GAD plasma-assisted synthesis of ZnO nanoparticles and their photocatalytic activity

All of the chemicals used, including zinc acetate Zn(CH₃COO)₂ H₂O, Congo Red (CR) (C₃₂H₂₂N₆Na₂O₆S₂) (696,663 g mol⁻¹), Brilliant Cresyl Blue (BCB) (C₁₇H₂₀ClN₃O)₂ (385.96 g mol⁻¹), and methylene bleu (MB) (C₁₆H₁₈ClN₃S) (319,852 g mol⁻¹) were purchased from Sigma-Aldrich and used exactly as specified. Demineralized water was used to prepare all of the solutions for the zinc oxide NP formation and the photodegradation studies.

The materials for the experiment were prepared with high precision. To start, 20 grams of Zn(CH₃COO)₂·2H₂O was dissolved in 1000 milliliters of demineralized water, using a magnetic stirrer, to create a zinc acetate solution. This solution was stirred for 10 min at room temperature, ensuring the mixture was well combined. The resulting solution was designed to yield approximately 6.6 grams of pure ZnO nanoparticles, which was required for the experiment. Once the zinc acetate solution was prepared, individual solutions of CR, MB, and BCB dyes were methodically formulated. Each solution was created to have a concentration of 7 ppm and was rigorously agitated for 30 min to ensure the dyes were completely dissolved.

In this work, ZnO nanoparticle synthesis utilizes a custom-designed gliding arc discharge (GAD) reactor (figure 1), drawing inspiration from various configurations reported in the literature for treating polluted liquids [23–32]. This setup operates on the principle of generating a non-thermal plasma plume at atmospheric pressure.

Zoom In Zoom Out Reset image size

The key components and their roles:

• A compressed air saturator: ensures adequate water content in the air stream entering the reactor.
• Inlet nozzle (1 mm diameter): controls the plasma gas flow and its interaction with the electrodes.
• Diverging electrodes with a 3 mm gap: Generate the electric arc and define the plasma plume volume.
• A high-voltage transformer from Aupem Sefli, with a voltage setting of 220 V/9 kV: provides the necessary voltage (9 kV) to initiate and sustain the arc.
• A plasma plume: sweeps along the electrode gap, generating active species.
• The target solution: uses zinc acetate as a source of Zn²⁺ ions. Optimization considerations: GAD performance depends on several factors, which were optimized in previous studies and maintained constant in this work [25–32]:
• Nozzle diameter: Affects plasma gas flow and species diffusion towards the target.
• Inter-electrode distance: Determines plasma plume size and stability.
• Plasma gas: Air saturated with water ensures cost-effectiveness and the generation of desired radicals.
• Gas flow rate: Set at 800 l/h for optimal plasma generation and interaction with the solution.
• Power input: Maintained at 0.9 KW based on established power estimation methods [23, 24].

In our experiment, we started by passing compressed air through a bubbler to ensure that the air became saturated with water. Then, the airflow passed through an inlet nozzle with a diameter of 1 mm between two diverging electrodes. These electrodes, with a minimum gap of 3 mm, were connected to a high-voltage. This setup generated an alternating voltage of 9 kV and a current of 100 mA. The whole setup consumed 0.9 kW of power, which was impressively efficient. When we turned on the high voltage, an electric arc formed between the electrodes. This happened because of the voltage difference between the electrodes. The arc did not stay in one place; instead, it moved away from where it started due to the flowing air. It traveled along the space between the electrodes, creating a larger area filled with a special kind of glowing air called plasma. This process happened repeatedly, with each new arc following the same steps as the previous one.

The result of this process is the creation of a non-thermal plasma under atmospheric pressure. This plasma generates various species, including ^•OH, O^•, H^•, NO, and HNOOH. In the subsequent phase of the experimental system, a flask containing zinc acetate is positioned 5 cm below the upper part of the plasma column. This arrangement is maintained for a duration of 60 min, resulting in the development of an opaque solution. This opaqueness serves as an indicator that the chemical constituents have participated in a redox mechanism, effectively converting ${{Zn}}^{2+}$ into ${{Zn}({OH})}_{2}.$

Following this treatment, the sample undergoes evaporation and drying at 100 °C for a span of 10 min to eliminate water content. Subsequently, a critical annealing process is undertaken at 600 °C, spanning a duration of 3 h. This annealing procedure serves multiple purposes, including the removal of organic components, the initiation of crystallization processes, and the production of zinc oxide nanoparticles.

Research indicates that temperatures at 600 °C play a crucial role in facilitating the complete decomposition of the precursor, resulting in the formation of well-defined ZnO crystals. Moreover, as the temperature increases, the crystalline shape transforms into a hexagonal structure [33]. The annealing process at 600 °C is instrumental in achieving optimal degradation of the organic group attached to the precursor, simultaneously fostering favorable crystallization, a conclusion corroborated by additional studies [34].

These processes were iterated ten times, with each iteration involving the utilization of 100 ml of solution. The aim was to achieve a cumulative quantity of approximately 6.6 g of pure ZnO nanoparticles.

Powder XRD data were obtained using a Proto AXRD-2 diffractometer with sample spinner and a Dectris Mythen 1 K (3.22° active length*) 1D-detector in Bragg–Brentano geometry with a Copper Line Focus x-ray tube and Ni kβ absorber. The Thermo Scientific TM Quattro SEM combines all-around imaging and analytics performance with an innovative environmental mode (ESEM) that allows materials to be examined in their natural condition. The powder samples' optical characteristics were measured using a Shimadzu 3101PC double-beam spectrophotometer. The spectrophotometer (Shimadzu model IR Iraffinity-1) measured FTIR absorption in the spectral region 400–4000 cm⁻¹. To discover the photocatalytic activity of the NPs, the Shimadzu Spectrophotometer UV-1800 was employed. METTLER TOLEDO's Simultaneous Thermal Analysis Device (TGA/DSC): TGA/DSC 3+ 1600 °C has been used to track changes in mass and thermal flow as a function of temperature. It allows users to evaluate a wide range of samples. The ability to conduct analyses in a controlled environment at temperatures ranging from 25 to 1600 °C (air, nitrogen or argon). Heater rate: 0.1 to 100 °C min⁻¹. Balance: 5 g measurement range with 1 ug creams available: aluminum and platinum creams (70 ul) and huge aluminum creams (600 and 900 ul). LUXMETER 400,000 LUX - USB used for measuring sunlight intensity.

Before the photocatalytic process, the dispersion was stirred constantly in the dark room for 30 min to allow the dye particles and the surface of the catalyst to reach a state of equilibrium between adsorption and desorption. After the dispersion was stirred for 30 min, it was exposed to UV light for 150 min. Every 15 min, a beaker was removed, and 4 ml was centrifuged to separate the nanoparticulate suspensions (NPs) of ZnO from the solution. The irradiation of a 365 nm, 30 W UV lamp was assured by placing it 15 cm above the solution at room temperature. Ten samples (25 ml) of each aqueous solution dye (CR, MB, and BCB) with initial concentrations of 7 ppm were mixed with 25 mg of prepared ZnO in an open glass (50 ml) used as a photo-reactor. The concentration of the dye was determined using a UV-visible spectrophotometer with wavelengths of 498 nm and 625 nm, respectively. The samples were taken at the prescribed time, centrifuged, and the dye concentration was measured at 498 nm and 625 nm. Using the following formula (equation (1)), the rate of degradation was determined.

$$\begin{eqnarray} &&\% \deg =100{\rm{\times }}\frac{{C}_{0}-{C}_{t}}{{C}_{0}}\end{eqnarray} \tag{ 1 }$$

Where C₀ represents the initial concentration (mg/l) and C_t represents the concentration of the dye at time t.

A gliding arc discharge is a type of ionized gas that is created by the passage of gas through an electric current. The gas is typically air, and it is often humidified to increase its conductivity. The arc moves along the surface of the electrodes, and it produces a variety of chemical species, including radicals, ions, and molecules. The chemical species that are produced in a gliding arc discharge depend on the gas composition, the humidity, and the operating parameters. In humid air (N₂, CO₂ , O₂, H₂O), the primary chemical species are ^•OH, O^•, H^•, ^•NO, O₃, ${{CO}}_{3}^{2-},$ and CH₃COOH [35–37]. These species are formed as a result of electron impact, dissociation, addition, transformation, collision events that occur in accordance with the gas vector, and chemical reactions.

The ^•OH and ^•NO radicals are of particular interest because they are very reactive. They can disperse in the treated target, and inducing both acidifying and oxidizing effects in aqueous solutions, react with other molecules to form new compounds, or they can initiate chemical reactions. This makes gliding arc discharge a promising technology for a variety of applications, such as water treatment, waste treatment, and material synthesis.

As a result of electron impact within the arc, the primary formation of ^•NO and ^•OH radicals is explained through (equations (2–4)) [37].

$$\begin{eqnarray}&&{H}_{2}O+{e}^{{-}}\to H{\rm{\bullet }}+{\rm{\bullet }}{OH}+{e}^{{-}}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{O}_{2}+{e}^{-}\to 2O{\rm{\bullet }}+{e}^{-}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{N}_{2}+O{\rm{\bullet }}\to {\rm{\bullet }}{NO}+N{\rm{\bullet }}\end{eqnarray} \tag{ 4 }$$

These species are capable of diffusing in aqueous solutions and interacting with molecules. The hydroxyl radical is the principal species responsible for the production of metal oxide nanoparticles [38], whose standard potential is E°(^•OH/H₂O) = 2.85 V N⁻¹H⁻¹E [39]. The hydroxyl radical ^•OH is a very reactive species that can react with other molecules. It can also react with an electron received from ions and molecules dissolved in water as impurities, as shown in (equation (5)). This reaction produces a hydroxide ion (OH-) [40].

$$\begin{eqnarray}&&{\rm{\bullet }}{OH}+{e}^{-}\to {{OH}}^{-}\end{eqnarray} \tag{ 5 }$$

The dissolution of zinc acetate in water is expressed by the following, (equation (6)) and (equation (7)) [41].

$$\begin{eqnarray}&&{{\rm{Zn}}({{\rm{CH}}}_{3}{\rm{COO}})}_{2}\to {{Zn}}^{2+}+2({{{\rm{CH}}}_{3}{\rm{COO}}}^{-})\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{{{\rm{CH}}}_{3}{\rm{COO}}}^{-}+{H}^{+}\to {{\rm{CH}}}_{3}{\rm{COOH}}\end{eqnarray} \tag{ 7 }$$

The zinc ion reacts with hydroxide ion and then zinc hydroxide is produced as (equation (8))

$$\begin{eqnarray}&&{{Zn}}^{2+}+2{\rm{O}}{H}^{-}+2{e}^{-}\to {{\rm{Zn}}\left({\rm{OH}}\right)}_{2}\end{eqnarray} \tag{ 8 }$$

Zinc oxide tends to coagulate and deposit in water, so we perform the vaporization process as follows: (equation (9))

$$\begin{eqnarray}&&{{\rm{drying\; Zn}}\left({\rm{OH}}\right)}_{2}\,{\rm{at}}\,100{\rm{^\circ }}{\rm{C}}\to {\rm{ZnO}}({\rm{s}})+{H}_{2}O\end{eqnarray} \tag{ 9 }$$

The reactions mentioned from (equations (2–9)) demonstrate the fundamental process of ZnO NPs creation by gliding arc discharge with the air as vector gas.

3.2.1. UV-visible

ZnO is a transition oxide with a hexagonal structure and a direct bandgap of 3.28 eV for n-type semiconductor materials. For example, the direct band gap of ZnO NPs is between 3.10 and 3.37 eV. Many parameters, including crystallinity, grain size, particle size, particle shape, and dislocation, affect the band gap of ZnO NPs. The UV–vis spectrum in figure 2 shows the absorption of ZnO nanoparticles produced using the GAD reactor. The high absorption peak at 362 nm is due to the surface plasmon resonance of ZnO NPs [42, 43].

Zoom In Zoom Out Reset image size

The following equation (equation (10)) calculates the nanoparticles' direct transition optical band gap energy [44]:

$$\begin{eqnarray}&&{({\alpha }{\rm{h}}\upsilon )}^{2}={\rm{A}}({\rm{h}}{\rm{\nu }}-{{\rm{E}}}_{{\rm{g}}})\end{eqnarray} \tag{ 10 }$$

where:

α is the absorption coefficient,

E_g is the optical band gap of the NPs,

hν is the photon energy,

A is a constant independent on hν.

As a result, Tauc's plot is used in order to compute the optical band gap. This is accomplished by plotting (hv)² versus (hv) and then extrapolating the linear section in order to determine the energy axis intercept figure 3(a). Synthesized ZnO NPs have an optical band gap of 3.28 eV, compared to bulk ZnO. According to the literature review [45], the band gap varied according to several parameters, such as the ZnO crystal defect, particle shape, and particle size.

Zoom In Zoom Out Reset image size

The optical, electrical, and catalytic properties of ZnO nanoparticles are influenced by the anisotropy of their band gap. The band gap refers to the energy difference between the valence and conduction bands of electrons. The variation in the ZnO nanoparticle band gap directly affects electrical conductivity, with ZnO having a wide band gap. By reducing the band gap, electrical conductivity can be improved, which is crucial for applications such as transparent conductive films, solar cells, and electronics. Furthermore, the band gap difference plays a significant role in determining ZnO's redox and catalytic properties, as the surfaces of ZnO nanoparticles catalyze various chemical reactions. The presence of oxygen vacancies enhances catalytic activity. Overall, ZnO nanoparticles possess the ability to clean, convert energy, and degrade pollutants due to their unique band gap properties.

Band tailing in the band gap commonly results from semiconductor impurity incorporation. Since optical transitions take place from occupied states in the valence band tail to unoccupied ones at the conduction band edge, the absorption coefficient α at the band edge is exponentially dependent on the photon energy. The empirical Urbach (equation (11)) describes local defects' band tail energy (E₀₀) [46]:

$$\begin{eqnarray}&&{\alpha }={{\alpha }}_{0}\left({\rm{h}}{\rm{\nu }}/{{\rm{E}}}_{00}\right)\end{eqnarray} \tag{ 11 }$$

Where α_o is a constant and E₀₀ is the gap area tail width of localized states. To calculate α_o and Ε₀₀, plot the logarithmic scale of the absorption coefficient as a function of photon energy hν. The slope of log (α) versus incident photon energy hν determines the Urbach tail [47]. From the inverse slope of the linear plot of ln(α) versus hν in figure 3(b), ZnO NPs' Urbach energy is 0.430 eV. Possible explanations for this result include structural and thermal disturbances.

3.2.2. FTIR spectroscopy

In order to identify the functional groups for ZnO NPs, FTIR analysis is used. The spectra of the treated zinc acetate solution by non-thermal plasma (GAD) before and after annealing at 600 °C for 3 h are shown in figure 4. The wavenumbers of the peaks that appeared in the FTIR and their functional origins are shown in table 2. As a result of plasma treatment of zinc acetate solution, FTIR spectra before annealing treatment revealed a number of absorption bands (697, 954, 1037, 1045, 1386, 1451, and 1542 cm⁻¹) corresponding to the functional groups of the organic molecules [48]. The broad and strong bands at 1451, and 1542 cm⁻¹ attributed to nitrogen-bonded (O=N, and C=N) respectively. The bands at 1037 and 1045 cm⁻¹ can be attributed to the (C=O) stretching and C-O-C stretching asymmetric vibrations and the (C-N) stretching vibrations of the aromatic amine.

Zoom In Zoom Out Reset image size

Table 2. IR peaks and their assignments for the NPs of ZnO system.

Assignments	Wavenumber (cm−1)	Wave number (cm−1)
	Before annealing	After annealing
Zn-O Stretching	< 600	< 600
N-O Stretching	697	Absent
C-N	954	Absent
C=O	1037	Absent
C-O-C/C-N	1045	Absent
O-H	1386	Absent
O=N	1451	Absent
C=N	1542	Absent
O≡N	2010	2009
C≡N	2158	2156

After annealing, the FITR spectrum of the ZnO NPs shows a sharp peak at 448 cm⁻¹ associated with Zn-O vibration and a broad peak at 3430 cm⁻¹ related to the O-H bonding due to moisture absorption.

In wurtzite hexagonal type ZnO crystals, the Zn-O stretching vibration of the oxygen sublattice (${E}_{2}H$) is responsible for the 426 and 565 cm⁻¹ absorption bands, while the 1386 cm⁻¹ oxygen vacancies are characterized as hydrogen-related defects on the surface of ZnO [49].

3.2.3. X-ray diffraction

An x-ray diffractometer was used to study the structure and elements of ZnO nanoparticles, and the results are displayed in figure 5. As can be observed, there are many peaks that correspond to the planes (100), (002), (101), (102), (110), and (103) for ZnO that is hexagonal in shape. The treated sample was made up of tiny particles, according to the XRD data.

Zoom In Zoom Out Reset image size

Figure 5 displays the XRD pattern of the ZnO NPs sample after preparation. The ZnO single crystal phase is verified by the diffraction pattern. The crystallographic plane (101) exhibits a hexagonal crystal structure, and all the diffraction peaks observed in the patterns (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) indicate a polycrystalline nature. These values closely correspond to those documented in the standard card (JCPDS number: 01-089-0551) [50].

Table 3 shows the ZnO NPs' crystallite size of 27.18 nm, calculated using Scherrer's formula (equation (12)) [51].

$$\begin{eqnarray}&&{\rm{D}}=\frac{k\lambda }{\beta \cos (\theta )}\end{eqnarray} \tag{ 12 }$$

The wavelength for a hexagonal crystal structure is λ =1.54 Å, the full-width at half-maximum (FWHM) is β, k = 0.91, and θ is the measured angle at the peak (101).

Table 3. Estimation of structural parameters of ZnO NPs.

Sample name	2θ (deg)	FWHM (deg)	D (nm)	Lattice parameter ($\hat{{\bf{A}}}$)		ε × 10–4	σ(GPa)	ξ × 1014 (line/m2)
ZnO pure	36.318	0.2952	27.18	a = b	c	1.921	−0.861	1.35
				3.2494	5.2051

In addition, the peak location deviation from the reference data of bulk ZnO reveals the presence of stress in the NPs. These equations (equations (13 and14)) were used to calculate an approximation of the stress level:

$$\begin{eqnarray}&&{\sigma }=\left[-\frac{{{\rm{C}}}_{33}\left({{\rm{C}}}_{11}+{{\rm{C}}}_{12}\right)}{{{\rm{C}}}_{13}}+2{{\rm{C}}}_{13}\right]{\varepsilon }\end{eqnarray} \tag{ 13 }$$

$$\begin{eqnarray}&&{\varepsilon }=\frac{c-{c}_{0}}{{c}_{0}}\end{eqnarray} \tag{ 14 }$$

Where, σ is strain, C₃₃(210 GPa) , C₁₁(210 GPa), C₁₂(120 GPa), and C₁₃ (105 GPa) are the elastic stiffness constants, ε is the uniform strain, ZnO NPs have a lattice parameter denoted by the letter c, which may be determined from the plane labeled (101), and the lattice parameter for the bulk material is denoted by c₀ [52]. Table 3 provides approximated values for 2θ, lattice parameter, FWHM, crystallite size, stress, strain, and dislocation. The negative signs of the stress suggest the existence of compressive stress [53]. This compressive stress is usually caused by distortions and defects in the lattice.

3.2.4. Scanning electron microscopy (SEM) and EDAX

Scanning electron microscopy (SEM) was employed to investigate the formation, size, shape, and morphology of the prepared sample's particles. Figures 6(a), and (b) show low-magnification SEM images revealing the typical microstructure of ZnO NPs, which is mainly of different sizes and agglomerated grains of nanopowder which consist of pseudo-spheres and non-uniform hexagonal particles [54], and show particle aggregation related to the self-assembly of the ZnO nanoparticles [55]. Figure 6(c) depicts the ZnO NPs size distribution histogram; the averaged grain size is 52 ± 1.26 nm, which is larger than the measurements reported by XRD analyses. While XRD solely reveals data on the crystalline components and not the amorphous ones, FESEM analysis delivers insights into the overall shape of the grains [54]. The energy-dispersive analysis x-ray spectra (EDAX) reveal the chemical composition and atomic percentage of nanoparticles (NPs). Figure 6(d) shows the EDAX spectra obtained for a sample of ZnO NPs. The elements identified are zinc and oxygen; however, the peak of carbon is not considered as it is absorbed by the superficial area of the ZnO NPs.

Zoom In Zoom Out Reset image size

3.2.5. Thermogravimetric analysis TGA / differential scanning calorimetry (DSC)

The DSC/TGA curve in the (figure 7) shows the product's behavior when subjected to a 10 °C/min heating rate from 25 °C to 700 °C. The TGA profile indicates a weight reduction interrupted by three distinct shifts. Meanwhile, the DSC curve shows three endothermic peaks, which correspond to different stages of decomposition: the release of physically adsorbed water in the initial stage and the release of chemically adsorbed water and carbonic matter in the second and third stages, respectively [12, 56, 57]. These findings are consistent with the peaks observed in the FTIR analysis of these groups. It is showing negligible mass loss of about 0.42% as a function of temperature, indicating thermal stability between 25 °C and 700 °C. Hence, we can conclude that the sample is pure.

Zoom In Zoom Out Reset image size

The effectiveness of synthesized ZnO NPs in removing organic pollutants from water was investigated using the cationic and anionic dyes BCB, MB, and CR (figure 8). The photo-degradation of these dyes was observed under UV light, and the results were analyzed in terms of contact time and the presence of ZnO NPs.

Zoom In Zoom Out Reset image size

The findings of the study demonstrate that after 2 h and 30 min of exposure to UV light, the rates of degradation were 92.9%, 71%, and 88% for BCB, MB and CR dyes, respectively. It was also observed that the photo-degradation efficiency of BCB was higher than that of MB and CR. The kinetics of the photodegradation reaction of the dyes were determined using the pseudo-first order equation (equation (15)) [58]:

$$\begin{eqnarray}&&\mathrm{ln}\left(\frac{{C}_{0}}{{C}_{t}}\right)={kt}\end{eqnarray} \tag{ 15 }$$

Where C_t and C₀ are the concentrations of dye at time t = 0 min and t, respectively, and k is the pseudo-first-order rate constant.

Figure 9 shows the relationship between ln (${C}_{0}$/${C}_{t}$) and the irradiation period for BCB, MB, and CR dyes. As seen in figure 9, a linear relationship was found between ln (${C}_{0}$/${C}_{t}$) and time, confirming that the photocatalytic degradation followed the first order mechanism. The value of the degradation rate constant (k) was estimated to be 0.0190 min⁻¹, 0.0081 min⁻¹, and 0.0086 min⁻¹ for BCB, MB and CR, respectively, based on the slope of the linear equation utilizing the above equation.

Zoom In Zoom Out Reset image size

Table 4 presents a comparison of the photocatalytic activities of various nanoparticles in several previous studies and ZnO synthesized by GAD plasma, for the degradation of BCB, MB, and CR. The results of this comparison indicate that ZnO nanoparticles produced through Gliding Arc Discharge plasma exhibited higher photocatalytic activity compared to other synthesis methods [13, 59–80].

The findings of this research could suggest that the method of synthesizing ZnO NPs can significantly affect their photocatalytic activity, which in turn affects their efficiency in degrading organic pollutants in water. Therefore, the use of ZnO NPs generated by the plasma GAD of air may be a more effective approach for water treatment applications.