Design and high efficient photoelectric-synergistic catalytic oxidation activity of 2D macroporous SNO2/1D TiO2 nanotubes

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Abstract

A novel catalyst was constructed by assembling 1D TiO2 nanotubes (TiO2 NTs) photocatalyst and 2D macroporous SnO2 electrocatalyst, which presents simultaneously the outstanding photocatalytic and electrocatalytic properties, and was applied to the photoelectric synergistic catalytic oxidation of biorefractory pollutants. Liquid crystal soft template was prepared by block copolymer self-assembly, and the macroporous SnO2 membrane grew orderly on the TiO2 NTs using the soft template by one-step assembly. The macroporous SnO2 has the pore size distribution between 150 and 400 nm, small particle size (14.2 nm), and high loading amount (27.3 g m−2). The band gap of the 2D macroporous SnO2/TiO2 NTs is 2.93 eV. Compared with the general TiO2 NTs and the SnO2/TiO2NTs, the 2D macroporous SnO2/TiO2 NTs possess better optical absorption and photocatalytic properties, with a photoelectric conversion efficiency of 35.2% at 365 nm. Moreover, the hybrid anode presents smaller surface impedance and solution interface impedance, larger electrochemical surface absorption volume and lower electrochemical reaction activation energy. In the photoelectrocatalytic process, the 2D macroporous SnO2/TiO2 NTs exhibited higher removal rate for 2,4-dichlorophenoxyacetic acid, the initial instantaneous current efficiency was 100%, and COD removal rate reached 90.1% within 3 h. The study showed that the intermediates was generated faster and removed quickly on the prepared catalyst.

Highlights

► 2D macroporous SnO2/1D TiO2 NTs prepared using the soft template method. ► Prepared anode presents excellent photocatalytic and electrocatalytic properties. ► 2D macroporous SnO2/1D TiO2 NTs with synergistic photoelectrocatalytic function.

Introduction

Photocatalytic (PC) and electrocatalytic (EC) oxidation are advanced oxidation technologies with different forms of energy conversion and distinguished catalytic characteristics used in the fields of environment, energy conversion/storage, and hydrolysis [1], [2], [3], [4]. Therefore, the photoelectrocatalytic (PEC) technology that combines the advantages of the two technologies is significant in the theoretical and application prospects. The key is to explore novel PEC material that simultaneously presents excellent PC and EC ability [5], [6].

In order for PC and EC to happen simultaneously at the same catalytic material surface, the catalytic species and the material properties are crucial. TiO2 is an excellent photocatalyst [7], [8], and the one-dimensional (1D) TiO2 nanotubes (TiO2 NTs) are more efficient than the other structure of TiO2, which can be easily prepared by electrochemical anodic oxidation [9]. With a highly ordered array arrangement, TiO2 NTs present larger specific surface area and higher surface energy, so it has high PC efficiency [10], [11]. However, TiO2 is a semiconductor material with low conductivity and poor EC performance, so TiO2 NTs are suitable to work as an efficient photocatalyst, but not suitable to be an electrocatalyst. In the view of electrochemical oxidation, dimensionally stable anodes (DSA) are widely used in the chlor-alkali industry for its lower chlorine evolution potential. However, the oxygen evolution potential of DSA is also low. While the Sb doped SnO2 coating anode has high oxygen evolution potential and superior EC performance [12], it will decrease the energy consumption of the hydrogen generation in the hydrolysis reaction and increase the current efficiency, so it is very suitable to use in the electrochemical oxidation synthesis, especially the electrochemical oxidation in aqueous media and environmental degradation of refractory pollutants [13], [14]. However, SnO2 is a semiconductor material, whose band gap is 3.88 eV [9], its PC efficiency is lower. So the key is to explore a novel integrated photoelectrocatalyst, which can realize that simultaneous PC and EC oxidation are synergisticly generated at one catalytic material surface and in the same reaction process.

Herein, we think that it can sufficiently use the structure of the TiO2 NTs. The ordered and vertical TiO2 NTs arrays in situ grown on the surface of pure Ti sheet not only have larger specific surface area and higher surface energy, but also have higher adsorption capacity and more active sites. It can act as an ideal “container” or “supporter” to assemble the Sb doped SnO2 electrocatalyst in the microstructure constructing a novel photoelectrocatalyst. Assembling the Sb doped SnO2 to the inside and surface of the TiO2 NTs by sol–gel method, it can easily form the SnO2/TiO2 NTs catalyst [15], [16]. The improved loading amount and dispersion of SnO2 are helpful to strengthen the PC and EC performance of the prepared catalyst [17], [18]. However, the preparation is complex and long-term, the loading was repeated several times [18]. Another is that a small quantity of antimony must be doped into SnO2 to achieve excellent EC properties, although SnO2 itself is light transmission agent, but Sb doped SnO2 will decrease the light absorption [18]. So the object of the research is that the loading of the Sb doped SnO2 will not affect the light absorption of TiO2 NTs, and right way can be found to enhance the catalytic oxidation effect of photoelectric synergy.

Therefore, it was started with the construction of a novel photoelectrocatalyst by assembling the 2D macroporous SnO2 to the 1D TiO2 NTs, which is the first time report. The soft template, with block copolymer (styrene phenol polyoxyethylene ether, abbreviated as SPPE) self-assembly into ordered liquid crystal (LC) structure, is adopted in the preparation, it is the necessary condition to form the specific structure. Sb doped SnO2 is assembled to the TiO2 NTs using the LC soft template. The soft template is removed by calcination, 2D periodically ordered macroporous SnO2 is successfully prepared. The preparation of the catalyst is one-step assembly, it is simple and time saving. The constructed way maybe have the following two advantages: one is that the 2D macroporous structure of SnO2 makes the light permeate directly and increases the light absorption greatly. And it is reverse phase doping for SnO2 and TiO2 according to the energy band theory, which perhaps is suitable for the separation of photogenerated electrons and holes, and improves the photoconversion efficiency. The other is that the prepared method perhaps endows SnO2 with small particle size, ordered arrangement and large loading amount, those will greatly improve the EC performance of the catalyst. Thus, in the experiment, we studied the fabrication of the catalyst, and the catalyst's photoelectric synergistic catalytic property. Furthermore, the properties of the catalysts were investigated by applying to the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D)[19], [20], 2,4-D is a phenoxy carboxylic acid herbicide, which is widely used in crop weeding and lawn maintenance in large amount and long time. It has mutagenicity and teratogenicity, and it is difficult to be biochemical degraded, whose chlorine-containing metabolic intermediates are of high toxicity [21]. Therefore, 2,4-D pollution has become an environmental problem needed to be resolved urgently [22], [23]. In the further study, it elaborated the high efficiency and oxidation mechanism of the 2D macroporous SnO2/TiO2 NTs from the generation and further oxidation of the intermediate. This study provides a new idea for exploring the catalyst with high photoelectric synergistic performance.

Section snippets

Preparation of 2D macroporous SnO2/TiO2 NTs

TiO2 NTs were prepared by the electrochemical anodic oxidation method according to the literature [18]. The preparation of precursor is as follows. 6.0 g of SPPE (number average molecular weight Mn¯=1622, relative molecular mass distribution width D = 1.10) is dissolved in 3.0 g water, forming solution A. 3.0 g SnCl2·2H2O and 0.15 g SbCl3 are dissolved in 3.0 g 18 wt% hydrochloric acid, forming solution B. Solution C is formed by mixing A and B solutions. TiO2 NTs are put into the buffer bottle and

Preparation and characterization of the novel catalyst

In the experiment, surfactant solution first form the micelles (Scheme 1b), then micelles spontaneously form the advanced homogeneous lyotropic LC with columnar structure when the macromolecule block copolymer surfactant of SPPE reaches certain concentration (Scheme 1c). The LC is a special orderly arranged structure, with high viscosity and transparent appearance. With LC as the template, it has strong interaction between the tin–antimony ions and the LC molecules (Scheme 1d), then the

Conclusion

In summary, a facile liquid crystal soft template method constructed the 2D macroporous SnO2/TiO2 NTs catalyst. The prepared 2D macroporous SnO2/TiO2 NTs catalyst simultaneously possesses superior photocatalytic and electrocatalytic performance. It displayed excellent photoelectrocatalytic synergistic oxidation ability applying in the degradation of 2,4-D. The present concept of preparing 2D macroporous SnO2/1D TiO2 NTs will open a new avenue to develop photoelectric functional materials with

Acknowledgments

This work was supported jointly by the National Natural Science Foundation P.R. China (project no. 20877058), 863 Program (project no. 2008AA06Z329) from the Ministry of Science, and Nanometer Science Foundation of Shanghai (project no. 0852nm01200).

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