Full Length ArticleEffect of ZnO core electrodeposition conditions on electrochemical and photocatalytic properties of polypyrrole-graphene oxide shelled nanoarrays
Introduction
Amongst the inorganic semiconductors, zinc oxide (ZnO) has been widely studied for potential application in functional devices thanks to the wide band gap, large exciton binding energy and transparency [1], [2]. Thanks to its unique structural anisotropy, various oriented nanostructures, including nanowires, nanorods, nanoplates, nanosheets, etc. can be fabricated and incorporated into devices [3], [4], [5]. The morphology-dependent properties of ZnO nanomaterials favorable in various applications such as electrochromics, sensing devices, or solar cells triggered the focus onto ZnO nanoarrays (NAs) with one-dimensional morphology because the efficiency of some applications is directly related to the surface area and orientation of the ZnO, e.g., improved electrolyte diffusion was reported for energy storage, fast response for electrochromic applications, improved sensing property due to increased surface area or enhanced performance of solar cells devices especially due to providing direct conduction pathway for electron transport [6]. The interest in fabrication of new ZnO nanostructures has been growing due to new applications relying on size and shape. Lately, pyramidal shape ZnO re-attracted interest in the solar cell community because of increased light scattering and simplicity of wet-chemistry approaches for which the key strategy is the tailoring the surface chemistry of the semipolar side facets during crystal growth.
On the other hand, surface engineering attracted much interest for boosting performance of ordered ZnO nanostructures, as it is commonly related to the presence of surface defects. Hybridization of ZnO with graphene may provide an ideal photocatalytic system to accelerate the charge transfer from photocatalyst to the liquid–solid interface by taking advantage of graphene’s unique electron transport property, superior chemical stability, high specific surface area and high transparency [7], [8], [9]. Many studies also reported enhanced interfacial charge transfer, minimized contact recombination, and improved collection capabilities in solar cells by modification of ZnO NAs with shells made of inorganic semiconductors [10], and various organic materials [11], [12]. The nanoarchitectures based on ZnO core and conducting polymer (CP) shells represent one of the most promising materials from both fundamental and technological view points as they exhibit attractive properties that favor their application in various fields such as energy storage, electronics, optics or catalysis [13], [14], [15], [16], [17]. One the main disadvantages of ZnO – the instability under illumination – can also be addressed by enveloping in CP shell [18]. The confinement in CPs or carbon shells was also indicated to boost the capacitance of metal oxides [19].
From the wealth of synthetic approaches including wet chemistry, magnetron sputtering, pulsed laser deposition, chemical vapor deposition, etc., the electrochemical deposition techniques have often been preferred for growing one-dimensional NAs of both ZnO and conducting polymers due to their low cost, mild conditions and accurate process control [20], [21]. Although synthesis of ZnO nanostructures has been widely explored, it is necessary to study the relationship between electrodeposition conditions and the growth of ZnO with well-controlled crystalline morphology for exploring a high effective and simple route to prepare novel nanostructures for practical applications. Moreover, in regards to ZnO/CP core/shell NAs, the studies till present involved ZnO NAs only as templates for the fabrication of oriented CP nanostructures [16], [17], [22], [23]. Thus, the research on ZnO/CP-rGO NAs lacks important understanding of the influence of hybrid interface on the capabilities and limitations of such NAs. On the other hand, graphene oxide (GO) could serve as a suitable filler for CP coatings thanks to its hydrophilicity and improved conductivity due to oxygen containing functional groups and further improve architecture performance [24].
Therefore, in this paper we report about the optimization of the properties of hybrid CP-GO coated ZnO NAs in relation to ZnO core morphology. Polypyrrole (PPy) was selected due to its many desirable features including low-cost, environmental stability and large scale processability [25]. The morphology of ZnO was tailored by electrodeposition from Zn(NO3)2 precursor in absence of a seed layer. As the ratio between the OH− generation rate and Zn2+ diffusion is the main parameter controlling the growth mechanism of ZnO from nitrate-based solutions, here the addition of NaNO3 and modulation of deposition potential and charge were carried on in order to induce varying morphologies. To the best of our knowledge, this is the first kind of report as an attempt to highlight the importance of adjusting the parameters of electrochemical procedures towards the tailoring of key properties of ZnO for enhancing properties of ZnO core/PPy-GO hybrid shell arrays.
Section snippets
Materials
The chemicals were purchased from Aldrich, Merck or Alfa Aesar and used as such. All solutions were prepared with double-distilled water. Indium-doped tin oxide (ITO) coated conducting glass slides (∼10 Ω/sq, Solar Energy Technology Co, Ltd, Wuhan Jinge, China) were degreased with detergent and consecutively cleansed in distilled water, acetone, and isopropanol before use by means of ultrasonic treatment.
Synthesis of GO
Graphite oxide was prepared by modified Hummers method as presented in our previous works
Electrochemical synthesis of the ZnO/PPy-GO core/hybrid shell NAs
Herein, Zn(NO3)2 was employed as a precursor of both the HO− ions and the Zn2+ ions and the growth of ZnO NAs was achieved in absence of a seed layer. The electrodeposition mechanism of ZnO from Zn(NO3)2 is based on the reduction of NO3− at the substrate resulting in the formation of HO− ions that react with Zn2+ ions to form Zn(OH)2 which is spontaneously dehydrated and forms ZnO, according to the equations:NO3− + H2O + 2e− → NO2− + 2OH−Zn2+ + 2OH− → Zn(OH)2 → ZnO + H2O
As the polar morphology-related surface
Conclusions
Hybrid ZnO/PPy-rGO core-shell nanoarchitectures were successfully fabricated by means of simple and fast electrochemical method. The FT-IR and XRD spectroscopy results indicated interactions between PPy chains with both ZnO and rGO sheets confirming successful organic interfacial modification. The electrochemical analysis indicated the electroactivity of the core/shell NAs improved with the crystallinity and c-axis oriented growth of ZnO nanorods tailored by increasing cathodic potential up to
Acknowledgements
The work described in this paper was supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (CityU 104512, CityU 7004388). AP acknowledges financial support from Romanian National Authority for Scientific Research and Innovation, CNCS – UEFISCDI (project number PN-II-RU-TE-2014-4-0806).
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