| Photocatalysis is an effective technique for environmental pollution control, for which photocatalytic activity and sunlight utilization are two key issues to research. TiO2 nanotube arrays (TiO2 NTs) is one of the most promising photocatalytic electrode materials. The special tubular array structure could provide more reactive sites and charge transport channels. Besides, the coupling of photocatalysis and electrocatalysis could further improve the photocatalytic activity. Despite those advantages, there are still two issues restricting their practical applications, the low efficiency of visible light utilization due to the wide band gap, and the poor quantum efficiency. To further promote the visible light utilization and quantum efficiency, CuInS2/TiO2 NTs composite electrodes were fabricated by different methods in the present study. The decoration of TiO2 NTs with a foreign semiconductor exhibiting excellent visible absorption is demonstrated as an effective strategy to broaden the spectral range of light-absorption. In particular, the incorporation of two semiconductors could form a kind of heterojunction, facilitating the reduction of electron-hole recombination. The detailed contents are as follows:(1) CuInS2/TiO2 NTs electrode was synthesized via ultrasound-assisted successive ionic layer adsorption and reaction (SILAR) method. The transfer capability of photon generated charge carriers was investigated on CuInS2/TiO2 NTs electrode. The decoration of TiO2 NTs with CulnS2 significantly broadened the range of visible light response and increased the effective separation of photo-generated electrons and holes. Compared with pure TiO2 NTs, the photoconversion efficiency increased about 5.62 times for the composite electrode under visible light irradiation. For the degradation of efficiency of 2-chlorophenol (2-CP) under visible light irradiation, CuInS2/TiO2 NTs electrode exhibited a much improved photoelectrocatalytic performance with an degradation efficiency of 27.5%higher than pure TiO2 NTs.(2) A novel CuInS2 modified TiO2 NTs heterostructure photoelectrode was prepared via an ultrasonication-assisted cathodic electrodeposition strategy. The stoichiometry measured by EDX, XPS and ICP-OES clearly illustrated the excess indium, which was confirmed as the characteristics of the elemental composition of the n-type CuInS2. Mott-Schottky analysis further demonstrated CuInS2 possessed an n-type feature. Optical characterizations indicated that the n-n type composite could extend the visible-light response range and enhance the visible light photocatalytic activity. Moreover, the electrochemical measurements attested to a significant improvement of the interfacial electron-transfer properties. Under visible light irradiation for 180 min, the degradation efficiency of 2-CP over n-type CuInS2/TiO2 NTs electrode is 1.80 times as high as that over CuInS2/TiO2 NTs electrode via ultrasound-assisted SILAR method. The results showed that CuInS2 could effectively enhance the visible-light activity of TiO2 NTs electrode and the n-n heterojunction structure could favor the surface- interface charge separation and transfer, thus dramatic suppressing of the electron-hole recombination.(3) CuInS2/TiO2 heterojunction electrode was prepared via a vacuum-assisted SILAR approach (VS-CuInS2/TiO2 NTs). Laser flash photolysis quantitatively elucidated the long-lived photoholes yielded over VS-CuInS2/TiO2 NTs, which demonstrated that CuInS2 modification via vacuum-assisted SILAR approach could prolong the lifetime of photogenerated charges, leading to the higher photocurrent of VS-CulnS2/TiO2 NTs compared with pure TiO2 NTs. This result showed that such a synthesis method derived materials can effectively improve the separation of photogenerated electrons and holes. Photocurrent density, photoelectric conversion efficiency and carrier density of VS-CuInS2/TiO2 NTs increased by 87.0%,78.1% and 35.3% than the conventional electrode prepared via SILAR. Under visible light irradiation for 180 min, the degradation efficiency of 2-CP for VS-CuInS2/TiO2 NTs electrode is 92.2%, which increased by 74.7% and 46.8%, compared with the above two methods derived composites.(4) The ESR and active species trapping experiments clearly demonstrated O2·-,·OH and holes species were generated over VS-CuInS2/TiO2 NTs and played roles in the PEC oxidation of chlorophenols. We attributed this improvement to the following factors:the uniform and dispersive decoration of CuInS2 contributing to the facilitated electron transfer; the extensive connection between CuInS2 and TiO2 by vacuum-assisted impregnation; the effective interfacial charge transfer and restraining recombination of photo-generated electron-hole pairs owing to a large area of heterojunction structure. The comparative study on photoelectrocatalytic degradation efficiency of 4-chlorophenol (4-CP),2,4-dichlorophenol (2,4-DCP) and 2,4,6-trichlorophenol (2,4,6-TCP) degradation for VS-CuInS2/TiO2 NTs electrode was conducted. The PEC degradation for these four chlorophenols results in their kinetic constants in an increasing order of 2-CP< 4-CP< 2,4-DCP< 2,4,6-TCP. LC-MS technique was employed for investigate the visible light degradation reaction process for 2,4-DCP. The degradation pathways of 2,4-DCP over VS-CuInS2/TiO2 NTs electrode were clarified. This study provides a valuable knowledge on design of TiO2 NTs-based highly efficient photoelectrodes for chlorophenols removal. |
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