Nanostructure modulation of Co3O4 films by varying anion sources for pseudocapacitor applications
Introduction
Transition metal oxides (TMDs) are of broad attention owing to their advantage electronic and magnetic properties [1,2]. For achieving desired performance, the controlling methods of morphologies and nanostructure have been applied in various research fields [3,4]. Nanostructured materials have been attracted more and more interests for their physical and chemical properties, and widely applied in advanced devices [[5], [6], [7]]. Among them, the preparation and performance research of hierarchical and secondary nanostructure thin films have attracted a lot of attentions in recent years for their high specific surface area, increased the amount of functional materials and formation of heterojunction [8,9]. A lot of research has been devoted to developed of diverse hierarchical or secondary nanostructured TMDs thin films.
The pseudocapacitance of electrode material depends on the interfacial redox reactions and intercalations. High specific surface area and ultra-small size can provide abundant active sites to store charge in the first few nanometers of the surface. A large number of previous works showed that some TMDs composites using Co3O4 as secondary structure presents excellent electrochemical performance [[10], [11], [12], [13]]. For the addition of secondary nanostructure, the electrochemical performance of these composites were enhanced owing to the higher specific surface areas, content of functional materials increased and large number of heterojunctions. Therefore, effectively preparing a secondary nanostructured Co3O4 layer on the surface of electrode materials is an effective method to enhancing electrochemical performance. Cobalt (II, III) oxides (Co3O4) as a typical TMDs were widely applied in various fields for its low cost, environment friendly, nice corrosion stability, high theoretical specific capacitance and the interesting intercalative pseudocapacitance properties [14,15].
Various nanostructured Co3O4 were prepared by different methods [[16], [17], [18], [19], [20], [21], [22], [23]]. Among these various methods [[24], [25], [26], [27], [28], [29]], Chemical bath deposition (CBD) is an advantageous thin films deposition method for preparing metal oxide thin films on different substrates [30,31]. In addition, the preparation of well-aligned heterojunction structure and thin films by using CBD method showed more advantages, such as the low operating temperature and pressure condition. Compared to other methods, CBD method can preserve the original nanostructure of the substrate materials without being destroyed [32].
In this work, CBD was applied to directly prepare different nanostructures of Co3O4 films at low temperature by changing the cobalt sources with different anions (SO42−, NO3−, Cl− and CH3COO−). We mainly studied the electrochemical property of the different Co3O4 functional layer to looking for a best preparation method of Co3O4 function film. The Co3O4 thin films with different hierarchical and porous nanostructures were in situ deposited on the surface of Ni foam. The electrochemical performances of the four different as-prepared materials were characterized and investigated the role of anions on the morphological evaluation and pseudocapacitance performance. Among these four different morphologies of Co3O4 thin films, the one prepared from cobalt acetate (A-Co3O4) showed a honeycomb like mesoporous structure and exhibited the higher special capacitance (743.8 F·g−1) at current density of 1 A·g−1 in the 2 M KOH solution. The method using cobalt acetate derived Co3O4 thin films will be beneficial to the preparation of the secondary structure of Co3O4 to enhancing the electrochemical performance of the pseudocapacitor. This low-temperature chemical solution deposition procedure is not only easy to mass produce, but also provides a convenient method for growing secondary nanostructures on different electrode materials surface.
Section snippets
Chemical reagents
Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O), cobalt chloride hexahydrate (CoCl2·6H2O), cobalt sulfate heptahydrate (CoSO4·7H2O), acetone, alcohol, potassium hydroxide (KOH) and urea (CO(NH2)2) were all analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd. All these chemical reagents were used as purchased without further purification.
Preparation of modified electrode
The Co3O4 modified electrodes were prepared by using a facile CBD method. In the preparation
Structure and morphology of samples
Fig. 1 schematically displays the structural evolution process and formation mechanism of the various Co3O4 films. In this CBD process, the formation of nanostructured Co3O4 is followed by steps of nucleation, aggregation and coalescence of particles to form the different hierarchical thin film [33]. In this reaction process, urea is used as the hydrolysis controlling agent which continuously decomposes to form OH− ion as the temperature increasing. The main reaction process performs that the OH
Conclusions
In summary, four different thin Co3O4 films were prepared by the CBD method varying cobalt compounds with four different anions (SO42−, NO3−, Cl− and CH3COO−) as the precursors. And these hierarchical and porous thin Co3O4 films were in situ deposited on the surface of Ni foam, which can enhance the pseudocapacitance performance by increasing the active electrochemical materials. The anion groups and substrates have a significant effect on the morphology of Co3O4 thin film prepared. The
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgements
We are grateful to the support from the PhD Start-up Fund of Zhengzhou University of Light Industry (No. 2017BSJJ039).
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