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C: Energy Conversion and Storage; Energy and Charge Transport
Defect Evolution Enhanced Visible-Light Photocatalytic Activity in Nitrogen Doped Anatase TiO Thin Films 2
Yun Jiang Jin, Jiajun Linghu, Jianwei Chai, Chin Sheng Chua, Lai Mun Wong, Yuan Ping Feng, Ming Yang, and Shijie Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04517 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018
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The Journal of Physical Chemistry
Defect Evolution Enhanced Visible-Light Photocatalytic Activity in Nitrogen Doped Anatase TiO2 Thin Films
Yun Jiang Jin1, †, Jiajun Linghu2, †, Jianwei Chai3, †, Chin Sheng Chua3, Lai Mun Wong3, Yuan Ping Feng2, Ming Yang3,*, and Shijie Wang3,*
1
State Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics and
Information Technology, Sun Yat-Sen University, 510062, China. 2
Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542,
Singapore. 3
Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Singapore
138634, Singapore.
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ABSTRACT Nitrogen (N) doped TiO2 is one of promising ways to extend the photocatalytic activity into visible light range, enabling to harvest more solar energy. In this study, we realize high concentration of N incorporated into anatase TiO2 films on ITO substrates. The band gap of TiO2 with high N substitutional doping is reduced to 1.91 eV, showing much improved photocatalytic reactivity as supported by degrading methyl orange solution radiated with visible light. Firstprinciples calculations further suggest that the form of dominant defects evolves from substitution N (NO) to the coexistence of NO and oxygen vacancies (OV) when the N doping centration is increased, which leads to the reduction of band gap into visible light range and more delocalized charge distribution. Our results demonstrate a novel synthesis route that can realize high concentration of N substitutional doping in TiO2 films, and provide an improved understanding of enhanced visible-light photocatalytic performance of N-doped TiO2.
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INTRODUCTION Titanium dioxide (TiO2) has attracted tremendous attention as a promising photocatalyst, which enables us to use sunlight for various applications such as water-splitting and environmental pollution removal.1-3 It is known that the most common polymorphs of TiO2 are semiconductors with band gaps larger than 3.0 eV, and thus they only can absorb sunlight in ultraviolet region.3 This significantly limits the photocatalytic efficiency as a smaller portion of sunlight can be utilized. Therefore, extensive efforts have been made in the past decades to engineer the electronic structure of TiO2 in order to extend its photocatalytic reactivity into visible light range.4-15 For examples, transition metal ions have been used to replace Ti4+ in the TiO2 lattices;46, 16
non-metal atoms are proposed to substitute the O2- ions;7-10, 12, 15, 17 incorporating hydrogen
into TiO2 (black TiO2) can improve the photocatalytic efficiency dramatically.13-14 Besides, surface modification or the formation of novel phases of TiO2 is also suggested to modify the bandgap to the visible light range.18-22 Among these attempts, doping nitrogen (N) into TiO2 is believed to be one of the most appealing methods to realize the reactivity in visible light range due to its stability, small ionization energy, and relatively low electron-hole recombination rate.23 Both physical and chemical methods have been proposed to incorporate N ions into TiO2, including plasma implantation, sputtering, sol-gel method, solvothermal method or direct hydrolysis of organic salts.7-9, 12, 23-32 In practical applications, a direct incorporation of N ions into substitution sites of TiO2 is highly desirable because it can minimize other unintended dopants, as well as its stability. 19, 23
Furthermore, TiO2 with anatase polymorph is found more desired for the photocatalytic
application as the excess electrons in this phase are less likely bounded to the host lattices.33 After nearly two decades of study, substitutional doping of N into anatase TiO2 has been realized, 3 ACS Paragon Plus Environment
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which shows much improved photocatalytic activity in visible light range.9, 11, 34 It is also noted that the electronic structures of N doped TiO2 are dependent on the N doping concentration.35 This infers that the interaction between N ions and host lattices of TiO2 is different under different doping concentrations. At high doping level, the electrons are much deficient in the TiO2 due to the substitutional N doping at O lattices, which might promote the formation other defects to compensate the reduced electrons. However, the studies on this aspect of the N-TiO2 are still limited. In this study, we report the study of controllable and tunable doping of N ions into anatase TiO2 using magnetic reactive sputtering process. We show that with high concentration of substitutional N ions, the N-TiO2 film exhibits much improved photocatalytic performance in visible light range, which is ascribed to the formation of oxygen vacancies accompanying the N incorporation as supported by the first-principles calculations. While at low N doping concentration, the bandgap reduction is less significant as N ions are incorporated mainly at substitutional sites and the formation of O vacancies is energetically unfavorable. METHOD Experimental Details: The magnetron sputtering growth of N-TiO2 films was conducted using Ti metal target and Ar, N2, and O2 gases. The ITO substrates were pre-cleaned using acetone in an ultrasonic bath following with DI water cleaning. The deposition was performed with a DC power of 120 W and an elevated substrate temperature of 450 °C. During the sputtering process, Ar and N2 pressure were fixed to 1.0×10-3 torr and 1.0×10-4 torr, respectively, while the O2 partial pressure was adjusted to modulate the N incorporation in the TiO2 films. As a result, three different batches of the N-TiO2 samples (S1, S2, and S3) were prepared under different N2/O2 ratios. The detailed growth parameters are summarized in Table 1. The structural, optical, and electronic properties of the samples were characterized by high-resolution x-ray diffraction (HR4 ACS Paragon Plus Environment
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XRD), UV-vis-NIR absorption spectra (UV-3600, Shimadzu), and X-ray photoelectron spectroscopy (VG-ESCA lab 220i-XL). The photocatalytic effect of the N-TiO2 films was evaluated by degrading methyl-orange solution (MO) under visible light illumination. Computational Details: First-principles calculations were performed using the Vienna ab-initio simulation package (VASP)36-37 with the projector-augmented wave (PAW)38 and PerdewBurke-Ernzerhof (PBE)39 format pseudopentials. The GGA+U method40 was adopted in all the calculations to obtain an improved band gap. We note that the calculated band gap of pristine anatase TiO2 bulk is sensitive to the applied Hubbard U value on the Ti d orbital,32, 41-42 as shown in Table S2. In this study, we set the U values to be 6.0 eV and 9.0 eV for Ti (d orbital) and O atoms (p orbital), respectively.43 With this setting of U, the band gap of anatase-TiO2 is increased from 1.97 eV (PBE result) to 3.18 eV, in good agreement with experimental value (~3.20 eV).44 Importantly, the shape of the band structure remains the same with that obtained by PBE, as shown in Figure S1, indicating the validity of these applied U values. A 3×3×1 supercell of anatase-TiO2, containing 108 atoms, was used for all the defectrelated calculations. The cutoff energy was set to 400 eV for the electronic plane-wave expansion. A 2×2×4 Monkhorst-Pack grid45 was adopted for the k-point sampling. During the structural optimization, all the atoms were relaxed until the force is smaller than 0.01 eV/Å. The formation energy of a defect D is defined by:46 = () − (ℎ) − ∑
(1)
where () is the calculated total energy of the host system with the defect D, and (ℎ) is the energy of the host supercell without defect. is the number of atoms of type (host atom or dopant) that has been added to ( >0) or removed from (
