###### Abstract  
High-intensity ultrasonic waves have great potential for the green synthesis of various nanomaterials under mild conditions and offer an excellent alternative for hazardous chemical methods. Herein a facile approach for the eco-friendly synthesis of Au-ZrO2 nanocatalyst with a high catalytic activity using a facile ultrasonic method is presented. Gold (Au) in the nanosize regime was successfully deposited on the surface of solvothermally synthesized monodispersed ZrO2 nanoparticles (ZrO2 NPs) in a very short period of time (5 min) at room temperature. Spherical shape small size Au nanoparticles that are uniformly dispersed on the surface of ZrO2 nanoparticles were obtained. Notably, in the absence of ZrO2 nanoparticles, HAuCl4 could not be reduced, indicating that nano-sized ZrO2 not only acted as support but also helped to reduce the gold precursor at the surface. The as-prepared Au-ZrO2 nanocatalyst was characterized by various techniques. The Au-ZrO2 nanocatalyst served as a highly efficient reducing catalyst for the reduction of 4-nitrophen. The reaction time decreased with increasing the amount of catalyst.  
gold NPs; ZrO2; nanocatalyst; 4-nitrophenol; 4-aminophenol +
Footnote †: journal: Applied Physics Letters  
18 December 2019 10 January 2020 10 January 2020 18 December 2019 9 January 2020 1  
Au nanoparticles deposited on metal oxide domains resulted in enhanced applications [8; 9; 10; 11]. Metal oxides based supports with large surface areas and porosities are known to enhance the catalytic activity and selectivity of gold-based catalysts [12; 13]. In this regard, zirconia (ZrO\({}_{2}\)) is of particular interest due to its tunable porosity and surface area, high thermal stability, unique amphoteric character and redox properties [14; 15]. Traditionally, the metal oxide supported Au based nanocatalysts were often prepared by impregnation, deposition-precipitation, physical blending or chemical precipitation followed by surface adsorption methods [16; 17; 18]. However, these methods usually yield insoluble materials with little control over the size, morphology and dispersion of the metal component. Furthermore, most of these methods also require high temperatures, rigorous reaction conditions, post-reaction treatment, long durations, and toxic reducing and stabilizing chemicals [19]. Therefore, due to environmental concerns, these methods have limited applications and the development of the non-toxic, facile, and green method is highly required to prepare supported gold-based nanocatalyst [20].  
Recently, sonochemical methods are considered as effective techniques for the preparation of different types of nanomaterials [21]. The dissociation of water molecules under ultrasonic conditions produces respective H and OH radicals from water vapors, that act as active species, which facilitates the oxidation and/or reduction of precursors [22]. Herein, we successfully prepared the Au-ZrO\({}_{2}\) nanocatalyst under ultrasonic irradiation using ZrO\({}_{2}\) support (cf. Scheme 1). The ZrO\({}_{2}\) nanoparticles were prepared to employ a solvothermal approach [23]. The reaction was performed under facile conditions at room temperature within a very short time period (i.e., 5 min). The preparation of Au-ZrO\({}_{2}\) was confirmed by various characterization techniques. The as-prepared Au-ZrO\({}_{2}\) sample was tested as a reduction catalyst for the conversion of 4-nitrophenol to 4-aminophenol under stirring at room temperature, and the progress of catalytic conversion was monitored using UV-Vis analysis.  
**Scheme 1.** Schematic representation of the preparation of Au-ZrO\({}_{2}\) nanocatalyst via the sonochemical method and its application towards the reduction of 4-nitrophenol.