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The anatase to rutile phase transformation via thermal and chemical (HF etching) routes of TiO 2 P25 has been investigated. The treatment parameters and properties of the resulting anatase and rutile nanoparticles are analyzed and discussed. Since the nature of TiO 2 surfaces plays a significant role in determining the physical and chemical properties of the TiO 2 nanoparticles, it is important to investigate the surface properties, including the morphology, the main exposed faces, the defectiveness, to be correlated with their peculiar properties, and then reactivity. Herein, we report an infrared spectroscopy investigation, employing the adsorption of CO probe molecule at low temperature, including 12 CO and 12 CO-13 CO isotopic mixtures, at the surface sites of TiO 2 P25, previously heated from room temperature to 1,023 K under vacuum conditions. The same FTIR experiments were adopted on HF-etched TiO 2 . X-ray diffraction and transmission electron microscopy analyses were adopted to elucidate the role played by the thermal and the HF-etching treatments in modifying not only the distribution of exposed surfaces, but even the phase composition of the pristine TiO 2 P25 samples, which are initially dominated by the most thermodynamically stable (101) facets of the anatase phase. The present study helps in the crystal and exposed facet engineering for the development of highly efficient photocatalysts.

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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.