Full length articleOne pot milling route to fabricate step-scheme AgI/I-BiOAc photocatalyst: Energy band structure optimized by the formation of solid solution
Graphical abstract
Introduction
Semiconductor photocatalysis has received increasing attention due to its broad application prospect in the energy crisis and sewage disposal [[1], [2], [3], [4]]. However, up to now, it is still far away from practical application due to low quantum efficiency. Fast recombination of photoinduced charge carriers for single component photocatalysts is a key issue. Various strategies have been developed to improve separation efficiency of carriers. The construction of heterostructure photocatalysts have been considered as a promising strategy [[5], [6], [7], [8], [9]].
Among diverse semiconductor heterojunctions, type-I heterostructured photocatalyst is not helpful to inhibit the recombination of the photogenerated electron-hole pairs because of embedded band alignment of two semiconductors [[10], [11], [12], [13]]. Although the charge carriers in type-II heterojunctions could be efficiently separated due to staggered band alignment, the redox capacity of charge carriers is weakened because the photogenerated electrons transfer to lower conduction band (CB) while the holes migrate to higher valence band (VB) [[14], [15], [16]]. In contrast, Z-scheme heterojunctions simultaneously own high separation efficiency and preserve the photogenerated electrons and holes with strong redox ability [[17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]], which act as an ideal system for photocatalytic degradation. The Z-scheme photocatalysts include two types. One is direct Z-scheme system by coupling two semiconductors [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]], and the other is mediator-containing ternary Z-scheme system [[30], [31], [32], [33], [34], [35]]. Relatively, the former is believed to be more stable and lower-cost in practical applications [36]. In addition, a new concept of step-scheme (S-scheme) heterojunction was proposed by Yu et al. [37]. In S-scheme heterojunction, comparatively useless electrons in the CB and comparatively useless holes in the VB are recombined and eliminated at the interface. While, useful holes in VB and useful electrons in CB are remained due to the presence of the internal electric field. Finally, photocatalytic oxidation and reduction reactions are initiated by these holes and electrons, respectively.
In order to construct S-scheme photocatalyst, the energy levels of the coupled semiconductors need to be matched, i.e., one semiconductor possesses less positive VB, which should be close to the CB of the other semiconductor. For the existing semiconductors, they are often difficult to meet the requirements. As is well-known, the formation of solid solutions could adjust energy band structures. For example, several solid solutions, such as BiOClxBr1–x [38,39], BiOIxBr1–x [40], Bi5O7Br0.5I0.5 [41], BiOClxI1–x [42] and BiO(ClBr)(1–x)/2Ix [43], have been demonstrated to exhibit enhanced photocatalytic activity through modulating band structures. Therefore, in this work, the S-scheme photocatalysts were designed by constructing solid solutions with adjustable energy levels.
CH3COO(BiO) (denoted as BiOAc) has been for the first time isolated and applied in photocatalysis for dyes degradation by our group [44]. Furthermore, in view of its wide band gap (3.28 eV) which restricts its utilization under sunlight, further modification by the formation of the heterojunctions BiOI/BiOAc and solid solutions BiOAcxI1-x (denoted as I-BiOAc) have been investigated [45,46]. Nevertheless, BiOAc and I-BiOAc were previously fabricated by solution method, which is not friendly to the environment. In this work, BiOAc and I-BiOAc were prepared using an eco-friendly and easy-handling solid-state grinding method.
In addition, for the construction of heterojunctions, a narrow bandgap semiconductor AgI (Eg = 2.8 eV) was employed as a promising candidate. Numerous AgI-based heterojunction photocatalysts, such as AgI/Bi5O7I [35], AgI/Bi2Sn2O7 [47], AgI-β-Bi2O3 [48], AgI/Bi12O17Cl2 [49], AgI/BiOI [[50], [51], [52]], Ag@AgI/CdWO4 [53], Ag3VO4/AgI [54], have been reported to possess the enhanced visible light photoreactivity. Herein, AgI was used to couple with BiOAc and I-BiOAc to improve their photocatalytic performance, based on the following reasons. Both of AgI and I-BiOAc solid solution contain common element I and could be in situ synthesized via an ion reaction, which thus ensure close interfacial contact. More importantly, the type-I AgI/BiOAc heterostructures can be transferred to S-scheme AgI/I-BiOAc photocatalyst by modulating the composition of the solid solutions, which greatly improve the photocatalytic activity under visible light due to the optimized energy band structures.
Section snippets
Materials preparation
All chemicals used in this work were reagent grade and have never been purified. Pure BiOAc was prepared via a milling route at room temperature. In a typical procedure, 1 mmol of Bi(NO3)3•5H2O (0.485 g) was mixed with 5 mmol of NaAc (0.410 g) in agate mortar and ground about 60 min. After that, the powders were washed thoroughly with deionized water and ethanol, and then dried in air at 60 °C for 12 h.
AgI/I-BiOAc composites were prepared through one-pot milling route. Typically, 1 mmol
Phase and structure
The crystal phase structures of the as-fabricated samples were firstly investigated by XRD. As shown in Fig. 2, all diffraction peaks of pure BiOAc could be indexed as tetragonal BiOAc (JCPDS 14-0800). After introducing I−, all the diffraction peaks of I-BiOAc were located between BiOAc and BiOI (Fig. S1a). Moreover, as KI amounts increasing, the distance between (102) and (110) peaks was gradually reduced and closer to those of BiOI, suggesting that I− has successfully incorporated into the
Conclusions
In summary, we demonstrate a facile and eco-friendly solid-state milling method to fabricate BiOAc-based heterojunctions. Specially, owing to the formation of the solid solutions I-BiOAc which composition could be adjusted to achieve suitable energy band structure, the energy levels of the valence and conduction band of I-BiOAc could perfectly match with those of AgI, and thus type-I heterojuction AgI/BiOAc was transformed into S-scheme AgI/I-BiOAc system. Furthermore, since S-scheme
Acknowledgments
The work is financially supported by the National Natural Science Foundation of China (51772155).
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