Pure Manganese Oxide Research Articles

Co-precipitation Synthesis and Characterization Studies of Manganese Oxide Doped with Nickel for High-Performance Energy Storage Supercapacitor Application

The advancement in nanotechnology research has facilitated the development of eco-friendly methods for the synthesis of nanoparticles. In this work, nickel (Ni)-doped manganese oxide was synthesized using the co-precipitation method. The prepared Ni-doped manganese oxide products have been characterized by X-ray powder diffraction, scanning electron microscopy (SEM), transmission electron microscope (TEM), and energy dispersive X-ray spectroscopy (EDS). The preliminary electrochemical characteristics include charge-discharge cycling, which improves the conductivity and capacitance of the high-performance aqueous asymmetrical supercapacitor. The CV analysis of the Ni-doped manganese oxide electrode demonstrated a distinctive pseudocapacitive behavior in 1 M KOH solution. The nickel (Ni) doped electrode has a higher specific capacitance value than the pure manganese oxide electrode, with a value of 2225.07 F×g-1 at a scan rate of 5 mV×s-1. Keywords: Supercapacitor, Co-precipitation method, Nanotechnology, Synthesized, Electrochemical

Investigation of the Activity and Stability of Manganese-Based Electrocatalysts for the Electrochemical Oxidation of Water and Small Organic Molecules in Acidic Electrolyte

Water electrolysis is a crucial process for hydrogen production, providing a clean and sustainable method for H2 production. Iridium-based catalysts are particularly effective in promoting the oxygen evolution reaction (OER) that takes place at the anode of electrolyzers. The unique properties of iridium make it well-suited for this purpose, but its high cost becomes a bottleneck in making water electrolysis economically viable on a large scale. Manganese-based electrocatalysts have emerged as promising candidates for the OER, mainly due to their earth abundance, low toxicity, and activity. It has been shown that manganese oxide can operate stably in acid, in a restricted potential window, where the formation of permanganate ions is avoided. Thus, for practical application, the stability must be improved, so a larger potential window can be employed. To address this challenge, some approaches have emerged from combining a catalytically active, but instable oxide, with the one that is stable, but inactive for the OER. In the presentation, we will show the results of activity, stability, and faradaic efficiency (FE) for O2 formation for water electro-oxidation on manganese oxide, and on manganese oxide combined with antimony, tin, titanium, niobium, and silicon oxide. The faradaic efficiencies, determined via EC-MS (Electrochemistry coupled to mass spectrometry) measurements revealed that manganese antimonate and pure manganese oxide have the highest values for O2 formation, with the former presenting higher stability at high overpotentials. We will discuss the obtained trends in stability and FEs for the other synthesized oxides. Additionally, for these two most efficient electrocatalysts, EC-MS measurements were conducted for the electrochemical oxidation of some small organic molecules, such as formic acid, methanol, ethanol, and ethylene glycol (electrochemical reforming for H2 production). The results showed that the selectivity for the electro-oxidation of water and/or of the organic molecule (monitoring the formation of CO2) strongly depends on the catalyst composition and on the number of carbon atoms of the molecule. We will further discuss, in the presentation, the factors that govern the selectivity and stability for each case. The obtained results of this study may be helpful for developing more efficient, selective and, mainly, more stable earth-abundant electrocatalysts for electrolyzers.

Effect of Annealing Process on the Morphological, Optical and Electrical Properties of Cu:MnO Films Prepared by PLD Technique

In this study, Nd:YAG laser pulses with a wavelength of 1064 nm, a power of 500 mJ, a pulse width of 9 ns, and a repetition frequency of 6 Hz were used to hit a manganese oxide (MnO) target surface 300 times. Pure MnO and copper Cu-doped MnO (Cu:MnO) with different amounts of Cu (0.03, 0.05, 0.07, and 0.09 wt%) produced by PLD were studied. Cu:MnO thin films were annealed at 473 K, and their morphological, optical, and electrical characteristics were studied. The results of the atomic force microscopic (AFM) investigation of morphological properties showed that Cu dopant impacted the creation of roughness and particle size in MnO2 films. The optical transmission was examined using a UV-Vis spectrophotometer. The highest optical absorption was noted at 0.09 dopant content. The dielectric constants' real (εr) and imaginary (εi) components, as well as the extinction coefficient (k), refractive index (n), and other optical constants, were studied. At an annealing temperature of (473 K), Hall effect studies demonstrate that all produced films exhibit a P-type conductivity.

Investigation on YSZ- and SiO2-Doped Mn-Fe Oxide Granules Based on Drop Technique for Thermochemical Energy Storage.

The Mn-Fe oxide material possesses the advantages of abundant availability, low cost, and non-toxicity as an energy storage material, particularly addressing the limitation of sluggish reoxidation kinetics observed in pure manganese oxide. However, scaling up the thermal energy storage (TCES) system poses challenges to the stability of the reactivities and mechanical strength of materials over long-term cycles, necessitating their resolution. In this study, Mn-Fe granules were fabricated with a diameter of approximately 2 mm using the feasible and scalable drop technique, and the effects of Y2O3-stabilized ZrO2 (YSZ) and SiO2 doping, at various doping ratios ranging from 1-20 wt%, were investigated on both the anti-sintering behavior and mechanical strength. In a thermal gravimetric analyzer, the redox reaction tests showed that both the dopants led to an enhancement in the reoxidation rates when the doping ratios were in an appropriate range, while they also brought about a decrease in the reduction rate and energy storage density. In a packed-bed reactor, the results of five consecutive redox tests showed a similar pattern to that in a thermal gravimetric analyzer. Additionally, the doping led to the stable reduction/oxidation reaction rates during the cyclic tests. In the subsequent 120 cyclic tests, the Si-doped granules exhibited volume expansion with a decreased crushing strength, whereas the YSZ-doped granules experienced drastic shrinkage with an increase in the crushing strength. The 1 wt% Si and 2 wt% Si presented the best synthetic performance, which resulted from the milder sintering effects during the long-term cyclic tests.

Co doping induced phase transition and its distinct effects on the catalytic performance of MnO2 toward toluene oxidation

Manganese oxides are very important and conventional catalysts that have demonstrated appreciable catalytic activity for the oxidation of volatile organic compounds (VOCs). Nevertheless, pure manganese oxides suffer from poor activity especially at low temperatures, making it difficult to meet industrial applications. In this work, Co species were successfully doped into the lattice of MnO2 aiming at constructing defects to boost its catalytic performance for VOCs oxidation. In combination with the results of systematic characterizations, we found that Co doping forced the catalyst to transform from α‐MnO2 to spinel phase (Co,Mn)(Co,Mn)2O4. During this process, the metal–oxygen bonds are significantly weakened, which induces the massive generation of oxygen defects, endowing the Co modified catalysts with enhanced redox property and improved ability to adsorb and activate gaseous oxygen. As a result, Co doped catalysts show much better catalytic activity compared with pristine α‐MnO2, among which Mn10Co10 exhibits the best performance showing a decrease of 41°C and 51°C in T50 and T90 compared with the raw sample, respectively. Furthermore, Mn10Co10 demonstrates excellent stability, water resistance, and reusability, illustrating a great potential for industrial applications. Moreover, path of toluene decomposition over Co10Mn10 was revealed by in situ DRIFTS experiment, which complies with the sequence of toluene → benzyl alcohol → benzaldehyde → benzoate → maleic anhydride → CO2 and H2O.

Perfectly dense ceramics of highly pure calcium manganese oxide

Immensely thermally stable and perfectly dense ceramics of highly pure calcium manganese oxide (CaMnO3) powders is realized at optimized minimal processing temperature and time using cost-effective ball-milling and hot-press techniques. The effect of ball milling time on the calcination time to prepare highly pure CaMnO3 powder and sintering time on the temperature adequate enough to achieve perfectly dense CaMnO3 ceramics is studied thoroughly and systematically. The as-synthesized single phase CaMnO3 is attained by mixing precursors of calcium carbonate and manganese oxide, using high energy ball milling for longer time followed by calcination for shorter time duration and vice-versa. A decrease in particle size in as-synthesized CaMnO3 is noticed for powders prepared with longer ball milling and shorter calcination times. The minimal time required for achieving highly pure and fine-grained powders containing sub-micrometer particles of CaMnO3 is accomplished by ball milling the precursor mixture for 12 h and calcining at 1273 K for 2 h. Relative density of 99.03% is achieved for the as-synthesized CaMnO3 powders hot pressed at 1073 K for 30 min.

Synthesis of manganese oxide from manganese ore of bajaur agency khyber Pakhtunkhwa (KPK), Pakistan

In the present work manganese sulphate developed from the Bajaur (KPK, Pakistan) manganese ore was utilized successfully for the synthesized Manganese Oxide chemically by an alkali-oxidation of manganese sulphate using the co-precipitation method. The advantages of this eco-friendly co-precipitation method include quick and easy preparation, simple control of particle size, and cost-effective composition using local ore. Because of the readily available raw materials and the controlled reaction conditions, this method is widely used in industry. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the synthetic manganese oxide. Fourier transforms infrared spectroscopy (FTIR) Infrared reveals that the O-Mn-O vibrational mode occurs in pure manganese oxide at wavelengths between 600 and 500 cm-1. The chemical composition was obtained by EDX analysis and confirmed the presence of Mn and O in the sample. Experimental optimization was done for the manganese oxide precipitation conditions. The findings show that a variety of parameters have an impact on the product yield; the precursor ratio and reaction temperature are the most important variables, and the reaction time and pH value have the greatest impact on the quality of crystallization. Bangladesh J. Sci. Ind. Res. 58(4), 221-230, 2023

Improvement in capacitive performance of ZnS with MnO2 via composite (ZnS/MnO2) strategy developed by hydrothermal technique

Nanocomposites based on manganese oxide (MnO2) exhibit a significantly wide working potential window, rendering them highly appealing for utilization in advanced energy transformation and storage technologies. The following article describes a simple method for synthesizing pure manganese oxide (MnO2), zinc sulfide (ZnS) and ZnS/MnO2 nanocomposites (NCs) for electrochemical supercapacitor applications. Various physiochemical approaches were employed to depict the structure and chemical features of the synthesized materials and electrochemical performance was determined with multiple electrochemical tests under 2.0 M KOH. The high pseudocapacitive nature and good conduction of the MnO2 in ZnS/MnO2 display a high specific capacitance (Cs) of 1002 F g−1 @ 1.0 A g−1. The ZnS/MnO2 display a higher discharge time of 467 s @ 1 A g−1 than 2 A g−1 (159 s) and 3 A g−1 (65 s). The symmetric behavior of the ZnS/MnO2 electrode materials displays the specific capacitance of 373 F g−1 and specific energy of 15.45 Wh kg−1 @ 1 A g−1. Hence, the synergistic behavior of ZnS/MnO2 nanocomposite produced via an effective and environment friendly hydrothermal method displays powerful performance toward energy storage application.

Mn- and Fe-spinels' formation through sintering of pure and waste materials: Mechanical, electrical and magnetic properties

This study focuses on fabrication of some spinel composites from pure and waste materials for electronic and magnetic applications. Six batches were designed from pure MgO, ZnO, Fe2O3, MnO2 and iron oxide extracted from alum sludge. The designed batches were sintered at 1250 and 1350 °C then investigated for their phase composition, microstructure, physical, mechanical, electrical and magnetic properties. The results indicated that, after sintering at 1350 °C, all batches formed spinel structures as main phases. The formed spinel structures were MgFe2O4, MgMn2O4, ZnMn2O4 and ZnFe2O4, in addition to some minor pervoskite structures. The batch that prepared from pure manganese and zinc oxides (B4), formed ZnMn2O4 spinel and exhibited the best densification parameters (bulk density; 4.7 g/cm3 and apparent porosity; 10%) as well as highest hardness (4.53 GPa). On the other hand, the specimen that fabricated from pure MgO, MnO2 and Fe2O3 (B2) formed MgFe2O4 and MgMn2O4 spinel structures and exhibited the highest electrical resistivity (1.3E + 09 Ohm.cm) as well as highest saturation magnetization (Ms = 24.948 emu/g).

MnO2 Doped with Ag Nanoparticles and Their Applications in Antimicrobial and Photocatalytic Reactions

A wide range of nanoparticles have been produced for photocatalysis applications. Nonetheless, degrading organic dyes requires nanoparticles that are efficient and excellent. As a photocatalyst, pure manganese oxide (MnO2) was prepared via a sol–gel method using silver (Ag) nanoparticles of transition metal oxide. In addition to X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX), the crystal structure and elemental composition were analysed. According to XRD data, the transition metal of MnO2 oxide is highly pure and has a small crystallite size. The presence of functional groups was confirmed and clarified using Fourier-transform infrared spectra (FTIR). By irradiating the transition pure and doped MnO2 photocatalysts with visible light, the UV-vis, μ-Raman, and surface areas were determined. As a result, of using the photocatalysts with aqueous methylene blue (MB) solutions under visible light irradiation, the MnO2 doped with Ag nanoparticles demonstrated high degradation efficiencies and were utilised to establish heterogeneous photocatalysis dominance. In this paper, we demonstrate that the photocatalytic efficiency of transition metal oxides is exclusively determined by the particle size and surface area of nano-sized materials. Due to their high surface charge ratio and different surface orientations, have the highest photocatalytic efficiency. Generally, MnO2 doped with Ag nanoparticles is resistant to bacteria of both Gram-positive and Gram-negative types (B. sublittus and Escherichia coli). There is still a need for more research to be performed on reducing the toxicity of metal and metal oxide nanoparticles so that they can be used as an effective alternative to antibiotics and disinfectants, particularly for biomedical applications.

Influence of Samarium on Structural, Morphological, and Electrical Properties of Lithium Manganese Oxide

Research and developments on preparing electrode material for lithium-ion batteries are burgeoning nowadays widely. In this study, we used the high energy ball-milling method to prepare pure and samarium-doped lithium manganese oxide (Li4Mn5O12) and investigated its structural, morphological, and electrical properties. The XRD spectrum for the produced materials confirmed the phase purity and crystallinity of the material. The fourier transform infrared spectrum was used to determine the different sorts of vibrations between molecules. The particle size with the presence of polyhedral-shaped morphology was certified by using SEM and TEM analysis. EDS mapping was used to assess the elemental composition and purity of the samples. Complex impedance spectroscopy analysis was used to investigate the temperature dependency of the materials’ electrical properties, and high conductivity (1.15 × 10−7 S cm−1) was reported for samarium-doped lithium manganese oxide at 100°C, and its dielectric relaxation behavior was examined.

Fractal Analysis of Pure and Fe-Doped Manganese Oxide Supercapacitor Electrodes

Fractal Analysis of Pure and Fe-Doped Manganese Oxide Supercapacitor Electrodes

The effect of deposition angle and thickness on structural and optical properties of manganese oxide thin films

Manganese oxide thin films were deposited on glass substrates by electron gun evaporation method under ultra-high vacuum condition. Thickness of the layers was measured 60 and 120 nm, by a quartz crystal method. Deposition conditions such as deposition rate, vacuum pressure, incidence of angle and substrate temperature were the same for all layers. After producing pure manganese oxide layers a post-annealing method was used in the presence of a uniform oxygen flow of 300 (sccm) and at 600 K annealing temperature. Optical reflectance and transmittance of the layers were measured in the wavelength of 350–850 nm by a spectrophotometer. Kramers–Kronig relations were used to calculate the optical constant. The influence of annealing temperature and oxygen flow on optical properties is investigated. It was found that film thickness and deposition angle plays an important role on the nanostructures as well as optical properties of layers and cause significant variations in behavior of thin manganese oxide films. The physical properties of materials were characterized by X-ray diffraction (XRD), FE-SEM, AFM, EDX, and UV–Vis techniques.

Impact of Zn-doped Manganese Oxide Nanoparticles on Structural and Optical Properties

In the present work, the structural and optical properties of pure and zinc-doped manganese oxide nanoparticles prepared by the sol-gel method have been studied for doping concentrations of 5, 10, and 15 %. The XRD patterns reveal that the cubic phase of pure manganese oxide nanoparticles is changed to tetragonal on doping with Zn. The surface morphology of pure and Zn doped manganese oxide nanoparticles has been studied by Scanning Electron Microscopy (SEM), which reveals appreciable changes in size and shape of the nanoparticles as the doping level of Zn is increased. The optical properties studied by UV-Visible spectroscopy further reveal that the energy band gap increases with a decrease in the size of nanoparticles.

Thermodynamic modelling of decomposition processes in the Mn-O and Mn-O-H systems

ABSTRACT An understanding of the decomposition processes in the Mn-O and the Mn-O-H systems is important for several applications such as the reduction of manganese ores or the production of pure manganese oxides. Precise thermodynamic data is required for modelling these thermal processes and in particular data is lacking for Mn5O8. It would be advantageous to more accurately model these processes, but this depends on the availability of reliable thermodynamic data. In the present research, the currently available data for the Mn-O system was evaluated in order to obtain consistent decomposition temperatures of the various manganese oxides, as calculated using the Equilibrium Composition program of HSC Chemistry® 7.1. Subsequently, these temperatures were compared to the experimentally determined values in the literature. Also, the effects of variables such as atmospheric composition and pressure on the temperatures were evaluated. Additionally, the thermal conditions required for the formation of Mn5O8, were delineated. The thermodynamic data utilized for Mn5O8 were as follows: ΔH° = −2444.5 kJ/mol, ΔS° = 280.2 J/(mol·K) and J/(mol·K). The selected data utilized in this research, can be used for accurate thermodynamic calculations and as examples, phase stability diagrams for the Mn-O, Mn-O-H and Mn-O-N systems were determined.

Oriented growth of δ-MnO2 nanosheets over core-shell Mn2O3@δ-MnO2 catalysts: An interface-engineered effects for enhanced low-temperature methanol oxidation

• The pure manganese oxides Mn 2 O 3 @δ-MnO 2 core-shell structures can be fine-manipulated via a novel synthesis method. • The synergistic effect of δ-MnO 2 nanosheets and Mn 2 O 3 microspheres is the key to enhance the deep methanol oxidation. • The Mn 2 O 3 @δ-MnO 2 interface structure exhibited more Mn 4+ , well redox ability and active surface lattice oxygen. • Activity study revealed that the Mn 2 O 3 @δ-MnO 2 -8h catalyst has excellent activity for low-temperature methanol oxidation. In order to elucidate the special role of interfacial effect over nanomaterials in the catalytic oxidation of volatile organic compounds (VOCs), a series of core-shell Mn 2 O 3 @δ-MnO 2 catalysts with different degree of oriented growth were prepared by a hydrothermal method, which were applied for methanol oxidation. Besides, the structure-activity relationship over the core-shell Mn 2 O 3 @δ-MnO 2 catalysts was further explained in details by multiple characterization techniques. It was found that the strong metal-support interaction promoted the formation of the interface between Mn 2 O 3 and δ-MnO 2 , which significantly changed the physicochemical properties and catalytic behaviours of core-shell Mn 2 O 3 @δ-MnO 2 catalysts. Among all the catalysts, the catalytic performances of these core-shell catalysts were obviously better than those of pure δ-MnO 2 nanosheets and Mn 2 O 3 microspheres, and the Mn 2 O 3 @δ-MnO 2 -8h catalyst exhibited the best outstanding activity for methanol oxidation (T 90 = 119 °C). In addition, the characterization analyses showed that the interfacial effect derived from the interaction between Mn 2 O 3 and δ-MnO 2 can increase the amount of Mn 4+ , and surface lattice oxygen, as well as optimize the low-temperature reduction ability, which was the crucial reason for the high activity of above core-shell Mn 2 O 3 @δ-MnO 2 catalysts.

Enhancement of Cd(II) electrosorption using electrosorption process with manganese oxide nanomaterial electrodeposited

The problem of water contamination by Cd(II) ions is considered as a major problem in the world. In this context, a clean, sustainable, soft and non-toxic approach was proposed based on the electrosorption process using electrodeposited pure manganese oxide on SnO2 subtract (Na-MnO2/SnO2) to remove Cd(II) ions without sludge formation like in the case of chemical and biological processes. Various characterization methods, such as scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and cyclic voltammetry were used to analyze the electrodeposited Na-MnO2/SnO2. The effects of the applied potential (0.0–1.0 V), electrodeposited charge quantities (Q = 0.45, 0.9, 1.5 and 3C) and the initial pH (2.0–9.0) on the electrosorption capacity of Cd(II) were examined and optimized. The highest electrosorption capacities were recorded for thin films with the lowest charge quantity (0.45C) and the highest Mn average oxidation state (3.84). Adsorption isotherms studies revealed that the nanomaterial (Na-MnO2/SnO2) has an electrosorption capacity of 2583.31 mg/g. The comparative study between the two techniques adsorption and electrosorption of Cd(II) by Na-MnO2/SnO2revealed the electrosorption to be better regarding the treatment kinetics and elimination capacity of Cd(II). During electrosorption, Na-MnO2/SnO2 with a charge of 0.45C released less manganese, indicating this process is more promising in capacity and stability. The nanomaterial got reused for several cycles. These low-cost efficient nanomaterials and methods are promising for the removal of heavy metals with a sustainable and environmentally friendly approach to produce clean water without secondary pollution or sludge formation.

Influence of thickness and deposition angle on structural and optical properties of manganese oxide thin films

Manganese oxide thin films were produced on glass substrates by resistive evaporation at room temperature. The layers with different thickness (30 and 90 nm) at different deposition angles (0 and 40°) were prepared by electron gun evaporation method under ultra-high vacuum condition. After deposition of pure manganese oxide thin films a post-annealing process was used in a uniform oxygen flow of 300 (sccm) and at 500 K annealing temperature. Optical transmittance and reflectance of the layers were measured in the wavelength of 350–850 nm by a spectrophotometer. Kramers–Kronig relations were used to calculate the optical constants. The influence of oxygen flow and annealing temperature on optical properties is investigated. It was found that film thickness and deposition angle plays a significant role on the nanostructures as well as optical properties of layers and cause major variations in behavior of thin manganese oxide films. The physical properties of materials were characterized by X-ray diffraction (XRD), FE-SEM, AFM, EDAX, and UV-VIS techniques.

Spray Pyrolysis Synthesis of Pure and Mg-Doped Manganese Oxide Thin Films

Pure and Mg-doped manganese oxide thin films were synthesized on heated glass substrates using the spray pyrolysis technique. The surface chemical composition was investigated by the use of X-ray photoelectron spectroscopy (XPS). Structural and morphological properties were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) and atomic force microscopy (AFM). Optical properties were characterized by UV-visible spectroscopy. XPS spectra showed typical Mn (2p3/2), (2p1/2) and O (1s) peaks of Mn3O4 with a slight shift attributed to the formation of different chemical states of manganese. XRD analysis revealed the tetragonal phase of Mn3O4 with a preferred (211) growth orientation that improved with Mg-doping; likewise, grain size is observed to increase with the Mg doping. SEM images of Mn3O4 films showed rough surfaces composed of uniformly distributed nanograins whose size decreases with the Mg-doping. The manganese oxide films surface observed in AFM show a textured, rough and porous surface. The combination of transmittance and absorption data in the UV-visible range allowed determining the energy values of the Eg band gap (1.5–2.5 eV). The decrease of the band gap with the Mg-doping increase is attributed to the influence of the greater size of the Mg2+ ion in the manganese oxide lattice.

Tuning the size and composition of manganese oxide nanoparticles through varying temperature ramp and aging time.

Manganese oxide (MnO) nanoparticles (NPs) can serve as robust pH-sensitive contrast agents for magnetic resonance imaging (MRI) due to Mn2+ release at low pH, which generates a ~30 fold change in T1 relaxivity. Strategies to control NP size, composition, and Mn2+ dissolution rates are essential to improve diagnostic performance of pH-responsive MnO NPs. We are the first to demonstrate that MnO NP size and composition can be tuned by the temperature ramping rate and aging time used during thermal decomposition of manganese(II) acetylacetonate. Two different temperature ramping rates (10°C/min and 20°C/min) were applied to reach 300°C and NPs were aged at that temperature for 5, 15, or 30 min. A faster ramping rate and shorter aging time produced the smallest NPs of ~23 nm. Shorter aging times created a mixture of MnO and Mn3O4 NPs, whereas longer aging times formed MnO. Our results indicate that a 20°C/min ramp rate with an aging time of 30 min was the ideal temperature condition to form the smallest pure MnO NPs of ~32 nm. However, Mn2+ dissolution rates at low pH were unaffected by synthesis conditions. Although Mn2+ production was high at pH 5 mimicking endosomes inside cells, minimal Mn2+ was released at pH 6.5 and 7.4, which mimic the tumor extracellular space and blood, respectively. To further elucidate the effects of NP composition and size on Mn2+ release and MRI contrast, the ideal MnO NP formulation (~32 nm) was compared with smaller MnO and Mn3O4 NPs. Small MnO NPs produced the highest amount of Mn2+ at acidic pH with maximum T1 MRI signal; Mn3O4 NPs generated the lowest MRI signal. MnO NPs encapsulated within poly(lactide-co-glycolide) (PLGA) retained significantly higher Mn2+ release and MRI signal compared to PLGA Mn3O4 NPs. Therefore, MnO instead of Mn3O4 should be targeted intracellularly to maximize MRI contrast.