Abstract

Zinc oxide (ZnO) nanoparticles are easily prepared from zinc solid using an electrochemical technique. A quick anodic oxidation under constant voltage of a zinc foil in an aqueous potassium chloride electrolyte leads to the fabrication of ZnO nanoparticles in a single-step process at room temperature and without the use of controversial chemical reagents. The crystallographic structure of the fabricated nanoparticles is characterized using X-ray diffraction (XRD) spectroscopy. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) are used to reveal the morphology and aggregate size of the nanopowder. The energy dispersive spectra obtained from the SEM-EDS confirm that the sample prepared is in the ZnO phase. The photocatalytic property of the synthesized ZnO nanoparticles is evaluated in studying the degradation of the Acid orange 7 (AO7) organic dye in the presence of the ZnO nanoparticles as a catalyst under UV-visible irradiation.

Keywords

Zinc Oxide Nanoparticles, Nanopowder, Anodic Oxidation, Photocatalysis

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1. Introduction

Since the mid-1960s, following the pioneering research of Peter Speiser [1], nanoparticles have attracted growing interest. The properties of finely divided matter offer a wealth of opportunities for discovery [2]. Innovative technological applications can be envisaged by the development of nanotechnology [3]. Among all the kinds of nanomaterials, nanoparticles are considered as building blocks of primary importance [4]. Numerous fields of applications from biology and medicine [5] [6], industrial [7]-[9] agriculture [10] [11] to environmental [12] and space applications [13] [14] are concerned. To develop these applications, nanoparticle production is of prime importance, including various top-down [15] and bottom-up approaches [16]. Among the many particles produced, ZnO nanoparticles are highly attractive considering their wide band gap [17], very suitable for optoelectronics applications [17]-[19] and solar cell devices [20]-[22]. Different synthesis methods are commonly used to produce ZnO nanoparticles including direct precipitation [23]-[25], sol-gel techniques [26]-[28], hydrothermal process [29]-[31], spray pyrolysis [32]-[34] for the most common ones, these approach need thermal post treatment to achieve a suitable crystallinity of the final products [35]-[37]. On the other hand, electrochemical anodization processes are mainly dedicated to the surface modification of bulk material [38]-[41], also called nanostructure formation. Few studies focus on the synthesis of nanoparticles using the anodization technique. In this paper, we elaborate on ZnO nanoparticles by chemical anodizing a Zn foil. This unusual synthesis method produces, in a few seconds, a large amount of ZnO nanoparticles and doesn’t require any thermal post-treatment. This method doesn’t require controversial chemical reagents or time-consuming steps.

2. Materials and Methods

2.1. Anodization Set-Up

The anodization setup is a custom-built apparatus (Figure 1). It includes a TDK^® Lambda GENH300-2.5 DC power supply, which serves as the voltage generator, and an Agilent^® 34401A multimeter for data recording during experiments. These instruments are interfaced with a computer. The lab has developed software named Nanobuilder to control the electrochemical cell via a GPIB interface, utilizing LabVIEW 8^® from National Instruments^® as the systems engineering tool. Nanobuilder regulates the target voltage applied to the anodizing cell by following a predefined voltage profile. Throughout the process, the generator provides feedback on the actual voltage applied, while the ammeter measures the anodizing current. The software logs these values in a file for real-time monitoring and subsequent analysis.

2.2. Sample Preparation

All chemicals are used without any post-treatment. Potassium chloride electrolyte solution 0.2 M is prepared by weighing and dissolving the appropriate amount of KCl (Fisher Chemical^®) in deionized water. We used zinc foil with a purity of 99.7% (Goodfellow^® England) and a thickness of 1 mm as starting material. A ribbon 5.5 cm long by 1 cm wide is cut from the zinc plate to form the anode. The zinc ribbon is degreased and cleaned by successive sonication in a series of baths during 5 minutes at each step: trichloroeth-ylene (Acros Organics^®), acetone (Fisher Chemical^®), ethanol (Prolabo^®) and deionized water are successively used to ensure a good surface finish prior to the anodization step. The zinc electrode is finally blown dry with dry air.

Figure 1. Schematic representation of the custom-built anodization setup.

2.3. Fabrication of ZnO Nanoparticles by Anodization of Zn Foil

The electrochemical cell is a classical electrolysis system consisting of a Pt foil as counter-electrode (cathode) and the zinc ribbon as the working electrode (anode). During the experiments, the stirring speed is kept constant (180 rpm). The zinc foil is anodized at room temperature under a constant voltage of 12 V. Anodization transforms the zinc foil into metal oxide nanoparticles. The process stops when the zinc bar is completely dissolved, thus opening the circuit. The produced nanoparticles are collected, separated from the electrolyte by centrifugation (artificial gravity acceleration: 100 g) and washed many times with deionized water to remove undesired salt incorporated during the anodization.

2.4. Pycnometer Density Method

The density of the produced powder is evaluated using the pycnometer method. The density of the xylene isomer blend (Fisher Chemical^®) is used as reference. All the adjustments are made at room temperature.

The method consists of a series of high-precision weighing operations:

m₁ is the mass of the empty pycnometer;

m₂ is the mass of the pycnometer filled with water;

m₃ is the mass of the pycnometer filled with xylene;

m₄ is the mass of the pycnometer containing 1 gram of powder;

m₅ is the mass of the pycnometer containing the powder and filled with xylene.

For this last measurement, the powder is immersed in xylene and degassed under vacuum and ultrasound for at least 10 minutes and 5 minutes, respectively. Then the adjustment with xylene is finished. As a consequence, the density expressions are the following:

$d_{\text{xylen}}=\frac{m_3-m_1}{m_2-m_1}\times d_{\text{water}}$ (1)

$d_{\text{powder}}=\frac{m_4-m_1}{\left(m_3-m_1\right)-\left(m_5-m_4\right)}\times d_{\text{xylen}}$ (2)

2.5. X-Ray Diffraction Spectroscopy

The crystal structure of the produced ZnO nanoparticles is investigated by Bruker^® D2 Phaser diffractometer using CuKα1 at 1.5406 Å with a nickel filter. The data are collected in a range that covers 2θ from 20˚ to 80˚. The dried nanopowder of ZnO is ground before analysis with an agate mortar and pestle.

2.6. Field Emission Scanning Electron Microscopy & Energy Dispersive X-Ray Spectroscopy

Observations are carried out using a field emission scanning electron microscope Regulus 8230 (Hitachi^®, Japan) at 2 kV with the secondary electron detector.

ZnO powder is deposited on stubs using adhesive carbon tabs and coated with platinum (Quorum^® Q150 TES, UK). For microanalysis using energy dispersive X-ray spectroscopy, an acceleration tension at 15 kV is used with the Ultim Max^® 170 mm² detector and AZtech^® software (Oxford, UK).

2.7. Transmission Electron Microscopy

TEM pictures are obtained using an 80 kV transmission electron microscope (Hitachi^® H-7650) with Hamamatsu^® camera AMT 40 (4k). ZnO powder was dispersed in distilled water. 40 µL of this dispersion is spread on a Nickel formvar-carbon grid.

2.8. Photocatalytic Test of the ZnO Nanoparticles

A 50 mL aqueous solution of 40 ppm Acid Orange 7 (Acros Organics^®) dye is prepared.

40 mg of dried ZnO nanopowder is suspended in the solution dye under stirring. The suspension is ultra-sonicated for 30 minutes in the dark to ensure a sufficient homogeneity of the medium as well as to reach the adsorption equilibrium. A homemade photo-reactor equipped with six 8-W low-pressure mercury vapor tubular lamps, a magnetic stirrer and a fan is used to perform the analysis. A control is made by measuring the absorbance of the dye solution in the same irradiator and over the same time as was made first. The absence of any change in absorbance after irradiation shows that radiation alone could not degrade the dye. The pH of the AO7 solution during the photocatalytic experiments is adjusted to 6. A 5 mL aliquot is taken out of the quartz reactor beaker at regular intervals, followed by centrifugation at 100 g for 5 minutes to separate ZnO nanoparticles from the AO7 solution. Each supernatant is analyzed using a Jenway^® model 7310 single-beam spectrophotometer. Permuted water ensures the blank. Spectral analyses are carried out between 300 and 700 nm wavelengths. Considering the AO7 dye, the maximum absorbance peak corresponds to a 486 nm wavelength. At the maximum absorbance wavelength, the percentage photocatalytic degradation is calculated using Equation (3).

$\text{Photodegradation }\left(\%\right)=\frac{A_0-A_t}{A_0}$ (3)

where A₀ is the initial maximum absorbance of the solution AO7, and A_t is the absorbance after a defined irradiation time t.

3. Results and Discussions

3.1. ZnO Nanoparticles Formation Process

When the DC power supply is switched on between the electrodes, instantaneously, the metallic zinc (anode) starts to be converted into metal oxide nanoparticles. The KCl electrolyte becomes instantly cloudy, and simultaneously, dihydrogen (H2) bubbles are emitted at the Pt surface (cathode). A rapid electrochemical reaction occurs, considering the high gas flow at the cathode. The anodization curve is plotted in Figure 2. This anodizing current versus time curve is totally different from the well-known curves relative to the formation of nanotubular structure via anodization in fluoride electrolyte [42]. In these as-mentioned cases, an initial sudden decrease in anodic current is the consequence of the building of an insulating anodic oxide layer: the ionic diffusion across this layer is reduced. A few times later, the anodic current starts to increase: it is the etching of the oxide layer. After this second step, etching and oxide formation reaction compete, leading to a steady state current flow [43].

Figure 2. Current-time curve of the ZnO nanoparticles formed in 0.2 M KCl electrolyte solution under 12 V anodic voltage.

In the present experiment, a constant high current of about 2.5 A is recorded all along the anodization, attesting to the continuous dissolution of the sacrificial zinc anode without the building of an insulating oxide layer, contrary to the usual nanotubular synthesis.

During the process, the atoms at the surface of zinc anode lose their peripheral electrons and become oxidized, following Equation (4):

$\text{Zn}\to {\text{Zn}}^{2+}+2{\text{e}}^{-}$ (4)

Anodic cation production is continuous during the applied voltage; the repulsive electrostatic force at the metallic surface between two positively charged particles leads to a drift in the electrolyte of the cathodic species and avoids the building of an insulating oxide layer. Simultaneously, numerous gas bubbles appear on the surface of the platinum cathode, dihydrogen is produced from the electrolysis of water following Equation (5) [44]:

$2{\text{H}}_{\text{2}}\text{O}+2{\text{e}}^{-}\to {\text{H}}_2+2{\text{OH}}^{-}$ (5)

The metal oxide nanoparticles are synthesized when metal cation reacts with the oxygen anions following the pathway described in Equations (6) and (7) [45] [46]:

${\text{Zn}}^{2+}+2{\text{OH}}^{-}\to \text{Zn}{\left(\text{OH}\right)}_2$ (6)

$\text{Zn}{\left(\text{OH}\right)}_2\to \text{ZnO}+{\text{H}}_{\text{2}}\text{O}$ (7)

After about 16 minutes of treatment, the current drops to zero, as a result of total etching of the zinc anode, opening the circuit. The total anodizing yield depends on a number of process parameters which are currently being investigated: the most important of these are the mass of the sacrificial electrode, its surface finish, the temperature and composition of the electrolyte, the surface and type of the counter-electrode, and the applied voltage. Once all these different parameters have been established, reproducible yields require a constant level of immersion in the electrolytic bath to ensure identical production performance. Full experimental data on the influence of these parameters are not yet available and will be investigated in further studies.

3.2. Pycnometer Density Measurement

The experiments based on precision weighing of the pycnometer lead to the determination of the densities. The xylene density is found to be 0.8684 and the density of the powder is measured at 5.42. This value is different from the bulk ZnO [47]. The nanoparticular nature of the sample as well as the measurement’s parameters [48] can explain the difference found with the data in the literature [49].

3.3. X-Ray Diffraction Analysis

Figure 3 shows the X-ray diffraction pattern of the synthesized nanoparticles. Sharp peaks are observed, suggesting a well-crystallized material. The peaks at 31.773˚, 34.441˚, 36.262˚, 47.559˚, 56.604˚, 62.889˚, 66.388˚, 67.968˚, 69.097˚, 72.610˚ and 76.978˚ correspond to the reflection from 100, 002, 101, 102, 210, 103, 200, 212, 201, 004, and 202 crystal planes indexed to the ZnO phase and indicate a hexagonal zincite type crystallite with the space group P63/mc. The lattice parameters of this wurtzite type unit cell structure are a = b = 3.24940 Å and c = 5.20380 Å.

The first peaks at the different scattering angle (2θ) are listed in Table 1 and compared to the JCPDS card N˚361451. The experimental data are consistent with the JCPDS data, and very small shifts of the experimental diffraction peaks are detected, less than 0.06% relative variation. The interplanar spacing dhkl for different observed (hkl) planes is calculated using Bragg’s equation:

Figure 3. Experimental XRD pattern of ZnO nanoparticles obtained by anodizing Zn foil.

$2\times d\times \sin \left(\theta \right)=n\times \lambda$ (8)

Very small differences are found between the calculated and reported values, see the calculation for the first peaks in Table 1.

Table 1. Comparison of the experimental XRD data with the JCPDS data (card N˚361451).

The crystallite size is evaluated using Scherrer’s equation:

$D=\frac{K\times \lambda }{\beta \times \cos \left(\theta \right)}$ (9)

D is the particle size (Å), λ = 1.54056 Å (CuKα radiation) k = 0.94 and β is the Full Width at half Maximum (FWHM). The crystallite size is found to be 27.7 nm.

3.4. Field Emission Scanning Electron Microscopy (FESEM) & Energy Dispersive X-Ray (EDS) Characterizations

Figure 4(a) shows the FESEM images of the produced material. The powder consists of micrometric-sized aggregates (Figure 4(a)). This agglomeration and clustering of the particles is an obvious result of their high surface energy [50]. Magnifications of the sample reveal a dual structure made up of micrometric grains on which nanometric crystallites are present, ranging in size from approximately 10 to 130 nm Figure 4(b)). These nanometric structures are designed as dots [51] [52]. These microscopic images of nanometric structures superimposed on micrometric aggregates significantly increase the specific surface area [53] [54]. The increase in specific surface area is an important parameter for catalytic applications [55] [56].

Figure 4. Field emission scanning electron microscopy images of ZnO nanoparticles obtained by anodizing Zn foil: (a) aggregate view; (b) Nanoparticles on micrometric grains.

In order to identify the element composition of the prepared sample, the EDS analysis is conducted during the FESEM observations. The results are presented in Figures 5(a)-(c).

The energy dispersive spectra from the FESEM-EDS analysis show prominent X-ray energies at 0.592 keV and 1.037 keV, respectively, representing the emissions from the K-shell of oxygen and L-shell of zinc [57] [58]. The additional X-ray energies at 8.44 keV and 9.337 keV are additional emission from Kα and Kβ core shells of zinc [59]. These results confirm that the sample prepared by the above method is indeed a ZnO phase.

(a)

(b) (c)

Figure 5. (a) Field emission scanning electron microscopy images of the localized spots of EDS analysis. (b) (c) Associated EDS spectra related to some of the spotted points of analysis.

3.5. Transmission Electron Microscopy (TEM) Characterizations

Figure 6 shows the TEM images of the synthesized sample. The images confirm the presence of a dual structure of the surface of the powder produced. It is seen that nanoparticles are superimposed on micrometric grains. ZnO nanoparticles have a particle size in the range of 20 - 80 nm, which is in line with previous analyses.

(a) (b)

Figure 6. TEM images of ZnO nanoparticles obtained by anodizing Zn foil.

3.6. Ultraviolet-Visible Analysis

The optical absorption spectrum of the synthesized ZnO nanoparticles is shown in Figure 7. The used sample is made by dispersion of a small amount of ZnO nanoparticles in deionized water. A sharp absorbance peak is found at 374 nm, which is in good agreement with published studies [60] [61] and confirms the presence of ZnO nanoparticles in the aqueous solution.

Figure 7. UV-vis absorbance spectrum of ZnO nanoparticles obtained by anodizing Zn foil.

The energy band gap is determined using the recorded UV-Visible data. Using the Tauc equation [62] [63], the plot of (αhν)2 versus photon energy (hν) is presented in Figure 8. The intersection of the tangent of the curve with the horizontal energy axis leads to an estimation of the ZnO nanoparticles’ energy band gap.

Here the empiric band gap is found to be around 3.17 eV. This value is slightly lower than the bulk ZnO (3.3 eV) reported in the literature [64] [65].

3.7. Photocatalytic Degradation of Acid Orange 7 Study Using ZnO Nanoparticles

Figure 9 shows typical UV-vis spectra obtained during UV irradiation of AO7 in the presence of ZnO nanoparticles. These spectra clearly show that the intensity of the characteristic band of AO7 at 486 nm decreases as a function of irradiation time. The ZnO nanoparticles are acting as photocatalyst in the degradation reaction of the AO7 dye. After 2 hours of irradiation, the degradation of the dye is significant as attested by Figure 10. With a low power density (48 W), a strong photoactivity is recorded.

Figure 8. Tauc plot of ZnO nanoparticles obtained by anodizing Zn foil in aqueous dispersion.

Figure 9. Absorption versus irradiation time for the acid orange 7 (AO7) for polychromatic light (350 - 400 nm) in the presence of ZnO nanoparticles obtained by anodizing Zn foil with exposure times of 0 min, 15 min, 30 min, 45 min, 60 min, 80 min and 120 min.

Figure 10. Percentage degradation of AO7 dye mixed with ZnO nanoparticles obtained by anodizing Zn foil.

Figure 11. Kinetic curve Ln(C0/C) = f(t) for the photodegradation of A07 dye using ZnO nanoparticles obtained by anodizing Zn foil as catalyst.

The kinetic behavior of the photodegradation of AO7 using ZnO nanoparticles as catalyst is studied by plotting ln(C0/C) versus t, where C0 is the initial concentration of AO7 dye and C is AO7 concentration at irradiation time “t” see Figure 11. A quasi-linear plot between ln(C0/C) and the irradiation time is obtained, which describes a pseudo-first-order kinetic reaction for the degradation of AO7 using ZnO NP as photocatalyst [66]. The slope of the line passing through origin determines the rate constant K. The K value and half-time life are found to be 0.0034 min⁻¹ and 204 min respectively. The incident light on the surface of ZnO NP creates electron-hole pairs through oxidation reduction reactions which induce radical production in the presence of dioxygen [67]. Hydroxyl and superoxide radicals are of primary importance in the photodegradation of A07 dye [68]. Oxygen vacancies in ZnO nanoparticles contribute to the photoreaction efficiency by limiting electron-hole recombination [69]-[71]. The aggregated state of the powder allows rapid natural settling, facilitating the first stage of separation from the supernatant and opening the possibility of reusability of our synthesized ZnO catalysts after an efficient cleaning procedure to desorb undesirable adsorbed by-products.

4. Conclusion

An innovative approach involving an anodization set-up is used to produce ZnO nanopowder. In this process, a large amount of ZnO is synthesized from a Zn sacrificial anode rod at room temperature without any post-treatment. X-ray diffraction reveals the hexagonal zincite-type crystallite of the sample, whereas the FESEM/EDS confirms the morphology and elemental composition of the powder. The produced material is tested to investigate its photocatalytic properties in regard to the kinetic degradation of a model organic dye AO7. Despite the highly aggregated state of the powder, a significant catalytic property is recorded. The dual morphology of nanometric structures superimposed on microscopic grains revealed by FESEM/TEM analysis contributes to an increase in specific surface area, which is conducive to the material’s photocatalytic activity. This original synthesis process stands out from others thanks to its ease of use, modularity and low cost, which will enable industrial applications by building a pilot reactor capable of producing ZnO nanoparticles rapidly and in large quantities This original synthesis process stands out from others thanks to its ease of use, modularity and low cost, which will enable industrial applications by building a pilot reactor capable of producing ZnO nanoparticles rapidly and in large quantities with minor adaptations relating to scale.

Acknowledgements

We would like to thank the Institut Universitaire de Technologie Clermont Auvergne for hosting our laboratory and the Clermont Auvergne Physics Laboratory for financing this work.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References