Journal of the Australian Ceramic Society https://doi.org/10.1007/s41779-018-00298-z
RESEARCH
Two-step synthesis of reduced graphene oxide with columnar-shaped ZnO composites and their photocatalytic performance with natural dye Rishikesh Yadav 1 & Vijay Kumar 1
&
Vipul Saxena 1 & Prabhakar Singh 2 & Vinay Kumar Singh 1
Received: 6 May 2018 / Revised: 2 November 2018 / Accepted: 13 December 2018 # Australian Ceramic Society 2019
Abstract Composites of ZnO with reduced graphene oxide were prepared in two-step synthesis process with constant temperature in variation with pH values. The synthesized composites were characterized and the results suggest that ZnO structure in the composites has a columnar morphology with an average diameter ranging 0.8–1.57 μm. The obtained properties of the composites with the present method confirmed that the material morphology influences the absorption and photocatalytic activity of natural dye under sunlight irradiation. The result shows that the maximum degradation efficiency is 64.40% achieved in 120 min. Keywords Zinc oxide . Columnar morphology . Reduced grapheme oxide . Photocatalytic activity . Natural dye
Introduction Reduced graphene oxide (rGO) is a potential candidate for many technological devices. Applications of this material can be summarized [1–6] as supercapacitors, fuel cells, and electrochemical detectors of inorganic and organic electroactive compounds and photoactive devices. rGO possesses exciting properties [7–12] such as high mobility and conductivity, high optical transparency, mechanical flexibility, chemical stability, and high specific surface area. The structure of rGO possesses a single layer of sp2 carbon atom and
Highlights • Reduced graphene oxide (rGO) was prepared by modified Hummers Hoffman method. • Columnar morphology of ZnO was developed in rGO-ZnO composites by two-step synthesis method. • UV-visible absorption capacity and photocatalytic activity of rGO-ZnO nanocomposites were performed using natural dye. • The results suggested that rGO-ZnO nanocomposites influenced the UV-visible absorption capacity and photocatalytic activities under sunlight irradiation. * Vijay Kumar
[email protected] 1
Department of Ceramic Engineering, IIT (BHU) Varanasi, Varanasi 221005, India
2
Department of Physics, IIT (BHU) Varanasi, Varanasi 221005, India
two-dimensional honeycomb lattice structure [13]. The oxide form of graphene connected to oxygen functional groups viz. carboxylic acid groups (-COOH), hydroxyl groups (-OH), and an epoxy group (-C-O-C) on both edges and hybridized surface of the basal plane through sp3 hybridization. The variation of carbon to oxygen ratio of graphene oxide affects the band gap ranging from semi-metal to semiconductor [14–18]. The optical, electrical, and mechanical properties of rGO can be altered by combining it with other materials like metals and semiconductors [19, 20]. Graphene oxide (GO)-based nanocomposites with ZnO and TiO2 have high electron mobility and also suitable photocatalytic applications [21, 22]. The ZnO is a promising semiconductor having an energy band gap of 3.37 eV which has a strong luminescence at room temperature. It has been suggested by Kamat et al. [22] that embedded ZnO can enhance the optical and electrical properties of rGO nanocomposites through the carboxylic functional groups. However, the morphology of ZnO has been considered in recent research activities to obtain structures such as nanorods, nanowires, nanobelts, and nanotubes. These types of structures influence the properties and their device applications [23–25]. For optical and semiconducting devices, ZnO is a better semiconductor material and it can be referred as an ntype transparent semiconductor material with a wide direct band gap and a large exciton binding energy. Its stability upon exposure to high energy radiation makes it superior
J Aust Ceram Soc
to the other materials [26]. The abovementioned properties make ZnO a desirable material for many optoelectronic applications [27, 28]. Keeping the abovementioned facts, the recent literature reveals that a particular structure such as tube or rod shapes ZnO could be utilized in making composites with rGO. Taking this idea in the present research work, columnar structures of ZnO were prepared and these were further embedded in GO by reduction process to obtain nanocomposite hybrid system. There are different processing routes for synthesis of ZnO nanoparticles with certain morphology such as sol-gel method [29], hydrothermal method [30], assisted solution route [31], two-step method [32], aqueous chemical route [33], and chemical precipitation method [34]. Among them, we have applied a two-step method as discussed in the section of materials and methods for the synthesis of the columnar morphology of ZnO in a hybrid system of reduced graphene oxide composites. This method comprises lower complexity, low cost, a higher degree of purity, and more possibility to obtain columnar structure compared to the other synthesis method. These composites were utilized for photocatalytic application and results revealed that composite materials had a good photocatalytic performance with natural dye under sunlight irradiation.
Experimental details Chemicals and materials All chemicals used in the present work were of analytical grade (AR) which was not further purified. Chemicals such as commercially available raw highly pyrolytic graphite (HOPG) flakes (99% carbon purity, Bay Carbon), ZnSO 4 ·7H 2 O (99%, Loba Chemie), H 2 SO 4 (98%, Loba Chemie), H 3 PO 4 (85%, Loba Chemie), NaNO 3 (98%, Loba Chemie), KMnO 4 (99%, Loba Chemie), and NaOH (98%, Loba Chemie) were used for synthesis of composites. Deionized water was used for the preparation of various salt solutions and in washing the precipitates.
Experimental procedure Reduced graphene oxide with columnar-shaped ZnO composites was synthesized by a two-step method by varying concentration of GO. In the initial stage, rGO was prepared by using the modified Hummers Hoffman method [35] with graphite powder as the starting material. In this method, a mixture of concentrated H2SO4 and H3PO4 in the volume ratio of 9:1 was used. Then, the mixed acid solution was cooled in an ice bath before taking it into use. In 120 mL of the ice-cooled acidic mixture, 2.5 g of
graphite powder and 2.5 g of NaNO3 were subsequently added. The slurry thus obtained was magnetically stirred in a continuous mode so that a proper suspension would be achieved and the powder might be dispersed uniformly. When this adequate dispersion was observed, then 15 g of KMnO4 was added in the suspension in a prolonged manner. The suspension was vigorously stirred while slowly adding KMnO4. In the next step, the suspension was kept in the ice bath, and it was allowed for reactions for at least 2 h. The suspension was taken out from the ice bath, and it was further stirred for another 2 h in a preheated water bath while maintaining the temperature in the range of 35–40 °C. Finally, the suspension was allowed to cool and also it was kept in an ice bath. In this stage, deionized water was added to maintain the volume of 400 mL. The precipitate was washed with deionized water and then thoroughly dried in an oven. In the second step for the preparation of composites, the concentrations of rGO were selected as 0.01, 0.03, and 0.05 mg/mL. These rGO concentrations were added separately in ethanol with continuous stirring in a beaker at room temperature. In another beaker, 50 mL of 0.05 M ZnSO4·7H2O was taken. These two solutions were mixed and then in this mixed solution NaOH (0.05 M) was added drop-by-drop with constant stirring and maintaining pH ~ 6, (acidic condition) to 9 and 12 (alkaline condition). The samples were coded as RZ-pH6, RZ-pH9, and RZ-pH12, respectively. Here, RZ is denoted to reduced graphene oxide and zinc composites. Each sample was centrifuged at 3000 rpm for 30 min. The precipitate was removed from the centrifuge then washed with dilute HCl and finally with deionized water. The precipitates were dried at a constant temperature of 90 °C for at least 24 h in an electric oven. The completely dried precipitates were ground and then sieved with the 37-μm sieve.
Characterization techniques Phases and microstructure analysis Phase analysis of the samples was done by using X-ray diffractometer (Model Bruker D8, UK Advance equipped) having Cu Kα radiation (λ ¼ 1:54178 A0 ). Angles selected were in the range of 10–80° with 5°/min scanning speed. The crystal size of RZ samples calculated by Scherer’s formula: D¼
Kλ βcosθ
ð1Þ
where D is the size of the crystallites and β is the full width at half maximum (FWHM) obtained by Lorentz fitting of a diffraction line located at an angle θ. The λ is X-ray wavelength, and K is a Scherrer constant (0.9), which is dependent on peak
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breadth, the crystallite shape, and crystallite size distribution. The lattice constants (a, b, and c) can be calculated according to the Bragg’ laws: 2dsinθ ¼ nλ
ð2Þ
And their lattice constant is given by equation [36]: λ λ and c ¼ a ¼ b ¼ pffiffiffi sinθ 3sinθ and unit cell volume calculated by using Eq. (4). pffiffiffi V ¼ 3a2 c=2
ð3Þ
ð7Þ
ð4Þ
ð5Þ
The microstrain (ε) was calculated by the following formula [37]: ε ¼ βcosθ=4
D ¼ ðC 0 −C t =C 0 Þ 100%
where C0 and Ct initial and final concentration, respectively.
The dislocation density (δ) was calculated by the following relation [37]: δ ¼ 1=D2
e, and f, respectively. Samples were exposed under sunlight for 0, 30, 60, 90, and 120 min, respectively. The catalysis test was carried out with the solution of ND each time and recollected after washing. The washed solution was further used in the photocatalytic experiment and this process was carried on cyclically. The degradation rate of ND is expressed as the following relation:
ð6Þ
Microstructure and phase distribution were studied by scanning electron microscopy (SEM), energy-dispersive Xray spectroscopy (EDX) using Model Zeiss EVO 18 (USA), and high-resolution transmission electron microscopy (HRTEM FEI Mod. No TECHNAI G220). Infrared IR spectroscopy (FTIR) using IS10 Nicolet, USA) at room temperature in the range of 400–4000 cm−1 was also performed. Evaluation of optical and photocatalytic properties Room temperature optical properties were studied with the help of Raman scattering measurements and UV-visible absorption spectroscopy. Absorption of the samples in powders form suspended in ethanol was recorded by absorption spectroscopy (Model Jasco V-770, Japan) at room temperature in the range of 200–800 nm and infrared region, respectively. For photocatalytic activity, the leaves of peepal tree known as Ficus religiosa were carefully washed twice with sterile distilled water and 5 g of leaves was dipped in 20 mL ethanol. The extract was filtered using filter paper no. 42 and was used immediately for photocatalytic activities. Here extracted chlorophyll is called nature dye and denoted as ND. Photocatalytic degradation of ND was performed in a closed box having stirrer inside with airtight inlet and outlet. The sunlight was used as the light source. In each experiment, the concentrations of the sample were kept at 0.01 mg/mL and were added to 20 mL ethanol with continuous stirring in a beaker at room temperature. For absorption studies, 1 mL ND and 1 mL RZpH6 were taken into 5 mL ethanol. The suspensions were stirred for 1 h in the dark to achieve the adsorption. Six samples were prepared in the above manner and coded as a, b, c, d,
Results and discussion Structure and morphological properties of ZnO columnar structures The structural characterizations of the samples were carried out by XRD. The XRD patterns of graphite (GR), GO, and RZ composites at different pH are shown in Fig. 1a. A very strong peak at an angle of 26.76° of the plane (002) in RG curve shows a typical pattern of the graphitic crystal structure with d-spacing of 0.335 Å. The intensities of the GO peaks were very low as compared to graphite as observed in the insert portion in Fig. 1b. No other peaks were observed, which indicates the removal of functional groups. The d-spacing of GO is 0.73 nm which is larger than the d-spacing (0.335 nm) of GR. This is the indication of removal of the oxygen-containing group on carbon nanosheets. The XRD patterns of RZ composites prepared at different pH values are showing the variations in results, confirming its influences on the crystallinity of the materials. In RZ composites, ZnO peaks are represented by peak marked as BΘ^ and additional peak are marked as BΔ.^ These additional peaks are similar to that of carbon black [38], and peak between 20° and 30° indicates the restacking of the reduced graphene sheets [39, 40]. The RZ-pH6 sample exhibited a very intense and sharp feature as compared with the XRD pattern of RZ-pH9 and RZpH12. This shows that the crystallinity is decreasing with an increase in the pH value of RZ composite. In RZ-pH6, the acidic nature dominates which restrain hydrolysis, while in RZ-pH9 and RZ-pH12, the alkali nature accelerates the hydrolysis present in the chemical reaction [41]. It can be concluded here that ZnO embedded in GO converts it to rGO and a decrease in pH improves the crystalline structure of the prepared composite samples. The major plane (101) of RZ-pH6 is significantly present, while other peaks having (hkl) values of (100), (002), (102), and (110) are seen with comparatively lower intensities. Estimation of ZnO crystalline size in RZ composites was done using Eq. (1), and lattice constant was calculated using Eq. (2)
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Fig. 1 a XRD patterns of graphite (GR), graphene oxide (GO), RZ-pH6, RZ-Ph9, and RZ-pH12. b Corresponding XRD peak shift of RZ-pH6, RZ-pH9, andRZ-pH12. c Williamson-Hall analysis of RZ-pH6, RZ-Ph9, and RZ-pH12
and listed in Table 1. Figure 1b shows that the crystalline peak of RZ-pH9 and RZ-pH12 is shifted when compared with RZ-pH6. This indicates the variation of lattice parameters because of the presence of carbonaceous materials. From the data listed in Table 1, it is also found that the
Table 1 The physical properties of the columnar structure of ZnO in RZ composites prepared in this work
average crystallite sizes of RZ-pH6 and RZ-pH9 are 43 and 13 nm, respectively, which shows that the lattice constant is also affected by the pH value of the samples. This variation of crystal size and lattice constant is due to alkali medium of RZ composite. These are also confirmed by
Sample
Unit cell parameters (Å)
c/a ratio
Cell volume (Å)3
Av. size (nm) and microstrain δ (line/m2) (1 × 10−4)
Dislocation density (line/m2)
Eg (eV)
RZ-Ph6
3.291
1.597
49.39
43
1 × 10−4
3.3
3 × 10−4
3.2
4 × 10−3
3.3
−4
RZ-Ph9
3.271
1.598
48.44
4 × 10 13
−4
RZ-Ph12
3.247
1.602
1.602
8 × 10 10
−3
4 × 10
d is the crystal size calculated by plane (101), a and c are lattice parameters calculated on the bases of plan (100) and (002), and the energy band gap Eg is obtained based on the absorption spectroscopy
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variation of dislocation density as given in Table 1. Thus, pH value and carbonaceous material present in RZ composite play a vital role in the material crystal size. The lattice strain and stress are believed to have a great influence on the physical properties of nanomaterial. XRD profile analysis is a well-executed technique for estimating the microstrain. From the available models, the WilliamsonHall (W-H) method is the most simplified and extensively studied technique [42] for analyzing the crystallite size and lattice strain. W-H analysis is an integrated method, in which the crystallite size and strain-induced line broadening of the XRD profile can be calculated from the peak width using abovementioned Eqs. (1) to (5) for W-H model [43]. From the linear plot of βhkl cos θ versus 4 sin θ, we can calculate the strain as well as the size of the crystallite. In principle, the slope of the graph gives the strain, whereas the reciprocal of the intercept should be the crystallite size [44]. Figure 1c represents the W-H plot of RZ-pH6 and RZ-pH9 of composite powder, respectively. A positive strain of RZ-pH6 and RZpH9 around the value 2.64498 × 10−4 and at 5.18 × 10−4 was observed from the slope of Fig. 1c, respectively. The crystallite sizes calculated from the intercept of RZ-pH6 and RZ-pH9 were 41 and 14 nm, respectively, which is also approximately similar to value as calculated by Eq. (1). The surface morphologies of RZ-pH6, RZ-pH9, RZ-pH12, and GO powder were investigated by SEM and shown in Fig. 2(a–e). Figure 2(a, b) displays SEM with EDX of RZpH6 which shows the formation of the columnar morphology of ZnO. The EDX analysis shows that only C, O, and Zn signals have been detected which indicates the absence of impurity. It has been reported that the presence of carbonaceous material changes the lattice constants of ZnO [45]. Similarly, in our sample, it is seen that with the increase in GO concentration from 0.01 to 0.05 wt.%, the agglomeration of the particles occurs. Another effect for the growth of ZnO in columnar shapes is its reaction in acidic medium because in the chemical reaction when ZnSO4·7H2O reacts with NaOH, it produces Zn (OH)2. This further hydrolyzed in [Zn(OH)4]2− and it acts as a growth unit [46]. The concentrated NaOH solution produces Na+ in composite solution which may intercalate ZnO growth unit. Due to their large surface-to-volume ratio and high surface energy, this ZnO morphology grows spontaneously from curled shapes to the columnar shape of ZnO structure. The average diameter of columnar shape measured with the help of scaling of SEM was obtained in the ranging 0.8–1.57 to 0.42–0.84 μm, respectively. With the increase in pH from acidic to the alkaline, columnar shape of ZnO agglomerated to each other as shown in Fig. 2c. The SEM image of the sample RZ-pH12 in Fig. 2d shows the shape of ZnO in a distorted manner, and it may be due to the NaOH. It has been observed that the structures are dependent on pH in all the SEM images which suggested that the pH value affected the ZnO structure of the samples in a hybrid system of RZ composite and also
altered the order of crystallinity. These results showed that the acidic medium is suitable for the development of the columnar shape of ZnO. In Fig. 2e, crumpling of GO sheets appeared, indicating the dehydration of sheets [47]. Figure 3a–d shows the HR-TEM image of RZ-pH9. The dark sections represent ZnO and appear as rod-like, belt-like which is shown in Fig. 3a, in Fig. 3b pencil-like, in Fig. 3c hammer- and palmlike, and Fig. 3d shows the tendency of the overlapping of two rods and forming a T-like shape. All the different types of ZnO structures have a dimension in nanoscale. So, sample RZ-pH9 has a different shape of ZnO morphology due to the agglomeration of ZnO nanoparticles with increasing pH value. Because in acidic medium the solution concentration of H+ ions is increased, this may be a favorable condition for the growth of columnar morphology of ZnO in RZ composite solution. In alkaline medium, the concentration OH− ions are increased which may be favorable to the agglomerate or overlap the growth of ZnO nanoparticles and form a different shape of ZnO. It is observed the distribution of different structure morphology of ZnO over the rGO sheets will improve the charge separation which will increase the photocatalytic activity. In the SAED pattern, preferred orientations are along the plane (100) and (002). In our work, we perform the photocatalytic activity using the catalytic RZ-pH6 because having the particular morphology as columnar-like. FTIR analyses of RZ-pH6, RZ-pH9, and RZ-pH12 composite samples are shown in Fig. 4. The band at 3428 cm −1 signifies the hydroxyl groups of absorbed H2O molecules. It is observed that peak width of sample RZ-pH6 decreases with increasing the pH value from acidic to alkaline medium and becomes the narrow peak of the sample RZ-pH12 as compared to the RZ-pH6 and RZ-pH9 because the additional amount of O-H from dropby-drop adding NaOH in composite solution reacts with the ZnSO4·7H2O to the pH values ≥ 6. The strong asymmetric stretching mode of variation of C=O was observed between 1628 and 1562 cm−1. It is also observed that the peak C=O was shifted which indicates the variation of structure morphology. The peaks C-OH (1172 cm−1) and C-O (1126 cm−1) are observed in the composites. Other groups such as carboxylic group at 1410 cm−1and hydroxide group at 1250 cm−1 disappeared in this spectrum. The standard broad peak at 450 cm−1 is analogous to pure ZnO, and it is vibrating in stretching modes [48]. It is also observed that stretching vibration of ZnO in the range 450–450 cm−1 is decreased with increasing pH value. This change indicates the changes in morphology of ZnO analogous to SEM and TEM results. Stretching mode in sample RZ-pH6, RZ-pH9, RZ-pH12 are similar. This pattern indicated the shifting peaks dramatically, and some of them disappeared after the reduction treatment, showing the absence of most of the functional groups that comprised of oxygen in the GO.
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Fig. 2 a SEM image of RZ-pH6. b EDX image of RZ-pH6. c SEM image of RZ-pH9. d SEM images of RZ-pH12. e SEM image of reduced grapheme oxide
Evaluation of optical and photocatalytic properties UV-visible absorption spectroscopy UV-visible normalized absorption spectra of RZ-pH6, RZpH9, and RZ-pH12 are shown in Fig. 5. The sharp characteristic of absorption peak signifies the crystalline structure and impurity present in ZnO structures. The absorbance of RZ composite depended on pH value when it changes from 6 to 9. On further increase in pH value, the absorbance dropped down. RGO contained in the composites increased the
absorbance in the visible region [49]. The optical band gap of the RZ composites at different pH value was determined by extrapolating the linear region of the αhυ2 versus energy plot as shown in the insert of Fig. 5b. The insert of Fig. 5b shows the band gap values of RZ-pH6 and RZ-pH9 about 3.3 and 3.25 eV, respectively. The variation in band gap was due to the morphology of ZnO and wt% of GO in RZ composites. Therefore, the presence of RGO in ZnO and obtained columnar morphology of ZnO were beneficial to increase the intensity of light absorption and for enhancement in the photocatalytic activity performance [50].
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Fig. 3 HR-TEM images of RZ-pH9 labeled by a rod- and belt-like, b pencil-like, c hammer- and palm-like, and d T shape-like in inset were the SAED pattern of ZnO
The Raman spectroscopy of RZ-pH6, RZ-pH9, and RZpH12 is shown in Fig. 6. There are two prominent peaks at
Fig. 4 FTIR spectra of RZ-pH6, RZ-pH9, and RZ-pH12
1350 cm−1 (D band) and 1580 cm−1 (G band) corresponding to the breathing mode of K-point phonons of A1g symmetry and the E2g phonon of sp2 C atoms, respectively [51]. The D′ peak in RZ-pH9 at 1604 cm−1 was obtained in our sample. The peaks D and D′ are defect-induced peak and the intensity ratio of the D to G band (ID/IG), as shown in Fig. 6, is a measure of the degree of graphitization and proportional to the average size of the sp2 domains in the samples. This ratio increased from 0.83 to 0.84 indicating the degree of reduction [52]. The higher intensity of the D band in prepared RZ composites reflected more defect than in GO associated with grain boundaries/vacancies [53, 54] and amorphous carbon [55]. The other Raman mode obtained at 2624 cm −1 (2D′ mode) in RZ-pH6 and at 2644 cm−1 in RZ-pH12. The 2D peak was slightly shifted as compared to the RZ-pH6 and disappeared in RZ-pH9. The G peak also shifted as compared to the RZ-pH6 and RZ-pH12. This indicated that RZ composite is reduced.
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Fig. 5 a UV-vis absorption spectra of RZ-pH6, RZ-pH9, andRZ-pH12 and b plot of αhυ2 versus photon energy for RZ-pH6, RZ-pH9, andRZ-pH12
Photocatalytic degradation activities of RZ-pH6 with ND The UV-visible spectrum of ND, RZ-pH6 with ND, its image, and their optical band gap was obtained by extrapolating the linear region of the αhυ2 versus energy plot shown in Fig. 7a. There were no absorption peaks in UV range of pure ND and
Fig. 6 Raman spectrum of RZ-pH6, RZ-pH9, andRZ-pH12
only appeared in the visible region at 664 nm. In the case of RZ-pH6 with ND, maximum absorption peaks appeared in the UV-visible region at 217 nm for rGO, 325 nm for ZnO, and 664 nm for ND. The insert in Fig. 7a shows the band gap values of RZ-pH6 and ND, found in the range of 3.0– 5.9 eV. This variation in band gap was due to the RGO, ZnO, and ND concentration in composite solution. The photocatalytic activity was performed in the photocatalytic experiment shown in Fig. 7b and recorded with UV-visible spectrum. The UV-visible spectrum of RZ-pH6 with ND and its image with increasing irradiation time between 0 and 120 min over the surface of a photocatalytic shown in Fig. 7c. The maximum absorption peaks of each sample occurred at 664 nm in the visible region. It can be seen that with an increased exposure time of sunlight, the absorbance of RZpH6 with ND was continuously decreasing but did not decrease under the dark condition and exhibit better photocatalytic performance. The absorption of light, transportation of charge, and separation during photocatalytic were key factors when RGO was introduced into ZnO [56]. The addition of RGO in the composite of ZnO increased the absorption intensity of visible light of the ZnO having a columnar morphology
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Fig.7 a UV-vis spectrum of ND and RZ-pH6 with ND solution and b insert of plot of (αhν)2 versus photon energy hν for ND and RZ-pH6 with ND solution. b Schematic diagram of photocatalytic experiment. c Timedependent absorption spectra of ND solution with RZ-pH6
photocatalytic. d Photo-stability of ND solution with RZ-pH6 by three times of cycling uses. e Photocatalytic degradation efficiency of ND solution with catalyst RZ-pH6. f Kinetic plot of ND solution with catalyst RZ-pH6
and there was a decrease pattern in the recombination of an electron-hole pair, resulting in enhanced photocatalytic
performance. The reusability of RZ-pH6 for three successive cycles of adsorption and desorption of ND was determined in
J Aust Ceram Soc
concentration, Langmuir-Hinshelwood first-order kinetic reaction is followed [57] and expressed as: InðC 0 =C Þ ¼ kt:
Fig. 8 Photocatalytic degradation process of ND by the RZ-pH6 composite under visible light illumination. Electrons from ND + (excited state of ND) flow to the conduction band (CB) of ZnO trough the RGO. Dotted lines represent the intra band gap energy levels of ZnO, which were narrowed by the interaction between ZnO and RGO during the synthesis of the RZ composites
ethanol as shown in Fig. 7d. It could be seen that the recycled use of RZ-pH6 for three times influenced photocatalytic activity and indicated that it can be reused for the adsorption of organic pollutants. It is also important to specify here that the particles size of RZ-pH6 in micron range having columnar morphology which is easily separated from the solution and could be further reused. However, the concentration of RZpH6 for the first cycle decreased with increasing irradiation time, and it further slightly decreases for the second and third cycles. This is due to the photo-corrosion effect of ZnO [56]. Figure 7e shows a percentage degradation curve for the catalysts RZ-pH6. It is observed that the degradation efficiency of ND with RZ-pH6 is 64.40% achieved in 120 min.
Kinetics of degradation Photocatalytic degradation kinetics for ND solution is analyzed with the RZ-pH6 catalyst which shown is in Fig. 7f. The kinetic study was carried out at 1 mL ND solution and catalyst loading of 0.01 mg/mL. For the low catalyst Table 2
ð8Þ
where k is the degradation reaction rate constant in min−1, C0 is the initial concentration, t the reaction time, and Ct the concentration at time t. The plot of In(C0/C) against t is a straight line with slope k which is determined via linear regression. The concentration (C/C0) varied during the photocatalytic experiment and depended on the maximum absorbance (A/A0). The major reaction steps in this photocatalytic mechanism under sunlight irradiation are summarized by the following Eqs. (9–12): sunlight
ZnO þ h v → ZnO ðe− þ hþ Þ −
ð9Þ −
ZnO ðe Þ þ RGO→ZnO þ RGOðe Þ þ
ð10Þ
−
ZnOðh Þ þ OH →ZnO þ :OH
ð11Þ
OH þ N D→degradation product
ð12Þ
Figure 8 shows the enhanced photocatalytic activity due to the narrow band gap of RZ. There was an adjustment in energy levels among the ND, RGO, and ZnO by charge transfer when there was an induced visible light absorption in ND. The absorption of ND molecules was also affected by the surface area of RGO. The electrons from the excited ND could be easily transferred to the rGO after sunlight illumination because of the conductive nature of rGO and its work function level [58]. It may be beneficial for electron transfer from CB of ZnO to graphene layer. The RZ composite served much better as a photocatalyst on degrading ND than the pure ZnO [59, 60], while rGO absorbed the ND on its surface by p-p stacking and affected the photodegradation process [61–64]. This can be concluded that the rGO did not directly involve in the photodegradation process as a reaction agent. As shown in Fig. 8, the electronic interaction between ZnO and RGO was
Comparative study of degradation efficiency of RGO-ZnO-based photocatalysts
Phocatalysts
Experimental c ondition Catalyst loading (mg L−1)
Dye
Light source
Irradiation times
rGO–ZnO rGO–ZnOpH~11 (nanoparticles and nanorods) rGO–ZnO (sphere) rGO–ZnO nanowire
30 20
Methylene blue Methylene blue
Xe lamp 200 W UV lamp 12 W
50 min 120 min
0.2 20
Methylene blue Rhodamine 6G
90 min 30 min
RGO-ZnOpH~6 columnar shape
0.01
Natural Dye (ND)
Visible irradiation Mercury lamp 150 W Sunlight
120 min
Degradation efficiency (%)
Ref.
80% -
[54] [38]
64.40%
[56] [57] Present study
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one the crucial factor for the efficiency of separation of generated charges. The photocatalytic activity performance of different morphology of ZnO with RGO-ZnO nanocomposite in previous report in the literature and our present morphology is listed comparatively in Table 2. In our work, although the columnar shape of ZnO structure samples with RGO was irradiated sunlight with lower catalyst loading (RZ-pH6), achieved degradation efficiency was increased with increasing exposure time under sunlight.
Conclusions A simple powder processing method has been successfully demonstrated for the preparation of columnar-shaped ZnO structures with an average diameter ranging 0.8–1.57 μm. The analysis of XRD, SEM, HR-TEM, and Raman spectroscopy results indicated that the RZ composite structures can be easily synthesized with improved crystallinity and higher degrees of reduction in the prepared samples could be obtained. The UV-visible and Raman spectroscopy results also showed that graphene oxide in the RZ composites is in reduced form. The obtained properties of the composites with the present method confirmed that the materials’ morphology has influenced the absorption and photocatalytic activity of natural dye under sunlight irradiation and resulted in maximum degradation efficiency of 64.40% achieved in 120 min and degradation efficiency increases with the increase in exposure time under sunlight. Funding The authors gratefully acknowledge the financial support of IIT (B.H.U) MHRD, New Delhi India.
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