Facile synthesis of zinc oxide nanoparticles decorated ...

Journal of Photochemistry & Photobiology, B: Biology 162 (2016) 500–510

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Journal of Photochemistry & Photobiology, B: Biology journal homepage: www.elsevier.com/locate/jphotobiol

Facile synthesis of zinc oxide nanoparticles decorated graphene oxide composite via simple solvothermal route and their photocatalytic activity on methylene blue degradation☆ Raji Atchudan a,⁎, Thomas Nesakumar Jebakumar Immanuel Edison a, Suguna Perumal b, Dhanapalan Karthikeyan c, Yong Rok Lee a,⁎ a b c

School of Chemical Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea Department of Applied Chemistry, Kyungpook National University, Daegu 41566, Republic of Korea Department of Chemistry, Arignar Anna College of Arts and Science, Krishnagiri 635 001, Tamilnadu, India

a r t i c l e

i n f o

Article history: Received 1 July 2016 Received in revised form 16 July 2016 Accepted 18 July 2016 Available online 20 July 2016 Keywords: Graphene oxide Zinc oxide Methylene blue Photocatalyst Irradiation Degradation

a b s t r a c t Zinc oxide nanoparticles decorated graphene oxide (ZnO@GO) composite was synthesized by simple solvothermal method where zinc oxide (ZnO) nanoparticles and graphene oxide (GO) were synthesized via simple thermal oxidation and Hummers method, respectively. The obtained materials were thoroughly characterized by various physico-chemical techniques such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Raman spectrum shows the intensity of D to G value was close to one which confirms the obtained GO and ZnO@GO composite possesses moderate graphitization. TEM images shows the ZnO nanoparticles mean size of 15 ± 5 nm were dispersed over the wrinkled graphene layers. The photocatalytic performance of ZnO@GO composite on degradation of methylene blue (MB) is investigated and the results show that the GO plays an important role in the enhancement of photocatalytic performance. The synthesized ZnO@GO composite achieves a maximum degradation efficiency of 98.5% in a neutral solution under UV-light irradiation for 15 min as compared with pure ZnO (degradation efficiency is 49% after 60 min of irradiation) due to the increased light absorption, the reduced charge recombination with the introduction of GO. Moreover, the resulting ZnO@GO composite possesses excellent degradation efficiency as compared to ZnO nanoparticles alone on MB. © 2016 Published by Elsevier B.V.

1. Introduction In recent years, graphene and its composite materials are attracted enormous attention due their wide range of potential applications in catalysis, solar cells, hydrogen storage, nanoelectronics, sensors and nanocomposites [1–6]. Graphene is an allotrope of carbon with twodimensional honeycomb sp2 crystalline lattice and good conductivity. The hexagonal atomic structure of graphene is able to supply a building platform for the epitaxial growth of other hexagonal nanostructures such as zinc oxide (ZnO), TiO2 and SnO2. In addition, graphene based semiconductor composites greatly improves the photocatalytic properties of these host materials. Among these semiconductor nanoparticles, ZnO received considerable attention of scientists owing to its size and

☆ Electronic Supplementary Information (ESI) available: XRD pattern of graphite powder and graphene oxide, UV–Vis spectra of MB with ZnO nanoparticles and degradation efficiency (%) of ZnO nanoparticles on MB. ⁎ Corresponding authors. E-mail addresses: [email protected] (R. Atchudan), [email protected] (Y.R. Lee).

http://dx.doi.org/10.1016/j.jphotobiol.2016.07.019 1011-1344/© 2016 Published by Elsevier B.V.

shape dependent photocatalytic properties. ZnO is an n-type semiconductor with bandgap ~3.37 eV, which has been applied in many fields such as photocatalysis, solar cells, gas sensors and photodetectors [7– 11]. In general, small size ZnO nanoparticles have increased specific surface area and more numbers of active surface sites where the photogenerated charge carriers are able to react with absorbed molecules. Graphene oxide (GO) with hydroxyl and carboxyl groups, which possesses excellent dispersibility in solvents and thus provides various opportunities for the fabrication of GO-based hybrid composites [12,13]. In general, dyes and pigments are hazardous and toxic to environment as well as for human beings. Upon high discharge of these environmentally unfriendly materials causes ulceration of skin and mucous membrane, dermatitis, perforation of nasal septum, severe irritation of respiratory tract and on ingestion may cause vomiting, pain, haemorrhage and sharp diarrhea [14]. So the degradation of organic dyes finds extensive application for environmental safety. The photocatalytic degradation is one of the best methods for detoxification of dyes and pollutants [15]. This process can mineralize organic dyes completely to H2O, CO2, and other nontoxic compounds without

R. Atchudan et al. / Journal of Photochemistry & Photobiology, B: Biology 162 (2016) 500–510

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causing a secondary pollution [16,17]. Methylene blue (MB) is a thiazine dye and finds various applications in medicine, biology and analytical chemistry. Owing to the solubility of MB in polar solvents such as water and ethanol, the effluents of those industries contain MB, which is toxic for aquatic organisms, nitrifying bacteria and fishes [18]. Therefore the degradation of MB is environmentally significant. Many researchers have been studied the degradation of MB as a model reaction for proving the photocatalytic ability of synthesized nanomaterials [19,20]. Various techniques have been adopted to synthesis photocatalytic ZnO nanoparticles and ZnO nanoparticles decorated graphene based nanostructured materials. However, it is a challengeable task to synthesize the nanomaterials with controlled particle size without agglomeration for better photocatalytic activity [21– 26]. In our work, zinc oxide nanoparticles decorated graphene oxide (ZnO@GO) composite has been prepared as a photocatalyst for the organic dye degradation. The zinc oxide (ZnO) nanoparticles were synthesized by simple thermal oxidation utilizing horizontal tubular furnace. GO was synthesized by Hummers method. The synthesized ZnO nanoparticles were impregnated over the GO sheets (layers) by simple solvothermal process. Moreover, the obtained ZnO@GO composite was used to enhance the photocatalytic performance on MB. The optical property was revealed by ultraviolet-visible (UV–Vis) spectroscopy techniques. The results show that the ZnO@GO composite plays an important role in the enhancement of photocatalytic performance and achieves a maximum degradation efficiency in a neutral solution under UV-light irradiation of 15 min as compared to pure ZnO nanoparticles due to the increased light absorption, the reduced charge recombination with the introduction of GO.

An appropriate amount (500 mg) of GO was dispersed in 75 mL of tetrahydrofuran in a 100 mL round bottom (RB) flask. 200 mg of ZnO nanoparticles were added in to the GO dispersion and sonicated for 20 min at ambient temperature. Subsequently the mixture was refluxed (65 °C) for 24 h under static condition. The final mixture was centrifuged and washed with tetrahydrofuran. The residue was dried at 100 °C for 5 h in a hot air oven. The resulting product was collected and used to enhance the photocatalytic acidity on degradation of organic dyes including methylene blue. The schematic representation of the synthesis of ZnO@GO composite was shown in Fig. 1.

2. Experimental

2.5. Characterization Methods

2.1. Materials

The synthesized ZnO, GO and ZnO@GO were characterized by various physico-chemical such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy- attenuated total reflectance (ATR), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), UV–Visible spectroscopy and Raman spectroscopy techniques. The XRD patterns were obtained on a PANalytical X'Pert diffractometer using Cu Kα radiation (K = 1.54 Å) equipped with a liquid nitrogen cooled germanium solid-state detector. The XRD patterns of resulted samples were recorded in the 2θ range of 10–90°, and at the step interval of 0.02° with the counting time of 5 s at each point. The FTIR spectra were recorded in transmittance mode on a perkin elmer spectrum two in the wavenumber range of 400– 4000 cm−1 by the co-addition of 32 scans at a resolution of 16 cm−1. Raman spectra were recorded with a Raman spectrometer Almega X/ Thermo using laser excitation line at 532 nm. FESEM was performed on Hitachi S-4800 at an accelerating voltage of 4 kV. The preparation of samples for FESEM, a small amount of samples was placing on conductive carbon tape. HRTEM images were obtained using a TITAN G2 ChemiSTEM Cs Probe electron microscope operated at 200 kV. Samples for HRTEM were prepared by placing droplets of a suspension of the sample in ethanol on a carbon-coated polymer micro grid supported on a Cu grid. X-ray photoelectron spectroscopy (XPS) was carried out using a K-Alpha spectrometer (Thermo Scientific). The spectra were baseline corrected using the instrument software. UV–Vis spectroscopy analysis was performed using an OPTIZEN 3220 UV.

Graphite powder (GP), zinc powder (b 50 nm) and MB were purchased from Sigma Aldrich. Potassium permanganate, barium chloride, hydrogen peroxide, sulfuric acid, hydrochloric acid, tetrahydrofuran, acetone, oxygen gas and hydrogen gas were purchased from Ducksan chemicals. The entire chemicals were used without any further purification. The deionized (DI) water was used throughout this study. 2.2. Synthesis of ZnO ZnO nanoparticles were synthesized using zinc powder by the simple thermal oxidation method with help of horizontal tubular furnace [27]. Typically, an appropriate amount of zinc powder (the mean size was b 50 nm) was placed in a quartz boat and inserts into the middle of quartz tube at the time of reaction temperature (800 °C) with heating rate of 10 °C/min. Prior to insert the quartz boat with zinc powder, the tubular furnace was purged under the mixture of argon gas and oxygen gas at the flow rate of 50 and 10 sccm (sccm denotes standard cubic centimeter per minute), respectively. After placing the zinc powder, the reaction temperature was maintained for 30 min. Then the furnace was cooled to room temperature under argon atmosphere, and then final ZnO nanoparticles were collected after the completion of the reaction. 2.3. Synthesis of GO The GO was synthesized using the modified Hummers method without any major modification [28]. 100 mL of concentrated sulfuric acid was taken in to a 1000 mL of beaker and it was cooled to 0 °C in an ice-bath as a safety measure. Subsequently 4 g of GP was added in to the sulfuric acid and vigorously stirred for 10 min. While maintaining vigorous stirring, 8 g of potassium permanganate was added slowly to the suspension. The rate of addition was controlled carefully to prevent the temperature of the suspension exceeding from 20 °C. The

suspension was stirred for 15 min below 20 °C and then the ice-bath was removed. Then the temperature of the suspension was raised to 40 °C, where it was maintained for 1 h. As the reaction progressed, the mixture gradually thickened with a diminishing in effervescence and the paste was brownish in color. At the end of reaction, 200 mL of DI water was slowly added into the paste and an increase in temperature to 98 °C. The diluted suspension was maintained at this temperature for 30 min. 560 mL of DI water added in to the suspension for further dilution and then 100 mL of hydrogen peroxide (28%) was added to reduce the residual permanganate and manganese dioxide to colorless soluble manganese sulfate. The obtained suspension was centrifuged and residue was thoroughly washed with 5% of hydrochloric acid until aqueous layer could not get salt with barium chloride. The final residue was washed with acetone and the obtained solid was dried at 70 °C for 5 h in a hot air oven under static condition. The dry form of resulted GO was ground well and stored for further use. 2.4. Synthesis of ZnO@GO Composite

2.6. Photocatalytic Activity of ZnO@GO Composite on MB Photocatalytic activity of the synthesized ZnO@GO composite were tested on MB under UV-light and degradation efficiency depends on irradiation time was revealed by using UV–Vis spectrophotometer (OPTIZEN 3220 UV). MB stock solution (1 mmol) was prepared in DI water. The MB solution with absorbance intensity of around 2.1 was prepared from MB stock solution. 50 mL of prepared MB solution

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Fig. 1. Schematic representation of the synthesis of ZnO, GO and ZnO@GO composite.

(reactant) was taken into a 100 mL beaker and kept on the magnetic stirrer. Subsequently, 10 mg of synthesized ZnO@GO composite was added into the reactant and irradiations was carried out under UVlight (wavelength 365 nm, distance 100 mm, power 40 W) under mild stirring for 15 min. UV–Vis measurement was carried for every 3 min (0–15 min) to find the absorbance of reaction mixture until complete of the reaction (absorbance of MB nearing to zero/until decolorize

the MB). After 12 min, visually absence of color observed in the reaction mixture. Similar reaction conditions were adopted for the photocatalytic activity of MB in the absence and presence of catalyst (synthesized ZnO nanoparticles). 10 mg of ZnO nanoparticles was used instead of ZnO@GO composite for the degradation of MB under UV-light irradiation. The degradation efficiency of MB (blank) was calculated and compared with synthesized ZnO nanoparticles and ZnO@GO composite.

Fig. 2. (a) and (b) Wide-angle XRD patterns, (c) FTIR spectra and (d) Raman spectra of synthesized GO, ZnO and ZnO@GO composite.


3. Results and Discussion 3.1. Characterization of the Synthesized Materials XRD is a one of the most significant tools to characterize the phase and purity of the synthesized nanomaterials. It gives average crystallite size, interlayer spacing and corresponding diffraction angle. Fig. 2(a) shows the wide angle XRD pattern of the synthesized ZnO nanoparticles and a standard JCPDS pattern. All of the diffraction peaks is wurtzite structured ZnO which were in good agreement with the standard JCPDS card no. 36-1451. The XRD pattern indicates that the synthesized nanomaterial is highly crystalline. The absence of peaks from other phases were observed which indicating the high purity of the prepared ZnO nanoparticles [29–31]. The mean size of the nanoparticles was calculated from the XRD pattern according to the line width of the (101) plane refraction peak using Scherrer Eq. (1): L¼

Kλ β cosθ

ð1Þ

where, L is the average size of the nanoparticles, K is a dimensionless shape factor (0.9), λ is the wavelength of incident X-ray (λ = 1.54 Å), β is the full width at half maximum (FWHM) of the diffraction peak and θ is the position of the (101) peak. The mean size of the prepared ZnO nanoparticle was 22 nm. The d-spacing value of ZnO nanoparticle was calculated using Bragg law (Eq.2) based on the position of (101) reflection peak. nλ ¼ 2dsinθ ðorÞ d ¼

nλ 2 sinθ

ð2Þ

where, n is a positive integer (1), λ is the wavelength of incident X-ray (λ = 1.54 Å) and θ is the position of the (101) peak. The d-spacing value is 0.255 nm. Fig. 2(b) shows the wide angle XRD pattern of the prepared GO and ZnO@GO composite. GO powder shows an intense peak at 2θ = 11.6° and weak peak at 2θ = 42° due to C(002) and C(100) plane, respectively which clearly indicates the typical graphite structure [32] and the interlayer spacing around 0.76 nm. The interlayer distance of graphene layers increased from 0.34 to 0.76 nm for graphite powder to GO powder, respectively (see the Fig. S1). The ZnO@GO composite reveals a broad peak at 2θ = 20.5° due to C(002) plane with the interlayer spacing of 0.42 nm, further ZnO nanoparticle peaks observed as accordance with the JCPDS card no. 36–1451. The above observation shows the GO during the solvothermal process, the reduction of GO which gives reduced graphene oxide (rGO). The mean size of the ZnO nanoparticle in synthesized ZnO@GO composite was calculated according to the line width of the (101) plane of about 20 nm. The mean size of the ZnO nanoparticle in ZnO@GO composite was little smaller than synthesized ZnO nanoparticle alone. This might due to splitting of bigger nanoparticles during the synthesis of ZnO@GO composite via solvothermal process. The results suggest that the fabricated ZnO@GO composite possess highly pure without major impurities [33,34]. The FTIR spectra of the synthesized ZnO, GO and ZnO@GO composite are shown in Fig. 2(c). The strong absorption band below 500 cm−1 corresponds to the Zn\\O stretching vibrations in ZnO nanoparticles [35]. Apart from this, there is no absorption band observed suggest that absence of such as hydroxyl and organic matters. In agreement with literature works, all IR bands reveal that the synthesized ZnO nanoparticles are without contamination. The FTIR spectrum of the GO powder absorption bands appeared at 1725 cm−1 and 1387 cm−1 due to the C_O and C\\O stretching vibration. The absorption band appeared at 1575 cm−1 due to the C_C (sp2-hybridized carbon atoms) stretching vibration of graphitic structure. The characteristic bands at 1245, 1040 and 585 cm−1 confirm the presence of the epoxy groups complying with the symmetric stretching, asymmetric stretching, and deformation vibrations, respectively. The broad absorption band at 3100–3400 cm−1 due to the O\\H stretching vibration of the absorbed water molecules

503

over the synthesized materials is observed [36]. The FTIR spectrum of the ZnO@GO composite are parent to the GO spectrum but the intensity of the O\\H and C_O (carbonyl group) stretching vibration band faintly decreased compared to GO powder. This might be due to ZnO nanoparticles occupied over the graphene layers and GO reduced to graphene sheets during the solvothermal process for fabrication of ZnO@GO composite. In addition, the absorption intensity of C_C slightly increased for ZnO@GO composite which supports the formation rGO during the solvothermal synthesis of ZnO@GO composite. The weak absorption band observed below 500 cm−1 corresponds to the Zn\\O stretching vibrations. The intensity of absorption band is low due to graphene layers finely constructed over the ZnO nanoparticles. Fig. 2(d) shows the Raman spectra of the prepared GO and ZnO@GO composite. The spectra show two bands at 1345 and 1595 cm−1 which are corresponding to the D-band and G-band, respectively. The D-band is related to the defects in the graphitic structure and the G-band to the vibration of the sp2-hybridized carbon atoms in the graphitic structure [37,38]. The peak intensities of the D-band and G-band are denoted as ID and IG, respectively. The structure ordering (graphitization) of materials was confirmed by the value of ID/IG and the value is approximately 1.21 and 1.23 for GO powder and ZnO@GO composite, respectively. The intensity ratio of D to G band (ID N IG) indicates that the obtained materials were moderate graphitization due to surface defects of the GO. The ID/IG ratio of ZnO@GO composite little higher than GO powder which indicates the structural defects faintly increase in the graphene lattice during the formation ZnO@GO composite. These defects will be favour for less aggregation of ZnO nanoparticles and also for stronger interaction of ZnO nanoparticles with the defect sites of the graphene layers [39]. The surface morphology of ZnO, GO and ZnO@GO composite were investigated by FESEM analysis. Fig. 3(a–f) shows the FESEM images of the synthesized ZnO nanoparticles with different magnifications. The ZnO nanoparticles exhibits an excellent near spherical shape with narrow size distribution. The mean size of the ZnO nanoparticle around 17 ± 5 nm and are good agreement with XRD results. Fig. 4(a–f) and Fig. 5(a–f) shows the FESEM images of the synthesized GO powder and ZnO@GO composite with different magnifications, respectively. The FESEM images showed that the set of individual layers with different orientations with smooth morphology for GO powder. The smooth surface morphology slightly decreased (rigidness faintly increased) for ZnO@GO composite (refer to Fig. 5) compared to GO powder due to the addition of ZnO nanoparticles over the graphene layers. The surface defect of the synthesized materials was slightly increased and the graphitization not much changed which was confirmed by XRD and Raman studies. To further investigate the morphology of synthesized GO powder and ZnO@GO composite were investigated by TEM analysis. Fig. 6(a–e) shows the TEM images of GO powder with different magnifications. The layers of GO are smooth and thin wrinkled layers which are clear from TEM images [40]. Fig. 6(f) shows the selected area electron diffraction (SAED) pattern of graphene layer in GO powder. The SAED pattern shows the two rings which is corresponds to C(002) and (100) planes for typical graphitic structure [41]. The obtained SAED pattern is more close to XRD pattern and this study suggested that the synthesized GO powder is moderate graphitic in nature. Fig. 7(a–h) shows the TEM images of ZnO@GO composite with different magnifications. The ZnO nanoparticles were dispersed over the wrinkled graphene layers which is clear from TEM images of ZnO@GO composite. The mean size of the decorated ZnO nanoparticles is around 15 ± 5 nm. HRTEM image clearly shows the lattice fringes and the inter layer distance is approximately 0.25 and 0.40 nm for ZnO nanoparticles and graphene layers, respectively. The calculated interlayer distance was closer to bulk ZnO and graphite. Inter layer distance is matches well with the interlayer distance calculated from XRD analysis. The interlayer distance of graphene layers is ~ 0.40 nm with in the synthesized ZnO@GO

504


Fig. 3. FESEM images of synthesized ZnO nanoparticles with different magnifications.

composite, this result supports the reduction of materials from GO to rGO. The SAED pattern of graphene layer within synthesized ZnO@GO composite is shown in Fig. 7(i). The graphene layers in fabricated ZnO@GO composite are moderate graphitic in nature which is clear from SAED pattern [42]. The SAED pattern of graphene layers obtained after synthesis of ZnO@GO composite even though, the graphitization was not much affected. High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDAX) spectroscopy with EDAX elemental mapping of ZnO@GO composite were shown in Fig. 8. The EDAX spectrum (Fig. 8(e)) is used as a quantitative analysis for the presence of components in the fabricated ZnO@GO composite. Carbon (C), oxygen (O) and zinc (Zn) are present in the ZnO@GO composite with absence of impurities, which confirms the obtained ZnO@GO composite is highly pure also confirmed from XRD pattern and elemental mapping. Apart from that C, O and Zn peak, the strong peak observed around 8 eV in the EDAX spectrum. This due to Cu obtained from Cu TEM grid [43]. EDAX mapping of the zinc distribution

in the composite sample and the results of C, O and Zn are clear from images (Fig. 8(b–d)). Fig. 8(d) shows the Zn was incorporated over the graphene layers. Based on this result, the ZnO nanoparticles were well distributed, which is also in good agreement with the XRD results. XPS is a one of the important tools to get a better understanding of the chemical state, functionality and composition of the resulted materials. Fig. 9(a) shows the XPS survey scan spectrum of synthesized ZnO@ GO composite. In the spectrum, four major binding energy peaks were observed around 285, 532, 1022 and 1046 eV, which were assigned to the orbital of carbon (C 1 s), oxygen (O 1 s), zinc (Zn 2p3/2) and zinc (Zn 2p1/2) levels, respectively. Other than that minor binding energy peaks were observed at 12, 90 and 141 eV corresponding to the electronic states of Zn 3d, Zn 3p and Zn 3 s, respectively. The high resolution XPS spectrum of C 1 s level (Fig. 9(b)) consisted of four binding energy appeared at 284.6, 285.1, 286.7 and 288.1 eV, which correspond to the C_C/C\\C (attributed to the sp2/sp3 carbon atom), C\\O bonds from C\\O\\C epoxy, C_O/C\\OH and O_C\\OH functional groups,

Fig. 4. FESEM images of synthesized GO with different magnifications.


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Fig. 5. FESEM images of synthesized ZnO@GO composites with different magnifications.

respectively [44,45]. Moreover, the absence of C\\O\\Zn peak in the composite indicated that there was no chemical C\\O\\Zn bond was formed between the graphene layers and ZnO nanoparticles [46]. The XPS spectrum of O 1 s level (Fig. 9(c)) yielded a three binding peak at 531.1, 532.5 and 533.5 eV, showing the presence of Zn\\O, C\\OH/ Zn\\OH and C_O group on the surface of graphene layers [47,48]. In addition, XPS of Zn 2p (Fig. 9(d)) showed a two major peak at the binding energy of 1022.4 and 1046.2 eV corresponding to Zn 2p3/2 and Zn 2p1/2 levels, respectively. This indicated that the Zn was present in the form of ZnO into synthesized ZnO@GO composite [49]. The atomic percentage of carbon, oxygen and zinc for synthesized ZnO@GO composite are approximately 65.73, 29.67 and 4.61%, respectively based on our XPS results. Therefore, it could be deduced that the synthesized material

composed of high purity under our preparation conditions. These XPS results are in good agreement with the previous reports in the literatures [50–52]. 3.2. Photocatalytic Activity of the Synthesized Materials The obtained ZnO@GO composite is used as photocatalyst for the degradation of MB by simple UV-light irradiation. The mechanism behind the photocatalytic degradation of MB with ZnO@GO composite under UV-light irradiation is illustrated in Fig. 10. The entire process can be described as follows: The electrons of ZnO nanoparticles are excited to generate electron–hole pairs (h+), the electrons (e−) in the valence band (VB) of ZnO are easily transferred to conduction band (CB) of

Fig. 6. (a)–(e) TEM images of synthesized GO with different magnifications and (f) SAED pattern of the corresponding graphene layer.

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Fig. 7. (a)–(h) TEM images with different magnifications of synthesized ZnO@GO composite and (i) SAED pattern of the corresponding graphene layer.

graphene layers under UV-light irradiation. The high separation rate of electron–hole pairs makes it easier to form O•− 2 active ion and OH• radicals on the surface of ZnO@GO composite to interact with MB, thus enhancing the photocatalytic performance of the ZnO@GO composite [53]. Subsequently, the ZnO nanoparticles and graphene layer within ZnO@ GO composite might involve in photocatalytic reaction under UV-light and MB degradation occur at the same time. Therefore, the incorporation of graphene layer into ZnO nanoparticle plays an important role to enhance the photocatalytic performance of ZnO@GO composite on MB clear from Fig. 10. The degradation percentage (efficiency) of MB was directly correlated with irradiation time which confirms the UV– Vis absorbance. This result demonstrates that the introduction of ZnO nanoparticles to graphene layers can dramatically enhance the photocatalytic activity through an interfacial interaction between ZnO nanoparticles and graphene layers [54]. The photocatalytic activity of synthesized ZnO@GO composite towards the degradation of MB was examined using UV–Vis spectroscopy. In General, MB displays blue color in water and absorb in the visible region at 612 and 664 nm. Fig. 11(a) shows the time dependent UV–Vis spectra of MB in presence of ZnO@GO composite under UV-light irradiation. After the addition of ZnO@GO to MB under UV-light, the absorbance intensity of MB decreased gradually with increase in irradiation

time and the solution turns to colorless in 12 min and the corresponding degradation efficiency is 92% (see Fig. 11(c) and inset photographic images), which reveals that the synthesized ZnO@GO composite shows excellent photocatalytic activity towards the degradation of MB [55]. The degradation of MB efficiency is 98.5% at the irradiation time of 15 min. At the same time (on the other hand) without UV-light, the degradation of MB did not occur, but adsorption of MB takes place due to the presence of GO, identified from the little decreasing in the absorbance intensity of MB after 30 min (degradation/adsorption efficiency is 21.1%, Fig. 11(b)). After 30 min, no change in absorbance was observed, which suggest that the decreasing of absorbance is might due to the adsorption of MB by GO present in the ZnO@GO composite [56]. Fig. 11(c) shows the percentage degradation efficiency (%) of MB in the presence of ZnO@GO composite with and without UV-light irradiation at different time intervals using the equation 3. The degradation of MB occurs rapidly under UV-light irradiation and reaches 92% after 12 min (the degradation efficiency achieved 98.5% after 15 min of irradiation), while in the case of normal (ambient) light, the degradation efficiency is very low (i.e. 21.1% even after 30 min). Degradation efficiencyð%Þ ¼

A0 ‐At X100 A0

ð3Þ


Fig. 8. HAADF-STEM image (a), the corresponding C (b), O (c), Zn (d) elemental maps and EDAX spectrum (e) of synthesized ZnO@GO composites.

Fig. 9. (a) XPS survey scan spectrum, high resolution XPS spectra of (b) C 1 s, (c) O 1 s and (d) Zn 2p of synthesized ZnO@GO composites.

507

508


Fig. 10. Photocatalytic degradation mechanism of MB with ZnO@GO composites under UV-light irradiation.

where A0 and At are the absorbance of MB at 664 nm in the dark and under UV-light irradiation (at t minutes), respectively. The photocatalytic activity of ZnO nanoparticles was studied on MB under UV-light irradiation and the degradation efficiency is around 49%

even after 60 min (the degradation efficiency is around 12% after 15 min of irradiation) which is clear from Fig. S2 (a) and (b). The degradation efficiency (%) of MB alone (absence of catalyst-Blank) is around 6.5% even after 180 min (the degradation efficiency is around 1.5% after

Fig. 11. UV–Vis spectra of (a) MB degradation and (b) MB adsorption with ZnO@GO composite depends on time, (c) catalytic efficiency (%) on MB degradation at different irradiation time and (d) first order kinetics plot of ln(At/A0) versus irradiation time.


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Table 1 Comparison of photocatalytic performances of ZnO nanoparticles-graphene based nanocomposites. Sample

Irradiation time (min)

Final MB degradation (%)

Graphene-ZnO composite Protein-capped ZnO nanoparticles ZnO/graphene composite

25 30 90

99.8 89.68 ~100

ZnO NPs/rGO nanocomposite ZnO/rGO hybrids Nanorods of ZnO/GO composite

180 30 30

99.5 99 98.7

ZnO-C nanocomposite

25

99.7

Graphene-ZnO nanofiber mats

240

80%

Graphdiyne-ZnO nanohybrids

120

68

ZnO@GO composite

15 min

98.5

15 min of irradiation, see the Fig. S3). These result revealed that the synthesized ZnO@GO composite is a promising candidate for photocatalytic degradation of MB. Absence of efficiency increment (nearly 100% degradation) after 15 min suggest the optimum time for the degradation of MB. Many researchers reported that the photocatalytic degradation of MB by ZnO nanoparticles follows pseudo-first order kinetics [57,58]. Hence, the degradation of MB was also analyzed by pseudo-first order kinetics according to our previous reports [59,60]. The kinetic of ZnO nanoparticle efficiency (%) on MB was studied and the calculated rate constant (k) is 0.011 min−1 (1.83 × 10−4 s−1). The detailed discussions were mentioned in supplementary data (see inset of Fig. S2 (b)). Fig. 11(d) shows the first order kinetics plots of ln(At/A0) versus irradiation time, which yield a straight line with the negative slope value as a rate constant. The derived straight line equation is (Y = − 0.254 × + 0.209), with R2 = 0.963. The calculated k is to be 0.254 min−1 (4.23 × 10−3 s−1), which is a maximum value when compared with synthesized ZnO nanoparticles on MB degradation. This may be due to the small and well dispersed ZnO nanoparticles over the GO support. In order to justify the superior activity of the synthesized ZnO@GO composite, the present study was compared with the recent reports of MB degradation by ZnO nanoparticle decorated graphene based nanomaterials and results were depicted in Table 1 [61–70]. UV–Vis absorbance spectra of MB alone (absence of catalyst-blank) at different time interval and degradation efficiency (%) of MB alone at different irradiation time under UV-light are shown in Fig. S3. The blue color was retained even after 180 min which was visually observed. The degradation efficiency of MB in absence of catalyst is nearly negligible compared to the degradation efficiency of synthesized ZnO@GO composite as well as ZnO nanoparticles (see Fig. S3(b) for more details).

4. Conclusions In summary, the ZnO@GO composite was synthesized successfully by a simple solvothermal method, where ZnO nanoparticles were synthesized via simple thermal oxidation. The resultant products were systematically characterized by various physico-chemical techniques. The mean size of the decorated ZnO nanoparticles was 15 ± 5 nm with an interlayer distance of 0.25 nm clear from TEM images. Interlayer distance of graphene layers based on C(002) plane within GO powder and ZnO@GO composite are approximately 0.76 and 0.42 nm, respectively. All the characterization techniques strongly support that the synthesized products were highly pure and well-constructed. Further the synthesized ZnO@GO composite plays an important role in the photocatalytic performance on MB and achieves a maximum degradation efficiency of 98.5% in a neutral solution under UV-light irradiation for 15 min. The rate constant of MB degradation with ZnO@GO composite was 0.254 min−1.

Rate constant k (s−1) 3.27 × 10−3 9.5 × 10−4 (0.057 min−1)

−4

7 × 10 (0.042 min−1) 2.38 × 10−3 (0.1432 min−1) 1 × 10−4 (6 × 10−3 min−1) 7.1 × 10−5 (0.00426 min−1) 4.23 × 10−3

Reference [58] [59] [60] [61] [62] [63] [64] [65] [66] This work

Acknowledgement This research was supported by the Nano Material Technology Development Program of the Korean National Research Foundation (NRF) funded by the Korean Ministry of Education, Science, and Technology (2012M3A7B4049675). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2014R1A2A1A11052391) and Priority Research Centers Program (2014R1A6A1031189). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jphotobiol.2016.07.019. References [1] H. Chen, K.C. Carroll, Metal-free catalysis of persulfate activation and organic-pollutant degradation by nitrogen-doped graphene and aminated graphene, Environ. Pollut. 215 (2016) 96–102. [2] K.Y. Cho, H.Y. Seo, Y.S. Yeom, P. Kumar, A.S. Lee, K.Y. Baek, H.G. Yoon, Stable 2Dstructured supports incorporating ionic block copolymer-wrapped carbon nanotubes with graphene oxide toward compact decoration of metal nanoparticles and high-performance nano-catalysis, Carbon 105 (2016) 340–352. [3] S.N. Kwon, C.H. Jung, S.I. Na, Electron-beam-induced reduced graphene oxide as an alternative hole-transporting interfacial layer for high-performance and reliable polymer solar cells, Org. Electron. 34 (2016) 67–74. [4] G. Xia, Y. Tan, F. Wu, F. Fang, D. Sun, Z. Guo, Z. Huang, X. Yu, Graphene-wrapped reversible reaction for advanced hydrogen storage, Nano Energy 26 (2016) 488–495. [5] C.S. Park, H. Yoon, O.S. Kwon, Graphene-based nanoelectronic biosensors, J. Ind. Eng. Chem. 38 (2016) 13–22. [6] N.R. Han, J.W. Cho, Click coupled stitched graphene sheets and their polymer nanocomposites with enhanced photothermal and mechanical properties, Compos. Part A 87 (2016) 78–85. [7] R. Wahab, S.K. Tripathy, H.S. Shin, M. Mohapatra, J. Musarrat, A.A. Al-Khedhairy, N.K. Kaushik, Photocatalytic oxidation of acetaldehyde with ZnO-quantum dots, Chem. Eng. J. 226 (2013) 154–160. [8] P.P. Das, S. Mukhopadhyay, S.A. Agarkar, A. Jana, P.S. Devi, Photochemical performance of ZnO nanostructures in dye sensitized solar cells, Solid State Sci. 48 (2015) 237–243. [9] L. Yu, F. Guo, S. Liu, B. Yang, Y. Jiang, L. Qi, X. Fan, Both oxygen vacancies defects and porosity facilitated NO2 gas sensing response in 2D ZnO nanowalls at room temperature, J. Alloys Compd. 682 (2016) 352–356. [10] A. Katoch, Z.U. Abideen, J.H. Kim, S.S. Kim, Influence of hollowness variation on the gas-sensing properties of ZnO hollow nanofibers, Sensors Actuators B 232 (2016) 698–704. [11] J. Saghaei, A. Fallahzadeh, T. Saghaei, Vapor treatment as a new method for photocurrent enhancement of UV photodetectors based on ZnO nanorods, Sensors Actuators B 247 (2016) 150–155. [12] J. Jang, V.H. Pham, S.H. Hur, J.S. Chung, Dispersibility of reduced alkylamine-functionalized graphene oxides in organic solvents, J. Colloid Interface Sci. 424 (2014) 62–66. [13] S. Rani, M. Kumar, R. Kumar, D. Kumar, S. Sharma, G. Singh, Characterization and dispersibility of improved thermally stable amide functionalized graphene oxide, Mater. Res. Bull. 60 (2014) 143–149. [14] C. Lavanya, R. Dhankar, S. Chhikara, S. Sheoran, Degradation of toxic dyes: a review, Int. J. Curr. Microbiol. App. Sci. 3 (2014) 189–199.

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