# Plasmonics in the Ultraviolet with Aluminum, Gallium, Magnesium and Rhodium
## 3 Low Oxidation Tendency: Gallium and Rhodium
### Rhodium  
Although Ga NPs form an oxide shell only 1-2 nm thick, oxide-free metals, like Pd, Pt, Ru or Rh, should be even more desirable. Of all these noble metals, Rh presents the strongest plasmonic response in the UV range. Moreover, the possibility of synthesizing Rh NP with sizes smaller than 10 nm through chemical methods [69; 70] and its potential for photocatalytic applications, makes this material very attractive for building plasmonic tools in which photon energies are well above 3 eV [69]. As a disadvantage, Rh is less abundant than metals like Al or Mg. This fact has a logical impact on the price: the price of Rh is more than 10\({}^{4}\) higher than that of Al or Mg.  
In our previous research, two different Rh NP have been synthesized: tripod stars [69; 71] and nanocubes [70]. On the one hand, [PERSON] et al. [69] synthesized and experimentally examined the properties of Rh tripods by means of SERS, surface enhanced fluorescence, and photoinduced degradation of a typical compound like p-aminothiophen (PATP). They found that Raman and fluorescence spectra were enhanced and photodegradation was accelerated in the presence of Rh under resonant UV excitation. In addition they observed that for more tightly focused UV illumination, fluorescence spectra often increased for many minutes. This is an indication that photo-excited hot electrons efficiently transferred from the Rh NPs to the attached PATP before photodamage ultimately quenched the fluorescence. These efficient photodegradation and photo-induced charge transfer processes confirmed the potential of Rh nanostructures for UV plasmonic and photocatalytic applications. On the other hand, a similar study was carried out by [PERSON] et al. [70]. They were able to synthesize uniform Rh NCs with controlled side length ranging between 10 to 59 nm. They observed that the UV LSPRs of the NCs accelerated the rate of PATP chemical decomposition.  
Rh has also been presented as a promising metal for plasmonic photocatalysis. Supported Rh nanoparticles and molecular complexes are widely used in industrial hydrogenation, hydroformylation and ammonia oxidation reactions. However, its main industrial application is in three-way catalytic converters to reduce NO\({}_{x}\), where Rh is often alloyed with Pt and Pd because of its corrosion resistance [72; 73; 74]. Very recently, [PERSON] et al. [75] reported that plasmonic Rh nanoparticles are photocatalytic, simultaneously lowering activation energies and exhibiting strong product photo-selectivity, as illustrated through the CO\({}_{2}\) hydrogenation reaction. These results have openedan exciting new pathway in the long history of heterogeneous catalysis by offering a compelling advantage of light over heat.  
The plasmonic response of a nanoparticle is very dependent on the shape of the nanostructure [7]. Through chemical synthesis and repeated use in photocatalytic processes, some defects may appear on the NPs. For example, in the case of the NCs, they present rounded edges and some concavities/convexities on the faces as it is shown in TEM images [70; 75]. In fact, it has been demonstrated that the concavity of Rh nanocubes can be controlled by using a site-selective etching strategy [76]. The effect that these deviations from the perfect shape has on the plasmonic performance has been studied by [PERSON] et al. [77].  
Other authors have proposed to used Rh nanoparticles for deep-UV bio-chemical sensing [78]. They proposed nanostructures composed of nanorings dimer-type antennas and an infinity shape structure built by a pair of split rings with a nanodisk. These nanostructures support Fano resonances that can be used for deep-UV biochemical sensing applications.