Photocatalytic activity of ZnO-Zn nanoparticles after nanosilver covering and annealing

The research evaluated the photocatalytic activity (PCA) of zinc oxide nanoparticles (NPs) produced by pulsed electron beam evaporation (PEBE). The ability of NPs to accelerate the photodegradation of methyl violet dye under UV exposure was considered. The paper presents an analysis of changes in catalytic activity after annealing and coating with nanosilver of the studied samples. It has been shown that as the annealing temperature increases, the PCA increases, especially for doped particles. The highest catalytic ability was shown by doped NPs ZnO-Zn+Ag annealed at a temperature of 400 °C. This sample allowed to increase the photodegradation of the organic dye by more than 6.5 times compared to the control.


Introduction
Photocatalysis is the process of initiating redox reactions when exposed to electromagnetic radiation of the optical spectrum on a semiconductor catalyst.Environmentally friendly technologies, based on photocatalysis, are widely used in water and air purification systems [1,2].At the same time, these technologies are constantly being improved.The search for the most effective photocatalysts is conducted.In this regard, nanoparticles (NPs) have several advantages.The high specific surface area of NPs provides a greater number of reactions per unit mass [3].In addition, the presence of structural and surface defects, as well as accelerated diffusion of free charge carriers to the particle surface, makes NPs the most promising material [3].
In studies of the photocatalytic activity (PCA) of nanomaterials, TiO2, SiO2, ZnO, CeO2 NPs are most often mentioned [1][2][3][4][5][6][7].Among them, zinc oxide NPs ZnO can be highlighted for non-toxicity, low cost, biocompatibility, chemical, thermal and photostability, high UV absorption [7].At the same time, it is possible to control the characteristics of NPs using annealing, doping and other types of treatment.Thus, it is possible to achieve the highest efficiency of the studied material.
In this work was evaluated the photocatalytic activity of zinc oxide NPs ZnO-Zn after annealing and coating with nanosilver.

Nanopartiles' characterization
Initial NPs were produced by pulsed electron beam evaporation (PEBE) method at the NANOBIM-2 installation and were described previously [8].NPs morphologically consist of small particles, which are connected into mesoporous agglomerates with a fractal structure.Particles often have a crystalline core and an amorphous shell.They are inherent in high specific surface area and various defects.This in turn inevitably leads to the desired high concentration of surface active centers per unit mass.

Nanosilver covering
Nanosilver covering of zinc oxide NPs ZnO-Zn+Ag was performed by radiochemical method, described previously [9].A suspension of nanosilver based on a sorbitol solution with the silicon oxide NPs was irradiated in Petri dishes using a nanosecond electron accelerator URT-0.5M[10].The irradiated suspensions were held for 96 hours.Then the solutions were drained, and the resulting powders were washed with distilled water three times and dried.

Annealing
Annealing of ZnO-Zn ZnO-Zn+Ag NPs was carried out in electrocorundum crucibles at temperatures of 200 and 400 °C.Annealed samples will be further designated S0, S200 and S400 depending on the annealing temperature.The isothermal exposure time was 10 minutes, the samples were cooled in the furnace to temperatures of 100-150 °C [11].

Photocatalysis experiment
For the research NPs were prepared by analogy with work [12].Samples of aqueous suspensions of NPs under investigation were made at a concentration of 300 mcg/ml and treated 30 min with ultrasound.After that the methyl violet (MV) dye, specification 6-09-945-86, was added to the samples, and acted as an imitator of the pollutant.The dye concentration was 10 mcg/ml.The control sample did not contain NPs.
All samples then were placed in quartz cuvettes with a volume of 3 ml and irradiated.To study the photocatalytic activity (PCA), a mercury lamp with arc discharge DRT-400, specification 16-90 IFMR.675610.002, was used as a radiation source.The power rate of the lamp is 400 W. Irradiation time was 30 min.After every 5 min of irradiation, the absorbance spectra were measured in the wavelength range of 500-700 nm using the SF-2000 spectrophotometer.The peak of the absorbance was observed at a wavelength of 584 nm.
Additionally, the stability of the studied suspensions was evaluated in a similar way.

Results and discussion
The decomposition of an organic dye over time of radiation exposure can be described by the linear equation y=kx+b.The coefficient k describes the rate of discoloration: the greater it is, the more efficiently the catalyst works.To determine the coefficient k and compare the results was carried out a linear approximation of the data obtained for the samples.
Figure 1 shows the dependence of the relative optical density on the UV exposure time for all the studied samples.
Analyzing the obtained results, we can see that an increase in the annealing temperature leads to the PCA enhancement of individual initial and doped samples.In the similar study [13], the change in the photoregradation of Rhodamine B is considered when using NPs ZnO produced by the chemical method.The authors [13] suggest that the low photodegradation rate in samples treated at low temperatures is due to the recovery of the particle surface by carboxylate groups, which reduce the availability of active centers.As the annealing temperature increases, the surface of the particles becomes cleaned, and the dye degradation rate increases.In our case, the difference in the activity of the ZnO S200 and S400 samples is insignificant.At the same time, the PCA of the ZnO-Zn+Ag S400 sample is much greater than for the ZnO-Zn+Ag S200.It can be assumed that after the annealing temperature increases above 400 °C, the surface of the studied particles is sufficiently cleaned so that the effect of surface plasmon resonance, observed in this heteropair [14], has a significant effect on the photocatalytic abilities of the ZnO-Zn+Ag sample.

Control
ZnO S0 ZnO S200 ZnO S400 ZnO+Ag S0 ZnO+Ag S200 ZnO+Ag S400 C/C 0 UV exposure time, min To quantify the photocatalytic images, we compare the coefficients k of the linear approximation.The obtained data are presented in Table 1.The results obtained show that the most effective photocatalyst was the sample ZnO+Ag S400.In its case, the photodegradation rate was increased by 6.77 times compared to the control.
Figure 2 shows an assessment of the stability of suspensions of the studied NPs.Samples of NPs ZnO-Zn suspensions showed relatively low stability for 30 minutes of exposure.The lowest is for the sample S0 and equal to 60%.Such a result may indicate the presence of microcrystalline agglomerations in these samples.This problem may negatively affect the efficiency of the nanomaterial.Along with the search for promising catalysts, it is necessary to provide all the necessary conditions for their most effective use.Nevertheless, the samples of ZnO-Zn NPs coated with nanosilver practically did not lose stability throughout the experiment.

Conclusion
Thus, the investigated zinc oxide NPs ZnO-Zn produced by the PEBE method were used for photodegradation of methyl violet dye under the influence of UV radiation.The effect of annealing (up to 400 °C), as well as the creation of a silver nanocoating on the activity was analyzed.
The results showed an increase in the rate of photodegradation with an increase in the annealing temperature.Nevertheless, in work [15], it was shown that the highest NPs of zinc oxide can be achieved at a temperature of 600 °C.For further research, it is necessary to expand the temperature range, as well as annealing in various environment to find the most effective processing conditions.
Samples of zinc oxide NPs coated with nanosilver ZnO-Zn+Ag S0 and S200 did not show an increase in PCA compared to the initial ones.However, the ZnO-Zn+Ag S400 sample showed the greatest ability to increase the photodegradation rate.In addition, the doped NPs showed significant sedimentation stability.

Figure 1 .
Figure 1.Relative optical density of ZnO-Zn NPs samples depending on the time of UV exposure.

Table 1 .
k coefficient values of examined samples relative to control.