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Visible light mediated photocatalytic activity of Ni-doped Al2O3 nanoparticles
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Visible light mediated photocatalytic activity of Ni-doped Al2 O3 nanoparticles S. Anbarasu , S. Ilangovan , K. Usharani , A. Prabhavathi , M. Suganya , S. Balamurugan , C. Kayathiri , M. Karthika , V.S. Nagarethinam , A.R. Balu PII: DOI: Reference:
S2468-0230(19)30259-7 https://doi.org/10.1016/j.surfin.2019.100416 SURFIN 100416
To appear in:
Surfaces and Interfaces
Received date: Revised date: Accepted date:
10 May 2019 15 November 2019 29 November 2019
Please cite this article as: S. Anbarasu , S. Ilangovan , K. Usharani , A. Prabhavathi , M. Suganya , S. Balamurugan , C. Kayathiri , M. Karthika , V.S. Nagarethinam , A.R. Balu , Visible light mediated photocatalytic activity of Ni-doped Al2 O3 nanoparticles, Surfaces and Interfaces (2019), doi: https://doi.org/10.1016/j.surfin.2019.100416
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Visible light mediated photocatalytic activity of Ni-doped Al2O3 nanoparticles S. Anbarasua,b, S. Ilangovana, K. Usharania, A. Prabhavathib,c, M. Suganyab, S. Balamuruganb, C. Kayathirib, M. Karthikab, V.S. Nagarethinamb, A.R. Balub,* a
PG and Research Department of Physics, Thiru Vi Ka Govt College, Thiruvarur, Tamilnadu, India
b
PG and Research Department of Physics, AVVM Sri Pushpam College, Poondi, Tamilnadu, India c
PG and Research Department of Physics, Sengamalathayar Educational Trust College, Mannarkudi, Tamilnadu, India
*Corresponding author Dr. A.R. BALU Ph: + 91 9442846351 Email: [email protected]
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Abstract Visible light mediated photocatalytic activity of Ni-doped Al2O3 nanoparticles synthesized via precipitation method has been reported in this paper. Ni doping concentration in Al2O3 is varied as 0, 2, 4 and 6 wt.%. The presence of diffraction peaks related to monoclinic crystal structure of θ- Al2O3 is evinced from the XRD patterns of the undoped and doped samples. The diffraction peak (0 0 2) appears to be dominant for all the samples and crystallite size decreased from 41 nm to 33 nm with Ni doping. A shift towards higher wavelength in the absorption edge of pure Al2O3 is evinced with Ni doping. PL spectra confirm the presence of peaks related to oxygen vacancies in the samples. Photocatalytic activity tests performed against metanil yellow dye confirmed that the doped samples exhibit better degradation efficiency than the undoped sample. The 4 wt.% Ni-doped Al2O3 catalyst exhibits a maximum degradation efficiency of 86.84% after 90 min visible light irradiation which was well acknowledged from its decreased crystallite size, band gap values and improved surface morphology. The results obtained confirmed that the Ni-doped Al2O3 nanoparticles are well suited as visible light mediated catalysts for the degradation of toxic organic dyes.
Keywords Doping; visible light; degradation; precipitation
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1.
Introduction Water contamination in recent years is a major threat to the human as well as aquatic
life due to the discharge of organic pollutants from the textile, food and cosmetics industries and other toxic industrial effluents. The techniques used to eliminate the contamination from the sewages such as coagulation, precipitation, flotation, activated carbon adsorption, reverse osmosis, electro dialysis, etc seems to be ineffective due to the mutagenic as well as carcinogenic nature of the toxic pollutants [1]. The advanced oxidation processes (AOPs) involving semiconductor photo catalysts seem to be effective in treating the organic pollutants due to the production of highly oxidizing species, such as H2O2, OH* and radicals which degrade the pollutants present in the waste water [2]. Metal oxide semiconductors such as TiO2 [3], CdO [4], ZnO [5], SnO2 [6], NiO [7], etc have been extensively used as catalysts for degrading organic pollutants. In this work, the role of aluminium oxide (Al2O3) as a photocatalyst under visible light was studied in detail. Aluminium oxide is a wide band gap material with interesting properties in pure and doped formats. Al2O3 generally exists in several phases such as γ-, δ-, θ- and α- and in all phases it exhibits high specific area, surface acidity and defects in the crystalline structure which makes it suitable for many technological applications [8]. Also, Al2O3 exhibits high optical, chemical and thermal stability under irradiation [9]. Al2O3 finds applications in electronics, catalysis and as structural ceramic due to its high mechanical strength and hardness, good alkaline and acidic corrosion resistance at high temperatures, excellent wear resistance and outstanding dielectric properties [10]. Al2O3 in γ-phase has been widely used as adsorbents in dye adsorption and also used as a support for adsorbents [11]. The non-toxic and hydrolytic stability make Al2O3 an effective photocatalyst for degrading toxic dye molecules [8]. However, under visible light, the efficiency of Al2O3 based catalyst seems to be appreciably low due to the limited generation of OH* and
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radicals which degrade the
dye molecules effectively. A possible reason for this might be due to its wide band gap. The band gap values of Al2O3 in κ, α and θ phases were reported to be 4.05, 8.8 and 5.15 eV, respectively [12]. By reducing the band gap value appreciably, Al2O3 could respond well under visible light. To shrink the band gap and improve the photocatalytic performance of Al2O3, doping with transition metal ions could be performed. Improved photocatalytic efficiency has been realized for Ag [13] and Zn [14] doped Al2O3 photocatalysts. Also, to be an efficient regenerable and reusable catalyst, Al2O3 should possess magnetic behavior. Hence, in this work a dopant ion has been selected in such away it can reduce the band gap and induce magnetic orderings in Al2O3. Ni doping has been performed to make Al2O3 to respond very effectively under visible light. Nickel (Ni2+) is a transition metal ion which has an ionic radius of 0.69 Å which is slightly higher than Al3+ (0.51 Å) ion. Also, the electro-negativity of Ni is 1.8 Pauling which is higher than that of Al (1.5 Pauling). Thus, Ni can create significant lattice defects in the Al2O3 matrix thereby improving its structural, optical, catalytic and magnetic properties significantly. It has been reported earlier that Ni-based catalysts have significant efforts on the catalytic performance against toxic dyes [15]. The present work aims to synthesize Nidoped (0, 2, 4 and 6 wt.%) Al2O3 nanoparticles via precipitation method. The structural, optical, catalytic and magnetic properties of the Ni-doped Al2O3 nanoparticles were studied and reported. 2.
Experimental details
2.1
Synthesis of undoped and Ni-doped Al2O3 nanoparticles Aluminium (III) chloride [AlCl3] and nickel chloride [NiCl2.6H2O] both of analytical
reagent grade (Sigma make) was used as the host and dopant precursor salts to synthesize Nidoped Al2O3 nanoparticles. De-ionized water (140 mL) and liquid ammonia (10 mL) were used as the solvent and precipitating agent, respectively. Undoped Al2O3 (0 wt.% Ni)
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nanoparticles was synthesized by dissolving 0.1M of aluminium (III) chloride in de-ionized water and liquid ammonia. After continuous stirring for 2 h, the solution was aged for 4 h to get white colored Al2O3 precipitates. The precipitates were separated, cleaned several times with water, calcined at 200°C for 1 h and then crushed to get Al2O3 nanoparticles. To achieve Ni doping, NiCl2.6H2O with 2, 4 and 6 wt.% of the weight of AlCl3 was added to the precursor solution used to synthesize pure Al2O3 and then by adopting the same procedure used to synthesize pure Al2O3, Ni-doped Al2O3 nanoparticles were synthesized. To protect the synthesized samples from long term oxidation by air they were kept under vacuum [16]. 2.2
Characterization The synthesized Ni-doped Al2O3 nanoparticles were characterized by techniques like
XRD, SEM, FTIR, PL, VSM and the details regarding the instruments used are compiled in Table 1. The specific surface area of the samples was evaluated by Barrett-Emmett-Teller (BET) method using ASAP2020C+M specific area and porosity analyzer. 2.3
Photocatalytic tests Photocatalytic experiments were performed under visible light irradiation against
metanil yellow (MY) dye. The visible light source used for the photocatalytic activity was a 100 W incandescent lamp which was kept vertically at a distance of 10 cm from the dye solution inside the photoreactor. Aqueous solution (100 mL) of 0.025 M MY and optimized value of the Ni-doped Al2 O3 nanoparticles (6 mg) was used as testing solution for the photocatalytic experiments. The dye solution with the catalysts was maintained in the dark for 30 min to ensure adsorption-desorption equilibrium. The pH value of the solution was maintained at 8. The photodegradation of the dye was followed by measuring absorption at regular intervals using a UV-Vis spectrophotometer (LAMBDA 35) at 439 nm wavelength.
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3.
Results and discussion
3.1
XRD Studies Fig.1 shows the XRD patterns of 0, 2, 4 and 6 wt.% Ni-doped Al2O3 nanoparticles. The
entire samples exhibit (2 0 1), (0 0 2), (3 1 1), (1 1 2), (6 0 0) and (5 1 1) diffraction peaks whose positions matched with monoclinic θ-Al2O3 crystal structure according to JCPDS cards No. 86-1410. The absence of Ni related peaks in the doped samples ascertained the entry of the dopant into the Al2O3 lattice without undergoing any chemical reactions with oxygen to form new compounds. The average crystallite size (D) of the samples was calculated by using the Debye-Scherrer equation,
(where
is the wavelength of the X- ray used
(1.5406 Å), θ is the diffraction angle) from the full-width at half maximum (β) value of the dominant (0 0 2) peak. The crystallite size values of the 0, 2, 4, and 6 wt.% Ni-doped Al2O3 nanoparticles were found to be 41, 38, 33 and 35 nm, respectively. The strain induced into the Al2O3 lattice due to the larger ionic radius of Ni2+ (0.69 Å) than Al3+ (0.51 Å) might have resulted in the decreased crystallite size values for the doped samples. Ni substitution may cause grain boundary restraining in Al2O3 matrix which due to symmetry-breaking effects at the boundary might have caused decrement in the crystallite size values [1]. Due to decreased crystallite size values, the doped samples exhibited better photocatalytic activity as can be seen in Section 3.6. 3.2
Surface morphology and Surface area analysis Fig. 2 shows the SEM images of a) 0 wt.% , b) 2 wt.% , c) 4 wt.% and d) 6 wt.% Ni-
doped Al2O3 nanoparticles. The surfaces of the Ni-doped Al2O3 samples are composed of irregular shaped grains with different sizes. Also, grains seem to be agglomerated for all the samples with plenty of empty sites. No significant variation is observed with Ni doping. However, agglomeration seems to be minimized and grains appeared to be tightly packed as Ni concentration increases. Compared to the undoped sample, decreased grain sizes were 6
evinced for the doped Al2O3 nanoparticles. Among the doped samples, the surface of the 4 wt.% Ni-doped Al2O3 (Fig. 2(c)) sample seems to tightly packed with nanosized grains without any empty sites confirming its improved surface morphology. BET calculation revealed much larger specific area for the 4 wt.% Ni-doped Al2O3 (126 m2g-1) sample. The BET values of the 0, 2 and 6 wt.% Ni-doped Al2O3 nanoparticles were found to be 87, 96 and 112 m2g-1, respectively. Due to increased surface area, the 4 wt.% Ni-doped Al2O3 nanoparticle exhibited enhanced photodegradation efficiency (Section 3.6). 3.3
FTIR Studies Fig. 3 shows the FTIR spectra of a) undoped (0 wt.% Ni) and b) 2, 4 and 6 wt.% Ni-
doped Al2O3 nanoparticles. The bands in the 3657-3140 cm-1 region and the peak at around 1634 cm-1 observed for both the undoped and doped samples are attributed to O-H stretching and bending vibrations, respectively [1]. The OH groups act as scavenger of photogenerated electrons and holes resulting in the formation of hydroxyl radicals OH * which degrades dye molecules effectively. N-H stretching vibration peak observed at 2055 cm-1 for the undoped Al2O3 and at 2018 cm-1 for the doped Al2O3 nanoparticles might have been originated from liquid ammonia which was used as precipitation agent to synthesize the nanoparticles [17]. Asymmetric stretching vibration of C=O is observed at 1404 cm-1 for all the Ni-doped Al2O3 nanoparticles [18]. C-O stretching vibration of adsorbed CO2 is observed at 1071, 1020 cm-1 for the undoped sample and at 1074 cm-1 for the doped samples [19]. C-H stretching vibration is observed at 980 cm-1 for the undoped sample and at 985 cm-1 for the doped samples [19]. Al-O vibrations were observed at 768, 620 cm-1 for the undoped Al2O3 and at 756, 621 cm-1 for the doped samples [16]. Ni related peak observed at 477 cm-1 confirmed the existence of Ni in the doped samples [20]. Metal-oxygen related peak is observed at 532 and 429 cm-1 for the undoped Al2O3 nanoparticles.
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3.4
Optical studies Fig. 4 shows the absorbance spectra of Al2O3 nanoparticles synthesized with 0, 2, 4 and
6 wt.% Ni doping concentrations. The absorption edge of pure Al2O3 shifted towards higher wavelength side with Ni doping inferring a reduction in the optical band gap (Eg) value. The Eg values of the Ni-doped Al2O3 nanoparticles were calculated from the absorption edge (λmax) values using the relation [14]: (1) The band gap values were 3.85, 3.79, 3.72 and 3.75 eV, respectively for the 0, 2, 4 and 6 wt.% Ni-doped Al2O3 nanoparticles. Clearly, the band gap of pure Al2O3 shifted towards lower value with Ni doping. Because of the red shift in the optical band gap, easy transfer of photoexcited electrons to the noble metal nanoparticles takes place leading to faster electronhole separation [21] and hence their recombination rate decreases, thereby improving the photocatalytic performance of the doped catalysts (Section 3.6). 3.5
PL studies Fig. 5 shows the room temperature photoluminescence spectra of Al2O3 nanoparticles
synthesized with 0, 2, 4 and 6 wt.% Ni doping concentrations, excited at λ = 320 nm. Seven emission bands were observed in the measured spectra. The UV emission peak at 360 nm ascertained to the extended band edge emission suggests the presence of lattice defects or impurities in the synthesized samples [22]. The near band edge emission at 377 nm may be attributed to the free excitonic emission of Al2O3 [23]. The excitonic emission peak observed at 410 nm may be related to the presence of oxygen vacancies in the synthesized Ni-doped Al2O3 nanoparticles [16]. The peaks at 475 nm and 494 nm are assigned to transitions occurring from metal interstitials (Ali) to the valence band [24]. Oxygen vacancies might have favored for the appearance of the peak at 494 nm as reported by Nallendran et al. [25].
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Deep-level or trap state emission is responsible for the emission peak at 521 nm [26]. Oxygen vacancies and OAl defects are the cause for the presence of the emission peak at 545 nm [27]. 3.6
Photocatalytic activity Photocatalytic activity of the Ni-doped Al2O3 nanoparticles was investigated for the
degradation of metanil yellow (MY), an anionic dye under visible light. The photocatalytic activity of MY dye by the Ni-doped Al2O3 nanoparticles is found to be influenced by various factors such as catalyst dosage, initial concentration of dye, pH, dopant content, etc. From our photocatalytic tests, we found that the degradation of MY dye is directly proportional to the dosage amount of the catalyst up to an optimum value of 6 mg / 100 mL. Above this optimized value, photocatalytic experiments showed negative results as the production of OH* radicals was inhibited due to the agglomeration of the particles which blocks the light illumination towards the surface. Therefore, 6 mg was fixed as the optimum dosage value for further experiments. 6 mg of the synthesized Ni-doped Al2O3 nanoparticles was mixed thoroughly in 100 mL aqueous solution containing 0.025 M MY dye. The dye solution with 0, 2, 4 and 6 wt.% Ni-doped Al2O3 catalysts was stirred for 30 min in dark to achieve adsorption-desorption and then subjected to visible light. As the light irradiation time increases, the color of the dye solutions with the catalysts start to fade confirming the degradation of the dye molecules. After 90 min light irradiation, the dye solutions with 2, 4, 6 wt.% Ni-doped Al2O3 catalysts seems to be faded better than that of the 0 wt.% Ni-doped Al2O3 catalyst and the dye solution with the 4 wt.% Ni-doped Al2O3 catalyst become almost colorless, confirming its enhanced degradation ability. The order of degradation ability was found to be in the order 4 wt. % Ni > 6 wt. % Ni > 2 wt. % Ni > 0 wt. % Ni In order to confirm that degradation is caused only by the catalyst used, prior to the photocatalysis experiment, a photolysis process is carried out for MY without the presence of
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any catalyst. Change in MY concentration at different time intervals in the presence and absence of Ni-doped Al2O3 catalysts as a function of irradiation time is shown in Fig. 6. There was no observable loss in the concentration of MY in the absence of photocatalyst and degradation takes place only in the presence of undoped and doped Al2O3 catalysts. The absorbance spectra of the 4 wt.% Ni-doped Al2O3 catalyst taken at λ = 439 nm after sampling 2 mL of the light irradiated dye solution is shown in Fig. 7. The absorption peak intensity decreases with increase in irradiation time, confirming the degradation nature of the dye molecules. Fig. 8 shows the degradation efficiencies (η) of the Ni-doped Al2O3 catalysts calculated using the formula [13]: (
⁄ )
(2)
where C0 and C are the concentration values of the dye under dark and light conditions, respectively. All the doped samples exhibited better degradation efficiencies and among them the 4 wt.% Ni-doped Al2O3 catalyst exhibited better degradation efficiency. The degradation efficiencies of the Al2O3 catalysts with 0, 2, 4 and 6 wt.% Ni doping concentrations after 90 min irradiation time were 57.14, 67.27, 86.84 and 78.72%, respectively. The photocatalytic ability of the Ni-doped Al2O3 nanoparticles depend on the generation of electron-hole (e-/h+) pairs from their surfaces when exposed to visible light. When the Ni-doped Al2O3 catalysts are exposed to light, the electrons in the valence band are excited to conduction band thereby generating electron-hole pairs [28]. The holes in the Nidoped Al2O3 catalysts react with water to form hydroxyl radicals (OH *); whereas the photoexcited electrons react with surrounding oxygen to form superoxy anion radicals ( These radicals attack the organic pollutant (MY) and degrade the dye molecules releasing CO2 and water [20]. Degradation efficiency in most cases reduces due to the undesirable recombination of electrons and holes. The dopant Ni2+ may accept an electron in the conduction band of Al2O3 to form Ni+ which may transport an electron to dissolved O2 to 10
).
produce superoxide radical anions, which inhibit the recombination of photoinduced electrons and holes. Thus, Ni2+ on the surface of Al2O3 nanoparticles may act as an electron scavenger. The schematic representation of the photocatalytic mechanism involved in the Ni-doped Al2O3 catalysts is pictured in Fig. 9. For efficient transfer of photoexcited electrons, the energy band gap (E g) must be appreciably small. Pure Al2O3 exhibits a high band gap value of 3.85 eV and hence its degradation efficiency is appreciably low (57.14%). However, when doped with Ni, the band gap of Al2O3 decreased appreciably which enhanced the effective transfer of photoexcited electrons and hence they exhibited better degradation efficiencies. Earlier research results [29-31] also showed a similar behavior. The creation of more vacancy defects into the Al2O3 matrix due to the substitution of Ni2+ ions results in the generation of more photoexcited electron-hole pairs and this might have also been a reason for the enhanced degradation efficiencies observed for the doped samples [32]. The photodegradation kinetics of the Ni-doped Al2 O3 catalysts were investigated based on the pseudo first-order kinetic model exemplified by Langmuir-Hinshelwood using the equation [14]: (
⁄ )
(3)
where k is the apparent first-order rate constant (min-1), t is the irradiation time, C0 and C are the concentrations of the dye under dark and light conditions, respectively. From the slopes of the graphs of
( ) vs. time (Fig. 10), the ‘k’ values of the Ni-doped Al2O3 catalysts were
calculated which was 0.00901, 0.01196, 0.02019 and 0.01713 min-1 for the 0, 2, 4 and 6 wt.% Ni-doped Al2O3 catalysts, respectively. The higher k values obtained for the doped samples than the undoped one supports for their enhanced degradation abilities. Higher degradation rate constant implies higher degradation efficiency. Among the doped samples, the 4 wt.% Ni-doped Al2O3 catalyst exhibited a higher degradation efficiency of 86.84% and this was 11
very much acknowledged with the high k value of 0.02019 min-1. Least band gap, crystallite size and improved surface morphology might have favored for the enhanced degradation efficiency observed for the 4 wt.% Ni-doped Al2O3 catalyst. Due to decreased crystallite size value, the specific surface area of the 4 wt.% Ni-doped Al2O3 catalyst increases and hence it had extra active sites to excite more photogenerated electron hole pairs under visible light, thereby decreasing their recombination rate. 3.7
Magnetic studies Fig. 11 shows the M-H curves of a) 0 wt.% b) 2 wt.%, c) 4 wt.%, and d) 6 wt.% Ni-
doped Al2O3 nanoparticles. Undoped Al2O3 and 2 wt.% Ni doped Al2O3 nanoparticles exhibit paramagnetic behavior (Fig. 10(a, b)). With further doping, well defined hysteresis loops was observed confirming their ferromagnetic nature. The para and ferro magnetic behaviors observed for the doped samples may be ascribed to oxygen vacancies and/or substitution of Al atoms by Ni atoms interstitially. Similar results have been reported by Aragon et al. [33] for Ni-doped SnO2 nanoparticles. The ferromagnetic behavior observed for the 4 and 6 wt.% Ni-doped Al2O3 nanoparticles might be due to the relatively high anisotropy contribution of Ni2+ ions and also due to the introduction of point defects (Ni vacancies) at dislocation cores of Al2O3 . This relies with the results obtained earlier for the SnS2-NiO nanocomposite [34]. Horizontal shift called exchange bias was observed for the 4 and 6 wt.% Ni-doped Al2O3 nanoparticles. The observed magnetic orderings with Ni-doping confirmed that the Ni-doped Al2O3 nanoparticles when used in photocatalytic applications can be easily separated from the dye solution by a magnet or an applied magnetic field, for taking absorbance spectra and to perform recycle tests.
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4.
Conclusion Ni-doped Al2O3 nanoparticles were synthesized via precipitation method with 0, 2, 4
and 6 wt.% Ni concentrations. The impact of Ni doping on the structural, optical, photocatalytic and magnetic properties of Al2O3 was studied systematically and the results obtained confirmed that Ni2+ ions strongly dominated them. Decreased crystallite size and band gap values were observed for the doped samples. Enhanced photocatalytic activity was observed for the doped samples and the 4 wt.% Ni-doped Al2O3 catalyst exhibited a maximum degradation efficiency of 86.84% against MY dye. Improved magnetic properties were observed for the Ni-doped Al2O3 nanoparticles. The ferromagnetic property observed for the Ni-doped Al2O3 nanoparticles is also an essential characteristic of a regenerable and reusable magnetic catalyst. Thus, Ni-doped Al2O3 nanoparticles are well suited for practical applications as efficient regenerable photocatalyst for the degradation of inorganic toxic dyes.
Author Contribution Statement S. Anbarasu: Conceptualization, S. Ilangovan: Conceptualization, K. Usharani: Methodology , A. Prabhavathi: Data curation , M. Suganya: Visualization, Investigation, S. Balamurugan: Formal analysis, C. Kayathiri: Resources, M. Karthika: Validation, V.S. Nagarethinam: Supervision, A.R. Balu: Data curation, Writing – Original draft preparation, Writing – Reviewing and Editing. The above descriptions are accurate and agreed by all the authors. Conflict of interest statement All the authors declare that the article is original and all are aware of its contents and they approve for its submission. The authors also confirm that the described work has not been published before; it is not under consideration for publication anywhere else and publication has been approved by all co-authors and the responsible authorities of the institute
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where the work has been carried out. No conflict of interest exists in the article. The authors declare that if the article is accepted it will not be published elsewhere in the same form in any language without the written consent of the publisher.
5.
Acknowledgements The Head, Alagappa University is very much thanked for the XRD and PL results.
Figure captions
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Fig. 1XRD patterns of the Ni-doped Al2O3 nanoparticles.
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Fig. 2SEM images of a) 0 wt.%, b) 2 wt.%, c) 4 wt.% and 6 wt.% Ni-doped Al2O3 nanoparticles.
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Fig. 3FTIR spectra of a) 0 wt.% and b) 2, 4 and 6 wt.% Ni-doped Al2O3 nanoparticles.
Fig. 4Absorbance spectra of the Ni-doped Al2O3 nanoparticles.
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Fig. 5PL spectra of the Ni-doped Al2O3 nanoparticles.
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Fig. 6Variation of C/Co as a function of illumination time for MY with the absence and presence of the catalysts.
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Fig. 7Absorbance spectra of the 4 wt.% Ni-doped Al2O3 catalyst recorded at different light irradiation time intervals.
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Fig. 8 Bar diagrams showing the photodegradation efficiencies of the Ni-doped Al2O3 catalysts.
Fig. 9Schematic representation of the photocatalytic mechanism involved in the Ni-doped Al2O3 catalysts.
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Fig. 10 Variation of
( ) as a function of irradiation time intervals.
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Fig. 11 M-H curves of a) 0 wt.%, b) 2 wt.%, c) 4 wt.% and d) 6 wt.% Ni-doped Al2O3 nanoparticles.
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Table 1 Instrument details used to characterize the Ni-doped Al2O3 nanoparticles Characterization technique
Instrument
XRD
X’pert PRO analytical PW 340/60 diffractometer
SEM
HITACHI S-3000H scanning electron microscope
FTIR
Perking Elmer RX-1 spectrophotometer
Optical
LAMBDA-35 UV-Vis-NIR double beam spectrophotometer
PL
Varian Cary Eclipse Fluorescence spectrophotometer
VSM
Lakheshore 7410 vibrating sample magnetometer
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Visible light mediated photocatalytic activity of Ni-doped Al2 O3 nanoparticles S. Anbarasu , S. Ilangovan , K. Usharani , A. Prabhavathi , M. Suganya , S. Balamurugan , C. Kayathiri , M. Karthika , V.S. Nagarethinam , A.R. Balu PII: DOI: Reference:
S2468-0230(19)30259-7 https://doi.org/10.1016/j.surfin.2019.100416 SURFIN 100416
To appear in:
Surfaces and Interfaces
Received date: Revised date: Accepted date:
10 May 2019 15 November 2019 29 November 2019
Please cite this article as: S. Anbarasu , S. Ilangovan , K. Usharani , A. Prabhavathi , M. Suganya , S. Balamurugan , C. Kayathiri , M. Karthika , V.S. Nagarethinam , A.R. Balu , Visible light mediated photocatalytic activity of Ni-doped Al2 O3 nanoparticles, Surfaces and Interfaces (2019), doi: https://doi.org/10.1016/j.surfin.2019.100416
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Visible light mediated photocatalytic activity of Ni-doped Al2O3 nanoparticles S. Anbarasua,b, S. Ilangovana, K. Usharania, A. Prabhavathib,c, M. Suganyab, S. Balamuruganb, C. Kayathirib, M. Karthikab, V.S. Nagarethinamb, A.R. Balub,* a
PG and Research Department of Physics, Thiru Vi Ka Govt College, Thiruvarur, Tamilnadu, India
b
PG and Research Department of Physics, AVVM Sri Pushpam College, Poondi, Tamilnadu, India c
PG and Research Department of Physics, Sengamalathayar Educational Trust College, Mannarkudi, Tamilnadu, India
*Corresponding author Dr. A.R. BALU Ph: + 91 9442846351 Email: [email protected]
1
Abstract Visible light mediated photocatalytic activity of Ni-doped Al2O3 nanoparticles synthesized via precipitation method has been reported in this paper. Ni doping concentration in Al2O3 is varied as 0, 2, 4 and 6 wt.%. The presence of diffraction peaks related to monoclinic crystal structure of θ- Al2O3 is evinced from the XRD patterns of the undoped and doped samples. The diffraction peak (0 0 2) appears to be dominant for all the samples and crystallite size decreased from 41 nm to 33 nm with Ni doping. A shift towards higher wavelength in the absorption edge of pure Al2O3 is evinced with Ni doping. PL spectra confirm the presence of peaks related to oxygen vacancies in the samples. Photocatalytic activity tests performed against metanil yellow dye confirmed that the doped samples exhibit better degradation efficiency than the undoped sample. The 4 wt.% Ni-doped Al2O3 catalyst exhibits a maximum degradation efficiency of 86.84% after 90 min visible light irradiation which was well acknowledged from its decreased crystallite size, band gap values and improved surface morphology. The results obtained confirmed that the Ni-doped Al2O3 nanoparticles are well suited as visible light mediated catalysts for the degradation of toxic organic dyes.
Keywords Doping; visible light; degradation; precipitation
2
1.
Introduction Water contamination in recent years is a major threat to the human as well as aquatic
life due to the discharge of organic pollutants from the textile, food and cosmetics industries and other toxic industrial effluents. The techniques used to eliminate the contamination from the sewages such as coagulation, precipitation, flotation, activated carbon adsorption, reverse osmosis, electro dialysis, etc seems to be ineffective due to the mutagenic as well as carcinogenic nature of the toxic pollutants [1]. The advanced oxidation processes (AOPs) involving semiconductor photo catalysts seem to be effective in treating the organic pollutants due to the production of highly oxidizing species, such as H2O2, OH* and radicals which degrade the pollutants present in the waste water [2]. Metal oxide semiconductors such as TiO2 [3], CdO [4], ZnO [5], SnO2 [6], NiO [7], etc have been extensively used as catalysts for degrading organic pollutants. In this work, the role of aluminium oxide (Al2O3) as a photocatalyst under visible light was studied in detail. Aluminium oxide is a wide band gap material with interesting properties in pure and doped formats. Al2O3 generally exists in several phases such as γ-, δ-, θ- and α- and in all phases it exhibits high specific area, surface acidity and defects in the crystalline structure which makes it suitable for many technological applications [8]. Also, Al2O3 exhibits high optical, chemical and thermal stability under irradiation [9]. Al2O3 finds applications in electronics, catalysis and as structural ceramic due to its high mechanical strength and hardness, good alkaline and acidic corrosion resistance at high temperatures, excellent wear resistance and outstanding dielectric properties [10]. Al2O3 in γ-phase has been widely used as adsorbents in dye adsorption and also used as a support for adsorbents [11]. The non-toxic and hydrolytic stability make Al2O3 an effective photocatalyst for degrading toxic dye molecules [8]. However, under visible light, the efficiency of Al2O3 based catalyst seems to be appreciably low due to the limited generation of OH* and
3
radicals which degrade the
dye molecules effectively. A possible reason for this might be due to its wide band gap. The band gap values of Al2O3 in κ, α and θ phases were reported to be 4.05, 8.8 and 5.15 eV, respectively [12]. By reducing the band gap value appreciably, Al2O3 could respond well under visible light. To shrink the band gap and improve the photocatalytic performance of Al2O3, doping with transition metal ions could be performed. Improved photocatalytic efficiency has been realized for Ag [13] and Zn [14] doped Al2O3 photocatalysts. Also, to be an efficient regenerable and reusable catalyst, Al2O3 should possess magnetic behavior. Hence, in this work a dopant ion has been selected in such away it can reduce the band gap and induce magnetic orderings in Al2O3. Ni doping has been performed to make Al2O3 to respond very effectively under visible light. Nickel (Ni2+) is a transition metal ion which has an ionic radius of 0.69 Å which is slightly higher than Al3+ (0.51 Å) ion. Also, the electro-negativity of Ni is 1.8 Pauling which is higher than that of Al (1.5 Pauling). Thus, Ni can create significant lattice defects in the Al2O3 matrix thereby improving its structural, optical, catalytic and magnetic properties significantly. It has been reported earlier that Ni-based catalysts have significant efforts on the catalytic performance against toxic dyes [15]. The present work aims to synthesize Nidoped (0, 2, 4 and 6 wt.%) Al2O3 nanoparticles via precipitation method. The structural, optical, catalytic and magnetic properties of the Ni-doped Al2O3 nanoparticles were studied and reported. 2.
Experimental details
2.1
Synthesis of undoped and Ni-doped Al2O3 nanoparticles Aluminium (III) chloride [AlCl3] and nickel chloride [NiCl2.6H2O] both of analytical
reagent grade (Sigma make) was used as the host and dopant precursor salts to synthesize Nidoped Al2O3 nanoparticles. De-ionized water (140 mL) and liquid ammonia (10 mL) were used as the solvent and precipitating agent, respectively. Undoped Al2O3 (0 wt.% Ni)
4
nanoparticles was synthesized by dissolving 0.1M of aluminium (III) chloride in de-ionized water and liquid ammonia. After continuous stirring for 2 h, the solution was aged for 4 h to get white colored Al2O3 precipitates. The precipitates were separated, cleaned several times with water, calcined at 200°C for 1 h and then crushed to get Al2O3 nanoparticles. To achieve Ni doping, NiCl2.6H2O with 2, 4 and 6 wt.% of the weight of AlCl3 was added to the precursor solution used to synthesize pure Al2O3 and then by adopting the same procedure used to synthesize pure Al2O3, Ni-doped Al2O3 nanoparticles were synthesized. To protect the synthesized samples from long term oxidation by air they were kept under vacuum [16]. 2.2
Characterization The synthesized Ni-doped Al2O3 nanoparticles were characterized by techniques like
XRD, SEM, FTIR, PL, VSM and the details regarding the instruments used are compiled in Table 1. The specific surface area of the samples was evaluated by Barrett-Emmett-Teller (BET) method using ASAP2020C+M specific area and porosity analyzer. 2.3
Photocatalytic tests Photocatalytic experiments were performed under visible light irradiation against
metanil yellow (MY) dye. The visible light source used for the photocatalytic activity was a 100 W incandescent lamp which was kept vertically at a distance of 10 cm from the dye solution inside the photoreactor. Aqueous solution (100 mL) of 0.025 M MY and optimized value of the Ni-doped Al2 O3 nanoparticles (6 mg) was used as testing solution for the photocatalytic experiments. The dye solution with the catalysts was maintained in the dark for 30 min to ensure adsorption-desorption equilibrium. The pH value of the solution was maintained at 8. The photodegradation of the dye was followed by measuring absorption at regular intervals using a UV-Vis spectrophotometer (LAMBDA 35) at 439 nm wavelength.
5
3.
Results and discussion
3.1
XRD Studies Fig.1 shows the XRD patterns of 0, 2, 4 and 6 wt.% Ni-doped Al2O3 nanoparticles. The
entire samples exhibit (2 0 1), (0 0 2), (3 1 1), (1 1 2), (6 0 0) and (5 1 1) diffraction peaks whose positions matched with monoclinic θ-Al2O3 crystal structure according to JCPDS cards No. 86-1410. The absence of Ni related peaks in the doped samples ascertained the entry of the dopant into the Al2O3 lattice without undergoing any chemical reactions with oxygen to form new compounds. The average crystallite size (D) of the samples was calculated by using the Debye-Scherrer equation,
(where
is the wavelength of the X- ray used
(1.5406 Å), θ is the diffraction angle) from the full-width at half maximum (β) value of the dominant (0 0 2) peak. The crystallite size values of the 0, 2, 4, and 6 wt.% Ni-doped Al2O3 nanoparticles were found to be 41, 38, 33 and 35 nm, respectively. The strain induced into the Al2O3 lattice due to the larger ionic radius of Ni2+ (0.69 Å) than Al3+ (0.51 Å) might have resulted in the decreased crystallite size values for the doped samples. Ni substitution may cause grain boundary restraining in Al2O3 matrix which due to symmetry-breaking effects at the boundary might have caused decrement in the crystallite size values [1]. Due to decreased crystallite size values, the doped samples exhibited better photocatalytic activity as can be seen in Section 3.6. 3.2
Surface morphology and Surface area analysis Fig. 2 shows the SEM images of a) 0 wt.% , b) 2 wt.% , c) 4 wt.% and d) 6 wt.% Ni-
doped Al2O3 nanoparticles. The surfaces of the Ni-doped Al2O3 samples are composed of irregular shaped grains with different sizes. Also, grains seem to be agglomerated for all the samples with plenty of empty sites. No significant variation is observed with Ni doping. However, agglomeration seems to be minimized and grains appeared to be tightly packed as Ni concentration increases. Compared to the undoped sample, decreased grain sizes were 6
evinced for the doped Al2O3 nanoparticles. Among the doped samples, the surface of the 4 wt.% Ni-doped Al2O3 (Fig. 2(c)) sample seems to tightly packed with nanosized grains without any empty sites confirming its improved surface morphology. BET calculation revealed much larger specific area for the 4 wt.% Ni-doped Al2O3 (126 m2g-1) sample. The BET values of the 0, 2 and 6 wt.% Ni-doped Al2O3 nanoparticles were found to be 87, 96 and 112 m2g-1, respectively. Due to increased surface area, the 4 wt.% Ni-doped Al2O3 nanoparticle exhibited enhanced photodegradation efficiency (Section 3.6). 3.3
FTIR Studies Fig. 3 shows the FTIR spectra of a) undoped (0 wt.% Ni) and b) 2, 4 and 6 wt.% Ni-
doped Al2O3 nanoparticles. The bands in the 3657-3140 cm-1 region and the peak at around 1634 cm-1 observed for both the undoped and doped samples are attributed to O-H stretching and bending vibrations, respectively [1]. The OH groups act as scavenger of photogenerated electrons and holes resulting in the formation of hydroxyl radicals OH * which degrades dye molecules effectively. N-H stretching vibration peak observed at 2055 cm-1 for the undoped Al2O3 and at 2018 cm-1 for the doped Al2O3 nanoparticles might have been originated from liquid ammonia which was used as precipitation agent to synthesize the nanoparticles [17]. Asymmetric stretching vibration of C=O is observed at 1404 cm-1 for all the Ni-doped Al2O3 nanoparticles [18]. C-O stretching vibration of adsorbed CO2 is observed at 1071, 1020 cm-1 for the undoped sample and at 1074 cm-1 for the doped samples [19]. C-H stretching vibration is observed at 980 cm-1 for the undoped sample and at 985 cm-1 for the doped samples [19]. Al-O vibrations were observed at 768, 620 cm-1 for the undoped Al2O3 and at 756, 621 cm-1 for the doped samples [16]. Ni related peak observed at 477 cm-1 confirmed the existence of Ni in the doped samples [20]. Metal-oxygen related peak is observed at 532 and 429 cm-1 for the undoped Al2O3 nanoparticles.
7
3.4
Optical studies Fig. 4 shows the absorbance spectra of Al2O3 nanoparticles synthesized with 0, 2, 4 and
6 wt.% Ni doping concentrations. The absorption edge of pure Al2O3 shifted towards higher wavelength side with Ni doping inferring a reduction in the optical band gap (Eg) value. The Eg values of the Ni-doped Al2O3 nanoparticles were calculated from the absorption edge (λmax) values using the relation [14]: (1) The band gap values were 3.85, 3.79, 3.72 and 3.75 eV, respectively for the 0, 2, 4 and 6 wt.% Ni-doped Al2O3 nanoparticles. Clearly, the band gap of pure Al2O3 shifted towards lower value with Ni doping. Because of the red shift in the optical band gap, easy transfer of photoexcited electrons to the noble metal nanoparticles takes place leading to faster electronhole separation [21] and hence their recombination rate decreases, thereby improving the photocatalytic performance of the doped catalysts (Section 3.6). 3.5
PL studies Fig. 5 shows the room temperature photoluminescence spectra of Al2O3 nanoparticles
synthesized with 0, 2, 4 and 6 wt.% Ni doping concentrations, excited at λ = 320 nm. Seven emission bands were observed in the measured spectra. The UV emission peak at 360 nm ascertained to the extended band edge emission suggests the presence of lattice defects or impurities in the synthesized samples [22]. The near band edge emission at 377 nm may be attributed to the free excitonic emission of Al2O3 [23]. The excitonic emission peak observed at 410 nm may be related to the presence of oxygen vacancies in the synthesized Ni-doped Al2O3 nanoparticles [16]. The peaks at 475 nm and 494 nm are assigned to transitions occurring from metal interstitials (Ali) to the valence band [24]. Oxygen vacancies might have favored for the appearance of the peak at 494 nm as reported by Nallendran et al. [25].
8
Deep-level or trap state emission is responsible for the emission peak at 521 nm [26]. Oxygen vacancies and OAl defects are the cause for the presence of the emission peak at 545 nm [27]. 3.6
Photocatalytic activity Photocatalytic activity of the Ni-doped Al2O3 nanoparticles was investigated for the
degradation of metanil yellow (MY), an anionic dye under visible light. The photocatalytic activity of MY dye by the Ni-doped Al2O3 nanoparticles is found to be influenced by various factors such as catalyst dosage, initial concentration of dye, pH, dopant content, etc. From our photocatalytic tests, we found that the degradation of MY dye is directly proportional to the dosage amount of the catalyst up to an optimum value of 6 mg / 100 mL. Above this optimized value, photocatalytic experiments showed negative results as the production of OH* radicals was inhibited due to the agglomeration of the particles which blocks the light illumination towards the surface. Therefore, 6 mg was fixed as the optimum dosage value for further experiments. 6 mg of the synthesized Ni-doped Al2O3 nanoparticles was mixed thoroughly in 100 mL aqueous solution containing 0.025 M MY dye. The dye solution with 0, 2, 4 and 6 wt.% Ni-doped Al2O3 catalysts was stirred for 30 min in dark to achieve adsorption-desorption and then subjected to visible light. As the light irradiation time increases, the color of the dye solutions with the catalysts start to fade confirming the degradation of the dye molecules. After 90 min light irradiation, the dye solutions with 2, 4, 6 wt.% Ni-doped Al2O3 catalysts seems to be faded better than that of the 0 wt.% Ni-doped Al2O3 catalyst and the dye solution with the 4 wt.% Ni-doped Al2O3 catalyst become almost colorless, confirming its enhanced degradation ability. The order of degradation ability was found to be in the order 4 wt. % Ni > 6 wt. % Ni > 2 wt. % Ni > 0 wt. % Ni In order to confirm that degradation is caused only by the catalyst used, prior to the photocatalysis experiment, a photolysis process is carried out for MY without the presence of
9
any catalyst. Change in MY concentration at different time intervals in the presence and absence of Ni-doped Al2O3 catalysts as a function of irradiation time is shown in Fig. 6. There was no observable loss in the concentration of MY in the absence of photocatalyst and degradation takes place only in the presence of undoped and doped Al2O3 catalysts. The absorbance spectra of the 4 wt.% Ni-doped Al2O3 catalyst taken at λ = 439 nm after sampling 2 mL of the light irradiated dye solution is shown in Fig. 7. The absorption peak intensity decreases with increase in irradiation time, confirming the degradation nature of the dye molecules. Fig. 8 shows the degradation efficiencies (η) of the Ni-doped Al2O3 catalysts calculated using the formula [13]: (
⁄ )
(2)
where C0 and C are the concentration values of the dye under dark and light conditions, respectively. All the doped samples exhibited better degradation efficiencies and among them the 4 wt.% Ni-doped Al2O3 catalyst exhibited better degradation efficiency. The degradation efficiencies of the Al2O3 catalysts with 0, 2, 4 and 6 wt.% Ni doping concentrations after 90 min irradiation time were 57.14, 67.27, 86.84 and 78.72%, respectively. The photocatalytic ability of the Ni-doped Al2O3 nanoparticles depend on the generation of electron-hole (e-/h+) pairs from their surfaces when exposed to visible light. When the Ni-doped Al2O3 catalysts are exposed to light, the electrons in the valence band are excited to conduction band thereby generating electron-hole pairs [28]. The holes in the Nidoped Al2O3 catalysts react with water to form hydroxyl radicals (OH *); whereas the photoexcited electrons react with surrounding oxygen to form superoxy anion radicals ( These radicals attack the organic pollutant (MY) and degrade the dye molecules releasing CO2 and water [20]. Degradation efficiency in most cases reduces due to the undesirable recombination of electrons and holes. The dopant Ni2+ may accept an electron in the conduction band of Al2O3 to form Ni+ which may transport an electron to dissolved O2 to 10
).
produce superoxide radical anions, which inhibit the recombination of photoinduced electrons and holes. Thus, Ni2+ on the surface of Al2O3 nanoparticles may act as an electron scavenger. The schematic representation of the photocatalytic mechanism involved in the Ni-doped Al2O3 catalysts is pictured in Fig. 9. For efficient transfer of photoexcited electrons, the energy band gap (E g) must be appreciably small. Pure Al2O3 exhibits a high band gap value of 3.85 eV and hence its degradation efficiency is appreciably low (57.14%). However, when doped with Ni, the band gap of Al2O3 decreased appreciably which enhanced the effective transfer of photoexcited electrons and hence they exhibited better degradation efficiencies. Earlier research results [29-31] also showed a similar behavior. The creation of more vacancy defects into the Al2O3 matrix due to the substitution of Ni2+ ions results in the generation of more photoexcited electron-hole pairs and this might have also been a reason for the enhanced degradation efficiencies observed for the doped samples [32]. The photodegradation kinetics of the Ni-doped Al2 O3 catalysts were investigated based on the pseudo first-order kinetic model exemplified by Langmuir-Hinshelwood using the equation [14]: (
⁄ )
(3)
where k is the apparent first-order rate constant (min-1), t is the irradiation time, C0 and C are the concentrations of the dye under dark and light conditions, respectively. From the slopes of the graphs of
( ) vs. time (Fig. 10), the ‘k’ values of the Ni-doped Al2O3 catalysts were
calculated which was 0.00901, 0.01196, 0.02019 and 0.01713 min-1 for the 0, 2, 4 and 6 wt.% Ni-doped Al2O3 catalysts, respectively. The higher k values obtained for the doped samples than the undoped one supports for their enhanced degradation abilities. Higher degradation rate constant implies higher degradation efficiency. Among the doped samples, the 4 wt.% Ni-doped Al2O3 catalyst exhibited a higher degradation efficiency of 86.84% and this was 11
very much acknowledged with the high k value of 0.02019 min-1. Least band gap, crystallite size and improved surface morphology might have favored for the enhanced degradation efficiency observed for the 4 wt.% Ni-doped Al2O3 catalyst. Due to decreased crystallite size value, the specific surface area of the 4 wt.% Ni-doped Al2O3 catalyst increases and hence it had extra active sites to excite more photogenerated electron hole pairs under visible light, thereby decreasing their recombination rate. 3.7
Magnetic studies Fig. 11 shows the M-H curves of a) 0 wt.% b) 2 wt.%, c) 4 wt.%, and d) 6 wt.% Ni-
doped Al2O3 nanoparticles. Undoped Al2O3 and 2 wt.% Ni doped Al2O3 nanoparticles exhibit paramagnetic behavior (Fig. 10(a, b)). With further doping, well defined hysteresis loops was observed confirming their ferromagnetic nature. The para and ferro magnetic behaviors observed for the doped samples may be ascribed to oxygen vacancies and/or substitution of Al atoms by Ni atoms interstitially. Similar results have been reported by Aragon et al. [33] for Ni-doped SnO2 nanoparticles. The ferromagnetic behavior observed for the 4 and 6 wt.% Ni-doped Al2O3 nanoparticles might be due to the relatively high anisotropy contribution of Ni2+ ions and also due to the introduction of point defects (Ni vacancies) at dislocation cores of Al2O3 . This relies with the results obtained earlier for the SnS2-NiO nanocomposite [34]. Horizontal shift called exchange bias was observed for the 4 and 6 wt.% Ni-doped Al2O3 nanoparticles. The observed magnetic orderings with Ni-doping confirmed that the Ni-doped Al2O3 nanoparticles when used in photocatalytic applications can be easily separated from the dye solution by a magnet or an applied magnetic field, for taking absorbance spectra and to perform recycle tests.
12
4.
Conclusion Ni-doped Al2O3 nanoparticles were synthesized via precipitation method with 0, 2, 4
and 6 wt.% Ni concentrations. The impact of Ni doping on the structural, optical, photocatalytic and magnetic properties of Al2O3 was studied systematically and the results obtained confirmed that Ni2+ ions strongly dominated them. Decreased crystallite size and band gap values were observed for the doped samples. Enhanced photocatalytic activity was observed for the doped samples and the 4 wt.% Ni-doped Al2O3 catalyst exhibited a maximum degradation efficiency of 86.84% against MY dye. Improved magnetic properties were observed for the Ni-doped Al2O3 nanoparticles. The ferromagnetic property observed for the Ni-doped Al2O3 nanoparticles is also an essential characteristic of a regenerable and reusable magnetic catalyst. Thus, Ni-doped Al2O3 nanoparticles are well suited for practical applications as efficient regenerable photocatalyst for the degradation of inorganic toxic dyes.
Author Contribution Statement S. Anbarasu: Conceptualization, S. Ilangovan: Conceptualization, K. Usharani: Methodology , A. Prabhavathi: Data curation , M. Suganya: Visualization, Investigation, S. Balamurugan: Formal analysis, C. Kayathiri: Resources, M. Karthika: Validation, V.S. Nagarethinam: Supervision, A.R. Balu: Data curation, Writing – Original draft preparation, Writing – Reviewing and Editing. The above descriptions are accurate and agreed by all the authors. Conflict of interest statement All the authors declare that the article is original and all are aware of its contents and they approve for its submission. The authors also confirm that the described work has not been published before; it is not under consideration for publication anywhere else and publication has been approved by all co-authors and the responsible authorities of the institute
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where the work has been carried out. No conflict of interest exists in the article. The authors declare that if the article is accepted it will not be published elsewhere in the same form in any language without the written consent of the publisher.
5.
Acknowledgements The Head, Alagappa University is very much thanked for the XRD and PL results.
Figure captions
14
Fig. 1XRD patterns of the Ni-doped Al2O3 nanoparticles.
15
Fig. 2SEM images of a) 0 wt.%, b) 2 wt.%, c) 4 wt.% and 6 wt.% Ni-doped Al2O3 nanoparticles.
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Fig. 3FTIR spectra of a) 0 wt.% and b) 2, 4 and 6 wt.% Ni-doped Al2O3 nanoparticles.
Fig. 4Absorbance spectra of the Ni-doped Al2O3 nanoparticles.
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Fig. 5PL spectra of the Ni-doped Al2O3 nanoparticles.
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Fig. 6Variation of C/Co as a function of illumination time for MY with the absence and presence of the catalysts.
19
Fig. 7Absorbance spectra of the 4 wt.% Ni-doped Al2O3 catalyst recorded at different light irradiation time intervals.
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Fig. 8 Bar diagrams showing the photodegradation efficiencies of the Ni-doped Al2O3 catalysts.
Fig. 9Schematic representation of the photocatalytic mechanism involved in the Ni-doped Al2O3 catalysts.
21
Fig. 10 Variation of
( ) as a function of irradiation time intervals.
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Fig. 11 M-H curves of a) 0 wt.%, b) 2 wt.%, c) 4 wt.% and d) 6 wt.% Ni-doped Al2O3 nanoparticles.
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Table 1 Instrument details used to characterize the Ni-doped Al2O3 nanoparticles Characterization technique
Instrument
XRD
X’pert PRO analytical PW 340/60 diffractometer
SEM
HITACHI S-3000H scanning electron microscope
FTIR
Perking Elmer RX-1 spectrophotometer
Optical
LAMBDA-35 UV-Vis-NIR double beam spectrophotometer
PL
Varian Cary Eclipse Fluorescence spectrophotometer
VSM
Lakheshore 7410 vibrating sample magnetometer
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