No abstract

No abstract

clean and renewable energy carrier, produced from sustainable and abundant energy sources, is a promising solution. [2] The combustion of hydrogen does not release any greenhouse gases into our atmosphere. [3] With focus on the photocatalytic production of hydrogen, the challenge is to find the right materials, synthesize them with the appropriate morphology and process them into a form that enables efficient photocatalysis. From a materials point of view, most of the research is dedicated to heterogeneous photocatalysis using semiconducting photo catalysts. [4] Kudo and Miseki compiled a large collection of different photocatalyst materials ranging from various metal oxides to metal (oxy)sulfides and metal (oxy)nitrides. [5] In spite of this immense compositional diversity, the largely available, cheap, stable, and nontoxic titanium dioxide (TiO 2 ) is still one of the most studied photocatalysts, regardless of its activity being limited to ultraviolet (UV) light illumination and its unfavorable fast electron hole recombination. [6] In addition to the materials selection, the morphology of the photocatalyst also plays an important role, because a large surface area, which exposes many adsorption sites to the environment, is crucial. [3] Nanostructures with particle-, [7][8][9] rod-, [10][11][12] tube-, [13][14][15] or sheet-like [16][17][18] morphology provide a large surface-to-volume ratio and thus have been found to be ideal structures for photocatalysis. However, most nanoparticles are used in powder form, which has the disadvantage that such photocatalytic nanostructures tend to agglomerate and that extraction of the photocatalyst from the reaction medium for recycling is challenging. [19] Consequently, processing of the nanoparticles into thin films [20,21] or their immobilization on 3D, photocatalytically nonactive templates such as foams, [22] sponges, [23] mesoporous silica, [24,25] electrospun nanofibers [26][27][28] or hydroxyapatite [29] has been pursued. [3] However, a significant reduction in surface area and number of adsorption sites, both of which are detrimental to photocatalytic activity, is inevitable. [19] A solution to this problem is the fabrication of templatefree, macroscopic, 3D structures entirely made of the photocatalytic material. Examples along these lines include 3D porous g-C 3 N 4 , [30] mesoporous TiO 2 foams, [31] graphene oxide (GO) sponges, [32] porous g-C 3 N 4 monoliths, [33] MoS 2 /rGO aerogels, [34] CN aerogels, [35] or Au-Pt-TiO 2 aerogels. [36] Unfortunately, the Monolithic aerogels composed of crystalline nanoparticles enable photocatalysis in three dimensions, but they suffer from low mechanical stability and it is difficult to produce them with complex geometries. Here, an approach to control the geometry of the photocatalysts to optimize their photocatalytic performance by introducing carefully designed 3D printed polymeric scaffolds into the aerogel monoliths is reported. This allows to systematically study and improve fundamental parameters in gas phase photocata...

Photocatalysis has the potential to make a major technological contribution to solving pressing environmental and energy problems. There are many strategies for improving photocatalysts, such as tuning the composition to optimize visible light absorption, charge separation, and surface chemistry, ensuring high crystallinity, and controlling particle size and shape to increase overall surface area and exploit the reactivity of individual crystal facets. These processes mainly affect the nanoscale and are therefore summarized as nanostructuring. In comparison, microstructuring is performed on a larger size scale and is mainly concerned with particle assembly and thin film preparation. Interestingly, most structuring efforts stop at this point, and there are very few examples of geometry optimization on a millimeter or even centimeter scale. However, the recent work on nanoparticle‐based aerogel monoliths has shown that this size range also offers great potential for improving the photocatalytic performance of materials, especially when the macroscopic geometry of the monolith is matched to the design of the photoreactor. This review article is dedicated to this aspect and addresses some issues and open questions that arise when working with macroscopically large photocatalysts. Guidelines are provided that could help develop novel and efficient photocatalysts with a truly 3D architecture.

Colloidal nanocrystals are the ideal building blocks for the fabrication of functional materials. Using various assembly, patterning or processing techniques, the nanocrystals can be arranged with unprecedented flexibility in 1-, 2- or 3-dimensional architectures over several orders of length scales, providing access to ordered or disordered, porous or non-porous, and simple as well as hierarchical structures. Careful selection of colloidal nanocrystals allows the properties of the final materials to be predefined. Moreover, by combining different nanocrystals, these properties can be fine-tuned for a specific application, opening up fascinating opportunities to create new materials for energy storage and conversion, catalysis, photocatalysis, biomedicine or optics. Indeed, functional materials made of preformed nanoparticles have been realized for metals, polymers, semiconductors, and ceramics, as well as for composites and organic-inorganic hybrids. In this review article, we introduce some concepts for the fabrication of colloidal nanocrystals and their assembly into dense and porous 3-dimensional structures. Porosity is a particularly important material property that strongly influences its application potential. Therefore, we pay special attention to this aspect and compare porous materials synthesized from nanoparticles with those from molecular routes. An additional focus is set on the degree of structural order that can be achieved on different length scales.