1
Zeta potential and photocatalytic activity of nitrogen doped TiO2 thin films

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

2
Nitrogen doped TiO2 films were fabricated by annealing anatase TiO2 films in gaseous NH3.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

3
Nitrogen atoms in these films were substitutionaly introduced into oxygen sites of TiO2 lattice.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

4
We have evaluated the zeta potential of these films and have found that it became negative with the amount of nitrogen doping.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

5
The photocatalytic decomposition of gaseous 2-propanol was examined under light-limited conditions, and the quantum yield (QY) was estimated while illuminating with ultraviolet (UV) and visible light (VIS).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

6
The QY values for the nitrogen doped TiO2 film were 15.4% (UV) and 0.41% (VIS), whereas those for the pure TiO2 film were 18.4% (UV) and 0% (VIS).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

7
In addition, the photocatalytic decomposition of ethylamine ions (CH3CH2NH3+) was evaluated in an aqueous solution under UV illumination in a light-rich condition.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

8
Consequently, the decomposition rate of the nitrogen doped TiO2 for aqueous ethylamine ions was higher than that of the pure TiO2 because the surface of the nitrogen doped TiO2 film was negatively charged and cationic ethylamine ions were efficiently adsorbed on its surface.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

9
The surface structure and the band structure of the nitrogen doped TiO2 are discussed in this paper.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

Introduction

10
When UV light is illuminated on TiO2, electron and hole pairs are generated in it and they reduce and oxidize adsorbates on the surface, respectively.1–4

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

11
Moreover, we recently reported that the surface of TiO2 becomes highly hydrophilic under UV illumination.5–7

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

12
A great deal of research has been conducted on these reactions and various TiO2 coated substrates have been applied to water and air purification,8–10 anti-bacterial agents,11 and self-cleaning surfaces.12,13

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

13
Thus far, these techniques are mainly applied to outdoor use since the photocatalytic reaction for these applications requires sufficient UV source, which is typically provided by the sun.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

14
The band gap of TiO2 (anatase) is 3.2 eV and the photocatalytic reaction proceeds under the UV illumination with wavelengths shorter than λ < 380 nm, which corresponds to the band gap energy.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

15
Numerous studies have attempted to extend the photosensitivity of TiO2 towards the visible light region by coupling with organic dye sensitizers14,15 or metal oxides,16–20 by doping with transition metals,21–23 and by reducing with hydrogen.24

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

16
Recently, visible light responses have been reported by nitrogen doping into a TiO2 lattice.25

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

17
R. Asahi et al. have reported that the substitutional doping of nitrogen into TiO2 lattice is effective because the N-2p states help narrow the band-gap by mixing with O-2p states.26,27

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

18
Using first-principle calculations and X-ray photoemission spectroscopy, they concluded that nitrogen doping into substitutional sites of TiO2 is indispensable for band-gap narrowing and photocatalytic activity.25

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

19
We previously reported the photocatalytic activities of nitrogen doped TiO2 powders under visible light and have concluded that the isolated N-2p narrow band above the O-2p valence band is responsible for the visible light response, when nitrogen is lightly doped (up to about 1%) into oxygen sites.28,29

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

20
Nitrogen doping into TiO2 lattice effectively controls the electric structure and is expected to control the surface structure of TiO2.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

21
The present paper focuses on the zeta potential of nitrogen doped TiO2.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

22
The zeta potential depends on the atomic arrangement of TiO2 surface and is crucial in controlling the adsorption properties of TiO2 surfaces.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

23
Substitutionally nitrogen doped TiO2 films are fabricated and the zeta potentials and photocatalytic oxidation activities are evaluated.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

24
The surface and the band structures of nitrogen doped TiO2 are also discussed.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj4

Experimental

Preparation of thin films

25
Pyrex glass plates coated with SiO2 were used as substrates for preparing thin films.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

26
Prior to coating the films, the substrates were ultrasonically degreased in ethanol for 30 min and then thoroughly rinsed with distilled water.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

27
These substrates were dipped into a titanium tetraisopropoxide solution (NDH-510C, Nihon Soda Co., Tokyo, Japan) and were slowly removed from the solution at a fixed rate of 15 cm min−1 under dry nitrogen.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

28
These samples were calcined at 500 °C for 30 min in air to obtain pure TiO2 films with anatase phase.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

29
The dip-coating and calcination procedures were repeated six times, which yielded films ca.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

30
600 nm thick.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

31
Afterwards, the films were annealed in gaseous ammonia (NH3) for 1 h at 400, 600, or 700 °C.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Characterization

32
The crystal phases of the thin films were evaluated by the grazing angle method of X-ray diffraction with Cu Kα rays (XRD: model RINT-2100, Rigaku Co., Tokyo, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

33
The incident angle was fixed at 0.5° and 2θ was scanned in the range of 10–70°.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

34
The surface morphologies and cross sections of thin films were observed using a scanning electron microscope (SEM: model S-4200, Hitachi Co., Tokyo, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

35
X-ray photoelectron spectroscopy with Mg Kα X-rays (XPS: model AXIS-HS, Kratos, Manchester, UK) evaluated the amount and states of the nitrogen atoms in these films.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

36
The photoelectrons of the N-1s, Ti-2p, O-1s and C-1s orbitals were recorded with a takeoff angle of 45°.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

37
The internal reference for the absolute binding energy was the C-1s peak of hydrocarbon contamination at 284.8 eV.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

38
The integrated peak areas of each orbital were calculated after subtracting the non-linear background.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

39
Quantitative analysis was based on the peak area multiplied by sensitivity factors supplied by Kratos, which considered the geometric configuration of the apparatus.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

40
In addition, the depth profiles of each element in the thin film were measured, following Ar+ ion sputtering with a 4.5 kV beam voltage.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

41
The ultraviolet-visible absorption spectra of the thin films were recorded on a spectrophotometer (UV-3100, Shimadzu Co., Kyoto, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

42
The absorption percentage (α, %) was obtained by measuring the transmittance (T, %) and reflectance (R, %) of the thin films, where α = 100 − (T + R).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

43
A 60 mm integrating sphere was used for the measurement, since the scattering effect was taken into account for the film absorption.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

44
The zeta potentials of the thin film surfaces were measured by the monitor particle method30 using an electrophoretic light scattering spectrophotometer (ELS-6000, Otsuka Electronics Co., Oosaka, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

45
A quartz cell was used to measure the electrophoretic mobility of polystylene latex reference particles, which had an average diameter of 520 nm.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

46
The cell was filled with a NaCl aqueous solution (0.1 mmol dm−3) containing latex reference particles.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

47
The pH value was adjusted using aqueous solutions of NaOH (0.1 mmol dm−3) and HCl (0.1 mmol dm−3).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

Photocatalytic activities

48
The photocatalytic activities of the thin films were evaluated by monitoring the decomposition of gaseous 2-propanol in air (25 °C).

Type: Method | Advantage: None | Novelty: None | ConceptID: Met8

49
2-Propanol was selected as the reactant because the oxidation of 2-propanol to acetone can determine the quantum yield.31

Type: Method | Advantage: Yes | Novelty: None | ConceptID: Met9

50
A thin film, 70 × 50 mm, was placed in a 0.8 dm3 Pyrex glass vessel.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

51
An O2(20%)–N2(80%) gas mixture, which was passed through a 30 °C water humidifier in order to adjust the relative humidity to 50%, was filled in a vessel.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

52
The headspace (20 cm3) of glass bottle reservoir, which contained liquid 2-propanol, was injected into the Pyrex glass vessel using a syringe.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

53
The initial concentration of 2-propanol was adjusted to 120 ppm.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

54
The photocatalytic decomposition of gaseous 2-propanol was evaluated while illuminating with ultraviolet (UV) and visible (VIS) light.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

55
A 10-W black light bulb (Toshiba Co., Tokyo, Japan) provided the UV illumination.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

56
The source for VIS illumination was a 250 W xenon lamp (LA-250Xe, Hayashi Watch-works Co. Ltd., Tokyo, Japan), which was used in conjunction with an optical fiber coupler, an UV cutting filter (Y-43, Toshiba Co. Ltd., Tokyo, Japan) and an IR cutting filter (V-40, Toshiba Co. Ltd., Tokyo, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp11

57
The number of absorbed photons in each film was fixed at 2.0 × 1014 (quanta s−1 cm−2) regardless of the illumination source.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

58
The amount of absorbed photons (Ia) was determined by eqn. (1). where I0 and α are the incident photon flux and the absorption percentage, respectively.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

59
The incident photon flux (I0) was measured by a spectro-radiometer (USR-40D, Ushio Co., Tokyo, Japan) and the absorption percentage (α) was obtained from UV-Vis spectra.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp12

60
The concentrations of 2-propanol, acetone, and carbon dioxide (CO2) were measured by a photoacoustic multi-gas monitor (model 1312, Innova, Ballerup, Denmark), which had a detection limit of approximately 0.1 ppm.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp13

61
The photocatalytic decomposition of ethylamine ions (CH3CH2NH3+) in an aqueous solution was also evaluated.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met12

62
Thin films were immersed in an aqueous solution of ethylamine ions.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met12

63
After bubbling N2 through the solution for 30 min, ultraviolet light illuminated the films.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met12

64
A capillary electrophoresis system (HP-3D, Hewlett Packard Co., California, USA) measured the concentration of ethylamine ions.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp14

65
The initial concentration of ethylamine ions was 10 ppm and the pH value of solution was 5.5 (20 °C).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

66
The UV illumination was provided by a 10-W black light bulb (Toshiba Co., Tokyo, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp15

67
The light intensity of UV illumination was 0.5 mW cm−2 or 1.0 mW cm−2, measured by a spectro-radiometer (USR-40D, Ushio Co., Tokyo, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp15

Results and discussion

68
Fig. 1 shows XRD patterns for the thin films.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

69
The crystal structures of pure TiO2 and the nitrogen doped TiO2 films annealed in NH3 below 600 °C all exhibited a single anatase phase.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

70
Annealing NH3 at 700 °C, however, changed the nitrogen doped TiO2 crystal phase to the cubic TiN phase.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

71
Fig. 2 shows SEM images for the surface of the thin films and the cross sections of pure TiO2 (E) and TiN film (F).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

72
Regardless of the annealing temperature in gaseous NH3, the surfaces exhibited nearly identical surface morphologies.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

73
The diameters of TiO2 particles were approximately 30 nm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

74
The films, which were 600 nm thick, were homogeneously deposited on glass substrates without cracks or pinholes.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

75
To investigate the doping density and state of nitrogen atoms, XPS spectra for N-1s band were measured (Fig. 3).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

76
A peak with the binding energy of 396 eV was observed for the nitrogen doped TiO2 films and the peak intensity depended on the annealing temperature in NH3.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

77
It has been reported that the peak at 396 eV is assigned to Ti–N bonding, whereas the peaks at 402 and 404 eV are assigned to molecularly chemisorbed N2.32

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

78
Prior to our XPS measurement, the surface of thin films was etched by Ar+ ion sputtering for 1 min, which eliminated chemisorbed or physisorbed N2 molecules on the surface.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

79
The XPS results indicate that the nitrogen atoms in the thin films were substitutionally doped into oxygen sites of TiO2 lattice.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

80
The doping densities of nitrogen atoms versus titanium atoms (N/Ti atom %) were 0% (pure TiO2), 0.9% (NH3 400 °C), 2.3% (NH3 600 °C), 36.4% (NH3 700 °C).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

81
The depth profiles of the nitrogen atoms in these films were evaluated using Ar+ ion sputtering, which indicated that the oxygen atom sites were homogeneously substituted by nitrogen atoms.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

82
Fig. 4 shows the absorption spectra of thin films.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

83
Nitrogen doped TiO2 films (annealed in NH3 at 400 °C and 600 °C) had shoulder peaks in the visible light region around 390–450 nm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

84
The pure TiO2 film does not absorb the visible light since the band gap of anatase type TiO2 is 3.2 eV.33

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

85
The TiN film was not transparent, owing to its metallic property.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

86
Fig. 5 shows the surface zeta potential versus pH for a pure TiO2 film and a film annealed in NH3 at 600 °C.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

87
The pH of the isoelectric point (IEP) for the pure TiO2 film was 5.8.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

88
The IEP for anatase TiO2 particles were previously determined by other procedures and ranged from 5.1 to 6..734–37

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

89
Therefore, the value of the pure TiO2 in the present paper is consistent with those measured by other procedures.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

90
However, the zeta potential of a nitrogen doped TiO2 film negatively shifted and the IEP decreased.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

91
Table 1 lists the zeta potentials for each film at a pH of 5.5.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

92
When the crystal phase remained anatase TiO2, the zeta potentials became more negative as the amount of nitrogen doping increased.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

93
When the crystal phase was transformed to the cubic TiN phase, the zeta potential, however, aproached zero potential (−4.65 mV) at a pH of 5.5.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

94
Previous studies reported that the IEP for TiN particles ranged from 4.0 to 5..038–40

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

95
Thus, the IEP of the TiN film in the present study is consistent with the literature.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

96
It is noteworthy that the surface zeta potentials of the nitrogen doped TiO2 films were more negative than that of pure TiO2, which indicates that the surface of nitrogen doped TiO2 films was highly acidic.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

97
It is known that OH groups on the TiO2 surface at terminal sites and those at bridging sites act as basic and acidic sites, respectively.41

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

98
The reported pKa value for OH groups at the terminal sites of TiO2 is 12.7 and that for OH groups at bridging sites is 2..934,42

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

99
Previous studies also reported that amounts of OH groups at terminal sites and those at bridge sites are nearly identical.43,44

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

100
Therefore, the IEP of TiO2 is a neutral value and the TiO2 exhibits amphoteric properties.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

101
The surface of nitrogen doped TiO2 films in this study, however, was highly acidic.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

102
Fig. 6 shows a possible surface structure for nitrogen doped TiO2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

103
Our XPS results indicated that the oxygen sites in the TiO2 lattice were substituted by the doped nitrogen atoms.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

104
Previous studies have reported that OH groups at terminal sites act as Brønsted acid sites on a TiO2 surface.45

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

105
The strength of the O–H bonds strongly affects the surface acidity.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

106
The strength between O–H bonding on metal oxides depends on the electronegativity of the metal ions, i.e. the strength between O–H bonding depends on the polarization of covalent electron pairs.46,47

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

107
It is presumed that the N–H bonding is weaker than the O–H bonding since the electronegativity of nitrogen (3.0) is less than oxygen (3.5).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

108
Therefore, the N–H sites of the nitrogen doped TiO2 surface act as acidic sites.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

109
In order to fully discuss the surface acidity of nitrogen doped TiO2, a comprehensive study on the states and amount of OH or NH groups should be conducted.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

110
The results of zeta potentials obtained in the present study, however, indicate that nitrogen atoms play an important role in controlling the surface adsorptive properties.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

111
Next, we have evaluated the photocatalytic decomposition of gaseous 2-propanol under light-limited conditions,31 to estimate the efficiency for the photocatalytically charged separation.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj5

112
Fig. 7 shows the concentration changes of gaseous 2-propanol, acetone, and CO2 for a pure TiO2 film under UV illumination.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

113
After 30 min, which was the time required for equilibrium between gaseous and adsorbed 2-propanol to be achieved, UV light illumination was initiated.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

114
The amount of generated acetone was equivalent to the amount of decomposed 2-propanol and very little CO2 was produced.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

115
The amount of CO2 was too small and insignificant to be considered.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

116
The oxidation reaction from 2-propanol to acetone is not a chain reaction.48,49

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

117
Hence, the quantum yield (QY) was calculated using eqn. (2).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met15

118
The number of absorbed photons obtained from eqn. (1) was fixed at 2.0 × 1014 (quanta s cm−2) under UV as well as VIS illumination.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met15

119
Table 2 lists the QY.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

120
When illuminating with UV light, the pure TiO2 film exhibited the highest QY and the QY decreased as the extent of nitrogen doping increased.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

121
Creating electron hole recombination centers such as oxygen vacancies50 caused the decrease in QY under UV illumination because the annealing procedure in a gaseous NH3 atmosphere is very reductive.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

122
Due to the metallic properties of the TiN film, which was obtained by annealing in NH3 at 700 °C, it did not display photocatalytic activity even under UV illumination.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs23

123
In contrast, when illuminating with VIS light, the nitrogen doped TiO2 films exhibited photocatalytic activity, but the pure TiO2 film did not.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

124
However, the QY values under VIS illumination were much lower than those obtained with UV illumination, although the number of absorbed photons was identical regardless of the illumination source.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

125
These results indicate that the photogenerated holes produced by the VIS light are localized at isolated levels between the valence and conduction bands.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

126
If the N-2p levels were mixed with O-2p orbital and the band-gap narrowed, then the photogenerated holes in the valence band would diffuse to the upper edge of the valence band by a radiative process.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

127
In these conditions, the oxidation power of photogenerated holes was supposed to be identical for both UV and VIS illumination.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

128
However, our experimental data exhibited that the QY of the nitrogen doped TiO2 film under VIS illumination was much lower than that under UV illumination.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

129
One possible speculation for the origin of the visible light activity is the oxygen vacancies.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

130
A previous study reported that oxygen vacancy states of TiO2 are located between 0.75 to 1.18 eV, which is below the minimum level of the conduction band.51

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

131
Therefore, the reduced TiO2−x can absorb a broad range of visible light above 500 nm.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

132
Indeed, nitrogen doped TiO2, which was annealed in NH3 at 600 °C, absorbed visible light above 500 nm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs25

133
The QY value under UV illumination was much lower than the pure TiO2 because the oxygen vacancies act as electron-hole recombination centers.50

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

134
In contrast, nitrogen doped TiO2 annealed in NH3 at 400 °C and pure TiO2 did not display optical absorption above 500 nm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

135
The UV-Vis spectrum of this film showed an absorption shoulder in the visible light region (390–450 nm), indicating that the absorption shoulder was due to isolated levels that consisted of N-2p orbitals in the band gap of TiO2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

136
We previously reported the optical absorption properties and QY values for nitrogen doped TiO2 powders and concluded that the isolated narrow band of N-2p orbitals, which is formed above the valence band, is responsible for the visible light activity.29

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met16

137
Fig. 8 shows the assumed band structure of nitrogen doped TiO2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

138
The isolated levels of the N-2p orbitals are situated above the valence band.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

139
Thus, holes produced by the VIS illumination are localized and have a slower mobility than those produced by UV illumination.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

140
Consequently, the photocatalytic activity under VIS illumination is much lower than that under UV illumination.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

141
Although the QY of the nitrogen doped TiO2 under VIS illumination was much lower than that under UV illumination, we have found the intriguing property of its surface acidity.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

142
The surface acidity plays an important role in determining its adsorptive property.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

143
Finally, we have evaluated the photocatalytic decomposition of aqueous ethylamine ions (CH3CH2NH3+) under light-rich conditions.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met17

144
Fig. 9 shows the changes in ethylamine ions under UV illumination for pure TiO2 and nitrogen doped TiO2 (annealed in NH3 at 400 °C).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

145
Previous studies reported that TiO2 photocatalyst could decompose ethylamine ions and produce several intermediates.52

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met18

146
It has also been reported that these intermediates could be completely mineralized into CO2, H2O, and HNO3 under UV illumination.52

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met19

147
In the previous study, the decomposition rate of the nitrogen doped TiO2 film was faster than those of the pure TiO2 film.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met19

148
In either film the decomposition rate was independent of the light intensity, indicating that the photocatalytic reaction proceeds under light-rich condition.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met19

149
As listed in Table 1, the zeta potentials at pH of 5.5 for pure TiO2 and nitrogen doped TiO2, which was annealed at 400 °C, were +15.5 and −46.9 mV, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs28

150
The pKa value of ethylamine is 10.63, thus almost all the ethylamine molecules (CH3CH2NH2) were dissociated into ethylamine ions (CH3CH2NH3+) in an aqueous solution.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

151
Ethylamine ions are positively charged in an aqueous solution at a pH of 5.5 and are supposed to be preferentially adsorbed onto the negatively charged surface of the nitrogen doped TiO2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

152
The concentration of ethylamine ions was very low (10 ppm), thus, the photocatalytic decomposition of these ions was provided by the light-rich conditions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

153
Under light-rich conditions, decomposition rates depend on the adsorptive property on the surface.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

154
Therefore, it is reasonable that the decomposition rate of the nitrogen doped TiO2 was higher than that of the pure TiO2 under UV illumination, even though the QY value of the nitrogen doped TiO2 (15.4%) obtained by the oxidation reaction of gaseous 2-propanol was slightly less than that of the pure TiO2 (18.5%) under UV illumination with the light-limited conditions.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

155
These results are quite interesting from not only a scientific standpoint, but also from an industrial one.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

156
Besides ethylamine ions, it is possible that nitrogen doped TiO2 can decompose other compounds and this photocatalyst has potential applications in water or air purification.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

Conclusion

157
We have fabricated nitrogen doped TiO2 films and have discovered that their zeta potentials became negative with the amount of nitrogen doping.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

158
The zeta potential of photocatalysts plays an important role in the adsorption property of aqueous contaminants.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

159
The effects of negatively charged nitrogen doped TiO2 surfaces were investigated by evaluating the photocatalytic decomposition of aqueous cations (CH3CH2NH3+) under UV illumination with light-rich conditions.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met20

160
The decomposition rate of nitrogen doped TiO2 was higher than that of the pure TiO2 because aqueous cations preferentially adsorbed onto the negatively charged surface of nitrogen doped TiO2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

161
To date surface state details of nitrogen doped TiO2 are unclear, but are currently under intensive investigation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

162
The present study, however, is quite valuable because it has the potential for scientific and industrial applications.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

163
Nitrogen doped TiO2 can efficiently adsorb the cations, owing to its negatively charged surface.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

164
Therefore, it is possible that this unique material will be used for water or air purification.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11