Co3O4 nanoparticles-loaded BiOCl nanoplates with the dominant {001} facets: efficient photodegradation of organic dyes under visible light
Graphical abstract
Introduction
Since Fujishima and Honda first reported photocatalysis on a TiO2 electrode, semiconductor-based photocatalysis has been attracting much attention for environmental remediation and renewable energy [1], [2]. As visible light occupies the major portion of solar light, improving the efficiency of visible light-responsive photocatalysts has become one of the most focused topics in contemporary photocatalysis field. As known, photocatalysis occurs when the photocatalyst absorbs a photon of energy equivalent to or higher than the band-gap energy (Eg) of the semiconductor, and photo-induced electron-hole pairs are then created [3], [4]. An electron in the valence band (VB) is transited into the conduction band (CB), leaving a hole in the VB, simultaneously. The photo-generated electron-hole pairs then react with the adsorbed molecules (e.g., water, oxygen, hydroxyl, etc.) on the surface/interface of the photocatalyst, ultimately producing the highly oxidizing radical species (e.g., •O2−, •OH), which can further oxidize organic molecules into carbon dioxide, water and inorganic products. However, there are still several pivotal questions concerning practical applications: (i) to reduce the cost of production for the purpose of commercialization, (ii) to develop photocatalytic ability for the degradation of various organic dyes, (iii) to decrease a high rate of electron-hole pair recombination and low interfacial charge transfer.
To overcome such formidable obstacles, we often consider employing two effective and simple strategies: (i) cation and anion doping (e.g., Fe, Pt, Ag) and (ii) creating a heterostructure by combining two semiconductors (e.g., LaVO4/TiO2 [5], TiO2/V-TiO2 [6], NiO/ZnO [7], etc.). Although cation and anion doping route shows enhanced photocatalytic activity under visible light, the rapid recombination of electron-hole pairs impairs photocatalytic activity [5], [8]. Because of photo-corrosion and an increase in charge carrier recombination rate, traditional visible light-active photocatalysts with narrow band gaps have low photocatalytic activity [8]. Thus, a great vision has come toward the creation of a heterojunction by combining two different semiconductors [4], [9].
Compared with the impurity doping method, the creation of a heterojunction with two different semiconductors having two different band gap structures is more flexible for broadening visible light absorption and less sensitive to the component homogeneity [10], [11]. Therefore, upon the formative heterojunction, the photo-induced electron-hole pairs can quickly migrate to the surface/interface, lessen the possibility of recombination, improving the photocatalytic performance [12], [13]. Nevertheless, the application of a heterojunction for enhancing the photodegradation of organic pollutants still faces some obstacles, namely: (i) the production process is complex and needs to be simplified and (ii) dye photodegradation by indirect semiconductor photosensitization process is low [4], [5], [6], [7]. Therefore, the creation of a heterojunction growing along the determined crystallographic direction will be another option to solve those issues. In this context, we therefore emphasize mainly on the enhancement of photocatalytic activity by creating the heterojunction between BiOCl nanoplates with the dominant {001} facets and Co3O4 nanoparticles.
With a wide band gap (Eg = 3.4 eV), bismuth oxychloride (BiOCl) has recently evoked much interest due to its good photocatalytic activity under UV light, unique layered structure, and high photo-corrosion stability [14], [15], [16], [17]. Generally, the intrinsic property of BiOCl limits its efficient utilization of visible light in the photodegradation of organic dye molecules. Nonetheless, the BiOCl has a layered structure composing of [Bi2O2]2+ layers between two sheets of Cl ions [18], [19], the self-induced internal electric fields or perpendicular to the {001} facets of the BiOCl caused by [Bi2O2]2+ cation layer [4], [18], [20]. This kind of architecture facilitates an easy transfer of electrons. Hence, the charge separation and transfer assisted by the internal electric field are more favorable along the [001] crystallographic direction. Moreover, the negative {001} facets terminated with oxygen can naturally enhance the adsorption of cationic dyes, such as RhB [21] and improve significantly the efficiency of photosensitized degradation process. Although it has shown the negative adsorption of ionic dye, such as methyl orange (MO) [18], the indirect dye-photosensitized degradation process can also describe the photo-reactivity of BiOCl [20], and the internal electric fields can inhibit the recombination of photo-induced electron-hole pairs in order to enhance the photodegradation of MO. Till now, BiOCl has been combined with other narrow band-gap semiconductors to improve its photocatalytic performance. Particularly, nano-flaked BiOCl heterostructured with Bi2O3 [12], [15], [22], BiOI [23], WO3 [24], and NaBiO3 [25] demonstrated efficient separation of electron-hole pairs and enhanced photocatalytic activity for the photodegradation of 1,4-terephthalic acid (TPA), nitric oxide (NO), RhB, and MO, respectively.
Co3O4 is a p-type semiconductor with interesting electronic properties [26] and has a narrow band gap (∼2.07 eV). Due to its excellent properties, Co3O4 can be applied as high-temperature solar selective absorber and efficient catalyst [27], [28]. Zhang et al. [29] reported that the Co3O4-coupled Ag3VO4 showed the improved photocatalytic activity for the photodegradation of RhB under visible light. The Co3O4/BiVO4 exhibited the enhanced photocatalytic performance for the photodegradation of acid orange II under visible light [30]. Like BiOCl, Co3O4 has also been combined with other semiconductors to create a heterojunction (Co3O4/CeO2 [31], Co3O4/Bi2WO6 [32] and so on), resulting in a high photocatalytic activity.
The band gap of Co3O4 (Eg = 2.07 eV) is smaller than that of BiOCl (Eg = 3.4 eV), and the conduction band (CB) of Co3O4 (ECB = −0.37 eV versus NHE) [28] is higher than that of BiOCl (ECB = 0.14 eV versus NHE) [17]. By considering the band gap positions of BiOCl and Co3O4, on one hand, the electrons photo-generated from the valence band (VB) of the Co3O4 can be transferred to BiOCl, and the electrons get accumulated and form internal micro-electric fields between two semiconductors, promoting the migration of photo-generated charge carriers, initiating a photocatalytic reaction. On the other hand, a cooperative effect of surface properties and heterojunction may also enhance the photocatalytic activity of the Co3O4/BiOCl for the photodegradation of organic dyes under visible light. Therefore, not only do the {001} facets represent the ability to transform the electrons but also the internal electric fields extend the interface electric field of a heterojunction which might be accounted for the Co3O4/BiOCl p–n heterojunction engendered along the [001] direction of the {001} facet of BiOCl.
In this work, a chemical co-precipitation method was applied to synthesize the Co3O4/BiOCl heterostructured photocatalyst composed of Co3O4 nanoparticles loaded on the BiOCl nanoplates with the dominant {001} facets. Under visible light irradiation, the Co3O4/BiOCl heterostructured photocatalyst exhibited efficient photocatalytic activity for the photodegradation of cationic (Rhodamine B) and ionic (methyl orange) organic dyes. The photodegradation mechanisms are also proposed.
Section snippets
Synthesis of Co3O4/BiOCl heterostructured photocatalyst
All the chemical reagents were of analytical grade and used without further purification. The Co3O4/BiOCl heterostructured photocatalyst was prepared by a simple chemical co-precipitation method at room temperature. In a typical procedure, CoCl2 was dissolved in 10 mL of 1 M HCl containing a stoichiometric amount of BiCl3. The pH of the solution was adjusted to 9–10 with the drop-wise addition of aqueous ammonia under vigorous stirring, and the gel-like mixture formed was aged for 2 h before
Structural, morphological and chemical characterization of the Co3O4/BiOCl heterostructured photocatalyst
Fig. 1 shows the XRD patterns of the Co3O4/BiOCl heterostructured photocatalyst samples with different amounts of Co3O4 ranging from 0 to 1.6 wt%. As can be seen in Fig. 1, the XRD patterns of the samples with a low amount of Co3O4 show only BiOCl crystal phase, and no diffraction peaks assignable to the Co3O4 phase can be noted. Nevertheless, we still believe that the Co3O4 nanoparticles were formed and dispersed on the surface of the BiOCl nanoplates. The crystallite sizes estimated from the
Conclusions
In summary, the Co3O4/BiOCl heterostructured photocatalyst was synthesized by a simple chemical co-precipitation method. The effect of the amount of Co3O4 nanoparticles on the enhancement of photocatalytic performance of Co3O4/BiOCl heterostructured photocatalyst was studied. Among all the samples prepared, the 0.8 wt% Co3O4/BiOCl sample demonstrated the highest photocatalytic activity for the photodegradation of RhB and MO under visible light. The enhanced photocatalytic performance of Co3O4
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Program nos. 51102160 and 51272148) and the State Ministry of Education College Students’ Innovative Projects (Program No. 201310718034, cx13035). MH would like to thank the Japan Society for the Promotion of Science (JSPS) for financial support.
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