Abstract
Photocatalysis has received special attention from researchers that focus on evaluating its potential applications in the fields of energy, environment, and therapeutics, in an economically sustainable way. The photocatalysts that are most often discussed in literature are the ones composed of semiconductors, such as TiO2, ZnO, CdS, ZnS, CeO2 and WO3, since they possess an exquisite combination of optical, physical and chemical properties. Among these semiconductors, TiO2 has been amply studied due to its high refraction index, bandgap of 3.0-3.2 eV, high chemical and photostability, low cost and high range of potential applications. Within this scope, the obtainment of hollow spherical structures of micro- and nanometer size has been heavily motivated by an array of advantages such as high surface area, low density, high charge capacity, high shell permeability, amongst others. When compared to their bulk counterparts, literature data indicates a significant increase in photonic efficiency by using hollow spheres, mostly due to multiple processes of diffraction and reflection of light. Moreover, selectivity is an important feature of photocatalysts, that also enhances photonic efficiency. It may be expressed in two different manners. The first one concerns with selective photocatalysts to the substrate, which focus on the required surficial phenomena of adsorption of a target molecule by the photocatalyst before the catalysis reaction can take place. The second one concerns the selective photocatalysts to the desired product. In this review, the developments accomplished in recent years (mostly 2018 onwards) are addressed, encompassing a quite productive period, both regarding selectivity, hollowness and their synergisms. Product selectivity has been relying mainly on the utilization of metallic dopants, mostly noble metals. However, doping with non-metallic elements (N, F, P) and pairs of metallic elements, such as Pd/Cu and Au/Ag, appears to be gaining an increased acceptance. New insights into the mechanisms of selectivity by doping, besides the usual consideration of the reduction of band gap, were produced. A great vitality could also be found in the usage of co-catalysts (metal, metal/polymer, graphene). Concerning substrate selectivity, morphology and surface structuring have been the approaches of choice since a long time already. A recent trend was also identified, namely that of seeking synergisms between increasing photonic efficiency and increasing selective capacity, although this has presented itself as a challenge. An additional trend recently observed was that of attempting a simultaneous substrate and product selectivity, in order to make the photocatalytic process even more effective. In the case of hollow TiO2, this review addresses the control of crystallinity of the shell which is frequently under the risk of bursting due to the harsh conditions applied. To circumvent the possibility of bursting, two main strategies have been proposed, "silica-protected calcination" and "acid pretreatment". Finally, the combination of hollowness and selectivity has captured the attention of researchers. Molecular imprinting took the lead when it came to structuring the hollow shell, mostly via the post-calcination deposition of an imprinted polymer. To introduce product selectivity, modifications both of the outer and of the inner surface of the shell were reported. The possibility of tuning the properties of the outer and inner surfaces separately, opens new creative avenues in the field of TiO2–supported photocatalysis, with subsequent modification of reaction mechanisms.