Preparation and enhanced visible light-driven catalytic activity of ZnO microrods sensitized by porphyrin heteroaggregate
Introduction
In recent years, zinc oxide, one of the most attractive functional semiconductor materials, has gained significant attention and emerged as a class of promising materials widely used in photodetectors, gas sensors, solar cells, transparent conductors, and short-wavelength optoelectronic devices or catalysts [1], [2], [3], [4].
It is well known that semiconductors with one-dimensional patterns, such as nano/micro-wires, rods or tubes, appear to be fascinating due to their novel optoelectronic properties [5], [6], [7], [8]. The one-dimensional ZnO rod has an ability to provide natural channels so as to leave little resistance for the charges during the redox reactions, which can facilitate the electron transfer and promote the reaction rate. From a practical point of view, it is possible for ZnO to be as a photocatalyst used in the semiconductor-assisted photocatalytic processes to deal with the severe water contamination caused by the synthetic textile dyes or other commercial colorants during the manufacturing operations [9]. Many studies have highlighted the performance of ZnO particles used as environmental photocatalysts to remove some organic compounds [10], [11], [12], [13], [14]. Nevertheless, the studies on photocatalytic property of ZnO with one-dimensional patterns are still limited [15]. Therefore, further exploration of the properties of one-dimensional ZnO is critical and necessary.
Unfortunately, ZnO has a wide band-gap and only absorbs UV light. In order to improve the photocatalytic performance of ZnO, a lot of efforts were made. It is known that the surface characteristics of ZnO determined by the different morphologies and sizes are of great significance in improving its photocatalytic efficiency [16]. Besides, the modified methods, such as doping or external sensitization of the wide band-gap semiconductors, coupling with a low band-gap semiconductor to extend the photoresponse, or increasing the external surface area of photocatalysts are all widely used to improve the photochemical performance of semiconductor materials [17], [18], [19], [20], [21]. Of the various kinds of methods, it has been found that dye sensitization appears to be a major pathway used in solar cells as well as photocatalytic systems. It can not only extend the absorption range to the visible region of the solar spectrum, but also easily obtain a higher efficiency of charge separation.
Efforts have been made in the past to employ porphyrin derivatives to sensitize semiconductors [22], [23], for porphyrin compounds possess good chromophore activities over the solar spectrum and good electron donating properties due to their large π-electron systems. It is reported that H- or J-type porphyrins aggregates play a role as light harvesting assemblies to gather and transfer energy to the assembled devices, and to obtain a higher incident photon-to-photocurrent generation efficiency [24]. The distinctive characteristics of one-dimensional ZnO rod and porphyrins aggregates make them good candidates in preparing new photocatalysts.
In this paper, we report the preparation and photocatalytic characteristics of a new photocatalyst, ZnO microrods sensitized by porphyrin heteroaggregate. Assisted by the porphyrin heteroaggregate, the dispersion in aqueous solution and the spectral response of the ZnO microrods are both improved. Importantly, the photocatalytic activity of ZnO under visible irradiation can be greatly enhanced.
Section snippets
Materials
Water-soluble porphyrins, tetrakis(4-trimethylaminophenyl) porphyrin iodide (TAPPI) and tetrakis(4-sulfonatophenyl) porphyrin cobalt(II) (CoTPPS) were prepared and purified according to the Ref. [25]. The structures of TAPPI and CoTPPS, and the interaction between two monomers are shown in Scheme 1. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and RhB were purchased from Shanghai Chemical Reagent Co. All the other reagents were obtained from commercial sources and used without further
Spectral characterization of porphyrin heteroaggregate
As shown in Fig. 1A, the Soret band of the TAPPI solution is at 410 nm. After being added the aqueous solution of CoTPPS and being placed for 50 h, the Soret band at 410 nm shifts to the longer wavelength and a new broad band at 421 nm is observed (Fig. 1A(c)). Besides, the Q bands are red-shifted and broadened corresponding to those of TAPPI and CoTPPS monomers, respectively. The noticeable changes in UV–vis spectra show that the porphyrin heteroaggregate is formed by electrostatic interaction
Conclusions
A new photocatalyst, the porphyrin heteroaggregate modified ZnO microrods, has been prepared by simply mixing ZnO microrods with the TAPPI–CoTPPS porphyrin heteroaggregate. The optical characteristics of the composite demonstrate that the heteroaggregate has been anchored on the surface of ZnO, and a strong interaction between ZnO and porphyrin heteroaggregate can be suggested, which are important to the electron transfer from Por* to the CB of ZnO. It can be concluded that ZnO/TAPPI–CoTPPS is
Acknowledgement
This work was financially supported by the Key Discipline Development Program of Shanghai Municipal Education Commission (No. J51503).
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