Cobalt Spinel Research Articles

Rechargeable Aqueous Polymer-Air Batteries Based on Polyanthraquinone Anode

Summary Aqueous metal-air batteries have attracted great interest because of their high safety and relatively high energy density. However, poor reversibility and instability of metal anodes limit their cycling life. Here, we report an aqueous polymer-air battery using conjugated organic polymer poly(1,4-anthraquinone) (P14AQ) in situ polymerized on carbon nanotubes (CNTs) as the anode and spinel cobalt manganese oxide supported on CNTs (CMO@CNT) as the catalyzed cathode. The P14AQ/CNT-air battery exhibits long cycling life of 500 cycles with 92% capacity retention and superior rate capability (147 mAh g−1P14AQ, 38.5 C) because of the high reversibility of the P14AQ/CNT anode and the excellent catalytic performance of the CMO@CNT cathode. We constructed a pouch-type P14AQ/CNT-air battery that displays a high capacity of 1,025 mAh (corresponding to an energy density of 165 Wh kg−1 based on total mass of P14AQ and CMO) and good capacity retention of 95% after 100 cycles, showing promise for potential large-scale application.

High-performance fiber optic gas sensor-based Co3O4/MWCNT composite by a novel microwave technique

A novel one-pot microwave irradiation method has been adapted to synthesize cubic spinel cobalt oxide/multi-walled carbon nanotube composite. X-ray diffraction and scanning electron microscope results reveal that cubic spinel structure of Co3O4 with nanodimensional and well implanted over the multi walled carbon nanotubes (MWCNTs) metrics. The N2 adsorption–desorption results exhibit that Co3O4/MWCNT composite confirms the highest specific area (135 m2 g−1) and lower pore size 7.8 nm compared with pure Co3O4 (surface area 88 = m2 g−1 and pore size = 10.5 nm). The XPS result demonstrates that Co is 2+ in state in the spinel Co3O4. The gas-detecting test displayed that Co3O4/MWCNT composite indicated high sensitivity toward ethanol (35 counts/ppm) and ammonia (51 counts/ppm) gases as compared with pure Co3O4 (24 counts/ppm and 43 counts/ppm for ethanol and ammonia gases, respectively). It also exhibits excellent long-term stability with small deviations (less than 5%) for the evaluation of a 2-month period. The enhanced gas-sensing mechanism of Co3O4/MWCNT was also discussed in detail.

Control of structural and magnetic characteristics of cobalt ferrite by post-calcination mechanical milling

The possibility to control the structural parameters of nanostructured spinel cobalt ferrite paves the way to prepare the ferrite with the anticipated magnetic properties for broad applications. This study is aimed at elucidating the correlation between structural parameters and magnetic properties of cobalt ferrite nanoparticles subjected to post-calcination high-energy ball milling. The milling energy was predicted at four different levels, using the standard collision model. The samples produced were characterized by X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and vibrating sample magnetometry (VSM). Both FTIR studies and Rietveld refinement of the XRD data shows re-distribution of cations in the spinel structure, induced by a critical level of milling energy. Also, the presence of the three ionization peaks, Co−2p, Fe−2p and O−1s, revealed by XPS confirms the formation of spinel ferrite structure in the cobalt ferrite nanoparticle. The maximum energy products (BH)max of the sample milled under high level ball milling energy was 2.18 MGOe which is almost 5 times higher than that of the un-milled calcined sample. Additionally, the magnetic anisotropy constant was calculated by the Law of Approach to Saturation (LAS) method and indicates a direct correlation between lattice strain and coercivity of samples.

Spinel Cobalt Titanium Binary Oxide as an All-Non-Precious Water Oxidation Electrocatalyst in Acid.

Replacing precious water oxidation electrocatalysts used in proton exchange membrane (PEM) water electrolyzers with the nonprecious and abundant electrocatalysts is still a poorly addressed issue in the field of hydrogen generation in acidic medium through water electrolysis. Herein we report such an all-nonprecious binary spinel metal oxide the "cobalt titanate" (Co2TiO4) as an efficient alternate to expensive IrO2 and RuO2 for PEM water electrolyzer. The synthesized Co2TiO4 octahedral nanocrystals of size 50 to 210 nm showed excellent oxygen evolution reaction (OER) activity in 0.5 M H2SO4, which was comparable to IrO2 and better than spinel Co3O4 when examined under identical experimental conditions. Overpotential of just 513 mV was sufficient enough to drive a kinetic current density of 10 mA cm-2, which is a significant figure of merit as far as acidic water oxidation electrocatalysis is concerned.

Structural and catalytic properties of Ni–Co spinel and its composites

Pure nickel–cobalt (Ni–Co) spinel and its two composites with active carbon and multi-walled carbon nanotubes (MWCNTs) were synthesized. X-ray diffraction confirmed the nickel cobaltites of cubic syngony and lattice constants for nanosized crystallites. Fourier transform infrared spectra confirmed an inverse spinel consisting of a tetrahedral site \(\hbox {Co}^{2+}\) and octahedral sites \(\hbox {Ni}^{2+}\) and \(\hbox {Co}^{2+}\). Scanning electron microscopy images demonstrated a surface texture typical for spinels and agglomerates of composite particles with active carbon and MWCNTs. All the synthesized samples have a surface area and porosity that are sufficient for the flow of heterogeneous catalytic processes. The micropore volume of the composite with MWCNTs constituted only 4% of the total porosity, while this percentage represented 25% for the composite on the basis of active carbon. The high catalytic activity of Ni–Co spinel is proved in the model reaction of borohydride hydrolysis. To create composite spinel catalysts, active carbon showed itself to be a more efficient carrier for the catalytically active mass of spinel, as compared to MWCNTs.

Electrophoretic co-deposition of Fe2O3 and Mn1,5Co1,5O4: Processing and oxidation performance of Fe-doped Mn-Co coatings for solid oxide cell interconnects

Fe-doped Mn1,5Co1,5O4 coatings on Crofer22APU were processed by an electrophoretic co-deposition method and the corrosion resistance was tested at 750 °C up to 2000 h.The “in-situ” Fe-doping of the manganese cobalt spinel was achieved by electrophoretic co-deposition of Mn1,5Co1,5O4 and Fe2O3 powders followed by a two-step reactive sintering treatment. The effects on the coating properties of two different Fe-doping levels (5 and 10 wt.% respectively) and two different temperatures of the reducing treatment (900 and 1000 °C) are discussed. Samples with Fe-doped coatings demonstrated a lower parabolic oxidation rate and thinner oxide scale in comparison with both the undoped Mn1,5Co1,5O4 spinel coating and bare Crofer 22 APU. The best corrosion protection was achieved with the combined effect of Fe-doping and a higher temperature of the reducing step at 1000 °C.

Photoactive Zn-Air-Batteries Suing Cobalt Oxide As the Photocatalyst at the Cathode

Herein, we introduce a new design for the photoactive Zn-air batteries (ZABs). Spinel-type cobalt oxide is synthesized, coated on a transparent conductive indium tin substrate (Co3O4/ITO), and used as a photoactive cathode. The optical absorption of the Co3O4/ITO electrode show direct and indirect optical bandgaps for which the direct and indirect band gap values are 2.20 eV and 1.35 eV, respectively. These values are in the visible light region (1.8-3.1 eV)1. Under the exposure of visible light, the Co3O4/ITO exhibits ca. 30% higher current density than that under dark condition and provides the lower overpotentials of oxygen evolution reaction (OER) and oxygen oxidation reaction (ORR) around 10% - 20% as compared to those under dark condition. By assembling the photoactive ZAB cell, the higher battery performance is clearly observed under the visible light illumination. This new idea can open up opportunities in many applications requiring the conversion and storage of energy within a single cell. Reference Walsh, A.; Ahn, K.-S.; Shet, S.; Huda, M. N.; Deutsch, T. G.; Wang, H.; Turner, J. A.; Wei, S.-H.; Yan, Y.; Al-Jassim, M. M., Ternary cobalt spinel oxides for solar driven hydrogen production: Theory and experiment. Energy & Environmental Science 2009, 2 (7), 774-782. Figure 1

Ammonia selective sensors based on cobalt spinel prepared by combustion synthesis

Nano-crystalline cobalt spinel was prepared by combustion synthesis and used as ammonia sensing material. After synthesis, the powder was calcined at 600 °C for 4 h and characterized by thermal analysis, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, nitrogen adsorption (B.E.T, Brunauer, Emmet, Teller and B.J.H., Barrett, Joyner, Halenda techniques), H2 temperature programmed reduction, H2O adsorption and field emission scanning electron microscopy. Sensors were screen-printed onto α-alumina substrates with platinum interdigitated electrodes and fired at 700 °C for 1 h in air, after drying overnight.The sensor response was measured in the range 150 °C–250 °C under 1–50 ppm of NH3. Best results were obtained at 225 °C, with R (the ratio between the impedance of the film under gas exposure at the equilibrium and the impedance under dry air) equal to 1.83 under 50 ppm NH3. Response time and recovery time (e.g., the times taken by the sensor to attain 90% of total impedance change from its initial impedance value) were determined, together with cross-sensitivity tests towards CH4, CO, N2O, humidity, O3, CO2 and NO2 at the best operating temperature.

Copper-Doped Cobalt Spinel Electrocatalysts Supported on Activated Carbon for Hydrogen Evolution Reaction.

The development of electrocatalysts based on the doping of copper over cobalt spinel supported on a microporous activated carbon has been studied. Both copper–cobalt and cobalt spinel nanoparticles were synthesized using a silica-template method. Hybrid materials consisting of an activated carbon (AC), cobalt oxide (Co3O4), and copper-doped cobalt oxide (CuCo2O4) nanoparticles, were obtained by dry mixing technique and evaluated as electrocatalysts in alkaline media for hydrogen evolution reaction. Physical mixtures containing 5, 10, and 20 wt.% of Co3O4 or CuCo2O4 with a highly microporous activated carbon were prepared and characterized by XRD, TEM, XPS, physical adsorption of gases, and electrochemical techniques. The electrochemical tests revealed that the electrodes containing copper as the dopant cation result in a lower overpotential and higher current density for the hydrogen evolution reaction.

Total oxidation of lean methane over cobalt spinel nanocubes—Mechanistic vistas gained from DFT modeling and catalytic isotopic investigations

Abstract Mechanistic investigations based on combination of catalytic isotopic experiments with DFT modeling were used to probe the pathways by which CH4 in transformed into H2O and CO2 over cobalt spinel nanocubes exposing the (100) surface exclusively. The catalyst synthetized by the hydrothermal method was thoroughly characterized by means of XRD, XRF, Raman and SEM techniques. Combustion of methane was performed in a TPSR mode using sole 16O2 or a 1:1 mixture of 16O2 and 18O2. Two distinct mechanistic pathways of the C H bond activation, and subsequent oxidation of the consecutive intermediates involve a suprafacial mechanism operating in the oxygen covered surface and an interfacial mechanism operating on the oxygen depleted/bare surface. It was found that depending on the conditions they may act separately in the early and late stages of the reaction, and simultaneously in the middle phase, in response to differences in the nature and accessibility of the surface and lattice oxygen species, however the activation of the first C– bond remains the kinetically relevant step. The cobalt-oxo CoT2c–O species exhibit the highest activity in the suprafacial C H bond breaking, and the peroxo O22– species are engaged in the intermediate reaction steps leading to formation of final H2O and CO2 products. On the oxygen exhausted surface, where the interfacial mechanism operates, dual Co–O sites constituted by CoO5c–O3O and CoT2c–O2O,1T pairs may activate the C H bond in a concerted way, with the former being more active. For all the mechanistic steps the transition and stable states of the involved intermediates and products were identified and quantified in terms of their structure and the associated free enthalpy profiles. The three dimensional (log(kintra/ksupra) versus (ΔGasupra-ΔGasupra) and ΘO) diagrams were constructed to discriminate between the suprafacial (Eley-Rideal/Langmuir-Hinshelwood) and interfacial (Mars-van Krevelen) mechanism of CH4 combustion. It is believed that the main features of the established mechanism are also applicable in the case of the methane oxidation over other electron rich transition metal oxides.

Shifting Oxygen Charge Towards Octahedral Metal: A Way to Promote Water Oxidation on Cobalt Spinel Oxides.

Cobalt spinel oxides are a class of promising transition metal (TM) oxides for catalyzing oxygen evolution reaction (OER). Their catalytic activity depends on the electronic structure. In a spinel oxide lattice, each oxygen anion is shared amongst its four nearest transition metal cations, of which one is located within the tetrahedral interstices and the remaining three cations are in the octahedral interstices. This work uncovered the influence of oxygen anion charge distribution on the electronic structure of the redox-active building block Co-O. The charge of oxygen anion tends to shift toward the octahedral-occupied Co instead of tetrahedral-occupied Co, which hence produces strong orbital interaction between octahedral Co and O. Thus, the OER activity can be promoted by pushing more Co into the octahedral site or shifting the oxygen charge towards the redox-active metal center in CoO6 octahedra.

Shifting Oxygen Charge Towards Octahedral Metal: A Way to Promote Water Oxidation on Cobalt Spinel Oxides

AbstractCobalt spinel oxides are a class of promising transition metal (TM) oxides for catalyzing oxygen evolution reaction (OER). Their catalytic activity depends on the electronic structure. In a spinel oxide lattice, each oxygen anion is shared amongst its four nearest transition metal cations, of which one is located within the tetrahedral interstices and the remaining three cations are in the octahedral interstices. This work uncovered the influence of oxygen anion charge distribution on the electronic structure of the redox‐active building block Co−O. The charge of oxygen anion tends to shift toward the octahedral‐occupied Co instead of tetrahedral‐occupied Co, which hence produces strong orbital interaction between octahedral Co and O. Thus, the OER activity can be promoted by pushing more Co into the octahedral site or shifting the oxygen charge towards the redox‐active metal center in CoO6 octahedra.

The influence of NOx presence on the catalytic N2O decomposition over the supported double-promoted cobalt spinel catalyst

The supported double-promoted cobalt spinel catalyst has been tested for N2O decomposition in the model and real gas streams (tail gas) from nitric acid plant. Activity tests carried out in the model gas stream showed a high activity of the developed supported catalyst. However, during catalytic investigations performed under real industrial conditions it was found that this catalyst is very sensitive to NOx poisoning. The negative influence of these oxides is visible even at their low concentration. However, the stability tests performed in real nitric acid plant gas streams showed that the developed catalyst preserves its initial activity for at least 40 days. Therefore, the authors propose to apply it in a dual-bed catalytic system consisting of deNOx and deN2O catalyst beds. At the first stage, tail gas is purified from NOx in the NH3-SCR process in the presence of commercial catalyst and at the second stage, catalytic decomposition of N2O into N2 and O2 takes place over the developed supported cobalt spinel catalyst. The deNOx catalyst bed applied in the proposed dual-bed catalytic system allowed for the complete conversion of NOx from the tail gases. After installation of the supported cobalt spinel catalyst downstream of the deNOx catalyst bed, a significant increase in deN2O catalyst activity in the real tail gas stream was observed. Under optimal conditions, this system (F = 150 L/h; T = 425–450 °C) allowed for the complete reduction of N2O emission. It is shown that the proposed catalytic system is robust and allows to obtain high conversions of both NOx and N2O.Graphical abstract

Ambient temperature operable Bi-Co ferrite NO2 sensors with high sensitivity and selectivity

The sol-gel chemical synthesis, reaction mechanism, structural, morphological and elemental scanning measurements of bismuth (Bi3+)-doped spinel cobalt ferrite (Bi-Co ferrite) film sensors are explored. The gas sensor studies of Bi-Co ferrite sensors performed on methanol, ethanol, hydrogen sulphide, ammonia and nitrogen dioxide (NO2) gases at 230 °C as operating temperature where, Bi-Co ferrite sensors are extremely selective towards NO2 gas compared with other gases which is attributed to its high electron donating power. Furthermore, a small 31/29 s response/recovery time value and low detection limit of 25 ppm are evidently been proved its practical application potential. These results helped us to understand the sensing mechanism in Bi-Co ferrite sensors and provided a hint for tuning the conductivity/resistance of metal oxide semiconductor-based gas sensors.

Cu-Promoted Cobalt Oxide Film Catalyst for Efficient Gas Emissions Abatement

Thin film catalysts have been recently reported as promising catalysts owing to their good catalytic activity and reduced material amount, leading to low-cost efficient catalysts for gaseous emissions control. Here, we report the slight loading of Cu in cobalt spinel using a one-step pulsed-spray evaporation chemical vapor deposition (PSE-CVD) synthesis technique for efficient short-chain volatile organic compounds (VOCs) emissions treatment. Crystalline structure and morphology analyses revealed nano-crystallite sizes and open-like morphology. The catalytic performance was evaluated through the complete oxidation of C3H6, as a short-chain representative model of VOCs, at a high gas hourly space velocity (GHSV). Very good activity was obtained towards the complete abatement of C3H6 at low temperature and no carbon monoxide (CO) was formed during the oxidation process. Slightly-promoted Co3O4 catalyst with Cu introduction resulted in high catalytic activity comparing to the performance of the catalysts in the literature, due to the high dispersion of Cu and high active surface oxygen amount. Moreover, to evaluate the capability of the used catalysts under near realistic reaction conditions, CO2 effect on the catalytic activity was performed and the catalyst exhibited very good results. Thus, the adopted slightly-doping strategy to tailor a high active catalyst at low temperature could establish a very promising route to strongly enhance the activity of such other catalysts towards gas emissions abatement at low temperature.

Excess of cations in the sol-gel synthesis of cobalt ferrite (CoFe2O4): A pathway to switching the inversion degree of spinels

Abstract Spinel cobalt ferrites (CoFe2O4) with varying inversion degrees were prepared via a citrate-nitrate sol-gel method. The gels, prepared with different cation stoichiometries, were dried at 110 °C, further treated at 850 °C and subjected to structural and magnetic characterization. X-ray diffractograms of the samples confirm the successful synthesis of the spinel. The synthesis conditions displayed a strong influence over the positioning of the cations in the crystal structure of these ferrimagnets. Mossbauer spectra yield inversion degrees of approximately 1.0 and 0.0 for the samples prepared with excess cobalt and excess iron, respectively. These cationic arrangements are also confirmed via Raman spectroscopy, which shows a larger splitting of the A1g signal in the sample with the highest inversion degree. The magnetic character of the samples at room temperature does not agree with what should be expect from the net magnetic moments per unit cell. However, this phenomenon was explained by the different exchange integrals at play in each of the systems, as shown by the magnetic critical temperature of each arrangement, which differed by approximately 100 °C. The proposed pathway has proven to be an accurate method for completely switching the cationic arrangement of CoFe2O4, further enabling the application of this material in advanced magnetoresistive and spintronic devices.

Assessing the magnetic, cytotoxic and photocatalytic influence of incorporating Yb3+ or Pr3+ ions in cobalt–nickel ferrite

Yb3+- and Pr3+-substituted cobalt–nickel ferrite nanoparticles were prepared through a sol–gel auto-combustion procedure and the effects on the cytotoxic, photocatalytic and magnetic characteristics of the resulting compounds were evaluated. As an initial step, the structure, morphology and magnetic properties of the produced particles were assessed using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), UV–visible diffuse reflectance spectroscopy (DRS), photoluminescence (PL), and vibrating sample magnetometery (VSM) techniques. The crystalline size and nanoparticles size of prepared nanostructures was in the range of 100–180 and 90–220 nm based on XRD and SEM results. Using the XRD results it was found that the product had a spinel cobalt–nickel ferrite phase structure, also containing some PrFeO3 and YbFeO3 phases as impurities. VSM results exhibited that Co0.5Ni0.5Pr0.1Fe1.9O4 has higher magnetic parameters than Co0.5Ni0.5Yb0.1Fe1.9O4, yet but the latter enjoys enhanced photocatalytic activity in degrading methyl orange (MO) under ultraviolet (UV) light irradiation.

Structural, optical and magnetic properties of Tm3+ substituted cobalt spinel ferrites synthesized via sonochemical approach

Co-Tm nano-spinel ferrite with chemical formula CoTmxFe2−xO4 (0.0 ≤ x ≤ 0.08) NPs were prepared via sonochemical approach. X-ray powder diffraction patterns, microscopic images (SEM and TEM) and infrared spectra proved the formation of Co spinel ferrite. The effect of Tm3+ substituted on spinal structure was evaluated by lattice parameters, tetrahedral and octahedral bond length and cationic distribution. The band gap energy (Eg) of samples were estimated by performing UV–Vis percent diffuse reflectance (% DR) and applying the Kubelka-Munk theory. Eg values are in an interval between 1.33 eV and 1.64 eV. The analyses of magnetization were performed at room (300 K; RT) and low (10 K) temperatures. Different magnetic parameters including coercivity Hc, saturation magnetization Ms, remanence Mr, squareness ratio (SQR = Mr/Ms) and magnetic moment nB were deduced and discussed. The results showed superparamagnetic (SPM) nature at RT for x = 0.00 and 0.02 samples. However, the other products exhibit ferromagnetic (FM) nature. At 10 K, all synthesized NPs display FM behavior. An amazing increase in the magnitudes of Ms, Mr and Hc was observed at 10 K in comparison to RT, which is principally due to the reduced thermal fluctuations of magnetic moments at lower temperatures. The Tm3+ substitution affects considerably the magnetizations data. An enhancement in the Ms, Mr, and nB was detected on increasing the Tm3+ concentration. The SQR values at RT are found to be smaller than 0.5 postulating a single domain nature with uniaxial anisotropy for all produced ferrites. However, SQRs are in the range 0.66–0.76 at 10 K, suggesting the multi magnetic domain at low temperature, except the x = 0.02 product where the SQR = 0.47 indicating the single magnetic domain. The obtained magnetic results were investigated deeply with relation to structural and microstructural properties.

Oxygen species stabilized on the cobalt spinel nano-octahedra at various reaction conditions and their role in catalytic CO and CH4 oxidation, N2O decomposition and oxygen isotopic exchange

Periodic spin unrestricted DFT+U calculations joined with atomistic thermodynamics and model catalytic experiments (TPD-O2, 18O2/16O4 exchange, N2O decomposition, CO and CH4 oxidation) were used to study the structure, stability and reactivity of various surface oxygen species and oxygen vacancies, produced under different thermodynamic conditions on the (1 1 1) surface exposed by the cobalt spinel nanooctahedra. We thoroughly analyzed the stability of differently oxygen covered terminations starting from ΘO = 0.86 nm−2 (isolated monoatomic species) up to ΘO = 5.21 nm−2 (full oxygen monolayer). The constructed 3-dimensional thermodynamic (γ, T, pO2) surface redox state diagram revealed that in typical catalytic pressures of O2 (pO2/p° ∼ 0.01– 1) three principal states of the spinel surface, including a region associated with the presence of μ-superoxo CoO3c–O2–CoT3c and metal-oxo CoO3c-O species (T < 450– 500 °C), a bare surface region (450– 500 °C to 550–600 °C), and a region were surface oxygen vacancies appear (T > 600 °C) may be distinguished. This diagram was used for quantitative assignment of the experimental TPD-O2 desorption peaks, and also, for interpretation of the role that various adoxygen and lattice oxygen species play in the investigated model catalytic reactions. It was shown that CO is primarily oxidized by the suprafacial CoO3c-O2-CoT3c diatomic oxygen and/or cobalt-oxo CoO3c-O species, whereas in low temperature activation of CH4 only the CoO3c-O adducts are involved. With the increasing temperature, the suprafacial methane oxidation route gradually changes into the interfacial Mars van Krevelen scheme, and the later pathway is accelerated above 600 °C, due to the onset of oxygen vacancy formation (in line with 18O2/Co316O4 isotopic exchange experiments). In the case of N2O decomposition the reaction occurs preferably on a bare surface, and the favorable thermodynamics provides a steady driving force for efficient removal of the recombined adoxygen intermediates in the form of gaseous O2. The phase diagram of the alterable surface oxygen states on the (1 1 1) termination was compared to the corresponding diagram for the (1 0 0) facet, to explain the nature and differences in catalytic redox behavior of the cobalt spinel nano-octahedra and nano-cubes. This allows also for prediction of oxygen speciation anisotropy on cubo-octahedral nanocrystals at various temperatures.

Superparamagnetic Behavior of Zn and Al Substituted Cobalt Nanoferrites

Nanostructured Zn- and Al-doped cobalt spinel ferrites ZnxCo1-xFe2-xAlxO4 (0 ≤ x ≤ 1.0) with particle size in the range of 7–13 nm were synthesized by glycol-thermal reaction. XRD analysis confirmed single-phase cubic spinel structure with no impurity phases. The Mossbauer spectra for Zn- and Al-substituted fine powders could be resolved into two quadrupole doublets associated with 57Fe nuclei in paramagnetic spin phase. An unusual reducing magnetization with increasing particle size has been observed. The field cooled and zero field cooled magnetization measurements show superparamagnetic nanoparticles in the compounds investigated. The blocking temperatures (TB) are sensitive to particle size. Larger particles are blocked at higher temperatures compared to smaller particles at the same field.