Simplistic Synthesis and Enhanced Photocatalytic Performance of Spherical ZnO Nanoparticles Prepared from Arabinose Solution

2.1 Green Synthesis of ZnO Spherical Nanoparticles

During the facile fabrication process, 100 mL of 0.3 M zinc nitrate dihydrate solution was stirred well for 30 min. Arabinose solution (100 mL; 33.3 mmol) was then added dropwise to the zinc nitrate solution under vigorous stirring at 90 °C for 6 h. A brown-white powder was formed. After a few minutes, the obtained powder was cooled at room temperature and then ground and annealed at different temperatures such as 400, 500, 600, and 700 °C for 3 h to obtain spherical ZnO nanoparticles, as illustrated in Figure 1. The ZnO powders were characterised in detail in terms of their structural, morphological, compositional, and optical properties, as well as photocatalytic activity, by employing different analytical techniques.

2.2 Characterisation Techniques

The phase structure and average crystallite size of the ZnO nanoparticles were investigated by examining the powder’s XRD patterns. The samples were recorded with a diffractometer (D8 Advance Bruker, Mannheim, Germany) using Cu–Kα radiation (λ = 0.15406 nm). The morphology of the obtained nanopowder was tested through SEM. The specimens were previously oven-dried at 105 °C and coated with a thin film of gold to create electrical conduction on the surface of the ZnO powder. The mode of chemical bonding in the prepared samples was studied by FTIR (Nicolet 6700, Thermo Fisher, Waltham, MA, USA) in the range of 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹.

2.3 Photodegradation of Mix Dye Waste

A mixture of methyl orange (MO), indigo carmine (IC), and malachite green (MG) at λ_max = 616 nm was selected to assess the photocatalytic performance of the spherical ZnO nanoparticles. A 254 nm UV lamp was employed as the light source and was located 20 cm away from the mixtures. During the test, 50 mg of the catalyst was dispersed in 100 mL of 11 mg/L of the combined dye solution in a 250 mL beaker. The reaction suspension was magnetically stirred for 30 min to achieve adsorption-desorption equilibrium. Afterwards, the suspension solution was illuminated with the UV light. At each time interval, 5 mL of the solution was withdrawn, centrifuged, and filtered through a membrane filter (0.2 μm), after which the residual concentration was measured. The degradation percentage of the dye waste was then calculated as follows [14]:

where c₀ and c_t represent the initial dye waste concentrations (11 mg/L) and time t, respectively.

3.1 Characterisations of Fabricated ZnO Nanoparticles

3.1.1 Thermal Stability

Figure 2 displays the TGA thermograph of the Zn-arabinose, which indicated two weight-loss stages of the Zn-arabinose during the heating process. In the first stage, between 160 and 295 °C weight loss was caused by the decomposition of the arabinose sugar to small organic compounds such as 2-furancarboxaldehyde, furan methanol, α-angelicalactone, and methyl ester of furoic acid [15]. The second stage took place between 300 and 400 °C, where the weight loss was due to the complete degradation of Zn/organic fragments to produce stable spherical ZnO nanoparticles. Therefore, 400 °C was the optimum heat-treatment condition at which all of the Zn-arabinose was transformed to ZnO nanoparticles.

Figure 2:

Thermal stability analysis of Zn-arabinose.

Figure 3:

XRD diffraction patterns of spherical ZnO nanoparticles at different annealing temperatures.

3.1.2 XRD Analysis

The XRD patterns of the prepared spherical ZnO nanoparticles at different annealing temperatures are displayed in Figure 3. The results of the XRD patterns revealed that the peaks at 31.71°, 34.38°, 36.30°, 47.52°, 56.56°, 62.91°, and 67.93° were well indexed to (100), (002), (101), (102), (110), (103), and (112) planes. These results were equivalent to wurtzite ZnO (JCPDS 36–1451) [16] with well-crystallised ZnO nanoparticles. The XRD patterns also confirm significant changes in the crystallite size with the increase in the annealing temperature from 400 to 700 °C (Fig. 4). This has a hexagonal wurtzite ZnO structure (JCPDS 36-1451) with high purity and crystallinity that increased with annealing temperature. The average crystallite size was calculated using Debye-Scherrer’s formula [17], [18].

(2)D=kλβcosθ

Also, the lattice parameters a and b were obtained via the following equations:

(3)a=λ3sinθand c=λsinθ

and the volume of unit cell was estimated through the following equation:

(4)ν=0.866×a2×c

Figure 4:

Relationship between full width at half maximum and crystalline size with annealing temperature.

d-spacing values were obtained from Braggs’s equation as follows:

(5)d=λ2sinθ

The variation of crystallite size, lattice parameters, and d-spacing results are presented in Table 1. It is worth noting that the average crystallite sizes of ZnO samples annealed at 400, 500, 600, and 700 °C was 18.23, 24.06, 27.64, and 31.19 nm, respectively. The average c/a ratio is in agreement with that of the hexagonal wurtzite structure of ZnO, indicating that the influences of heat treatment still preserve the ZnO structure.

Table 1:

d-spacing, crystalline size (D; nm), lattice parameters, and unit cell (v) of ZnO nanoparticles with different annealing temperatures.

Temperature (°C)	2Θ(101)	d-spacing	FWHM	Crystalline size (nm)	Lattice parameters		c/a	Unit cell (ν)
					a	c
400	36.13	2.484	0.479	18.23	3.257	5.222	1.6033	47.972
500	36.16	2.482	0.363	24.06	3.263	5.221	1.6030	48.139
600	36.20	2.479	0.316	27.64	3.254	5.214	1.6023	47.811
700	36.22	2.478	0.280	31.19	3.253	5.212	1.6022	47.762

1. FWHM, full width at half maximum.

The average crystallite size, lattice parameters, and unit cell volume estimated from all aforementioned equations are present in Table 1.

3.1.3 SEM Image Analysis and Energy-Dispersive X-Ray (EDX) Investigation

The morphology of the specimens was investigated using field emission scanning electron microscopy (FESEM) and is shown in Figure 5a–d. It can be noted that the samples showed spherical nanoparticles at room temperature with particle sizes increasing with incremental increases in the annealing temperature. To check the formation and purity of ZnO prepared, the EDX analysis was employed. The spectra of ZnO at 700 °C (Fig. 5f) clearly showed a strong response at 1 and 0.5 keV attributed to Zn and O, respectively. Moreover, a weak peak at 8.6 and 9.5 were also due to Zn. Besides, the weight percentages of the elements Zn and O achieved from EDX results were 78.30 % and 21.7 %, respectively.

Figure 5:

(a–d) FESEM images of ZnO nanoparticles at different annealing temperatures from 400 to 700 °C and (f) EDX spectra of ZnO nanoparticles at 700 °C.

3.2 Photocatalytic Activity of Spherical ZnO Nanoparticles

To scrutinise the photocatalytic activity of the spherical ZnO catalysts, they were probed through photodegradation of a mixture of three dyes (MO, IC, and MG) as typical environmental toxins. A spectrum of the mixtures of dye waste at 616 nm (maximum adsorption wavelength) is illustrated in Figure 8a. The degradation and kinetics of the photodegradation of dye mixtures were evaluated using ZnO nanoparticles annealed at different temperatures under UV light irradiation with standard experimental conditions as displayed in Figure 8b, c. It is a well-known truth that an incident photon with l < 390 nm can make an exciton, i.e. abound electron-hole pair in spherical ZnO photocatalyst. These electron-hole pairs on the surface of the ZnO nanoparticles via a series of oxidation-reduction reactions produce intermediates such as O2−*, HO2*, and H2O2, which lastly breed OH^* radicals in the existence of dissolved O₂. These OH^* radicals are extremely strong oxidizing agents that can cause the disruption of the aromatic rings of the mixed dye molecules, resulting in their decomposition [25].

Figure 6:

(a) UV-Vis absorption spectra and (b) energy gap of ZnO at different annealing temperatures.

Figure 7:

FTIR spectra of ZnO nanoparticles at different annealing temperatures.

Figure 8:

(a) The UV-Vis spectra of the dye waste, (b, c) pseudo-first-order kinetics graph of ZnO at various annealing temperatures for the dye mixture photodegradation, and (d) the photodegradation percentage of ZnO for the dye waste.

The effect of annealing of ZnO nanoparticles on the photocatalytic degradation of pollutants was demonstrated thanks to the Langmuir-Hinshelwood kinetic equation as follows [26]:

where t represents the illumination time (min) and k is the rate of degradation constant.

It can be observed that the photocatalytic degradation rate of the model pollutants decreased with the increase in the annealing temperature from 400 to 700 °C (Fig. 8a, b; Tab. 2). This observation may be due to the diminution of surface area and increases in the agglomerations of the ZnO nanoparticles, which is proved by SEM results (Fig. 5). Furthermore, this finding could be also because a ZnO nanoparticle at 400 °C has fine and small particle size. A similar result was observed in [19], [27]. Indeed, the spherical ZnO nanoparticles annealed at 400 °C eliminated 84 % of the simulated dye wastes after 25 min of illumination with a rate constant equal to 0.0867 nm⁻¹ as shown in Figure 8d and Table 2.

Figure 9:

Proposed mechanism of photocatalytic degradation of mix dyes.

The major agent responsible for the forming of an electron-hole pair on the surface of the ZnO photocatalyst is the energy gap between the valence and CB, which is referred to as the band gap. The larger particle size of ZnO resulted in a minimal band gap [28], [29]. It can be noticed that with the increase in the annealing temperature, the particle size increases, which results in the diminution of the energy band gap. In addition, with smaller particle size, the surface area of the ZnO photocatalyst is also increased. Both of these agents favour improved photocatalytic activities of a ZnO photocatalyst.

Moreover, the introduction of the native defects in the ZnO particles in the form of neutral (V_O), singly charged (VO+), or doubly charged (VO++) oxygen vacancies at elevated annealing temperatures up to 700 °C also plays a crucial role in the improvement of photocatalytic efficiencies of the fabricated spherical ZnO nanoparticles. These defects in the ZnO diminish the electron-hole recombination, increase the quantum yield, and result in better photocatalytic activities of ZnO nanoparticles. For our results, the photocatalytic performance for the spherical ZnO photocatalyst with higher annealing temperatures of 500–700 °C decreased, and this could be ascribed to the reduction of surface area due to the agglomerations of the ZnO particles [30].

3.3 Photocatalytic Degradation Mechanism

A possible photocatalytic mechanism for the degradation of organic pollutant by spherical ZnO is shown in Figure 9. By means of irradiating the as-fabricated ZnO catalyst with visible light, the electrons in the VB is excited to the CBs. Then, a corresponding number of holes are formed in the VB, as shown in Figure 9. In addition, the CB reacts with the dissolved oxygen to offer a superoxide radical, which resulted in the creation of hydroxide radicals [31]. Afterwards, the VB reacts with H₂O molecules to make hydroxide radicals. As a result, the ^⋅OH radical produced in the solution is a physically influential oxidizing agent for mixed dye degradation. Therefore, the organic pollutant mixture is mineralised owing to the joint action of H⁺ and the powerful oxidizing species ^⋅OH.