Copper and Nitrogen Co-Doping Effect on Visible-Light Responsive Photocatalysis of Plasma-Nitrided Copper-Doped Titanium Oxide Film

In order to clarify the visible-light responsive photocatalysis of TiO2 co-doped with Cu and N atoms, plasma-nitridation was taken place to Cu-doped TiO2 film. Cu-doped TiO2 films were prepared by dip-coating method and they were nitrided by nitrogen plasma in the plasma-enhanced CVD system. Cu-doped TiO2 films before and after plasma-nitridation show similar X-ray diffraction peaks of anatase TiO2. XPS analysis reveals that the ionic states of Ti and Cu in the Cu-doped TiO2 films are Ti and Cu, respectively. After nitrogen plasma treatment, oxygen atoms are released by substitution of nitrogen atoms in the TiO2 matrix, so that Cu is oxidized to generate Cu and at the same time oxygen vacancy is formed. The absorption edge of both Cu-doped and plasma-nitrided Cu-doped TiO2 did red shift. Visible-light responsive photocatalytic activity of the Cu-doped TiO2 film degraded after nitrogen plasma treatment.


Introduction
Titanium dioxide (TiO 2 ) is well known as chemically stable and harmless material, and has been applied widely in various fields such as photocatalysts [1] [2] [3], dye-sensitized solar cells [4] [5] [6], water splitting [3] [7], and so on.In recent years, it has been received a great deal of attention especially as an environmental purification materials, because of its photocatalytic decomposition of

Experimental
The Cu-doped TiO 2 films were prepared by dip-coating using sol solution mixed titanium tetraisopropoxide, ethanol, hydrochloric acid, water, and metallic salt.
The procedure of preparation of the sol solution was as follows; the mixture of titanium tetraisopropoxide (TTIP) and ethanol (C 2 H 5 OH) was stirred for 2 hours, and copper (II) chloride dihydrate (CuCl 2 •2H 2 O) was dissolved in the mixture of C 2 H 5 OH, deionized water (H 2 O), and hydrochloric acid (HCl) and also stirred for 2 hours, after then both solutions were mixed and stirred for more 2 hours.The ratio of each reagent was TTIP:C 2 H 5 OH:H 2 O:HCl = 1:5:5:0.4,and 2 mol% of metallic salt against TTIP was added in case of deposition for the Cu-doped TiO 2 films.The flow of preparation of the sol solution is shown in Figure 1.In the sol-gel reaction using TTIP as a raw material, TiO 2 is formed by a hydrolysis reaction and a condensation-polymerization reaction as shown in Equations ( 1) and (2).In the case of doping Cu atoms, it is taken into TiO 2 in the monovalent or divalent oxidation state during the sol-gel reaction.

(
) ( ) ( ) The substrate used was Si wafer.In case of transmission spectra measurements, quartz substrate was used.Dip-coating and pre-annealing at 120˚C for 10 min were After post-annealing, film thickness was about 200 nm.All heat treatments were carried out under nitrogen ambient.Figure 2 shows the capacitance coupled plasma-enhanced CVD equipment which uses a high frequency of 13.56 MHz.In this work, nitrogen gas was used for the plasma-nitridation.Flow rate of nitrogen gas was kept constant of 100 sccm.RF power, substrate temperature, and gas pressure were 100 W, 350˚C, and 133 Pa, respectively.Plasma-nitridation was carried out for the sol-gel derived Cu-doped TiO 2 films after post-annealing.
Photocatalytic activities of the Cu-doped TiO 2 films deposited on quartz substrate before and after plasma-nitridation were evaluated by pigment degradation measurement.The fluorescent light with UV-cut filter (<420 nm) was used for visible-light sources.The degradation rate of metylene blue as a pigment was evaluated by measuring the changes in absorbance of 654 nm using UV-Vis spectrometer.The films were soaked in the 1 mmol/L metylene blue solution for 60 min, and then the samples were dried in the dark after the metylene blue on the back surface was wiped off.The absorbance at 654 nm was measured every 15 min visible-light irradiation.

Depth Profile of Nitrogen Atoms
Nitrogen distribution in the plasma-nitrided Cu-doped film was evaluated by XPS, and depth profile of nitrogen atoms was shown in   structure of the plasma-nitrided film becomes dense after post-annealing at 500˚C for 30 min.respectively, which implies that the ionic state of titanium is Ti 4+ [19].The prominent sharp O1s peak at 529.9 eV is associated with the oxygen of TiO 2 [20].The Cu2p 3/2 peak at 932.2 and Cu2p 1/2 peak at 952.1 eV are both assigned to

Structural Evaluation of the Films
Cu + [20].
The XPS spectra of the plasma-nitrided Cu-doped TiO 2 film are shown in At a same time, oxygen vacancies are considered to be formed in the TiO 2 matrix.

Visible-Light Responsive Photocatalysis
Figure 8 shows transmission spectra of non-doped TiO The visible-light responsive photocatalysis was evaluated by measuring degradation of metylene blue using UV-Vis spectroscopy.Figure 9 shows the transmittance of metylene blue solution at 654 nm as a function of visible-light irradiation time.The transmittance decreases exponentially with increase of the irradiation time in both cases of the Cu-doped TiO 2 and plasma-nitrided Cu-doped TiO 2 films.
Assuming that the decomposition reaction of methylene blue is the first-order reaction, the natural logarithm of the transmittance ratio, i.e. absorbance, before and after irradiation as a function of the irradiation time is shown in Figure 10.The Cu doping into the TiO 2 films is effective for the visible-light responsive photocatalysis, however, the nitrogen-plasma treatment of the Cu-doped TiO 2 film leads to formation of impurity level and oxygen vacancy.Because these act as recombination centers of carriers, the Cu and N atoms co-doping effect for enhancement of visible-light photocatalysis using nitrogen plasma treatment of the Cu-doped TiO 2 film was not observed unfortunately.

Conclusions
Copper and nitrogen co-doping effect on visible-light responsive photocatalysis of plasma-nitrided copper-doped titanium oxide film was investigated.In this study, it is found that oxygen atoms were released from TiO 2 in the plasma-nitridation process of the Cu-doped TiO 2 , and leads to formation of impurity level and oxygen vacancy, as a result, the photocatalytic activity degraded.

Cu exists as
In order to suppress release of oxygen atoms, it is considered that using of mixed gas plasma of oxygen and nitrogen gases is effective.

Figure 1 .
Figure 1.The flow of preparation of the sol solution.

Figure 3 .
During XPS measurements, the film was step-etched in 2% HF solution.The vertical axis shows normalized N1s peak intensity against the film before step-etching.Almost all the nitrogen atoms exist within 30 -40 nm of surface region.It is considered that the nitridation takes place only around the surface region, because

Figure 2 .
Figure 2. Schematic of the capacitance coupled plasma-enhanced CVD equipment.

Figure 4
Figure 4 shows the AFM image of the surface morphology of the Cu-doped TiO 2 film after post-annealing at 500˚C for 30 min.Surface shape with fine dense particles of several tens of nanometers in diameter and about 30 nm in height are observed.There was no significant difference in the surface shape after the nitrogen plasma treatment.The crystalline structure of the films was evaluated by XRD.The X-ray diffraction patterns for the non-doped, Cu-doped and plasma-nitrided Cu-doped TiO 2 films are shown in Figure 5.All the films show the diffraction peaks of anatase (101), (112), and (200) of TiO 2 with each same intensities.It is found that the crystallization of the TiO 2 is hardly affected by Cu doping and nitrogen-plasma treatment.

Figure 7 .Figure 6 .Figure 7 .
Figure 7. Ti2p signals are hardly changed compared with those of the Cu-doped TiO 2 film, however, O1s of N-O bonds at 531.3 eV, and N1s of Ti-N bonds at 397.1 eV are observed [15].Moreover, two peaks at 934.5 eV (Cu2p 3/2 ) and 954.7 eV (Cu2p 1/2 ), assigned to Cu 2+ are obviously observed as shown in Figure6(d)[20].In the Cu-doped TiO 2 films, Cu exists as Cu 2 O in the nearly stoichiometric TiO 2 matrix and its content is estimated about 2 at% by calculation using sensitivity factor.When the Cu-doped TiO 2 films are treated in the nitrogen-plasma,

Figure 9 .
Figure 9. Transmittance of metylene blue solution at 654 nm as a function of visible-light irradiation time.

Figure 10 .
Figure 10.Natural logarithm of the absorbance ratio before and after visible-light irradiation as a function of the irradiation time.
2 , Cu-doped TiO 2 , and plasma-nitrided Cu-doped TiO 2 films, respectively.The absorption edges of Cu-doped TiO 2 and plasma-nitrided Cu-doped TiO 2 films do red shift compared with that of non-doped TiO 2 film.Slightly large red shift is observed with the plasma-nitrided Cu-doped TiO 2 film.Bandgap energies of TiO 2 , Cu 2 O, and CuO are 3.2, 2.1, and 1.76 eV, respectively [21].So, visible-light absorption of the Cu-doped TiO 2 is due to Cu 2 O exist in the film.In case of the plasma-nitrided Cu-doped TiO 2 , oxidation state of a part of Cu 2 O changes to CuO, and in addition, bandgap of TiO 2 decreases because of replacement of O atoms with N atoms in the TiO 2 matrix, as a result visible-light absorption enhances compared with the Cu-doped TiO 2 .