Photon Factory Activity Report 2014 #32 (2015) B
BL-3B, BL-11A/2013G569

Electronic Structures of Nb and N co-doped Anatase TiO2Akiko Sakino, Masato Emori and Hiroshi Sakama*
Department of Physics, Sophia University, Tokyo 102-8554, Japan
1 Introduction
Titanium dioxide (TiO2) has been attracted muchattention as a photocatalyst for its high photocatalytic
activity. However, TiO2 can be excited only by ultraviolet
light because of the wide band gap. For more efficient useof solar energy, the visible-light active photocatalyst isdesired.
One of the effective methods synthesizing visible lightactive photocatalyst is co-doping of nonmetal and metalelements[1]. High activity under visible light is expected
because of the compensation effect[2]. Compensation
effect is the transfer of electron between non-metallicions and metallic ions.
In previous studies, co-doping TiO2 samples weremainly fabricated by liquid phase growth method.
However, since a lot of impurities are contained in theproducts fabricated by this method, it was difficult tocontrol the concentrations of non-metallic and metallic
dopants accurately. In this research, nitrogen (N) andniobium (Nb) doped anatase TiO2 were fabricated byvapour growth method that enables to control the dopantconcentrations.
2 Experiment
The Nb -doped anatase TiO2 thin films were fabricatedby pulsed laser deposition (PLD) using the sintered pellet
composed of titanium dioxide and niobium oxide withvarious Nb concentrations. Nb-N-doped anatase TiO2 thinfilms were prepared by annealing Nb-doped TiO2 samples
in ammonia gas flow at 870 or 970K.
X-ray photoelectron spectroscopy (XPS) measurements
were performed utilizing synchrotron radiation at BeamLine 3B and 11A of Photon Factory to examine theelectronic structures of Nb-N-doped anatase TiO2 thinfilms.
3 Results and Discussion
Figure 1 shows N1s XPS spectra of Nb-N-doped
anatase TiO2 thin films with various Nb and N
concentrations. Two different peaks are observed around
400 eV. The peak at the binding energy of 400 eV isderived from interstitial nitrogen (Ni) and that at 396.8 eVis due to substitutional nitrogen (Ns). According to ourprevious study, the slope of the valence band edge
became gentle and the states in the band gap just abovebalance band appeared by N doping. The states in theband gap just above balance band were successfully
attributed to Ns[3]. N 1s spectra in Fig.1 indicate that Ns
concentration increases with the increase in annealing
temperature. However, annealing above 870K inammonia was shown to result in the formation of oxygen
vacancies, which reduces the photocatalytic activities. Infig.1(a), Ni is dominant in the film without Nb annealed at870K in ammonia. In contrast, Ns becomes dominant in
the film with Nb (9wt.%) annealed at 870K in ammonia.
This suggests that the Nb doping promotes thesubstitutional incorporation of N atoms.
Figure 2 shows the valence band edge XPS spectra of
Nb-N-doped anatase TiO2 thin films with Nb 9wt.%. The
states in the band gap just above balance band clearly
appear at the annealing temperature of 870K.
In summary, Nb doping is shown to promote thegeneration of substitutional N atoms which increase thevisible-light activities of anatase TiO2 films.

Fig. 1: N1s XPS spectra of anatase TiO2 thin films (a)without Nb and (b) with Nb 9wt.%.

Fig. 2: Valence band edge XPS spectra of Nb-N-doped
anatase TiO2 thin films with Nb 9wt.%.
References[1] M.Z.Zelcuk et al., Reac. Kinet. Mech. Cat. 106, 313(2012).
[2]W.Zhu et al., Phys. Rev. Lett. 103, 226401 (2009).
[3] M.Emori et al., PF Activity Report 30, 152 (2013) .
* h-sakama@sophia.ac.jp