The pursuit of innovative TiO₂-based photocatalysts able to efficiently absorb visible light, as well as to convey photocarriers from the bulk region to the surface reaction sites, constitutes an active research domain for a myriad of applications. In particular, intense research effort has been devoted to enhancing the photocatalytic properties of this semiconductor in the form of a thin film, which can be directly applied as a self-cleaning surface in building facades, windows, vehicles and so on. Two main approaches are used to extend the TiO₂ spectral response to the visible light region and improve the separation between the photocarriers, namely i) doping with metal and non-metal elements; and ii) coupling TiO₂ to graphene derivatives.

In this work, we investigate the photocatalytic and photoelectrochemical properties of TiO₂-based thin films on glass prepared by different techniques such as RF magnetron sputtering and spin-coating. Doping with nitrogen (N) and copper (Cu) was attempted in sputtered films and the performance was also studied by combining the two doping in a bilayer structure, pointing out how the best arrangement is by far the one with Cu-TiO₂ as the top layer. As far as the spin-coated films are concerned, the influence of graphene was assessed on N-TiO₂ photocatalytic activity. Structural, optical and morphological properties were evaluated by XRD, Raman, UV-Vis spectroscopy, photoluminescence (PL), time-resolved PL, XPS, SEM and TEM. The separating and transferring efficiency of the charge was investigated by electrochemical impedance spectroscopy (EIS) and Mott-Schottky experiments were conducted to assess the capacitance behavior under DC potential polarization. Linear sweep voltammetry and photocurrent under light irradiation steps were also carried out to go into more detail in the photoresponse to UV and visible light. Photoelectrochemical data were finally correlated to photocatalytic tests performed both in gas and liquid phase using 2-propanol and 4-nitrophenol as target molecules, respectively.

In the case of spin-coated films, the introduction of graphene in N-TiO₂ films led to a significant increase in the film reactivity due to the extended absorption range of light wavelengths and suppressed recombination of photogenerated electrons and holes. With regard to sputtered films, the bilayer with Cu-TiO₂ as the top layer was found to exhibit the best photocatalytic performance both under solar simulated and visible light, where the performance of the bilayer with N-TiO₂ as the top layer is instead even worse than N-TiO₂ single layer (Fig.1) [1]. These results suggest that Cu-TiO₂ binds to the N-TiO₂ through its defects leading to their saturation. The superior reactivity is thus ensured by optimal arrangement of the two doped semiconductors within the stack ensuring a copper-driven electron trapping, which is crucial for the generation of a large amount of radicals at film surface. This work marks an important step ahead for the improvement of the performance of multifunctional TiO₂-based coatings targeting a plethora of applications, since it gives evidence on how the efficiency of these materials can be boosted not only by the use of dopant species, as amply demonstrated hitherto, but also through the combination of different dopants in a layered structure with a specific stacking order.

References

[1] C. Garlisi, J. Szlachetko, C. Aubry, D.L. Fernandes, Y. Hattori, C. Paun, M.V. Pavliuk, N.S. Rajput, E. Lewin, J. Sá, G. Palmisano, N-TiO₂/Cu-TiO₂ double-layer films: Impact of stacking order on photocatalytic properties, Journal of Catalysis, 353 (2017) 116-122.