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Enhanced photocatalytic Activity of Ferromagnetic Fe-doped NiO nanoparticles
Journal Pre-proof Enhanced Photocatalytic activity of ferromagnetic Fe-doped NiO Nanoparticles Hur Abbas, K. Nadeem, A. Hassan, S. Rahman, H. Krenn
PII:
S0030-4026(19)31535-9
DOI:
https://doi.org/10.1016/j.ijleo.2019.163637
Reference:
IJLEO 163637
To appear in:
Optik
Received Date:
1 August 2019
Accepted Date:
13 October 2019
Please cite this article as: Abbas H, Nadeem K, Hassan A, Rahman S, Krenn H, Enhanced Photocatalytic activity of ferromagnetic Fe-doped NiO Nanoparticles, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163637
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Enhanced Photocatalytic activity of ferromagnetic Fe-doped NiO Nanoparticles Hur Abbasa, K. Nadeema*, A. Hassanb, S. Rahmanc, H. Krennd
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Nanoscience and Technology Laboratory, Department of Physics, International Islamic University, Islamabad 44000, Pakistan. b
Department of Physics, Allama Iqbal Open University, Islamabad 44000, Pakistan.
c
Department of Material Science and Engineering, University of Science and Technology of China Hefei, Anhui 230026, China. Institute of Physics, Karl-Franzens University, Universitätsplatz 5, A-8010 Graz, Austria.
*
Corresponding author’s email: [email protected] (K. Nadeem)
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Phone # 0092-51-9019714
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Abstract
The Ni1-xFexO (x= 0.00, 0.02, 0.04, 0.06 and 0.08) nanoparticles (NPs) has been
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synthesized via composite hydroxide mediated (CHM) synthesis approach. The single phase formation of NiO NPs was achieved without any impurity phase confirmed by XRD and average crystallite size showed minor decreasing trend with increasing Fe doping
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concentration. FTIR spectra showed dip in 417-432 cm-1 range which confirmed the existence of Ni-O bonding. In Raman spectra, the weak two-magnon (2M) band at 1472 cm-1 in pure NiO NPs indicate suppressed antiferromagnetism (AFM) due to smaller average
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crystallite size and superparamagnetic contribution from the uncompensated surface/core spins. The absence of 2M band in Fe doped samples is due to onset of ferromagnetism in
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these NPs. The M-H loops showed the room temperature ferromagnetism in Fe doped samples due to double exchange interaction of mixed valance states of Fe. The energy band gap showed minor increasing trend with increasing Fe doping concentration and is mainly attributed to Moss-Burstein effect. The photocatalytic activity showed an increasing trend with increasing Fe doping due to generation oxygen vacancies which served as energy traps and restricted the electron-hole recombination. Therefore, the bifunctional ferromagnetic Fe
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doped NiO NPs are more favourable for photocatalysis along with their ability of collection for re-use in the degradation process.
Keywords: Nickel oxide; Raman; ferromagnetic; Band gap; Photocatalytic.
1. Introduction Bulk nickel oxide (NiO) is a transition metal oxide which has intriguing physical and chemical properties. It has face centred cubic (FCC) structure and antiferromagnetic (AFM)
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ordering well above room temperature TN=523 K [1]. It is a direct band gap p-type
semiconductor [2]. The recent ongoing research at nanoscale confirmed its fascinating
optical, magnetic and photocatalytic properties [3-5] and wide range of applications in
energy storage devices [6], p-type transparent conducting films [7], dye sensitized solar cells
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[8], battery electrodes [9], supercapacitors [10], gas sensors [11], catalysis [12] etc. The most extensive method to fabricate the nanoparticles (NPs) for desired application is to dope by
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ions of foreign element. The suitable dopant not only breaks the crystal symmetry and produces strain in the structure but may also disrupt the magnetic nature of the NPs. The
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presence of two-magnon (2M) band ~1500 cm-1 in Raman spectra reveal the AFM nature of bulk NiO. Mironova et al. [13] observed 2M peak for larger sized NiO NPs and confirmed its AFM nature by magnetic measurements also. Similarly, the absence of 2M peak can be
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attributed to the superparamagnetic (SPM) nature of NPs. Liu et al. [14] reported the disappearance of 2M peak in NiO nanocrystals due to size reduction and attributed to broken symmetry of AFM. The room temperature ferromagnetism (RTFM) in NiO NPs can be
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achieved by incorporation of suitable metal ions [15]. Lin et al. [16] reported RTFM in NiO NPs with Fe doping and ascribed to double exchange mechanism. The localized energy states
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generated in the host material by dopant can also have great impact on optical nature of the material. Patel et al. [17] observed increased optical energy band gap in Fe doped NiO NPs and attributed it to quantum confinement effect. Among different metal oxides, NiO NPs are reported as efficient photocatalyst [18-20] due to their higher degradation rate, better chemical stability, presence of oxygen vacancies, non-toxicity and relatively low cost of production. In order to improve the photocatalytic ability of NiO NPs, the incorporation of suitable metal ions can possibly generate oxygen vacancies that slows down the electron-hole recombination rate. The oxygen vacancies serve as charge carrier traps for electrons and is 2
vital for photocatalytic process. Sankar et al. [21] reported the improved photocatalytic degradation performance of Mn doped NiO NPs and attributed it to generation of localized electronic states which possibly served as charge carrier traps. Therefore, it would be beneficial to investigate the photocatalytic ability of Fe doped NiO NPs that will be collected easily for reuse due to their magnetic nature and are not found to be reported in literature. In this article, we observed the enhanced photocatalytic activity of NiO NPs with increasing Fe doping concentration and also observed RTFM in these doped NPs.
2. Experiment
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The Fe doped (0, 2, 4, 6 and 8% at.%) NiO NPs have been synthesized by composite hydroxide mediated (CHM) approach described elsewhere [22]. The appropriate ratio of
nickel nitrate hexahydrate and iron nitrate nonahydrate (Sigma Aldrich, 99.9%) were added to the eutectic mixture of NaOH and KOH in a ratio of 51.5 : 48.5, respectively. All the
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chemicals were added to teflon vessel and placed in oven at 200oC for 24 h. Finally, the
obtained material was washed several times to get Fe doped NiO NPs. The samples were labelled as NF0, NF2, NF4, NF6 and NF8 with corresponding 0, 2, 4, 6 and 8% Fe doping
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into NiO NPs.
The purity and phase formation of synthesized samples were examined by X-ray
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diffractometer (XRD), vibrational modes of metal oxide bonds were observed by Fourier transform infrared (FTIR) spectrometer, the surface morphology of NPs was observed by scanning electron microscopy (SEM) accompanied by energy dispersive X-ray spectroscopy
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(EDS) for chemical composition. The phonon and magnon modes were observed by Raman spectroscopy with laser wavelength of 532 nm. The magnetic measurements were taken by
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using SQUID magnetometer (Quantum design). The percentage reflectance spectra were taken by for the estimation of optical energy band gap by using diffuse reflectance spectroscopy. The photocatalytic degradation of MO dye has been observed by UV-Vis
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spectrophotometer.
3. Results and discussion: Fig. 1 (a) and (b) shows the X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra of NF0, NF2, NF4, NF6 and NF8 NPs. The diffraction peaks at 37.12o, 43.14o, 62.72o, 75.12o and 79.18o correspond to (111), (200), (220), (311) and (222) planes of FCC structure of NiO, respectively. The obtained pattern was found in accordance 3
with the standard pattern of NiO (JCPDS No. 01-073-1523). The ferromagnetic Ni phase was not observed which is most probable impurity phase observed at low temperature [23]. The other possible impurity phases, like NiFe2O4, Fe2O3 and Fe3O4 which have magnetic origin
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were also found absent.
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Fig. 1: (a) XRD pattern and (b) FTIR spectra of NF0, NF2, NF4, NF6 and NF8 NPs.
The average crystallite size was found to be 33, 32, 31, 29 and 30 nm for NF0, NF2,
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NF4, NF6 and NF8 NPs, respectively. A very small change in average crystallite size has been observed with Fe doping due to insignificant difference in ionic radii of Ni2+ (0.69Å)
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and Fe2+ (0.74Å) ions. No shift in the peak position has been observed which confirmed that doping did not produced any strain in the NiO structure. In FTIR spectroscopy, a dominant dip in the range of 417-432 cm-1 is consigned to stretching vibration mode of Ni-O molecule
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[24].
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Fig. 2: (a, b) SEM images of NF0 and NF8 NPs, respectively (c, b) EDS spectra of NF0 and NF8 NPs,
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respectively.
Fig. 2 (a, b) displays the scanning electron microscopy (SEM) images of NF0 and
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NF8 NPs, respectively at 500 nm scale. It is clear that most of the NPs are spherical in shape with little agglomeration present. Fig. 2 (c, d) shows the EDS spectra of NF0 and NF8 NPs,
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respectively. The presence of Ni, Fe and O peaks confirmed the Fe doping whereas the existing Au peak is due to gold sputtering for achieving better resolution. Raman scattering technique is a useful probe to investigate the lattice disorder,
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structural transitions, cationic distribution and magnetic ordering. Fig. 3 shows the Raman
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spectra of pure and Fe doped NiO NPs at room temperature.
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Fig. 3: Raman spectra of NF0, NF2, NF4, NF6 and NF8 NPs. Insets show zoom portion of 2M band.
The band in the range 400-600 cm-1 is due to the overlapping of first order phonon
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(1P) (1TO = 440 cm-1 and 1LO = 560 cm-1) [25]. The 1TO and 1LO scattering modes are not observable in bulk NiO and stoichiometric NiO, therefore the presence of these modes in NiO NPs are attributed to surface disorder, interstitial oxygen vacancies, broken symmetry and
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imperfectness in crystal structure [26]. Mironova et al. [13] observed the increased intensity of this band with decrease in crystallite size. Chen et al. [27] observed red shift in 1P mode of
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the silicon nanowires with decrease in diameter of wires and attributed it to size induced phonon confinement effect. No such red shift has been observed for our Fe doped samples due to very small difference in average crystallite size (33-29 nm). The second order phonon (2P) band observed at 1025 cm-1 is attributed to longitudinal optical (2LO) stretching mode of NiO [28]. The two-magnon (2M) band has a magnetic origin and is characteristic feature of a bulk AFM material below TN=523 K [29]. This band also gets disappear in small AFM NPs at room temperature due to reduced coordination of antiparallel spins [30]. In Fig. 3, insets show very weak band at 1472 cm-1 in NF0 that confirms weak AFM coupling in the core of 6
NiO NPs. However, this band gets suppressed for Fe doped samples due to magnetic nature of Fe ions. Liu et al. [26] perceived RTFM in Fe doped NiO nanofibers and did not observed 2M peak in these samples. Therefore, absence of 2M peak in Fe doped NiO NPs is an indication of RTFM in these samples.
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Fig. 4 shows the M-H loops of pure and Fe doped NiO NPs at 300 K.
Fig. 4: M-H loops of NF0, NF2, NF4, NF6 and NF8 NPs at 300 K.
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Bulk NiO is AFM below Neel temperature (TN=573 K) but when its dimensions are reduced to nanoscale, the spins at the NPs surface progressively become unequal and
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remained uncompensated which get enhanced on reducing further crystallite size. Therefore, RT weak ferromagnetism in pure NiO NPs can be attributed to finite size effect [31]. It is clear from M-H loops that pure NiO NPs exhibited weak RTFM due to uncompensated
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surface spins with coercivity of 2614 Oe and remanence 0.025 emu/g. Tadic et al. [32] reported RTFM-like behaviour in NiO NPs with coercivity of 115 Oe and remanence 0.0087 emu/g. Continuous linear increase in magnetization with applied field is an indication of AFM core which is in agreement with the weak signal of 2M peak induced in NF0 as shown in Fig. 3. The net magnetic moment increased for Fe doped samples and found maximum for NF6 sample as shown in Fig. 5 (a). The doping of different metal ions induces structural disorder and have influence the average crystallite size. Therefore, induced magnetization can 7
be due to finite size of pure NiO NPs [33-35]. Ponnusamy et al. [36] observed RTFM in Fe doped NiO NPs and attributed it to decrease in average crystallite size. In our case, Fe doping did not produced a significant change in the average crystallite size so this aspect cannot be accounted for doped samples. From XRD and Raman results, it is also clear that no other secondary phase of Fe such as Fe2O3 and Fe3O4 is present, therefore this aspect of enhancement in net magnetization is also neglected. Many researchers [37-40] reported the enhanced magnetization in Fe doped NiO NPs due to ferrimagnetic NiFe2O4 phase formation. Raja et al. [37] observed the ferrimagnetic NiFe2O4 phase for more than 2% Fe doping into NiO NPs but in our case no such impurity phase has been observed even up to 8% of Fe doping. The disorder generated by substitution of Ni2+ by Fe2+ (or Fe3+) breaks the original
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translational symmetry of magnetic ions and super-exchange interaction (Ni2+-O2-- Ni2+) in crystallites is interrupted [16]. There are two stable valance states of Fe ions exist, i.e. Fe2+ and Fe3+. Douvalis et al. [39] observed the existence of Fe3+ ions into NiO matrix by
Mossbauer spectroscopy, therefore enhanced net magnetic moment may be due to double
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exchange interaction of mixed valance states [41]. Khemprasit et al. [42] also observed the
enhanced ferromagnetism in Fe doped NiO NPs and attributed it to generation of Ni vacancy
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defects and double exchange interactions. The net magnetic moment did not saturate up to 50 KOe which confirmed the presence of AFM phase in these NPs. The higher value of
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coercivity for pure NiO NPs is attributed to the presence of surface uncompensated spins. However, coercivity started decreasing with increasing Fe doping concentration due to the
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contribution of magnetization from core of the NPs as shown in Fig. 5(b).
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Fig. 5: (a) Magnetization vs Fe doping (%) at ±5T and (b) Coercivity (Hc) vs Fe doping (%).
Before analysing the photocatalytic activity of Fe doped NiO NPs, it is necessary to investigate their optical band gap energy so that information about the activation energy of 8
photocatalyst may be collected. The energy band gap of pure and Fe doped NiO NPs was
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evaluated from well-known Tauc’s relation [43] and is shown by Tauc’s plot in Fig. 6.
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Fig. 6: Tauc’s plot for NF0, NF2, NF4, NF6 and NF8 NPs.
The DRS technique was employed to calculate the optical absorption coefficient [44] (1−𝑅)2 2𝑅
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𝐹(𝑅) =
(1)
Where 𝐹(𝑅) is Kubelka Munk function and R is the absolute reflectance of the material. The
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estimated energy band gaps were 3.47, 3.51, 3.52, 3.53 and 3.57 eV for NF0, NF2, NF4, NF6 and NF8 NPs, respectively. The smaller crystallites have larger band gap according to
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quantum confinement effect [2]. Hashem et al. [45] reported the increase in energy band gap from 3.51 to 3.60 eV when crystallite size of pure NiO is reduced from 34 to 14 nm. However, finite size effect cannot be considered for our Fe doped NiO NPs due to very small difference in average crystallite size as calculated from XRD. Therefore, slight enhancement in energy band gap for Fe doped samples can be attributed to Moss-Burstein effect [46] in which Fermi level shift close to the conduction band due to doping which increases carrier density and leads to increase in energy band gap.
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The photocatalytic activity of synthesized samples was observed by using Methyl orange (MeO) dye which is widely used as model pollutant. The 70 ml MeO solution (15 mg/l) was taken in a beaker and 30 mg catalyst (Fe doped NiO NPs) was added to test photocatalytic degradation under UV illumination. In order to maintain absorption-desorption equilibrium, the MeO solution was stirred for 30 minutes in the dark. Fig. 7 (a-e) shows the absorbance spectra of MeO dye at various time intervals in the presence of NF0, NF2, NF4, NF6 and NF8 NPs used as photocatalyst. The maximum intensity of absorbance spectra of MeO dye was observed at 464 nm [20] which gradually get decreased with the increasing irradiation time and is attributed to the degradation of MeO dye molecules which confirms
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the photocatalytic ability of pure and Fe doped NiO NPs.
Fig. 7: (a-e) Absorbance spectra of MeO dye in the presence of NF0, NF2, NF4, NF6 and NF8 NPs.
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Fig. 8 (a) shows the variation in degradation percentage of MeO dye with irradiation time when pure and Fe doped NiO NPs were used as photocatalyst. The enhanced photocatalytic activity with Fe-doping may be attributed to following reasons: (1) more absorption of photons due to generation of localized states, (2) higher doping rate increased concentration of oxygen vacancies that serve as energy traps to delay electron-hole recombination. Wang et al. [47] reported an enhanced photocatalytic activity of ZnO nanorods and attributed it to generation of oxygen vacancies that reduced the recombination rate of electron and holes. Fig. 8 (b) shows that after 90 minutes of UV irradiation, NF0 NPs
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only degraded 68% where NF8 completely degraded the model pollutant.
Fig. 8: (a) Variation of Photo degradation percentage of NF0, NF2, NF4, NF6 and NF8 NPs with UV irradiation
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time and (b) Photo degradation percentage at 90 min UV irradiation time.
When ultraviolet (UV) light of sufficient energy (equal or greater than the bandgap
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energy) falls on the surface of catalyst, the electron-hole pairs are generated which are vital to initiate the photocatalytic process. The excited electrons in the C.B and holes in V.B react
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with nearby oxygen (O2) molecules and surface bound water to produce superoxide radical anion (·O2-) and hydroxyl radical (·OH), respectively. These radicals are known as reactive oxygen species (ROS) and strongly react with organic pollutant molecule to degrade [48] (∙ 𝑂2− ) 𝑜𝑟 (∙ 𝑂𝐻) + 𝑀𝑒𝑂 𝑑𝑦𝑒 → 𝐻2 𝑂 + 𝐶𝑂2 + 𝑑𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠
(2) The schematic diagram of the photocatalytic process is shown in Fig. 9.
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Fig. 9: Schematic illustration of the photocatalytic process.
The photocatalytic dye degradation percentage was calculated by equation [49]. 𝐴𝑜 −𝐴𝑡 𝐴𝑜
) × 100
(3)
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Degradation (%) = (
Where Ao and At are the maximum value of absorbance of MO dye at 0 and t minutes,
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respectively.
Conclusion
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The Fe doped (0, 2, 4, 6 and 8%) NiO NPs has been synthesized by low temperature CHM approach. The phase formation of pure and Fe doped NiO NPs has been confirmed by XRD and possible impurities which were commonly observed in literature such as Ni, Fe2O3,
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Fe3O4 and NiFe2O4 were not found. The Fe doping did not produced any significant change in the average crystallite size due to small difference in ionic radii of Ni2+ and Fe2+ ions.
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FTIR spectroscopy revealed the presence of Ni-O bonding whereas SEM images showed the spherical shape of the both pure and Fe doped NiO NPs. The small 2M peak in Raman spectra reveal weak AFM in pure NiO NPs due to contribution from the core whereas high
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coercivity is attributed to uncompensated spins present on its surface. The M-H loops confirmed the RTFM in Fe doped NiO NPs and magnetization get increased with Fe doping and found maximum at 6% Fe doping. The optical energy band gap was found to be increased from 3.47 to 3.57 eV with increasing Fe doping which is attributed to shift in fermi level towards conduction band due to higher carrier concentration. The photocatalytic activity of NiO NPs was observed to be increased with increasing Fe doping concentration due to
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generation of oxygen vacancies and localized energy states that suppressed the electron-hole recombination rate.
Acknowledgement K. Nadeem acknowledges International Islamic University for providing research funds
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under project no. IIUI/ORIC/RP/HRSC/2016-518.
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and Fe-doped NiO NPs synthesized by wet-chemical process, Mater. Charact., 114 (2016) 166-171. [37] S.P. Raja, C. Venkateswaran, Investigation of magnetic behaviour of Ni–Fe–O prepared by the solid state method, J. Phys. D: Appl. Phys., 42 (2009) 145001. [38] J. He, S. Yuan, Y. Yin, Z. Tian, P. Li, Y. Wang, K. Liu, C. Wang, Exchange bias and the origin of room-temperature ferromagnetism in Fe-doped NiO bulk samples, J. Appl. Phys., 103 (2008) 023906. [39] A. Douvalis, L. Jankovic, T. Bakas, The origin of ferromagnetism in 57Fe-doped NiO, J. Phys.: Condens. Matter, 19 (2007) 436203. [40] A. Mishra, S. Bandyopadhyay, D. Das, Structural and magnetic properties of pristine and Fe-doped NiO NPs synthesized by the co-precipitation method, Mater. Res. Bull., 47 (2012) 2288-2293. [41] C. Zener, Interaction between the d-shells in the transition metals. II. Ferromagnetic compounds of manganese with perovskite structure, PhRv, 82 (1951) 403. [42] J. Khemprasit, S. Kaen-Ngam, B. Khumpaitool, P. Kamkhou, Effects of calcining temperature on structural and magnetic properties of nanosized Fe-doped NiO prepared by diol-based sol− gel process, J. Magn. Magn. Mater., 323 (2011) 2408-2412. [43] M. Kahouli, A. Barhoumi, A. Bouzid, A. Al-Hajry, S. Guermazi, Structural and optical properties of ZnO NPs prepared by direct precipitation method, Superlattices Microstruct., 85 (2015) 7-23. [44] G. Kortüm, Reflectance spectroscopy: principles, methods, applications, Springer Science & Business Media 2012. [45] M. Hashem, E. Saion, N.M. Al-Hada, H.M. Kamari, A.H. Shaari, Z.A. Talib, S.B. Paiman, M.A. Kamarudeen, Fabrication and characterization of semiconductor nickel oxide (NiO) NPs manufactured using a facile thermal treatment, Results in Physics, 6 (2016) 10241030. [46] S. Fabbiyola, V. Sailaja, L.J. Kennedy, M. Bououdina, J.J. Vijaya, Optical and magnetic properties of Ni-doped ZnO NPs, J. Alloys Compd., 694 (2017) 522-531. [47] C. Wang, D. Wu, P. Wang, Y. Ao, J. Hou, J. Qian, Effect of oxygen vacancy on enhanced photocatalytic activity of reduced ZnO nanorod arrays, Appl. Surf. Sci., 325 (2015) 112-116. [48] K. Anandan, V. Rajendran, Effects of Mn on the magnetic and optical properties and photocatalytic activities of NiO NPs synthesized via the simple precipitation process, Materials Science and Engineering: B, 199 (2015) 48-56. [49] A.K. Ramasami, M. Reddy, G.R. Balakrishna, Combustion synthesis and characterization of NiO NPs, Materials Science in Semiconductor Processing, 40 (2015) 194202.
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List of Figures
Fig. 1: (a) XRD pattern and (b) FTIR spectra of NF0, NF2, NF4, NF6 and NF8 NPs. Fig. 2: (a, b) TEM images of NF0 and NF8 NPs, respectively (c, b) EDS spectra of NF0 and NF8 NPs, respectively. Fig. 3: Raman spectra of NF0, NF2, NF4, NF6 and NF8 NPs. Insets show zoom portion of 2M band. Fig. 4: M-H loops of NF0, NF2, NF4, NF6 and NF8 NPs at 300 K. Fig. 6: Tauc’s plot for NF0, NF2, NF4, NF6 and NF8 NPs.
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Fig. 5: (a) Magnetization vs Fe doping (%) at ±5T and (b) Coercivity (Hc) vs Fe doping (%).
Fig. 7: (a-e) Absorbance spectra of MeO dye in the presence of NF0, NF2, NF4, NF6 and NF8 NPs.
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Fig. 8: (a) Variation of Photo degradation percentage of NF0, NF2, NF4, NF6 and NF8 NPs with UV irradiation time and (b) Photo degradation percentage at 90 min
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UV irradiation time.
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Fig. 9: Schematic illustration of the photocatalytic process.
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PII:
S0030-4026(19)31535-9
DOI:
https://doi.org/10.1016/j.ijleo.2019.163637
Reference:
IJLEO 163637
To appear in:
Optik
Received Date:
1 August 2019
Accepted Date:
13 October 2019
Please cite this article as: Abbas H, Nadeem K, Hassan A, Rahman S, Krenn H, Enhanced Photocatalytic activity of ferromagnetic Fe-doped NiO Nanoparticles, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163637
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Enhanced Photocatalytic activity of ferromagnetic Fe-doped NiO Nanoparticles Hur Abbasa, K. Nadeema*, A. Hassanb, S. Rahmanc, H. Krennd
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Nanoscience and Technology Laboratory, Department of Physics, International Islamic University, Islamabad 44000, Pakistan. b
Department of Physics, Allama Iqbal Open University, Islamabad 44000, Pakistan.
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Department of Material Science and Engineering, University of Science and Technology of China Hefei, Anhui 230026, China. Institute of Physics, Karl-Franzens University, Universitätsplatz 5, A-8010 Graz, Austria.
*
Corresponding author’s email: [email protected] (K. Nadeem)
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Phone # 0092-51-9019714
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Abstract
The Ni1-xFexO (x= 0.00, 0.02, 0.04, 0.06 and 0.08) nanoparticles (NPs) has been
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synthesized via composite hydroxide mediated (CHM) synthesis approach. The single phase formation of NiO NPs was achieved without any impurity phase confirmed by XRD and average crystallite size showed minor decreasing trend with increasing Fe doping
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concentration. FTIR spectra showed dip in 417-432 cm-1 range which confirmed the existence of Ni-O bonding. In Raman spectra, the weak two-magnon (2M) band at 1472 cm-1 in pure NiO NPs indicate suppressed antiferromagnetism (AFM) due to smaller average
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crystallite size and superparamagnetic contribution from the uncompensated surface/core spins. The absence of 2M band in Fe doped samples is due to onset of ferromagnetism in
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these NPs. The M-H loops showed the room temperature ferromagnetism in Fe doped samples due to double exchange interaction of mixed valance states of Fe. The energy band gap showed minor increasing trend with increasing Fe doping concentration and is mainly attributed to Moss-Burstein effect. The photocatalytic activity showed an increasing trend with increasing Fe doping due to generation oxygen vacancies which served as energy traps and restricted the electron-hole recombination. Therefore, the bifunctional ferromagnetic Fe
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doped NiO NPs are more favourable for photocatalysis along with their ability of collection for re-use in the degradation process.
Keywords: Nickel oxide; Raman; ferromagnetic; Band gap; Photocatalytic.
1. Introduction Bulk nickel oxide (NiO) is a transition metal oxide which has intriguing physical and chemical properties. It has face centred cubic (FCC) structure and antiferromagnetic (AFM)
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ordering well above room temperature TN=523 K [1]. It is a direct band gap p-type
semiconductor [2]. The recent ongoing research at nanoscale confirmed its fascinating
optical, magnetic and photocatalytic properties [3-5] and wide range of applications in
energy storage devices [6], p-type transparent conducting films [7], dye sensitized solar cells
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[8], battery electrodes [9], supercapacitors [10], gas sensors [11], catalysis [12] etc. The most extensive method to fabricate the nanoparticles (NPs) for desired application is to dope by
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ions of foreign element. The suitable dopant not only breaks the crystal symmetry and produces strain in the structure but may also disrupt the magnetic nature of the NPs. The
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presence of two-magnon (2M) band ~1500 cm-1 in Raman spectra reveal the AFM nature of bulk NiO. Mironova et al. [13] observed 2M peak for larger sized NiO NPs and confirmed its AFM nature by magnetic measurements also. Similarly, the absence of 2M peak can be
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attributed to the superparamagnetic (SPM) nature of NPs. Liu et al. [14] reported the disappearance of 2M peak in NiO nanocrystals due to size reduction and attributed to broken symmetry of AFM. The room temperature ferromagnetism (RTFM) in NiO NPs can be
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achieved by incorporation of suitable metal ions [15]. Lin et al. [16] reported RTFM in NiO NPs with Fe doping and ascribed to double exchange mechanism. The localized energy states
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generated in the host material by dopant can also have great impact on optical nature of the material. Patel et al. [17] observed increased optical energy band gap in Fe doped NiO NPs and attributed it to quantum confinement effect. Among different metal oxides, NiO NPs are reported as efficient photocatalyst [18-20] due to their higher degradation rate, better chemical stability, presence of oxygen vacancies, non-toxicity and relatively low cost of production. In order to improve the photocatalytic ability of NiO NPs, the incorporation of suitable metal ions can possibly generate oxygen vacancies that slows down the electron-hole recombination rate. The oxygen vacancies serve as charge carrier traps for electrons and is 2
vital for photocatalytic process. Sankar et al. [21] reported the improved photocatalytic degradation performance of Mn doped NiO NPs and attributed it to generation of localized electronic states which possibly served as charge carrier traps. Therefore, it would be beneficial to investigate the photocatalytic ability of Fe doped NiO NPs that will be collected easily for reuse due to their magnetic nature and are not found to be reported in literature. In this article, we observed the enhanced photocatalytic activity of NiO NPs with increasing Fe doping concentration and also observed RTFM in these doped NPs.
2. Experiment
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The Fe doped (0, 2, 4, 6 and 8% at.%) NiO NPs have been synthesized by composite hydroxide mediated (CHM) approach described elsewhere [22]. The appropriate ratio of
nickel nitrate hexahydrate and iron nitrate nonahydrate (Sigma Aldrich, 99.9%) were added to the eutectic mixture of NaOH and KOH in a ratio of 51.5 : 48.5, respectively. All the
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chemicals were added to teflon vessel and placed in oven at 200oC for 24 h. Finally, the
obtained material was washed several times to get Fe doped NiO NPs. The samples were labelled as NF0, NF2, NF4, NF6 and NF8 with corresponding 0, 2, 4, 6 and 8% Fe doping
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into NiO NPs.
The purity and phase formation of synthesized samples were examined by X-ray
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diffractometer (XRD), vibrational modes of metal oxide bonds were observed by Fourier transform infrared (FTIR) spectrometer, the surface morphology of NPs was observed by scanning electron microscopy (SEM) accompanied by energy dispersive X-ray spectroscopy
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(EDS) for chemical composition. The phonon and magnon modes were observed by Raman spectroscopy with laser wavelength of 532 nm. The magnetic measurements were taken by
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using SQUID magnetometer (Quantum design). The percentage reflectance spectra were taken by for the estimation of optical energy band gap by using diffuse reflectance spectroscopy. The photocatalytic degradation of MO dye has been observed by UV-Vis
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spectrophotometer.
3. Results and discussion: Fig. 1 (a) and (b) shows the X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra of NF0, NF2, NF4, NF6 and NF8 NPs. The diffraction peaks at 37.12o, 43.14o, 62.72o, 75.12o and 79.18o correspond to (111), (200), (220), (311) and (222) planes of FCC structure of NiO, respectively. The obtained pattern was found in accordance 3
with the standard pattern of NiO (JCPDS No. 01-073-1523). The ferromagnetic Ni phase was not observed which is most probable impurity phase observed at low temperature [23]. The other possible impurity phases, like NiFe2O4, Fe2O3 and Fe3O4 which have magnetic origin
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were also found absent.
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Fig. 1: (a) XRD pattern and (b) FTIR spectra of NF0, NF2, NF4, NF6 and NF8 NPs.
The average crystallite size was found to be 33, 32, 31, 29 and 30 nm for NF0, NF2,
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NF4, NF6 and NF8 NPs, respectively. A very small change in average crystallite size has been observed with Fe doping due to insignificant difference in ionic radii of Ni2+ (0.69Å)
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and Fe2+ (0.74Å) ions. No shift in the peak position has been observed which confirmed that doping did not produced any strain in the NiO structure. In FTIR spectroscopy, a dominant dip in the range of 417-432 cm-1 is consigned to stretching vibration mode of Ni-O molecule
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[24].
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Fig. 2: (a, b) SEM images of NF0 and NF8 NPs, respectively (c, b) EDS spectra of NF0 and NF8 NPs,
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respectively.
Fig. 2 (a, b) displays the scanning electron microscopy (SEM) images of NF0 and
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NF8 NPs, respectively at 500 nm scale. It is clear that most of the NPs are spherical in shape with little agglomeration present. Fig. 2 (c, d) shows the EDS spectra of NF0 and NF8 NPs,
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respectively. The presence of Ni, Fe and O peaks confirmed the Fe doping whereas the existing Au peak is due to gold sputtering for achieving better resolution. Raman scattering technique is a useful probe to investigate the lattice disorder,
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structural transitions, cationic distribution and magnetic ordering. Fig. 3 shows the Raman
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spectra of pure and Fe doped NiO NPs at room temperature.
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Fig. 3: Raman spectra of NF0, NF2, NF4, NF6 and NF8 NPs. Insets show zoom portion of 2M band.
The band in the range 400-600 cm-1 is due to the overlapping of first order phonon
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(1P) (1TO = 440 cm-1 and 1LO = 560 cm-1) [25]. The 1TO and 1LO scattering modes are not observable in bulk NiO and stoichiometric NiO, therefore the presence of these modes in NiO NPs are attributed to surface disorder, interstitial oxygen vacancies, broken symmetry and
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imperfectness in crystal structure [26]. Mironova et al. [13] observed the increased intensity of this band with decrease in crystallite size. Chen et al. [27] observed red shift in 1P mode of
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the silicon nanowires with decrease in diameter of wires and attributed it to size induced phonon confinement effect. No such red shift has been observed for our Fe doped samples due to very small difference in average crystallite size (33-29 nm). The second order phonon (2P) band observed at 1025 cm-1 is attributed to longitudinal optical (2LO) stretching mode of NiO [28]. The two-magnon (2M) band has a magnetic origin and is characteristic feature of a bulk AFM material below TN=523 K [29]. This band also gets disappear in small AFM NPs at room temperature due to reduced coordination of antiparallel spins [30]. In Fig. 3, insets show very weak band at 1472 cm-1 in NF0 that confirms weak AFM coupling in the core of 6
NiO NPs. However, this band gets suppressed for Fe doped samples due to magnetic nature of Fe ions. Liu et al. [26] perceived RTFM in Fe doped NiO nanofibers and did not observed 2M peak in these samples. Therefore, absence of 2M peak in Fe doped NiO NPs is an indication of RTFM in these samples.
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Fig. 4 shows the M-H loops of pure and Fe doped NiO NPs at 300 K.
Fig. 4: M-H loops of NF0, NF2, NF4, NF6 and NF8 NPs at 300 K.
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Bulk NiO is AFM below Neel temperature (TN=573 K) but when its dimensions are reduced to nanoscale, the spins at the NPs surface progressively become unequal and
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remained uncompensated which get enhanced on reducing further crystallite size. Therefore, RT weak ferromagnetism in pure NiO NPs can be attributed to finite size effect [31]. It is clear from M-H loops that pure NiO NPs exhibited weak RTFM due to uncompensated
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surface spins with coercivity of 2614 Oe and remanence 0.025 emu/g. Tadic et al. [32] reported RTFM-like behaviour in NiO NPs with coercivity of 115 Oe and remanence 0.0087 emu/g. Continuous linear increase in magnetization with applied field is an indication of AFM core which is in agreement with the weak signal of 2M peak induced in NF0 as shown in Fig. 3. The net magnetic moment increased for Fe doped samples and found maximum for NF6 sample as shown in Fig. 5 (a). The doping of different metal ions induces structural disorder and have influence the average crystallite size. Therefore, induced magnetization can 7
be due to finite size of pure NiO NPs [33-35]. Ponnusamy et al. [36] observed RTFM in Fe doped NiO NPs and attributed it to decrease in average crystallite size. In our case, Fe doping did not produced a significant change in the average crystallite size so this aspect cannot be accounted for doped samples. From XRD and Raman results, it is also clear that no other secondary phase of Fe such as Fe2O3 and Fe3O4 is present, therefore this aspect of enhancement in net magnetization is also neglected. Many researchers [37-40] reported the enhanced magnetization in Fe doped NiO NPs due to ferrimagnetic NiFe2O4 phase formation. Raja et al. [37] observed the ferrimagnetic NiFe2O4 phase for more than 2% Fe doping into NiO NPs but in our case no such impurity phase has been observed even up to 8% of Fe doping. The disorder generated by substitution of Ni2+ by Fe2+ (or Fe3+) breaks the original
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translational symmetry of magnetic ions and super-exchange interaction (Ni2+-O2-- Ni2+) in crystallites is interrupted [16]. There are two stable valance states of Fe ions exist, i.e. Fe2+ and Fe3+. Douvalis et al. [39] observed the existence of Fe3+ ions into NiO matrix by
Mossbauer spectroscopy, therefore enhanced net magnetic moment may be due to double
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exchange interaction of mixed valance states [41]. Khemprasit et al. [42] also observed the
enhanced ferromagnetism in Fe doped NiO NPs and attributed it to generation of Ni vacancy
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defects and double exchange interactions. The net magnetic moment did not saturate up to 50 KOe which confirmed the presence of AFM phase in these NPs. The higher value of
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coercivity for pure NiO NPs is attributed to the presence of surface uncompensated spins. However, coercivity started decreasing with increasing Fe doping concentration due to the
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contribution of magnetization from core of the NPs as shown in Fig. 5(b).
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Fig. 5: (a) Magnetization vs Fe doping (%) at ±5T and (b) Coercivity (Hc) vs Fe doping (%).
Before analysing the photocatalytic activity of Fe doped NiO NPs, it is necessary to investigate their optical band gap energy so that information about the activation energy of 8
photocatalyst may be collected. The energy band gap of pure and Fe doped NiO NPs was
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evaluated from well-known Tauc’s relation [43] and is shown by Tauc’s plot in Fig. 6.
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Fig. 6: Tauc’s plot for NF0, NF2, NF4, NF6 and NF8 NPs.
The DRS technique was employed to calculate the optical absorption coefficient [44] (1−𝑅)2 2𝑅
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𝐹(𝑅) =
(1)
Where 𝐹(𝑅) is Kubelka Munk function and R is the absolute reflectance of the material. The
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estimated energy band gaps were 3.47, 3.51, 3.52, 3.53 and 3.57 eV for NF0, NF2, NF4, NF6 and NF8 NPs, respectively. The smaller crystallites have larger band gap according to
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quantum confinement effect [2]. Hashem et al. [45] reported the increase in energy band gap from 3.51 to 3.60 eV when crystallite size of pure NiO is reduced from 34 to 14 nm. However, finite size effect cannot be considered for our Fe doped NiO NPs due to very small difference in average crystallite size as calculated from XRD. Therefore, slight enhancement in energy band gap for Fe doped samples can be attributed to Moss-Burstein effect [46] in which Fermi level shift close to the conduction band due to doping which increases carrier density and leads to increase in energy band gap.
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The photocatalytic activity of synthesized samples was observed by using Methyl orange (MeO) dye which is widely used as model pollutant. The 70 ml MeO solution (15 mg/l) was taken in a beaker and 30 mg catalyst (Fe doped NiO NPs) was added to test photocatalytic degradation under UV illumination. In order to maintain absorption-desorption equilibrium, the MeO solution was stirred for 30 minutes in the dark. Fig. 7 (a-e) shows the absorbance spectra of MeO dye at various time intervals in the presence of NF0, NF2, NF4, NF6 and NF8 NPs used as photocatalyst. The maximum intensity of absorbance spectra of MeO dye was observed at 464 nm [20] which gradually get decreased with the increasing irradiation time and is attributed to the degradation of MeO dye molecules which confirms
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the photocatalytic ability of pure and Fe doped NiO NPs.
Fig. 7: (a-e) Absorbance spectra of MeO dye in the presence of NF0, NF2, NF4, NF6 and NF8 NPs.
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Fig. 8 (a) shows the variation in degradation percentage of MeO dye with irradiation time when pure and Fe doped NiO NPs were used as photocatalyst. The enhanced photocatalytic activity with Fe-doping may be attributed to following reasons: (1) more absorption of photons due to generation of localized states, (2) higher doping rate increased concentration of oxygen vacancies that serve as energy traps to delay electron-hole recombination. Wang et al. [47] reported an enhanced photocatalytic activity of ZnO nanorods and attributed it to generation of oxygen vacancies that reduced the recombination rate of electron and holes. Fig. 8 (b) shows that after 90 minutes of UV irradiation, NF0 NPs
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only degraded 68% where NF8 completely degraded the model pollutant.
Fig. 8: (a) Variation of Photo degradation percentage of NF0, NF2, NF4, NF6 and NF8 NPs with UV irradiation
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time and (b) Photo degradation percentage at 90 min UV irradiation time.
When ultraviolet (UV) light of sufficient energy (equal or greater than the bandgap
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energy) falls on the surface of catalyst, the electron-hole pairs are generated which are vital to initiate the photocatalytic process. The excited electrons in the C.B and holes in V.B react
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with nearby oxygen (O2) molecules and surface bound water to produce superoxide radical anion (·O2-) and hydroxyl radical (·OH), respectively. These radicals are known as reactive oxygen species (ROS) and strongly react with organic pollutant molecule to degrade [48] (∙ 𝑂2− ) 𝑜𝑟 (∙ 𝑂𝐻) + 𝑀𝑒𝑂 𝑑𝑦𝑒 → 𝐻2 𝑂 + 𝐶𝑂2 + 𝑑𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠
(2) The schematic diagram of the photocatalytic process is shown in Fig. 9.
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Fig. 9: Schematic illustration of the photocatalytic process.
The photocatalytic dye degradation percentage was calculated by equation [49]. 𝐴𝑜 −𝐴𝑡 𝐴𝑜
) × 100
(3)
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Degradation (%) = (
Where Ao and At are the maximum value of absorbance of MO dye at 0 and t minutes,
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respectively.
Conclusion
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The Fe doped (0, 2, 4, 6 and 8%) NiO NPs has been synthesized by low temperature CHM approach. The phase formation of pure and Fe doped NiO NPs has been confirmed by XRD and possible impurities which were commonly observed in literature such as Ni, Fe2O3,
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Fe3O4 and NiFe2O4 were not found. The Fe doping did not produced any significant change in the average crystallite size due to small difference in ionic radii of Ni2+ and Fe2+ ions.
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FTIR spectroscopy revealed the presence of Ni-O bonding whereas SEM images showed the spherical shape of the both pure and Fe doped NiO NPs. The small 2M peak in Raman spectra reveal weak AFM in pure NiO NPs due to contribution from the core whereas high
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coercivity is attributed to uncompensated spins present on its surface. The M-H loops confirmed the RTFM in Fe doped NiO NPs and magnetization get increased with Fe doping and found maximum at 6% Fe doping. The optical energy band gap was found to be increased from 3.47 to 3.57 eV with increasing Fe doping which is attributed to shift in fermi level towards conduction band due to higher carrier concentration. The photocatalytic activity of NiO NPs was observed to be increased with increasing Fe doping concentration due to
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generation of oxygen vacancies and localized energy states that suppressed the electron-hole recombination rate.
Acknowledgement K. Nadeem acknowledges International Islamic University for providing research funds
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under project no. IIUI/ORIC/RP/HRSC/2016-518.
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List of Figures
Fig. 1: (a) XRD pattern and (b) FTIR spectra of NF0, NF2, NF4, NF6 and NF8 NPs. Fig. 2: (a, b) TEM images of NF0 and NF8 NPs, respectively (c, b) EDS spectra of NF0 and NF8 NPs, respectively. Fig. 3: Raman spectra of NF0, NF2, NF4, NF6 and NF8 NPs. Insets show zoom portion of 2M band. Fig. 4: M-H loops of NF0, NF2, NF4, NF6 and NF8 NPs at 300 K. Fig. 6: Tauc’s plot for NF0, NF2, NF4, NF6 and NF8 NPs.
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Fig. 5: (a) Magnetization vs Fe doping (%) at ±5T and (b) Coercivity (Hc) vs Fe doping (%).
Fig. 7: (a-e) Absorbance spectra of MeO dye in the presence of NF0, NF2, NF4, NF6 and NF8 NPs.
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Fig. 8: (a) Variation of Photo degradation percentage of NF0, NF2, NF4, NF6 and NF8 NPs with UV irradiation time and (b) Photo degradation percentage at 90 min
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UV irradiation time.
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Fig. 9: Schematic illustration of the photocatalytic process.
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