Fabrication of hierarchical bismuth oxyhalides (BiOX, X = Cl, Br, I) materials and application of photocatalytic hydrogen production from water splitting
Graphical abstract
Introduction
The technology of semiconductor-based photocatalytic water splitting, photoelectro-chemically splitting water into H2 and O2, has been considered as one of the most important approaches to achieve the conversion from solar energy to hydrogen energy [1]. In addition, much effort has been devoted in recent years to develop high activity heterogeneous photocatalysts for environmental applications, such as air purification, water disinfection, hazardous waste remediation, and wastewater treatment [2]. Bismuth oxyhalides (BiOX, X = F, Cl, Br, and I) have been studied recently due to their hierarchical structures (unique layered structure), adequate chemical stability, and superior photocatalytic activity as photocatalysts for water splitting and their ability to decompose toxic pollutants into harmless inorganic substances upon solar light irradiation [3], [4], [5]. The combination of NaBiO3 and BiOCl has a higher photocatalytic activity compared with single NaBiO3 or BiOCl photocatalyst, due to more effective photo-excited electron-hole separation by the heterojunction semiconductor structure [6]. Thin film of BiOBr showed high photocatalytic activity in the degradation of RhB due to {001} facets, which are the more active planes [7]. The CNT-BiOI electrode would have an advantage for simultaneously achieving an enhanced electricity generation efficiency and degradation efficiency for recalcitrant organic pollutants [8]. BiOI also has a tetragonal crystal structure with exposed {001} facets, which is favorable to enhance the visible-light photocatalytic reaction [9]. In addition, the photocatalystic activities of the composite photocatalysts, such as BiOClxBr1-x, BiOClxI1-x, and BiOBrxI1-x, were much stronger than the of the individual compounds (BiOCl, BiOBr, and BiOI) [10]. Up to now, the synthesis of the bismuth oxyhalide composite photocatalysts is still challenging because of the difficulty to obtain the tunable band gaps and the controllable morphological structures by an inexpensive and simple-operation method [10]. The microwave-assisted hydrothermal/solvothermal process is advantageous over the conventional hydrothermal process in synthesizing some metal oxide phases. Some significant advantages are: (i) it can increase the reaction rate by one to two orders of magnitude, (ii) it can lead to novel phases, (iii) it can eliminate metastable phases, and (iv) it can lead to a rapid heating. Rapid heating along with rapid kinetics can also lead to energy savings [11]. Therefore, bismuth oxyhalide hierarchical microspheres (BiOX, X = Cl, Br, and I) and composite photocatalysts (BiOClxBr1-x, BiOClxI1-x, and BiOBrxI1-x) were prepared in this work via a facile one-pot microwave-assisted solvothermal synthesis using the solvent mixture containing EG and EtOH at the ratio of 75%:25%, where it has been reported at our earlier publication [12]. In addition, we explored its catalytic performance by photocatalytic splitting of water via the irradiation of visible light. The major factors, such as the pH values of water, the dosages of bismuth oxyhalide photocatalyst, and the different types of bismuth oxyhalide composite photocatalysts, to affect hydrogen evolution efficiency via photocatalytic splitting of water were also evaluated. To the best of our knowledge, this is the first report by comparing the photo-splitting of water with bismuth oxyhalide composite photocatalysts.
Section snippets
Materials
All chemicals were of the highest purity available and were used as received without further purification. Bismuth (III) nitrate pentahydrate [Bi(NO3)2·5H2O], potassium chloride (KCl), potassium bromide (KBr), and potassium iodide (KI) were respectively obtained from Acros. Yakuri Pure Chemical, Merck, and Shimakyu. Ethylene glycol (HOCH2CH2OH, EG), polyethylenglycol (PEG, molecular weight = 20,000), and ethanol (C2H5OH, EtOH) were purchased from FERAK Inc. Unless otherwise specified, all the
Characterization of bismuth oxyhalide composite photocatalysts
According to our previous report, many factors in the reaction, such as reaction time, reaction temperature, and solvent ratio, can affect the morphology of the self-assembled BiOBr particles [12]. In this work, the obtained solution was transferred into a 100 mL EasyPrep Teflon vessel via a facile one-pot microwave-assisted solvothermal procedure with various irradiation time and temperature. The morphologies of self-assembled BiOI particles in time sequence are shown in Fig. 1. From the FE-SEM
Conclusions
In summary, we have successfully synthesized BiOX composite catalysts with hierarchical flower-like microspheres structure. The obtained surface and optical characteristics demonstrate that BiOI can act as the best visible-light-driven photocatalyst among BiOXs used in this research. Its visible light assisted photocatalytic ability has been shown by the photosplitting of deionized water, reaching the maximum hydrogen evolution rate of 1,316.9 μmol h−1 g−1. In addition, the BiOAxB1-x composite
Acknowledgments
The authors wish to thank for the financial support by the Ministry of Science and Technology (MOST) in Taiwan under the contract number of MOST-104-2221-E-035-004-MY3. The support in providing the fabrication and measurement facilities from the Precision Instrument Support Center of Feng Chia University is also acknowledged.
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