Controlled vertical growing of Bi2O3 nano sheets on diatomite disks and its high visible-light photocatalytic performance
Graphical abstract
Introduction
Ever increasing amount of wastewater discharged by dye manufacturing and textile industries leads to severe environmental problems [[1], [2], [3], [4], [5]]. Most of the organic dyes in wastewater are hazardous to humans as well as aquatic life [6,7], and their weathering process under natural conditions also produces toxic metabolites [8]. Rhodamine B (RhB) is one of the most extensively used dyes which is hazardous to human and animal [9,10]. Many researchers have tried various methods like oxidation, adsorption, degradation and catalytic reduction for removal of various dyes from wastewater [11,12]. Among those methods, degradation by semiconductor photocatalyst seems very attractive due to its reusability, eco-friendly and universality for nearly all kinds of organic pollutants.
Bismuth oxide, which has narrower band gap comparing with TiO2 [13], has been proved to be a potential excellent photocatalyst for photocatalytic degradation of water and organic pollutants under visible light [[14], [15], [16]]. As an important type of layered structured P-type semiconductor, the photocatalytic behavior of Bi2O3 is significant different from N-type semiconductor TiO2, and needs to be better understand [17]. As known, Bi2O3 has four polymorphic forms generally, monoclinic α-phase, tetragonal β-phase, body-centered cubic γ-phase, and face centered cubic δ-phase. Among them, the α-phase is the most stable phase at room temperature; the δ-phase is the stable phase at high temperatures; and the β-phase Bi2O3 displays better photocatalytic activity than others [17]. Therefore, obtaining highly active lattice structure is the key point for Bi2O3 synthesis. Another effective way to improve the activity of Bi2O3 is to refine its particle size into nanometer [18]. Considering the convenience of recycling, these highly active nano-scaled catalysts need to be loaded on porous carriers during application, which usually have large adsorption capacity for pollutants as well. Natural diatomite is a competitive candidate among various Bi2O3 carriers, due to its hierarchical porous structure, large specific surface area, strong adsorption performance, high temperature resistance, low costs and abundant in reserves [[19], [20], [21], [22], [23], [24]].
Since diatomite is the secretion of ancient organisms, biomasses are usually found in natural diatomite mineral. After experienced long-term geological deposition process, the biomasses are tightly combined with diatomite, which provides ideal inseparable carbon source on diatomite surface. It has been reported that carbon materials can improve the light utilization of loaded TiO2, ZnO, C3N4 and Bi2WO6, and hence increase their visible-light photocatalytic activity [[25], [26], [27]]. Therefore, controlling the amount and condition of in-situ biomass material on natural diatomite provides a promising way to improve the visible-light photocatalytic activity of loaded Bi2O3. However, the contributions of in-situ biomass to catalytic activity and adsorption performance of Bi2O3/diatomite are often ignored in previous researches [28,29].
In this work, a special kind of natural diatomite mineral, whose in-situ biomass content is higher than 20 wt%, is employed to provide enough in-situ carbon precursors between the interface of Bi2O3 and diatomite. Three different pre-treating methods, carbothermal reduction, air calcination, and hydrogen peroxide pretreatment, for raw diatomite have been compared in order to reveal the role of in-situ biomasses (and their products) of natural diatomite in improving the photocatalytic performance of Bi2O3/diatomite composites. The effects of total amount and static electricity of in-situ carbon materials on the geometric growth tendency of Bi2O3 nano-sheets are also carefully examined. The photocatalytic performance of various Bi2O3/diatomite composites are evaluated by the degradation of two light-insensitive organic pollutant, Rhodamine B (RhB) and levofloxacin (LEV), which are common in polluted water environment.
Section snippets
Materials
Natural diatomite mineral from Keshiketeng Banner (Inner Mongolia Province, China) was used as porous carrier. The main phase in porous diatomite mineral is amorphous SiO2. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), Sodium hydroxide (NaOH), hydrogen peroxide (H2O2) and Rhodamine B (RhB) were purchased from Beijing Reagent Co. (Beijing, China), which were all analytical reagent grade without any further purification before used. Deionized water was used throughout all experimental procedures.
Synthesis of Bi2O3/diatomite composites
DSC analysis
Fig.1 shows the DSC curves of various treated diatomite supports. There is only a wide exothermic peak for every curve between 300 °C and 400 °C except CD, caused by oxidative decomposition of biomass adsorbed on the surface of diatomite. With the increasing temperature of H2O2 pretreatment, the areas of these exothermic peaks decrease due to the removal of biomass in raw diatomite. Thus, the in-situ biomass could be thoroughly removed from diatomite when the H2O2 pre-treating temperature
Conclusions
In this article highly visible-light photocatalytic active nanosheets with β-Bi2O3-like phase structure have been composed onto natural diatomite disks by hydrothermal synthesis. Diatomite provides ideal pore channels and abundant in-situ biomasses, so that in-situ carbon materials can be obtained at the interface region between porous diatomite and Bi2O3. Various pretreatments are carried out to control the in-situ biomass of diatomite carriers to obtain vertical growth of Bi2O3 nano sheets on
Novelty statement
The main innovation of this article is that the diatomite carrier benefits the formation and refinement β-Bi2O3-like phase nano sheets, which has much higher photocatalytic activity than common Bi2O3 phases, resulting in the improved visible-light photocatalytic performance of Bi2O3/diatomite composites. Furthermore, the in-situ biomasses and their products on natural diatomite improve the vertical-growth tendency of Bi2O3 nano sheets, resulting in higher active area and hence better adsorption
CRediT authorship contribution statement
Guihua Ren: Conceptualization, Methodology, Writing - original draft. Xuanyu Ren: Methodology, Writing - original draft, Writing - review & editing, Validation. Wentao Ju: Data curation. Yinshan Jiang: Conceptualization, Supervision. Minglei Han: Validation. Zhiqiang Dong: Data curation. Xiaodong Yang: Supervision. Kuizhou Dou: Writing - review & editing. Bing Xue: Supervision. Fangfei Li: Conceptualization, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgments
We acknowledged the funding support from the financial support of the National Natural Scientific Foundation of China (NSFC, grant No. 41702036, No. 41472035, and No. 51304080), and the Science and Technology development Project of Jilin Province (Grant No. 20170201002GX).
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These authors contributed to the work equally and should be regarded as co-first authors.