Abstract
Micro- and nanoscale ZnO tetrapods provide an attractive support for metallic nanostructures since they can be inexpensively produced using the flame transport method and nanoparticle synthesis schemes can take advantage of a coupled response facilitated by the formation of a semiconductor-metal interface. Here, we present a light-mediated solution-based growth mode capable of decorating the surface of ZnO tetrapods with nanostructures of gold, silver, copper, platinum, palladium, ruthenium, iridium, and rhodium. It involves two coupled reactions that are driven by the optical excitation of electron-hole pairs in the ZnO semiconductor by ultraviolet photons where the excited electrons are used to reduce aqueous metal ions onto the ZnO tetrapod as excited holes are scavenged from the surface. For the most part, the growth mode gives rise to nanoparticles with a roundish morphology that are uniformly distributed on the tetrapod surface. Larger structures with irregular shapes are, however, obtained for syntheses utilizing aqueous metal nitrates as opposed to chlorides, a result that suggests that the anion plays a role in shape determination. It is also demonstrated that changes to the molarity of the metal ion can influence the nanostructure nucleation rate. The catalytic activity of tetrapods decorated with each of the eight metals is assessed using the reduction of 4-nitrophenol by borohydride as a model reaction where it is shown that those decorated with Pd, Ag, and Rh are the most active.
Highlights
As a wide bandgap (3.37 eV) semiconductor exhibiting high electron mobility, room temperature luminescence, and piezoelectricity, ZnO has garnered intense interest (Kumar et al, 2017; Laurenti and Valentina, 2017; Chaudhary et al, 2018; Vishnukumar et al, 2018)
Tetrapods are typically formed using a Nanostructures on ZnO Tetrapodal Backbones non-catalytic growth mode that sees the nucleation of a zinc blende crystal core from which four wurtzite crystal arms emerge, a growth mode that is common to II–VI semiconductors (Newton and Warburton, 2007)
With a high Young’s modulus, the ability to withstand high temperatures, and the capability to be produced in bulk quantities from earth abundant materials, ZnO tetrapods can act as inexpensive structural backbones (Newton and Warburton, 2007; Janotti and Van de Walle, 2009; Mecklenburg et al, 2012; Silva et al, 2017)
Summary
As a wide bandgap (3.37 eV) semiconductor exhibiting high electron mobility, room temperature luminescence, and piezoelectricity, ZnO has garnered intense interest (Kumar et al, 2017; Laurenti and Valentina, 2017; Chaudhary et al, 2018; Vishnukumar et al, 2018). Tetrapods are typically formed using a Nanostructures on ZnO Tetrapodal Backbones non-catalytic growth mode that sees the nucleation of a zinc blende crystal core from which four wurtzite crystal arms emerge, a growth mode that is common to II–VI semiconductors (Newton and Warburton, 2007) Such structures are of intense interest due to potential applications in electronics, sensing, biomedicine, catalysis, and composites (Bai et al, 2008; Abdulgafour et al, 2012; Castillejos et al, 2012; Papavlassopoulos et al, 2014; Naghizadeh-Alamdari et al, 2015; Picciolini et al, 2015; Alsultany et al, 2016; Gröttrup et al, 2017a; Mishra and Adelung, 2017). Decorating micro- and nanoscale ZnO tetrapods with metal nanostructures is a intriguing prospect in the respect that the tetrapod provides a retrievable support for use in liquid-phase heterogeneous catalysis while simultaneously providing the potential for a coupled response at the semiconductor-nanometal interface
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