J Electroceram DOI 10.1007/s10832-013-9846-4
Facile green synthesis, optical and photocatalytic properties of zinc oxide nanosheets via microwave assisted hydrothermal technique Faten Al-Hazmi & Nadia Abdel Aal & Ahmed A. Al-Ghamdi & F. Alnowaiser & Zarah H. Gafer & Abdullah G. Al-Sehemi & Farid El-Tantawy & F. Yakuphanoglu
Received: 2 May 2013 / Accepted: 10 July 2013 # Springer Science+Business Media New York 2013
Abstract Novel hexagonal two dimensional ZnO nanosheets were successfully and economically synthesized using zinc acetate and urea based on a facile microwave hydrothermal method. The structure, morphology and size of the ZnO nanosheets were investigated by X-ray diffraction (X-ray), field emission scanning electron microscopy (FESEM), energy dispersive analysis of x-ray (EDS), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and Fourier transform infrared spectroscopy (FTIR). X-ray analysis showed that the obtained ZnO nanosheets are crystalline corresponding to the pure ZnO phase with an average particle size of 12 nm. Optical properties of ZnO nanosheets were investigated
F. Al-Hazmi : A. A. Al-Ghamdi : F. Yakuphanoglu (*) Faculty of Science, Department of Physics, King Abdulaziz University, P. O. 80203, Jeddah 21589, Saudi Arabia e-mail:
[email protected] N. A. Aal Department of Chemistry, Faculty of Science, Suez Canal University, Ismailia, Egypt F. Alnowaiser Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia Z. H. Gafer : A. G. Al-Sehemi Chemistry Department, Faculty of Science, King Khalid University, Abha, KSA, Saudi Arabia F. El-Tantawy Department of Physics, Faculty of Science, Suez Canal University, Ismailia, Egypt F. Yakuphanoglu Department of Physics, Faculty of Science, Firat University, Elazig 23169, Turkey
by UV-Vis absorption and photoluminescence (PL) techniques. The band gap energy of ZnO nanosheets was found to be 3.29 eV. The photoluminescence (PL) measurement shows a strong UV emission, blue emission and blue-green emission bands. ZnO nano sheets possess a higher photocatalytic activity leading to the degradation of methylene blue (MB). The ZnO nanosheets are expected to have new opportunities in vast research areas and for application in catalysts and optoelectronic devices. Keywords ZnO nanosheets . Microwave hydrothermal process . Microstructure . Optical and photo catalytic properties
1 Introduction Nanostructure semiconductor metal oxides have been received much attention owing to their size and morphology and to their distinct and novel physical and chemical properties that differ from their bulk counterparts [1–4]. Zinc oxide (ZnO) is a transparent n-type semiconductor which exhibits a wide band gap of 3.37 eV at room temperature equivalent to that of GaN with a large exciton binding energy of 60 meV and efficient ultraviolet luminescence [4–10]. Nanostructure ZnO is one of semiconductor metal oxides , which indicates the quantum confinement effects in the experimentally accessible size [5]. Due to its fascinating physical and chemical features, ZnO finds a vast range of applications as photonic material in the blue-UV region, gas sensor devices, additive to concrete and rubber tires, white pigment in paints and glazes, UV blocker in sun creams, anti-inflammatory component in creams and ointments, biomedicine, electronics, antibacterial agents, piezoelectricity transducer, photocatalysts,
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high radiation resistance, solar cell windows, varistors, cathode—ray phosphor and more [1–9]. Low dimensional semiconducting oxides nanostructures such as nanotubes, nanowires, nanosheets, nanoribbon rods, helices, combs, tetrapods, and so on, have successfully yielded attracted extraordinary attention for their potential applications in device and interconnect integration in nanoelectronics and molecular electronics [1–5]. Among them, two-dimensional nanosheets, which are an important category of nanostructured materials, are expected to represent important building blocks for nanoscale devices possessing various interesting functions. In fact, the synthesis of low-dimensional metal oxides has employed harsh synthetic routes such as high temperatures, low pressures, and the use of costly equipments [6]. Various methods such as vapor liquid—solid, template-assisted, colloidal micellar, electrochemical processes and others have been developed for the synthesis of one-dimensional nano materials [2–4]. Recently, demands for new green synthesis strategies have become key issues in material chemistry to control the morphology and understand the growth mechanisms of nanostructures. Microwave heating is concerned as a promising route for fast bulk heating which can result in high reaction rates in short reaction time and an increase in the product yield in comparison with conventional heating routes [8, 9]. Therefore, microwave hydrothermal method is found to be an important method for synthesis of nanostructures with uniform particle size distribution and versatile morphology [10, 11]. To ameliorate the synthesis problem of low dimensional nanostructures, the aim of our study is to synthesize two dimensional ZnO nanosheets by a facile, green, and cost effective approach through microwave assisted technique using zinc acetate and urea. The optical and photocatalytic degradation properties of synthesized ZnO nanosheets were investigated in detail.
2 Experimental details 2.1 Synthesis of ZnO nanosheets The raw materials are analytic grade reagents and purchased without further treatment. For the synthesis for ZnO nanosheets, 0.6 mol of zinc acetate dihydrate Zn(CH3COO)2 ⋅ 2H2O and 0.4 mol urea were dissolved in 50 ml distilled water. The mixture was magnetically stirred vigorously for 10 min and then transferred into Teflon-lined autoclave of 50 mL capacity. The autoclave was sealed and kept at 220 °C in microwave oven for 15 min, and then allowed to cool down to room temperature naturally. The obtained solid white precipitates were filtered and rinsed with water and absolute alcohol several times. Finally, after drying at 80 °C overnight, the white powders were calcined at 400 °C for 90 min.
2.2 Physical techniques The structure of the synthesized ZnO nanosheets was analyzed by X-ray powder diffraction (X-ray) using a Shimatzou X-ray diffractometer (Shimatzou, XRD-6000), with Cukα radiation and wavelength of 0.15147 nm, working at 30 mA and 20 kV. The phases were identified using the JCPDS database. Fourier transform infrared spectra (FTIR) were obtained on KBr pellets at room temperature using a Bruker FTIR spectrometer (TENSOR 37). The morphology and particle size of the synthesized ZnO nanosheets were examined by means of field emission scanning electron microscope (FSEM) and transmission electron microscopy (TEM) using a JEOL EM- 2100 F) operated at 200 kV. The optical absorbance was measured using a Lambda 35, Perkin-Elmer double beam UV-visible absorption spectrometer. Photoluminescence (PL) measurements were carried out at room temperature excited by a He-Cd laser operating at a wavelength of 325 nm with 40 mW. The spectra were resolved by a mono chrometer (SPEX 500 M) and recorded by a luminescence spectrometer (LS/55, PerkinElmer). The photocatalytic tests on the ZnO nanosheets for decomposition of methylene blue (MB) solution were examined at room temperature. The photocatalytic experiments were carried out as the following: 0.05 g of ZnO catalyst was placed in erlenmeyer flask (photoreactor vessels) which contained measured volume of methylene blue (MB) aqueous solution (200 ml of 1×10−5 M). Then, the solution was magnetically stirred in the dark (in tightly closed wooden compartment) for 50 min to reach the adsorption- desorption equilibrium of MB on the ZnO surfaces and to avoid interference from ambient light. The suspension solutions were then irradiated with visible light using a fluorescent lamp (FL15D-T25) with the wavelength range of 400–800 nm and a maximum intensity of 550 nm, producing a power of 15 W with constant magnetic stirring. At certain irradiation time intervals (10, 20, 30, 40 and 50 min), 5 ml of each aqueous solution sample was collected and centrifuged to remove the ZnO powders. The absorbance of MB solution at 665 nm was measured using a SPECORD S100 spectrophotometer. The concentration of aqueous MB solution was evaluated by recording the change in the absorbance centered at 665 nm.
3 Results and discussion 3.1 Microstructure analysis The crystal phases and crystallinity of the synthesized ZnO nanosheets were analyzed by XRD patterns, as shown in Fig. 1. As seen in Fig. 1, there are eight diffraction peaks at 2θ values of 31.73∘, 34.40∘, 36.21∘, 47.49∘, 56.52∘, 62.80∘,
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Fig. 1 X- ray powder diffraction pattern of ZnO nanosheets obtained by Microwave assisted hydrothermal route
67.86∘ and 68.99∘ which correspond to (100), (002), (101), (102), (110), (103), (112), (201) planes of ZnO, respectively. The observed lattice values are in well agreement with the standard spectrum of hexagonal (Wurtzite) ZnO structure with space group of P6 3 mc and lattice constants a = b=0.3249 nm and c=0.5206 nm (JCPDS card number 800074). No characteristic peaks related to other impurities were detected, indicating that high-purity ZnO nanoparticles are formed without secondary phases via microwave hydrothermal process. The average particle size (d) of the synthesized ZnO nanosheets was calculated by the following relation: d¼
0:9λ βcosθ
where I(hkl) is the diffraction line intensity of the (hkl) reflection of as synthesized ZnO powder, ∑I(hkl) is the sum of the intensities of all the diffraction lines monitored. The I0 refers to intensity of the reference ZnO sample. The degree of crystallinity of the synthesized ZnO nanosheets was to be about 87 %. FTIR spectra of the synthesized ZnO nanosheets are shown in Fig. 2. It is observed that the band at wave numbers 487 cm− 1 is attributed to ZnO vibrations [1, 2]. The presence of band at 1,611 cm−1 is attributed to C=O stretching vibration mode [3, 4]. The band at wavenumber 3,453 cm−1 is referred to a stretching vibration of hydrogen bond, indicating the existence of hydroxyl group (−OH) [1–5]. Field emission scanning electron micrographs (FESEM) of ZnO nanosheets with different magnification is depicted in Fig. 3(a) and (b). It is clear that the nanosheets were well defined. The well-proportioned ZnO nanosheets have an average thickness of 12 nm and lateral dimensions of 70 nm and are closely matching with XRD results. Furthermore, it is clear that the surfaces of ZnO nanosheets are very smooth. Energy dispersive x-ray spectroscopy (EDS) analysis was performed to determine the stoichiometry of the synthesized ZnO nanosheets, as shown in Fig. 3(c). The spectra reveal the strong peaks at 0.5, 1.2, 8.7, and 9.6 KeV which are attributed to O Kα, Zn Lα, Zn Kα, and Zn Kβ, respectively, suggesting that the nanoparticles are indeed made up of Zn and O only [1–5]. Quantitative analysis of the atomic and weight concentration (at% and wt%) is given in Table inset in Fig. 3(c). It is seen that no foreign ions are present in the final product. This
ð1Þ
where λ is the x-ray wavelength, θ is the Bragg diffraction angle and β is the broadening of the diffraction line measured in radians at the full width at half of its maximum intensity (FWHM) on 2-theta scale. From X-ray data, the average crystallite size was found to be 12 nm. Interestingly, the synthesized ZnO nanosheets are highly crystallized according to the sharp and intense diffraction peaks. To confirm the above fact, the degree of crystallinity (Dc) was calculated from X-ray data using the following relation:
X I 0 ðhklÞ I ðhklÞ Dc ¼ X I ðhklÞ I 0ðhklÞ
ð2Þ
Fig. 2 FTIR spectra of ZnO nanosheets obtained by Microwave assisted hydrothermal route
J Electroceram Fig. 3 (a) Low magnification FESEM image of ZnO nanosheets, (b) High magnification FESEM image of ZnO nanosheets, and (c) EDS of ZnO nanosheets
suggests that the synthesized ZnO nanosheets contain 100 % of ZnO and matched well with the X-ray data. Other peak observed for carbon was due to the supporting carbon-coated grid used for sample preparation. Transmission electron microscopy (TEM) measurements of the samples were performed to visualize the size and morphology of ZnO nanoparticles. Figure 4(a) and (b) show the low and high resolution images of the synthesized ZnO nanosheets. High resolution images show that the nanosheets are structurally perfect and confirm the direction of zone axis which is parallel to [110]. The lattice space distance to 0.26 nm is in accordance with the interplanar distance of (002) plane of hexagonal like zinc oxide. The selected area electron diffraction (SAED) pattern in Fig. 4(c) indicate that the ZnO nanosheets are polycrystalline.
3.2 Optical properties Optical measurements can provide a useful knowledge for understanding the physical properties of materials, provision of information about the structure and energy gap in both crystalline and non-crystalline materials and the possibility of potential applications in optoelectronic devices. UV-Vis spectra of the synthesized ZnO nanosheets are shown in Fig. 5(a). It is clearly observed that a well defined excitonic absorption peak at 389 nm was observed and it corresponds to wurtzite hexagonal phase bulk zinc oxide [2]. The band edge of the ZnO nanosheets appears at 439 nm. Interestingly, there is no other peak observed in the spectrum, this indicates that the synthesized ZnO nanosheets possess good optical properties.
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The optical band gap energy (Eg) of the synthesized ZnO nanosheets can be evaluated by Davis and Mott equation, which gives an expression for the absorption coefficient (α), as a function of photon energy (hυ) through the following [1–3]:
(a)
n ðαhυÞ ¼ α0 hυ −Eg
(b)
0.51 nm
(c)
(100) (002) (101) (102) (110) (103)
ð3Þ
where α0 is a constant related to the extent of the band tailing, which depends on the width of localized states in the band gap, the exponent n=1/2 for allowed direct transition, n=2 for allowed indirect transition, n=3/2 for a direct forbidden transition, n=3 for an indirect forbidden transition and α is the absorption coefficient. After fitting all the values of n in the Davis and Mott relation, the n value of 1/2 was found to hold well, leading to direct transitions. In order to obtain the band gap energy (Eg) of the synthesized ZnO nanosheets, we plotted the curve of (αhυ)2 vs., (hυ), as shown in Fig. 5(b). The optical band gap was determined by extrapolating the linear portion of the plot to (αhυ)2 =0 i.e. absorption equal to zero, as shown in (Fig. 5(b)) and was found to be 3.22 eV. Obviously, the Eg value of the synthesized ZnO nanosheets is lower than that of 3.40 eV reported in the literature of ZnO [3, 12]. The difference in the band gap value is attributed to the imperfections formations which are represented by localized states on the forbidden band of ZnO particles. This suggests that the Eg is due to the electronic transitions between the filled valence states to the energy level of the generated imperfections instead of the transition between the valence band to the conduction band as usual [2, 12]. A room temperature photoluminescence (PL) spectrum of the synthesized ZnO nanosheets is depicted in Fig. 5(c). It is clear that PL spectra mainly are consisted of three emission bands in the spectrum: one is located at around 389 nm in the ultraviolet region, a blue-green band located at around 460 nm and a green band located at 531 nm. The emission peak at 389 nm at the ultraviolet range is ascribed to the direct recombination (photogenerated charge carriers) of the free exciton through an exciton—exciton collision process and related near—band edge emission of ZnO [13, 14]. In addition, it is seen that blue emission with a major peak was centered at 460 nm, and a blue-green emission, was centered at 560 nm. The blue luminescence for ZnO nanosheets is attributed to oxygen vacancies (Vo) [14], and the blue-green emission at 531 nm is related to oxygen interstitials (Oi). Based on the spectra obtained, the higher emission intensity is obtained from a smaller size or higher surface area to volume ratio, indicative of high-quality ZnO nanosheets. 3.3 Photocatalytic activity study
Fig. 4 (a) Typical low magnification TEM image of ZnO nanosheets, (b) Typical high resolution HRTEM image of ZnO nanosheets and c SAED pattern of ZnO nanosheets
The photocatalytic decomposition was studied by monitoring the absorbance at regular time intervals and wavelength
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(a)
(a)
80
1.6 t = 0 min t= 10 min t = 20 min
60
t = 30 min
1.2
t = 40 min t = 50 min
Absorbance (a. u.)
Absorbance (%) (a. u.)
70
50
40
30
20 200
300
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500
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700
0.8
0.4
800
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(b) 120
0 200
100
300
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2
( hυ ) (eV/cm)
2
Wavelength (nm) 80
(b)
60
120
1.2 Absence of ZnO
40
100
1
20
80
0 3 hυ υ (eV)
4
0.8
5
(c)
60 D (%) I/I0
0.6
I/I0
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Degradation (%)
1
40 0.4
Intensity (a. u.)
20 Absence of ZnO
0.2
0
-20 0
20
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Fig. 5 (a) UV-visible absorption spectrum of ZnO nanosheets, (b) (αhυ)2 versus photon energy (E) and the estimated optical energy gap is 3.40 eV. (c) Room temperature PL spectra of ZnO nanosheets (excited at 385 nm)
Fig. 6 (a) Variation of the absorption spectra for MB solution in the presence of ZnO nanosheets under UV light at different time intervals and (b) Typical plot for the change in the absorption intensity and change in % degradation as a function of irradiation time of ZnO nanosheets containing methylene blue
of 665 nm using UV-Visible spectrophotometer. UV-Visible absorption spectra of methylene blue (MB) at different time intervals in the presence of ZnO nanosheets are shown in
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Fig. 6(a). It is seen that the maximum absorption intensity of MB is decreased sharply with the increase in UV irradiation time. Rapid degradation of MB dye in the presence of ZnO nanosheets is attributed to the high aspect ratio and 2 D layer structure of the synthesized ZnO nanosheets. After 50 min of UV irradiation, the maximum absorption intensity was disappeared. This indicates that MB dye was completely degraded in presence of the synthesized ZnO nanosheets photocatalyst. This is attributed to the breaking of the conjugated π− system in the MB chain and degraded auxochromic groups in the MB dye [12, 15]. Figure 6(b) shows the absorption intensity and degradation (%) as a function of irradiation time for MB in presence and absence of ZnO photocatalysts. The degradation (%) of ZnO nanosheets was obtained by the following equation, C 0 −C t Degradation ð%Þ ¼ 100 C0
ð4Þ
where C0 is the initial MB concentration and Ct is the MB concentration after irradiation at a given time. It is observed that in the absence of the synthesized ZnO nanosheets photocatalyst, no degradation of MB dye was occurred, but under UV light irradiation, it is observed and no detected loss of the dye can be seen. On the other hand, in the presence of ZnO nanosheets, MB was degraded marginally and it was completely degraded in 50 min. This is a strong clue that the synthesized ZnO nanosheets are working as a high and efficient photocatalyst for the photocatalytic degradation of MB, comparable to that of hydrothermally synthesized samples as well as samples synthesized by other methods [3, 15]. It is worthy to note that the degradation of MB was reached up to 99.2 % within 50 min of photocatalytic reaction performed under UV light irradiation.
4 Conclusions Zinc oxide nanosheets were successfully synthesized with zinc acetate and urea as raw materials by microwave hydrothermal process at 200 °C for 15 min for the first time. The
optical energy gap of as synthesized ZnO nanosheets is about 3.22 eV. PL spectra consist of UV emission peak at 389 nm, blue-green band located at around 460 nm and a green band located at 531 nm. ZnO nanosheets as photocatalyst, almost complete degradation of MB was observed within 50 min under UV-light irradiation. Acknowledgments The authors gratefully acknowledge and thank the Deanship of Scientific Research, King Abdulaziz University (KAU), Jeddah, Saudi Arabia, for providing financial support through a research project grant (Grant No. 337/247/1431).
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