Aerosol spray assisted assembly of TiO2 mesocrystals into hierarchical hollow microspheres with enhanced photocatalytic performance
Graphical abstract
Introduction
Photocatalysis is one method of the advanced oxidation processes (AOP), which is also a potential technology for solving environmental and energy problems including degradation of pollutants, water splitting and dye-sensitized solar cell [1], [2], [3], [4]. It is well established that an efficient photocatalyst is the principal part for photocatalysis process. Among various semiconductor photocatalysts, TiO2 is desirable for photocatalytic application owing to its peculiarities of stability, low cost, and nontoxicity [5], [6], [7]. TiO2 mesocrystals have recently emerged and been paid more and more attention as a new class of porous TiO2 materials with oriented superstructures arranged by TiO2 nanocrystal building blocks [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Recently, our work showed that TiO2 mesocrystals could largely enhance charge separation and had remarkably long-lived charges, thereby exhibiting greatly increased photoconductivity and photocatalytic activity [8], [9], [10], [11].
Varied synthetic strategies have been emerged to successfully synthesize TiO2 mesocrystals in the past few years [9], [11], [12], [13], [14], [15], [16], [17], [18]. It is important to note that TiO2 mesocrystals were first synthesized by O’Brien’s group using topotactic conversion from NH4TiOF3 mesocrystals [17]. Recently, hydrothermal/solvothermal approaches were most commonly used to synthesize TiO2 mesocrystals [11], [12], [13], [14], [15], [16], [17], [18]. Although it is beneficial to obtain high quality TiO2 mesocrystals at a specific temperature and for a period, these methods included complicated procedures which lead to the limitation in their large-scale applications.
On the other hand, aerosol spray has received a lot of interest mainly because it is a rapid method for synthesis of nano-materials [19], [20], [21], [22], [23], [24], [25], [26], [27]. The atomizer creates an aerosol mist of liquid droplets including the precursor of nano-materials. In a certain temperature, the droplets evaporate rapidly, resulting in very fine particles of nano-materials. Until now, by varying the precursors, surfactant and solvent, SiO2, TiO2, carbon, Bi2Ti2O7 and other nano-materials have been successfully fabricated with mesoporous structure [19], [20], [21], [22], [23], [24], [25], [26], [27]. Despite the excellent potentialities of aerosol spray for the preparation of nano-materials, there have been very few studies on the preparation of single crystal materials. Very recently, Ohtani et al. demonstrated that single-crystalline anatase particles with decahedral structure could be synthesized by a gas-phase reaction [28]. However, the gas-phase reaction described by Ohtani et al. can only obtain bulk TiO2 single crystals with no porosity. Porous TiO2 single crystals with a relatively large specific surface area is still highly desired through a facile and easy to handle method.
Here, we developed an ultrasound-assisted aerosol spray approach to synthesize hierarchical TiO2 mesocrystals hollow microspheres. By controlling the spray drying temperatures, TiO2 nanosheets mesocrystals were self-assembled into hierarchical hollow microspheres due to the template effect of the spray droplets. The results revealed that the hollows microspheres had an average diameter in the range of 0.5–2 μm. The TiO2 mesocrystal hollow microspheres were displayed enhanced photocatalytic activity in comparison with TiO2 nanosheet mesocrystals. Furthermore, we found that it was a general method to incorporating guest metal nanoparticles such as Au in TiO2 mesocrystal hollow structures when the metal precursor solution was involved. The surface modification of ultrafine Au nanoparticles greatly improved the visible light responsive photocatalytic activity.
Section snippets
Materials
P123 ((EO)20(PO)70(EO)20)), TiF4 were purchased from Aldrich. NH4NO3, NH4F, HAuCl4, methyl orange (MO) and p-chlorophenol (4-CP) (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the reagents were used as received without further purification.
Preparation of hierarchical TiO2 mesocrystals
A precursor solution containing TiF4, H2O, NH4NO3, and P123 with molar ratio of 90:30000:450:1 was prepared. The precursor solution was ultrasonically atomized into aerosol droplets by a household ultrasonic humidifier (1.7 MHz, 50VA),
Results and discussion
The XRD pattern of as-made microspheres shows strong characteristic diffraction peaks ascribing to NH4TiOF3 and weak ones of NH4NO3 (Fig. 1), which indicating that the NH4TiOF3 is growth during the fast aerosol-spray annealing process. With further a thermal treatment, NH4TiOF3 was easily transformed into TiO2. When the as-made sample is calcined at 200 °C, pure NH4TiOF3 phase then appears (Fig. 1). With the annealing temperature increasing, NH4TiOF3 phase then completely transforms into anatase
Conclusions
We have demonstrated an ultrasound-assisted aerosol spray method followed by topotactic transformations upon calcination for preparing hierarchical TiO2 hollow microspheres which assembled from TiO2 mesocrystal nanosheets. We found the crystallinity, specific surface area and hollow structure were responsible for the photocatalytic performance of TiO2 mesocrystal hollow microspheres. Furthermore, ultrafine Au nanoparticle with diameter of 3–4 nm were easily incorporated and well-dispersed on the
Acknowledgments
This work is supported by National Natural Science Foundation of China (21407106, 21522703), Shanghai Government (14ZR1430800, 13SG44, 15520711300), and International Joint Laboratory on Resource Chemistry (IJLRC). Research is also supported by The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and Shuguang Research Program of Shanghai Education Committee.
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Equal contribution as the first author.