Self-Sensitized Photocatalytic Degradation of Colorless Organic Pollutants Attached to Rutile Nanorods—Experimental and Theoretical DFT+D Studies

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• 作者：Kezhen Qi ; Filip Zasada ; Witold Piskorz ; Paulina Indyka ; Joanna Gryboś ; Mateusz Trochowski ; Marta Buchalska ; Marcin Kobielusz ; Wojciech Macyk ; Zbigniew Sojka
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：March 17, 2016
• 年：2016
• 卷：120
• 期：10
• 页码：5442-5456
• 全文大小：842K
• ISSN：1932-7455

文摘

Utilization of visible light by photosensitization of semiconductor photocatalysts via surface attachment of small colorless organic pollutants (COP) is an effective way to stimulate their photocatalytic degradation. Herein, by means of the spectroscopic, photoelectrochemical, spectroelectrochemical, and photocatalytic studies combined with the DFT+D molecular modeling, we show how disubstituted benzene derivatives, like catechol (CAT), salicylic acid (SAL), phthalic acid (PTA), and terephthalic acid (TPA), can tune the photocatalytic properties of rutile nanorods with the dominant (110) termination. We elucidated in a systematic way the COP ligand-binding configurations, the alignment of energy levels, and the charge-transfer pathways from the organic admolecules to the titania substrate. The pDOS structures of the COP@r-TiO₂(110) assemblies were interpreted in terms of electronic interactions between the titania photocatalyst and the COP adspecies. It was shown that the appearance of additional states within the band gap and in the conduction band allows for a one-step HOMO → CB ligand to metal charge transfer and a two-step HOMO → LUMO → CB sensitization. Screening of the photocatalytic performance of the COP@r-TiO₂ samples revealed that the self-degradation efficiency gauged by the initial rate constant varies in the following order: catechol (−OH, –OH; 0.024 min^–1) > salicylate (−OH, –COOH; 0.013 min^–1) > phthalates (−COOH, –COOH; 0.005 and 0.004 min^–1 for PTA and TPA, respectively), showing a beneficial role of hydroxyl functionalities at an early stage of degradation process. It was found that the higher activity of the OH-bearing catechol and salicylate adspecies was associated with the direct HOMO → CB electron-transfer pathway operating in the visible light. The two-step HOMO → LUMO → CB mechanism (requiring UV light) characteristic of carboxyl-bearing functionalities, despite favorable energy level alignment and coupling, is less efficient due to low density of the electronic states at the top of the conduction band, and low flux of the solar radiation in that energy region. The in situ diffuse reflectance spectroscopic (DRS) measurements revealed that at early stages of the photocatalytic degradation the aromatic rings of the COP moieties are readily photohydroxylated, fostering the visible light utilization via the HOMO → CB electron transfer route. Such latent autocatalytic hydroxylation processes are relevant for photocatalytic degradation of those pollutants that originally do not exhibit hydroxyl functionalities provided that a photogenerated hole is localized at the organic moiety.