Substitution effects of zinc porphyrin-sensitized TiO2 nanoparticles for photodegradation of AB1
Graphical abstract
Introduction
With the rapid development of global industry and comprehensive experimental industrialization, organic fuel pollutants have become one of the serious threats to human public health and safety [1,2]. To properly address of industrial wastewater problem, photodegradation is an environmentally friendly and promising technology [3], [4], [5], [6]. Porphyrin-based semiconductor nanostructured catalysts, normally consisted of metalloporphyrin molecules and semiconductors, are emerging photocatalysts for degradation of dyes with the merits of high efficiency, low cost and no secondary pollution [7], [8], [9], [10], [11]. TiO2 is a cheap, safe, stable and widely-used semiconductor material, who has strong UV absorption ability and is commonly used as one of the key components in a photocatalyst to degrade harmful substances in wastewater [12]. Another key component, typically being introduced to enhance the light harvesting ability of the photocatalyst, can be selected from the porphyrin family due to their remarkable photon absorption in the entire visible wavelength range [13,14].
Porphyrins naturally possess strong photosensitivity, ultrafast electron transfer properties and unique biocatalytic activity, due to the heteroaromatic nature of the π-system [15], [16], [17]. The preparation of new porphyrin composite materials can further develop research in biology, chemistry, optoelectronics and medicine [18,19]. When using as a functional molecule to construct photocatalysts, also known as photosensitizers, the porphyrin ring can be easily tailored at the meso-positions, β-positions as well as the central metal species [20,21]. Consequently, the electronic state and the spatial morphology of the molecule can be rational designed to allow facilitated charge transfer at the catalytic interface. For example, Manivannan et al. reported a highly stable, reusable TiO2 photocatalyst composite containing tetraphenylporphyrin sulfonic acid, which was found to perform a 99% reduction of eosin yellow dye within 50 min under visible photon irradiation [22]. Another tetracarboxyl iron porphyrin/TiO2 composite photocatalyst was designed and synthesized by Yao et al., exhibiting a good photodegradation activity for antibiotics such as norfloxacin, tetracycline and sulfapyridine [23].
In general, for efficient photocatalysts consisted of porphyrin molecules, metalloporphyrins rather than free-base porphyrins are exploited since their more feasible energy levels for electron injection/dye regeneration as well as better stability [24], [25], [26]. At present, the most common metalloporphyrins for photocatalysis are iron porphyrins, cobalt porphyrins, nickel and zinc porphyrins, and among which, zinc porphyrins have shown quite promising application potential in terms of light harvesting, stability and interfacial electron transfer kinetics [27,28].
To design zinc porphyrins for photocatalytic utilization, selecting proper substituents on the porphyrin macrocycle is an effective protocol. Carbazole (Cb) and triphenylamine (TPA) derivatives are often used as building blocks for construction of photosensitive or electronic compounds; both possess strong electron-donating ability [29,30]. In particular, TPA has a propeller structure formed by the three benzene rings, i.e., 3-dimensional morphology [31,32]. This structural feature of TPA, on one hand, inhibits the agglomeration of the molecules, and on the other hand, endows the molecules with facilitated collision frequency to the reactive species in electrolyte and consequently accelerate the interfacial charge transfer rate [15,33]. To check the efficacy of the two functional groups in affecting the photodegradation property of zinc porphyrins, and more importantly, to better understand the structure-function relationships, we designed and synthesized two tetraarylporphyrin analogues for comparison. The porphyrin molecule attaching a Cb unit is nominated as Cb-Ph-ZnP, and the one with a TPA group is nominated as TPA-BiPh-ZnP; phenyl and biphenyl units are used to covalently link the two parts in the former and latter sensitizers, respectively (Fig. 1). The zinc porphyrins are chemisorbed onto house-made TiO2 nanoparticles for reduction of black acid (AB1) in aqueous solution under visible light.
Section snippets
Synthesis of Zn porphyrins
Compounds 1 and 2 were synthesized according to literature reports [33,34]. The aldehyde (compound a, 0.5 mmol), 2, 2′-(2, 4, 6-trimethylphenylmethylene) dipyrrole (compound 1, 1 mmol) and 4-(5, 5-dimethyl-1, 3-dioxan-2-yl) benzaldehyde (compound 2, 0.5 mmol) were fully dissolved in dichloromethane (DCM, 99.9%) at a molar ratio of 1:2:1 under nitrogen protection. Then, trifluoroacetic acid (TFA, 2.42 mmol, 99%) was slowly added, and the mixture was stirred at room temperature for 1 h. After
Physical properties of the porphyrins and the composites
The optimized structures and DFT calculation results of Cb-Ph-ZnP and TPA-BiPh-ZnP are shown in Fig. 2. It is found that the highest occupied molecular orbitals (HOMO) of Cb-Ph-ZnP is localized on both the porphyrin core and the Cb substituent, while the lowest unoccupied molecular orbitals (LUMO) is mainly located on the anchoring group with relatively weaker electron distribution on the porphyrin cycle. As for TPA-BiPh-ZnP, a better separation of the electron density of HOMO and LUMO is
Conclusions
Carbazole and triphenylamine were selected as the building blocks to construct two meso-substituted zinc porphyrins. The nonplanar nature of the molecules was suggested by the DFT calculations, showing the more dispersed electron delocalization on TPA-BiPh-ZnP than Cb-Ph-ZnP. The two zinc porphyrin molecules were decorated onto TiO2 nanoparticles, whose surface morphology, electronic status and absorption spectroscopy were investigated by SEM, XPS and DRS techniques, respectively. The
CRediT authorship contribution statement
Yuqin Wei: Methodology, Data curation, Writing – original draft. Yan Chen: Data curation, Validation. Rui Yuan: Methodology. Zhaoli Xue: Data curation, Formal analysis. Long Zhao: Supervision, Funding acquisition, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We are grateful for support from the National Natural Science Foundation of China (No. 22075110) and the Postdoctoral Foundation of Jiangsu Province (No. 2021K068A).
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