Improved photoelectrochemical performance of Z-scheme g-C3N4/Bi2O3/BiPO4 heterostructure and degradation property
Introduction
Photoelectrochemical (PEC) measurement is implemented in developing techniques for water splitting [1], [2], sensing platforms [3] and mineralization of organic pollutants [4], [5], [6] due to its low processing cost, simple instrumentation, and ability to provide a more accurate miniaturization method compared to other optical and electrochemical detection methods [7], [8]. In summary, a PEC measurement works by coupling photo-irradiation with electrochemical detection [9], thus providing promising analytical applications. The PEC performances are mainly affected and regulated by the charge separation and transfer mechanism of the photoactive component during the process of the reaction. In the severe situation of growing environment pollution of the world, the treatment of pollutants is essential for human beings. Hydrogen production under solar energy via photoelectrochemical and photocatalytic water splitting has been studied for decades as a promising solution to the problems [10], [11], [12], [13], [14]. There are many similarities between the working process of water splitting and the degradation of organic pollutants. The reaction efficiency is determined by the active groups (O2−, OH, OH− and H+) [15] which were produced with the help of photo-generated electrons and holes.
BiPO4 is an excellent semiconductor photocatalysts, which presents high activity for the degradation of organic pollutants under ultraviolet light irradiation [16], [17]. However, similar to TiO2, BiPO4 is also a wideband gap (ca. 3.85 eV) photocatalyst and the quantum efficiency is not high enough to meet the requirement of industrial application. Thus, it still needs to improve the photo-performances of BiPO4 photocatalyst [18]. Bi2O3 is extensively used in photo-electrochemical sensing applications because of its narrow band gap, effective light-harvesting media, and excellent charge separation properties [19]. In recent years, graphite C3N4 (g-C3N4) material has attracted much attention in the field of photocatalyst due to its nontoxic, inexpensive and easy to prepare by heating of melamine, dicyandiamide and urea [20], [21], [22], [23], [24], [25], [26], [27]. It is known that the band gap of g-C3N4 is about 2.7 eV, which can absorb visible light up to 460 nm. However, it exhibits low photocatalytic activity because of the high recombination rate of photo-generated electron–hole pairs.
However, the PEC performances of those catalysts are always suffer from two major problems: (1) the restriction of light utilization due to their large band gaps which usually limit the light absorption just in the ultraviolet (UV) region; and (2) the fast recombination of photo-generated excitons because of the short diffusion paths of charge carries [28]. Therefore, it is quite necessary to design a high-efficiency PEC system, which can simultaneously combine the advantages of large light utilization (UV and visible regions) and fast carrier transport. The Z-scheme can provide the opportunity to keeping eCB−/hVB+ with stronger reduction/oxidation capability on different moieties by quenching eCB−/hVB+ with weaker reduction/oxidation potentials through redox mediators [29], [30], [31]. In order to achieve those goals, we designed a combination structure of Z-scheme and p–n heterojunction and discussed the PEC performance toward the dye degradation. The g-C3N4/Bi2O3/BiPO4 was fabricated in a two-step method. The direct Z-scheme photocatalyst g-C3N4/Bi2O3 exhibited excellent PEC activity because of the formation of the Z-scheme system between Bi2O3 and g-C3N4 could result in the photoexcited electrons of g-C3N4 with a high reducibility and photoexcited holes of Bi2O3 with a high oxidizability participating in oxidation and reduction reactions, respectively [32]. The coupling of a broad band gap semiconductor with a narrow one is a good method to take advantage of both the two semiconductors [33]. The p-type semiconductors contain more holes, the n-type semiconductors own more free electrons, the p–n junction was fabricated after the combining of n-type and p-type two semiconductors with appropriate band positions. In this mechanism, eCB−(or hVB+) can be smoothly transferred from one semiconductor with a higher CB minimum (lower VB maximum) to another with a lower CB minimum (higher VB maximum), which can consequently reduce the recombination probability of eCB− and hVB+ by keeping reduction and oxidation processes in different regions [34]. The p–n heterojunction was built between Bi2O3 and BiPO4, the two semiconductors possess matched energy level and the synergistic effect was achieved. High utilization efficiency of solar energy can be harvested by the narrow band gap semiconductor; at the same time, low recombination rate of electrons and holes can be realized by the interaction of the two semiconductors. The absorption and utilization of solar light are improved via the introduction of g-C3N4.
This work intends to take the most use of the high efficiency and strong mineralization ability of BiPO4, the efficient electron transfer and separation of Bi2O3 and the dramatic visible light absorption ability of g-C3N4 to obtain a highly efficient solar light photocatalyst with strong mineralization ability. The PEC performance of BiPO4, g-C3N4/BiPO4, Bi2O3/BiPO4 and g-C3N4/Bi2O3/BiPO4 were systematically investigated.
Section snippets
Synthesis of g-C3N4/Bi2O3/BiPO4
All reagents for synthesis and analysis were used without any further treatments. BiPO4 was synthesized by a simple one step hydrothermal process. All chemicals used were in analytic grade reagent. 8 mmol of Bi(NO3)3·5H2O was dissolved in 100 mL distilled water and stirred for 30 min at room temperature, while 8 mmol of Na3PO4·12H2O and 30 mL distilled water were put in a beaker under magnetic stirring for 10 min, then was added into Bi(NO3)3 solution dropwisely. The final mixture was transferred
XRD analysis
The XRD patterns of as-prepared powder g-C3N4/Bi2O3/BiPO4 composites are shown in Fig. 1. It can be seen that a basic diffraction peak of g-C3N4 was at around 27.8°, it was well indexed as (002) diffraction plane (JCPDS 87-1526) [35]. The crystallization performance of g-C3N4 was so poor that the strength of the diffraction peak was so weak. The diffraction peaks of pure Bi2O3 were accord with Bi2O3 (JCPDS 65-2366). As compared with the pure Bi2O3,for the Bi2O3/BiPO4 and g-C3N4/Bi2O3/BiPO4
Conclusions
The g-C3N4/Bi2O3/BiPO4 was fabricated via a two-step method, and its photoelectric activities and photocatalytic activity had also been investigated under simulated sunlight illumination. It was found that different crystal phases of Bi2O3 can be prepared when the reaction environment and process changed. The excellent photoelectric performance of g-C3N4/Bi2O3/BiPO4 could be attributed to the Bi2O3/BiPO4 p–n struncure which improved the separation efficiency of photo-induced carriers and
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 51202136), Special Fund from Shaanxi Provincial Department of Education (2013JK0939), Natural Science Foundation of Shaanxi (2015JM5213), the Academic Backbone Cultivation Program of Shaanxi University of science & technology (XSGP201202) and the Postgraduate Innovation Fund of Shaanxi University of Science and Technology.
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