Article pubs.acs.org/IECR
A Generalizable Multigram Synthesis and Mechanistic Investigation of YMnO3 Nanoplates Coray L. McBean,† Crystal S. Lewis,† Amanda L. Tiano,‡ Jack W. Simonson,§ Myung-Geun Han,∥ William J. Gannon,¶ Shiyu Yue,† Jonathan M. Patete,⊥ Adam A. Corrao,† Alexander C. Santulli,⊥ Lijun Wu,∥ Meigan C. Aronson,¶ Yimei Zhu,§,◆ and Stanislaus S. Wong*,†,∥ †
Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, United States US Nano, LLC, 1748 Independence Boulevard, Building A, Sarasota, Florida 34234, United States § Department of Physics, Farmingdale State College, Farmingdale, New York 11735-1021, United States ∥ Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory, Building 480, Upton, New York 11973, United States ¶ Department of Physics and Astronomy, Texas A&M University, College Station, Texas 77843-4242, United States ⊥ Department of Chemistry, Manhattan College, Riverdale, New York 10471, United States ◆ Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, New York 11794-3800, United States ‡
S Supporting Information *
ABSTRACT: The reproducible gram-scale synthesis of crystalline nanoscale multiferroics is critical for the development of the next generation of commercially relevant electronic devices. Of the subset of multiferroic materials, yttrium manganese oxide (YMnO3) is highly attractive, because of its large magneto-electric coupling constants and the recent observation of giant polarization under pressure in these types of rare earth manganites. Utilizing a unique synthetic methodology that combines metal−oleate thermal degradation with the use of a molten salt protocol, we were able to reproducibly generate monodisperse distributions of morphologically distinctive yttrium manganese oxides. Specifically, using a molten NaCl flux, we were able to synthesize phase-pure, single-crystalline hexagonal YMnO3 nanoplates, measuring 441 ± 241 nm in diameter and 46 ± 6 nm in height. Moreover, these nanoplates gave rise to multiferroic behavior, which was confirmed by the observation of a ferroelectric phase from a combination of high-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) analysis. Magnetic measurements are consistent with the onset of a spin glass state below 5 K. To highlight the generalizability of the synthetic method we have developed herein, as a demonstration of principle, we have also successfully used the same protocol to produce nanocubes of lanthanum aluminum oxide (LaAlO3).
1. INTRODUCTION
A number of metal oxides with the chemical composition of ABO3, wherein “A” and “B” are cations of different sizes, are known to be multiferroic in nature. For example, (i) BiFeO3, (ii) several Mn-based metal oxides such as BiMnO3, TbMnO3, and HoMnO3, as well as (iii) many of the rare-earth manganites exhibit magneto-electric coupling.2,5,6 Within the family of rareearth manganites, characterized by the general formulae of RMnO3 and RMn2O5 (wherein “R” is a rare-earth element), the yttrium manganese oxides represent a particularly attractive group of materials. Indeed, of the yttrium manganese oxides, hexagonal-phase YMnO3 (h-YMnO3)5 displays multiferroic
Concurrent with recent technological advancements, multiferroic materials have attracted significant interest owing to their uniquely coupled magnetic and electric parameters, simultaneously exhibiting both ferromagnetism and ferroelectricity.1,2 Albeit still relatively rare, many existing bulk multiferroics have been studied for applications, including but not limited to magnetically tunable high-frequency filters, oscillators, and highly efficient magnetic field sensors to replace existing superconducting quantum interference device (SQUID) technology.2,3 The most promising uses of these unique and industrially relevant materials tend to be associated with spintronics, sensors, and data storage,2,4 wherein their intrinsically coupled electronic and magnetic properties have become highly valuable attributes. © XXXX American Chemical Society
Received: January 9, 2017 Revised: April 1, 2017 Accepted: April 25, 2017
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DOI: 10.1021/acs.iecr.7b00113 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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dimensional (2-D) morphologies comprising hexagonal nanoplate morphologies of YMnO3. The practical and commercial relevance of our methodology resides in the fact that we can generate morphologically distinctive, homogeneous, and monodisperse distributions of these nanoscale motifs on a large scale using a protocol that is relatively easy, rapid, and intrinsically generalizable to the production of other chemically distinctive and industrially significant metal oxide compositions. It is worth noting that the process developed herein does not necessarily require the use of either specialized or complex equipment, such as an electrospinning apparatus or Schlenk lines for precise atmospheric control. Moreover, we do not rely on either lithographic, patterning, sputtering, epitaxy, or vapor deposition (e.g., CVD) techniques, which are commonly used as part of a number of industrial processes. Furthermore, our reasonably high-yield method not only avoids the use of either expensive precursor reagents or specialty gases, such as O2-rich mixtures, but also minimizes the generation of potentially harmful and unwanted byproducts. As relevant literature precedence, it is worth noting that there have been analogous reports of hexagonal plate formation, for example, for various metal oxides and “mixed metal” oxides. The production of Co2V2O7 submicron plates,24 Fe2O3 plates,25 as well as of Co3O426 and Al2O327 hexagonal plates comes to mind. Nevertheless, to the best of our knowledge in the literature, given the importance of multiferroic materials in general and YMnO3 in particular, there are neither reports of producing platelike motifs of YMnO3 nanostructures nor of using the idea of thermally degrading organometallic precursors with subsequent growth in a molten salt medium to do so, thereby emphasizing the novelty and potential scalability of this current work. The thermal degradation of metal−oleate complexes represents a fairly common wet solution technique for metal oxide nanoparticle synthesis. In fact, this methodology has already been used to synthesize 5 to 22 nm manganese oxide (MnO) and iron oxide nanoparticles.28 Similarly, various lanthanum oxide nanodisks with sizes in the range of 4.2 to 8.5 nm have been fabricated via thermal degradation of an intermediate organometallic complex.29 However, the widespread adoption of thermal degradation techniques is limited for several reasons. First, the maximum reaction temperature is constrained by the boiling point of the solvent; this is significant, because “mixed metal” oxides often require relatively high temperatures to form. Second, these techniques tend to be commonly initiated within an inert atmosphere, because an uncontrolled ignition of organic degradation products would otherwise occur, if carried out in air. As such, thermal degradation protocols have issues associated with relatively low yields per run and the necessity of using sophisticated equipment to initiate the reaction. Finally, the ultimate product is usually coated with an organic capping agent that must then be subsequently removed by additional processing steps, such as chemical washing and/or thermal annealing.30 In fact, while sodium oleate has been commonly implemented for metal oxide nanoparticle synthesis using the wet-solution method, there have been comparatively fewer if any reports regarding the use and incorporation of this organic surfactant species (i.e., sodium oleate) in combination with a molten flux protocol to generate a pure “mixed metal” oxide sample. Therefore, we report on the thermal degradation/molten salt coupled synthesis of single-crystalline, multiferroic YMnO3 nanostructures, which are not only characterized by a
behavior, and as such, has drawn significant research interest. Although multiple crystallographic phases of YMnO3 exist, only the noncentrosymmetric structure (P63cm) is multiferroic, due to the loss of inversion symmetry, whereas the centrosymmetric structure (P63/mmc) is paraelectric. In fact, the structure of multiferroic YMnO3 consists of alternating layers of Y3+ ions and manganese oxide trigonal bipyramids (MnO5).7 Below the Curie temperature (TC) of 1270 K, the yttrium ions are displaced, due to trimerization and tilting of the MnO5 bipyramids.8 In fact, a net polarization arises from this displacement of the Y3+ ions, which not only constitutes the origin of the ferroelectric behavior of h-YMnO37 but also has been experimentally confirmed by both Raman and infrared spectroscopy data.9,10 As a result, h-YMnO3 exhibits useful and interesting ferroelectric, magnetic, and optical properties.11−14 These properties coupled with their intrinsic nonvolatile nature have rendered this material as a highly desirable candidate for information storage and magneto-electric sensors. Examples of additional and equally intriguing multiferroic materials of interest include but are not limited to Pb(Zr,Ti)O3−Pb(Fe,Ta)O3,15 Bi6Ti2.8Fe1.52Mn0.68O18,16 as well as (BiTi0.1Fe0.8Mg0.1O3)0.85(CaTiO3)0.15.17 However, many of these aforementioned compounds are either compositionally complex and/or incorporate either toxic or expensive metals. Hence, the focus on YMnO3 reflects its comparatively simpler (and presumably more cost-effective to produce) chemical composition, because it incorporates relatively abundant and potentially less toxic elements, such as yttrium and manganese, which cost as little as $35 kg−1 and $2 kg−1, respectively.18 Hence, there is a real need to synthesize commercially relevant and “less complex” metal oxides, such as multiferroic YMnO3, on a large scale with simultaneous morphological and chemical control. Not surprisingly, whereas significant efforts have already been successfully expended toward generating thin films of YMnO3,4,19 the reported synthesis of other nanoscale motifs of YMnO3 is still quite limited. To date, nanoparticles of YMnO3 have been fabricated by hydrothermal,20 citric acid,21 and sol−gel22 methods, with a variety of different product sizes. Interestingly, the sol−gel method reported by Bergum et al. demonstrated the successful production of both the orthorhombic and hexagonal phases of YMnO3, depending on the annealing temperature employed,22 of varying crystallite sizes, ranging from 49 to 467 nm. By comparison, Liu and co-workers synthesized polydisperse YMnO3 nanoparticles, ranging from 25 to 92 nm in size, by utilizing a multistep protocol with various annealing stages.21 Furthermore, Wang et al. have employed a hydrothermal technique, running at 230 °C for 2 days, to manufacture polycrystalline ∼90 nm YMnO3 nanoparticles.20 Recently, our own group has been able to create anisotropic, one-dimensional (1-D) motifs of YMnO3, using a sol−gel technique within a template framework. Specifically, we were able to isolate not only (i) 276 ± 52 nm diameter polycrystalline nanotubes, composed of 17 nm particulate constituent grains, but also (ii) 125 ± 21 nm diameter single crystalline multiferroic nanowires of this material, with overall lengths for both 1-D geometries of up to several microns.23 These synthetic successes allowed us to investigate the effect of confinement coupled with axial elongation upon the magnetic properties of these unique anisotropic structures. To complement this prior work, we have sought to probe the corresponding properties and synthesis of flattened, quasi twoB
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The ensuing product was ultimately annealed at 820 °C for 4 h, using a ramp rate of ∼27 °C/min. The samples of “YMO-D” and “YMO-E” were annealed for 2 and 1 h, respectively. By contrast, the sample of “YMO-F” was quenched immediately, after the initial “ramp up” in temperature. After the heat treatment, the product was washed with 2 M HCl over a 2 h period at room temperature in order to remove any metal oxide impurities. The sample was further washed twice with distilled water and then air-dried at 60 °C. Additional systematic experimental variations to this typical protocol were performed, and the resulting samples have been designated as “S-A” through “S-K”. Figure S1 illustrates a set of representative optical images, highlighting the formation of intermediate species associated with isolated samples, obtained after selected successive steps within a typical multigram “scale up” of the synthetic procedure described above. Moreover, we systematically explore precise chemical and physical variations in the samples as a result of corresponding alterations in the experimental parameters, such as annealing duration, the identity of the surfactant, and so forth, in the accompanying SI section. 2.2. Structural Characterization Protocols. To determine crystallinity, XRD data were obtained using a Rigaku Miniflex diffractometer with an incident Cu Kα radiation (i.e., λ = 1.54 Å). Powder X-ray photoelectron spectroscopy (XPS) samples were transferred into the vacuum chamber of a homebuilt XPS surface analysis system, furnished with a model SPECS Phoibos 100 electron energy analyzer for electron detection. The obtained data were analyzed using the XPS PEAK 4.1 software. The morphology, size, and shape of the final products were investigated using a field-emission scanning electron microscope (FE-SEM Leo 1550), with data collected at an accelerating voltage of 20 kV; this instrument also had the capability for acquiring energy dispersive X-ray spectroscopy (EDS) data. In addition, low resolution TEM was carried out with the assistance of a FEI Tecnai12 BioTwin G2 instrument, equipped with an AMT XR-60 CCD digital camera system. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) data were achieved using both a JEOL 3000F as well as an aberrationcorrected Titan 80−300 microscope, operated at accelerating voltages of 300 kV. Dynamic light scattering was used to estimate particle size using the Zetasizer Nano ZS instrument, purchased from Malvern Instruments. To ascertain chemical functionality, FTIR spectra were obtained using a Nexus 670 instrument (ThermoNicolet) with the Smart Orbit diamond ATR accessory stage holder, a KBr beam splitter, and a DTGS KBr detector. A TGA Q500 instrument (TA Instruments) was employed in order to gain insights into the reaction dynamics as well as the thermal stability of our products. To probe the magnetic properties of our nanostructures, DC magnetic measurements were carried out in a Quantum Design Magnetic Properties Measurement System using the Reciprocating Sample Option. Additional details can be found in the SI section.
geometrically hexagonal nanoplate motif but also synthesized in reasonable quantities, the latter point of which is particularly relevant and important for possible translation into practical applications. Specifically, we have generated nanoplates of YMnO3 by exploring a variety of annealing temperatures and surfactants. That is, by utilizing a novel oleate-mediated, molten salt procedure, we have been able to create monodisperse, hexagonally shaped YMnO3 nanoplates characterized by smooth, continuous edges, with heights measuring 46 ± 6 nm and diameters of 441 ± 241 nm. Specifically, these products were made, when NaCl was used as the underlying reactive salt medium, and they were isolated upon rapid quenching after annealing at 820 °C for 4 h. Magnetic, vibrational, and structural properties of these nanomaterials were subsequently characterized to probe and correlate the effects of chemical composition and morphology. Though our paper is primarily focused on YMnO3, we have also demonstrated the generalizability of our protocol by initiating the equally successful synthesis of cubes of lanthanum aluminum oxide (LaAlO3). Our synthesis and characterization data on LaAlO3, discussed at length in the Supporting Information (SI) section, highlight the inherent viability, adaptability, and flexibility of using our in-house procedure to create distinctive morphological motifs of other relevant and similar metal oxide systems. 2. Experimental Section. 2.1. Materials and Methodology. All chemicals were used as purchased, without any further purification. The complete list of materials used in the work are listed as follows: Mn(NO3)2·4H2O (Alfa Aesar, 98%), Y(NO3)3·6 H2O (Alfa Aesar, 99.9%), NaCl (EM Reagents), sodium oleate (TCI Chemicals, 97%), sodium benzoate (Fisher Scientific), NaOH (EMD Millipore, ≥ 99%), dimethyl sulfoxide (Acros Organics, 99.9%), oleylamine (Sigma-Aldrich, 70%), sodium thiophenolate (Sigma-Aldrich, 90%), ethanol (Alfa Aesar, 90%), Al(NO3)3·9H2O (Mallinckrodt, Analytical Reagent), La(NO3)3·6H2O (Alfa Aesar, 99.9%), NaNO3 (Fisher Scientific, ACS), NaI (Alfa Aesar, ≥ 99%), HCl (Fisher Scientific, NF/FCC grade), and Igepal CO-630 (Aldrich). A typical hexagonal nanoplate (“YMO-A”) synthesis proceeds via an optimized molten salt synthesis (MSS) method in which equimolar amounts of 0.43 mmol of yttrium nitrate and manganese nitrate were added to 1 mL of distilled water in a test tube. In parallel, 2.15 mmol of sodium oleate was dissolved in 4 mL of distilled water in a second test tube and gently heated, thereby enabling the formation of an aqueous solution. Both test tubes were subsequently sonicated in order to disperse the precursors in solution. Upon the ensuing dropwise addition of the metal nitrate mixture into the sodium oleate solution, the color and texture evolved from that of a clear, gold-colored liquid to a pink, highly viscous wax. The “sticky” and “waxy” metal oleate precipitate was presently washed with distilled water. Separately, 2.0 g of sodium chloride (NaCl) was ground to a fine powder, and then 1.08 mmol of additional sodium oleate powder, in the specific role of a surfactant, was added to sodium chloride and thoroughly mixed, until a homogeneous mixture was ultimately obtained. For the YMnO3 nanoparticle synthesis (i.e., the “YMO-B” sample), we did not use any sodium oleate surfactant, whereas with the “YMO-C” sample, we utilized 2.16 mmol of sodium oleate. Thereafter, the sticky metal oleate precipitate was combined together with the contents of the NaCl/sodium oleate mixture and ground for an additional 30 min.
3. RESULTS AND DISCUSSION 3.1. Product Characterization. We systematically investigated the use and effect of several independent reaction parameters utilized to synthesize a monodisperse distribution of pure YMnO3 nanostructures. Specifically, the roles of reaction temperature, reaction time, ramp rates, and surfactant were all separately examined and highlighted in Tables S1, Schemes S2, C
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Figure 1. SEM micrographs of YMnO3 created with increasing amounts of sodium oleate surfactant which allows for the production of (A) particles when no added oleate as well as of nanodisks when 1.1 mmol (B) and 2.2 mmol (C) of oleate is added. The micrograph panels A, B, and C correspond to samples “YMO-B”, “YMO-A”, and “YMO-C”, respectively. The corresponding XRD profile (D) is indicative of high purity as compared with the JCPDS standard for h-YMnO3.
Figure 2. (A) A HAADF STEM image of as-prepared YMnO3 nanoplates with highlighted region 1 (R1) and region 2 (R2) along the c-axis and aaxis, respectively. (B) HAADF STEM image of R1 taken along the c-axis. Hexagonal arrangements of Y (red) and Mn (blue) atomic columns are clearly shown. (C) High-resolution image of R2 along the a-axis. The direction of polarization is indicated by the white arrow. R3 refers to region 3. (D) Atomic resolution HAADF STEM image of R3 shows the ferroelectric displacement of Y atoms (red circles)Mn is illustrated as blue circles. The unit cell is highlighted with a white rectangle.
manipulation of this array of reaction variables, we found that we could synthesize additional anisotropic motifs, such as nanoflowers (Figures S2A-C and S3B) and nanorods (Figure
S3, as well as in Figures S2, S3 and S14−S17 of the Supporting Information section with the thermodynamics of possible intermediate formation, described in Table S2. Based upon D
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Figure 3. XPS data of as-prepared, hexagonally shaped YMnO3 (“YMO-A”) sample. (A) XPS survey spectrum of an as-prepared YMnO3 sample. Gaussian−Lorenztian line fitting of (B) Y 3d, (C) Mn 2p, and (D) O 1s spectra.
annular dark field (HAADF) scanning transmission electron microscopy (STEM) (Figure 2A) confirmed the presence of highly faceted YMnO3 hexagons, consistent with SEM observations (Figure 2A). Two “YMO-A” nanoplates were imaged, namely a plate oriented along the c-axis (Region 1, R1) and a plate oriented along the a-axis (Region 2, R2), as shown in Figure 2B,C, respectively. In our HAADF STEM images, we only noted signals attributable to columns of Y (red-circled) and Mn (bluecircled), respectively, due to the relatively weak scattering of O. Hexagonal arrangements of both Y and Mn ions were observed in the plate, oriented along the c-axis, as shown in Figure 2B. When the hexagonal plate is viewed along the a-direction (Figure 2C,D), the displacement of the Y ions along the c-axis becomes more evident. The displacement of the Y ions is ∼0.05 nm (Figure 2D), a value which is consistent with a previous report.23 In Figure 2C, as two-thirds of the Y ions in the unit cell are displaced downward with the remaining third of the Y ions analogously displaced upward, the net polarization is therefore directed downward, as highlighted by the white arrow. This result clearly supports the notion that the nanoplates possess a ferroelectric phase (P63cm), as opposed to a high-temperature paraelectric phase (P63/mmc: a = 0.361 and c = 1.139 nm), typically characterized by the lack of any apparent Y displacement. To complement and corroborate the structural order information obtained via HRTEM and powder XRD, XPS measurements were acquired on “YMO-A”. We specifically honed in on binding energies associated with the Y 3d, Mn 2p, and O 1s transitions to ultimately gain insight into the nature of the Mn valence environment. Figure 3A highlights a broad scan, wherein identifiable elements, such as Y, Mn, O, and C, were observed. Figure 3B focuses on the region of the spectrum that is typical of Y 3p electrons. Our deduced values for the Y 3p1/2 and 3p3/2 transitions are 312.7 and 300.8 eV, respectively, which are in close agreement with the previously reported literature values of 312.95 and 301.17 eV,32 respectively. In Figure 3C, the observed binding energy values for the Mn 2p regions are 652.8, 655.3, 641.5, and 644.0 eV, respectively, which also closely correspond with the results of prior studies.33,34 Generally, elements with a higher electron density give rise to a lower excitation energy peak. In our case, peaks at
S4), respectively. A detailed discussion of the effects of the various reaction variables analyzed coupled with insights into the additional motifs formed are highlighted in the SI section. Interestingly, we found that by using a combination of a reaction temperature of 820 °C and a reaction time of 4 h, we were able to produce pure, crystalline h-YMnO3 (P63cm) nanoparticles, likely through the mediation of an intermediate species consisting of a “mixed metal” oleate complex. The XRD data confirmed that all of the observable peaks were attributable to the JCPDS standard (no. 25-1079) for hexagonal-phase YMnO3 with no impurity signals observed for either manganese oxides or yttrium oxides (Y2O3), as highlighted in Figure 1A,D. In the absence of the sodium oleate precursor, polydisperse YMnO3 nanoparticles (“YMO-B”) were produced, with an average diameter of 68 ± 24 nm. More pertinently, however, when this reaction is performed in the presence of excess sodium oleate, all else being equal, these experimental conditions gave rise to the formation of highly faceted, hexagonally shaped structures whose inherent height and width dimensions correlated with the precise quantity of excess sodium oleate used, as shown in Figure 1B,C. Specifically, the use of 1 mM (“YMO-A”) and 2 mM (“YMOC”) sodium oleate surfactant yielded hexagonally shaped structures with average diameters of 441 ± 241 nm and 567 ± 259 nm with corresponding heights of 46 ± 6 and 62 ± 19 nm, respectively. That is, the particles became larger in size with increasing oleate content. Our observation of a larger plate dimension, directly correlating with a corresponding increase in surfactant concentration, is consistent with the prior report from Garcia et al., regarding ZnO hexagons.31 As per XRD analysis, we point out that our as-prepared nanodisks also maintained the desired h-YMnO3 structure (Figure 1D). It is also worth noting that both the polydisperse spherical particles as well as the more uniform, hexagonally shaped structural motifs were synthesized using similar “ramping up” heating rates (i.e., 27 °C/min). Hence, the significance of our study is that hexagonally shaped YMnO3 particles, especially nanoplates, have rarely been observed in the prior literature, and our protocol can reproducibly generate these less-conventional structures. These motifs (“YMO-A”) were further analyzed by highresolution electron microscopy. In particular, high-angle E
DOI: 10.1021/acs.iecr.7b00113 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. Proposed reaction schematic for the thermal decomposition of the metal oleate complex (A) and the morphological evolution (B) of YMnO3 over a period of 4 h.
652.8 and 641.5 eV were attributed to Mn4+, whereas the signals located at 655.3 and 644.0 eV were ascribed to the 2p1/2 and 2p3/2 orbitals, respectively, of Mn3+. Moreover, Mn4+ ions emanating from MnO2 also have reported values within this region. For example, the Mn4+ 2p1/2 peak has been previously noted at 653.8 eV,35 while observed positions for the 2p3/2 peak have ranged from 641.1 eV36 through 643.4 eV, denoting data which correlate well with our own results herein.37 We hypothesize that the Mn4+ ions originate from defectcontaining areas within the h-YMnO3 lattice of the sample.38 There have been reports of the presence of Mn4+ within presumably pure YMnO3.39 Relevant literature discussing analogous systems such as LaMnO3 propose the idea of “oxidative non-stoichiometry”, which postulates the presence of metal vacancies within the metal oxide lattice. That is, in principle, each vacant lanthanide RE3+ ion can be compensated for by the presence of 3 Mn4+ ions.39−42 Because of the multiple valence states potentially available to Mn, these metal oxides can possibly incorporate a substantial degree of oxidative nonstoichiometry. The O 1s region consists of a convolution of 3 different individual peaks located at 528.1, 530.0, and 531.9 eV, respectively, which have been subsequently described as Oa, Ob, and Oc in Figure 3D. Although it is generally agreed upon that the lowest energy peak (i.e., Oa) is likely associated with lattice oxygen atoms,43 the origin of the higher energy peaks is still controversial. For example, it has been suggested that the highest energy peak (i.e., Oc) is associated with adsorbed moisture.43 By contrast, the peak centered at 531.9 eV (i.e., Ob) may be ascribed to the presence of surface absorbed oxygen species43,44 and/or surface crystal defects, associated with vacancies.45−47 Nevertheless, our O, Mn, and Y XPS data collected herein are consistent with results previously reported for both YMnO3 and analogous metal oxides.48,49 3.2. Investigating the Reaction Mechanism. In our systematic efforts to generate pure, homogeneous, and uniform YMnO3 or LaAlO3, in terms of size, shape, and morphology, we carefully tested out and varied a number of significant reaction parameters such as (i) the nature of the surfactants, (ii) the
reaction times, and (iii) the reaction temperatures. An additional and extensive detailed discussion of all of these various reaction variables has been provided in the SI section. Moreover, a summary of our data from all of these collective runs as well as the complementary, associated structural characterization results (i.e., SEM and X-ray diffraction, for instance) are provided in Table S1 as well as in Figures S2−S22 and Schemes S1−S3 of the Supporting Information section, respectively. As for the “optimized” synthetic process that effectively yields the metal oxide materials discussed in this paper, this tailored protocol can essentially be considered as a culmination of three discrete and distinctive segments, the results of which we initially summarize and then describe in greater detail. In the f irst step of the reaction, a “mixed metal” precursor oleate complex is prepared at room temperature by the dropwise addition of yttrium and manganese nitrates into a solution of sodium oleate, thereby yielding a mixed Y3+/Mn2+ oleate complex (Figure 4A and eqs 1 and 2).50 This process can be summarized in the following equations: Y3 + + 3C18H33O2− → Y(C18H33O2 )3
(1)
Mn 2 + + 2C18H33O2− → Mn(C18H33O2 )2
(2)
Scheme S1 postulates the proposed formation of both yttrium and manganese oleate species, after isolation from the aqueous environment and drying in air. In support of this scheme, FTIR data were collected for the bare sodium oleate precursor (Figure S5). In addition to the presence of a peak at ∼3008 cm−1, associated with the C−H vibrational mode of the oleate anion, two major bands are present within the metal carboxyl region, and are centered at ∼1548 and 1462 cm−1. These can be ascribed to the carboxylate symmetric and asymmetric stretching modes, respectively,51 It has been previously established that the distance between these two peak positions within the metal carboxyl region provides an indication of the nature of the coordinative environment of the complex.52,53 On the basis of our data, the distance between the symmetric and asymmetric stretching peaks is
