Thin Films - American Chemical Society

L. K. Randeniya,* A. Bendavid, P. J. Martin, and E. W. Preston. CSIRO Materials Science and Engineering, P.O. Box 218, Bradfield Road, Lindfield, NSW ...
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18334

J. Phys. Chem. C 2007, 111, 18334-18340

Photoelectrochemical and Structural Properties of TiO2 and N-Doped TiO2 Thin Films Synthesized Using Pulsed Direct Current Plasma-Activated Chemical Vapor Deposition L. K. Randeniya,* A. Bendavid, P. J. Martin, and E. W. Preston CSIRO Materials Science and Engineering, P.O. Box 218, Bradfield Road, Lindfield, NSW 2070, Australia ReceiVed: July 27, 2007; In Final Form: September 23, 2007

Amorphous TiO2 thin films were synthesized using a pulsed direct current plasma deposition technique. The films were prepared in the discharges of Ar, Ar + O2, and Ar + N2. The original and annealed samples were characterized using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, ultraviolet and visible spectroscopy (UV-vis), and electrochemistry. The presence of oxygen was found to be essential in the annealing medium for the formation of polycrystalline films. By optimizing the experimental conditions, we obtained TiO2 films that showed 80% incident photon conversion efficiency for splitting of water near 300 nm. Films containing up to 12 atom % N were obtained when mixtures of Ar and N2 were used as the plasma source gas. Although there is evidence for the presence of Ti-N bonds in the amorphous film, the annealing in the presence of oxygen to obtain polycrystalline films led to decomposition of these bonds. The resulting polycrystalline films contained 1-2 atom % of N (XPS peak at 399.5 ( 0.5 eV) and showed visible light absorption. However, in contrast to recent reports in the literature for powdered materials with similar XPS and UV-vis characteristics oxidation of water or formic acid could not be achieved using these films under visible light illumination. There is evidence that the holes generated in the occupied N 2p midgap levels are recombining efficiently at the carbon-impurity sites.

1. Introduction Titanium dioxide (TiO2) continues to be a material of great interest to researchers. Its photocatalytic properties, biocompatibility, and photoinduced superhydrophilic properties have been extensively investigated. Its potential importance in technologies such as air and water purification, industrial waste management, self-cleaning surfaces, and bioactive materials has been well documented.1,2 A main deficiency of TiO2 as a photocatalyst is the wide band gap (3.0-3.2 eV), which limits its light harvesting capacity to about 1.3-2.3% of the incoming solar radiation at the surface of the Earth. Therefore, various doping techniques have been investigated to lower the band gap. Anion doping with elements such as C, N, and S is currently envisaged as having the potential to improve the light-harvesting capacity of TiO2. Chemical vapor deposition (CVD) techniques are attractive because of their ability to produce materials under nonequilibrium conditions. They also give good step coverage at low deposition temperature. A great majority of the metal-organic plasma-assisted chemical vapor deposition (MO-PACVD) techniques have employed radio frequency generators with capacitive coupling to obtain the discharge conditions.3, 4,5,6 However, the radio frequency techniques require fairly expensive power generators and their application for deposition in large areas is limited. Also, there is only a small number of studies where doping of TiO2 with anions (e.g., C, N, S) has been reported using MO-PACVD methods. In this paper, we report the synthesis of TiO2 and N-doped TiO2 thin films using a pulsed direct current PACVD technique. This technique has advantages over the conventional radio * To whom correspondence should be addressed. E-mail: Lakshman. [email protected].

frequency (RF) methods in that the cost of the power generator is lower and the potential for scaling up for industrial applications is higher. The discharge was produced using a sequence of direct current (dc) voltage pulses. The magnitude, frequency, and the polarity of the pulses can be easily varied over a range of values. Therefore, this technique is useful in producing a range of plasma conditions for novel material synthesis. The technique has the potential for deposition of large areas and therefore is attractive for industrial applications. We find that polycrystalline TiO2 films with very high incident photon conversion efficiency (IPCE) can be produced at relatively high-deposition rates using this technique. We also prepared N-doped TiO2 polycrystalline films with 1-2 atom % of N, which showed visible light absorption. However, in contrast to recent reports in the literature for powdered materials with similar X-ray photoelectron spectroscopy (XPS) and ultraviolet and visible spectroscopy (UV-vis) characteristics, oxidation of water or formic acid could not be achieved using these films under visible light illumination. 2. Experimental Section A schematic of the PACVD deposition system is shown in Figure 1. The substrate electrode inside of the stainless-steel chamber was powered by a dc pulse generator (Rubig Model MP120) operated in the voltage range of 300-420 V. A sequence of ten 10 µs negative voltage pulses separated by 10 µs intervals is applied to the electrode to generate the plasma. The sequence of ten negative pulses is separated by a single positive voltage pulse of the same magnitude and duration.7 This sequence of pulses is repeated through the experiment. The substrates, namely, fluorine-doped tin oxide-conducting glass and semiconductor grade silicon (100), were firmly secured on to a steel disc fitted to the end of the Cu electrode (see Figure

10.1021/jp075938u CCC: $37.00 Published 2007 by the American Chemical Society Published on Web 11/17/2007

PACVD Synthesis of TiO2 and N-Doped TiO2 films

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18335

Figure 1. A schematic of the MO-PACVD apparatus used for the syntheses of TiO2 films.

TABLE 1: Typical Flow Conditions Used for Film Depositions

Ar Ar/O2 Ar/N2

primary gas flow rate

organic precursor carrier gas flow rate

20 sccm (Ar) 20 sccm (Ar) 20 sccm (N2)

15 sccm (Ar) 15 sccm (Ar) 15 sccm (Ar)

other gas flow rate 5 sccm (O2)

1). No external heating of the substrates was implemented in the current set of experiments. Prior to deposition, the substrates were cleaned in acetone, ethanol, and deionized water using an ultrasonic bath. The gases were introduced into the chamber through a gas distributor using mass flow controllers. The pressure inside the chamber is maintained at a chosen value between 65 and 185 Pa (measured by a capacitance manometer) with the aid of a pumping system equipped with a throttle valve, a roots blower and a rotary pump. The throttle valve allows the total pressure of the chamber to be set independent of the gas flow. The glass vessel containing the metal-organic precursor, titanium isopropoxide (Sigma Aldrich 97%), was heated to 80 °C in a water bath. Argon was used as the carrier gas for the liquid precursor. The stainless steel tubing connecting the glass vessel containing the liquid precursor and the chamber was heated to 100 °C to prevent condensation. During a typical experiment, the chamber is pumped down to a pressure below 5 × 10 -4 hPa. Argon gas is then introduced and a plasma is generated by applying the voltage pulse sequence. After 10 min of etching of the substrates to remove physisorbed contaminants, the liquid precursor and the other relevant gases are introduced into the chamber. For experiments in Ar/N2 plasmas, a nitrogen flow is introduced, and the Ar flow is turned off or reduced prior to the introduction of the liquid precursor and the carrier gas (Ar). Typical flow rates for experiments in different discharge conditions are shown in Table 1. Depending on the required thickness of the film, the deposition time varied from 30 min to 2 h. The film thickness was measured using a step-edge profilometer (Sloane Instruments Dektak 3030). The residual stress in the films was determined by measuring the radius of curvature of the substrate before and after deposition using the profilometer. The composition of the films was determined by XPS in a SPECS150 system operated with Mg KR X-ray source.8

The peaks were referenced to the adventitious C 1s peak at 284.6 eV to compensate for surface charging. The fitting of the peaks to Gaussian/Lorentzian product formula with a Shirley background9 was carried out using CASA-XPS V2.3.13 software. X-ray diffraction (XRD) measurements were carried using a Panalytical X′Pert Pro diffractometer with an X′cellerator multichannel detector and a real time multiplier strip. The scanning electron microscopy (SEM) was performed using a JEOL JSM 5400LV spectrometer. UV-vis spectra were obtained using a Varian Cary 5 Spectrophotometer equipped with a diffuse reflectance accessory. Both reflectance and transmittance were measured for the thin films on conducting glass substrates; the diffuse reflectance accessory was operated in the total (“S”) mode (specular plus diffuse) to allow as much of the transmitted and reflected light as possible to be captured. The absorbance Aλ at wavelength λ was calculated from the reflectance Rλ and transmittance Tλ as Aλ ) 1 - Rλ - Tλ. Photoelectrochemical measurements were made using a conventional three-electrode arrangement with a Pt wire counter electrode and a saturated calomel electrode (SCE) reference as described elsewhere.10 The electrolyte (either 1 M NaOH solution or 0.5 M H2SO4 solution) was purged with nitrogen prior to and during the measurements. For the studies involving the oxidation of formic acid, a freshly prepared 1 M solution of formic acid in 0.5 M H2SO4 was used as the electrolyte. A Voltalab PGZ 3000 potentiostat with a scanning rate of 1 mV s-1 was used to measure photocurrents as a function of applied voltage. The light source was a 1000 W ozone-free xenon lamp (Oriel 6271) fitted with an Oriel 61945 water filter. The power density on the electrode was typically 100 mW cm-2. For measurement of IPCEs, an Oriel Model 7750 monochromator was used. The impedance measurements were carried out in dark conditions by applying an alternating current (ac) signal (amplitude of (10 mV) of a given frequency superimposed on the applied dc potential. 3. Results and Discussion 3.1. Film Quality and Deposition Rates. We obtained films with good adhesion both on Si and on conducting glass. The compressive stresses for the films on Si substrates were determined to be in the range of 0.7-1.0 G Pa, which is an agreement with the results reported by Bendavid et al.11 Using the flow rates shown in Table 1 and setting the chamber pressure

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Figure 2. XRD spectra for films prepared in the plasmas of Ar, Ar/ N2, and Ar/O2. All films annealed in air for 4 h at 400 °C. Peaks unmarked are due to ITO glass substrate.

at 66 Pa, we obtained deposition rates of 20-25 nm/min for Ar discharges, 15-20 for nm/min Ar/O2 discharges, and 510 nm/min for Ar/N2 discharges. These rates are comparable to those obtained by RF techniques by Battiston et al.3 for similar gas mixtures. Other studies using RF techniques produced lower rates; however, the deposition rates are greatly dependent on the power and the composition of source gas 4,5,6 The deposition rates decreased with increasing pressure in the chamber but showed only a small dependence on the voltage in the range of 320 to 420 V. For Ar/O2 plasmas, the deposition rates increased with increasing amount of O2 in the plasma gas mixture; a deposition rate of 35 nm/min was obtained for O2 flow rate of 25 sccm. The deposition rate decreased monotonically when the precursor carrier gas flow rate was reduced. The use of higher carrier gas flow rates (>25 sccm) led to films with poor adhesion and larger residual stresses. 3.2. XRD and SEM Results. XRD results confirmed that the films as prepared were amorphous. After annealing in air at 400 °C, films with diffraction patterns characteristic of anatase were obtained (Figure 2). We found that the presence of O2 during the annealing process was essential for the crystallization of the films. Heat treatment in Ar, N2, or in vacuum for long periods of time (up to 48 h) failed to produce polycrystalline films (discussed below). The films prepared in Ar/O2 required a minimum of 2 h of annealing in air before anatase peaks could be detected in the XRD spectrum. Films prepared in Ar and Ar/N2 plasmas required 4 h or longer periods of annealing in air to induce crystallization. Using the Scherrer equation, we estimated the volume-averaged particle sizes to be in the range of 25-40 nm for the films prepared in Ar and Ar/N2 discharges and annealed at 400 °C for 4 h in air. Smaller values of 1020 nm were obtained for the films made in Ar/O2 and annealed in air for 4 h at 400 °C. There are differences in the surface morphology between the films prepared in different plasma gases as seen in the SEM images (Figure 3). The grains in the film prepared in Ar/O2 (Figure 3c) with the average sizes of 10-20 nm as determined by the XRD appear to have packed closer together to form larger microstructures. This may have resulted in a reduction in the density of grain boundaries in the film and associated slower recombination rates for charge carriers. There is less such agglomeration of particles occurring in the films prepared in Ar and Ar/N2 (Figure 3a,b). Therefore, larger density of grain boundaries can be expected for these films in comparison to that of a film prepared in Ar/O2. Therefore, a greater resistance

Randeniya et al. for charge carriers may be present in the films prepared in Ar and Ar/N2. The results of annealing experiments and electrochemical experiments discussed in Sections 3.4 and 3.6 support these assertions. However, it is not straightforward to relate the relatively low-resolution surface features shown in Figure 3 to the surface areas or the density of grain boundaries of the films. 3.3. XPS Results. Table 2 shows the elemental composition of the surface determined by XPS before and after annealing for selected films prepared in Ar, Ar/O2, and Ar/N2 discharges (carbon not included in the calculation). Some N is incorporated into the films during the air-annealing process. The nitrogen level drops dramatically in the film prepared in Ar/N2 plasma after annealing in air. Before annealing, O is present in the amorphous films well in excess of what would be expected for stoichiometric TiO2. The reason for this is the presence of partially oxidized carbon radicals resulting from the breakdown of the metal-organic precursor. The existence of C-O and C-O-O radicals was confirmed by the presence of a XPS peak at 288.5 ( 0.6 eV for C 1s core electrons.12 Chemisorbed H2O and OH radicals can also contribute to additional oxygen. Annealing in air removes the majority of these species and consequently the atomic ratio of O to Ti reduced to values closer to 2. For all samples, the O 1s peak comprised of a dominant peak at 529.2 ( 0.4 eV and a shoulder peak at 531.2 ( 0.4 eV (Figure 4a). The lower energy peak is due to Ti-O bonding in TiO2 and the higher energy peak is due to C-O, C-O-O, and O-H bonds on the surface.12 The percentage contribution to the total peak from the component at 531.2 eV decreased dramatically after annealing in air as a result of removal of the chemisorbed species. The Ti 2p3/2 peak for amorphous films prepared in Ar and Ar/O2 plasmas appeared at 458.5 ( 0.2 eV and had full width half-maximum (fwhm) values of 1.4-1.5 eV. The amorphous films prepared in Ar/N2 plasma showed the peak at slightly lower energy (458 ( 0.5 eV) with larger fwhm values of 1.62.4 eV (Figure 4b). The lowering of the energy of the peak position for Ti 2p3/2 for N-doped TiO2 has been observed by others.13-17 An unresolved shoulder peak observed at 456 ( 0.5 eV for the amorphous films prepared in Ar/N2 contributes to the broadening of the peak. The appearance of a shoulder peak is attributed to the presence of Ti-N linkages. For pure TiN, the peak for Ti 2p3/2 is normally observed in the range of 455-455.6 eV.18,19,20 However, the higher electronegativity of O in comparison to N would increase the ionic character of Ti, which can explain a slightly higher binding energy for Ti 2p3/2 electrons for the shoulder peak observed in our samples. Airannealed polycrystalline films, in spite of the original gas composition used in the plasma to generate them, showed peaks at 458.3 ( 0.2 eV with fwhm values of 1.3-1.4 eV. For films prepared in Ar/N2 this indicates the dissociation of the Ti-N linkages during air-annealing process. Polycrystalline films prepared in Ar/O2 plasma and Ar plasma contained approximately 1 atom % of N, which gave a peak at 399.5 ( 0.5 eV for N 1s core electrons. For the films prepared in Ar/N2 plasma, the total nitrogen content in the film increased with the increasing fraction of N2 in the discharge. For the flow conditions shown in Table 1 for Ar/N2 discharges, we obtained up to 8-12 atom % of N in the amorphous film. As shown in Table 2, the nitrogen content in the film reduced when films were annealed in air. In Figure 5, we show the N 1s XPS spectra for such films measured before and after annealing in air. The measured N 1s peak before annealing is wide and extends from 394 to 403 eV. With annealing, the profiles became narrower.

PACVD Synthesis of TiO2 and N-Doped TiO2 films

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18337

Figure 3. SEM images for films prepared in (a) Ar (b) Ar/N2 and (c) Ar/O2. All films annealed in air for 4 h at 400 °C. The scale shown is 0.5 µm.

TABLE 2: Typical Surface Compositions Measured by XPS (Atomic %) for Films Prepared under Different Plasma Conditions Ar Ar/O2 Ar/N2

before annealing annealed in air at 400 °C for 4 h before annealing annealed in air at 400 °C for 4 h before annealing Heated in air for 4 h at 400 °C

Ti

O

N

22 32 28 32 24 30

78 67 72 67 64 68

0 1 0 1 12 2

In the majority of the films prepared, the N 1s peak for the samples before heat treatment could be fitted into two peaks, one at 396 ( 0.5 eV (fwhm ∼ 1.6 eV) and a second at 399.5 ( 0.6 eV (fwhm ∼ 2.5 eV). We concluded that the original amorphous films contained a fraction of N in the form of nitride (N3-, peak at 396 eV). The polycrystalline films did not show a peak at 396 eV confirming the removal of Ti-N linkages during the air-annealing process. Remaining N (1-2 atom %) showed a peak centered at 399.5 eV (dotted curve, Figure 5). The position and the width of the peak at 396 eV is indicative of N3-, however, it is lower in binding energy to N 1s in pure Ti-N bond, which is normally observed at 397.0397.8 eV.18,19,20 Saha and Tompkins18 proposed that a peak near 396 eV can be due to β-N (chemisorbed atomic nitrogen) trapped in grained boundaries. Following the work of Esaka et al.19 and the work more recently by Batzill et al.,21 we assign the peak at 396 eV to anionic N in N-Ti-O type bonding where the ionic character of the Ti-N bond in comparison to pure TiN is higher due to the presence of O. The assignment of the N 1s peak at 399.5 eV is not straightforward as a wide range of interpretations for the peaks observed in the vicinity of this binding energy are available in the literature. Earlier studies argued that this peak resulted from chemisorbed N218 although a contribution from oxynitride bonds (Ti-O-N) cannot be ruled out.22 More recently, the peaks in the vicinity of 399 ( 1 eV were assigned to oxynitride (TiO-N) by Okato et al.23 and to hyponitrite (N2O2 2-) by Sakthivel et al.24 (following Navio et al.25). Others have argued that they observed anionic N (N-) in O-Ti-N linkages at 399.6,26 399.2,17 398.2,13 and at 401.3 eV.27-29 In our case, films prepared in Ar and Ar/O2 and annealed in air at 400 °C also showed a peak at the same binding energy amounting to up to 0.6 atom % of nitrogen after annealing in air. Therefore, we concluded that there is a substantial contribution to the peak at 400 eV from surface-chemisorbed N2. However, the UV-vis absorbance studies shown below suggest that N is incorporated into the TiO2 matrix in a form other than chemisorbed N2. 3.4. Further Details of Annealing Experiments. It was found that the XPS peak at 396 eV seen in the amorphous films prepared in Ar/N2 was very sensitive to annealing at temperatures of 200 °C and above. Dissociation of Ti-N bonds in the presence of oxygen at high temperatures is to be expected as

the formation for TiO2 is thermodynamically more favorable. In an attempt to prepare polycrystalline films containing Ti-N bonds, we annealed samples prepared in Ar/N2 plasma in the absence of oxygen. Interestingly, even after heating for long periods (48 h) at temperatures of up to 500 °C, crystallization did not eventuate in vacuum, argon, or nitrogen. However, the Ti-N bonds dissociated during the annealing process in all these cases. This suggests that there is a sufficient amount of available oxygen embedded in the amorphous film to allow replacement of Ti-N bonds with Ti-O. A plausible explanation for the resistance for crystallization is that the presence of residual carbon and partially oxidized carbon radicals in the bulk of the amorphous film create steric hindrance to crystallization process. This hypothesis is supported by the fact that the films prepared in the absence of oxygen in plasma, which contained a greater concentration of C and its radicals in the bulk of the film, required longer periods of annealing in the presence of oxygen for the initiation of the crystallization process. 3.5. UV-Vis Results. The UV-vis spectrum is often used to illustrate the band gap extension into the visible region by N-doped TiO2. In Figure 6, we show absorbance for films (thickness of approximately 300-500 nm) prepared in Ar, Ar/ N2, and Ar/O2 plasma and annealed subsequently in air for 4 h. Some absorption of radiation at wavelengths above 420 nm is seen for all three films. This absorption for the films prepared in the Ar and Ar/O2 plasma is most likely caused by the presence of C in the films.30 The absorbance in the visible region however is clearly prominent for the film prepared in the Ar/N2 plasma and the absorption edge has been clearly shifted. It is most likely that the presence of N is leading to the visible light absorption in all three films (see Table 2), although a contribution of carbon cannot be ruled out. The higher absorption of visible light seen in Figure 6 for the film prepared in Ar/N2 should be a result of higher atomic percentage of nitrogen present in the film. Whether the shift in the absorption edge observed for N-doped TiO2 represents an actual extension of the band gap is debatable.21,31,32,33,34,35 3.6. Electrochemistry Results. Measured IPCE curves for water splitting in a 1M NaOH solution are shown in Figure 7 for the films prepared in Ar /O2, Ar/N2, and Ar discharges. All films used for electrochemical experiments discussed below were annealed in air at 400 °C for 4 h. The thicker film (300 nm) prepared in Ar/O2 plasma shows up to 65% photon conversion efficiency near wavelengths of 300 nm. Thinner films (120 nm) made in Ar/O2 plasma gave over 80% conversion efficiency near 300 nm but the IPCE values were smaller in comparison to the thicker film at higher wavelengths. The reason for this is the tradeoff between the probability of recombination of the charge carriers and the extent of photon absorption within the film. The probability of photon absorption increases with increasing path length and therefore higher IPCE values are observed at longer wavelengths for thicker films. The differences

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Randeniya et al.

Figure 4. XPS spectra for (a) O 1s core level for a film prepared in Ar/O2 and (b) Ti 2p core level for a film prepared in Ar/N2, measured before annealing and after annealing in air at 400 °C for 4 h.

Figure 5. N 1s XPS spectra for films prepared in Ar/N2 plasma before annealing (amorphous film, thick curve) and after annealing in air for 4 h (polycrystalline film, dashed curve).

Figure 6. UV-vis absorbance for films prepared in different plasma. All films annealed in air for 4 h. Anatase spectra was obtained using powder packed (1 mm) in a cell.

observed for the two films at the shorter wavelengths fall within the estimated uncertainties for our IPCE measurements.10 The IPCE values for photoelectrodes shown here for films prepared

Figure 7. Measured IPCE values for films prepared in Ar /O2, Ar/N2, and Ar plasmas in 1M NaOH solution. The measurements were made by an applied voltage of -0.1 V vs SCE. The measured thicknesses of the films are shown.

in Ar/O2 are about a factor of 2 larger in comparison to IPCE values reported by Battiston et al.3 for films prepared using RF plasma-enhanced chemical vapor deposition. Also, our values are comparable to “highly efficient” photoelectrodes prepared by Kavan and Gra¨tzel36 using an aerosol pyrolysis technique. The films with highest IPCE values for water splitting were obtained by optimizing the conditions used in the PACVD experiment (given earlier in the experimental section). The total pressure inside the chamber, the gas composition in the plasma, and the carrier gas flow rate for titanium precursor (which affects the rate of deposition) were all found to be important for depositing films that showed high quantum efficiencies. Such optimization of experimental conditions resulted in films with low trap-state concentrations, high crystallanity (after calcination), and low resistance for charge carriers. They are all important factors for the optimum catalytic performance of the semiconductor. As discussed earlier, recent reports have argued that there exists a clear correlation of photocatalytic activity of N-doped TiO2 in the visible region to a XPS peak observed either at 396-397 eV or at 398.2-401.3 eV. Yates et al. synthesized

PACVD Synthesis of TiO2 and N-Doped TiO2 films

Figure 8. Photocurrent densities as a function of wavelength for oxidation of 1 M CH2O2 in a 0.5 M H2SO4 solution. Photocurrent at each wavelength is normalized to light power of 1 mW/cm2. Apllied potential for measurements was 0.8 V with respect to SCE.

N-doped TiO2 films using chemical vapor deposition techniques that showed a peak at 396 eV for N 1s core electrons but no photocatalytic activity was observed in the visible region.15 The films examined here contained 1-2 atom % N with a XPS peak mainly centered at 399.5 eV. As can be seen in Figure 7, there is no evidence that doping of N in the present form is leading to water-splitting activity for TiO2 in the visible region. We have further investigated the photocatalytic properties of the film containing N by measuring the photocurrents in a solution of 1 M formic acid in 0.5 M H2SO4. Neumann et al.30 recently showed that C-doped TiO2 electrodes are capable of oxidizing formic acid under visible light illumination. Figure 8 shows the photocurrent densities as a function of wavelength obtained for films prepared in Ar/O2 and Ar/N2 plasma. The photocurrents were normalized to light intensity of 1 mW/cm2 at each wavelength to avoid complications from the wavelengthdependent intensity variations in the lamp. Figure 8 clearly shows that although the films containing N showed absorption in the visible region (Figure 6), there is no evidence for corresponding photocatalytic activity. It also shows that for this film the oxidation power in the UV region is smaller in comparison to the film prepared in Ar/O2. This result contrasts the observations of others, who prepared TiO2 electrodes with similar N atomic percentages and similar XPS and visible light absorption characteristics and showed the presence of visible photoactivity for the decomposition of organic molecules.17,24 We note that these latter studies of oxidation of organic compounds in the visible region were done with powders and the mechanisms (whether the oxidation is caused by surface intermediates of oxygen reduction or the holes generated in either the midgap levels or valence band) are still under debate.35 As proposed by others, it is likely that the visible light absorption in our samples is caused by occupied midgap N 2p states located slightly above the valence band.21,33,34 The oxidation in the visible region would then occur via the formation of holes in these states.35 Our photochemical results suggest that the reactivity of these holes is extremely low. We suggest that the recombination rates at the sites of carbon impurities are faster for the holes generated in the midgap states in comparison to the holes generated in the valance band. This can explain the difference between the curves shown in Figure 8.

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18339

Figure 9. Mott-Schottky plots for films prepared in Ar and Ar/O2 plasmas. Measurements made in 0.5 M H2SO4 solution.

Figure 7 and 8 show that the films of similar thicknesses prepared in the absence of O2 lower the oxidation power in the UV region. This can be due to more efficient recombination of holes occurring in the films prepared in the absence of O2. To investigate this, we measured the impedance of films prepared in Ar and Ar/O2 as a function of applied voltage in the electrolyte solution in the frequency range of 1-1000 Hz. Assuming a simple model of a capacitor and a resistor in series to represent the depletion layer and the solution resistance of a semiconductor electrolyte interface, one can obtain the capacitance of the depletion layer from the imaginary part of the measured impedance.36 The capacitance can be related to the donor concentration (ND) and the flat-band potential of the semiconductor using the Mott-Schottky equation

1 2 kT ) |∆φ| 2 e Csc r0NDe

(

)

(1)

where Csc is the capacitance of the space charge layer, r is the relative permittivity (or dielectric constant) of the semiconductor, and 0 is the permittivity of free space () 8.85 pF m-1). The charge of an electron is given by e, Boltzmann constant is given by k, and the absolute temperate is given by T. The difference in potential across the space-charge layer is denoted by |∆φ|. Figure 9 shows the plot of 1/C2sc versus applied potential at 100 Hz for two electrodes prepared in Ar and Ar/O2 plasma. Similar linear plots are obtained for the measurements made at other frequencies in the range of 1-1000 Hz. Linear regression fits to data obtained at different frequencies gave ND ) (1.22.2) × 1019 cm-3 for a film prepared in Ar/O2 and ND ) (1.93.2) × 1020 cm-3 for the film prepared in Ar. It is clear that the donor density obtained this way is a factor of 15 larger for the samples prepared in the absence of oxygen. The estimation of donor density made by Kavan and Gra¨tzel36 for their highly efficient TiO2 films using a similar technique compares well with the values we obtained for the films prepared in Ar/O2. We hypothesize that the higher donor densities calculated for the films prepared in Ar and Ar/N2 plasmas resulted from the presence of additional surface trap states. A similar correlation between measured donor densities and the presence of surface states was proposed by Kikuchi et al.37 for TiO2 films prepared using radio frequency magnetron deposition in Ar and in Ar/ O2 media. The presence of surface states limits the charge carries available for the oxidation of water at the surface.

18340 J. Phys. Chem. C, Vol. 111, No. 49, 2007 When the amorphous films are synthesized in the absence of oxygen in plasma, greater concentrations of carbon radicals get incorporated into the amorphous TiO2 film. Although annealing in air removes most of the carbon-containing species, it is likely that a greater concentration of these radicals remain trapped in the grain boundaries of the polycrystalline films originated from Ar and Ar/N2 plasmas. These radicals can act as efficient recombination sites for holes. We also see in Figure 7 that the impact of recombination is smaller for films prepared in Ar/N2 plasma in comparison to those made in Ar. On the basis of our analysis of XPS results for a large number of samples, we explain this difference as follows. In Ar/N2 plasma, there is competition between carbon and nitrogen compounds for adsorption sites in TiO2 matrix during the deposition process. Therefore, smaller carbon amounts get incorporated into the film for films prepared in Ar/N2 in comparison to films prepared in Ar. Because nitrogen is relatively easily removed during the annealing process, it is likely that the films resulting from Ar/N2 plasma have relatively smaller concentrations of carbon radicals trapped in the grain boundaries. Therefore, the recombination rates of charge carriers are smaller. This can explain the higher IPCE values obtained for the film prepared in Ar/N2 plasma in comparison to film derived from Ar (Figure 7). 4. Conclusions Thin films of TiO2 and N-doped TiO2 were synthesized by a PACVD technique using a pulsed dc power generator. The films showed good adhesion and the observed deposition rates were higher than those obtained by conventional RF techniques. The films prepared in Ar/O2 plasma after annealing in air gave high incident photon conversion efficiencies (up to 80%) in the UV region. Polycrystalline films containing 1-2% of N were obtained by annealing the samples prepared in Ar/N2 plasma. Although visible light absorption was observed, the films did not show corresponding photocatalytic activity toward water splitting or oxidation of formic acid. We propose that the holes created in the occupied N 2p midgap states are efficiently trapped by carbon radicals present in the grain boundaries. The presence of oxygen was found to be essential for the induction of crystallization by annealing. We hypothesize that carbon and partially oxidized carbon radicals in the bulk of the amorphous film are hindering the crystallization process. This possibility is supported by the fact that the films prepared in Ar and Ar/N2 plasma required longer periods of annealing in the presence of oxygen to induce crystallization in comparison to the films prepared in Ar/O2 plasma. Acknowledgment. The authors thank their colleagues Dr. Ian Plumb and Ms. Julie Glasscock for making their electrochemical equipment available for measurements. We also thank Ms. Glasscock for her assistance with SEM imaging. References and Notes (1) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1-21.

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