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Microwave-Assisted Hydrothermal Synthesis of Zn 2 SnO 4 Nanostructures for Photocatalytic Dye Degradation

: Zinc-tin oxide (ZTO) nanostructures appear as one of the most promising material systems for a new generation of nanodevices. In this work, a microwave-assisted hydrothermal synthesis to produce different shapes of Zn 2 SnO 4 nanostructures (nanoparticles, octahedrons and nanoplates) is presented. Reproducible and homogeneous results were obtained with the advantage of reducing up to 20 h the synthesis time when compared to using a conventional oven. Further-more, the photocatalytic activity of the Zn 2 SnO 4 nanostructures in the degradation of rhodamine B under UV light was studied. Zn 2 SnO 4 nanoparticles demonstrated better performance with >90% of degradation being achieved in 2.5 h.


Introduction
The zinc-tin oxide (ZTO) material system possesses a wide range of attractive properties for a new generation of multifunctional nanodevices. It can crystallize in Zn2SnO4 and ZnSnO3 phases, with different types of nanostructures possible for each phase. Each has unique properties suitable for applications in catalysis, sensors, transistors, memories, or energy harvesting devices [1][2][3][4][5].
Hydrothermal synthesis is one of the best methods to obtain complex oxides due to its characteristic pressure and temperature conditions. These processes are commonly performed using conventional ovens; nevertheless, microwave-assisted synthesis is an interesting alternative for the inorganic nanostructures' fabrication, due to its faster reactions and consequences of its heating process. This contributes to an enhancement of the product purity and yield [6][7][8][9]. Several ZnO and SnO2 nanostructures have been reported by microwave-assisted synthesis [7,[10][11][12][13][14][15], however, the advantageous multicomponent ZTO is significantly more difficult to obtain. Nevertheless, ZTO nanostructures synthesized in microwave systems have already been reported [8,[16][17][18]. A simple method to produce quantum dots using a conventional microwave was demonstrated by Lehnen et al. [8], while Nehru et al. produced Zn2SnO4 nanostructures in a microwave system by using a urea-based combustion process [16]. Recently, Jain et al. reported a microwaveassisted hydrothermal synthesis of Zn2SnO4 nanorods using a power of 600 W for 15 min, and an annealing treatment at 700 °C for 5 h [18]. In this work, a seed-layer free microwave-assisted hydrothermal synthesis of Zn2SnO4 nanostructures with different morphologies (nanoparticles, octahedrons and nanoplates) is presented. The Zn2SnO4 nanostructures were obtained in ≤60 min without employing any annealing treatment. The photocatalytic performance of these nanostructures in degradation of rhodamine B (RhB) under  UV light is also shown, with better performance being observed when using the Zn2SnO4 nanoparticles.

Materials and Methods
The Zn2SnO4 octahedrons and nanoplates were synthesized through the hydrothermal process described in reference [19]. The Zn2SnO4 nanoparticles were synthesized based on the synthesis of Annamalai et al. [20]. After its preparation, the solutions were kept in the microwave (CEM Focused Microwave Synthesis System Discover SP), in a dynamic mode at 200 °C, establishing a maximum pressure of 270 PSI and a maximum power of 100 W for the nanoparticles and 80 W for the octahedrons and the nanoplates. The nanostructures were washed several times alternately with IPA and H2O, and then dried at 60 °C for 2 h in a vacuum [21]. A PANalytical's X'Pert PRO MRD diffractometer (with Cu Ka radiation) was used to determine the crystalline structure of the Zn2SnO4 nanostructures. The data was acquired in the 10-90° 2θ range with a step size of 0.033°. The morphology of the nanostructures was carried out by a Carl Zeiss AURIGA CrossBeam (FIB-SEM) workstation. The photocatalytic performance of the ZTO nanostructures in the degradation of rhodamine B (RhB) was evaluated at room temperature. For each experiment, 40 mg of each powder containing the different nanostructures was dispersed in 50 mL of the RhB solution (5 mg/L). The RhB used is from Sigma Aldrich (Lisbon, Portugal). The photocatalysis tests were performed under UV light using 3 lamps (Osram, HNS L 95 W 2G11, Munich, Germany) with an emission wavelength range of 200-280 nm (ozone free), aligned in parallel at a distance of 10 cm from the solutions with the nanostructures.

Microwave-Assisted Synthesis of Zn2SnO4 Nanostructures
In a previous work, Zn2SnO4 octahedrons were synthesized by a hydrothermal method in a conventional oven at 200 °C for 24 h [19]. A posteriori, a shorter duration of 12 h was also studied and it was found that this is not enough time to purely produce the Zn2SnO4 phase, leading instead to the predominance of Zn2SnO4 nanoplates, which are an intermediate structure in the formation of the Zn2SnO4 octahedrons [22]. Following these results, Zn2SnO4 octahedrons and nanoplates were synthesized replacing the conventional oven with a microwave system. Figure 1 shows the XRD patterns and the SEM images for the ZTO nanostructures obtained with these microwave-assisted syntheses. The same trend observed for the oven syntheses is verified for microwave syntheses even if employing significantly shorter times, i.e., longer synthesis leads to the achievement of pure phase Zn2SnO4 octahedrons, while some mixture of phases is achieved for shorter syntheses, producing mainly Zn2SnO4 nanoplates [22]. Based on the synthesis described by Annamalai et al. [20] in a conventional oven, and aiming to reduce the synthesis time, Zn2SnO4 nanoparticles were also synthesized using the microwave system. This synthesis was performed at 200 °C, establishing a maximum power of 100 W and a maximum pressure of 270 PSI. Two synthesis durations were considered, 20 min and 60 min.
In Figure 2, the SEM images and the XRD patterns are presented, showing that in both cases small nanoparticles with a pure Zn2SnO4 phase were obtained. This shows that it is possible to reduce at least 5 h of synthesis time, compared to the synthesis reported by Annamalai et al. [20], and still produce Zn2SnO4 nanoparticles with good quality. In general, these studies showed the efficiency of the microwave heating method to produce high-quality Zn2SnO4 nanoplates, octahedrons and nanoparticles, without postprocessing annealing treatment and with the advantage of allowing for much shorter syntheses than usually needed when using conventional ovens.

Photocatalytic Activity of Zn2SnO4 Nanostructures
The photocatalytic activity of the Zn2SnO4 octahedrons, nanoplates and nanoparticles produced by microwave-assisted hydrothermal synthesis in only 60 min was studied in the degradation of RhB under UV light. Figure 3 shows the absorbance spectra variation of the RhB solution with the UV light exposure time in the presence of each ZTO nanostructure. It is possible to observe that the nanoparticles have a much higher degradation rate (0.01659 min −1 ) compared to octahedrons (0.01069 min −1 ) and the nanoplates (0.01097 min −1 ). This could be due to the higher surface area of the nanoparticles compared with the other two nanostructures, which also could explain the smaller degradation rate of the octahedrons compared with the nanoparticles and the nanoplates. The degradation rate was obtained through the pseudo-first-order reaction kinetic model, which is represented by ln C /C kt, where C0 is the initial concentration (mg·L −1 ) [18,23,24].

Conclusions
While Zn2SnO4 nanostructures such as octahedrons, nanoplates and nanoparticles present great interest for different applications, sometimes the reaction times and/or yield are not practical, especially when considering devices that require a high volume of particles. Thus, microwave-assisted synthesis was explored in this work due to its faster and more uniform heating rate. This method has shown to be able to result in different ZTO nanostructures (Zn2SnO4 octahedrons, nanoplates and nanoparticles), allowing to decrease the synthesis time up to 20 h, when comparing with the conventional oven. Furthermore, the photocatalytic activity of the ZTO nanostructures synthesized in only 60 min was studied in the degradation of rhodamine B under UV light, revealing a higher degradation rate using the nanoparticles (0.01659 min −1 ) as photocatalyst rather than using the nanoplates (0.01097 min −1 ) and the octahedrons (0.01069 min −1 ).

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.