Elsevier

Solid State Communications

Volume 164, June 2013, Pages 27-31
Solid State Communications

First principle study of CuN, Cu and N-doped anatase TiO2

https://doi.org/10.1016/j.ssc.2013.04.005Get rights and content

Highlights

  • CuN, Cu and N-doped anatase TiO2 are studied by using DFT.

  • The Cu interstitial and N substitutional co-doped TiO2 are most favorable structure.

  • The band gap is the smallest in V region after CuN co-doped TiO2.

  • The activity of CuN co-doped is highest than that of corresponding single atom doped.

Abstract

Cu, N and CuN-doped anatase TiO2 are studied using density functional theory (DFT). It is found the band gap decreases after CuN co-doped. The intensity of adsorption spectra increases in the following order: CuN codoped⪢Cu-doped⪢N-doped⪢pure anatase in visible region, indicating that the electrons on the valence band easily transit to the conduction band. In other words, theoretic calculation shows that the photocatalytic activity of CuN co-doped is higher than that of the corresponding single Cu or N-doped.

Introduction

TiO2 has received more and more attention as a promising material for catalyst carrier, dye-sensitized solar cells and photochemical applications [1], [2], [3], [4], [5]. TiO2 phase have three distinct structures at the nature state: rutile, anatase and brookite, of which rutile is the most stable one. Although anatase is less stable than rutile, it is more efficient and more widely used in catalysis and photoelectrochemistry at lower temperature about 873 K [6], [7]. For example, some researchers find that metals/TiO2-anatase show higher activities than metals/TiO2–rutile catalysts [4], [5], [8].

As for the photochemical applications of pure anatase, the band gap energy is about 3.2 eV, which is mainly activated by ultraviolet (UV) lights. The result shows that the application of UV radiation alone for photoelectrochemistry is not an economically feasible option, because this part of the spectrum accounts for only 3–4% of solar radiation [9]. Therefore, in order to harvest the energy from the visible (V) region of sunlight, the absorption edge of anatase must be shifted to this region. Many studies have been carried out in the attempt to shift the absorption edge to the V region by implanting an adequate amount of cation or anion such as Co, Cu, Pd, V, N, C and F [10], [11], [12], [13], [14], [15]. The band gap of TiO2 for the photo-excitation is reduced by implanting an adequate amount of cation or anion. More recently, the simultaneous doping of two kinds of atoms into TiO2 has attracted considerable interest, since it can result in a higher photocatalytic activity and peculiar characteristics compared with single element doping into TiO2, such as CoN, CeN and NTa co-doped TiO2 [16], [17], [18], [19]. Therefore, in order to further improve the photocatalytic activity, simultaneous doping of two kinds of atoms in TiO2 is necessary.

The Cu- or N-doped anatase was studied by many experiments, which lead to significant enhancement of photocatalytic activity compared with pure anatase. In order to explain the reason of Cu- or N-doped anatase samples, the Cu- or N-doped anatase were studied by theorists [11], [14]. They found that the band gap decreases due to N or Cu-doped, thereby, the photocatalytic efficiency of anatase increases. The calculation results are inconsistent with experiment. To the best of our knowledge, two kinds of ions doped in anatase lattice have been studied by some experimenters and theorists, such as F and Zr, N and F, N and Zr [12], [20], [21], but the co-doped of N and Cu in anatase lattice is lacking. To understand the microscopic mechanisms of CuN co-doped and give advice for the experiments and enhance anatase's photocatalysis in the future, Cu and N co-doped in anatase is studied in the paper.

Section snippets

Theoretical methods

Cambridge Serial Total Energy Package (CASTEP) has been used in our calculations, which is performed within DFT plane-wave pseudopotential method [22]. The general gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) [23] functional and ultra-soft pseudo-potentials was used to describe the exchange correlation effects and electronion interactions, respectively. A (2×2×1) anatase supercell (Ti16O32) was studied, in which 3×3×3 meshes of k points was used to sample the Brillouin zone. A

Result and discussion

The equilibrium lattice constants of anatase are a=3.806 Å, c=9. 874 Å after optimization, compared to an experimental value of a=3.786 Å, c=9.514 Å [24]. The calculated band gap is 1.91 eV, which is smaller than that of the experimental value (3.20 eV) [25]. This difference mainly arises from the use of GGA for the exchange-correlation functional, which generally underestimates the energy gap [11], [26]. Therefore, DFT+U (Hubbard coefficient) approach should be used to compensate for the

Conclusion

In summary, the stability of Cu interstitial system is higher than that of Cu substitutional system, thereby N and Cu co-doped anatase is prefer to formation of N substitution O and Cu interstitial system. An isolated energy state about 1.6 eV occurs due to Ti 3d and N 2p hybridization after CuN doped anatase, the VB maximum and CB minimum shifts to high energy and low energy respectively due to O 2p and Cu 3d hybridization. The photocatalytic activity shows the order CuN co-doped⪢Cu-doped⪢N

Acknowledgments

The authors gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China (Grant nos. 51175363, 51274149) and the Program for Chang jiang Scholar and Innovative Research Team in University (IRT0972).

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