Enhancing Multifunctionality through Secondary Phase Inclusion by

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Enhancing Multifunctionality Through Secondary Phase Inclusion by Self-Assembly of MnO Nanostructures With Superior Exchange Anisotropy and Oxygen Evolution Activity 3

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Bharati Debnath, Ashwani Kumar, Hemant G. Salunke, and Sayan Bhattacharyya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09157 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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The Journal of Physical Chemistry

Enhancing Multifunctionality through Secondary Phase Inclusion by Selfassembly of Mn3O4 Nanostructures with Superior Exchange Anisotropy and Oxygen Evolution Activity Bharati Debnath,a Ashwani Kumar,a Hemant G. Salunke,b and Sayan Bhattacharyya*a a

Department of Chemical Sciences, and Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India b Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India * Email for correspondence: [email protected] Phone: +91-33-6634-0000 Ext. 1275; Fax: +91-33-25873020 Abstract While altering physical properties by self-assembly is a common phenomenon, controlled inclusion of a secondary phase that in turn enhances the properties of the ensemble is a rare occurrence. Herein monodisperse Mn3O4 spherical nanoparticles were self-assembled into hierarchical flakes and cubes by regulating the surfactant - metal precursor molar ratio, reaction atmosphere and time. The secondary phase of Mn2O3 was incorporated differently, depending on the type of self-assembly as 2, 3.5 and 6.5 wt% in the flakes, spherical and cubic morphologies, respectively. The highest percentage of Mn2O3 in the cubes boosts its multifunctionality in terms of enhanced magnetic exchange coupling and oxygen evolution reaction (OER) activity. With 2 T cooling field, the hysteresis loop shift corresponding to coupling between antiferromagnetic Mn2O3 and ferrimagnetic Mn3O4 reached 3813 ± 2 Oe for the cubes, which is record high for any reported Mn3O4 – Mn2O3 system. The presence of eg1 electron due to higher Mn2O3 fraction in the cubes facilitated high structural flexibility for optimum strength of interaction between the catalyst and intermediate ions during OER. Likewise, a current density 10 mAcm–2 was reached at overpotential of 0.946 ± 0.02 V for the cubes, which is superior to spherical and flakes.

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1. Introduction The design of multifaceted nano-objects with a range of properties and application potential is the need of the hour as is evident from the recent global research with metals, their alloys, oxides and composites, and polymers for applications in catalysis, electronics, energy, environmental technologies and biomedical research.1-4 The magnetic materials are of special importance since beside exploiting their intricately rich spin alignment for high-density magnetic data recording and energy storage, they have been employed in sensitive diagnostics and therapeutics, as well as highly efficient catalysis.1,5-9 Impurity doping in semiconductors results in concomitant electronic and magnetic properties for spin filter devices that can be controlled by magnetic and electrical tuning.10-12 However there has been an increasing demand to explore earth-abundant multifunctional materials mostly based on carbon and transition metal oxide nanostructures with controlled size and shape.13-19 In this context, manganese oxides with mixed oxidation states of the metal, are attractive owing to their variable magnetic ordering,20 and applicability in a range of catalysis reactions for clean energy conversion and storage devices.17,18,21-23 Mn3O4 with a normal spinel structure is the most stable among other manganese oxide phases especially at high temperatures. To obtain monodisperse nanoparticles (NPs) of these metal oxides that can act as building blocks of multifunctional ordered nanostructures,22,24 the steady intricately formulated self-assembly processes have become highly relevant. Among various synthesis methodologies such as hydrothermal, sol-gel and thermal decomposition etc., only few could attain the self-assembly of ordered two- or three-dimensional (2D or 3D) nanostructures with monodisperse NPs,25 overcoming the challenge of spontaneous aggregation. While most of the Mn3O4 self-assembled nanostructures were obtained by solvothermal technique aided by high pressure inside the 2 ACS Paragon Plus Environment

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autoclave,22,25 there are fewer studies on self-assembled association of individual NPs into hierarchical architectures by soft chemical methods using surfactants,26-28 and most of them result in a non-ordered assembly. To the best of our knowledge, self-assembly of Mn3O4 NPs into 2D or 3D ordered architectures prepared by soft chemical methods are not reported till date. Herein a one pot, template-free, solution based method using cost-effective inorganic reagents was employed to prepare monodisperse Mn3O4 spherical (MS) NPs which were further self-assembled into hierarchical Mn3O4 flakes (MF) and cubes (MC) by optimizing the surfactant-metal precursor molar ratio, reaction atmosphere and time. Incorporating different fractions of the self-assembly controlled Mn2O3 secondary phase boosts the multifunctionality of the resultant Mn-O nanostructures as demonstrated with enhanced magnetic exchange bias (EB) coupling and OER activity. EB coupling is a pertinent solution to overcome the unstable spin orientation that limit the room temperature applications of superparamagnetic NPs whereby ferromagnetic (FM) / ferrimagnetic (FiM) phases are interfaced with antiferromagnetic (AFM) counterparts, and an exchange anisotropy is created by pinning the FM/FiM spins by AFM spins that can dominate the thermal energy.29,30 The exchange anisotropy is measured from the shift of hysteresis loop along the magnetic field axis when conventionally field cooled through Néel temperature, TN, of the AFM phase such that the FM/FiM Curie temperature, TC, is greater than TN.30 In the nano-regime, EB coupling is particularly complex due to the uneven FM/FiM – AFM interfaces and uncompensated AFM spins.31-34 Herein exchange coupling was made possible due to the presence of AFM Mn2O3, where the FiM Mn3O4 spins are coupled with Mn2O3 and the size-dependent TC of Mn3O4 is greater than TN of Mn2O3 phase.35 EB as high as 3813 Oe with 2T applied cooling field has been observed for the MC nanostructures, which is the highest so far in a Mn-O system. On the other hand, these nanostructures have shown promise as low-cost 3 ACS Paragon Plus Environment


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earth-abundant OER catalysts for applications in energy storage and conversion systems such as water splitting, metal-air batteries, and fuel cells.36 OER is considered to be the bottleneck in water splitting, because of its complex reaction that demands removal of four electrons and four protons from two water molecules to produce one oxygen molecule.37 The manganese-based materials are extensively investigated, because of their ability to manage the four-electron transfer for the oxidation of water to oxygen in photosystem II.38 Among MS/MF/MC nanostructures, MC demonstrates the highest OER activity. 2. Experimental Section 2.1. Materials Manganese (II) acetate tetrahydrate (Mn(CH3COO)2.4H2O; Merck, ≥99.5%), ODA (Aldrich, technical grade 90%), o-DCB (Merck, ≥98%), potassium hydroxide pellets (KOH; Merck, ≥ 85 %), ethanol (C2H5OH; Merck ≥ 99.9 %), Toray Carbon Fiber Paper (CFP; Alfa Aesar) and nafionperfluorinated resin solution (5 wt%, Sigma Aldrich) were used without further purification. 2.2. Synthesis Mn3O4 MF and MS Nanostructures: 0.14 g Mn(CH3COO)2.4H2O, x g ODA and 15 mL of oDCB were mixed in a 25 mL round bottom flask and heated at 185°C for 2 h in air. After 2 h it was cooled to room temperature and washed with ethanol. Then it was dried at 60°C overnight in the oven. When x = 11.32 g, Mn3O4 MF nanostructures were obtained, whereas MS nanoparticles were obtained with x = 0.75 g. Mn3O4 MC: 0.14 g Mn(CH3COO)2.4H2O, 15.12 g ODA and 15 mL of o-DCB were mixed in a 25 mL round bottom flask. Then it was stirred under N2 flow for 30 min and heated at 185°C for 4 ACS Paragon Plus Environment

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2 h in N2. After 2 h it was cooled to room temperature, washed with ethanol and oven dried at 60°C overnight. All the nanostructures were calcined in air at 120°C for 15 min with a heating rate of 5°C/min followed by normal cooling. 2.3. Methods The calcination of the as-synthesized products was carried out in a Carbolite wire-wound tube furnace-single zone; model MTF 12/38/400. The X-ray diffraction (XRD) measurements were carried out with a Rigaku (mini flex II, Japan) powder X-ray diffractometer having Cu Kα = 1.54059 Å radiation.

The 3D crystal structures were obtained using VESTA 3.

Thermogravimetric analysis (TGA) data were collected on a Mettler Toledo STARe, under air atmosphere at 5°C/min heating and cooling rates. Transmission electron microscopy (TEM) images were obtained by JEOL, JEM 2100F model. Field emission scanning electron microscopy (FESEM) images were recorded in Carl Zeiss SUPRA 55VP FESEM. The magnetic properties were studied using the Cryogenics - Physical Property Measurements System (PPMS) with VSM probe in the temperature range 5-300 K and applied fields up to 5 T. The temperaturedependent zero-field cooled (ZFC) curve was measured by cooling the samples to 5 K under zero magnetic field after which 100 Oe field was applied and data were collected from 5 to 300 K. In case of field cooled (FC) measurements, the samples were cooled in the presence of 100 Oe applied fields. All the electrochemical measurements were carried out using an electrochemical workstation (CH Instruments, Model CHI604D) and the plots were analyzed with inbuilt software in the electrochemical workstation.

2.4. Electrochemical measurements



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All electrochemical tests were carried out in a conventional three-electrode electrochemical cell in 1 M KOH using CHI604D electrochemical workstation. A commercial CFP fabricated with catalyst ink was used as a working electrode. A platinum wire and Ag/AgCl (3M KCl) served as a counter electrode and reference electrode, respectively. The catalyst ink was prepared by dispersing 5 mg of catalyst into 500 µl of ethanol containing 10 µl of 5 % Nafion solution and sonicated for 60 min. Then a certain amount of catalyst ink was drop-casted onto CFP and left to dry in air (loading amount: 1 mg/cm2). Finally, the working electrode was coated with 20 µl of 0.5 % Nafion solution in ethanol and dried at 60°C overnight. Before recording the electrochemical activity of the catalyst, all the working electrodes were saturated using bulk electrolysis for few seconds and also using cyclic voltammetry (CV) scans at a scan rate of 100 mV s-1. Linear sweep voltammetry (LSV) measurements were conducted with a scan rate of 10 mV s-1 in order to minimize the capacitive curren,46 and this scan rate is slow enough to ensure steady-state behavior at the electrode surface.41 Electrochemical impendence spectroscopy (EIS) studies were performed to compare RCT in the faradaic region since RCT is inversely proportional to the rate of charge transfer. All the potentials shown in this study were calibrated to RHE using the equation: E(RHE) = E(Ag/AgCl) + E0(Ag/AgCl) + 0.059*pH,47 in order to exclude the influence of pH, reference electrodes and internal solutions e.g. KCl. When the current density was calculated, the working surface area was calculated on a single side.47 The durability of the catalyst was tested by bulk electrolysis (chronoamperometry). To measure the faradaic efficiency, the actual amount of O2 produced was measured using the water displacement method in an air-tight vessel.48 Electrochemically active surface area (ECSA) was obtained from electrochemical double-layer capacitance (Cdl) by collecting CVs at various scan rates in a non-faradaic region since ECSA is directly proportional to Cdl value in the same


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material according to the following equation: Cdl = Ic/ν, where Cdl, Ic and ν are the double layer capacitance (F cm-2) of the electroactive materials, charging current (mA cm-2), and scan rate (mV s-1), respectively. All the potentials in three-electrode OER measurements were 85% iRcorrected with respect to the ohmic resistance of the solution unless specified and calibrated to reversible hydrogen electrode (RHE) based on the equation: E(RHE) = E(Ag/AgCl) + E0(Ag/AgCl) + 0.059*pH – 85%iRs. 3. Results and Discussion 3.1. Structural Characterization At first glance, the XRD patterns show a pure Mn3O4 phase with tetragonal crystal structure of I41/amd space group (JCPDS 24-0734) (Figure S1, Supporting Information). A secondary Mn2O3 phase is ascertained by best fits in the Rietveld refined XRD patterns (Figure 1a-c) and the fraction of this orthorhombic Mn-O phase with Pcab space group is determined to be 2, 3.5 and 6.5 % in MF, MS and MC, respectively (Table 1). Figure 1d shows the representative unit cell of Mn3O4 and Mn2O3. The 3D morphologies studied by FESEM show that the self-assembled 2D flakes (MF) have a lateral dimension of ~1.5 µm, the diameter of the MS NPs is ~15 nm and the edge length of the 3D cubes (MC) is ~70 nm (Figure 2a-c). In Figure 2d-f, the TEM images clearly indicate that the self-assembled ordered nanostructures are composed of ‘small’ 9-14 nm monodisperse NPs. The interplaner distances in MF, MS and MC are 0.25, 0.16 and 0.16 Å corresponding to (111), (220) and (220) lattice planes of the Mn3O4 phase, respectively (Figure 2g-i). The interparticle distance varies with the 18-carbon saturated fatty acid chain of the octadecylamine (ODA) surfactant used during their synthesis. To examine the quantity of surfactant attached over the NP surface, TGA was performed on the final products, where the weight loss is observed up to different temperatures although the boiling point of ODA is ~350oC 7 ACS Paragon Plus Environment


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(Figure S2, Supporting Information). In MF, the decomposition is complete at 295oC with a net weight loss of 24.4% whereas in MS, the weight loss is 12.6% at 390oC. In MC, with the surfactant molecules trapped on the self-assembled NP clusters, the decomposition continues up to 800oC and 29% loss is observed in the plateau at 505oC. From these results it is evident that MS contains the least quantity of surfactant and MC the highest. The schematically presented interparticle distances in Figure 2j-l obtained from ~50 NPs in the high resolution TEM images corroborate this fact where the NPs are found to be 1.56 ± 0.2 nm, 0.95 ± 0.3 nm and 1.99 ± 0.3 nm apart in MF, MS and MC, respectively. While monodispersity is retained in all the samples, the inert conditions during the synthesis of MC nanostructures retain a higher fraction of the surfactant, thus separating the NPs further apart. The fast Fourier transform (FFT) patterns in Figure 2m-o show the corresponding reflections of Mn3O4 phase confirming high crystallinity, even though the calcination of the as-prepared samples was performed at only 120oC. The appearance of the secondary Mn2O3 phase depends on the extent of NP-surface oxidation. More the exposure of NPs out of surfactant coating, higher will be this rate of oxidation. Thus the surface of spherical 9 nm NPs in MS is more prone to air oxidation than the stacked flakes in MF, and in case of MC, the NPs at the edges and corners are more active towards surface oxidation and generation of this secondary phase. As a result the wt% of Mn2O3 phase in MC is higher than MF and MS. The secondary Mn2O3 phase is however not detected by TEM analyses. 3.2. Self-assembly Process The nucleation of Mn3O4 NPs by thermal decomposition of Mn-acetate is rather straightforward, however post nucleation stage, the NP self-assembly process is strictly governed by the surfactant/precursor molar ratio, reaction atmosphere and duration (Figure 3). A series of control experiments were performed by varying the reaction conditions in order to elucidate the


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formation of flakes and cubes from the nucleation stage. The formation of meta-stable structures is possible only under certain optimized conditions, beyond which they break down into irregular shaped aggregates. For example, increasing the surfactant/precursor ratio converts the spherical NPs into 2D flakes whereas excess reaction time disintegrates these 2D morphologies. Reactions were performed both in air and N2 at a constant temperature but with different equivalents of the surfactant, ODA, dissolved in an appropriate volume of the solvent, 1,2-dichlorobenzene (oDCB) followed by temporal monitoring of the consequences (Table S1, Supporting Information). When the reactions were performed in air, the extent and rate of decomposition of the precursor and surfactant are higher than when conducted under N2. At first the outcome of the control experiments in air will be discussed followed by those in N2. In air, when 5 equivalents of ODA are used, spherical NPs are obtained at different time intervals due to a predominant Ostwald ripening (Figure 3 and Figure S3, Supporting Information). With 10 equivalents, NP-assemblies are obtained by agglomeration at the nucleation stage of 1.5 h which self-assembles to flake-like structures after 2 h (Figure 3 and Figure S4, Supporting Information). This observation suggests that the growth of flakes is possible in the presence of an adequate surfactant concentration and a reaction time not less than 2 h. By extending the reaction time to 4 h and giving enough time to form the stable morphologies, the meta-stable flakes disintegrate into agglomerated particles (Figure S4, Supporting Information). Interestingly self-assembly starts at a shorter reaction time by incorporating higher equivalents of ODA. With both 50 and 75 equivalents of ODA, flakes are observed to grow within 1.5 h, completion of growth within 2 h and break down into particles with longer reaction times (Figure 3 and Figure S5, S6, Supporting Information). The selfassembly is even faster with 100 equivalents of ODA where after an initial nucleation of NPs



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within 30 min, the flakes start to assemble within 1 h (Figure 3 and Figure S7, Supporting Information). According to the earlier pattern of events, the self-assembly is complete within 2 h and these flakes start disintegrating within 2.5 h and completely transform into agglomerated spherical particles at 4 h of reaction. When the reactions are conducted in N2 atmosphere, the NP-assembled cubes are formed only with 100 equivalents of ODA whereas with lower equivalents, agglomerated particles are obtained even after 2 h of reaction (Figure S4-S6, Supporting Information). In the first 30 min of reaction, NPs are obtained and after 1 h, cubes start to nucleate by NP-assembly and at 2 h, growth of these cubes gets completed (Figure 3 and Figure S7, Supporting Information). Unlike the flakes prepared in air, when cubes are obtained in N2, the cubes remain stable irrespective of the length of time the reaction is subjected to. The stability of Mn3O4 cubes inside the reaction medium owes to limited decomposition of ODA under inert conditions. For property analyses all the nanostructures of MF, MS and MC are chosen after 2 h of reaction (Figure 3), MS from a reaction with 5 equivalents of ODA in air, MF from the reaction with 75 equivalents of ODA in air and MC from the reaction with 100 equivalents of ODA in N2. 3.3. Magnetic Exchange Bias Coupling Because of the presence of 2, 3.5 and 6.5 wt% of AFM Mn2O3 in MF, MS and MC nanostructures, respectively (Figure 1a-c and Table 1) exchange coupling between the spins at the FiM Mn3O4 – AFM Mn2O3 interface is expected to be favored. The EB coupling is studied at 5 K and at this temperature the spins remain blocked as shown from the temperature-dependent magnetization (M-T) curves (Figure S8, Supporting Information). Since magnetic transitions are largely dependent on the morphology of the nanostructures, the superparamagnetic blocking varies as 33.4, 39.8 and 39.9 K for MF, MS and MC, respectively. From the field dependent


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magnetization (M-H) plots in Figure 4 it can be observed that under all conditions of zero-field cooling and presence of 2 or 4 T cooling fields, MS has the highest magnetization followed by MF and then MC (Table S2, Supporting Information). From the unsaturated M-H loops, the magnetization at 5 T applied field is 37.2-38.6 emu/g for MS, 22.3-23.2 emu/g for MF, and 13.113.2 emu/g for MC. Magnetic interactions are facilitated by shorter interparticle distances and with a spacing of 0.95 nm, the MS NPs show better magnetization than MF, with spacing 1.56 nm. With interparticle spacing of 1.99 ± 0.3 nm, magnetization of MC is the least even though the NP size is 14 nm (Figure 2j-l). In addition a higher percentage of surfactant binders on the surface of self-assembled NPs in MC along with the presence of 6.5 wt% of an AFM phase decrease the magnetization of MC in comparison to MS or MF. EB depends on the extent of AFM/FiM coupling which improves with smaller 9-14 nm NPs in the present study due to better interfacial contact between the AFM and FiM phases. The extent of EB coupling however shows a different trend in comparison to their magnetization and this trend is directly proportional to the wt% of AFM Mn2O3 phase present in the nanostructures. Due to the presence of highest fraction of Mn2O3 in MC as compared to MF and MS, MC shows higher EB coupling. In the absence of any cooling field, the hysteresis loop shift is minimum i.e. 27 ± 0.2, 103 ± 0.4 and 196 ± 0.4 Oe for MF, MS, and MC, respectively. When the samples are cooled in the presence of 2 T magnetic field from above TC of Mn3O4 to below TN of Mn2O3 the loop shift increases in the negative direction of the field axis due to the unidirectional anisotropy generated at the FiM AFM interface.31,32 With 2 T cooling field, the EB increases to 130 ± 0.4, 375 ± 0.4 and 3813 ± 2 Oe for MF, MS, and MC, respectively. When the cooling field is increased to 4 T, the loop shifts decrease slightly to 125 ± 0.5, 315 ± 0.5 and 3721 ± 1 Oe for MF, MS, and MC, respectively probably due to a competing force against the interfacial spin coupling.31 It is important to note



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that EB in MC is almost ten times than that of MS and this enhanced EB coupling is so far the highest among any Mn3O4 – Mn2O3 system. Additionally, these self-assembled nanostructures demonstrate high coercive fields of 5818 ± 4, 7976 ± 4 and 7301 ± 3 Oe for MF, MS, and MC, respectively. The above results denoting high magnetic exchange anisotropy attest to the advantageous inclusion of a secondary Mn2O3 phase in Mn3O4 nanostructures. 3.4. Electrochemical OER Activity

The electrocatalytic OER performance of these self-assembled nanostructures was investigated in an alkaline medium and Figure 5a demonstrates the LSV polarization curves recorded at a scan rate of 10 mVs-1. Since the as-measured current do not directly reflect the intrinsic behavior of the catalyst due to ohmic resistance, an iR correction was applied for further analysis.39 In order to compare the OER performance of the electrocatalysts, we used the overpotential at a current density of 10 mA cm–2 which is the current density expected for a 10% efficient solar-to-fuel conversion device under 1 sun illumination.40 MC reaches 10 mA cm–2 at an overpotential of 0.946 ± 0.02 V, which is superior to MS and MF which require overpotentials of 1.06 ± 0.04 and 1.41 ± 0.04 V, respectively. Moreover MC has a much sharper increase of current density implying a faster reaction rate. To get further insight into the OER kinetics, Tafel plots are derived from the LSV polarization curves using the Tafel equation [η = b*log j + a, where η is the overpotential, j is the current density and b is the Tafel slope]. MC has the least Tafel slope of 126 mV dec-1 implying more favorable reaction kinetics towards OER than MS and MF with Tafel slopes of 175 and 211 mV dec-1, respectively (Figure 5b). To study the electrode kinetics under OER conditions, EIS measurements were performed at an overpotential of 1.067 V (iR uncorrected). Nyquist plots in Figure 5c shows the charge transfer resistance (RCT in the equivalent circuit), determined from the semicircle at low 12 ACS Paragon Plus Environment

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frequency (high Z´). MC exhibits the lowest RCT in comparison to other samples, demonstrating faster electron transfer kinetics at the interface between electrode and electrolyte. The better catalytic performance of a catalyst is usually ascribed to the better availability of active sites which can be further confirmed by Cdl, and thereby from the estimation of ECSA because Cdl is directly proportional to ECSA (Figure S9, Supporting Information).41 The Cdl values are 1.57 ± 0.23, 1.22 ± 0.21 and 0.079 ± 0.004 mF cm-2 for MC, MS and MF, respectively, which confirms that the improved catalytic activity of the MC originates from its higher ECSA. The long-term stability of the best catalyst, investigated by chronoamperometric measurements is shown from the current density-time (I vs t) curve at a constant overpotential of 0.953 V (Figure 5d). MC exhibits an excellent stability for 12 h and the LSV plot taken after stability test shows almost similar activity with a negligible decrease in the current density. The FESEM image (Figure 5e) of the working electrode taken after the stability test reveals that the cube morphology remains intact even after 12 h of bulk electrolysis. To check the energy conversion efficiency from electrical energy to chemical energy for MC, the Faradaic efficiency is measured by collecting the actual amount of oxygen gas at a constant potential of 1.4 V versus Ag/AgCl, by using a water displacement method. MC shows a Faradaic efficiency of 98 ± 1.04 % for OER, indicating sufficient energy conversion efficiency (Figure 5f). The excellent OER durability, as well as a high Faradaic efficiency of MC, has been associated with the unique structural and chemical stability of transition metal oxides in an alkaline medium. Since Mn3O4 is particularly an unconventional OER catalyst, the incorporation of 6.5 wt% of Mn2O3 in MC is particularly helpful in enhancing its OER activity. Mn2O3 on the other hand is the most active OER catalyst in the manganese family demonstrating overpotentials of 550-600 mV at 10 mA cm–2,42,43 because of the high activity of Mn3+ (t32g e1g) sites owing to a beneficial eg orbital occupancy



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close to unity, similar to Co3+ (t52g e1g), and Ni3+ (t62g e1g).44 Mn3+ species with eg1 configuration facilitates the Mn–O and Mn–Mn bond enlargement with appropriate structural flexibility and disorder in the Mn-O structure which enhances the water oxidation process.45 With an average 1.5 eg orbital electrons in Mn3O4 it can be envisaged that a collective eg orbital occupancy in MF, MS and MC is the summation of the product of wt% of the respective phases and the number of eg electrons in that phase. Therefore the collective eg orbital occupancy in MF, MS and MC is 1.49, 1.48 and 1.46, respectively. Arguably, 1.46 is the closest to the optimal eg orbital occupancy of 1.2, resulting in a relatively superior OER performance of MC.

4. Conclusions

In summary, we have shown how self-assembly control can enhance the properties of nanostructures through induction of a secondary phase. The Mn3O4 NPs undergo in situ selfassembly by one pot thermal decomposition of Mn-acetate in o-DCB into Mn3O4 flakes and cubes by controlling the surfactant/precursor ratio, reaction time and atmosphere, and in the process a secondary Mn2O3 phase is also formed. The magnetic properties of the flakes, spherical and cubic nanostructures are different in terms of their magnetization and exchange anisotropy. The Mn3O4 cubes with the highest fraction of AFM Mn2O3 phase provide an exceptionally high EB of 3813 ± 2 Oe, one of the highest in Mn-O family. The cubic morphology also shows a better performance towards OER than compared to the spherical NPs and flakes. The improved activity of the cubes can be ascribed to its higher ECSA, as well as presence of highest density of Mn3+ in t2g3eg1 state as compared to particles and flakes. Mn3+ species in eg1 configuration results in better flexibility and disorder in the Mn-O structure and hence enable a higher water oxidation activity. Overall the findings of this work can be summarized point wise as (i) self-assembly of


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monodisperse Mn3O4 NPs in a one-pot synthesis, (ii) a rare dependence of the wt% of a secondary phase on morphology of the nanostructures, (iii) large EB coupling in Mn3O4/Mn2O3 system when the AFM fraction is lower, and (iv) uncommon multifunctionality in Mn-O nanostructures. Supporting Information XRD patterns; TGA plots; Table and FESEM images of morphological evolution; ZFC/FC curves; M-H results; ECSA calculation. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements

BD thanks Department of Science & Technology - INSPIRE program for her fellowship. AK thanks the Academic and Research Fund for his fellowship. The financial support from Department of Science and Technology – Science and Engineering research Board (DSTSERB) under sanction No. EMR/2016/001703 is duly acknowledged.

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Table 1: XRD-Rietveld refinement parameters. ∑ ( ∑[ ]

Here Rwp =

, wi=

, χ2 =

, RObs = ∑

] [!

, N = number of data

points, Iobs = observed intensity, Ical = calculated intensity, P = number of parameters. Sample [space group] MF

Lattice parameters (Å), Bond angle (°)

Atomic positions (x, y, z)

Occupation number

98 % Mn3O4 [I41/amd]

a = b = 5.7767 ± 0.0001Å, c = 9.4519 ± 0.0001Å α = β = γ = 90°

Mn1 (0, 0.5, 0. 5) Mn2 (0, 0.25, 0.875) O3 (0, 0.474, 0.254)

Mn1 = 1 Mn2 = 1 O3 = 1.0

2% Mn2O3 [Pcab]

a = 9.410 ± 0.0002Å b = 9.450 ± 0.0001Å c = 9.370 ± 0.0003Å α = β = γ = 90°

Mn1 (0, 0, 0.351) O2 (0, 0.3102, 0.25)

Mn1 = 1 O2 = 1

a = b = 5.7697 ± 0.0002Å, c = 9.4573 ± 0.0002Å α = β = γ = 90°

Mn1 (0, 0.5, 0. 5) Mn2 (0, 0.25, 0.875) O3 (0, 0.474, 0.254)

Mn1 = 1 Mn2 = 1 O3 = 1.0

Goodness of fit (reduced χ 2)

Weighted profile (RWP)

1.281

1.21%

1.099

1.65 %

1.058

1.06 %

MS 96.5% Mn3O4 [I41/amd] 3.5% Mn2O3 [ Pcab ]

a = 9.410 ± 0.0001Å b = 9.450 ± 0.0001Å c = 9.370 ± 0.0003Å α = β = γ = 90°

Mn1 (0, 0, 0.351) O2 (0, 0.3102, 0.25)

Mn1 = 1 O2 = 1

a = b = 5.7573 ± 0.0001Å, c = 9.2180 ± 0.0003Å α = β = γ = 90°

Mn1 (0, 0.5, 0. 5) Mn2 (0, 0.25, 0.875) O3 (0, 0.474, 0.254)

Mn1 = 1 Mn2 = 1 O3 = 1.0

a = 9.409 ± 0.0002Å b = 9.451 ± 0.0001Å c = 9.372 ± 0.0001Å α = β = γ = 90°

Mn1 (0, 0, 0.351) O2 (0, 0.3102, 0.25)

Mn1 = 1 O2 = 1

MC 93.5% Mn3O4 [I41/amd] 6.5% Mn2O3 [ Pcab ]


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Figure 1: XRD-Rietveld analyzed patterns of (a) MF, (b) MS and (c) MC. The legends: diff (difference plot between observed and calculated patterns); Obs (observed pattern); Calc (calculated pattern); and Bckgr (background plot). (d) Representative unit cells of (i) Mn3O4 and (ii) Mn2O3.



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Figure 2: (a-c) FESEM, and (d-f) TEM images of MF, MS and MC nanostructures, respectively. Inset of (d-f) show the corresponding diameter histograms. High resolution TEM images showing lattice fringes of the Mn3O4 phase and the individual self-assembled NPs of (g) MF, (h) MS and (i) MC nanostructures. (j-l) Schematic representation of interparticle distance of surfactant coated self-assembled NPs and (m-o) selected area electron diffraction (SAED) patterns of MF, MS and MC, respectively.


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Figure 3: Schematic illustration of the formation and self-assembly of Mn3O4 NPs under different synthesis conditions.



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Figure 4: (a) M−H loops under ZFC conditions at 5 K and (b) enlarged view of the corresponding hysteresis loop shift and coercivity. (c) M−H loops at 5 K after applying 2T cooling field and (d) enlarged view of the corresponding hysteresis loop shift and coercivity. (e) M−H loops at 5 K after applying 4T cooling field and (f) enlarged view of the corresponding hysteresis loop shift and coercivity of MF, MS and MC nanostructures.


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Figure 5: (a) LSV polarization curves for OER. (b) Tafel plot for the OER process. (c) Nyquist plots obtained at 1.067 V overpotential for OER. Inset shows the equivalent circuit. (d) Chronoamperometric durability test of MC at a constant overpotential of 0.953 V. Inset shows the LSV curves before and after durability test. (e) FESEM image of MC after durability test. (f) Amount of gas theoretically calculated and experimentally measured versus time at 1.4 V vs Ag/AgCl for OER of MC catalyst.



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TOC Graphic


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Enhancing Multifunctionality through Secondary Phase Inclusion by

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