Elsevier

Chemical Engineering Journal

Volumes 204–206, 15 September 2012, Pages 158-168
Chemical Engineering Journal

Facile synthesis of CuO nanomorphs and their morphology dependent sunlight driven photocatalytic properties

https://doi.org/10.1016/j.cej.2012.07.012Get rights and content

Abstract

Cupric oxide (CuO) nanostructures having different morphologies such as spherical, vesicular, nanosheet and platelet were synthesized from copper sulfate in presence of polyvinyl pyrrolidone (PVP) and oxalic acid utilizing chemical precipitation and hydrothermal methods. Variation in reaction parameters such as synthesis method and the complexing agent on the morphology of the nanoparticles were investigated. These nanostructures were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), thermo–gravimetric–differential thermal analysis (TG/DTA), UV–Vis DR spectroscopy, energy dispersive X-ray analysis (EDAX), Fourier transform infra-red spectroscopy (FT-IR), BET specific surface area analysis and particle size distribution. Effort has been made to explain the plausible growth mechanism for the formation of morphological structures. The catalytic activities of as-prepared CuO nanostructures were evaluated by monitoring degradation of methylene blue. The sunlight driven photocatalytic degradation rates of methylene blue were found to be 72.57%, 93.48%, 49.71% and 95.71% for spherical, vesicular, sheet like and the platelet like morphologies respectively.

Highlights

► Spherical, vesicular, platelet type and sheet-like CuO nanomorphs are synthesized. ► PVP and oxalic acid mediated chemical precipitation and hydrothermal methods are employed for synthesis. ► CuO nanomorphs mediated sunlight driven photocatalysis of methylene blue is studied. ► Platelet like CuO showed highest photoactivity towards MB degradation.

Introduction

Nanocrystalline semiconductor particles have drawn considerable interest in recent years because of their special properties such as a large surface to volume ratio, increased activity, special electronic properties and unique optical properties compared to those of the bulk materials [1], [2]. The oxides of transition metals are an important class of semiconductors. Among these transition-metal oxides, copper oxide (CuO); one of the important p-type semiconductors with a narrow band gap of 1.4 eV [3] has attracted much attention. CuO has complex magnetic phases and forms the basis for several high-Tc superconductors and materials with giant magneto resistance [4], [5]. In addition, CuO is used as optical switch, pigment, fungicide, metallurgy reagent, gas sensor, magnetic storage media, field emission (FE) emitter and solar cells owing to its photoconductive and photochemical properties [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Recently, many studies have focused on its applications in photocatalysis. However, in most reports, CuO was used only as a sensitizer of a composite photocatalyst such as CuO/TiO2 [17], [18], [19], CuO/SnO2 [20], CuO/ZnO [21], CuO/BiVO4 [22], CuO/SiO2 [23], and CuO/zeolite [24], [25]. In these studies, the activity was not satisfactory when CuO alone was used as the photocatalyst in the control experiment. Since 2008, several groups [26], [27], [28], [29] have reported CuO of various morphologies used as the photocatalyst. The light sources were only limited to ultraviolet (UV) or ultraviolet–visible (UV–vis) light. Disadvantages of these methods are: the price of consuming scarce resources and generating pollutant emissions and waste, associated to the electricity needed. Moreover, some of the wavelengths involved are harmful and require protective measures during their use; this limits the utility of the photocatalyst. Sunlight is the key source of illumination to perform different photocatalytic reactions. Visible light is a major component of sunlight as compared to UV–vis light, which is only about 4% of total solar radiation. Due to this factor, development of a stable photocatalytic system, which should be effective in the presence of visible light, is indispensable from the view point of efficient utilization of sunlight.

Generally, the catalytic properties are affected by the morphology. This implies the importance of necessity of controlled morphological synthesis of CuO for enhanced practical applications. Similar to many functional semiconductors, many novel CuO micro/nanostructures such as networks [30], sheaf-like structures [31], nanoribbons [32], as well as caddice clew-like [33], rectangular nanobat-like [34] and peanut-like [35] structures have been fabricated by a range of techniques, including chemical vapor deposition, hydrothermal methods and some solution processes. Among the various synthesis methods utilized, hydrothermal and chemical precipitation methods are of special interest because they are safe and environmental-friendly processes performed at moderate temperatures without use of any expensive equipment.

In the present paper, we report synthesis of CuO nanostructures with different morphologies such as spherical, vesicular, nanosheet like and platelet, using chemical precipitation and hydrothermal methods. Polyvinyl pyrrolidone (PVP) and oxalic acid were used as the templates to control growth of CuO crystals. The feature of this work lies in its simplicity in controlling the growth of the various integrated nanostructures from single metal ion source using simple chemical routes. Further, the natural sunlight driven photocatalytic activities of as prepared CuO crystals were evaluated using methylene blue as a model dye contaminant. Plateletlike CuO nanostructures were found to have the best catalytic activity towards MB dye degradation.

Section snippets

Synthesis of CuO nanostructures

As summarized in Table 1, there were four major synthetic schemes used in the current work:

  • (a)

    3.319 g CuSO4⋅5H2O and 2 g PVP were dissolved separately in 25 ml of double distilled water. These two solutions were mixed together using magnetic stirrer at room temperature. The initial pH of the solution was 5.7. Then to this solution 10 ml of 30% hydrazine hydrate solution was added drop wise with constant stirring. The final pH of the solution was 8.5. The brown colored precipitate so formed was

Physicochemical characterization

The ability of the chemical precipitation and hydrothermal methods used to promote the formation of oxide nanostructures with low reaction temperature and ambient processing time was evaluated by characterizing the synthesized material using different characterization techniques. The crystallinity and the crystal phases of the synthesized materials were examined by X-ray diffraction (XRD) measured with Cu Kα radiation. Fig. 1 shows the XRD patterns of products obtained by four different methods.

Conclusion

CuO nanomorphs were synthesized using chemical precipitation and hydrothermal methods. Compared with the other methods such as sonochemical and sol–gel methods which require higher temperature and expensive equipments, this procedure does not require pressure controlling and higher temperature. By changing the reaction parameters such as, precipitating/reducing agent, complexing agent and the reaction temperature, CuO nanostructures of different morphologies can be grown. The properties of the

Acknowledgements

The authors would like to thank Mr. Palani Raja for experimental assistance.

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