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Kirkendall Growth of Hollow MnO Nanoparticles Upon Galvanic Reaction of MnO with Cu and Evaluation as Anode for Lithium-Ion Batteries 2+

Shelton J.P. Varapragasam, Choumini Balasanthiran, Ashim Gurung, Qiquan Qiao, Robert M. Rioux, and James D Hoefelmeyer J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on April 30, 2017

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The Journal of Physical Chemistry

Kirkendall Growth of Hollow Mn3O4 Nanoparticles upon Galvanic Reaction of MnO with Cu2+ and Evaluation as Anode for Lithium-Ion Batteries Shelton J. P. Varapragasam,a Choumini Balasanthiran,b Ashim Gurung,c Qiquan Qiao,*,c Robert M. Rioux,*,b,d James D. Hoefelmeyer*,a a

Department of Chemistry, University of South Dakota, 414 E. Clark St., Vermillion, SD 57069 Department of Chemical Engineering, The Pennsylvania State University, 165 Fenske Laboratory, University Park, PA 16802-4400 c Department of Electrical Engineering, South Dakota State University, 020 EECS, Brookings, SD 57007 d Department of Chemistry, The Pennsylvania State University, 165 Fenske Laboratory, University Park, PA 16802-4400 b

corresponding author email: [email protected]; [email protected]; [email protected]

Abstract We report the formation of high surface area hollow Mn3O4 nanoparticles that form as a result of the galvanic reaction of Cu2+ with MnO nanocrystals concomitant with a nanoscale Kirkendall effect. The MnO nanocrystals were prepared according to the ultra-large scale synthesis reported by Hyeon, which allowed the preparation of hollow Mn3O4 in multigram quantities. Ex-situ analyses with transmission electron microscopy and powder X-ray diffraction show the morphology and phase stability of the hollow particles that correlate with DSC-TGA data and show collapse of the hollow particles at temperatures greater than 200°C. Electrodes fabricated from hollow Mn3O4 exhibited excellent initial Li ion storage capability (initial discharge capacity = 1324 mAh/g) but poor cycling performance (97% loss of discharge capacity after 10th cycle); whereas, Mn3O4-MWCNT electrodes exhibited good reversibility and discharge capacity of 760 mAh/g after 100 cycles. Introduction Mn3O4 nanomaterials find use in multiple applications that include lithium ion batteries, 1 - 5 oxygen reduction catalysis, 6 - 8 magnetic resonance imaging, 9 , 10 and supercapacitors. 11 - 16 The properties of nanomaterials are highly dependent on morphology; therefore it is worthwhile to develop synthetic routes that yield new Mn3O4 nanostructures and to develop the relationship between morphology and reactivity. Recent efforts have been dedicated toward the preparation of nanoscale Mn3O4 materials with attention to scalability and careful control of the morphology.17 Mn3O4 crystallizes in the spinel structure with tetragonal crystal habit defined by octahedra elongated along the [001] direction due to Jahn-Teller distortion of the octahedral Mn3+ highspin d4 centers. 18 , 19 The unpaired spins contribute paramagnetism in the solid, and at temperatures below ~40K the spins undergo ferromagnetic ordering. 20 - 22 The equilibrium structure is bound by {011} facets and nanocrystals with this morphology were obtained using a 1 ACS Paragon Plus Environment

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simple hydrothermal method.23 Spherical Mn3O4 nanocrystals with different size were prepared in order to study size-dependent magnetic properties.22 Mn3O4 nanocrystals with varying thickness along the c axis were prepared, including nanoplates enclosed by two (001) facets and four (100)/(010) facets or truncated bipyramids with {011} facets.24 Facile synthesis of highly uniform Mn3O4 nanocrystals yielded dots, rods, and wires with growth along the direction.25 Uniform Mn3O4 nanocrystals with various shapes (spheres, plates, nanowires, and nanokites) with limiting dimension of ~5 nm were prepared using a low temperature hydrothermal reaction.9 A biphasic reaction was used to prepare uniform spherical or cubic Mn3O4 nanocrystals with tunable size range of ~5-15 nm. 26 There are numerous reports of Mn3O4 nanorods and nanowires in which the crystallite grows along the c axis.27-30 Sponge-like nanosized Mn3O4 was synthesized by a simple precipitation method.31 There are several reports on the preparation of hollow Mn3O4 nanoparticles.10,32-38 Attributes of hollow nanoparticles include storage and delivery of reagents, 39-43 and the ‘cage-effect’ 44 for reactions within the hollow interior.45-48 Conceptually, the most direct path toward the synthesis of hollow metal oxide nanocrystals occurs upon oxidation of a metal nanocrystal followed by Kirkendall growth of the metal oxide;49 however, as of yet there are no reports of well-defined metallic Mn nanocrystals in the literature. A related strategy for the preparation of hollow oxide nanoparticles is galvanic replacement reaction in metal oxide nanocrystals. 50 So far, hollow Mn3O4 nanoparticles have not been prepared using this method. Polysaccharide microspheres32 and carbon nanospheres37 were used as a templates to prepare hollow Mn3O4 nanoparticles with diameters of ~100-200 nm. It is known that oxidative etching of MnO (with trioctylphosphine oxide) is a route to hollow nanoparticles; however, the composition was amorphous metal phosphate. 51 Later work showed that simple air oxidation (over many days) of aqueous dispersion of MnO nanocrystals led to hollow Mn3O4 nanoparticles.10 In this work, we demonstrate a simple, reproducible synthesis of hollow (~30 nm) Mn3O4 nanoparticles. We report the thermal stability of the hollow nanostructure and demonstrate its applicability as an anode material for lithium ion batteries. Experimental General. Manganese(II) chloride tetrahydrate (MnCl2. 4H2O, 99% Aldrich), copper(II) chloride dihydrate (reagent grade, Fisher Scientific), sodium oleate (>97%, TCI America), hexanes (98%, Acros), and isopropanol (98%, Acros) were used without further purification. Multi-walled carbon nanotubes, denoted MWCNT, (ID = 2-5 nm; OD = < 8 nm; length = 10-30 µm; surface area = 500 m2/g; conductivity > 100 S/cm) were purchased from Cheap Tubes Inc., USA. 1Octadecene (technical grade, 90%, Acros), oleic acid (90%, Fisher Scientific), and oleylamine (>50%, TCI America) were degassed at 120°C under vacuum for 1 h to remove volatile impurities prior to use. The synthesis of MnO and Mn3O4 were performed under a N2 atmosphere using Schlenk and glovebox techniques. Synthesis of MnO nanocrystals. MnO nanocrystals were synthesized according to the method reported by the Hyeon group.52 A flask was charged with 7.91 g MnCl2.4H2O, 36.5 g sodium oleate, 60 mL ‘nanopure’ 18.2 MΩ water, 80 mL ethanol and 140 mL hexanes. The contents were heated for 4 h at 70°C. The contents were allowed to cool to room temperature after which an organic and aqueous layer was apparent. The organic layer was separated and washed with 3 2 ACS Paragon Plus Environment

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× 30 mL water. The organic solution containing manganese(II)oleate was transferred to a 1L 3neck round bottom flask. The solvent was removed in vacuo, leaving behind a brown waxy residue. To this 6.3 mL degassed oleic acid and 250 mL degassed 1-octadecene were added. The flask was fitted with a reflux condenser and heated to 320°C at 3.3°C/min and kept for 0.5 h after which the contents were allowed to cool slowly to room temperature. Note: The contents were off-white in color under inert atmosphere, then became dark brown when exposed to air, which is due to oxidation of the surface of MnO to form a thin Mn3O4 layer.53,54 Isopropanol (200 mL) was added to induce precipitation of the nanocrystals. The mixture was centrifuged at 3500 rpm for 8 minutes and the supernatant was discarded. Solids were redispersed in hexanes. In a typical reaction, the isolated, purified MnO nanocrystals were dispersed in 40 mL hexanes. From this solution, a 1 mL aliquot was taken and placed in a fired and weighed ceramic crucible. The solvent was removed by evaporation, and the residue was calcined at 450°C for 6 h in air (which yields Mn5O8). The mass of the inorganic product was 0.0822g, and based on the volume fraction, the mass of MnO in the stock solution was calculated as 2.90g (theoretical yield = 2.83g). It was critical to determine the exact mass of inorganic MnO in a stock dispersion in order to perform subsequent reactions with precise control of the Cu:Mn ratio. Synthesis of Mn3O4. In a typical experiment, 103 mg (1.45 mmol) MnO (mass determined as described above) was transferred into a three-neck round bottom flask, and the hexane was evaporated under vacuum. To this, 20 mL degassed ODE was added. Then CuCl2.2H2O was added to achieve Cu:Mn values according to Table S1. Degassed oleylamine was added to achieve a 12:1 N:Cu ratio. The flask was fitted with a reflux condenser and nitrogen was passed into the flask to blanket the solvent. The reaction mixture was heated to 150°C at a rate of 10°C/min, and kept at that temperature for 3 h. Once the reaction mixture was cooled to room temperature, 25 mL of isopropanol was added, and the product was isolated by centrifugation at 3500 rpm for 8 min to precipitate the dark brown product. The supernatant was discarded and the precipitated nanoparticles were redispersed in hexanes. In order to determine the influence of copper in the synthesis of hollow Mn3O4, a successive set of experiments were conducted at constant temperature (150°C) versus Cu:Mn mole ratio (Table S1). Additionally, another series of reactions were performed at constant Cu:Mn ratio (0.4:1) versus temperature (100°C, 125°C, 150°C, and 175°C) to determine the influence of temperature. Characterization. Powder X-ray diffraction (PXRD) data collection was performed with a Rigaku Ultima IV instrument. The X-ray tube produced Cu Kα radiation (λ = 1.54 Å), and the generator was set to 44 kV and 44 mA during data collection. Data was collected from 20−80° (2θ) with a step size of 0.02°. Nanoparticle samples were purified upon precipitation/redispersion three times using a hexanes/isopropanol solvent/non-solvent pair. From the purified dispersion in hexanes, the solvent was removed in vacuo to leave a dry powder suitable for diffraction measurements. Transmission electron microscopy (TEM) images were obtained using a Tecnai Spirit G2 Twin (FEI Company) instrument with a LaB6 filament operating at 120 kV. Dilute dispersions of nanoparticle samples were drop-casted onto 200 mesh Cu grids with thin film carbon support (Electron Microscopy Sciences). High-resolution TEM images and energy-dispersive spectroscopy (EDS) data were obtained using a FEI Titan G2 XFEG operating at 200 kV. Dilute dispersions of nanoparticle samples were drop-cast onto 300 mesh Mo grids with a thin film 3 ACS Paragon Plus Environment


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carbon support. EDS acquisition times were 5 min and data were analyzed using Esprit software. The distribution of elements was calculated from the integrated intensities of the relevant peaks after background subtraction. Physisorption analyses were measured using a Micrometritics Gemini V surface and pore size analyzer. Prior to the measurement, the samples were kept for 12 h at 100°C under vacuum to remove any residual gas and moisture from the sample. N2 adsorption-desorption isotherms were analyzed according to the BET55 method to obtain surface area and the BJH56 method to obtain pore size distribution. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out with a TA Instruments Q600 SDT Thermal Analyzer. The samples were placed in an Al2O3 crucible and heated from room temperature to 800°C with a heating rate of 2°C/min in air or N2 at a flow rate of 100 mL/min. Elemental analysis (EA) was performed via inductively coupled plasma optical emission spectroscopy (ICP-OES) (Agilent Technologies 700 Series). The samples for EA were prepared via digestion with aqua regia. The electrochemical measurements were carried out using CR-2032 coin cells with a working electrode, lithium foil as counter electrode, Celgard as separator, and 1M LiPF6 dissolved in 1:1:1 volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) as electrolyte assembled in an argon atmosphere. The working electrode consisted of Mn3O4 or Mn3O4-MWCNT (Mn3O4-MWCNT mixture was prepared by combining Mn3O4 and MWCNT in 2:1 weight ratio. The mixture was physically ground using a ceramic mortar and pestle) as the active material mixed with super P carbon black and polyvinylidene fluoride (PVDF) at a weight ratio of 8:1:1 respectively in N-methyl-2-pyrrolidone (NMP) solvent coated on a copper foil and vacuum-dried overnight at 80°C.

Results and Discussion Results of MnO synthesis We prepared uniform MnO nanocrystals in multi-gram scale according to the literature.52 The decomposition of Mn-oleate gave a white suspension of product that became dark brown-black upon exposure to air. The sample was easily isolated upon precipitation from the reaction mixture, centrifugation, and removal of the supernatant. The nanocrystals readily dispersed in non-polar solvents, such as hexanes, and were purified with three precipitation/redispersion steps. Powder X-ray diffraction indicated exclusively the presence of MnO (JCPDS card no. 78-0424), and analysis with TEM indicated uniform nanocrystals 31.5 ± 2.0 nm having cubic morphology with rounded corners (Figure 1).

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Figure 1. (a) TEM image of MnO nanocrystals with frequency vs. diameter/nm histogram (inset); (b) Powder X-ray diffraction data of MnO nanocrystals. MnO is a Mott insulator with experimental bandgap ~4 eV, 57-60 which is consistent with the initial color of the product under N2 atmosphere. It has been noted in the literature that exposure of freshly prepared MnO particles to air leads to surface oxidation to form a thin shell of Mn3O4, which contributes unique magnetic properties.53,54 It has even been shown that at 200°C air oxidation of MnO nano-crosses to Mn3O4 occurs with conservation of nanoparticle shape.61 The observation of pure MnO phase in our nanocrystal sample indicates the extent of surface oxidation was limited and did not result in a thick shell of Mn3O4. Furthermore, our TEM observations did not indicate a core-shell MnO-Mn3O4 structure, which is evident in samples from other studies.53 Our characterization data are in agreement with the literature (i.e. that the sample is essentially phase-pure MnO and the extent and depth of surface oxidation is low).52



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Results of Cu2+ addition to MnO We had previously demonstrated a procedure that allowed precise control of the adsorption of transition metal or lanthanide ions on the surface of TiO2 nanocrystals.62,63 We were motivated to show similar behavior on the surface of MnO nanocrystals. However, initial experiments in order to determine adsorption of Cu2+ on MnO nanocrystals showed the unexpected result of formation of hollow nanoparticles. Powder X-ray diffraction data show the presence of MnO and Mn3O4 phases in the material. We suspected a galvanic reaction was taking place that led to a nanoscale Kirkendall effect to produce the hollow nanoparticles.49,50,64,65 Subsequent efforts focused on determining the chemical reaction leading to the Kirkendall effect and to study how reaction conditions influenced the final nanoparticle morphology. In order to investigate the role of copper(II) in the formation of hollow Mn3O4 nanoparticles, we prepared a series of reactions in which the Cu:Mn ratio was carefully varied as 10, 20, 30, 40, 50, 67, and 100 mol% at a reaction temperature of 150°C. Powder X-ray diffraction data (Figure 2) of the products from the reaction series indicated a trend in which the intensity of the MnO reflections decreased while new reflections appeared that corresponded to Mn3O4 (hausmannite, JCPDS card no. 24-0734).

Figure 2. Powder X-ray diffraction data of the products from reaction of MnO nanocrystals with Cu2+; Cu:Mn = a) 0, b) 0.1, c) 0.2, d) 0.3, e) 0.4, f) 0.5, g) 0.67, h) 1.0. The TEM data from the series of reactions (Figure 3) show increasing ratio of hollow:solid nanoparticles as the Cu:Mn ratio increases from 0-0.4. The outer diameter of the hollow nanoparticles is similar to the diameter of the solid nanoparticles (see Figure S1). With increasing Cu:Mn ratio there was a higher frequency of hollow nanoparticles in the product mixture. Interestingly, the shell thickness in the hollow nanoparticles was very similar (~7 nm) upon varying Cu:Mn from 10-40 mol%. At Cu:Mn = 0.5, a population of particles with diameter 6.9 ± 1.3 nm was found in addition to the mixture of solid and hollow particles with diameter ~30 nm. Some open shells were found, which suggested the small particles form upon breakingup of the hollow particle shells. At Cu:Mn = 0.67, there are no ~30 nm solid particles, very few intact hollow nanoparticles, and the sample is dominated by the small particles and open shells. 6 ACS Paragon Plus Environment

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At Cu:Mn = 1, there are no hollow nanoparticles or open shells. The average particle size and size distribution increased (8.2 ± 1.4 nm) compared to the small fragments that initially form upon disintegration of the hollow particles. This may be due to rapid oxidation and break-up followed by slow Ostwald ripening of the Mn3O4 fragments.

Figure 3. Transmission electron micrographs of nanoparticle products isolated after reaction of MnO nanocrystals with Cu2+ at 150°C. Cu:Mn = a) 0.1, b) 0.2, c) 0.3, d) 0.4, e) 0.67, f) 1.0.

The observations of oxidation (from MnO to Mn3O4) and that Cu:Mn < 0.67 gave hollow particles whereas Cu:Mn >= 0.67 gave fragments suggest the product forms as a result of a oneelectron transfer between Cu2+ and Mn2+. The relevant chemical equilibria are summarized below: Cu2+ + e- = Cu+ ∆E = +0.158V MnO + H2O = Mn(OH)2 Mn(OH)3 + e- = Mn(OH)2 + OH∆E = -0.40V Mn(OH)3 = MnO(OH) + H2O MnO + 2 MnO(OH) = Mn3O4 + H2O Standard heats of formation of the relevant species are summarized in Table 1.66 Species

∆Gf (kcal/mol) 7 ACS Paragon Plus Environment


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11.94 Cu+ 2+ Cu 15.65 H2O -57.80 OH-54.96 MnO -86.74 Mn(OH)2 -147 Mn(OH)3 -181.0 MnO(OH) -133.3 Mn3O4 -306.69 Table 1. Standard heats of formation for relevant species in the reaction of Cu2+ with MnO. From this, the overall reaction between Cu2+ and MnO (eqn. 1) has ∆G = -14.01 kcal/mol. Eqn. 1:

2 OH- + 2 Cu2+ + 3 MnO = Mn3O4 + H2O + 2 Cu+

In further support of this conclusion, we note that no metallic copper precipitate or colloids was observed in the samples or supernatant solutions. In fact, the supernatant was nearly colorless under inert atmosphere and turned to green color within several seconds upon being exposed to air (Figure S2), which is consistent with air-oxidation of Cu+ to Cu2+. It is worth noting that the reaction shown in eqn. 1 is thermodynamically driven by ∆E = + 0.558V (-12.9 kcal/mol) in alkaline aqueous conditions that arise from the presence of oleylamine and water that originated from the CuCl2.2H2O reagent. The sample obtained at 150°C and Mn:Cu ratio of 40 mol% had a high occurrence of hollow nanoparticles and appeared to have the highest structural integrity based on the TEM data. At higher ratio of Mn:Cu the smaller nanoparticles and broken fragments were the dominant sample morphology. More detailed investigation of the hollow nanoparticles formed at Mn:Cu = 0.4 were carried out using HAADF-STEM, energy dispersive spectroscopy (EDS), and Cu analysis with ICP-OES. The high-resolution image indicated polycrystalline shells (Figure 4). Lattice fringes with spacing of 0.49 nm, 0.31 nm and 0.26 nm correspond to the (101), (112), and (211) planes of Mn3O4 were identified. 67 An EDS map obtained over an area containing hollow nanoparticles showed the concentration of Mn and O throughout the shell as well as small amounts of Cu. According to the EDS data, the relative abundance of Cu was ~8%; however, ICP-OES data of the same sample indicated Cu content of 1.5%. Given the difficulties in obtaining quantitative analysis from EDS data, the ICP-AES data is a more reliable value.


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Figure 4. (a) High-resolution HAADF-STEM image of hollow Mn3O4 nanoparticles; identical areas of hollow Mn3O4 nanoparticles were imaged with (b) HAADF-STEM and (c) EDS. To investigate the influence of temperature, a series of experiments in which Cu:Mn was kept at 0.4 while T was varied as 100°C, 125°C, 150°C, and 175°C were performed. TEM data of the products of this reaction series are shown in Figure S3. The product formed at 100°C showed a bimodal size distribution with nanoparticle diameter centered at ~5 nm and ~30 nm with very few hollow nanoparticles. Some core-shell MnO-Mn3O4 nanoparticles were evident in the sample. At 125°C, there were far fewer smaller nanoparticles compared to the sample obtained at 100°C and there was a slight increase in the frequency of hollow nanoparticles. At 150°C the sample consisted of a majority population of hollow nanoparticles and minority population of solid nanoparticles, both with outer diameter ~30 nm. At 175°C the sample was composed of a mix of ~5 nm particles, ~30 nm hollow nanoparticles, a low frequency of solid or core-shell nanoparticles, and significant frequency of broken or deformed hollow nanoparticles. The observation of broken shells of hollow particles simultaneous with the increasing frequency of smaller nanoparticles somewhat resembled the samples obtained at 150°C at a higher ratio of Cu:Mn. It is known Cu+ can reduce carboxylic acids,68 prompting us to believe that at higher temperature there is some reduction of oleic acid and re-oxidation of Cu+ to Cu2+ in the reaction that contributes additional oxidation of MnO. According to powder X-ray diffraction data (Figure S4), the extent of the oxidation and phase transformation from MnO to Mn3O4 increased as a function of temperature. The reaction conditions that led to the highest frequency of hollow Mn3O4 nanoparticles were T = 150°C and Cu:Mn = 0.4. Thermal stability 9 ACS Paragon Plus Environment


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We assessed the thermal stability of hollow Mn3O4 nanoparticles using DSC-TGA, ex-situ TEM, powder XRD analyses, and physisorption. The hollow particles have a surface layer of oleic acid and oleylamine that contributes surface stabilization and high dispersibility in non-polar solvent. In order to obtain physicochemical data for the nanoparticle samples, we chose to remove the surfactant layer and focus on the inorganic material of interest. The organic layer would contribute large mass loss in the TGA results and distort the physisorption analysis. A sample of the product obtained at 150°C and Mn:Cu = 0.4 was subjected to 20 precipitation/redispersion cycles using isopropanol/hexanes in order to remove the organic passivating layer after which the material was no longer able to redisperse into hexanes. We chose this option over a calcination protocol in order to protect the sample from thermal damage or oxidation. A sample of MnO nanocrystals was similarly treated to remove the surfactant layer. Nitrogen physisorption analysis (Figure 5) showed the BET surface area of the MnO nanocrystals and hollow Mn3O4 nanoparticles were 27.7 m2/g and 93.3 m2/g, respectively. Given that the outer diameters of the particles in both samples are very similar (~30 nm), the larger surface area of the Mn3O4 sample is consistent with a hollow structure. The pore size distributions of the samples showed striking contrast. The MnO nanocrystals had much lower pore volume and a broad bimodal distribution (likely due to interparticle spacing) whereas the Mn3O4 sample had an intense, narrow pore size distribution centered at ~9 nm that is diagnostic of the hollow interior of the particles.

Figure 5. Nitrogen physisorption analysis of the MnO nanocrystals and hollow Mn3O4 nanoparticles. Data recorded from DSC-TGA measurements of MnO and Mn3O4 under N2 and air atmosphere are shown in Figure 6. Under N2 atmosphere, and up to ~300°C, both MnO and Mn3O4 show gradual mass loss with near-zero heat flow. This feature is assigned to desorption of residual organic material from the surface of the inorganic nanoparticles. At ~350°C there is an inflection point in the weight loss and heat flow traces, after which the rate of mass loss was lower and the process was endothermic. We assign this feature to reduction of the sample with loss of oxygen atoms. Under air atmosphere, both MnO and Mn3O4 show an abrupt mass loss from ~150-250°C in an exothermic process. This feature is assigned to oxidative decomposition of residual organic material on the nanoparticle surface. Above ~250°C (in air), there was a mass increase in both 10 ACS Paragon Plus Environment

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samples that we assign to slow oxidation of the inorganic nanoparticles. We note that the MnO sample shows significant mass gain after the oxidative desorption of the residual organic material; whereas, the Mn3O4 showed a smaller mass gain. Furthermore, the hollow Mn3O4 particles showed some mass loss above ~550°C. This is likely due to the loss of surface oxygen upon collapse and sintering of the hollow particles.

Figure 6. DSC-TGA traces of (top left) MnO in N2, (top right) Mn3O4 in N2, (bottom left) MnO in air, (bottom right) Mn3O4 in air. We performed ex situ TEM and powder XRD measurements of Mn3O4 samples calcined in air at various temperatures in order investigate the stability of the crystal phase and particle morphology. Samples of Mn3O4 (after ligand removal) were calcined at 200°C, 250°C, 300°C, 350°C, 400°C, and 450°C for 2 h in air. After calcination, powder X-ray diffraction (Figure 7) and TEM (Figure 8) data were obtained. At room temperature, the sample showed MnO and Mn3O4 phases. Samples calcined at 200-300°C showed exclusively the Mn3O4 hausmannite phase. Samples calcined at 350°C showed a mixture of Mn3O4 and Mn5O8. Above 350°C, the sample was exclusively the Mn5O8 phase. Recently, well-defined Mn5O8 nanoparticles were prepared via annealing under an oxygen atmosphere at 375°C.69 The phase changed observed in the series of ex situ powder X-ray diffraction data correlate with the observed mass gain of MnO and Mn3O4 nanoparticles in the DSC-TGA analysis. From TEM data, it is evident the hollow Mn3O4 nanoparticles retained their hollow morphology after calcination at 200°C. At 250°C, the hollow structure had clearly collapsed.70 As the calcination temperature increased, the sample showed further sintering. 11 ACS Paragon Plus Environment


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Figure 7. Powder X-ray diffraction data of hollow Mn3O4 nanoparticles (a) after calcination for 2 h in air at T = b) 200°C, c) 250°C, d) 300°C, e) 350°C, f) 400°C, and g) 450°C.

Figure 8. Transmission electron micrographs of hollow Mn3O4 nanoparticles after calcination for 2 h in air at T = a) 200°C, b) 250°C, c) 300°C, d) 350°C, e) 400°C, and f) 450°C. Electrochemical measurements MnO and Mn3O4 materials are of interest as anode materials for lithium ion batteries with theoretical capacities of 756 mAh/g and 936 mAh/g, respectively.1-4,36, 71 We assessed the electrochemical performance of hollow Mn3O4 nanoparticles in a coin cell with Li foil as counter 12 ACS Paragon Plus Environment

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electrode. Figure 9a shows the galvanostatic charge-discharge voltage profiles of hollow Mn3O4 nanoparticles as the working electrode. Measurements were obtained at a constant current density of 40 mA/g for 1st, 2nd and 10th cycles cycled between 3.0 and 0.01 V vs Li+/Li. The 1st discharge exhibited multiple voltage plateaus between ~1.6 to 0.4 V that can be attributed to formation of a solid electrolyte interphase (SEI) layer and initial reduction of Mn3O4.1,36 The large voltage plateau at ~ 0.3-0.4 V corresponds to the main lithiation reaction: Mn3O4 + 8Li+ + 8e- → 3Mn0 +4Li2O (theoretical capacity ~ 936 mAh/g). This led to a high 1st discharge capacity of 1324 mAh/g. This demonstrates that the hollow Mn3O4 has the capability of Li ion storage. No distinct plateau was observed in the 1st charge reaction, which implies that only partial oxidation of Mn0 occurred. This led to a severe decrease in the 1st charge capacity to 255 mAh/g. Similar results were observed by Wang et al.1 for free Mn3O4 nanoparticles without graphene support. The 2nd and 10th discharge capacities were 290 mAh/g and 35 mAh/g, representing 78% and 97% loss of capacity relative to the first cycle. The observations indicate that the hollow Mn3O4 exhibited excellent initial Li ion storage capability but poor cycling performance. Poor cycling performance can be attributed to the insulating property of Mn3O4, aggregation of particles, and severe decomposition of electrolyte. Figure 9b shows the galvanostatic chargedischarge voltage profiles of hollow Mn3O4/MWCNT electrode. Measurements were obtained at a constant current density of 40 mA/g for 1st, 2nd, and 10th cycles cycled between 3.0 and 0.01 V vs Li+/Li. Similar to the hollow Mn3O4 electrode, plateaus occurring at ~ 1.6 to 0.4 V can be ascribed to initial reduction of Mn3O4 and formation of the SEI layer. The voltage plateau at ~ 0.3-0.4 V corresponds to the main lithiation reaction: Mn3O4 + 8 Li+ + 8 e- → 3 Mn0 + 4 Li2O. A high 1st discharge capacity of 1224 mAh/g was observed. Unlike the hollow Mn3O4, the 1st charge profile had two distinct falling points of inflection in the voltage ranges of 1.0-1.5 V and 2.0-2.5 V corresponding to the oxidation of Mn0 to Mn2+ and Mn2+ to Mn3+, respectively.36 The 1st charge capacity was 578 mAh/g. The discharge profiles of the 2nd and 10th cycles showed a prominent plateau at ~0.5V. Figure 10 shows the electrochemical cycling performance of hollow Mn3O4-MWCNT compared to hollow Mn3O4 only electrode. As can be seen, the hollow Mn3O4MWCNT electrode greatly outperforms the hollow Mn3O4 only electrode. The hollow Mn3O4 electrode showed poor discharge/charge capacities after only a few cycles, diminishing ~97% after 10 cycles. The discharge/charge capacities remained steady from the 10th-100th cycles, and were measured as 23/22.5 mAh/g on the 100th cycle. In contrast, the hollow Mn3O4/MWCNT electrode showed high capacity out to 100 cycles. After the initial charge/discharge cycle, the subsequent charge and discharge capacities rapidly converged to a value of 668 mAh/g on the 10th cycle. Interestingly, the reversible discharge/charge capacities increased steadily up to 971/953 mAh/g at the end of the 69th cycle. Jian et al. observed a capacity of ~980 mAh/g after 140 cycles at 200 mA/g for hollow Mn3O4 spheres,36 and Wang et al. observed a capacity of 592 mAh/g after 50 cycles at 100 mA/g for Mn3O4 nanocrystals anchored on MWCNTs.5 After the 69th cycle, our electrode showed steadily diminishing reversible discharge/charge capacity out to 100 cycles, and final values of 760/747 mAh/g were observed. Overall, the hollow Mn3O4/MWCNT electrode showed high discharge/charge capacity and good reversibility over 100 cycles. The improved cycling performance can be attributed to improved electrical conductivity within the hollow Mn3O4/MWCNT composite, and reduced Li-ion diffusion length facilitated by the nanoscale conductive network of MWCNT. The MWCNT-only electrode demonstrated a capacity of 257/250 mAh/g after 50 cycles (Figure S5).



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Figure 10. Electrochemical cycling performance of hollow Mn3O4 and Mn3O4-MWCNT electrodes. Open and closed circles represent charge and discharge capacity, respectively. Cyclic voltammograms of hollow Mn3O4-MWCNT (Figure 11a) and hollow Mn3O4 (Figure 11b) were obtained upon potential sweep in the range of 0.01-3.0 V at a scan rate of 0.2 mV/s. The first cycle of both electrodes showed cathodic peaks at 1.25 V indicating reduction of Mn3+ to Mn2+, 0.6 V indicating decomposition of electrolyte and formation of SEI layer and below 0.25 V indicating reduction of Mn2+ to Mn0. The cathodic peak at 0.6 V is more prominent for Mn3O4-MWCNT due to the lithiation in MWCNT. Subsequently, both electrodes showed an 14 ACS Paragon Plus Environment

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anodic peak at 1.3 V indicating oxidation of Mn0 to Mn2+. In subsequent sweeps of the hollow Mn3O4 electrode, the intensity of the anodic and cathodic peaks diminished. In contrast, the hollow Mn3O4/MWCNT electrode showed increasing anodic peak current at ~1.3V and increasing cathodic peak current at ~0.3V through 10 cycles. A very weak anodic peak was observed at ~2.3V for Mn3O4/MWCNT that is due to the partial oxidation of Mn2+ to Mn3+.

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Figure 11. Cyclic voltammograms of (a) Mn3O4-MWCNT and (b) hollow Mn3O4 electrodes. Conclusion



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We report the reaction of MnO nanocrystals with Cu2+ leads to a galvanic reaction and nanoscale Kirkendall effect that results in the formation of hollow Mn3O4 nanoparticles with outer diameter ~31 nm and inner diameter ~ 15 nm. With increasing ratio of Cu:Mn the frequency of hollow particles increased while the inner and outer diameters of the hollow particles remained nearly constant. Upon complete conversion of MnO to Mn3O4, the sample consisted of smaller nanoparticles that appear to be fragments of Mn3O4 shells that have crumbled apart. Optimum conditions for the preparation of hollow Mn3O4 nanoparticles were found at 150°C and Cu:Mn = 0.4. Physisorption analyses of MnO and hollow Mn3O4 nanoparticles showed dramatic differences. While particles in both samples have nearly equal outer diameter, the hollow Mn3O4 nanoparticles showed a marked increase in surface area and increased porosity with narrow size distribution diagnostic of the hollow cavity. We perform ex-situ analyses of hollow Mn3O4 nanoparticles calcined in air. Analysis of powder X-ray diffraction data indicate a phase transformation of the hollow Mn3O4 to Mn5O8 while TEM analyses show the hollow nanoparticle morphology collapses above 200°C. Electrodes fabricated from hollow Mn3O4 exhibited excellent initial Li ion storage capability (initial discharge capacity = 1324 mAh/g) but poor cycling performance (97% loss of discharge capacity after 10th cycle); whereas, Mn3O4MWCNT electrodes exhibited good reversibility and discharge capacity of 760 mAh/g after 100 cycles. Supporting Information. Size histograms, photographs of reactions, TEM vs. temperature, powder XRD vs. temperature, and electrochemical performance of MWCNT supplied as Supporting Information. Acknowledgements This work was supported by the National Science Foundation (CHE-0840507, CHE-0722632, DGE-0903685) and NASA EPSCoR (NNX14AN22A and NNX16AQ98A). C. B. and R. M. R. are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Catalysis Science program under Award Number DE-SC0016192. TOC Graphic

References Cited


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