Enhanced visible light photocatalytic performance of polyaniline modified mesoporous single crystal TiO₂ microsphere

Highlights

• The mesoporous single crystal microsphere of PANI/MS-TiO₂ improved the light absorption.

• The mesoporous structure of MS-TiO₂ can increase the loading amount of PANI.

• The synergistic effect between PANI and MS-TiO₂ promoted the separation of charges.

• Improved photocatalysis was achieved via PANI modified mesoporous single crystal TiO₂ microsphere.

Abstract

Polyaniline (PANI) modified mesoporous single crystal TiO₂ microsphere (PANI/MS-TiO₂) with excellent photocatalytic activity was successfully prepared by a simple method of solution evaporation and chemisorption. The X-ray diffraction characterization demonstrated that the whole MS-TiO₂ kept the crystal type of anatase. The nitrogen adsorption-desorption characterization coupled with scanning electron microscopy indicated that the MS-TiO₂ possessed a unique mesoporous structure with high specific surface area, which resulted in the increased load of PANI on the surface of MS-TiO₂ and multiple light reflection in the photocatalyst. The UV–vis diffuse reflectance spectra confirmed that PANI/MS-TiO₂ presented more absorption ability in the visible light range than that of the pristine MS-TiO₂. The transient photocurrent responses and electrochemical impedance spectroscopy (EIS) indicated the high photo responses and fast photogenerated charge separation efficiency of PANI/MS-TiO₂. The photocatalytic activity of the PANI/MS-TiO₂ was evaluated by the photodegradation of RhB and MB under visible light irradiation. MS-TiO₂ photocatalyst with different molar ration of PANI had been prepared, and the results showed that the optimal photocatalyst (PANI/MS-TiO₂ (1:40)) exhibited the highest photocatalytic efficiency which is nearly three times as great as that of pristine MS-TiO₂ for the degradation of the RhB and MB under visible light irradiation. The remarkable performance of the PANI/MS-TiO₂ under visible light was attributed to its mesoporous single crystal structure with large surface, conductivity, as well as the synergistic effect between PANI and MS-TiO₂.

Introduction

In recent decades, many researchers have been engaged in finding and developing a more effective method for the treatment of organic dye wastewater or other environmental pollutants. Photocatalysis, based on clean solar energy and semiconductor as the photocatalyst, has become a very promising strategy with low cost, high activity and no secondary pollution in the environment [1], [2], [3], [4], [5], [6], [7]. TiO₂ has been proved to be among the most efficient photocatalysts due to its extraordinary photostability, good mechanical flexibility, high photocatalytic activity and non-toxicity [8], [9]. However, because of the relative wide band gap (∼3.2 eV), TiO₂ can only utilize the ultraviolet light, which just occupied 4% of the whole solar light, so the wide use of TiO₂ is limited severely. Therefore, in order to change the unfavorable situation and improve the solar light utilization and photocatalytic efficiency of TiO₂, much efforts shifting the photoresponse area of TiO₂ from ultraviolet to the visible light range have been made, such as doping with noble metal [10], [11], metal ion [12], non-metal ion [13] and coupling with relative narrow band-gap semiconductors [14], [15], [16], [17], [18]. Though the modification mentioned above can partly enhance the photocatalytic performance of TiO₂, there still exist some unresolved problems. For example, decorating with other materials will bring a threat to the thermal stability for the whole photocatalytic system and the increase of recombination probability between photoelectron and carrier.

Recently, conducting polymers, such as polypyrrole, polythiophene, and polyaniline (PANI) have been widely studied as excellent sensitizers to improve the visible light utilization and photocatalytic efficiency of TiO₂ photocatalysts [19], [20], [21]. The improvement of the photocatalytic performance of TiO₂ by conducting polymers decoration is mainly due to the increased visible light absorption property and rapid charge separation as a result of the synergistic action between the conducting polymers and TiO₂. Zhang et al. prepared PANI/TiO₂ composites via in situ polymerization and chemisorption method [22]. The prepared PANI/TiO₂ photocatalyst presented remarkably higher photocatalytic performance than that of untreated TiO₂ particles on degradation of RhB and MB under UV and visible light irradiation. Salem and coworkers also found that the photocatalytic efficiency of TiO₂ had been improved when modified by PANI [23]. However, the photocatalytic activity of these PANI modified TiO₂ nanoparticles were inhibited by the low visible light utilization efficiency. Hence, it is necessary to promote the visible light utilization efficiency of the PANI modified TiO₂ nanoparticle hybrid photocatalyst for higher photocatalytic activity under visible light irradiation.

Large surface area is a key point to promote the photocatalytic efficiency of modified TiO₂ hybrid photocatalyst in the visible light region, which can increase the load amount of the sensitizer, thus improving the visible light absorption in return. In view of that reason, mesoporous semiconductors and materials have attracted great attention due to their large surface structure for environmental and energy applications [24], [25], [26], [27], [28], [29]. However, most of the mesoporous materials own polycrystalline or amorphous structure, which are not conducive to the transportation of the photogenerated electron and increase the recombination efficiency. Hence, the photocatalytic activity based on these polycrystalline or amorphous semiconductor was greatly restricted. To solve this problem, Liao and coworker carried out the research on core-shell mesoporous TiO₂ modified by PANI. The specific core-shell mesoporous structure can provide large surface area, and the single crystal type can bring about excellent charge separation efficiency. When sensitized by PANI, the hybrid composites exhibited remarkably photocatalytic activity for the degradation of RhB and phenol under visible light irradiation [30]. Very recently, Liu et al. successfully developed a type of dye sensitized solar cell based on mesoporous single crystal TiO₂ microsphere (MS-TiO₂) and obtained great energy conversion efficiency compared with the conventional TiO₂ nanoparticles based solar cells, which is mainly contributed to the high surface area, efficient charge transfer and separation properties of MS-TiO₂ [31]. Due to the great performance of MS-TiO₂ exhibited in the prepared new type solar cell in Liu’s study, we have reasons to expect that MS-TiO₂ may be a good matrix for conducting polymer decoration for its large surface area and unique mesoporous single crystal structure.

In this work, we developed polyaniline (PANI) modified mesoporous single crystal TiO₂ microsphere (PANI/MS-TiO₂) by a simple method of solution evaporation and chemisorption. The synergistic action between PANI and the special TiO₂ microsphere coupled with mesoporous single crystal structure could promote the separation efficiency of photogenerated electron and hole. Its good visible light response is expected. Meanwhile, the effective charge separation property and efficient visible light utilization could improve the photocatalytic activity of the PANI/MS-TiO₂ hybrid composite remarkably under visible light irradiation. To evaluate the photocatalytic activity of the as-prepared PANI decorated MS-TiO₂, rhodamine B (RhB) and methylene blue (MB) were used as objective contaminants for photocatalytic degradation reactions under visible light irradiation (λ > 400 nm).