Efficient Charge Carrier Separation in l-Alanine Acids Derived N-TiO2 Nanospheres: The Role of Oxygen Vacancies in Tetrahedral Ti4+ Sites.

Yongjuan Chen,Xiu Luo,Jiaqiang Wang,Liang Jiang,Peiwen Xu,Yao Luo,Zhiying Yan,Junjie Li,Jiao He

doi:10.3390/nano9050698

Abstract

N-doped TiO2 with oxygen vacancies exhibits many advantages for photocatalysis, such as enhanced visible light absorbency, inhibition of the photogenerated charge carrier recombination, etc. However, preparation of N-doped TiO2 with oxygen vacancies under mild conditions is still a challenge. Herein, N-doped TiO2 nanospheres with tetrahedral Ti4+ sites were synthesized by using dodecylamine as template and assisted by l-alanine acids. The obtained samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–Vis diffuse reflectance spectra (UV–Vis DRS). It was found that the dodecylamine as a neutral surfactant controlled the structure of TiO2 spherical, while l-alanine acids provided a nitrogen source. The existence of tetrahedral Ti4+ sites in N-doped TiO2 was also confirmed. The N-doped TiO2 sample with tetrahedral Ti4+ sites exhibited significantly improved photocatalytic performance for degradation of methylene blue solution under UV light or visible light irradiation. A combined time-resolved infrared (IR) spectroscopy study reveals that the enhanced photocatalytic performance could be attributed to a large amount of photogenerated charge carriers and efficient charge separation. It is demonstrated that the shallow donor state produced by oxygen vacancies of tetrahedral Ti4+ sites can effectively promote separation of charge carriers besides capturing electrons.

Highlights

TiO2-based photocatalysts have been most widely investigated in the past decades
Some specific acid or alkali additives can act as a cotemplate along with surfactants/polymer assemblies and this may play a significant role in precisely tailoring both TiO2 structure and morphology [9]
The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained by JEM Fas-TEM-3010 electron microscope instrument (JEOL, Tokyo, Japan) at the accelerating voltage of 200 kV

Summary

Introduction

TiO2-based photocatalysts have been most widely investigated in the past decades. practical applications of TiO2 are still very limited because its fast recombination of photogenerated electron-hole pairs and wide band gap (3.2 eV) lead to a low quantum efficiency of photocatalytic reaction. The sol–gel method is usually preferred because doping amount of N and the size of the sample can be controlled [8] In this doping process, some specific acid or alkali additives can act as a cotemplate along with surfactants/polymer assemblies and this may play a significant role in precisely tailoring both TiO2 structure and morphology [9]. Durupthy et al [11] demonstrated the presence of the amino acids has an effect on the kinetics and thermodynamics during the crystalline TiO2 formation These accomplishments have inspired us to use amino acids in preparing N-doped TiO2 with unique structure, and amino acids can serve as good sources of nitrogen. The construction of tetrahedral Ti4+ sites is an effective strategy to produce stable oxygen vacancies. The sample with tetrahedral Ti4+ sites in N-doped TiO2 exhibited significantly photocatalytic performance, whether under UV light or visible light

Synthesis of Samples

Characterizations

Photocatalytic Activity

Time-Resolved IR Measurement