Investigation of Laser-Based Mo-Doped ZnO Nanoparticle Production and Photocatalysis Application ^†

Abstract

One of the most sensible, economical, and ecologically friendly methods for treating wastewater is photocatalytic treatment. The most widely used and easily accessible photocatalyst for wastewater treatment is zinc oxide (ZnO). This study used laser ablation to create ZnO, Mo, and Mo-doped ZnO photocatalysts. The nanoparticles were then characterized using linear absorbance, X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The degradation of methylene blue under UV-Vis spectroscopy was used to evaluate the photocatalytic activity of the photocatalysts and the reaction’s kinetics. The Mo doping of ZnO enhanced photocatalytic degradation efficiency, according to the analytical data. This study’s 90 min photocatalytic degradation experiments showed about 94.11% methylene blue degradation efficiency. Mo-doped ZnO nanoparticle photocatalysts have a promising future for treating wastewater, according to this study, which calls for more research in this area.

1. Introduction

The high adverse effects of the industry have caused the pollution of our world and waters. Scientists reveal ways with which to attain a clean world and clean water by solving this negative situation. Nowadays, the process of cleaning industrial wastes from water with fast and simple techniques without creating chemical waste, such as with the green synthesis method in material technology, is a vital issue [1,2].

It is known that zinc oxide (ZnO), which is mostly used in semiconductor material technology, has a very good degradation effect in photocatalysis studies as well as in diode, solar cell, and optoelectronic application areas. Studies have shown that the photocatalytic effect increases with the addition of different metals to ZnO [3]. Among these, studies on ZnO doped with Molybdenum (Mo) are noteworthy. Given the variety of special qualities that the doping procedure imparts, molybdenum-doped zinc oxide (Mo-doped ZnO) nanoparticles have drawn attention for their photocatalytic uses [4]. The modification of ZnO’s electrical structure, which facilitates visible light absorption and improves charge carrier dynamics, is the main way that Mo increases photocatalytic activity. Under different light circumstances, this doping can result in increased photocatalytic efficiency by introducing new energy levels within ZnO’s bandgap [5].

In impacting ZnO’s basic hexagonal structure, the addition of molybdenum modifies its crystalline structure. Mo-doped ZnO nanoparticles maintain the characteristic diffraction peaks related to zinc oxide, as shown by Xu et al., confirming that phase transition is not a function of the doping [6]. This is essential for preserving the structural stability required for efficient photocatalysis. Additionally, the analysis of lattice characteristics shows that Mo can cause lattice dislocations, which raise the number of photocatalytic active sites and improve the overall degradation efficiency of contaminants like organic dye [7]. Mo-doped ZnO nanoparticles’ improved light absorption and decreased electron-hole recombination rates have been linked to their photocatalytic efficacy. According to studies, this doping arrangement enhances the charge carrier density and visible light absorption, which leads to improved interfacial charge transfer kinetics [8]. These materials are very useful in water treatment applications because they produce reactive oxygen species (ROS), which are essential for the breakdown of organic molecules in the presence of light [9].

Furthermore, a variety of synthesis methods have been used to guarantee the ideal doping concentrations and particle shapes for efficient photocatalysis. To maximize photodegradation processes, for example, a uniform distribution of molybdenum within the ZnO matrix has been achieved using sol–gel techniques. Interestingly, Mo bonding with ZnO has a synergistic impact that stabilizes the nanoparticles and increases their surface area, which allows for more interaction with the target pollutants during photocatalytic activities [1].

In summary, within the scope of this study, ZnO and Mo targets were used separately in a quartz container, and at the same time, ZnO targets were used with Mo doping in three different forms using a nanosecond Nd:YAG laser at a 1064 nm wavelength and with a 10 cm focal length lens to obtain nanoparticles. The obtained ZnO, Mo, and Mo-doped ZnO nanoparticles were analyzed by considering both their crystal structures, vibration states, and electron microscope images. All nanoparticles were obtained under the same conditions in ultrapure water. Photocatalysis studies were carried out by carrying out the methylene blue degradation process. The obtained data are presented in detail.

2. Materials and Methods

Nanosecond Laser AblationBased ZnO, Mo and Mo-Doped ZnO Nps Productions and Their Photocatalytic Degradation in Methylene Blue

Nd:YAG lasers (Continuum Minilite laser, New York, NY, USA) operating at 1064 nm were used to prepare both samples. Both times, metal targets with spot sizes of 100 µm were the focus of the laser beam. The target, a ZnO metal plate, was positioned within a quartz cuvette, which was 10 mL of pure deionized water without any surfactants [10].

The degradation efficiency of methylene blue in the presence of Mo-doped ZnO was examined under visible light to ascertain the effect of the Mo-doped ZnO on the photocatalytic capabilities of the photocatalysts, and the mechanism is shown in Figure 1.

When a light source is bright enough, VB electrons move to the CB, creating a hole and electron on the photocatalyst’s surface. Free charge carriers are created at a photocatalyst’s interface during a redox reaction that produces hydroxyl radicals. In addition to their strong reactivity, free radicals can break down contaminants by destroying organic bonds and dye molecules’ aromatic rings. The inclusion of additional free radicals enhanced the photocatalytic performance. The following is the suggested mechanism for the synthesized material’s photocatalytic activity shown in Figure 1a. The photocatalytic effect of Mo-doped ZnO is seen in the 90 min experiment obtained with methylene blue degradation, as shown in Figure 1b.

3. Results

3.1. Linear Absorbance of Mo Nps, ZnO Nps, and Mo-Doped ZnO Nps

The nanosecond laser ablation method is accepted as a clean nanoparticle production technique compared to chemical synthesis systems. The speed and economy of production techniques in ultrapure water are seen as important features, especially for revealing the photocatalyst effect. With this motivation, when Uv-Vis absorbance parameters for nanoparticles produced using ZnO and Mo targets were obtained between 400 nm and 800 nm, as indicated in Figure 2, it was observed that the absorbance peak intensity increased, especially between 400 nm and 500 nm. At the same time, it was observed that the absorption peak increased between 550 nm and 700 nm for nanoparticles produced as Mo-doped ZnO under the same conditions.

3.2. XRD Results of ZnO Nanoparticles

Using XRD analysis, the structural properties of ZnO-NPs are ascertained. The XRD spectrum is shown in Figure 3.

The crystal lattice of the obtained ZnO nanoparticles is obtained as (1 0 0), (0 0 2), (1 0 1), and (1 1 0), respectively. This characteristic lattice structure was found to be compatible with that in the literature [11,12].

3.3. FTIR Results of Mo-Doped ZnO Nanoparticles

As shown in Figure 4, FTIR was used to analyze Mo-doped ZnO to determine their chemical structure and identify functional groups. The stretching vibrations of ZnO, which were shifted to the lower frequency in Mo-doped ZnO, are identified by the absorption peaks seen at 1008 cm⁻¹.

The presence of Mo ions on the crystallite structure of the nanoparticle can be shown by the decreasing peak intensity at 1261 cm⁻¹, 1390 cm⁻¹, and 1460 cm⁻¹ in the Mo-doped ZnO sample. The stretching vibration of the O-H group was linked to the broad peak seen at 3438 cm⁻¹.

3.4. TEM, Hr-TEM, and SEM Images of ZnO and Mo Nanoparticles Obtained with the Laser Ablation Process

Figure 5 shows transmission electron microscopy (TEM) and high-resolution TEM scanning electron microscopy images for ZnO and Mo nanoparticles.

When the distribution of ZnO and Mo nanoparticles is examined, it is observed that they are homogeneous in terms of morphology and particle size in Figure 5a. Using the HR-TEM image of the ZnO Nps lattice, the distance was measured at 0.21 nm. Mo Nps elemental mapping analysis results exhibit clear distributions on the surface, as shown in Figure 5b.

3.5. Photocatalytic Degradation of Mo Nps, ZnO Nps, and Mo-Doped ZnO Nps

In the presence of the obtained ZnO, Mo, and Mo-doped ZnO Nps photocatalysts, the degradation process of methylene blue was examined. In Figure 6, the absorbance performances show the absorption curve changes obtained in 90 min for Mo-doped ZnO (Figure 6a), ZnO (Figure 6b), and Mo (Figure 6c) nanoparticles obtained using the nanosecond laser ablation method. In particular, studies in the literature have shown that Mo doping increases the degradation effect of ZnO nanoparticles, and this is explained as Mo increasing the hydroxyl radicals and strengthening the hydrogen formation mechanism [8,11,13].

The experiments were performed under visible light using methylene blue (10 mgL⁻¹) dye solution. During the photocatalysis process, all samples for pH:10 were interpreted by taking absorption spectra under visible light. In photocatalysis experiments, a 250 W metal halide lamp (GE ARC250/T/H/960E40, CN) that mimics daylight was used. To investigate the photocatalytic performance of Nps under visible lighting at room temperature, photocatalysts (5 mg), weighed in a certain amount of the methylene blue dye of our choice, were added and mixed for 30 min for adsorption–desorption balance in the experiments. The distance from the light source to the liquid surface was set to 15 cm. At specific illumination ranges, the methylene blue concentration was analyzed with an absorbance spectrophotometer. The following equation was used to evaluate the degradation kinetics of methylene blue:

ln (C_t/C₀) = −kt

(1)

Here, k, t, C₀, and C_t are the kinetic constant, the time, and the initial equilibrium concentration of MB, and the given “t” is the concentration of methylene blue in minutes. The photocatalytic degradation efficiency percentage and decay kinetic values mentioned above were examined by considering the degradation mechanism shown by all of the obtained nanoparticles in methylene blue, and these values are presented in detail in Figure 7, and the results are given in

Table 1. ZnO, Mo, and Mo-doped ZnO Nps for pH:10 kinetic constants and degradation efficiency values.

Photocatalyst	Kinetic Constants, k (min−1)	Photocatalytic Degradation Efficiency (% in 90 min)
Mo Nps	0.008	83.125
ZnO Nps	0.013	93.125
Mo doped ZnO Nps	0.014	94.110

.

Figure 7a exhibits the concentration (C_t/C₀) changes, and Figure 7b shows the negatively logarithmic concentration changes (−ln(C_t/C₀)) of methylene blue in 90 min. The Mo doping of ZnO, depending on catalysis time, indicates that the degradation rate of methylene blue also tends to decrease. The pseudo-first-order fitting of photocatalytic dye degradation is shown in Figure 7c.

The effect of ZnO and Mo nanoparticles on methylene blue degradation is known from the literature [14,15]. In this study, when Mo nanoparticles were added to ZnO nanoparticles by laser ablation [8], the degradation effect was more effective for methylene blue in 90 min.

4. Conclusions

The goal of the current work was to analyze the photocatalytic degradation of methylene blue in an aquatic environment using ZnO Nps, Mo Nps, and Mo-doped ZnO Nps. The findings show that the Mo-doped ZnO nanoparticle photocatalytic degradation efficiency was 94.110 % in 90 min in methylene blue. Additionally, Mo-doped ZnO Nps demonstrated excellent stability and could be used repeatedly with high efficiency. The synthesis and characterization of Mo-doped ZnO nanoparticles using the laser ablation method exhibit promising potential for hydrogen generation and methylene blue dye degradation applications. The eco-friendly Mo-doped ZnO nanoparticle photocatalysts have great potential as affordable energy generation and wastewater treatment solutions Their favorable characteristics make them materials with a wide range of uses in materials science, energy production, healthcare, and environmental remediation.