Enhanced photocatalytic activity of Al and Fe co-doped ZnO nanorods for methylene blue degradation

Enhanced photocatalytic activity of Al and Fe co-doped ZnO nanorods for methylene blue degradation

Ceramics International xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate...

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Ceramics International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Enhanced photocatalytic activity of Al and Fe co-doped ZnO nanorods for methylene blue degradation N.R. Khalid*, A. Hammad, M.B. Tahir, M. Rafique, T. Iqbal, G. Nabi, M.K. Hussain Department of Physics, University of Gujrat, HH Campus, Gujrat, 50700, Pakistan

A R T I C LE I N FO

A B S T R A C T

Keywords: Al–Fe/ZnO Nanorods Hydrothermal synthesis Optical properties MB degradation

Photocatalysis is a promising technique for the degradation of harmful organic dyes present in the waste water. Therefore, in this study Al, Fe co-doped ZnO (Al–Fe/ZnO) nanorods were synthesized via simple and costeffective hydrothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) showed that synthesized photocatalyst Al–Fe/ZnO has high crystallinity, nanorods-like morphology and compositionally pure. UV–visible absorption and photoluminescence (PL) spectroscopies were utilized to study the optical properties of samples. The results of UV–visible spectroscopy demonstrate that Al and Fe doping has decreased the band gap energy from 3.37 to 2.62 eV. This decrease in ZnO band gap allows it to absorb more photons for the excitation of valence band electrons and consequently, higher efficiency of the photocatalyst under light illumination. The PL results show that the doping of Al and Fe have played an efficient role to inhibit the recombination of electrons/holes pairs in ZnO during photocatalysis. Moreover, the visiblelight driven photocatalytic efficiency of the samples was evaluated using methylene blue (MB) as model contaminant. It was observed that Al–Fe/ZnO photocatalyst exhibited seventeen (17) time higher degradation rate constant (k) for MB dye as compare to pure ZnO. This excellent photocatalytic activity of Al–Fe/ZnO sample could be attributed to rod-shaped morphology, improved visible light absorption and inhibited charge carrier's recombination due to synergistic effects of Al and Fe metal dopants.

1. Introduction Now a day, harmful organic pollutants of water produced by some industries have become more and more toxic for environment and human health [1,2]. Photocatalytic degradation of organic compounds is a capable strategy to resolve many recent energy and environmental problems through its wide applicability [3]. With fast growth in the field of nanoscience and nanotechnology, nanostructured photocatalytic materials have attained lot of attraction due to their ability for remediation of organic pollutants [4,5]. Amongst the most studied photocatalytic semiconductors, zinc oxide (ZnO) photocatalyst is one of the best choices due to its low cost, easy preparation, and nontoxic nature [6,7]. However, semiconductor ZnO has wide band gap (3.37 eV) and is only UV-light active material, which restricts ZnO role in practical applications [8]. Secondly, for photocatalytic reactions high oxidizing power of photo-generated holes in the valence band and high oxidation power of electrons in conduction band is required. Unfortunately, ZnO photocatalyst has lower efficiency due to very short life-times (0.322 ns) of photogenerated electrons and holes because it causes higher recombination of charge carriers during photocatalysis

*

[9]. Therefore, it is highly required to enhance the performance of ZnO photocatalyst not only by reducing the recombination of charge carriers but also improving its visible light absorption. Thus, to increase ZnO light absorption in whole visible light region for maximum utilization of solar spectrum, various modifying strategies have been tried in the recent past [10,11]. The most effective technique to advance the performance of ZnO photocatalyst in visible-light range, is doping of metal ions into ZnO [12–14]. This technique has the advantage to overcome electron/hole pairs recombination effectively along with higher visible light absorption. Numerous metal ions such as Ni, Fe, Co, Bi, Mn and Al etc. [15–23] have been doped into ZnO to enhance its performance as photocatalyst. Due to non-toxic nature, easy availability and suitability, Al and Fe metals are good choice as dopant materials to improve ZnO photocatalytic efficiency. Zhang et al. [23] showed that 20% Al doping into ZnO has increased the optical absorption capacity as well as photocatalytic performance for methyl orange degradation. Habba et al. [11] have synthesized Fe-doped ZnO nanowires and they observed reduction in ZnO band gap energy and enhanced photocatalytic performance in the degradation of organic dyes due to Fe dopant.

Corresponding author. E-mail address: [email protected] (N.R. Khalid).

https://doi.org/10.1016/j.ceramint.2019.07.132 Received 28 May 2019; Received in revised form 7 July 2019; Accepted 11 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: N.R. Khalid, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.07.132

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The synthesis method affects the surface morphology of the material leading to change in photocatalytic activity. There are various techniques to prepare ZnO based nanostructured materials including sol-gel process, hydrothermal synthesis, electrochemical, chemical vapor deposition and magnetron sputtering [24–26]. Among them, the hydrothermal method is the cheapest and most convenient way to prepare large-area samples, so it is widely used [27]. Researchers have prepared one dimensional nanostructures including nanorods, nanobelts, and nanowires which improve the charge carrier mobility and thus, improved photocatalytic activity [11,28]. The synthesis of Fe– Al/ZnO nanorods based photocatalyst via hydrothermal process is not reported in the literature to the best of our knowledge. This research aims to prepare the Al–Fe/ZnO nanorods using facile hydrothermal process. The influence of Al and Fe dopants on structural, morphological, UV–visible and photoluminescence properties of ZnO were studied. The loading of Al and Fe dopants has decreased the ZnO band gap and inhibited the recombination of charge carrier's. Moreover, photocatalytic test showed superior visible-light-responsive efficiency of Al–Fe/ZnO photocatalyst. Fig. 1. XRD patterns of ZnO, Fe/ZnO, AL/ZnO and Al–Fe/ZnO nanorods.

2. Experimental

ZnO nanorods analyzed by XRD. The detected peaks of all samples confirm the wurtzite structure (hexagonal) of ZnO according to JCPDs Card No. 36–1451. The doped and co-doped ZnO XRD patterns show that there are only ZnO peaks without any due to Fe or Al or metal oxides (Fe2O3 or Al2O3) phases. Secondly, there is no peak shift towards lower or higher angle in doped ZnO samples, only slight reduction in intensity of diffraction peaks is observed. The Debye-Scherrer equation was used to calculate the average crystallite size of sample and is estimated to be 20 nm for bare ZnO and 18 nm, 17 nm and 15 nm for Fe/ ZnO, Al/ZnO and Al–Fe/ZnO samples respectively. These results show that crystallite size of doped or co-doped samples decreases due to Al and Fe incorporation, indicating that the Fe or Al doping inhibited the crystal growth of ZnO structure. The morphology of Al–Fe/ZnO sample was studied by means of SEM. Fig. 2(a–c) displays the SEM of Al–Fe/ZnO nanorods at different magnifications. Interestingly, Al–Fe co-doped ZnO nanorods have sharp tip-shaped like a needle. Furthermore, rods are of different sizes having diameter of 200–500 nm. The EDX spectrum of Al–Fe co-doped sample is shown in Fig. 2d, which confirms the presence of Al, Fe, Zn with weight percentages as displayed in the inset of Fig. 2d. It can be seen that there is also an extra peak of carbon in EDX spectrum which is due to the carbon-tape substrate. Thus, it is concluded that the hydrothermal method was very effective to prepare Al–Fe co-doped ZnO nanorods in this study. UV–visible absorption spectra measurements were made in the wavelength range of 200–800 nm as displayed in Fig. 3a. The values of band gap energy of ZnO, Fe/ZnO, Al/ZnO and Al–Fe/ZnO nanorods are extracted from absorption spectra using Tauc plot equation [29],

2.1. Al–Fe/ZnO nanorods synthesis Al–Fe/ZnO nanorods were prepared by simple hydrothermal using zinc nitrate (Zn (NO3)2·6H2O), iron nitrate Fe(NO3)3.9H2O, aluminum nitrate (Al (NO3)3.9H2O) as precursor and doping source. For the preparation of samples, 0.2 M zinc nitrate solution were dissolved in deionized containing required amount of NaOH in continuous 2 h stirring. Then aluminum nitrate and/or iron nitrate solutions were mixed drop by drop in above solution to prepare doped and co-doped samples. After that, the above solution containing white precipitate was transported into Teflon lined autoclave (100 ml) in the oven at 180 °C for 12 h for hydrothermal reaction. The resulted solution after hydrothermal process was washed several time to obtain the pH neutral. Finally, the product was dried at 80 °C overnight for 12 h and calcined to 450 °C for 3 h. The molar ratios of Al to Zn and Fe to ZnO were 6% as previously optimized in our study [22]. 2.2. Characterization XRD analysis was made by powder X-rays diffraction (XRD) equipped with D/max2500 and a CuKα source (λ = 0.1541 nm). The morphological and elemental analysis of Al–Fe/ZnO photocatalyst was completed with scanning electron microscope (SEM; LEO1530). The measurement of UV–visible absorption spectra of the photocatalysts was made by Shimadzu, UV-1800 spectrometer. The photoluminescence spectra were measured with FP-8200, JASCO spectrometer. The excitation wavelength was fixed at 280 for all samples. 2.3. Photocatalytic efficiency evaluation

(αh ϑ)2 = A (h ϑ − Eg )

The efficiency of synthesized photocatalysts was estimated by monitoring the methylene blue degradation as a model compound. A cylindrical pyrex reaction cell of 100 ml capacity equipped with water circular system was used in the experiment. A 50 mg amount of synthesized nanorods was dissolve into 50 ml de-ionized water containing 10 mg dye concentration under continuous magnetic stirring of 10 min before visible-light (λ ≥ 400 nm) illumination with Xenon lamp 300 W (Au Light, CEL-HXF-300, Beijing). The samples were taken out after equal interval of time, filtered and examined via UV–vis spectrometer to evaluate the degraded amount of MB dye.

where, “hϑ ” represents incident photon energy, “α ” is absorption coefficient, “ A ” is a proportionality constant and “Eg ” is for band gap energy. The band gap energy of all samples was found from the plot of (αhϑ )2 versus hϑ (Fig. 3 (b)). The calculated band gap energies are 3.37 eV (pure ZnO), 2.98 eV (Al/ZnO), 2.87 eV (Fe/ZnO) and 2.62 eV (Al–Fe/ZnO) which show clear reduction in band gap energy due to the incorporation of Al and Fe into ZnO matrix. The narrowing of ZnO band gap due to Al doping was due to the introduction of defects/impurity states within the band gap of ZnO. Therefore, such impurity band development give rise to new donor electronic states just below ZnO conduction band due to hybridization of Al dopant and ZnO matrix [29]. Secondly, “s-d, p-d” exchange-interaction between ZnO band electrons and localized “d” electrons of Fe present in ZnO matrix has reduced the band gap of ZnO [30]. This, “s-d”, “p-d” exchange

3. Results and discussion Fig. 1 displays patterns of pure ZnO, Fe/ZnO, Al/ZnO and Al–Fe/ 2

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Fig. 2. (a–c) SEM images of hydrothermally synthesized Al–Fe/ZnO nanorods at different magnifications (d) EDX of Al–Fe/ZnO nanorods sample.

The performance of all photocatalysts under visible light irradiation (λ ≥ 400 nm) was explored via degradation of methylene blue (MB) dye. Fig. 5 displays the photocatalytic efficiency results of photocatalysts and it is obvious that Al–Fe co-doped ZnO photocatalyst has degraded 90% MB dye in just 75 min. The activity of the photocatalysts follows the order Al–Fe/ZnO Al/ZnO, Fe/ZnO and pure ZnO. For further investigation, The Langmuir–Hinshelwood (L-H) model was used to evaluate degradation rate constant of methylene blue dye [34]:

interactions cause negative correction to the conduction band edge and positive correction to the valence band edge of ZnO matrix resulting into narrowing of band gap [11]. The photoluminescence spectra were obtained to study the charge transfer and migration properties in pure ZnO, Fe/ZnO, Al/ZnO and Al–Fe/ZnO nanorods. The PL emission intensity results from the recombination of photo-excited electron/hole pairs, therefore, lower and higher PL emission intensities show lower and higher recombination of photogenerated charge carriers [31,32]. Fig. 4 presents PL emission spectra of all synthesized photocatalysts. The obtained spectra can be differentiated into two regions, one which is called band edge emission region have the range from 365 to 385 nm. This first region of emission spectrum results from the conduction band electrons and valence band holes’ recombination [33]. The second emission region in the wavelength range of 385–525 nm could be ascribed to the defects emission region where emission results from the existence of defects in the ZnO structure. Moreover, pure ZnO shows highly intensive peaks but as ZnO is doped or co-doped, a noteworthy reduction in the intensity of PL spectra is observed. The emission intensity is greatly reduced in Al–Fe/ ZnO nanorods as compare to Al or Fe doped ZnO samples. It follows therefore that Al–Fe co-doping into ZnO has effectively reduced the defects in structure of ZnO by removing surface defects. It is known that defects are generally concentrated on the surface of material rather than in bulk phase and become suppressed due to grafting material surface by surfactant or dye molecules. In our case, the emission intensity was decreased by doping ZnO either by Al or Fe and/or both. Thus, it can be concluded that dopants presence on the surface of ZnO nanorods have reduced the surface defects of ZnO and hence charge carriers recombination.

ln

Co = kt C

here, “Co ” is initial MB concentration, "C" is concentration of MB after illumination time “t” and k represents rate constant of pseudo-firstC order. After plotting the graph between ln Co versus time, t, the rate constants for all photocatalysts were found from the slope of straight line (Fig. 5b). It is noticeable that all samples show almost linear curve which verifies that degradation of MB dye follows pseudo-first-order reaction kinetics. It is known that there will be higher photocatalytic activity if first order constant is higher [35]. The obtained values rate constant (k) are 0.0024 min−1, 0.0036 min−1, 0.0068 min−1 and 0.014 min−1 for bare ZnO, Fe/ZnO, Al/ZnO and Al–Fe/ZnO photocatalysts respectively. This data shows that Al–Fe/ZnO sample has seventeentime superior performance as compare to bare ZnO. The excellent improvement in photocatalytic performance of Al–Fe/ZnO might be attributed to the cooperative effects of Al and Fe dopants. The stability of the Al–Fe/ZnO photocatalyst was determined by recycling experiment under the same conditions for degradation of methylene blue. Fig. 6 demonstrate no obvious decline in the degradation efficiency of the photocatalyst after five cycles, suggesting 3

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Fig. 3. (a) UV–visible absorption spectra (b) Tauc plot of different samples.

Fig. 5. (a) Photocatalytic test of different photocatalysts for MB dye degradation under visible-light illumination (λ ≥ 400 nm), (b) The plot of ln (Co/C) versus irradiation time at different initial methylene blue (MB) concentrations. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4. PL spectra of ZnO, Fe/ZnO, AL/ZnO and Al–Fe/ZnO nanorods photocatalyst.

that the Al–Fe/ZnO photocatalyst has high stability for MB degradation. Photocatalytic reaction mechanism of Al–Fe/ZnO is shown in the schematic diagram of Fig. 7. Upon absorption of light by the photocatalyst, the electrons from valence band transfer to the conduction band along with the production of holes in the valence band. The photo-created electrons reduce the oxygen to form superoxide radicals (O2−) and at the same time holes present in valence band oxidize the water molecules/OH− ions to produce hydroxyl radicals (OH.). These

Fig. 6. Cyclic test of Al–Fe/ZnO for its stability.

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Fig. 7. Schematic diagram of proposed reaction mechanism for MB dye degradation using Al–Fe/ZnO photocatalyst.

superoxide and hydroxyl radicals participate in degradation process of MB dye. It was previously observed that photogenerated electrons that move to the conduction band are mostly unstable and quickly go back to valence band. Therefore, they can recombine with the holes by going to valence band and overall decreases the quantum efficiency of photocatalysis. In this study, enhancement in photocatalytic performance of Al–Fe/ZnO can be attributed to combine effect of Al and Fe doping. The reason may be that the localized electronic states of dopant ions (Al3+, Fe3+) acted as charge traps for photocreated electron-hole pairs (Fig. 6), and reduced the recombination of charge carriers, thereby, improved activity by Al–Fe/ZnO photocatalyst [11,34]. 4. Conclusions In summary, Al and Fe co-doped ZnO nanorods have been prepared via hydrothermal method. The samples were tested as photocatalysts in MB dye degradation under visible light excitation. A superior efficiency of Al–Fe/ZnO photocatalyst was found compare to Al/ZnO, Fe/ZnO and bare ZnO. The improved activity of Al–Fe/ZnO photocatalyst could be attributed to the synergistic effects of Al and Fe dopants. In fact, life time of charge carriers was noticeably increased due to Al and Fe doping into ZnO matrix as confirmed by PL which played an important role in the generation of required radicals (superoxide and hydroxyl) for the degradation of toxic organic compounds into harmless compounds. The PL analysis showed that incorporation of Al and Fe into ZnO structure have greatly inhibited the recombination of electron-hole pairs. The absorption measurements confirmed the reduction in ZnO band gap after introducing Al and Fe into ZnO. Thus, higher photocatalytic activity of Al–Fe/ZnO nanorods proves that the prepared novel photocatalyst could have potential applications in the field of energy and environment. References [1] Hall-Spencer, M. Jason, Riccardo Rodolfo-Metalpa, Sophie Martin, Emma Ransome, Maoz Fine, Suzanne M. Turner, Sonia J. Rowley, Dario Tedesco, MariaCristina Buia, Volcanic carbon dioxide vents show ecosystem effects of ocean acidification, Nature 454 (2008) 7200 96. [2] Denghui Jiang, Yuegang Zhang, Xinheng Li, Folded-up thin carbon nanosheets grown on Cu2O cubes for improving photocatalytic activity, Nanoscale 9 (34) (2017) 12348–12352. [3] Zhigang Xiong, Li Li Zhang, Jizhen Ma, X.S. Zhao, Photocatalytic degradation of dyes over graphene–gold nanocomposites under visible light irradiation, Chem. Commun. 46 (33) (2010) 6099–6101.

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