Electro-Optic Modulation Induced Enhancement in Photocatalytic

The Journal of Physical Chemistry C 2016 120 (1), 631-638 .... hollow tetragonal nanocones with large exposed (1 0 1) facets for excellent photocataly...
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Electro-Optic Modulation Induced Enhancement in Photocatalytic Activity of N‑Doped TiO2 Thin Films Avesh Kumar and T. Mohanty* School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India ABSTRACT: In the presence of ultraviolet light, TiO2 photocatalyst finds a lot of applications in degradation of organic and inorganic particles present in environment as well as in water. TiO2 nanoparticles can not be utilized effectively in presence of solar light since solar radiation has very little contribution in UV region. The photoresponse of TiO2 can be shifted to the visible region by nitrogen doping. In the nitrogen-doped TiO2 lattice, the oxygen vacancies are formed below the conduction band in the form of donor energy level. These vacancies increase with increasing nitrogen concentration in TiO2, which results in an increase in the trapping rate. This leads to enhancement of photocatalytic activity of nitrogendoped TiO2 film. Nitrogen doping results in the creation of defects in nitrogendoped TiO2, which in other way leads to a modification of the work function of thin films. The observed changes in work function of these films, measured using scanning Kelvin probe microscopy, indicates the shifting of Fermi level with the introduction of defects. The study of work function as well as surface properties of thin films helps in better understanding of the photo activity of nitrogen-doped TiO2 film.

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conduction band minimum. When the thin film is irradiated by UV light (λ < 390 nm), electrons are excited from O 2p to Ti 3d energy levels (CB).7 Under UV irradiation photocatalytic efficiency of nitrogen doped TiO2 (N-TiO2) remains same as that of undoped TiO2 catalyst. While under visible light irradiation, electrons are excited from N 2p to Ti 3d energy levels7 leading to enhancement of photocatalytic activity. Similarly under irradiation of solar light (consisting of both UV and visible light), the electrons are excited from both O 2p and N 2p energy levels.10 Thus N-TiO2 NPs shows higher photocatalytic activity under solar irradiation than the UV light irradiation. In addition to creation of energy level, oxygen vacancies are created below conduction band.11−13 The presence of oxygen vacancies below CB is expected to make changes in work function of N-TiO2 thin film and hence shifting of Fermi level. The work function of nitrogen-doped TiO2 thin film can be measured using scanning Kelvin probe microscopy technique which is based on vibrating parallel capacitor mode. When two metallic plates are electrically in contact, the electrons flow from lower work function to higher work function until their Fermi levels equalize. This flow of electrons creates a potential between tip and sample named as contact potential difference (CPD).14 The difference in the surface potential creates electrostatic force which is the rate of change of the energy with tip−sample separation distance. The force can be compensated by applying an external bias dc voltage to the sample which has same magnitude as CPD with opposite direction.3,15,16 The external compensated voltage

INTRODUCTION Titanium dioxide is one of the most promising materials for its applications in photocatalysis, solar cell, environmental, and water purification.1−4 Photocatalytic efficiency of pure TiO2 under solar irradiation is very low due to narrow UV region (300−390 nm) of the incoming solar energy.5 But there is a possibility of improving photocatalytic efficiency by doping nitrogen in TiO2 nanoparticles (NPs).6 Nitrogen (N) atoms are incorporated into the crystalline lattice of TiO2 NPs due to its comparable atomic size with oxygen, which changes the electronic band structure and optical property of TiO2.7,8 Photocatalytic efficiency of TiO2 thin films can be estimated from photodegradation of methyl blue under light irradiation. Nitrogen-doped TiO2 thin films exhibit a greater photocatalytic efficiency and strong absorption under solar light irradiation. In nitrogen-doped TiO2, oxygen atoms present in substitutional level are replaced by nitrogen atoms from their regular lattice site, leading to formation of energy level as well as oxygen vacancies (defects) below the CB. Whereas in interstitial position, N atom fit into the open space between the atoms of the lattice structure of TiO2 NPs. Incorporation of nitrogen causes formation of deep energy level located at 1.18 eV below the conduction band (CB) and N 2p impurity energy level above O 2p energy level, extending to width of valence band.7 Since N 2p state has a higher potential energy or less binding energy above valence band (VB) maximum, mixing of N 2p states with O 2p states leads to narrowing of band gap of nitrogen doped TiO2 film.7−9 Optical absorption spectra are extended to the visible light region (red shift) which can be estimated from UV−vis absorption measurement. It is reported that the electron emission from TiO2 is enhanced by nitrogen doping due to presence of oxygen vacancies below the © 2014 American Chemical Society

Received: October 21, 2013 Revised: March 11, 2014 Published: March 11, 2014 7130

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irradiation for 60 min using TiO2 and different percentages of N-TiO2 catalyst. In other experiments, MB solution with TiO2 and N-TiO2 catalysts was irradiated for different amounts of time, with an interval of 30 min under solar light. The variations in the absorbance of MB under irradiation were monitored using a Shimadzu-UV-2450 spectrophotometer. Photodegradation of MB solution can be estimated by the following relation:1

reduces the Kelvin current to zero. As both work function as well as photocatalytic activity is surface phenomena a change in work function (i.e., surface potential) of N-doped TiO2 thin film can be correlated with its photocatalytic activity. In the present work, we report a correlation of surface potential as well as shifting of Fermi level with photocatalytic activity of N-doped TiO2 film. The experimentally measured work function value of TiO2 decreases with increasing nitrogen dopant concentration indicating shifting of Fermi level toward conduction band and enhance conductivity of N-TiO2 film. Enhancement of photocatalytic efficiency by nitrogen doping in TiO2 is observed. The mechanism of charge carriers generation in N-TiO2 under UV and solar light excitation is discussed. A detailed study is presented on variations of contact potential difference as well as photocatalytic efficiency of TiO2 with doping of different concentration of nitrogen.

⎛ A ⎞ degradation percentage = ⎜1 − t ⎟ × 100 A0 ⎠ ⎝

(1)

where A0 is the initial absorbance of methyl blue without any exposure and At is the absorbance of MB after photo irradiation for time t.

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RESULTS AND DISCUSSION X-ray Diffraction Studies. X-ray diffraction (XRD) studies of TiO2 and nitrogen-doped TiO2 thin films were carried out using an X-ray diffractometer (PAN analytical X’pert PRO) to determine the crystalline phase of samples. The XRD spectrum for samples is shown in Figure 1. The anatase phase of the TiO2

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EXPERIMENTAL SECTION Materials. Titanium isopropoxide (TIP, C12H28O4Ti), acetic acid (C2H4O2), ammonium chloride (NH4Cl), and methyl blue were purchased from Sigma-Aldrich. Ethanol was purchased from Merck. The single crystal silicon ⟨100⟩ wafer was used for thin film preparation. Synthesis Technique. TiO2 solution was prepared by sol− gel method at room temperature, a mixing solution of 40 mL of ethanol, 10 mL of acetic acid, and 10 mL of titanium isopropoxide were added in a beaker and stirred vigorously for 3 h. In the case of N-TiO2, different mol percentages of NH4Cl were added with mixing solution and stirred vigorously on a magnetic stirrer for 3 h. These solutions were used for thin film preparation. The nanocrystalline thin films were grown on silicon (Si) substrates, followed by spin-coating technique. The TiO2 and N-TiO2 films were dried in air at 100 °C for 1 h and were cooled at room temperature. Again, these films were heated at 500 °C for 3 h with heating rate of 1.66 °C min−1, then the films were cooled at room temperature. Characterizations. TiO2 and N-TiO2 thin films are analyzed using different characterization techniques. Such as, the crystalline phase and crystallinity of TiO2 and N-TiO2 thin films were identified by glancing angle X-ray diffraction (GAXRD) technique with Cu Kα (1.54 Å). The glancing angle of the X-ray with respect to the sample surface was fixed at 1°. The scan rate was ∼1°/min. The morphology and structural changes in N-TiO2 thin films were observed by highresolution transmission electron microscopy (HRTEM) using 200 keV JEOL 2100F TEM. The work function (WF) of TiO2 and N-TiO 2 thin films with a varying dopant mole concentration of nitrogen were carried out using scanning Kelvin probe (SKP) microscopy from KP Technology, United Kingdom.17 Optical characterizations of these films were carried out using UV/visible absorption technique (UV 2401 PC, Shimadzu) in transmittance mode to determine the band gap of nitrogen-doped TiO2 thin films with different concentrations. Measurements of Photocatalytic Activity. The photocatalytic efficiency of TiO2 and N-TiO2 nanocrystalline thin films was estimated in terms of the degradation of methyl blue under light exposure. A photocatalysis experiment was performed in a glass beaker containing 8 mL of methyl blue (MB; C37H27N3Na2O9S3) solution, and only one thin film was dispersed in one beaker. A xenon lamp (15 W lamp, Phillips, G15T 8) was used as the UV light irradiation source. Photodegradation of MB was carried out under UV and solar

Figure 1. XRD spectra of undoped and nitrogen-doped TiO2 thin films.

nanocrystalline is identified from its diffraction peaks at (101), (103), (200), (105), (211), and (204) in the XRD spectrum. This phase does not vary with dopant concentration of nitrogen in TiO2. A small peak at angle (2θ) = 31.53°, marked in the circular shape, corresponds to the nitrogen doped in TiO2, as reported by other research groups.18 The crystalline size of the samples can be calculated using the Scherrer relation.1 These were found to be ∼13 and ∼22 nm for TiO2 and nitrogendoped TiO2 films, respectively, which confirms the nanocrystalline structure for anatase TiO2 and N-doped TiO2. The (101) plane peak intensity of N-doped TiO2 is increased as compared to undoped TiO2, which demonstrates that the crystallinity of TiO2 could be improved by N doping. The 2θ peak position of N-TiO2 (7% N concentration) at 52.5° has a minor shift from it’s exact value, which may be due to lattice compressive strain, because the atomic radius of N is smaller than the Ti radius.19,20 This type of shift has been observed by other groups, which suggests that the oxygen or Ti atoms in the TiO2 lattice may be substituted by nitrogen atoms.20−22 Transmission Electron Microscopy Studies. The morphological structure of films is investigated using TEM 7131

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Figure 2. TEM images of TiO2 and nitrogen-doped TiO2 thin films: (a) TiO2, (b) 1% nitrogen, (c) 3% nitrogen, (d) 5% nitrogen, (e) 7% nitrogen in TiO2 film, and (f) lattice spacing of TiO2 thin film.

Figure 3. TEM-EDAX of nitrogen-doped TiO2 film: (a) image shows the distribution of Ti, N, and O particles with red, green, and yellow colors, respectively, and (b) EDAX analysis of nitrogen-doped TiO2 film.

spectrum was taken at the selected positions of the sample in order to confirm the chemical composition of the nanocrystalline TiO2 thin film. The observed elements are Ti, O, and N present in the given sample. Morphology and Work Function Measurements by SKP Microscopy. The estimation of the work function of the material can be used for investigating its electronic behavior. The work function can be expressed as1

technique. TEM images of TiO2 and N-doped TiO2 are shown in Figure 2. These images confirmed the spherical shape of the nanosized particles. The sizes of TiO2 and the nitrogen-doped TiO2 particles are 14 ± 2 and 20 ± 2 nm, respectively. Figure 2f shows the interplanar spacing of TiO2 thin film to be 0.165 nm. This matches exactly with the spacing of plane (105) of anatase TiO2, as already indicated by the XRD spectrum. The colored Figure 3a shows the distribution of titanium (Ti), oxygen (O), and nitrogen (N) atoms with red, green, and yellow colors, respectively. It has been reported by different research groups23−25 that the incorporation of a N atom into the TiO2 lattice modifies the electronic band structure and changes the absorption spectrum of TiO2. In our TEM-EDAX experiment, the presence of Ti, O, and N atoms with color identity in N-TiO2 is observed. Hence, we can expect an introduction of nitrogen into the TiO2 lattice. The energydispersive X-ray analysis (EDAX) image of the N-doped TiO2 nanocrystalline thin film is shown in Figure 3b. This EDAX

Φsample = χs + (EC − E F)

(2)

where χs is the electron affinity of the sample, and EC and EF are the conduction band energy and Fermi energy of the semiconductor, respectively. The Fermi energy level is located in the band gap of the semiconductor, which is the chemical potential for electrons. The Fermi level (quasi) is related to the density of conduction band electrons and is given by1,26 7132

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Figure 4. (a) CPD image of TiO2 thin film and (b−h) CPD images of N-TiO2 thin film at different concentrations (mol %) of nitrogen: (b) 1, (c) 2, (d) 3, (e) 4, (f) 5, (g) 6, and (h) 7% nitrogen.

nc = NCe−(EC − EF)/ kBT

WF was found to be 4.92 eV which lies in the range of 4 to 6 eV and matches well with the reported results.1,3 The observed CPD value for N-TiO2 nanocrystalline thin films increase with increasing dopant concentration of nitrogen for N-TiO2 is shown in Figure 4b−h. Average value of CPD versus nitrogen dopant % concentration for N-TiO2 is plotted, as shown in Figure 5. The CPD value for N-TiO2 films increases from 246.72 to 888.22 mV with increasing dopant concentration of nitrogen. In our previous work, we have observed already that the CPD value of TiO2 increases with increasing concentration of defects.3 It is already reported that defect concentration and nitrogen concentration are proportional to each other.28 Therefore, in the present case, the observed increase in CPD value may be due to an increase in the concentration of defects in N-doped TiO2. These nitrogen doping induced surface defects are distributed randomly on the surface of TiO2, causing a change in the surface morphology.29

(3)

where NC is the density of states in a conduction band and nc is the density of electrons. The estimation of Fermi level shifting is measured in terms of WF measurement using scanning Kelvin probe microscopy.27 The change in WF of the TiO2 thin film is related from the variation of the contact potential difference (CPD) between the TiO2 and the Au tip as reference. It is given by3 VCPD =

(Φtip − Φsample) e

(4)

where Φtip and Φsample are the work functions (5.1 eV) of the Au tip and TiO2 sample, respectively. The CPD values of pure TiO2 and nitrogen-doped TiO2 nanocrystalline thin films with varying concentrations are shown in Figure 4. The average CPD value for nanocrystalline TiO2 thin film was calculated to be 179.92 mV (Figure 4a). The 7133

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Optical Analysis and Band Gap Calculation. UV−vis absorption spectra for TiO2 and nitrogen-doped TiO2 thin films are shown in Figure 6. The red shift of the absorption

Figure 5. Variation of the CPD values with varying nitrogen concentrations of N-TiO2 thin films.

Before explaining the origin of defects in N-TiO2, it is necessary to have detailed description of defects presenting in TiO2. Titanium dioxide possesses point defects in the form of titanium interstitial/titanium vacancies or oxygen vacancies or both. Ti interstitials (Ti3+) are associated with donor energy levels below the conduction band of TiO2. These are the positive ionic defects, which act as trapping sites for electrons.30−32 In addition, titanium vacancies presented at or near the surface of TiO2 are identified as the active sites for water adsorption.32 These vacancies act as negative ionic defects; therefore, these defects may be considered in terms of the reactivity between TiO2 and water.30,32 The modifications in electronic structure and defect type, as well as concentration and surface structure, for TiO2 can be induced by doping of nitrogen.33 Generally, three lattice oxygen atoms of N-TiO2 crystal lattice are replaced by two nitrogen atoms from their lattice site. In this one, oxygen vacancy is formed below the conduction band in the form of donor energy level. These donor levels get shifted below the CB with increasing nitrogen concentration for N-TiO2. With the introduction of nitrogen dopant concentration, a deep energy level of ∼1.18 eV is generated below the conduction band of TiO2. Consequently, the defects concentration is also increased with increasing nitrogen dopant concentration for N-TiO2 film.7,8,34 In our previous work, we have observed a linear variation of CPD with concentration of defects.3 Therefore, as per our expectation, a similar variation is observed in the present case, where the CPD value increases with increasing nitrogen dopant concentration for N-TiO2. Hence, the CPD and defects both are linearly proportional to each other with increasing the nitrogen dopant concentration. The experimental results of variation of CPD with nitrogen concentration are very well fitted to a quadratic polynomial, with an error of 2% (Figure 5); it is given as Y = Y0 + C1X + C2X2 + C3X3

Figure 6. UV−vis absorption spectra of nitrogen doped TiO2 thin films with different nitrogen concentrations. Inset shows the plot of (αhν)1/2 vs hν for these films.

spectra may be due to a variation in the dopant concentration of nitrogen. The band gap of TiO2 and N-TiO2 can be estimated from the slope of Tauc’s plot [(αhν)1/2 vs hν] on the x-axis, as shown in the inset in Figure 6. The Tauc’s relation is given by the relation35 αhν = C(hν − Eg )n

(6)

where α is the optical absorbance coefficient, Eg is optical band gap of the material, C is the material-dependent absorption constant, and hν is the incident photon energy in eV. The value of “n” used in Tauc’s relation governs the transition processes. Its value depends on the nature of the transition. Its values are 1/2, 3/2, 2, and 3 for direct allowed, direct forbidden, indirect allowed, and indirect forbidden transitions, respectively.35,36 For the present case, the value of n is taken as 2. The calculated band gap is found to be 3.19, 3.10, 3.06, 3.03, and 3.05 eV for 0, 1, 3, 5, and 7% dopant concentrations of nitrogen in TiO2, respectively. It can be seen that the band gap of the N-TiO2 slowly decreases with an increasing concentration of nitrogen in TiO2. At 7% N, it slightly increases with dopant concentration, which indicates the modifications in TiO2 with nitrogen dopant concentrations. This modification may be due to the substitution of the lattice oxygen by a nitrogen atom in TiO2 thin films, thus, narrowing the band gap of TiO2 by mixing the N 2p and O 2p states.37,38 It can be said that the defect levels are responsible for the change in band gap with respect to that of pure TiO2. The rate of photocatalytic reaction is proportional to the number of photons absorbed by the photocatalyst and the efficiency of the band gap transition. The narrow band gap produces more photo excited electrons, which are favorable for photoactivity. Thus, changes in band gap play an important role in both visible light absorption and photocatalytic activity. Photodegradation of Methyl Blue. Photocatalytic efficiency of TiO2 and N-TiO2 thin films were evaluated by degradation of methyl blue solutions under UV and solar light irradiation. The rate of degradation was estimated with change in intensity of absorption peak. The decrease in the intensity of absorption peak of methyl blue indicates the degradation of the dye molecule. The maximum intensity of absorbance peak for MB is about 664 nm. Therefore, this peak is used to determine

(5)

where Y0 = 173.67, C1 = 83.82, C2 = 11.94, and C3 = −1.36, with the R2 value as 0.98. The value of the intercept on the Yaxis is 173.67, which is close to the CPD value (179.92 mV) of pure TiO2. The CPD of N-doped TiO2 increases with an increasing concentration of nitrogen due to the evolution of defects. The increase in CPD, in other words, the decrease in work function of TiO2, with increasing nitrogen dopant concentration (eq 2) indicates the shifting of the Fermi level toward the conduction band. 7134

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Figure 7. (a) Absorbance spectra of MB after UV exposure for 60 min in the presence of pure TiO2 and different mol % concentrations of N-doped TiO2 thin films, and (b) photodegradation of methyl blue on N-doped TiO2 thin films.

Figure 8. (a) Absorbance spectra of methyl blue after solar light exposure for 60 min in the presence of pure TiO2 and different mol % concentrations of N-doped TiO2 thin films, and (b) photodegradation of methyl blue on N-doped TiO2 thin films.

photodegradation efficiency of MB remains nearly saturated below a 5% concentration of nitrogen for N-TiO2 thin film. Beyond this, photodegradation efficiency is decreased with increasing concentration of nitrogen (Figure 7b). Figure 8a shows the absorption spectra of methyl blue solution after solar light irradiation with N-TiO2 catalyst. The effect of dopant concentration of nitrogen for N-TiO2 film on photodegradation of MB under solar irradiation is shown in Figure 8b. The photodegradation percentage increases from 8.93 to 79.86 with increasing nitrogen dopant concentration up to 5% for N-TiO2 because the trapping rate of charge carrier increased with nitrogen doping due to an increase in oxygen vacancies. The observed data shows an enhancement in photocatalytic activity of N-TiO2 than TiO2 nanocrystalline thin film. On a further increase in nitrogen concentration of NTiO2 the photodegradation percentage decreases, as deep trapping centers play a role of recombination center for charge carrier.7,9 The degradation percentage variation with nitrogen doping concentration is fitted to a quadratic polynomial, and it is given as

the degradation of MB. The photodegradation percentage of MB was studied in terms of changes in the absorption spectra using eq 1. UV−visible absorption spectra of MB and the effect of nitrogen concentration for N-TiO2 on the photodegradation of MB under UV light irradiation are shown in Figure 7a and b, respectively. It can be seen from Figure 7b that the photodegradation efficiency of MB remains nearly saturated for low concentrations (below 5%) of nitrogen for N-TiO2 nanocrystalline film. This is because under UV light irradiation, the photogenerated electrons are excited from valence band to conduction band (Ti 3d ← O 2p), similar to TiO2. Therefore, under UV light irradiation, the behavior of the N-TiO2 catalyst is similar to the undoped TiO2 catalyst. At a certain concentration (above 5%) of nitrogen for N-TiO2, the photocatalytic activity of N-TiO2 is decreased and it acquires a lesser value than that for TiO2 film. Because the population of surface-trapped electrons is decreased due to deep trapping by additionally induced oxygen vacancies and the associated defect states below the CB.7,8,39 Hence, the photocatalytic activity of N-TiO2 is decreased. The degradation percentage variation of MB with nitrogen doping concentration is fitted to a quadratic polynomial; it is a well fitted equation and is given by

Y = Y0 + B1X + B2 X2

Y = Y0 + A1X + A 2 X2

(8)

where Y0 = 13.55, A1 = 26.05, and A2 = −2.60. This equation can be written in vertex form, as given below

(7)

Y = A1(X − h)2 + k

where Y0 = 19.53, B1 = 4.22, and B2 = −0.73. The maxima of the curve calculated from coefficients B1 and B2 are found to be 25.52, which appears in the range between 21.63 to 25.75, for corresponding dopant concentrations of 0 to 5%. It is observed in our experiment that, under UV light irradiation, the

h=− 7135

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(e− − h+) rapidly diffuse on the surface of TiO2 unless they are trapped or recombine.7,9 The CB electrons can transfer to new Ti3+ states and then to deep states of lower potential below CB. The trapped electrons are localized at Ti3+ states or on surface sites in CB, while trapped holes are localized at oxygen anion (O−) centers in VB. Ti3+ as well as surface trapped electrons and surface trapped holes both contribute to photo absorption in the N-TiO2 photocatalytic mechanism (Figure 10). The charge transfer process can be expressed as follows:31,41−43 Charge carrier generation:

where (h, k) is the vertex of the parabola. The calculated values of vertex (h, k) are found to be (5, 79.8). This is the maximum value (79.8%) of degradation at 5% concentration of nitrogen for N-TiO2. Therefore, after a limit (5%) of nitrogen concentration, the photodegradation of MB is decreased. Figure 9 shows the calculated values of kinetics of methyl blue with linear fitting under solar light irradiation. The

TiO2 + hv → e− + h+

(10)

Charge carrier trapping: e− + Ti4 + → Ti 3 + h+ + O2 − → O−

trapped electron trapped hole

(11) (13)

4+

where Ti is the lattice trapping sites for an electron. Approximately 90% of the photo excited charge carriers can recombine and energy release44 in term of heat. The remaining few charge carriers (i.e., electrons) are trapped by defect active sites (surface traps, shallow or deep traps) because the defect states on the surface of catalyst act as trapping sites.31 The trapped electrons are transferred to absorbed oxygen molecules (O2) on the surface of catalyst, which act as electron scavengers and produce superoxide anion radicals. The role of O2 in electron scavenging is due to higher in redox potential of O2 (−0.33 eV vs NHE: normal hydrogen electrode) than the redox potential (−0.6 V vs NHE) of conduction band electrons of TiO2.45 Anion radicals (O2•−) form OH•. On the other side, holes are trapped by defect active sites and these trapped holes react with the OH− group or H2O molecules producing OH• radicals, which are responsible for the degradation of organic pollutant.1 Higher the OH− content on the surface of TiO2, more the OH• is produced, which leads to the enhancement of the photocatalytic activity of TiO2. Hence, holes, OH• and O2, can play an important role in the photocatalytic reaction mechanisms. The photomechanism of N-doped TiO2 film under solar light irradiation is shown in Figure 10. We will now discuss the effect of nitrogen concentration on efficient electron trapping in N-TiO2 thin film under solar irradiation. The donor energy levels associated with defects are shifted below the CB of TiO2 with increasing nitrogen dopant concentration for N-TiO2. In N-doped TiO2, deep energy levels, 1.18 and 1.20 eV, of oxygen vacancies are generated below the TiO2 conduction band.6,7 It is already reported28 that the defect concentration increases with increasing nitrogen

Figure 9. Kinetic rate constant analysis of methyl blue for undoped and N-doped TiO2 thin films after solar light exposure for different times.

photocatalytic degradation can be described by the first order kinetics as40 ⎛A ⎞ ln⎜ 0 ⎟ = kt ⎝ At ⎠

(9)

where k is the reaction rate constant of photocatalytic degradation of MB. The correlation coefficient for the fitted line was calculated to be R2 = 0.96123 and 0.97679 for undoped and N-doped TiO2, respectively. The reaction rate constant was calculated to be 1.2 × 10−4 s−1 and 4.4 × 10−4 s−1 for undoped and N-doped TiO2 thin films, respectively. It is obvious that the rate constant k increases with increasing concentration of nitrogen dopant. Mechanism for Photocatalytic Activity of N-Doped TiO2 Film. When N-TiO2 photocatalyst is exposed to UV light, the electrons are excited from O 2p states to conduction band (Ti3d ← O2p), leaving behind holes. Whereas, after visible light irradiation of N-TiO2, it is observed that the electrons are excited from N 2p states to conduction band (Ti3d ← N2p), leaving behind holes in the N 2p states. These charge carriers

Figure 10. Schematic illustration of a photocatalytic mechanism of N-doped TiO2 under solar irradiation. 7136

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Notes

dopant concentration. Therefore, the CPD values are increased with N dopant concentration. Under solar light irradiation, the defects are generated in N-doped TiO2, which acts as the electron trapping center. The concentration of the electron trapping center increases with increasing N dopant concentration, leading to an increase in absorbed sites for oxygen molecules. Thus, the CPD value and photocatalytic efficiency of N-TiO2 increased simultaneously with dopant concentration up to a certain limit (5%) of nitrogen concentration. Beyond this limit, the CPD value is increasing due to defects in evolution with incorporation of nitrogen as dopant into TiO2. But under solar light irradiation, it is expected that the defects present in the energy level (nearly in middle of band gap) act as recombination center for photoexcited electrons as well as holes. The energy level in middle of band gap strongly interacts with both CB and VB, thus, reducing the photocatalytic activity.7,31,34,46 Therefore, the photocatalytic activity of NTiO2 film decreases beyond an optimal limit of nitrogen concentration in TiO2. It is observed that the CPD value and photocatalytic activity are inversely proportional to each other above 5% concentration of nitrogen. Increase in CPD value (i.e., decrease in WF) indicates shifting in Fermi level toward conduction band of N-TiO2 film with dopant concentration of nitrogen (eqs 2 and 3). After certain dopant concentration, the defects generated in N-doped TiO2 act as recombination centers for electrons and holes,6,28 which leads to a decrease in photocatalytic activity. Thus, the position of the Fermi level plays an important role in enhancing the photocatalytic efficiency of TiO2.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors T.M. thanks DAE-BRNS, Mumbai, India, for financial support to purchase scanning Kelvin probe (SKP) unit via Project No. 2008/37/9/BRNS. The authors express their gratitude to Prof. Subhasis Ghosh and Dr. Sobhan Sen, of SPS, JNU for UV/visible absorption study of films and methyl blue, respectively. We are thankful to AIRF, JNU, for providing XRD and TEM characterizations. A.K. is thankful to the council of scientific and industrial research (CSIR), India, for providing SRF fellowship.

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CONCLUSIONS In conclusion, defects are generated below the conduction band of N-doped TiO2 and with increase in nitrogen dopant concentration leading to decrease in band gap. The donor states as well as number of defects increases with increasing nitrogen concentration in TiO2. Electron emission from TiO2 is enhanced by nitrogen doping due to the presence of donor states below the conduction band minimum. It also results in a decrease in work function and hence increases in CPD of NTiO2 film. The CPD value increases linearly with increasing concentration of nitrogen in N-TiO2, indicating the shifting of Fermi level toward conduction band. Simultaneously, the trapping rate of charge carrier increases with increasing nitrogen dopant concentration due to an increase in the defects concentration up to a certain limit. Beyond this limit, the trapping rate decreases due to a deep energy level by additionally induced oxygen vacancies. The oxygen vacancies of deep level act as recombination center, reducing the charge carrier lifetime. Therefore, the decay rate decreases with increasing nitrogen concentration. Under UV light irradiation the photocatalytic activity of N-TiO2 remains nearly unchanged with a variation of nitrogen concentration, whereas in the presence of solar irradiation the photocatalytic activity of NTiO2 increases up to the 5% of nitrogen concentration in TiO2. Nitrogen-doped TiO2 shows excellent photocatalytic activity under solar irradiation than the UV irradiation.

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