Comparative study of Ag/ZnO nanopowders obtained by solvothermal and precipitation methods

Abstract

Beside wide range of applications, zinc oxide is recently recognized as a promising photocatalyst mainly used to degrade organic water pollutants. The great advantage of ZnO over TiO2, which is the most common photocatalyst, is the absorption in wider solar spectrum, partly in the visible region [1]. Generally, the photocatalytic properties usually depend on the crystallinity and crystallite size, particle size, morphology and pore sizes, surface area, band gap energy, availability of active sites, number and nature of trapped sites, as well as on adsorption/desorption characteristics [2]. The modification of photocatalysts by noble metals can enhance their photocatalytic activity. This paper discuss the influence of synthetic method on photocatalytic activity of obtained powders. Solvothermal and precipitation methods were chosen for the preparation of Ag/ZnO nanopowders. For the first one, the Zn(CH3COO)2·2H2O with different AgNO3 content (0, 0.75, 1.5 and 3 mol.%) was solvothermally treated at 120 °C for 18 h in the presence of poly(vinyl pyrrolidone), ethylene-glycol and NaOH. The obtained samples were rinsed out with distilled water, centrifuged and dried at 105 °C for 3h. In precipitation method, Zn(CH3COO)2·2H2O was dissolved in ethylene-glycol in the presence of chitosan soluted in acetic acid, which was followed by addition of AgNO3 in the same quantity as used for the solvothermal synthesis. The obtained suspension was heated up to 150 °C for 2 h and then calcined at 400 °C for 1 h (Fig.1). The prepared samples were characterized by XRPD, FESEM, UV-vis techniques, while photocatalytic properties were tested on Reactive Orange 16 textile azo dye and compared to the commercial ZnO (the average particle size: 20 nm). 84 Based on XRPD, the predominant phase in all samples is ZnO with the weak reflections belonging to Ag for 0.75%Ag/ZnO, which become clearly visible for samples with 1.5 and 3% of Ag. The average crystallite size was calculated using Williamson-Hall method giving 20 ± 2 and 25 ± 2 nm for powders obtained by solvothermal and precipitation method, respectively. The calculated strains were insignificant probably due to low concentration of defects. On the other hand, this is not the case for commercial ZnO with strain of 0.25(3) %. This sample consists of nanoparticles with a slightly elongated spherical shape while all obtained ZnO nanopowders contain mutually similar nanoparticles with approximately spherical shape, which however create diverse forms of agglomerates. The average particle size was around 25 nm for solvothermally synthesized samples and approximately 30 nm for samples obtained by precipitation method. Comparing to both unmodified ZnO and commercial one, all prepared Ag/ZnO samples showed a broad band at 450 nm (visible region) which causes the narrowing of band gap. The existence of this band is the confirmation that metallic Ag particles were formed, as indicated by XRPD, since they can absorb visible light and activate photocatalyst in visible spectrum. The photocatalytic measurements showed that all synthesized nanopowders have higher adsorption power and photocatalytic activity than the commercial ZnO (Fig.1). This could be attributed to slightly elongated grains of commercial ZnO and the existence of significant strain. If compare the samples obtained by two different methods, Ag modified ZnO powders from solvothermal method are more efficient than the ones from precipitation method due to the smaller particles of silver.Larger Ag particles on ZnO could cause negative effects, such as light blocked, which result in the decrease of photocatalytic activity. The unmodified ZnO powders have the same efficiency regardless of the synthetic method. Comparing the photocatalytic activity of samples obtained by same method, the Ag distribution and its particle size seems to play the major role. Summary, no matter which procedure was used, 1.5% Ag/ZnO powders have performed the highest effciency in degradation of RO16. It means that the photocatalytic activity does not depend only on Ag content but the metal distribution is important as well. In this work, it is shown that the optimized content of Ag amounts about 1.5 mol.% regardless of synthetic method and it could be recommended for further research in terms of economy. References [1] B. Simović et al., J. Serb. Chem. Soc., 2014, 79, 1433. [2] B. Simović et al., Process. Appl. Ceram., 2017, 11, 27.