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CuO@ZnO core-shell nanocomposites: Novel hydrothermal synthesis and enhancement in photocatalytic property
Accepted Manuscript CuO@ZnO core-shell nanocomposites: Novel hydrothermal synthesis and enhancement in photocatalytic property Mohammadreza Mansournia, Leila Ghaderi PII:
S0925-8388(16)32667-6
DOI:
10.1016/j.jallcom.2016.08.267
Reference:
JALCOM 38769
To appear in:
Journal of Alloys and Compounds
Received Date: 13 May 2016 Revised Date:
1 August 2016
Accepted Date: 26 August 2016
Please cite this article as: M. Mansournia, L. Ghaderi, CuO@ZnO core-shell nanocomposites: Novel hydrothermal synthesis and enhancement in photocatalytic property, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.08.267. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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CuO@ZnO Core-Shell Nanocomposites: Novel Hydrothermal Synthesis and Enhancement in Photocatalytic Property
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Mohammadreza Mansournia*, Leila Ghaderi Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P.O. Box 87317-53153, Kashan, I. R. Iran
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*Corresponding author. Tel.: +98 315 591 2339; fax: +98 315 591 2397,
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E-mail address: [email protected] Abstract
The efficient charge separation and interfacial charge transfer in the semiconductors are of great significance to photocatalytic performance. Herein, we report a facile and cost-effective
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synthesis of the CuO@ZnO core-shell nanocomposites with the aim to enhance the catalytic activity of ZnO in photodegradation of methylene blue (MB) under UV irradiation. Using a simple two-step hydrothermal method, CuO nanostructures with different thicknesses are
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grown on ZnO nanoparticles (0.4, 2, 10 and 50% with respect to the ratio of initial molar concentrations of copper to zinc). Significant improvement in the photocatalytic efficiency
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was observed with utilizing 0.4 and 2%CuO@ZnO nanocomposites compared to the pristine ZnO nanoparticles. The structural and optical properties of the as-prepared samples were investigated by XRD and DRS analysis, respectively. Further, the morphology of the assynthesized nanostructures was studied by FESEM and TEM imaging, and the high purity and the characteristic functional groups of the final products were also explored using EDX and FTIR spectroscopies, respectively. Keywords: Two-step hydrothermal, Core-shell, Nanocomposites, Photocatalysis
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ACCEPTED MANUSCRIPT 1. Introduction Environmental protection is one of the main topics of scientific societies that water treatment forms an important part of this topic. Photocatalytic purification of water has many
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advantages over the conventional procedures such as electrochemical oxidation, chlorination and oxidation by KMnO4-H2O2 [1]. Photocatalysis, a protuberant off-shoot of advanced oxidation processes, has emerged as one of the most efficient method for the complete mineralization of toxic organic pollutants [2-4]. The use of low-cost resources like
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authenticates its efficacy and economic viability [5-9].
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semiconductor materials, water and light photons for the production of O2•‾ and •OH further
Among different semiconductor nanomaterials, metal oxides including TiO2 and ZnO have been investigated intensively as photocatalysts because of their appropriate band gap (bulk band gaps of 3.4 and 3.2 eV, respectively), high photocatalytic activity, and stability against
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photocorrosion [5,10-18]. Especially, a few distinct advantages of ZnO such as the direct band gap, simple tailoring of structures, ease of crystallization, anisotropic growth, and higher exciton binding energy and electron mobility has given it an edge compared to TiO2
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[12-16]. Nevertheless, the low photocatalytic efficiency has been recognized as a major obstacle to the degradation of pollutant treatments in large-scale. To improve the
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photocatalytic efficiency of ZnO nanostructures, many efforts have been done by change its morphology [19,20], modification of ZnO by non-metal doping [21], addition of transition metals [22] as well as use of coupled semiconductors. Previous researches have demonstrated that the recombination rate of the photo-induced electron-hole pairs in ZnO plays an important role in the photocatalytic degradation process [23], which is usually faster than surface redox reactions. The coupled semiconductor materials have two types of energy-level systems which play a main role in achieving charge separation. Thus coupling of different semiconductor oxides is considered to be one of the most effective strategies to suppress the 2
ACCEPTED MANUSCRIPT electron-hole recombination, leading to high photocatalytic efficiency. As a significant p-type narrow bandgap semiconductor (1.7 eV), copper(II) oxide has been applied to increase the photocatalytic efficiency of some wide bandgap semiconductors [24-26]. In this study, with a view to improve the photocatalytic activity, ZnO nanoparticles were modified by coating
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them with CuO to form CuO@ZnO core-shell nanocomposites. The effect of increasing thickness of CuO layer on photocatalytic activity under UV light irradiation was optimized
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for the degradation of methylene blue (MB).
2.1. Materials and instruments
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2. Experimental
Zn(NO3).4H2O, Cu(NO3)2.3H2O, methylene blue (C16H18ClN3S), ammonia solution (NH3 25%) and absolute ethanol were purchased from Merck and used in the experiments without
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further purification. Powder X-ray diffraction (XRD) measurements were conducted on a X'Pert Pro X-ray diffractometer (Phillips company) using Ni-filtered Cu Kα radiation at scan
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range of 10 < 2θ <80 with a step time of 1 seconds and step size of 0.02°. Fourier transform infrared (FTIR) spectra were determined on a magna Nicolet 550 spectrophotometer in KBr
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pellets. A Zeiss ΣIGMA VP field-emission scanning electron microscope (FESEM), working at an accelerating voltage of 15.00 kV, was used to obtain the SEM images of the products in order to their morphological investigation. Compositional analysis was performed by an energy dispersive X-ray spectroscope (EDX) attached to the SEM microscope. The size distribution histograms of typical samples were provided by measuring the diameter of more than 100 particles on the SEM image by using Digimizer software. Transmission electron microscopy (TEM) images were taken on a Philips cm30 transmission electron microscope operated at 150 KV. The UV-Vis diffuse reflectance spectra (DRS) of samples were obtained 3
ACCEPTED MANUSCRIPT over an Ava spec-2048 TEC spectrophotometer and transformed to the absorption spectra according to the Tauc relationship.
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2.2. Synthesis of nanostructures The synthetic procedures were on the base of a modified and developed version of the method which we have recently reported for the preparation of ZnO nanostructures [27]. ZnO nanoparticles: In the beginning, 2.5 mmol of Zn(NO3).4H2O was dissolved in 25 mL
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distilled water (beaker B1). Subsequently, 10 mL of ammonia solution was transferred into
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other beaker (B2). In the next stage beakers B1 and B2 were placed in a Teflon-lined autoclave at room temperature for 15 min. Then the precipitate was separated by centrifugation, and washed with distilled water and absolute ethanol several times. Lastly to form pure ZnO nanoparticles, the as-prepared product was calcined at 150 °C in air for 6 h.
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CuO nanoplates: Briefly, two beakers, one containing 25 mL of 0.01 M Cu(NO3)2.3H2O aqueous solution and another containing 25 mL of the ammonia solution were put into a Teflon-lined autoclave at 70 °C for 1 h. The black product was separated and rinsed
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fine powder.
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repeatedly with distilled water and ethanol, respectively, and then dried at 75 °C to obtain the
CuO@ZnO nanocomposites: Two-step hydrothermal method was used to prepare CuO coated ZnO nanostructures. The coated samples loaded with 0.4, 2, 10, and 50% CuO due to the ratio of initial molar concentrations of copper to zinc were prepared. In a typical synthesis, for 0.4% CuO coated ZnO nanoparticles, the as-synthesized ZnO and 0.0125 mmol of Cu(NO3)2.3H2O were dispersed in 25 mL of distilled water and ultrasonicated for 30 min. Next, the beaker containing of above-mentioned solution, and another beaker with 10 mL of the ammonia solution were transferred to a Teflon autoclave. After hydrothermal reaction at
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2.3. Photocatalytic Measurements The photocatalytic activity of the as-made ZnO nanoparticles, CuO nanoplates, and CuO@ZnO core-shell nanocomposites were investigated by exposing 100 mL of 5 ppm of
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respective MB solution containing 20 mg of the catalyst with constant magnetic stirring (300 rpm) in a glass beaker under UV light source (a series of six Philips UV lamps 15 W having a
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center wavelength of 254 nm positioned at 20 cm a distant over the suspension surface) at ambient temperature and atmospheric pressure. In order to ensure adsorption/desorption equilibrium, the solution was stirred for 30 min in dark, prior to the irradiation. During the irradiation, samples were taken out every 15 min, centrifuged, and analyzed at 663 nm,
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corresponding to the maximum absorption wavelength of MB, using a single beam spectrophotometer (Spectronic 20D, Milton Roy). Moreover, the blank experiment in the
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absence of catalyst was done.
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3. Results and discussion
3.1. Structural and morphological study The comparison of the XRD patterns of the as-synthesized nanostructures is presented in Fig. 1. For the pristine ZnO, the diffraction peaks appeared at 2θ values of 31.9, 34.6, 36.4, 47.7, 56.8, 63.03, 66.5, 68.1, 69.2, 72.7 and 77.1 are matched with (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes of the hexagonal zinc oxide, respectively (JCPDS: 80-0075, space group: P63mc, cell constants: a = b = 3.2 Å, c = 5.2 Å). 5
ACCEPTED MANUSCRIPT Meanwhile the XRD pattern of copper oxide nanostructures exhibits the reflection lines at 2θ values of 32.5, 35.6, 38.8, 48.8, 53.6, 58.3, 61.6, 66.3, 68.1, 72.4 and 75.2 which can be wellindexed to the monoclinic phase of CuO with JCPDS card No. 74-1021, space group C2/c and cell constants of a = 4.6 Å, b = 3.4 Å, c = 5.1. No additional peaks are also detected in
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the XRD patterns of these synthesized ZnO and CuO nanostructures, confirming that the obtained samples are single phase and consequently of very high purity. Further, the average crystallite size (D) of the as-made ZnO and CuO are about 26 and 21 nm, respectively,
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evaluated from the XRD data of their main reflections through the Scherrer’s equation 1. (1)
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D = K λ / β cos θ
Where λ is the wavelength of Cu Kα radiation (1.5406 Å), θ and β are the Bragg’s angle and corrected half-width of the diffraction peak (in radian), respectively, and K is the so-called shape factor which usually equals to 0.89.
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As seen in Fig. 1, the XRD patterns of typical CuO@ZnO nanocomposites indicate all the above-mentioned reflections of hexagonal phase ZnO core but no a recognizable existence of the diffraction lines corresponded to CuO shell. For 50%CuO@ZnO sample, with increasing
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CuO thickness, the intensities of ZnO peaks are significantly decreased and the main
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reflection of monoclinic CuO at 2θ ≈ 39 is appeared. In lower percentages of copper, it seems that the XRD technique is not able to verify the presence of CuO, due to the small amount of this compound coated on zinc oxide core. In order to further prove the formation of CuO@ZnO core-shell nanocomposites, the energy dispersive X-ray spectroscopy (EDX) was used. This technique revealed that all the synthesized core-shells were consisted of the elements of zinc, copper and oxygen. For instance, the EDX spectrum of the core-shell nanostructures containing the smallest amount of copper (0.4%CuO@ZnO ) is depicted in Fig. 2.
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ACCEPTED MANUSCRIPT On the other hand, the present functional groups in products were investigated using FTIR spectroscopy. According to Fig. 3, the broad band centered on about 3435 cm-1 and the band at around 1625 cm-1 is attributed to the stretching and bending vibrations of the hydroxyl groups of adsorbed water, respectively. The absorption peak at around 1380 cm-1 corresponds
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to the stretching modes of nitrate anions. In all the spectra, the bands appearing in the range of 400-600 cm-1 are assigned to the metal-oxygen (M–O) stretching vibrations [27,28]. Therefore, FTIR results are in accordance with those of that were obtained from the XRD and
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EDX techniques.
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The size and morphology of the as-prepared products were characterized by field-emission scanning electron microscopy and transmission electron microscopy. Fig. 4 presents the FESEM images of ZnO and CuO nanostructures at different magnifications. High magnification images in Figs. 4a and 4b clearly show the particle morphology and plate-like structures organized from nanoparticles for ZnO and CuO, respectively. Moreover, Figs. 4c
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and 4d show the particle size distribution of these samples. It is observed that their particle sizes possess a narrow distribution. Also, the average particle diameter of ZnO and CuO are
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estimated to be about 110 and 25 nm, respectively. Additionally, Fig. 5 indicates the comparison of high resolution FESEM images of CuO coated ZnO nanostructures. According
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to the images of pure CuO and ZnO samples as mentioned above, Figs 5a, 5b and 5c reveal the homogeneous deposition of CuO at the surface of ZnO nanoparticles without the formation of individual CuO nanoplates, for the cases of 0.4, 2 and 10%CuO@ZnO nanocomposites. Whereas, the picture of the sample loaded with 50% CuO (Fig. 5d) shows the formation of CuO nanoplates along with the CuO@ZnO nanoparticles. Therefore, it is deduced that by increase in the Cu2+ concentration, the plate-like CuO nanostructures can be separately obtained. Furthermore, Fig. 6 shows typical TEM images of 10%CuO@ZnO
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ACCEPTED MANUSCRIPT sample. There is a contrast difference in this nanocomposite with a dark inner center and relative light edge, confirming its core-shell structure. The room temperature DRS spectrums for pristine ZnO nanoparticles, CuO nanoplates and
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typical core-shell nanocomposites are shown in Fig. 7. The absorption intensity in UV and visible regions is higher for pure CuO and CuO@ZnO core-shells in comparison with pure ZnO. The electronic transitions from the 2p orbitals of O2- (valence band) to metal ion (3d/4s orbitals, conduction band) are responsible for the aforementioned absorption in pristine ZnO
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and CuO. In fact, the lower energy transitions of O2- (2p) → Cu2+ (3d) compared to those of
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O2- (2p) → Zn2+ (3d/4s) increase the absorption in these regions. With the coating of CuO at the surface of zinc oxide, two closely spaced conduction bands are formed derived from the sharing of surface oxide entities between Cu2+ and Zn2+ surface states. Furthermore, the role of d-d transitions between the closely spaced Zn2+ and Cu2+ states for enhanced absorption
shell thickness.
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3.2. photocatalytic study
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cannot be neglected [29]. So, the magnitude of the absorption increases with increase in the
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In photocatalysis, a particular case is the photochemical oxidative process, where a semiconductor surface is activated by UV light to generate free active oxygen species radicals such as •OH, •O2‾ and •O2H. These species are regarded as the major contributors in the degradation process of organic contaminants. Generally, the photo-induced electron-hole pairs are separated from each other in ZnO under UV irradiation. The electrons (e-) transit from the valence band to the conduction band and leave positive holes (h+) in the valence band. After separation of electrons and holes, the dissolved oxygen (O2) adsorbed on ZnO surface will react with photo-induced electrons to form superoxide anion radical (•O2‾) and 8
ACCEPTED MANUSCRIPT the holes (h+) in the valence band will be positive enough to produce hydroxyl radicals from the adsorbed hydroxide ions and water molecules at the surface of catalyst [27,30,31]. The photocatalytic activity of bare ZnO nanoparticles, CuO nanoplates and different
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CuO@ZnO core-shell nanocomposites was assessed by investigating the degradation of MB aqueous solution. The results have been depicted in Fig. 8, where Aₒ is the absorption of MB aqueous solution before UV irradiation and A is its absorption after definite illumination times. As shown in Fig. 8a, the negligible change in the concentration of MB solution is
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occurred in the absence of catalyst (blank test) or in the presence of CuO nanoplates,
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whereas, the significant decrease of MB concentration is observed under UV light via using other catalysts. It was observed that the blue color of MB solution disappeared completely within 90 min illumination in the presence of the 0.4%CuO@ZnO and 2%CuO@ZnO nanocomposites. Fig. 8b compares the degradation efficiencies of all the photocatalysts at 90
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min, determined using the equation 2. Degradation (%) = 100 [1 – (A / Aₒ)]
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According to Figs. 8a and 8b, the order of degradation rate is as follows: CuO <
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50%CuO@ZnO < ZnO < 10%CuO@ZnO < 2%CuO@ZnO ~ 0.4%CuO@ZnO.
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The improved photocatalytic efficiency of CuO@ZnO compared to pure ZnO results from the significant charge separation and interfacial charge transfer. As illustrated in Fig. 9, the valence and conduction band edges for ZnO and CuO are located at +2.89 and -0.31 V, and +1.25 and +0.45 V, respectively. Generally, it can be visualized that the valence band of copper oxide layer on the surface of ZnO forms the trap centers for the photo-excited electrons in zinc oxide valence band. Also, the surface CuO adds an extra conduction band which augments the probability of the excited electron capture. Besides, as the Zn2+ and Cu2+ entities are closely spaced, the lifetime of the excited states is further augmented by the inert
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ACCEPTED MANUSCRIPT transfer of electrons between the two states. With increasing the surface coating, the direct absorption of photons by CuO shell to a fair reduction in the direct photon absorption by ZnO core.
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At moderate CuO coating (10%CuO@ZnO), with the possibility of transfer of excited electrons from the conduction band of ZnO to that of CuO as an energetically allowed process, the yield of the •O2‾ production is drastically reduced. Additionally, the energetically favored transfer of h+ from the valence band of ZnO to that of CuO affects the oxidation of
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adsorbed water and decreases the generation of •OH radicals. At higher CuO coating
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(50%CuO@ZnO), with the possibility of the majority of photons directly absorbed by CuO, the occurrence of reverse processes, transfer of e- and h+ from CuO to ZnO, is energetically forbidden. Therefore, further CuO coating leads to decreasing in the extent of generation of superoxide anion radicals (•O2‾) and the degradation activity of the catalyst [29,32-34]. Moreover, in the case of 50%CuO@ZnO, the numerous nanoplates of CuO are synthesized in
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addition to the goal core-shell nanocomposites, as found already in Fig. 5d. Therefore, its activity will be less than pure zinc oxide nanoparticles in photocatalytic degradation of MB.
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As a result, it is expected that the core-shell CuO@ZnO nanostructures with thinner layer of CuO (0.4 and 2%) indicate the best efficiencies in photocatalytic experiments (Fig. 8). On the
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other hand, the decreased efficiency of the catalysts with increasing surface layer of CuO revealed that the increased spectral response (Fig. 7) does not essentially mean the increased photocatalytic activity too.
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ACCEPTED MANUSCRIPT 4. Conclusions We successfully fabricated the highly efficient CuO@ZnO core-shell photocatalysts using the low-cost two-step hydrothermal method. The lower copper oxide coatings enhanced the yield
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of photocatalytic degradation process while the higher coatings were detrimental. In fact the as-prepared CuO@ZnO core-shell nanostructures with the lower thickness of CuO coating exhibit more effective separation and transport of photo-induced electron-hole pairs than
applications in organic pollutants degradation.
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Acknowledgments
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pristine ZnO nanoparticles, which can make them a promising candidates for photocatalytic
The authors are grateful to University of Kashan for supporting this work by Grant No. 363030/6.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1 XRD patterns of the as-prepared samples: ZnO, 10%CuO@ZnO, 50%CuO@ZnO and CuO.
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Fig. 2 EDX spectrum of a typical core-shell product (0.4%CuO@ZnO). Fig. 3 FTIR spectra of the as-synthesized ZnO, CuO and all of the core-shell samples.
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Fig. 4 FESEM images with different magnifications and size distribution histograms of the as-obtained ZnO nanoparticles (a and c) and CuO nanoplates (b and d), respectively.
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Fig. 5 FESEM images with different magnifications of the as-synthesized nanocomposites: 0.4% CuO@ZnO (a), 2% CuO@ZnO (b), 10% CuO@ZnO (c) and 50%CuO@ZnO (d). Fig. 6 TEM images of a typical nanocomposite (10% CuO@ZnO).
and CuO nanostructures.
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Fig. 7 Solid-state absorption spectra of the as-prepared ZnO, 2%CuO@ZnO, 50%CuO@ZnO
Fig. 8 Degradation rate of MB solution under UV light in the presence of different samples,
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versus irradiation time (a) at the time of 90 min (b).
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Fig. 9 Schematic diagram illustrating the valence and conduction bands and charge carrier transfer mechanism at moderate CuO coating ZnO nanoparticles
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Research highlights Facile two-step hydrothermal synthesis of the CuO@ZnO core-shell nanocomposites
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Significant improvement in catalytic activity of ZnO in photodegradation of MB
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Nanostructures with lower coatings (0.4 and 2%CuO) indicate the best catalytic yields
S0925-8388(16)32667-6
DOI:
10.1016/j.jallcom.2016.08.267
Reference:
JALCOM 38769
To appear in:
Journal of Alloys and Compounds
Received Date: 13 May 2016 Revised Date:
1 August 2016
Accepted Date: 26 August 2016
Please cite this article as: M. Mansournia, L. Ghaderi, CuO@ZnO core-shell nanocomposites: Novel hydrothermal synthesis and enhancement in photocatalytic property, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.08.267. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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CuO@ZnO Core-Shell Nanocomposites: Novel Hydrothermal Synthesis and Enhancement in Photocatalytic Property
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Mohammadreza Mansournia*, Leila Ghaderi Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P.O. Box 87317-53153, Kashan, I. R. Iran
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*Corresponding author. Tel.: +98 315 591 2339; fax: +98 315 591 2397,
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E-mail address: [email protected] Abstract
The efficient charge separation and interfacial charge transfer in the semiconductors are of great significance to photocatalytic performance. Herein, we report a facile and cost-effective
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synthesis of the CuO@ZnO core-shell nanocomposites with the aim to enhance the catalytic activity of ZnO in photodegradation of methylene blue (MB) under UV irradiation. Using a simple two-step hydrothermal method, CuO nanostructures with different thicknesses are
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grown on ZnO nanoparticles (0.4, 2, 10 and 50% with respect to the ratio of initial molar concentrations of copper to zinc). Significant improvement in the photocatalytic efficiency
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was observed with utilizing 0.4 and 2%CuO@ZnO nanocomposites compared to the pristine ZnO nanoparticles. The structural and optical properties of the as-prepared samples were investigated by XRD and DRS analysis, respectively. Further, the morphology of the assynthesized nanostructures was studied by FESEM and TEM imaging, and the high purity and the characteristic functional groups of the final products were also explored using EDX and FTIR spectroscopies, respectively. Keywords: Two-step hydrothermal, Core-shell, Nanocomposites, Photocatalysis
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ACCEPTED MANUSCRIPT 1. Introduction Environmental protection is one of the main topics of scientific societies that water treatment forms an important part of this topic. Photocatalytic purification of water has many
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advantages over the conventional procedures such as electrochemical oxidation, chlorination and oxidation by KMnO4-H2O2 [1]. Photocatalysis, a protuberant off-shoot of advanced oxidation processes, has emerged as one of the most efficient method for the complete mineralization of toxic organic pollutants [2-4]. The use of low-cost resources like
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authenticates its efficacy and economic viability [5-9].
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semiconductor materials, water and light photons for the production of O2•‾ and •OH further
Among different semiconductor nanomaterials, metal oxides including TiO2 and ZnO have been investigated intensively as photocatalysts because of their appropriate band gap (bulk band gaps of 3.4 and 3.2 eV, respectively), high photocatalytic activity, and stability against
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photocorrosion [5,10-18]. Especially, a few distinct advantages of ZnO such as the direct band gap, simple tailoring of structures, ease of crystallization, anisotropic growth, and higher exciton binding energy and electron mobility has given it an edge compared to TiO2
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[12-16]. Nevertheless, the low photocatalytic efficiency has been recognized as a major obstacle to the degradation of pollutant treatments in large-scale. To improve the
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photocatalytic efficiency of ZnO nanostructures, many efforts have been done by change its morphology [19,20], modification of ZnO by non-metal doping [21], addition of transition metals [22] as well as use of coupled semiconductors. Previous researches have demonstrated that the recombination rate of the photo-induced electron-hole pairs in ZnO plays an important role in the photocatalytic degradation process [23], which is usually faster than surface redox reactions. The coupled semiconductor materials have two types of energy-level systems which play a main role in achieving charge separation. Thus coupling of different semiconductor oxides is considered to be one of the most effective strategies to suppress the 2
ACCEPTED MANUSCRIPT electron-hole recombination, leading to high photocatalytic efficiency. As a significant p-type narrow bandgap semiconductor (1.7 eV), copper(II) oxide has been applied to increase the photocatalytic efficiency of some wide bandgap semiconductors [24-26]. In this study, with a view to improve the photocatalytic activity, ZnO nanoparticles were modified by coating
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them with CuO to form CuO@ZnO core-shell nanocomposites. The effect of increasing thickness of CuO layer on photocatalytic activity under UV light irradiation was optimized
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for the degradation of methylene blue (MB).
2.1. Materials and instruments
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2. Experimental
Zn(NO3).4H2O, Cu(NO3)2.3H2O, methylene blue (C16H18ClN3S), ammonia solution (NH3 25%) and absolute ethanol were purchased from Merck and used in the experiments without
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further purification. Powder X-ray diffraction (XRD) measurements were conducted on a X'Pert Pro X-ray diffractometer (Phillips company) using Ni-filtered Cu Kα radiation at scan
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range of 10 < 2θ <80 with a step time of 1 seconds and step size of 0.02°. Fourier transform infrared (FTIR) spectra were determined on a magna Nicolet 550 spectrophotometer in KBr
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pellets. A Zeiss ΣIGMA VP field-emission scanning electron microscope (FESEM), working at an accelerating voltage of 15.00 kV, was used to obtain the SEM images of the products in order to their morphological investigation. Compositional analysis was performed by an energy dispersive X-ray spectroscope (EDX) attached to the SEM microscope. The size distribution histograms of typical samples were provided by measuring the diameter of more than 100 particles on the SEM image by using Digimizer software. Transmission electron microscopy (TEM) images were taken on a Philips cm30 transmission electron microscope operated at 150 KV. The UV-Vis diffuse reflectance spectra (DRS) of samples were obtained 3
ACCEPTED MANUSCRIPT over an Ava spec-2048 TEC spectrophotometer and transformed to the absorption spectra according to the Tauc relationship.
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2.2. Synthesis of nanostructures The synthetic procedures were on the base of a modified and developed version of the method which we have recently reported for the preparation of ZnO nanostructures [27]. ZnO nanoparticles: In the beginning, 2.5 mmol of Zn(NO3).4H2O was dissolved in 25 mL
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distilled water (beaker B1). Subsequently, 10 mL of ammonia solution was transferred into
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other beaker (B2). In the next stage beakers B1 and B2 were placed in a Teflon-lined autoclave at room temperature for 15 min. Then the precipitate was separated by centrifugation, and washed with distilled water and absolute ethanol several times. Lastly to form pure ZnO nanoparticles, the as-prepared product was calcined at 150 °C in air for 6 h.
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CuO nanoplates: Briefly, two beakers, one containing 25 mL of 0.01 M Cu(NO3)2.3H2O aqueous solution and another containing 25 mL of the ammonia solution were put into a Teflon-lined autoclave at 70 °C for 1 h. The black product was separated and rinsed
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fine powder.
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repeatedly with distilled water and ethanol, respectively, and then dried at 75 °C to obtain the
CuO@ZnO nanocomposites: Two-step hydrothermal method was used to prepare CuO coated ZnO nanostructures. The coated samples loaded with 0.4, 2, 10, and 50% CuO due to the ratio of initial molar concentrations of copper to zinc were prepared. In a typical synthesis, for 0.4% CuO coated ZnO nanoparticles, the as-synthesized ZnO and 0.0125 mmol of Cu(NO3)2.3H2O were dispersed in 25 mL of distilled water and ultrasonicated for 30 min. Next, the beaker containing of above-mentioned solution, and another beaker with 10 mL of the ammonia solution were transferred to a Teflon autoclave. After hydrothermal reaction at
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2.3. Photocatalytic Measurements The photocatalytic activity of the as-made ZnO nanoparticles, CuO nanoplates, and CuO@ZnO core-shell nanocomposites were investigated by exposing 100 mL of 5 ppm of
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respective MB solution containing 20 mg of the catalyst with constant magnetic stirring (300 rpm) in a glass beaker under UV light source (a series of six Philips UV lamps 15 W having a
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center wavelength of 254 nm positioned at 20 cm a distant over the suspension surface) at ambient temperature and atmospheric pressure. In order to ensure adsorption/desorption equilibrium, the solution was stirred for 30 min in dark, prior to the irradiation. During the irradiation, samples were taken out every 15 min, centrifuged, and analyzed at 663 nm,
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corresponding to the maximum absorption wavelength of MB, using a single beam spectrophotometer (Spectronic 20D, Milton Roy). Moreover, the blank experiment in the
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absence of catalyst was done.
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3. Results and discussion
3.1. Structural and morphological study The comparison of the XRD patterns of the as-synthesized nanostructures is presented in Fig. 1. For the pristine ZnO, the diffraction peaks appeared at 2θ values of 31.9, 34.6, 36.4, 47.7, 56.8, 63.03, 66.5, 68.1, 69.2, 72.7 and 77.1 are matched with (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes of the hexagonal zinc oxide, respectively (JCPDS: 80-0075, space group: P63mc, cell constants: a = b = 3.2 Å, c = 5.2 Å). 5
ACCEPTED MANUSCRIPT Meanwhile the XRD pattern of copper oxide nanostructures exhibits the reflection lines at 2θ values of 32.5, 35.6, 38.8, 48.8, 53.6, 58.3, 61.6, 66.3, 68.1, 72.4 and 75.2 which can be wellindexed to the monoclinic phase of CuO with JCPDS card No. 74-1021, space group C2/c and cell constants of a = 4.6 Å, b = 3.4 Å, c = 5.1. No additional peaks are also detected in
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the XRD patterns of these synthesized ZnO and CuO nanostructures, confirming that the obtained samples are single phase and consequently of very high purity. Further, the average crystallite size (D) of the as-made ZnO and CuO are about 26 and 21 nm, respectively,
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evaluated from the XRD data of their main reflections through the Scherrer’s equation 1. (1)
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D = K λ / β cos θ
Where λ is the wavelength of Cu Kα radiation (1.5406 Å), θ and β are the Bragg’s angle and corrected half-width of the diffraction peak (in radian), respectively, and K is the so-called shape factor which usually equals to 0.89.
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As seen in Fig. 1, the XRD patterns of typical CuO@ZnO nanocomposites indicate all the above-mentioned reflections of hexagonal phase ZnO core but no a recognizable existence of the diffraction lines corresponded to CuO shell. For 50%CuO@ZnO sample, with increasing
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CuO thickness, the intensities of ZnO peaks are significantly decreased and the main
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reflection of monoclinic CuO at 2θ ≈ 39 is appeared. In lower percentages of copper, it seems that the XRD technique is not able to verify the presence of CuO, due to the small amount of this compound coated on zinc oxide core. In order to further prove the formation of CuO@ZnO core-shell nanocomposites, the energy dispersive X-ray spectroscopy (EDX) was used. This technique revealed that all the synthesized core-shells were consisted of the elements of zinc, copper and oxygen. For instance, the EDX spectrum of the core-shell nanostructures containing the smallest amount of copper (0.4%CuO@ZnO ) is depicted in Fig. 2.
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ACCEPTED MANUSCRIPT On the other hand, the present functional groups in products were investigated using FTIR spectroscopy. According to Fig. 3, the broad band centered on about 3435 cm-1 and the band at around 1625 cm-1 is attributed to the stretching and bending vibrations of the hydroxyl groups of adsorbed water, respectively. The absorption peak at around 1380 cm-1 corresponds
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to the stretching modes of nitrate anions. In all the spectra, the bands appearing in the range of 400-600 cm-1 are assigned to the metal-oxygen (M–O) stretching vibrations [27,28]. Therefore, FTIR results are in accordance with those of that were obtained from the XRD and
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EDX techniques.
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The size and morphology of the as-prepared products were characterized by field-emission scanning electron microscopy and transmission electron microscopy. Fig. 4 presents the FESEM images of ZnO and CuO nanostructures at different magnifications. High magnification images in Figs. 4a and 4b clearly show the particle morphology and plate-like structures organized from nanoparticles for ZnO and CuO, respectively. Moreover, Figs. 4c
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and 4d show the particle size distribution of these samples. It is observed that their particle sizes possess a narrow distribution. Also, the average particle diameter of ZnO and CuO are
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estimated to be about 110 and 25 nm, respectively. Additionally, Fig. 5 indicates the comparison of high resolution FESEM images of CuO coated ZnO nanostructures. According
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to the images of pure CuO and ZnO samples as mentioned above, Figs 5a, 5b and 5c reveal the homogeneous deposition of CuO at the surface of ZnO nanoparticles without the formation of individual CuO nanoplates, for the cases of 0.4, 2 and 10%CuO@ZnO nanocomposites. Whereas, the picture of the sample loaded with 50% CuO (Fig. 5d) shows the formation of CuO nanoplates along with the CuO@ZnO nanoparticles. Therefore, it is deduced that by increase in the Cu2+ concentration, the plate-like CuO nanostructures can be separately obtained. Furthermore, Fig. 6 shows typical TEM images of 10%CuO@ZnO
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ACCEPTED MANUSCRIPT sample. There is a contrast difference in this nanocomposite with a dark inner center and relative light edge, confirming its core-shell structure. The room temperature DRS spectrums for pristine ZnO nanoparticles, CuO nanoplates and
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typical core-shell nanocomposites are shown in Fig. 7. The absorption intensity in UV and visible regions is higher for pure CuO and CuO@ZnO core-shells in comparison with pure ZnO. The electronic transitions from the 2p orbitals of O2- (valence band) to metal ion (3d/4s orbitals, conduction band) are responsible for the aforementioned absorption in pristine ZnO
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and CuO. In fact, the lower energy transitions of O2- (2p) → Cu2+ (3d) compared to those of
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O2- (2p) → Zn2+ (3d/4s) increase the absorption in these regions. With the coating of CuO at the surface of zinc oxide, two closely spaced conduction bands are formed derived from the sharing of surface oxide entities between Cu2+ and Zn2+ surface states. Furthermore, the role of d-d transitions between the closely spaced Zn2+ and Cu2+ states for enhanced absorption
shell thickness.
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3.2. photocatalytic study
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cannot be neglected [29]. So, the magnitude of the absorption increases with increase in the
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In photocatalysis, a particular case is the photochemical oxidative process, where a semiconductor surface is activated by UV light to generate free active oxygen species radicals such as •OH, •O2‾ and •O2H. These species are regarded as the major contributors in the degradation process of organic contaminants. Generally, the photo-induced electron-hole pairs are separated from each other in ZnO under UV irradiation. The electrons (e-) transit from the valence band to the conduction band and leave positive holes (h+) in the valence band. After separation of electrons and holes, the dissolved oxygen (O2) adsorbed on ZnO surface will react with photo-induced electrons to form superoxide anion radical (•O2‾) and 8
ACCEPTED MANUSCRIPT the holes (h+) in the valence band will be positive enough to produce hydroxyl radicals from the adsorbed hydroxide ions and water molecules at the surface of catalyst [27,30,31]. The photocatalytic activity of bare ZnO nanoparticles, CuO nanoplates and different
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CuO@ZnO core-shell nanocomposites was assessed by investigating the degradation of MB aqueous solution. The results have been depicted in Fig. 8, where Aₒ is the absorption of MB aqueous solution before UV irradiation and A is its absorption after definite illumination times. As shown in Fig. 8a, the negligible change in the concentration of MB solution is
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occurred in the absence of catalyst (blank test) or in the presence of CuO nanoplates,
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whereas, the significant decrease of MB concentration is observed under UV light via using other catalysts. It was observed that the blue color of MB solution disappeared completely within 90 min illumination in the presence of the 0.4%CuO@ZnO and 2%CuO@ZnO nanocomposites. Fig. 8b compares the degradation efficiencies of all the photocatalysts at 90
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min, determined using the equation 2. Degradation (%) = 100 [1 – (A / Aₒ)]
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According to Figs. 8a and 8b, the order of degradation rate is as follows: CuO <
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50%CuO@ZnO < ZnO < 10%CuO@ZnO < 2%CuO@ZnO ~ 0.4%CuO@ZnO.
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The improved photocatalytic efficiency of CuO@ZnO compared to pure ZnO results from the significant charge separation and interfacial charge transfer. As illustrated in Fig. 9, the valence and conduction band edges for ZnO and CuO are located at +2.89 and -0.31 V, and +1.25 and +0.45 V, respectively. Generally, it can be visualized that the valence band of copper oxide layer on the surface of ZnO forms the trap centers for the photo-excited electrons in zinc oxide valence band. Also, the surface CuO adds an extra conduction band which augments the probability of the excited electron capture. Besides, as the Zn2+ and Cu2+ entities are closely spaced, the lifetime of the excited states is further augmented by the inert
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ACCEPTED MANUSCRIPT transfer of electrons between the two states. With increasing the surface coating, the direct absorption of photons by CuO shell to a fair reduction in the direct photon absorption by ZnO core.
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At moderate CuO coating (10%CuO@ZnO), with the possibility of transfer of excited electrons from the conduction band of ZnO to that of CuO as an energetically allowed process, the yield of the •O2‾ production is drastically reduced. Additionally, the energetically favored transfer of h+ from the valence band of ZnO to that of CuO affects the oxidation of
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adsorbed water and decreases the generation of •OH radicals. At higher CuO coating
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(50%CuO@ZnO), with the possibility of the majority of photons directly absorbed by CuO, the occurrence of reverse processes, transfer of e- and h+ from CuO to ZnO, is energetically forbidden. Therefore, further CuO coating leads to decreasing in the extent of generation of superoxide anion radicals (•O2‾) and the degradation activity of the catalyst [29,32-34]. Moreover, in the case of 50%CuO@ZnO, the numerous nanoplates of CuO are synthesized in
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addition to the goal core-shell nanocomposites, as found already in Fig. 5d. Therefore, its activity will be less than pure zinc oxide nanoparticles in photocatalytic degradation of MB.
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As a result, it is expected that the core-shell CuO@ZnO nanostructures with thinner layer of CuO (0.4 and 2%) indicate the best efficiencies in photocatalytic experiments (Fig. 8). On the
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other hand, the decreased efficiency of the catalysts with increasing surface layer of CuO revealed that the increased spectral response (Fig. 7) does not essentially mean the increased photocatalytic activity too.
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ACCEPTED MANUSCRIPT 4. Conclusions We successfully fabricated the highly efficient CuO@ZnO core-shell photocatalysts using the low-cost two-step hydrothermal method. The lower copper oxide coatings enhanced the yield
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of photocatalytic degradation process while the higher coatings were detrimental. In fact the as-prepared CuO@ZnO core-shell nanostructures with the lower thickness of CuO coating exhibit more effective separation and transport of photo-induced electron-hole pairs than
applications in organic pollutants degradation.
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Acknowledgments
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pristine ZnO nanoparticles, which can make them a promising candidates for photocatalytic
The authors are grateful to University of Kashan for supporting this work by Grant No. 363030/6.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1 XRD patterns of the as-prepared samples: ZnO, 10%CuO@ZnO, 50%CuO@ZnO and CuO.
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Fig. 2 EDX spectrum of a typical core-shell product (0.4%CuO@ZnO). Fig. 3 FTIR spectra of the as-synthesized ZnO, CuO and all of the core-shell samples.
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Fig. 4 FESEM images with different magnifications and size distribution histograms of the as-obtained ZnO nanoparticles (a and c) and CuO nanoplates (b and d), respectively.
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Fig. 5 FESEM images with different magnifications of the as-synthesized nanocomposites: 0.4% CuO@ZnO (a), 2% CuO@ZnO (b), 10% CuO@ZnO (c) and 50%CuO@ZnO (d). Fig. 6 TEM images of a typical nanocomposite (10% CuO@ZnO).
and CuO nanostructures.
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Fig. 7 Solid-state absorption spectra of the as-prepared ZnO, 2%CuO@ZnO, 50%CuO@ZnO
Fig. 8 Degradation rate of MB solution under UV light in the presence of different samples,
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versus irradiation time (a) at the time of 90 min (b).
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Fig. 9 Schematic diagram illustrating the valence and conduction bands and charge carrier transfer mechanism at moderate CuO coating ZnO nanoparticles
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Research highlights Facile two-step hydrothermal synthesis of the CuO@ZnO core-shell nanocomposites
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Significant improvement in catalytic activity of ZnO in photodegradation of MB
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Nanostructures with lower coatings (0.4 and 2%CuO) indicate the best catalytic yields