, followed by thermal treatment to achieve TiO2 crystallinity. Synthesis strategies were based on the oil-in-water microemulsion reaction method. Thermally treated nanomaterials were characterized in terms of structure, morphology, and composition, to confirm hybrid nanoparticles formation and relate with the synthesis approaches; textural, optical, and magnetic properties were evaluated. X-ray diffraction revealed coexistence of cubic spinel-type CoFe2O4 and tetragonal anatase TiO2. Electron microscopy images depicted crystalline nanoparticles (sizes below 25 nm), with homogeneous Ti distribution for the hybrid nanoparticles synthesized by seed approach. EDX microanalysis and ICP-AES corroborated established chemical composition. XPS evidenced chemical states, as well as TiO2 predominance over CoFe2O4 surface. According to BET measurements, the hybrid nanoparticles were mesoporous. UV-Vis spectroscopy showed optical response along the UV-visible light region. Magnetic properties suggested the breaking order of magnetic domains due to modification with TiO2, especially for mediated seed approach sample. The properties of the obtained hybrid nanoparticles were different in comparison with its individual components. The results highlight the usefulness of designed microemulsion approaches for the straightforward synthesis of CoFe2O4-TiO2 nanostructured hybrids.
In the last years, synthesis of nanomaterials has been expanded to the development of more complex structures than single phase nanoparticles (NPs), due to the possibility of coupling a wide range of nanosized building blocks (bounded together by their chemical interfaces) in order to design new materials with enhanced characteristics and improved or innovative properties, combined in one single multifunctional platform [1–4]. These types of nanomaterials are known as hybrid nanoparticles or nanocomposites and emerge as an attractive alternative for technological or scientific applications where conventional NPs cannot compete [5–7]. For instance, in the field of heterogeneous photocatalysis, titanium oxide (TiO2) based magnetic nanoparticles are used as vehicle for simple and easy separation, from liquid reaction media, by application of an external magnetic field [7–9]. Interestingly, TiO2 has been one of the most studied semiconductors for photocatalytic applications, standing out due its high photoreactivity, great chemical stability, nontoxicity, and relatively easy and low cost production [10, 11]. Among magnetic components, different spinel-type ferrites (MFe2O4, M: divalent metallic ion) have been combined with titanium oxide, to develop highly and stable magneto-photocatalytic nanocomposites, suitable for the photocatalytic degradation of water pollutants [9, 12–15]. Currently, cobalt ferrite (CoFe2O4) is a promising candidate for the design of TiO2 hybrid nanoparticles, owing to its good chemical and thermal stability, and principally magnetic response, moderate magnetic saturation, and low magnetic remanence, ideal for recovery application purposes [16–21]. The proper combination of CoFe2O4 and TiO2 has resulted in UV-visible light active nanocomposites (extending the light absorption of titanium oxide), with an improved photocatalytic response in some cases [16, 17, 20]. In light of tailoring the CoFe2O4-TiO2 hybrid nanoparticles configuration, the capability of designing complex nanostructures with controlled characteristics (crystallinity, size, surface area, etc.) has been most frequently demonstrated by the combination of two wet chemical methods. For instance, a sol-gel process in reverse microemulsion combined with solvothermal technique , coprecipitation/sol-gel , hydrothermal/sol-gel , and hydrothermal/coprecipitation methods . Other authors have synthesized hybrid CoFe2O4-TiO2 system only using one wet chemical route, such as the polymeric precursor method , coprecipitation , and hydrothermal technique . A great part of these studies carried out the thermal treatment of CoFe2O4 phase, prior to the integration of titanium oxide, for posterior coupling with TiO2, and then a second thermal treatment to achieve titanium phase crystallization. The separation of cobalt ferrite from its liquid media increments the possibility of forming bigger CoFe2O4 agglomerations, decreasing the available surface area to decorate; moreover, it implies more synthesis stages. In this point, the present work proposes a new alternative route for CoFe2O4-TiO2 hybrid nanoparticles preparation, through the expansion of the novel oil-in-water (O/W) microemulsion reaction method [23, 24] (reported by our research group), without the necessity of cobalt ferrite prethermal treatment, and adding the benefits of microemulsion synthesis routes. The O/W microemulsion reaction method consists in the use of organometallic precursors, dissolved within nanosized oil droplets (2–50 nm), stabilized by surfactant, and dispersed in a continuous aqueous phase. When a precipitating agent is added, as an aqueous solution directly to the microemulsion without compromising its stability, the reagents will contact each other at the interface, and they will react to form precipitates of nanometric size (NPs); in this context, the water droplets and the interface act as “nanoreactors.” The O/W microemulsion method has several advantages; namely, it allows a high control of particle size, a high purity, and good chemical homogeneity under mild conditions. Since water is the continuous and major phase, the method may be considered as more ecologic since the concentration of oil needed is considerably lower and only one microemulsion is needed, instead of two as normally used with the traditional water-in-oil (W/O) microemulsion method [23–25]. Several inorganic nanoparticles have been prepared using the O/W microemulsion method [26–30], including photocatalysts [31, 32] and magnetic nanoparticles [33, 34]. In this sense, the synthesis of the CoFe2O4-TiO2 hybrid nanoparticles through the O/W microemulsion method represents a new approach to prepare potential magneto-photocatalytic nanomaterials. Through this study we propose two synthesis strategies; the first one (impregnation approach) consists in the intimate mixing of already formed and crystalline CoFe2O4 nanoparticles, well-dispersed in oil-in-water microemulsion, with amorphous TiO2 nanoparticles, obtained, and contained in a microemulsion system with the same oil-surfactant-water composition; the second one is based on a seed mediated approach, where cobalt ferrite (synthesized by the O/W microemulsion method) is employed as nucleation substrate (seed) for the in situ growing of amorphous TiO2 in a microemulsion reaction system. According to these, two types of hybrid nanoparticles configurations were expected: partial decoration of CoFe2O4 with TiO2 and much more uniform wrapping or full coverage of CoFe2O4 with a TiO2 nanoparticles layer. The confined self-assembled media at nanoscale, created by the microemulsion environment, may lead to an enhanced synergy between magneto-photocatalytic nanoparticles at their interface, which can result in hybrid CoFe2O4-TiO2 nanostructures with good structural characteristics and improved properties. In-depth characterization was performed to evidence CoFe2O4-TiO2 hybrid nanoparticles formation, evaluate their properties, and relate obtained characteristics with the developed synthesis approaches.
2. Materials and Methods
Iron(III) 2-ethylhexanoate (C24H45FeO6, nominally 50% in mineral spirits) and titanium(IV) 2-ethylhexanoate (C32H60O8Ti, 97%) were purchased from Alfa Aesar. Cobalt(II) 2-ethylhexanoate (C16H30CoO4, 65 wt% solution in mineral spirits), isooctane (C8H18, CHROMASOLV® Plus for HPLC, ≥99.5%), tetramethylammonium hydroxide pentahydrate TMAH (C4H13NO·5H2O, ≥97%), and ammonium hydroxide NH4OH (H5NO, solution 28.0–30.0%) were purchased from Sigma-Aldrich. Synperonic™ 91/5 (C19O6H40) was purchased from CRODA.
2.2. CoFe2O4-TiO2 Hybrid Nanoparticles Synthesis
In order to obtain the dual phase nanoparticles, two synthesis approaches were developed. First, we introduce the microemulsions preparation and synthesis of single cobalt ferrite and titanium oxide nanoparticles and then the microemulsion reaction strategies named as “impregnation” and “seed” approaches, based on the synthesis of single CoFe2O4 and TiO2 phases. We used a 1 : 3 CoFe2O4 : TiO2 molar ratio for both cases.
2.2.1. CoFe2O4 NPs: Microemulsion Preparation and Synthesis
Cobalt ferrite nanoparticles were synthesized by the oil-in-water microemulsion reaction method; for this purpose we formulate a pseudo-ternary system composed of oil (O)/surfactant (S)/deionized water (W) with a 20/20/60 weight percent ratio (wt%). As oil we employed isooctane and as surfactant Synperonic 91/5. The metalorganic precursors iron(III) and cobalt(II) 2-ethylhexanoates (Fe : Co 2 : 1 molar ratio) were previously dissolved in isooctane; Fe and Co were in 0.98 and 1.86 wt% with respect to the nonpolar phase. Constituents were weighted and properly mixed on a closed vessel and placed in a water bath with controlled temperature. A brown translucent solution (due to precursors coloration) with a low viscosity and lack of birefringence was obtained, in a temperature range from 45 to 55°C. These physical characteristics suggested the formation of an oil-in-water microemulsion, which was confirmed by conductivity measurements (see Supporting Information in Supplementary Material available online at https://doi.org/10.1155/2017/2367856). Below 45°C, a dark-brown and milky mixture was formed, while heating up to 55°C led to phase separation. Once that microemulsion composition and temperature were studied, the confined reaction was carried out at 46°C by adding a 1 M TMAH solution to the microemulsion media, under magnetic stirring until reaching a pH of 12; a dark-brown precipitation evidenced the formation of cobalt ferrite nanoparticles. Stirring and temperature conditions were maintained for 24 hours.
2.2.2. TiO2 NPs: Microemulsion Preparation and Synthesis
Titanium oxide nanoparticles were also synthesized employing a pseudo-ternary system formulated with a 20/20/60 wt% composition of O/S/W. In this case, titanium(IV) 2-ethylhexanoate was previously dissolved in isooctane, considering 1.19 wt% of Ti with respect to the oil phase. Constituents were weighted and properly mixed on a closed vessel and placed in a water bath with controlled temperature. Using this pseudo-ternary system, we obtained a transparent, fluid, and isotropic liquid solution at a temperature range from 19 to 27°C. Out of this interval, a white milky solution was observed; but interestingly, if temperature was increased from 40 to 43°C a transparent, fluid, and isotropic liquid solution was also obtained. Based on conductivity measurements (Supporting Information), an oil-in-water microemulsion was formed in the 19–27°C range, while a bicontinuous microemulsion was obtained in the 40–43°C range. We decided to carry out the confined reaction at 41°C, due to the proximity to the Co-Fe O/W microemulsion formation. The reaction was conducted by adding NH4OH concentrate solution (under magnetic stirring) to the Ti colloidal system until reaching a pH of 11; titanium oxide nanoparticles formation was indicated by the formation of a white precipitate. Stirring and temperature conditions were maintained for 24 hours.
2.2.3. CoFe2O4-TiO2 Hybrid NPs: Impregnation Approach
Simultaneously, cobalt ferrite and titanium oxide NPs were synthesized in separated vessels, as described. After 24 hours of reaction for CoFe2O4 NPs, microemulsion temperature was adjusted to 41°C (cobalt ferrite colloidal system was stable at this temperature due to dilution effect through precipitating agent addition); then the TiO2 NPs microemulsion was poured over cobalt ferrite system under vigorous magnetic stirring. Agitation was maintained for 15 minutes, and then acetone (1/1 ratio in volume) was added to promote CoFe2O4 impregnation with TiO2. The color of prepared CoFe2O4-TiO2 hybrid nanoparticles was clearly lighter than single CoFe2O4 nanoparticles, due to the incorporation of titanium oxide phase. Through this approach we expected at least partial CoFe2O4 NPs decoration with TiO2 nanoparticles.
2.2.4. CoFe2O4-TiO2 Hybrid NPs: Seed Approach
In a first step, CoFe2O4 NPs were synthesized by the O/W microemulsion method as described; then the required amount of NH4OH solution for the subsequent titanium oxide precipitation (determined in impregnation approach) was added directly to the cobalt ferrite microemulsion; its temperature was adjusted to 41°C. As a second step, Ti precursor microemulsion was prepared and poured over the CoFe2O4 nanoparticles microemulsion system. At this stage on its surface, cobalt ferrite is used as nucleation and growing seed for TiO2 phase. Agitation and temperature conditions were maintained for 24 hours. Similarly, to hybrid nanoparticles prepared by the impregnation approach, the color of as-obtained product was clearly lighter than single CoFe2O4 nanoparticles. Through this approach, a uniform coating of TiO2, in the shape of small nanograins, all around CoFe2O4 surface is attempted.
Both prepared CoFe2O4-TiO2 hybrid nanoparticles were separated from their microemulsion media and washed by several cycles of centrifugation (acetone-water mixture, isopropanol, and chloroform) to remove byproducts and impurities. The obtained powders were dried at 70°C and finally thermally treated at 450°C during 5 hours with a heating rate of 5°C/minute under air atmosphere. A CoFe2O4-TiO2 NPs physical mixture was also prepared, by mixing (1 : 3 molar ratio) thermally treated cobalt ferrite and thermally treated titanium oxide powders. This sample served as reference material. Figure 1 broadly illustrates the developed synthesis approaches.
2.3. CoFe2O4-TiO2 Hybrid NPs Characterization
The crystalline structure of as-synthesized and thermally treated CoFe2O4-TiO2 hybrid nanoparticles was determined by X-ray diffraction (XRD) using a PANalytical Empyrean diffractometer, with CuKα radiation ( nm) at 45 kV and 40 mA. Diffractograms were recorded in the 5–100° range of 2θ, with a step size of 0.0167113° and a time per step of 59.69 s. The morphological characterization was performed by high-resolution transmission electron microscopy in scanning mode (HRTEM-STEM), using a field emission transmission electron microscope, JEM-2200FS (with 0.1 nm resolution) operated at 200 kV. Additionally, energy dispersive X-ray microanalysis (EDX) was carried out, in order to assess the local chemical composition of the nanomaterials. Complementary to this study, inductively coupled plasma atomic emission spectroscopy (ICP-AES) was employed to evaluate the overall chemical composition of the samples, and Thermo Jarrell Ash iCAP 6000 equipment was used. On the other hand, the textural properties of the thermally treated hybrid NPs were determined by nitrogen adsorption-desorption isotherms (at 77 K), employing an automatic Quantachrome Autosorb instrument; prior to the N2 adsorption, all the samples were outgassed at 150°C for 3 hours. Surface area and the mean pore size diameter were calculated using the Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The optical properties were determined by diffuse reflectance (DR) UV-Vis measurements, using UV-Vis-NIR spectrophotometer from Agilent Technologies, with an integrating sphere configuration; the DR UV-Vis spectra were acquired in the 200–1000 nm range. The magnetic properties of thermally treated cobalt ferrite and CoFe2O4-TiO2 hybrid nanoparticles were recorded at 300 K using a vibrating sample magnetometer (quantum design PPMS with VSM option) with a maximum magnetic field of ±60,000 Oe. The chemical surface composition was studied by X-ray photoelectron spectroscopy (XPS), analyses were carried out by means of a Thermo Scientific Escalab 250 Xi instrument, with a working pressure of ~10−10 mbar, and the photoelectrons were generated with an AlKα (1486.68 eV) source. The X-ray voltage and power were 14 kV and 350 W, respectively. The spectra were obtained using a pass energy of 50 eV. Peak fitting with Voight profiles was performed using the Thermo Avantage software V 5.41. The baseline corrections were made using the Shirley-Sherwood method. Binding energies were referenced to the C (1s) peak fixed at 284.8 eV.
3. Results and Discussion
3.1. CoFe2O4-TiO2 Hybrid Nanoparticle Synthesis
The mechanism of CoFe2O4-TiO2 hybrid nanoparticles formation can be explained by means of the oil-in-water microemulsion reaction principles [23, 24]. For CoFe2O4 synthesis, the Fe(III) and Co(II) 2-ethylhexanoates were contained in nanosized isooctane droplets, stabilized by a monolayer of hydrophilic Synperonic 91/5 surfactant, and dispersed in a continuous aqueous phase. After the addition of a water soluble base, as TMAH (a source of OH- species), a precipitation reaction occurs (presumably through the oil/water interface), leading to the formation of an iron-cobalt oxide, inside the oil droplets that act as nanoreactors, confining the formed compound at the nanoscale level. Once the first inorganic nuclei were formed and after 24 hours of reaction, the CoFe2O4 cubic-spinel phase is formed and dispersed in the colloidal system, at the adequate temperature conditions. After cobalt ferrite formation, the two planed strategies (impregnation and seed) were followed, using CoFe2O4 NPs as substrates, attempting to attach the titanium oxide on its surface. By the impregnation approach, the cobalt ferrite is mixed with already formed amorphous TiO2 NPs, which were also obtained by a microemulsion reaction process. Assuming the small size of microemulsion droplets as well as obtained products and the huge oil/water interfacial area, there must be a great level of interaction at the nanoscale level between the inorganic phases, leading to a good distribution of titanium oxide-cobalt ferrite nanoparticles. After the destabilization of the resulting colloidal system (through acetone addition), the coagulation of the nanoparticles must promote intimate incorporation of TiO2 amorphous nanoparticles over the cobalt ferrite system. On the other hand, by the seed approach, the first nuclei of amorphous TiO2 must grow directly on cobalt ferrite surface nanoparticles, due to the in situ confined reaction of Ti(IV) 2-ethylhexanoate, driven by the collisions amongst oil droplets (containing CoFe2O4 NPs) and the oil/water interface reaction with unreacted NH4OH, contained in the aqueous phase of CoFe2O4 NPs microemulsion. This strategy must produce a more efficient coating over cobalt ferrite nanoparticles than the impregnation approach. Finally the thermal treatment of both synthesized heterostructures led to the decomposition of organic byproducts and complete crystallization of inorganic phases.
3.2. CoFe2O4-TiO2 Hybrid NPs Characterization
3.2.1. X-Ray Diffraction
Figure 2 shows the XRD patterns of as-obtained CoFe2O4 and TiO2 single NPs and CoFe2O4-TiO2 hybrid nanoparticles synthesized by impregnation and seed approaches, before thermal treatment. As it can be seen, as-obtained CoFe2O4 pattern corresponds to a polycrystalline material, whose signals () at , and 63.18° can be assigned to the (111), (220), (311), (400), (511), and (440) crystalline planes of cubic spinel-type cobalt ferrite (JCPDS 04-006-4150). On the other hand, as-obtained titanium oxide pattern showed well broadened peaks indicating that the prepared phase is amorphous. Results in Figure 2 coherently show that as-obtained CoFe2O4-TiO2 hybrid nanoparticles barely exhibit the crystalline planes related to the cobalt ferrite phase (), as shown in the corresponding XRD patterns. The broadened profile and poor intensity of these signals are consequence of the amorphous TiO2 and the nanosized CoFe2O4. These results confirm the capacity of the O/W microemulsion method to produce structures with certain crystallinity under mild conditions, in our case at least for the single cobalt ferrite phase when using TMAH 1 M as precipitating agent, in contrast to other synthesis methods where thermal treatment is needed to produce crystalline CoFe2O4 [35–40], even if compared with some water-in-oil microemulsion synthesis methods [41, 42]. Otherwise, the lack of long-range ordering for as-obtained TiO2 is a common feature at mild conditions as reported [24, 31]. In the same figure, XRD patterns of thermally treated samples are presented. The corresponding cobalt ferrite pattern presents more defined peaks, revealing an improved crystallinity as a result of the atomic ordering promoted by the heating stage. However, the effect of thermal treatment is more evident in the pattern of the thermally treated titanium oxide, in contrast with as-obtained TiO2. The related XRD profile exhibits peaks (x) at , , , , , and that are in good agreement with the (101), (044), (200), (105), (211), and (204) crystalline planes of the TiO2 in the tetragonal phase known as anatase (JCPDS 04-002-8296), the most photocatalytic active TiO2 phase according to literature [43, 44], without evidence of other titanium polymorphs. In order to understand the diffraction effect of the crystalline TiO2 contribution to the cobalt ferrite phase and the influence of the synthesis approach, the reference CoFe2O4-TiO2 NPs physical mixture was analyzed. The resulting pattern is a simple sum of the diffraction from cubic spinel-type CoFe2O4 and tetragonal anatase TiO2 which is evident if compared with corresponding XRD profiles (thermally treated CoFe2O4 and TiO2). In the XRD profiles of thermally treated CoFe2O4-TiO2 hybrid nanoparticles, obtained by the impregnation and seed approaches, it is possible to observe similarities with the CoFe2O4-TiO2 NPs physical mixture. At least the three main reflections from each crystalline phase (CoFe2O4 (): (220), (311), and (440); TiO2 (x): (101), (200), and (204)) appear in the thermally treated hybrid nanoparticles. This fact makes sense due to the nanosized character of samples with intrinsic broadening and poor peaks. This condition limits the possibility of observing minor intensity peaks. Thermal treatment improves crystallinity evidenced through the higher intensity if compared with as-obtained samples; however this crystallinity is lower if compared with XRD profile from CoFe2O4-TiO2 NPs physical mixture; the lower intensity of diffraction signals has also been reported for the coating of a magnetic phase with TiO2 nanoparticles , although not at the same proportion compared to this work. If CoFe2O4-TiO2 XRD profiles are compared from impregnation and seed approaches we found a difference. For instance, the thermally treated CoFe2O4-TiO2 hybrid nanoparticles obtained by impregnation approach present a more defined reflection. This result could be due to the fact that TiO2 nanocrystals are not completely wrapping the cobalt ferrite phase. In the case of the thermally treated CoFe2O4-TiO2 hybrid NPs obtained by seed approach, a less defined reflection is observed, surely due to a more intimate wrapping of CoFe2O4 phase within TiO2 nanocrystals, as a consequence of the titanium in situ reaction carried out over cobalt ferrite seeds. The mixing of already formed CoFe2O4 and TiO2 NPs, dispersed in microemulsions as in the impregnation synthesis approach, results in a better strategy to obtain a more uniform wrapping of cobalt ferrite with TiO2, if it is compared with the simple NPs physical mixture, but less effective if it is compared with the seed approach. The crystallite mean size () of CoFe2O4 and TiO2 on thermally treated samples was estimated by Rietveld refinement (Table 1), and instrumental contribution using a CeO2 NIST reference standard (Code SRM 674b) was previously determined in order to assess values; related cell parameters were also estimated and reported in Table 1.
As it can be seen, the seed synthesis approach yielded a smaller CoFe2O4 crystallite mean size than impregnation approach; this can be attributed to a growing inhibition effect (even after thermal treatment), caused by the uniform TiO2 wrapping over cobalt ferrite surface. Meanwhile, the TiO2 crystallite mean size estimated for the seed sample is slightly larger than that obtained for the impregnation approach; thus it is possible to assume that titanium oxide crystallization is not as affected as for supporting ferrite, under the decoration process and the applied thermal treatment carried out by the synthesis approaches. In the case of CoFe2O4-TiO2 NPs physical mixture, it is evident that crystallite mean sizes for CoFe2O4 and TiO2 phases are almost the same compared to the corresponding single nanoparticles; these results are coherent since this sample is powder-powder mixing of magneto-photocatalytic phases. In regard to estimated cell parameters, it is possible to observe deviations from the cubic spinel structure cell parameters, for CoFe2O4, and from the tetragonal structure cell parameters, for anatase TiO2. These deviations can be explained by the probable substitution of Fe3+ (0.64 Å) and/or Co2+ (0.65 Å) for Ti4+ ions (0.68 Å) into the tetragonal lattice structure of anatase TiO2 as a consequence of their similar ion sizes, due to ion diffusion (at the interfaces) because of thermal treatment conditions, as similarly reported by Raju and Murthy . However, the absence of extra peaks in the XRD patterns and thus the lack of secondary phases confirm that not solid reaction occurred between cobalt ferrite and titanium oxide NPs. Finally, it is worth mentioning that the thermal treatment temperature (450°C) was established taking into consideration the fact that for temperatures above 550°C iron diffusion into the TiO2 layer may occur, forming Fe2TiO5 (PDF #01-070-2728) as a secondary phase (Supporting Information); this commonly happens with Fe rich compositions in the presence of titania, as reported by Gao et al. for the synthesis of TiO2/γ-Fe2O3 . Moreover, the thermal treatment time was chosen taking into account the fact that TiO2 crystallization in the CoFe2O4-TiO2 hybrid nanoparticles did not occur at the same rate compared to pure TiO2; hence, in order to achieve a higher crystallinity we extend the thermal treatment of the materials for 5 hours, as shown in the XRD patterns. The delay of TiO2 crystallization could be related to the nanosized character of hybrid NPs after thermal treatment. Probably, intimate wrapping between CoFe2O4 and TiO2 nanocrystals and a higher melting point for nanostructures could control in some ways the intrinsic diffusion processes.
3.2.2. High Resolution Electron Transmission Microscopy-STEM Mode and EDX Analysis/ICP-Atomic Emission Spectroscopy Analysis
Figure 3 presents representative bright field HRTEM-STEM electron micrographs of thermally treated samples: cobalt ferrite, titanium oxide single nanoparticles, and CoFe2O4-TiO2 hybrid nanoparticles, prepared by impregnation and seed approaches. First, CoFe2O4 analyzed zone depicts agglomerated nanoparticles, with a semiglobular morphology and sizes below 25 nm; similarly, TiO2 exhibits agglomerated nanoparticles but with smaller sizes, below 15 nm, and also semiglobular morphology. As it can be seen, the corresponding inset images display the presence of nanocrystals, as denoted by their lattice fringes, making evident the crystallinity of CoFe2O4 and TiO2 nanoparticles. The measured interplanar spacings are in good accordance with cubic-spinel phase of cobalt ferrite and anatase phase of titanium oxide. In the same figure, bright field micrographs of synthesized hybrid nanoparticles (impregnation and seed approaches) depict nanostructured materials apparently composed of smaller nanoparticles, in comparison with bare cobalt ferrite, due to the surface integration of titanium oxide over cobalt ferrite NPs surface. According to Fu et al., who synthesized core-shell CoFe2O4-TiO2 heterostructures by coprecipitation/sol-gel route, cobalt ferrite nanoparticles must absorb electrons more strongly than TiO2 (because CoFe2O4 possess a higher atomic number); consequently darker regions on electron micrographs could be assigned to Co-Fe phase ; however, in our case obtained nanoparticles are much smaller and tend to form agglomerates (thus thicker regions), which provides also image contrast, as shown in single nanoparticles images, making it difficult to carry out a proper identification of the magnetic and photocatalytic components at view. Moreover, through titanium oxide modification, the ferrite nanoparticles show diffuse boundaries, as similarly reported by Stefan et al.  for core-shell Fe3O4-TiO2 nanoparticles of sizes below 20 nm. In order to assess the distribution of Fe, Co, and Ti, on the hybrid CoFe2O4-TiO2 nanoparticles, an elemental mapping was performed; corresponding images are exhibited in Figure 3. As expected, the impregnation synthesis approach led to a less uniform elemental distribution, compared to the hybrid NPs synthesized by seed approach, that assures a more intimate coupling with titanium oxide phase. Related semiquantitative analysis by EDX is shown in Table 2, and it is evident that there is a deviation from the expected nominal values; nonetheless obtained Co : Fe ratio is close to the 1 : 2 stoichiometry (CoFe2O4). In the case of Ti, the experimental value is higher; this could be attributed to the wrapping of titanium oxide over cobalt ferrite surface that partially screened the signals of cobalt and iron. Complementary to this study, ICP-atomic emission spectroscopy analysis of the samples is also shown in Table 2; as it can be seen, the overall experimental composition is very close to the CoFe2O4-TiO2 nominal values; thus it is possible to affirm that the microemulsion synthesis method conducted to a well-defined chemical composition. On the other hand, Figure 4 presents higher magnification electron micrographs of hybrid nanoparticles. As observed, the images also display the presence of nanocrystals. Fast Fourier transformation was applied in the highlighted zones (using Digital Micrograph software), with the aim of obtaining the corresponding electron diffraction patterns (EDPs). For the EDPs from the impregnation hybrid nanoparticles, the calculated -spacings ~2.5 and ~3.5 Å are in agreement with () (311) and (101) interplanar distances of the cubic spinel-type CoFe2O4 (JCPDS 04-006-4150) and the anatase TiO2 (JCPDS 04-002-8296), respectively. In accordance with the impregnation microemulsion synthesis approach, the image of the hybrid nanoparticles displays an apparently random distribution of CoFe2O4 and TiO2 nanocrystals, as a result of the microemulsion-microemulsion mixing of CoFe2O4 with amorphous TiO2 nanoparticles, and the posterior thermal treatment. Otherwise, for the EDPs from the seed hybrid nanoparticles, the calculated -spacings ~2.5 and ~3.5 Å are in agreement with () (311) and (101) interplanar distances of the CoFe2O4 and TiO2. In accordance with the seed approach, the image of the hybrid nanoparticles exhibits a TiO2 nanocrystal jointed directly on the surface of a cobalt ferrite nanocrystal, in addition to a wrapping that could be associated with the titania phase, as a result of the in situ reaction of Ti precursor over the CoFe2O4 nanocrystals used as a seed. In general, the HRTEM-STEM results confirm the coexistence of CoFe2O4 and TiO2 in one platform and therefore the achievement of the CoFe2O4-TiO2 hybrid nanoparticles by the two planned synthesis approaches. As demonstrated, the characteristics of the hybrid nanoparticles are dependent on the synthesis approach.
3.2.3. Optical Properties
In Figure 5 UV-Vis diffuse reflectance spectra of thermally treated samples, single CoFe2O4 and TiO2 and hybrid CoFe2O4-TiO2 nanoparticles, are presented. As evident, TiO2 is absorbed strongly in the UV region, below a band edge of ~400 nm, whereas the optical absorption of CoFe2O4 spreads along the UV-visible spectrum, decreasing near the infrared region. Similarly, a broad absorption starting from UV and extended to the visible light region is evident from the spectra of hybrid CoFe2O4-TiO2 nanoparticles prepared by the seed approach, indicating a red shift compared with pure TiO2, as a result of the coupling with cobalt ferrite phase. According to similar works, the increasing of TiO2 optical absorption is attributed to the substitution of Fe3+ (0.64 Å) and/or Co2+ (0.65 Å) for some Ti4+ (0.68 Å) ions in the titanium oxide lattice structure, as shown previously from the calculation of CoFe2O4 and TiO2 parameter cells. This ion substitution implies a change into the TiO2 electronic structure, and the Fe3+/Co2+ modification acts as a new impurity level between the valence or conduction band, narrowing the original bandgap, and enhancing the visible light absorption response [18, 20, 48]. In the same way, the optical response of the thermally treated hybrid nanoparticles prepared by the impregnation approach is favored to the visible light, and although the spectra of both hybrids are overlapped at the ~200–350 region, it is possible to notice that impregnation hybrid nanoparticles exhibit a slight band edge displacement to the near infrared region, in addition to a small increment in absorption intensity. These differences are a consequence of their peculiar structural characteristics; for instance, the impregnation sample has not the same arrangement of TiO2 nanocrystals if compared with the material synthesized by the seed approach (as demonstrated by HRTEM-STEM results); however, both materials are well coupled. This fact is more evident if we compare the hybrid nanoparticles spectra against the spectra of the NPs physical mixture, as it can be seen the CoFe2O4-TiO2 physical mixture spectra clearly exhibit a decay in absorbance below 450 nm, the band edge absorption of pure TiO2, in contrast with hybrid nanoparticles whose titania band edge absorption is apparently modified due to the strong interaction between inorganic phases.
Taking into consideration the optical response showed by the hybrid CoFe2O4-TiO2 nanoparticles, it is possible to affirm that the synthesized nanomaterials meet the optical requirements to be active under ultraviolet and/or visible light illumination, in contrast with single TiO2 nanoparticles.
3.2.4. Textural Properties
Nitrogen adsorption/desorption isotherms curves (at 77 K) of thermally treated samples, single CoFe2O4 and TiO2 nanoparticles and hybrid CoFe2O4-TiO2 nanoparticles, are exhibited in Figure 6. It is well known that the shape of the adsorption/desorption isotherm is related to the texture of the solid. In this sense, TiO2 nanoparticles depict a type IV isotherm and an H-2 hysteresis loop, in the relative pressure range of 0.5–1 [44, 49, 50]. This fact implies that thermally threated TiO2 is presumably mesoporous (pore sizes between 2 and 50 nm), as estimated by a pore size distribution centered at 14.8 nm and a pore volume of 1.6 cm3/g, employing the BJH (Barrett-Joyner-Halenda) method. Moreover, the specific surface area of TiO2 was calculated using the BET (Brunauer-Emmett-Teller) method, obtaining a value of 403 m2/g. In the case of the CoFe2O4 nanoparticles isotherm curve, it is evident that the amount of N2 being adsorbed (as function of the relative pressure) is much lower than pure TiO2, mainly due to the inherent nonporous character of cobalt ferrite  compared with titanium oxide. On the other hand the CoFe2O4-TiO2 hybrid nanoparticles isotherms display wider hysteresis loops at a high relative pressure, compared with pure CoFe2O4, indicating also a larger volume of gas being adsorbed. The profiles of the corresponding curves exhibit a mesoporous behavior due to the integration of titanium oxide phase; however, from calculated textural properties (Table 3), it is clear that thermally treated CoFe2O4-TiO2 samples possess surface area values closer to the values presented by cobalt ferrite phase. Thus the textural properties of the hybrid CoFe2O4-TiO2 nanoparticles are likely due to the combination of inherent porosity and the interparticle arrangement, achieved from the packing of neighbouring nanoparticles. The reported surfaces areas of microemulsion prepared hybrid CoFe2O4-TiO2 nanoparticles are higher to the value reported for CoFe2O4-TiO2 nanomaterials synthesized by methods such as the polymeric precursor method (75.29 m2/g) , with a CoFe2O4 : TiO2 weight ratio of 56 : 44.
In addition, it should be noted that the slight differences of the textural properties of hybrid CoFe2O4-TiO2 nanoparticles can be related to the microemulsion synthesis approaches and therefore to the TiO2 arrangement on cobalt ferrite phase supporting core. Obtained results confirm the capacity of the O/W microemulsion method to produce materials with good textural properties.
3.2.5. Magnetic Properties
Figure 7 shows the room-temperature magnetization versus applied field curves ( versus ) of thermally treated samples: cobalt ferrite NPs and hybrid CoFe2O4-TiO2 nanoparticles, as well as the NPs physical mixture. Magnetization values are reported per gram of CoFe2O4, considering the 1 : 3 CoFe2O4-TiO2 stablished molar ratio. First, it can be seen that the nanosized CoFe2O4 shows a typical ferrimagnetic hysteresis loop ; from the corresponding curve, the spinel ferrite displays a maximum magnetization () of 65 emu/g and a magnetic remanence () of 22 emu/g, with a coercitivity () of 1191 Oe. The obtained value is lower than the reported value for the bulk counterpart ( 80 emu/g) [40, 53], but close to the range recorded for the CoFe2O4 nanoparticles (with average sizes of less than 50 nm), synthesized by water-in-oil microemulsions [42, 53] and even some other wet methods such as sol-gel , precipitation, and coprecipitation . Besides, the obtained coercivity is also in the reported range. Second, compared with the curve of single CoFe2O4 NPS, the hybrid CoFe2O4-TiO2 nanoparticles display a clearly different behavior, for instance, a very slim hysteresis loop (inset of Figure 7). The magnetic properties of the corresponding materials are summarized in Table 4; as it can be seen the and values are lower than those obtained from bare CoFe2O4. The decrement on magnetic properties is a typical behavior of magnetic nanoparticles coated with titania nanocrystals, as reported in [18, 20, 21, 48]. This is attributed to the presence of nonmagnetic TiO2 nanocrystals that act as a dead layer, breaking the long range order of the magnetic CoFe2O4 domains [54, 55]. Thus, it is possible to affirm that the hybrid CoFe2O4-TiO2 NPs with weaker magnetic properties possess less magnetic dipole-dipole coupling, as a result of a more uniform titania wrapping on the cobalt ferrite surface. In this context, the seed microemulsion synthesis approach is a better option to develop a more efficient coating, in agreement with previous results (XRD and HRTEM-STEM). With the purpose of explaining the effect of the TiO2 wrapping on cobalt ferrite surface, the CoFe2O4-TiO2 NPs physical mixture magnetic curve is also presented. As can be seen, its hysteresis loop is almost overlapped with the CoFe2O4 NPs curve (Figure 7 inset), in contrast to the microemulsion synthesized hybrid CoFe2O4-TiO2 nanoparticles. Therefore, the results coherently suggest that the CoFe2O4 magnetic domains in this sample are not disrupted, in contrast to the CoFe2O4 materials that are wrapped by the layer of TiO2 nanocrystals, as in the synthesized CoFe2O4-TiO2 hybrid nanomaterials, that presumably tend to behave like magnetic monodomains (especially the seed sample). The magnetic response shown by the impregnation and seed hybrid CoFe2O4-TiO2 nanoparticles is attractive for magnetic recovery applications in heterogeneous photocatalysis. If a suitable magnetic field is applied near to the CoFe2O4-TiO2 nanomaterials, the magnetic-photocatalytic nanocomposite powders could be easily collected, as shown in Figure 7. Furthermore, removal of the magnetic field would leave almost no residual magnetism on the hybrid nanoparticles, as a consequence of their low remanent magnetization; thus the powders could be redispersed for recycling, presumably without complications.
3.2.6. Surface Composition
To find out about chemical composition of obtained hybrid CoFe2O4-TiO2 nanoparticles, X-ray photoelectron spectroscopy measurements were performed. Figure 8 shows the XPS spectra (Fe, Co, Ti, and O species) of the samples after thermal treatment. High-resolution XPS spectra for Fe 2p are presented on the first row. The Fe 2p3/2 main peak, shouldering with a satellite peak, is fitted into two signals, indicating the existence of Fe3+ species in two different lattice sites, as expected in a cubic spinel-type crystalline structure . Each peak has associated a characteristic spin orbit splitting (doublet). For the impregnation sample, the binding energy at 709.6 eV arises from in octahedral sites (), while the binding energy at 710.99 eV is caused by in tetrahedral sites (). Similarly, in the seed sample, the two signals at 710.49 eV and 712.63 eV can be related to the octahedral and tetrahedral iron(III) [56, 57]. According to the integrated intensity of the fitted doublets, the distribution of in octahedral sites is 60% and 40% in tetrahedral sites. On the other hand (second row), the Co 2p3/2 XPS spectra of analyzed samples were fitted using three peaks; each peak has associated a characteristic spin orbit splitting (doublet) which leads to six-peak representation. First, for the impregnation sample, the binding energies at 779.07 and 780.90 eV are assigned to ions in octahedral sites () and tetrahedral sites (), respectively. Second, for the seed sample, the signal is situated at 781.32 eV and the signal on 784.58 eV. The ratio of ions in octahedral to tetrahedral sites is obtained, from the integrated intensity of the fitted peaks, to be about 2 : 3. Additionally, the signals at 784.5 eV (I) and 788.09 (S) are characterized to be the satellite peak of Co 2p3/2 main line. Finally, corresponding XPS spectra for Ti2p and O1s signals are shown in the third and fourth rows. In the case of titanium, main 2p3/2 peak is observed at 457.498 eV (I) and 458.18 (S) with a spin orbit splitting of 5.86 eV and 5.73 eV, respectively; these peak positions were identified as that of Ti4+ from TiO2. Binding energies slightly deviate from those reported for pure TiO2 at 458.59 eV . This could be an evidence of slight changes for right stoichiometry due to oxygen vacancies. Ti2p3/2 peak was fitted with one peak in the case of the impregnation sample, which proves no other adsorbed species or additional phases; in the case of the seed approach sample, a second very low intensity peak can be identified (); this can be attributed to the Fe-O-Ti contribution, also inferred by XRD and UV-Vis results. With respect to oxygen, O1s peak is divided into three peaks. The main peak at 529.86 eV (I) and 529.89 eV (S) is attributed to the contribution of the crystal lattice oxygen (oxygen bonded to metal). However, the exact assignment of the two higher binding energy peaks is rather complex and controversial as numerous factors like surface defects, contaminants, impurities, or chemisorbed species could result in the appearance of the shoulder peaks. It is suggested that remaining contributions come from chemisorbed species like carbon (C-O) or hydrogen (O-H).
According to some studies, the architecture of core-shell type nanoparticles can be envisaged by XPS data interpretation [4, 59]. If CoFe2O4-TiO2 hybrid nanoparticles are composed of a core-shell structure, the core signal would be partially or totally screened (as a function of layer thickness) by the signal of the corresponding shell. In this sense, Figure 9 displays the Fe 2p3/2 and Ti 2p3/2 detected peak intensities as a function of thermally treated hybrid nanoparticles. For the impregnation sample, the intensity of Fe signal is higher than the signal detected for titanium; in contrast, the Ti signal on the seed sample is by large above the iron signal. Therefore, it is coherent to assume that hybrid CoFe2O4-TiO2 nanoparticles synthesized by the seed approach present a TiO2 thicker layer over cobalt ferrite surface (or it is more uniformly coated) in comparison with the hybrid nanoparticles synthesized by the impregnation approach.
CoFe2O4-TiO2 hybrid nanoparticles were successfully prepared by the impregnation and seed approaches, based on the oil-in-water microemulsion reaction method conditions of single CoFe2O4 and TiO2 nanoparticles. Applied thermal treatment allowed the crystallization of anatase TiO2 NPs layer within the cubic spinel-type CoFe2O4 nanoparticles, for both synthesis cases, as demonstrated by XRD patterns; the shape and intensity of main diffraction signals suggested differences on CoFe2O4-TiO2 heterostructures, due to the arrangement of TiO2 onto the cobalt ferrite phase, as confirmed by electron microscopy images, which depicted partial decoration, for the impregnation approach, and a more uniform coverage (core-shell like), for the seed approach. In the same way, the elemental mapping of seed-mediated sample exhibited a homogeneous titanium distribution in contrast to the impregnation approach; however a well-defined CoFe2O4-TiO2 chemical composition was obtained for the two developed approaches, as proved by atomic absorption spectroscopy and EDX microanalysis. The study of optical properties showed the ultraviolet and visible light absorption capability of hybrid nanoparticles, as a consequence of bandgap energies modification, due to cobalt ferrite and titanium oxide coupling. The assessed textural properties exposed the high surface areas of CoFe2O4-TiO2 hybrid nanomaterials (as well as single phase nanoparticles); the integration of titanium oxide within CoFe2O4 phase resulted in a major gas absorption capability, as indicated by the obtained adsorption/desorption isotherms. CoFe2O4 magnetic properties were clearly altered with the attaching of nonmagnetic TiO2 layer; thus it was possible to assume that the hybrid nanoparticles synthesized by the seed approach possessed a much more continuous coverage coating due to the apparently magnetic monodomain behavior displayed by the corresponding magnetic curve. XPS analysis shows the presence of Fe3+, Co2+ (CoFe2O4 phase), and Ti4+ (TiO2 phase) chemical states on the surface of synthesized and thermally treated samples, and the titanium peak intensities suggested its predominance over CoFe2O4 nanoparticles surface, especially for the seed approach. In addition, the characterization of the CoFe2O4-TiO2 NPs physical mixture provided enough evidence of the characteristics possessed by a hybrid nanomaterial with a poor interaction between its components, in comparison with the microemulsion synthesized hybrid nanomaterials. In summary, this report provides new microemulsion synthesis strategies for the straightforward design of CoFe2O4-TiO2 hybrid nanoparticles. The characteristics of synthesized hybrid nanoparticles are suitable for potential applications in the field of photocatalysis.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors are grateful to CONACYT (CB Project Grant no. CB2011/166649 and Proyecto Redes Temáticas no. 194451). The authors also acknowledge Aleyda Cavazos Cavazos (CIMAV Monterrey), Luis G. Silva Vidaurri (CIMAV Monterrey), Luis de la Torre Sáenz (CIMAV Chihuahua), Gilberto H. López (CIQA), Omar Ali Carrasco Jaim (UANL), and Dr. Isaías Juárez Ramírez (UANL) for their help with atomic absorption spectroscopy, XPS BET, VSM, and UV-Vis measurements, respectively.
Fig. S1 displays the conductivity measurements (as a function of temperature) of the pseudo-ternary systems, prepared for the microemulsion synthesis of cobalt ferrite (CoFe-2EH microemulsion) and titanium oxide (Ti-2EH microemulsion) nanoparticles. This study served to confirm the type of microemulsion formed, at specific temperature conditions, along with visual inspection. On the other hand.
Fig. S2 shows the XRD diffraction pattern of the CoFe2O4-TiO2 hybrid nanoparticles thermal treated at a higher temperature (550°C), obtained pattern served to choose the thermal treatments conditions.
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