Date pits extracts assisted synthesis of magnesium oxides nanoparticles and its application towards the photocatalytic degradation of methylene blue

3.1 Morphology and chemical element of MgO nanoparticles

Fig. 2(a, b) shows scanning electron microscopy (SEM) images of MgO nanoparticles annealed at (a) 700 and (b) 500 °C, respectively. Spherical nanograins of irregular sizes (both small and big) are observed from SEM images. Usually irregular nanograins tend to agglomerate (see insets) due to their surface energy leading to the formation of large surface area. It is seen that MgO nanoparticles annealed at 700 °C exhibit bigger nanograins that form agglomeration (see Fig. 2(a)). Researchers report that bigger grains could be attributed to the Oswald ripening process with limited porosity and crystallinity.

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Fig. 2

SEM images showing the morphology of synthesised MgO nanoparticles annealed at (a) 700 and (b) 500 °C. Insets show spherical agglomerated nanograins.

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The SEM images of samples annealed at lower annealing temperature of 500 °C is shown in Fig. 2(b). A breakdown of agglomeration revealing much smaller nanograins is seen. Thus by visual inspection only, it is seen that at lower annealing temperature, smaller size nanograin is achieved.

To identify the elemental composition of synthesised MgO nanoparticles, EDS analysis was conducted and results are shown in Fig. 3(a-d). Obtained from EDS mapping in high percentages are Magnesium (Mg) and Oxygen (O), which are the main chemical elements in the samples. This confirms the purity of MgO nanoparticles devoid of impurities. The presence of carbon observed arises from carbon coating samples before SEM analysis. It is noted that atomic percentages of chemical elements of MgO nanoparticles vary with annealing temperature. For instance, percentage of Mg is higher in MgO nanoparticles annealed at 500 °C as compared with MgO annealed at 700 °C. In the same way, O is in abundance in MgO nanoparticles annealed at 700 °C than in MgO nanoparticles annealed at 500 °C.

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Fig. 3

EDS spectra showing the mapping of elements present in samples with respective atomic percentages of synthesized MgO annealed at (a, b) 700 °C, (c, d) 500 °C.

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High-resolution transmission electron microscopy (HRTEM) images of un-annealed MgO nanoparticles are presented in Fig. 4(a-c). As one can see, un-annealed MgO nanoparticles reveal agglomerated spherical nanograins evident in Fig. 4(a). It is observed that SAED pattern (see Fig. 4(c)) taken from un-annealed MgO nanoparticles shows no diffraction peak, indicative of the amorphous nature of un-annealed MgO nanoparticles.

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Fig. 4

HRTEM images of un-annealed MgO nanoparticles showing (a) nanograins, (b) singular nanograins, (c) SAED pattern.

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Upon annealing MgO nanoparticles, the structure becomes refined. This can be observed from the HRTEM images of MgO nanoparticles annealed at 700 °C, as shown in Fig. 5(a). One can see both small and big size nanograins of spherical shapes. For a singular nanograin shown in Fig. 5(b), series of distinguishable planes with approximate dhkl distances of 0.32 and 0.34 nm are observed. Bright diffraction peaks (Fig. 5(c)) with d_hkl distances of 0.22, 0.149and 0.121 nm which correspond to (0 0 2), (0 2 2), and (2 2 2) planes, respectively, of face-centred cubic Periclase MgO, are seen on SAED image.

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Fig. 5

HRTEM images of annealed at 700 °C MgO nanoparticles showing (a) nanograins, (b) singular nanograins, (c) SAED pattern.

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As annealing temperature is reduced to 500 °C, MgO nanoparticles become less agglomerated. This is evident from Fig. 6(a-c) which shows an HRTEM image of nanograins with average size of 14.0 nm. Lattice resolved HRTEM image presented in Fig. 6(b) illustrates the interplanar distances of 2.10 and 1.49 Å of (0 0 2) and (0 2 2) crystal planes, respectively. Corresponding SAED pattern characterized by bright diffraction spots with each spot arising from Bragg reflection from an individual crystallite is shown in Fig. 6(c).

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Fig. 6

HRTEM images of annealed at 500 °C MgO nanoparticles showing (a) nanograins, (b) singular nanograins, (c) SAED pattern.

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Observation from SAED patterns of un-annealed and annealed MgO nanoparticles present different outputs. Un-annealed MgO nanoparticles SAED image reveals diffuse ring patterns confirming the amorphous nature of nanograins. While SAED patterns of annealed MgO nanoparticles reveal bright diffraction spots, and small spots making up rings. This confirms crystalline and poly-nano-crystalline of nanograins.

The histogram of particle size distribution estimated via image J software for each sample is plotted in supporting document Fig. 7(a-c). The results presented in Fig. 7(a-c) give estimates average particle sizes. It is estimated that un-annealed MgO nanoparticles (Fig. 7(a)) recorded random particle distribution. However, as MgO nanoparticles are annealed at (b) 700 °C and (c) 500 °C, particles are evenly distributed. Lowest average particle size is recorded for annealed at 500 °C MgO nanoparticles. The average particle size agrees with observations from SEM and HRTEM analysis.

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Fig. 7

Histogram of the particle size distribution of MgO (a) un-annealed, annealed at (b) 700 (c) 500 °C.

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3.2 Phase identification: XRD, Raman, ATR

XRD patterns of un-annealed and annealed MgO nanoparticles synthesised via the green chemistry approach are shown in Fig. 8(a). From the XRD patterns, un-annealed MgO nanoparticles show amorphous structure while annealed samples exhibit well crystalline structures. Precisely, the XRD pattern of un-annealed MgO nanoparticles shows no diffraction peak. Meanwhile annealed MgO nanoparticles exhibited five (5) distinguishable peaks at 2θ = 36.889°, 42.856°, 62.216°, 74.577°, 78.510° which correspond to crystal lattice planes (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) respectively. The observed peaks are perfectly matched to a Face-centered cubic Periclase MgO with lattice perimeters of a = 4.2171 Å, S.G. = Fm-3m (2 2 5), Z = 4. These values are in good agreement with standard values reported by ICSD code 9863.

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Fig. 8

(a) The XRD patterns for un-annealed and annealed MgO nanoparticles, (b) W-H plot of MgO nanoparticles annealed at 500 °C and (c) W-H plot of MgO nanoparticles annealed at 700 °C.

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X-ray profile was used to estimate crystallite size. The average crystallite size (<$\varphi$ ˃_m) was calculated by employing the Debye–Scherrerformula:

By considering the peak width as a function of 2θ, the lattice strain of annealed MgO nanoparticles are estimated via the Eq. (2).

A graph of $4sin\theta$ and $\beta cos\theta$ along × and y-axis respectively, for annealed MgO nanoparticles, is shown in Fig. 8(b,c). From linear fit, microstrains (ɛ), are estimated from the slope of fit. Annealed MgO nanoparticles reveal positive strain while a minimal strain characterized by negative value was obtained for MgO nanoparticles annealed at 500 and 700 °C respectively.

The dislocation density (δ) is estimated via equation below:

To investigate vibrational modes of samples, ambient temperature Raman spectra was performed and results are shown in supporting document Fig. s1. Raman line observed at 423 cm⁻¹ from 500 °C annealed MgO nanoparticles coincide with one of the prominent peaks in Chen’s calculation Karthik et al. (2019). Such a line is attributed to transverse optical (TO) phonon at the zone centre. Also, a prominent peak at 2433 cm⁻¹ ascribed to C–H carbon bonded with MgO at the surface is identified on MgO nanoparticles annealed at 500 °C yet absent on MgO nanoparticles annealed at 700 °C. Raman spectrum obtained on MgO nanoparticles annealed at 700 °C shows strong peaks at various regions indicating cubic structure designated to MgO.

ATR spectra recorded with wavelength ranging from 400 to 4000 cm⁻¹ on annealed MgO nanoparticles presented (see supporting document Fig. s2). The peaks seen at 3692 and 1428 cm⁻¹ correspond to OH stretching and H₂O adsorption. The bands observed at 2990 and 2922 cm⁻¹ are attributed to aliphatic group CH₂ asymmetric and symmetric vibration respectively. The medium appearing at 1259 and 1069 cm⁻¹ are assigned to φ-O vibration and C–O stretching vibrations, respectively. The absorption band observed at 884 cm⁻¹ is a characteristic vibration of a cubic MgO Wu et al. (2008).These peaks further confirmed the successful synthesis of MgO phase.

3.3 Optical properties: DRS, UV–Vis absorbance, PL

Diffuse reflectance spectra (DRS) of synthesised un-annealed and annealed MgO nanoparticles are shown in Fig. s5 of supporting document. DRS spectra in the UV–VIS-NIR range of annealed and un-annealed MgO nanoparticles show fairly constant reflectance and a broadband at 320 nm with fringes respectively. It is reported that pure MgO nanoparticles record sharp absorption band at 185 nm which correspond to optical excitation of five-fold coordinated O²⁻ anion (Suresh and Arivuoli, 2011). Such wavelength is below the accessible wavelength of spectrophotometer used in this analysis. In our spectra, absorption bands are observed at 205 and 276 nm which have been designated by regions I& II (See supporting document Fig. s3). These bands correspond to 6.04 and 4.49 eV respectively. Such bands are identified with excitation of four-fold and three-fold coordinated O²⁻ anions in edges and corners of MgO respectively Gu et al. (2008).

From DRS measurement, the optical band gap (supporting document Fig. s4) was calculated via Kubelka–Munk function described elsewhere and values are given in Table 2 (see supporting document). The band-gap obtained for un-annealed MgO nanoparticles is less than the standard value. The decreased bandgap could be due to the quantum confinement effect.

The band edge potential, i.e. the potential of conduction and valence band is determined via the equations below;

The valence band is expressed as

The values of the calculated conduction and valence bands of the MgO nanoparticles are shown in Table 2 (see supporting documents).

UV–Vis absorbance was used to investigate the formation and nanoparticle growth dynamics of the MgO nanoparticles. The absorbance spectra of aqueous extracts of date pits and mixture (magnesium acetate and extracts) is shown in supporting document (Fig. s5). The UV–Vis absorbance spectrum of aqueous date pits extracts shows two peaks at 280 and 330 nm. Such peaks originate from phenolics such as gallic acid and flavonoids which serve as reducing agents. UV–Vis absorbance spectrum of mixture shows a broad and shift in wavelength at 290 nm. The peak observed at 290 nm is the characteristic peak of MgO nanoparticles Mabry et al. (1970).It should be noted that the UV–Vis absorbance of the mixture was measured immediately magnesium acetate was added to extracts. This implies that at 0 h reaction time, MgO nanoparticles are formed.

To further understand the optical properties and various structural defects of MgO nanoparticles, room temperature photoluminescence spectra (PL) were recorded. An ultraviolet light source of 240 nm was used to excite the MgO nanoparticles. The PL spectra (Fig. s6 of supporting document) revealed two emission bands. One broad blue band emission is at 424 nm (2.92 eV) which is attributed to ³B_1u→¹Ag transition of F₂²⁺center in D_2h symmetry. Such centres consist of two oxygen vacancies in nearest neighbour position along 〈1 1 0〉, each having a trapped electron with the ground and first excited states as ¹Ag and ³B_1u, respectively Kumar et al. (2011). The second emission band is observed at 541 nm which would favour the existence of large quantities of oxygen vacancies to acts as deep defect donors to create new energy levels in the bandgap of MgO nanoparticles.

3.4 Photocatalytic activity OF MgO

Photocatalytic activity of annealed MgO nanoparticles was studied by measuring the degradation ability with aqueous solutions of methyl blue (MB) under the UV lamp. UV–Vis absorbance at different concentrations of MB (2, 4, 6, 8, 10 ppm) were measured on a wavelength range of 200–800 nm. The spectra shown in Fig. 9(a), recorded the highest absorbance (10 ppm) value at a unique wavelength of 663.4 nm.

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Fig. 9

UV–Vis absorbance spectra for (a) different MB concentrations, (b) photocatalytic degradation of MB in contact time with MgO annealed at 500 °C (c) photocatalytic degradation of MB in contact time with MgO annealed 700 °C.

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Fig. 9(b,c) show spectra for the photocatalytic activity of annealed MgO nanoparticles under visible light at sampling time. As can be seen, UV–Vis absorption spectra of MB by annealed MgO nanoparticles under a UV lamp at sampling time changes. The peak levels decrease with an increase in a contact time of visible light irradiation. The photocatalytic degradation of MB dye by annealed MgO nanoparticles can be attributed to its spherical-like nano grain structures.

After 150 min of contact time of visible light irradiation, the shape of the UV–Vis absorbance peak remains the same. This shows that our material is still active even after such a long time. For stability and reproducibility, the experiment was repeated three times and it was noticed that each run did not show any significant difference in the results.

The photocatalytic efficiency was evaluated by the equation below:

3.4.1

3.4.1 Removal kinetic of MB

The photocatalytic of MgO nanoparticles obeys pseudo-first-order kinetics which was calculated by the equation.

(5)

$$ln\frac{A_t}{A_o}=-kt$$where k is rate constant and t is sampling t. The slope of $ln\frac{A_t}{A_o}$ verses $t$ (see Fig. 10 (a) gives the values of k. The k values were estimates as 0. 0027 mins⁻¹ (R² = 0.78309) and 0.00085 min⁻¹ (R² 0.97142) for annealed at 500 and 700 °C MgO nanoparticles. The values of rate constant increase with photocatalytic degradation of annealed MgO nanograins. The photocatalytic degradation efficiency versus irradiation is shown in Fig. 10(b). The mechanism of photocatalysis on MB is explained in Fig. 10(c).

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Fig. 10

Plot showing (a) photocatalytic degradation efficiency of MgO nanoparticles versus irradiation time, (b) removal Kinetic of MB, (c) the mechanism of photocatalysis of MgO nanoparticles on MB.

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As a follow up of this study, it is intended to use the same procedure to bio-synthesize several other functional nanoscaled oxides and even nanocomposites (Simo et al., 2014; Kaviyarasu et al., 2015; Magdalane et al., 2018; Maaza et al., 2005; Khenfouch, et al., 2012).