Molecules and heterostructures at TiO2 surface: the cases of H2O, CO2, and organic and inorganic sensitizers

Abstract

TiO2-based (nano)materials are widely exploited in systems and devices of actual technological interest, because of their outstanding physical and chemical properties, including chemical stability, long durability, non-toxicity abundance and low cost. For this, they are considered ideal for many practical applications including energy-related devices, photocatalysis, but are known to have some limitations. To improve their performance and then to find more efficient materials in the energy and environmental remediation fields, at first the investigation of the surface/interface properties at the molecular scale is required. In this contribution, a critical review of advances in the field of the TiO2 surface chemistry, highlighting the role of interactions at the molecular level, grafting and assembling/fabrication of suitable heterostructures, is reported. A few case studies, from the H2O, CO2 and acetylene interactions until to the grafting of organic/inorganic systems (graphene, MoS2) at the TiO2 surface, are highlighted. The discussed case studies are argued from their principles to the technological relevance. KeywordsTiO2 photocatalysis H2O, CO2 and C2H2 adsorption Graphene and MoS2/TiO2 hybrids Surface heterostructures IR spectroscopy STM and AFM HR-TEM Electronic spectroscopy Quantum modelling Preliminary remarks: motivation and structure of this review Particles and nanoparticles resulting from the combination of titanium, the second transition element more common in Earth's crust after iron, with oxygen to form TiO2, mainly in the anatase and rutile phases, are used in a wide array of products, devices and technologies. As reported by the Titanium Dioxide Manufacturers Associations (TDMA, https://tdma.info), gathering since 1974 the major producers of this material in Europe, the commercial and research area representing the "first coordination sphere" of the authors of this review, application for TiO2 across the EU, as well as across the globe, include paints, papers, pharmaceuticals, sunscreen and food (where it is listed as colourant E171). As a photocatalyst, titanium dioxide can be added to paints, cements, windows and tiles in order to decompose environmental pollutants, and as a white pigment (in this case called titanium white, Pigment White 6 or CI 77891), TiO2 is one of the most important raw materials for paints and coatings. Moreover, the range of innovative uses for TiO2 keeps on increasing. In recent years, for example, it has been used in the manufacture of cheap and efficient solar cells and building facades that can neutralize air pollution.

Experimental and computational insights into the aminopropylphosphonic acid modification of mesoporous TiO2 powder: The role of the amine functionality on the surface interaction and coordination

“…The fact that molecular adsorption of H2O is favored over dissociative adsorption on a defect-free anatase (101) facet has already been described in the literature. [106][107][108][109] In a first step, the impact of a single H2O molecule (i.e. a monohydrated 22 surface) is evaluated.…”

Surface Structure and Phase Composition of TiO2 P25 Particles After Thermal Treatments and HF Etching

“…Although the structure of the surface can be investigated by using microscopy techniques, including electron and scanning tunneling microscopies, details concerning the surface sites of TiO 2 polytypes still remain elusive. In general, an accurate and deep understanding of the active sites, their Lewis acidity, coordination states, including local defects and more extended terminations at the surface, can be gained by adsorption of small molecular probes (Mino et al, 2014(Mino et al, , 2019Zaera, 2014;Cravanzola et al, 2017), like CO (Desjonqueres and Spanjaard, 1996;Hadjiivanov and Klissurski, 1996;Scarano et al, 2004;Groppo et al, 2006). In this contest, we report a Fouriertransform infrared (FTIR) spectroscopic study of the surface sites present on TiO 2 P25 treated at different temperatures (RT,473,673,773,873,973, and 1,023 K) or etched with HF solutions at different level of concentrations.…”

Low Temperature Synthesis of Photocatalytic Mesoporous TiO2 Nanomaterials

“…Under these conditions, bicarbonate and carbonated anions, similarly to The occurrence of brookite as the prevalent phase immediately after the purification step can be ascribed to a high stochiometric excess of bicarbonate and carbonate anions present in the reaction medium with respect to the TiO 2 precursor (approximately 5:1 according to the investigated experimental condition). Under these conditions, bicarbonate and carbonated anions, similarly to oxalate anions, can act as rigid bidentate ligands [26,27], able to bridge two Ti centers, thus leading to a chelate complex that promotes the stabilization of the oxide, namely the metastable brookite phase, as previously reported [28]. Several reports, indeed, indicated that brookite generation takes place through the formation of water-soluble titanium complexes including oxalates [28], citrates [29] and lactates [30].…”

“…HRTEM analyses do not allow to us distinguish between the anatase and brookite phases, due to both the inherent accuracy of magnification calibration in HRTEM, not higher than 5% [26], and the similarity of the two polymorphs of TiO 2 , that share the octahedral coordination of Ti atoms but slightly differ in the way the TiO 6 octahedra are linked [27]. For this reason, the indexing of the fast Fourier transforms (FFTs) of HRTEM images in Figure 5 is done based on both phases. )…”