Research Paper
Self-floating amphiphilic black TiO2 foams with 3D macro-mesoporous architectures as efficient solar-driven photocatalysts

https://doi.org/10.1016/j.apcatb.2017.01.059Get rights and content

Highlights

  • Self-floating amphiphilic macro-mesoporous black TiO2 foam prepared by freeze-drying.

  • With exceptional visible light photocatalytic activity and long-term stability.

  • Super amphiphilic character favors the rapid adsorption and photocatalysis.

  • Complete mineralization of floating insoluble hexadecane.

  • Favoring solar-light-harvesting directly and recycling easily.

Abstract

The recycle and light-harvesting of powder photocatalysts in suspended system are bottlenecks for practical applications in photocatalysis. Herein, we demonstrate the facile synthesis of self-floating amphiphilic black TiO2 foams with 3D macro-mesoporous architectures through freeze-drying method combined with cast molding technology and subsequent high-temperature surface hydrogenation. Ethylenediamine plays bifunctional roles on acid-base equilibrium and “concrete effect” on stabilizing the 3D macro-mesoporous networks against collapsing, which also inhibit the phase transformation from anatase-to-rutile and undesirable grain growth during hydrogenation at 600 °C. The resultant black TiO2 foams, which can float on the water, extend the photoresponse from UV to visible-light region and exhibit excellent solar-driven photocatalytic activity and long-term stability for complete mineralization of floating insoluble hexadecane and some typical pesticides. Especially for floating contaminant hexadecane, the photocatalytic reaction apparent rate constant k is ∼7 times higher than that of commercial Degussa P25 under AM 1.5 irradiation. This enhancement is attributed to the 3D macro-mesoporous networks facilitating mass transport, the super amphiphilicity benefiting rapid adsorption, the floating feature and Ti3+ in frameworks favoring light-harvesting and spatial separation of photogenerated electron-hole pairs. The novel self-floating photocatalyst will have real practical applications for mineralizing floating contaminants in natural environment.

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Self-floating amphiphilic macro-mesoporous black TiO2 foams are prepared via freeze-dried method combined with molding technology and hydrogenation, which extend photoresponse to visible-light region and exhibit excellent solar-driven photocatalytic activity due to the 3D amphiphilic macro-mesoporous networks facilitating mass transport and adsorption, the floating feature and Ti3+ favoring light-harvesting and the separation of photogenerated electron-hole pairs.

Introduction

Semiconductor photocatalysis has attracted increasing attention due to great potentials for solving serious environmental issues [1], [2], [3], [4], [5]. In particular, porous TiO2 materials have triggered tremendous research interests because of their nontoxicity, low-cost, environmental friendliness, high chemical stability, tunable macro/mesostructured networks, large surface areas and pore volumes, excellent electronic and optical properties [6], [7], [8], [9]. Numerous efforts have been carried out to prepare high-performance porous TiO2 materials, including improving crystallinity, tuning bandgap, fabricating heterojunctions, etc [10], [11], [12], [13], [14]. However, the separation efficiency of photogenerated charge carriers and solar light utilization are still not high enough until this date. What’s worse, the recycle of powder photocatalysts in suspended system is extremely difficult and becomes a bottleneck for practical application [15], [16]. Light-weight floating photocatalysts, which not only favor catalysts recycle, but also illuminate directly in aqueous solution and increase the light-harvesting efficiently, should be good candidates. Since then, various floating photocatalysts have been constructed on the floating supporters, such as expanded perlite, high-surface area vermiculite, fly-ash cenosphere, low density polyethylene, etc [17], [18], [19], [20], [21], [22], [23]. Although the photocatalytic performance was indeed improved obviously, it still existed some unsolved problems, including the effective loading, the firmness on the supporter, and blocking light transmission [24], [25]. Inspired by the formation of volcanic rocks under extreme quenching conditions, which could float on the surface of water due to quantities of closed pores in frameworks [26], to design and synthesize support-free self-floating porous TiO2 is possible.

As is known that the wide bandgap for anatase TiO2 (∼3.2 eV) greatly hinders the utilization of solar light [27]. Fortunately, the recent discovery of black TiO2 materials by Chen and coworkers via hydrogenation has caused great concern and opened up new era for tuning the TiO2 bandstructures, which extended the photoresponse to visible light and/or near infrared region [28]. The highly localized nature of the midgap states resulted in efficient spatial separation of photogenerated electron-hole pairs in black TiO2 based on density functional theory, which led to high solar-driven photocatalytic performance [29]. Since then, great efforts have been paid to synthesize various black TiO2 materials and tried to reveal the mysterious structure [30], [31], [32], [33], [34], [35], [36], [37]. Although the exact working mechanism of black TiO2 is still under debating, an indisputable fact is that the solar-driven photocatalytic performance and separation efficiency of photogenerated charge carriers are indeed improved obviously, which represents a great breakthrough in photocatalysis [38], [39], [40], [41]. However, the light-harvesting in aqueous solution and the recycle issue for photocatalysts are still unsettled problems. Moreover, the photocatalyst with super amphiphilic character would favor the adsorption and photocatalysis for various contaminants, which is also crucial for photocatalytic reaction. Therefore, it is still a great challenge to fabricate self-floating amphiphilic porous black TiO2 materials.

Herein, we demonstrate the facile synthesis of self-floating amphiphilic black TiO2 foams with 3D macro-mesoporous architectures through freeze-drying method combined with cast molding technology and surface hydrogenation at 600 °C. The resultant black TiO2 foams, which can float on the water, extend the photoresponse from UV to visible light region and exhibit excellent solar-driven photocatalytic activity and long-term stability for complete mineralization of floating insoluble hexadecane and some typical pesticides, which is higher than that of commercial Degussa P25 TiO2 under AM 1.5 irradiation. The novel light-weight self-floating black TiO2 foams will have widespread practical applications in environmental fields.

Section snippets

Materials

Titanyl sulfate (TiOSO4, CAS: 123334-00-9), ethanediamine (C2H8N2, CAS: 107-15-3), ethanol (C2H6O, CAS: 64-17-5) and polyacrylamide (C3xH5xNxOx, CAS: 9003-05-8) were of analytical grade and purchased from Aladdin Reagent Corp. All chemicals were used as received without any further purification. Deionized water was used for all experiments.

Synthesis

In a typical synthetic procedure, 2 g of TiOSO4 was dissolved in 60 mL deionized water with stirring for 18 h at room temperature. Then the solution was turned

Crystal structure and morphology of self-floating amphiphilic macro-mesoporous black TiO2 foam

In this paper, we demonstrate the facile synthesis of self-floating amphiphilic black TiO2 foams with 3D macro-mesoporous architectures through freeze-drying method combined with cast molding technology and surface hydrogenation at 600 °C. The illustrated formation process of self-floating amphiphilic macro-mesoporous black TiO2 foams (SAMBTFs) is shown in Scheme 1. In this procedure, polyacrylamide is dissolved in Ti precusor and ethanediamine aqueous solutions uniformly, which aggregates and

Conclusions

In summary, we demonstrated a facile strategy for fabricating SAMBTFs via freeze-drying method combined with cast molding technology and subsequent surface hydrogenation. The SAMBTFs materials with narrow bandgap could float on water surface and extend the photoresponse from UV to visible-light region due to the Ti3+ in frameworks and surface disorders, which increased the light-harvesting and resolved the recycle issues. The SAMBTFs exhibited excellent solar-driven photocatalytic activity and

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21631004, 21376065, 21371053, 51672073), the Project for Foshan Innovation Group (2014IT100062), Application Technology Research and Development Projects in Harbin (2013AE4BW051), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2015014) and the International Science & Technology Cooperation Program of China (2014DFR41110).

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