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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Effect of the TiO2-ZnO Heterostructure on the Photoinduced Hydrophilic Conversion of TiO2 and ZnO Surfaces Aida V. Rudakova, Alexei V Emeline, and Detlef W. Bahnemann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12125 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019
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The Journal of Physical Chemistry
Effect of the TiO2-ZnO Heterostructure on the Photoinduced Hydrophilic Conversion of TiO2 and ZnO Surfaces
Aida V. Rudakova,1* Alexei V. Emeline,1 Detlef W. Bahnemann1,2 1Laboratory
“Photoactive Nanocomposite Materials”, Saint-Petersburg State University, Saint-
Petersburg, 198504 Russia 2Laboratory
“Photocatalysis and Nanotechnology”, Institut fuer Technische Chemie, Gottfried
Wilhelm Leibniz Universitaet Hannover, Callinstrasse 3, D-30167 Hannover, Germany *Corresponding author:
[email protected] 1
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Abstract The present study demonstrates the effect of the composition and of the structure of TiO2ZnO heterostructured coatings on the photoinduced hydrophilic conversion of TiO2 and ZnO surfaces. It has been observed that the kinetic parameters of the photoinduced hydrophilic conversion can be significantly changed. Based on the analysis of the experimental data and of the proposed mechanism it is concluded that the ratio between the surface concentrations of electrons and holes participating in the photoinduced generation and deactivation of hydrophilic surface sites plays the major role in this alteration of the surface hydrophilicity of such heterostructured coatings. The ratio between the surface concentrations of electrons and holes can be changed considerably by the formation of the so-called type-II heterojunction between TiO2 and ZnO yielding an effective charge separation. The proposed approach based on the creation of these type-II heterostructures seems to be promising and productive for both, fundamental studies of the photoinduced surface superhydrophilicity as well as for the application of self-cleaning coatings with controlled wettability.
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The Journal of Physical Chemistry
Introduction Self-cleaning coatings have attained considerable attention due to their practical applications in energy and environmental areas.1-5 The application of the photoinduced alteration of the surface hydrophilicity is one of the most widely used approaches for the development of such coatings.5-8 In general, the photoinduced hydrophilic conversion depends on the intrinsic physicalchemical properties of photoactive materials and, in particular, of their surface states. Undoubtedly, the alteration of the material’s wettability upon irradiation is a multi-parameter process. Among various influencing factors, the surface morphology,5,9-11 the surface acidity,12,13 and the surface modification5 have been explored earlier. It has also been reported that the intensity and the spectral composition of the activating light can significantly affect the photoinduced hydrophilic conversion, including the controllable switching from hydrophobic to hydrophilic state and back by changing the irradiation conditions.5,12-15 The fundamental problem of mechanistic study of the photoinduced superhydrophilicity phenomenon has not been solved yet and remains very actual. Obviously, the photoinduced hydrophilic conversion is caused by the alteration of the surface free energy which can be a sequence of a reconstruction of the surficial water multi-layer.1,13,16-19 It has been suggested earlier that such a reconstruction can occur as a result of a change of the charge and/or energy of surface states induced by trapping photogenerated carriers at these states.5,12,14,19,20 The following mechanism of the processes responsible for the photoinduced transformation of surface states into new hydrophilic states was proposed earlier:12-14 S + h(e) → S*
k1 (1)
S* + e(h) → S
k2 (2)
S* → H
k3 (3)
H→S
k4 (4)
3
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In this mechanism, the determinative step of the photoinduced conversion of a “normal” surface state into another hydrophilic surface state is the trapping of photogenerated holes (h) and/or electrons (e) by the surface sites (S) depending on the type of these surface sites: hole or electron traps, respectively. This initiation step (eq. 1) is called photoactivation of the surface states. Equation 3 describes the conversion of the photoactivated surface to a new hydrophilic state (H) resulting from a reconstruction of the surface water multi-layer. For TiO2 surfaces, the average concentration of the surface defect sites responsible for the UV-induced changes of the surface energy was estimated earlier to be approximately 10-4‒10-3 of the regular sites.12 There are also two deactivation pathways for a newly formed hydrophilic surface state: a photoinduced deactivation of the active surface sites (S+ or S-) (eq. 2) and a non-photostimulated deactivation of the hydrophilic surface state (H) due to a thermal disordering of the reconstructed adsorbed water multi-layer (eq. 4). Obviously, these deactivation processes are oppositely directed to the activation step and the conversion process. At the same time, the rate constant k4 of the non-photostimulated deactivation step is rather small as compare with the rate constants of other three steps, especially at light intensities higher than 4 mW/cm2.12 Hence, according to presented mechanism and taking in account that k4 0, provided that (A/B).S0 > H0. This can be realized if either H0 → 0 that means that initial hydrophilicity is very low, or A >> B even for very high surface hydrophilicity. The latter condition exists particularly when the surface concentrations of charge carriers n1 >> n2. The opposite direction of the surface hydrophilicity alteration (that is the surface changes towards higher hydrophobicity), H < 0, can be achieved provided that (A/B).S0 < H0. This can be achieved provided that H0 is sufficiently high and A
