In photocatalysis or catalysis in general, selfassembly can be crucial for the positioning of catalytic nanoparticles in an ordered way. This also allows the functionalization of individual nanoparticles towards better activity or selectivity before their selfassembly to form a large-scale interface. For example, hybrid Au core-ZnO shell nanoparticles can be self-assembled into a monolayer or multilayer films that work as photoelectrodes in water-splitting, Figure . In such a configuration, the plasmonic cores not only help in enhancing the photocatalytic activity of the ZnO shells, but the individual nanoparticles can also couple among themselves to exhibit lattice resonances that further improve this light-matter interaction . As shown in Figure , the enhancement of the hybrid nanoparticle film over pure ZnO is ~6 times. Similarly, Kim et al. showed that self-assembly is an effective strategy to obtain uniform catalytic interfaces from nanoparticles. Their self-assembled films of Au-Cu alloy nanoparticles also allowed unambiguous comparisons, Figure ). In heterogeneous catalysis, self-assembled monolayer (or multilayer) films can form highly active interfaces from more than one constituent nanoparticles that also facilitate comparison between different systems for normalized active surface area. Selfassembly also facilitates facile fabrication of photonic crystal structures of photocatalysts by using assembly of sacrificial templates. As shown in Figure ), inverse opal crystals of TiO2 obtained by using polystyrene nanosphere films as sacrificial templates have significantly higher photocatalytic activity than normal TiO2 due to the slow light or slow photon effect in the photonic crystal. The exploitation of the slow light effect in photocatalytic activity enhancement has also been reported in number of investigations. Self-assembly into 3D clusters provides another strategy to combine two different types of nanoparticles for a specific application. For instance, as shown in Figure ) emulsion directed clustering of Au and CdSe nanoparticles results in clusters with improved activity by plasmonic enhancement. Shi et al. showed that such hybrid clusters yielded a multi-fold increase in the H2 evolution reaction over CdSe nanoparticles . As evident in the synthesis of self-assembled multilayered films, the void space in-between the nanoparticles inherently renders the structures porous, and the porosity itself can hence be controlled by the nanoparticle size and the inter-particle spacing. Thus, self-assembled structures can be promising as porous materials in applications where high porosity is advantageous. For example, in catalysis, porous self-assembled substrates can be useful for highly dispersed catalyst loading. Similarly, in electrocatalysis, self-assembled nanoparticle films can be effective electrode materials. Wang et al. showed that self-assembled electrodes obtained by layerby-layer assembly of lithium titanate nanoparticles (Figure ) can deliver a specific capacity of 167 mAh g -1 at 0.1 C. The electrodes exhibit comparable performances to conventional slurry-cast electrodes at different current densities with greater flexibility to design advanced multi-layered nanostructures with cationic polyelectrolytes. In electrocatalytic applications, self-assembled nanoparticles can form electrodes with high coverage of active sites . Self-assembly of iron oxide nanoparticle-COOH on the APTES-ITO electrodes for instance facilitates electrochemical detection of ppb level concentration of Cd(II) and Pb(II) ions in seawater, Figure (f) . As discussed above, when plasmonic nanoparticles form clusters, the increased optical interaction results in higher photothermal temperatures. In plasmonic photocatalysis, cluster enhanced photothermal temperature or hot-electron generation may significantly enhance catalytic processes . It has been shown by Lee et al. that clustering induced the generation of strong electromagnetic hotspots that enhance the light-tochemical energy conversion process. This is achieved by trimers of Au nanoparticles coated with catalytic Pd and Pt shells, where the plasmonic Au cores couple electromagnetically. Thus, in plasmonic catalysis, self-assembled clusters are especially promising due to the plasmonic coupling among the constituting nanostructures.