Undoped Co3O4 Research Articles

Tuning of the physical properties by various transition metal doping in Co3O4: TM (TM = Ni, Mn, Cu) thin films: A comparative study

This paper presents the results of an experimental work devoted to the synthesis and the characterization of cobalt oxide (Co3O4) thin layers doped with transition metal (TM = Ni, Mn, Cu) in order to investigate their structural, optical and electrical properties. The materials were synthesized by the sol–gel method and deposited by dip-coating process. The synthesized samples were characterised by Ultraviolet-visible spectroscopy (UV–visible), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and Complex impedance spectroscopy to elucidate the optical, structural, vibrational and electrical properties. Our structural results show that the materials kept their cubic spinelle structure with preferred orientation of (311), Our optical results show that the transmittance of all doped samples decreases compared to undoped Co3O4 and the doping leads to a reduction of their band gap energies. The complementary phase information is provided by FT-IR spectroscopy. FT-IR spectra of TM-doped Co3O4 confirms the presence of Co2+−O and Co3+−O vibrations in the spinel lattice. The Nyquist plots suggest that the equivalent circuits (parallel circuit of Rp and Cp) of our films are not qualitatively affected by the doping, but it was found that the effective resistance Rp decreases whereas the effective capacitance Cp increases with doping.

Room temperature NO2 gas sensor based on porous Co3O4 slices/reduced graphene oxide hybrid

The porous flake-like Co3O4 and Co3O4/graphene hybrids with different graphene contents were prepared through a facile two-step method. The field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) characterizations proved the tight adherence of graphene sheets to the surface of porous Co3O4 slices. Gas sensors based on the as-synthesized products were then fabricated to explore their potential applications. The results indicated that the optimal hybrid exhibited a response of 26.8% to 5 ppm of NO2 at room temperature (RT), which was 2.27 times higher than that of undoped Co3O4 (100 °C). The hybrid sensor also showed fast response, excellent selectivity, long-term stability and extremely low detection limit toward NO2 at RT. The significantly enhanced sensing properties to NO2 could be attributed to larger specific surface area, more chemisorbed oxygen species and the coupling effect between Co3O4 and graphene in the hybrid. As we all know, as a typical p-type semiconductor, the gas-sensing properties of Co3O4 toward oxidizing gases including NO2 was rarely reported. We believe that our work could pave the way for the future design of Co3O4-based gas sensors for oxidizing gases detection at low or even room temperature.

Promotion effects of alkali- and alkaline earth metals on catalytic activity of mesoporous Co3O4 for 4-nitrophenol reduction

Mesoporous cobalt oxides doped with alkali (Li, Na, K, Cs) and alkaline earth (Mg, Ca) metals were synthesized and evaluated for their catalytic activity in the reduction of 4-nitrophenol. The prepared materials were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Brunauer–Emmet–Teller (BET) and hydrogen temperature programmed reduction (H2-TPR) analyses. The characterization techniques used showed the materials to consist of mono-dispersed nanoparticle aggregates with connected, well defined intra-particle voids and the crystalline phase of the cobalt oxide to be cubic Co3O4, while the pore diameters ranged from 12.1 to 19.2nm depending on the metal ion dopant. The reduction of 4-nitrophenol was chosen as a well-controlled model reaction allowing us to determine the catalytic activity as a function of alkali or alkaline earth dopant. Calcium doped cobalt oxide was found to be the most catalytically active with an apparent rate constant of 3.76×10−3s−1 and the order with respect to dopant was Ca>Cs>Mg>Li>K>Na. From characterization of the catalysts by SEM, TEM and H2-TPR promotion effects of the dopants were found to be due to electronic changes in the catalysts as a result of doping rather than structural changes. The kinetics of the most active calcium doped catalyst was modeled in terms of Langmuir–Hinshelwood kinetics. The Langmuir–Hinshelwood surface rate constant for Ca-doped Co3O4 was 5.47×10−5molm−2s−1 compared to the undoped Co3O4 at 5.33×10−6molm−2s−1. Activation energies were calculated to be 51.3kJmol−1 and 50.7kJmol−1 for undoped and Ca-doped Co3O4.

The design and synthesis of polyhedral Ti-doped Co3O4 with enhanced lithium-storage properties for Li-ion batteries

Polyhedral Ti-doped Co3O4 nanoparticles with a diameter of about 100–300 nm have been easily synthesized by a co-heat precipitated method. The structure and morphology of the materials were characterized by X-ray diffraction, field-emission scanning electron microscopy, and transmission electron microscopy. The electrochemical measurements were implemented on half coin cells. Galvanostatic charge, discharge performance, cyclic voltammetry and impedance measurement were utilized to investigate the electrochemical properties. The Ti-doped Co3O4 electrodes showed superior performance compared with the undoped Co3O4 electrodes, including the enhanced rate capability, and better capacity retention. At current densities of 500 mA g−1, the Ti-doped Co3O4 electrodes exhibited initial capacities of 1173.6 and 849.0 mAh g−1, and the capacities were maintained at 850.3 and 838.6 mAh g−1 after 120 cycles. These excellent electrochemical properties can be attributed to the nanoscale structure and Ti doping.

The Effect of the Preparation Method of Pd-Doped Cobalt Spinel on the Catalytic Activity in Methane Oxidation Under Lean Fuel Conditions

In this work a series of cobalt spinel catalysts was synthesized by different methods and doped with 1 wt% of palladium via incipient wetness impregnation or solution combustion synthesis (SCS). Both undoped and Pd-doped spinels were characterized from the structural and surface point of view (XRD, XRF, μRS, BET, FESEM) and tested toward the oxidation of methane in lean conditions, in gas mixtures containing from 0.5 to 2 vol% of CH4. These methane concentrations are typical of processes involving combustion of CH4 in lean conditions as the after-treatment of unconverted CH4 in compressed natural gas-fed vehicle exhausts, or the treatment of fugitive CH4 from leaks in coal mines. The temperature window for the catalytic activity of the undoped Co3O4 materials spread from 350 to 600 °C. The preparation method affected the catalytic activity of undoped Co3O4, with the best catalytic activity related to the spinel prepared by SCS employing urea. The addition of 1 wt% of Pd lowered the temperature window for the catalytic activity to the range of 250–500 °C, with insignificant differences for the various preparation methods. The beneficial effect of Pd-doping was assigned to the enhancement of the reduction of cobalt in the oxide structure.

Template‐Based Engineering of Carbon‐Doped Co3O4 Hollow Nanofibers as Anode Materials for Lithium‐Ion Batteries

Co3O4 anode materials exhibit poor conductivity and a large volume change, rendering controlling of their nanostructure essential to optimize their lithium storage performance. Carbon‐doped Co3O4 hollow nanofibers (C‐doped Co3O4 HNFs), for the first time are synthesized using bifunctional polymeric nanofibers as template and carbon source. Compared with undoped Co3O4 HNFs and solid Co3O4 NFs, C‐doped Co3O4 HNFs feature a remarkably high specific capacity, excellent cycling stability, and superior rate capacity as anode materials for lithium‐ion batteries. The superior performance of C‐doped Co3O4 HNFs electrodes can be attributed to their structural features, which confer enhanced electron transportation and Li+ ion diffusion due to C‐doping, and tolerance for volume change due to the 1D hollow structure. Density functional theory calculations provide a good explanation of the observed enhanced conductivity in C‐doped Co3O4 HNFs.

Undoped and Mn-doped Co3O4 nanorods for ethanol sensing

Undoped and Mn-doped (1 at.%) Co3O4 were synthesized by simple precipitation method. The morphology of the material was investigated by scanning electron microscopy which confirmed the formation of nanorods (NRs). The NRs were formed from interconnected nanoparticles. X-ray diffraction analysis confirmed the presence and crystalline nature of the Co3O4 NRs. FTIR studies revealed that the defects are present in undoped Co3O4. Raman spectra confirmed the presence of five Raman active modes and the doping of Mn shifts the Raman peaks to higher wave number. Spectral responses of clad modified fiber optic gas sensor are studied for various concentrations (0–500 ppm) of ethanol at room temperature, where the clad portion is replaced with undoped and Mn-doped Co3O4 and used as gas sensing material. Since the refractive index of the samples calculated from UV-Vis absorption spectra is greater than that of core, the fiber optic sensor works under leaky mode condition. The sensor showed linear variation in the spectral peak intensity with ethanol concentration. Undoped Co3O4 exhibits higher sensitivity compared to Mn-doped Co3O4 due to defects present in the undoped sample which offers more reactive sites for ethanol adsorption.

Effect of Pr, Sm, and Tb Doping on the Morphology, Crystallite Size, and N2O Decomposition Activity of Co3O4 Nanorods

Cobalt(II,III) oxide, Co3O4, is a promising catalyst for nitrous oxide direct decomposition. In this paper we report effect of doping with some rare earth (RE) elements (Pr, Sm, and Tb) on the morphology and crystallite size of Co3O4 nanorods. The various precursors (RE/Co oxalates) were prepared via the microwave assisted method and subsequent calcination. The decomposition pathway of these precursors was followed using thermogravimetric analysis (TGA). Based on thermal analysis results, Pr‐, Sm‐, and Tb‐doped Co3O4 samples were obtained via the calcination in static air at 500°C for their oxalate precursors. X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FT‐IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and hydrogen temperature programmed reduction (H2‐TPR) were used to characterize the RE‐doped cobalt oxide catalysts. The activity of the prepared catalysts was investigated for N2O direct decomposition and compared with that of the undoped Co3O4 catalyst. It was shown that the promoted Co3O4 catalysts revealed higher activity compared to the unpromoted one. The dependence of the activity on both the catalysts particle size and the reduction behaviour was discussed.

Promotive effect of CNT on Co3O4–SnO2 in a semiconductor-type CO sensor working at room temperature

Abstract Sensing materials of Co3O4–SnO2 system were prepared from CoOOH and tin(II) 2-ethylhexanoate in air. The prepared samples were characterized using X-ray diffraction (XRD), temperature program reduction (TPR), and thermo-gravimetric/differential thermal analysis (TG/DTA) techniques. The resulting Co3O4–SnO2 material was used as a semiconductor p-type CO sensor. The maximum sensor response to CO in the working temperature range from 25 to 100 °C was found at 25 °C. The mixture with a Co3O4–SnO2 ratio of 1:4 had the highest sensor response among various ratios of Co3O4–SnO2 at the working temperature. Doping 0.1% CNT into Co3O4–SnO2 enhanced the CO response of undoped Co3O4–SnO2. The sensor response varied under CO concentrations ranging from 20 to 1000 ppm. An explanation for the CO sensing characteristics of the sensor was proposed by referring to the adsorption and desorption processes of CO.