Effects of Interface Defects on Charge Transfer and Photoinduced

Nov 8, 2012 - as interface defective TiO2 (Id/TiO2). For comparison, a normal TiO2 bilayer film (N/TiO2) was prepared by dip coating twice but without...
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Effects of Interface Defects on Charge Transfer and Photoinduced Properties of TiO2 Bilayer Films Jiandong Zhuang,*,†,‡ Sunxian Weng,‡ Wenxin Dai,‡ Ping Liu,*,‡ and Qian Liu† †

The State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, P. R. China ‡ State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, P. R. China S Supporting Information *

ABSTRACT: By a combination of cold plasma treatment and sol−gel dip-coating technology, defective TiO2 bilayer films were successfully prepared, and then the defect was characterized by X-ray photoelectron spectroscopy and electron spin resonance. The photocatalytic activity and photoinduced superhydrophilicity of the films were evaluated by the photocatalytic degradation of organic dyes and water contact angle measurement in air, respectively. Compared to the conventional TiO2 film, the TiO2 bilayer films with defects provide much higher efficiency in photocatalysis and photoinduced hydrophilicity. Moreover, it is found that the introduction of defect sites at the interface of two TiO2 layers results in a significant improvement in the photonic efficiency due to the generation of electron trapping states. The defect sites can inhibit the recombination of electron−hole pairs and thereby increase the concentration of photogenerated carriers at the film surface, leading to an enhancement of quantum efficiency of TiO2 films under UV irradiation. Furthermore, the defects located at the interface between two TiO2 layers show higher stability as compared with that located at the outermost surface of TiO2 films. In addition, the fabrication of the interface defective TiO2 multilayer film is convenient and of low cost. The study demonstrates that this modified strategy is practical for promoting the photoinduced properties of TiO2 multilayer films.

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TiO2 film is one important task for the applications of TiO2 photocatalyst in the future. Most studies indicate that the functional properties of TiO2 are closely related to its defect disorder.10−13 Upon UV excitation of TiO2, electrons are excited from the valence band to the conduction band. The photogenerated electrons (e) and holes (h) migrate from bulk to surface, where electrons reduce adsorbed electron acceptor and holes oxidize adsorbed donor species. It is well-known that the competition among the recombination, trapping, and transfer of photogenerated electron−hole (e-h) pair determines the overall quantum efficiency of TiO2, and an improved separation (trapping and transfer) of this e-h pair is beneficial for enhancing the

INTRODUCTION Over the past decades, titanium dioxide (TiO2) has become one of the most extensively studied metal oxides due to its important application in photocatalysis for environmental cleanup, solar cells, clean H2 energy production, antimicrobial activity, and more.1−6 Among them, TiO2 photocatalysis has demonstrated efficacy as a treatment process for water and air purification and remediation. Two excellent properties of TiO2, photocatalysis and photoinduced hydrophilicity (PIH), do indeed make it suitable for a variety of thin film applications.7−9 Compared with TiO2 powder, the application of such films in environmental treatment is beneficial: allowing for more efficient distribution of light, showing antimist and self-cleaning properties in an outdoor environment, and providing more convenience in actual applications. However, the photocatalytic performance of TiO2 film is still low because of its small loading amount. Therefore, further improving the photon efficiency of © 2012 American Chemical Society

Received: August 8, 2012 Revised: November 5, 2012 Published: November 8, 2012 25354

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photoreactivity of TiO2.14−16 In principle, the e-h recombination and trapping occur in the defect sites (surface/bulk defects). Although the exact effects of surface/bulk defects on photocatalysis are still unclear, it is generally accepted that surface defects serve as charge carrier trapping sites that can improve e-h separation, whereas bulk defects only act as charge carrier recombination sites.17,18 Recently, it was reported that increasing the relative concentration ratio of surface defects to bulk defects in TiO2 nanocrystals could significantly improve the separation efficiency of photogenerated electrons and holes.19 However, surface defects are unstable in the common circumstance or aqueous media, and the direct contact with outside medium (O2 and/or H2O) will make these defects (e.g., oxygen vacancies) readily removed.10,20−22 So, it is of significant importance to improve the stability of surface defects in enhancing the photocatalytic efficiency of TiO2. To contrast with surface defects, defects located in TiO2 materials (subsurface/bulk defects) may exist more stably in a heterogeneous photocatalytic process. This basic concept inspires us to create defects at the inner framework, rather than at the outermost surface of TiO2 films, to prevent the direct outside contact of these defects with O2 or H2O. Our strategy is therefore to fabricate a TiO2 layer, on the basis of which surface defects are created, and subsequently coat a second TiO2 layer onto the first defective layer. Then, multilayer TiO2 films with interface defects (between layers) can be obtained finally. We hope that this type of subsurface defect can not only promote the photonic efficiency of TiO2 but also enhance the stability of this promoted effect. Moreover, this strategy also provides a means of investigating the role of subsurface defects in the transfer of charges across the semiconductor interface. Cold plasma treatment (CPT) technology, an efficient technique that can create defects on metal oxide surfaces at low temperature without affecting the bulk material, is adopted to introduce the surface defects on the TiO2 layer. In a previous study23 we succeed in synthesizing the defective TiO2 bilayer films by a combination of sol−gel dip coating method and CPT technology and found the locations of defects can greatly affect the photodegradation process of Rhodamine B dye over TiO2 films. Nevertheless, it is well-known that efficient transfer of charges across the semiconductor interface is the key for converting light energy into electricity or fuel.24 To understand the characteristics of the defects in more detail, studies are made on the role of interface defects toward photoinduced properties (photocatalytic activity and PIH) of TiO2 bilayer thin films.

have been located at the interface of two TiO2 layers (surface of inner TiO2 layer), and called the as-prepared TiO2 bilayer films as interface defective TiO2 (Id/TiO2). For comparison, a normal TiO2 bilayer film (N/TiO2) was prepared by dip coating twice but without the plasma treatment. Moreover, an outer surface defective TiO2 bilayer film (Sd/TiO2) was also prepared by cold plasma treatment over the surface of N/TiO2 sample. The average film thickness per coating cycle was measured to be about 0.09 μm.22 The schematic diagram of the preparation process of TiO2 bilayer samples is shown in Scheme S1 of the Supporting Information. Characterizations. The surface roughness and morphologies of TiO2 thin films were evaluated by atomic force microscopy (AFM, on a Nanoscope Multimode IIIa microscope, Veeco Instruments). The root-mean-square roughness values (Rrms) were estimated by spectral analysis on 1 μm2 areas. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.15418) operating at 40 kV and 40 mA. The TiO2 bilayer film coated on the substrate is too thin to obtain a fine XRD signal. Therefore, the layer number of TiO2 film was increased to 5. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scienctific) at 1.2 × 10−9 mbar using Al Kα Xray beam (1486.6 eV). The attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra were acquired using a Thermo Nicolet Nexus 670 FTIR spectrometer with a Ge single bounce ATR accessory. A Bruker model A300 spectrometer equipped with a xenon lamp (with 254 nm filter) was used for measurements of the electron-spin resonance (ESR) signals of radicals spin-trapped by 5,5-dimethyl-1pyrroline-N-oxide (DMPO, Alfa Aesar). The settings were center field, 3512.48 G; microwave frequency, 9.86 GHz; modulation amplitude, 1.0 G; power, 6.35 mW. The TiO2 powders used for ESR tests were carefully scratched off the TiO2-coated quartz slides before the experiments. Diphenylpicrydrazide (DPPH) was used as a g-marker (g = 2.0036) for the calibration of the spectra. A conventional three-electrode cells using a ZENNIUM electrochemical workstation (Zahner, Germany) equipped was used to determine the flat-band potential (Ufb). The catalyst sample was deposited on a FTO conducting glass serving as a working electrode, with the Ag/ AgCl electrode as the reference electrode, Pt as the counter electrode, and 1 M NaOH as the electrolyte (pH = 14). The working electrode with exposed area of 0.025 cm2 was illuminated from the back side (through the FTO substrate − TiO2 interface). The Mott−Schottky plot to evaluate the Ufb of the semiconductor space charge region was obtained by measuring impedance spectra at fixed frequency of 1 kHz. Test of Photocatalytic Activity. The as-prepared TiO2 films were examined for their catalytic activity toward photodegradation of dyes, including methyl orange (MO, 10 ppm) and rhodamine B (RhB, 10 μM). This test was conducted in a quartz tube, which was illuminated by four surrounding wideband lamps (4 W, PhilipsTL/05) with a predominant wavelength at 365 nm. The system was cooled by air to maintain it at room temperature. Two TiO2-coated slides (TiO2 content ≈ 1.2 mg) were immersed in 80 mL of a dye solution. During the experiment, an aliquot (2 mL) of the solution was taken at a certain time interval and analyzed on a Varian UV−vis spectrophotometer (Cary-50). After every assay, the analyzed aliquot was quickly poured back into the reactor to ensure a roughly equivalent volume of solution. The

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EXPERIMENTAL SECTION Preparation of TiO2 Sol and Films. The preparation procedure was described in detail in our previous paper.23 TiO2 anatase sol was prepared via a modified sol−gel processing of titanium tetraisopropoxide.25 The TiO2 films were deposited onto the carefully cleaned quartz slides by consecutive steps of dip coating and/or cold plasma treatment. In a typical preparation of TiO2 bilayer film with interface defects, the slide was dipped in the TiO2 sol and then pulled out at a constant rate. After being dried at 393 K, the obtained TiO2 monolayer film was treated with a cold plasma discharge to create surface defects. Then the treated film was coated with a second TiO2 layer on its surface via repeating the coating and drying processes. Finally, a transparent TiO2 bilayer film, inner layer treated by CPT, was obtained. We presume the defects 25355

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dimensional images of the surface of the as-prepared samples N/TiO2, Sd/TiO2, and Id/TiO2, respectively. The surface morphologies evaluated by AFM demonstrated that all three samples show similar morphology and particle size. Moreover, AFM image analysis also gives the values for the surface roughness. The root-mean-square roughness value (Rrms) of a normal TiO2 bilayer thin film (N/TiO2) is 4.145 nm. With treatment of cold plasma, the surface roughness of the TiO2 bilayer films decreased, and the Rrms values of Sd/TiO2 and Id/ TiO2 are 3.049 and 3.117 nm, respectively. Figure 1d shows the XRD patterns of the as-prepared TiO2 thin films. It is interesting to note that for both N/TiO2 and Id/TiO2 films, the diffraction patterns are almost the same and exhibit diffraction peaks exclusively ascribed to TiO2 crystals with the anatase phase. The average TiO2 crystallite sizes were calculated via the Debye−Scherrer equation based on the main diffraction (101) peak, and the primary crystallite size of the TiO2 thin films is about 5 nm. The results above indicate that the cold plasma treatment can slightly affect the surface roughness of TiO2 bilayer films but does not change the crystalline structure or as the grain size of the TiO 2 nanoparticles. XPS and ESR Analysis. Because of its high sensitivity to surface, XPS is a powerful technique for surface research. To investigate surface states of the TiO2 films, O 1s core levels were measured by X-ray photoelectron spectroscopy (XPS). Before carrying out the XPS measurements, the film surface was etched by Ar ion accelerated by 1 kV for 5 s. As shown in Figure 2a, the O 1s core level spectra are asymmetric and exhibit a broad shoulder to the high binding energy side, indicating that several oxygen species are present in the nearsurface region. The O 1s spectra can be deconvoluted by four asymmetric Gaussian curves. The main peak at about 530 eV is attributed to oxygen in the TiO2 crystal lattice (OL), while the other three oxygen peaks can be assigned to the Ti−O bonds of Ti2O3 (OTi3+, 530.9 eV), the hydroxyl groups (OH, 531.9 eV), and the C−O bonds (OC, 532.9 eV), respectively.26 Upon comparing the contribution of each type of oxygen, it is found that the relative peak intensity of OTi3+ in the Sd/TiO2 sample is stronger than that in the N/TiO2 sample, indicating that more defects have been created on the surface of the Sd/TiO2 film. For the Sd/TiO2 film, moreover, the OC content and the OH content are, respectively, calculated to be 9.3 and 15.2%, while for the N/TiO2 film, the contents of OC and OH are 4.7 and 13.3%, respectively. The higher OC content means a higher

change in absorbance in the dye solution was used to monitor the extent of reaction at given irradiation time intervals. The final photodegradation efficiency of dyes over catalysts was calculated by the following equation: Et(%) = (1 − Ct/C0) × 100%, where C0 and Ct stand for the initial and final concentration of dyes, respectively. Test of Water Contact Angle. The PIH of TiO2 film was evaluated by the change of the contact angle of a water droplet on the UV illuminated TiO2 surface. The films were illuminated with two UV lamps (4 W, 365 nm), and the light intensity reaching the film was about 2 mW/cm2. The sessile and captive drop method was used for the contact angle measurements with a commercial contact angle meter (OCA20 Optical Contact Angle Device, Dataphysics Ltd.). The water droplet size used for the measurements was 5 μL. Water droplets were placed at five different positions for one sample, and the average value was adopted as the contact angle.

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RESULTS AND DISCUSSION Surface Morphologies, Roughness, and Crystal Phase of TiO2 Films. Shown in parts a−c of Figure 1 are AFM three-

Figure 1. Three-dimension AFM images of the surface of (a) N/TiO2, (b) Sd/TiO2, (c) Id/TiO2, and (d) XRD patterns of as-prepared TiO2 films.

Figure 2. XPS spectra of the thin films: (a) O1s and (b) Ti 2p. 25356

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shown), meaning that the MO is fairly stable to UV 365-nm irradiation. The inset of Figure 4 shows the temporal evolutions in the UV−vis adsorption spectra of MO mediated by different TiO2 films. It can be found that the concentration of MO decreases over TiO2 films under UV (365 nm) irradiation, and no new absorption peak emerges. Temporal changes in the concentration of MO, which are directly monitored by examining the variations in maximal absorption in UV−vis spectra at 464 nm, are shown in Figure 4. Clearly, the Id/TiO2 sample shows better photocatalytic activity in the degradation of MO as compared with the N/TiO2. After UV irradiation for 7 h, the degradation efficiency of MO over Id/TiO2 is about 88.6%, which is much higher than that over N/TiO2 films (33.2%). The effect of interface defects on the PIH of TiO2 thin films was also evaluated. We measured the changes of water contact angle of Id/TiO2 and N/TiO2 films under illumination by 365 nm UV lamps with a self-made apparatus. Before test, the samples were stored in the dark for 2 weeks. It is seen in Figure 5 that with the introduction of UV irradiation on the sample surface, the water contact angle at each sample decreases and is down to ∼5° and ∼30° at Id/TiO2 and N/TiO2 films after 15 min, respectively. The slope of the contact angle decreasing line can be defined as the rate constant for the hydrophilic conversion process under irradiation. It is obvious that the hydrophilic conversion rate of Id-TiO2 is much faster than that of N/TiO2. Until recently, however, the mechanism of the PIH effect on the TiO2 film is still debated in the academic world, and several mechanisms have been proposed, including defect production,7,31,32 rupture of Ti−OH bonding,33 and photo-oxidation of organic layers.34 But in the final analysis, we proposed that no matter which mechanism involved, the separated efficiency of photogenerated electron−hole pairs may be the decisive factor in the PIH process on TiO2 films. In summary, the above results indicate that the interface defect sites can promote the quantum efficiency of TiO2 bilayer films, which corresponds to the photocatalytic activity and the PIH effect of TiO2. This quantum efficiency of the Id-TiO2 film was further examined by ESR. It is well-known that, for the photocatalysis system, the photogenerated holes on irradiated TiO2 can oxidize OH− to give •OH, while the photogenerated electrons can reduce O2 to give •O2−. The generation of both •OH and • O2− radicals is confirmed by the ESR spin-trap with the DMPO technique. As shown in Figure 5, four characteristic peaks of DMPO-•OH can be observed in the irradiated TiO2 suspension, while six characteristic peaks from the DMPO-•O2− species can be obviously observed in TiO2 methanolic dispersion. It is apparent that the intensities of the signals corresponding to DMPO-•OH and DMPO-•O2− are much stronger in irradiated Id/TiO2 than in irradiated N/TiO2 under similar conditions. Furthermore, the DMPO spin-trapping ESR results can also be backed up by the fluorescence analysis shown in Figure S4 of the Supporting Information. The results show that the interface defect sites can promote the separation of photogenerated electron−hole pairs and enhance the quantum efficiency of Id-TiO2 system subsequently. Electrochemical Analysis. Complementing the photoinduced reactivity experiments, we also carried out electrochemical analysis of TiO2 films, including Mott−Schottky plots and photocurrent response, to investigate their electronic properties. Figure 6 exhibits typical Mott−Schottky plots in the dark for TiO2 bilayer films. The positive slope of the linear

content of adsorbed carbonaceous matter, which can also be proved by the C 1s core level XPS spectra (Figure S2 of the Supporting Information). In addition, the hydroxyl groups which result from the dissociation of adsorbed water on TiO2 are further dependent on the surface defects (e.g., oxygen vacancies) of TiO2.9,10 Thus, the higher OH content means that more surface defects present on the Sd/TiO2 films. This result indicates that direct cold plasma treatment over TiO2 film can create surface defect sites efficiently. As a matter of fact, these defect sites are most likely created at the two-coordinated bridging sites of TiO2 surface to form oxygen vacancies,27 and meanwhile some Ti3+ defects are also formed, which can be proved by Ti 2p core level XPS spectra (Figure 2b). A shoulder 1.5 eV to the right of the main Ti 2p2/3 peak appears after the sample was treated with cold plasma for 5 min confirms the creation of Ti3+ defects on the surface. According to the analysis of the XPS results, it is found that CPT is an efficient method for creating oxygen vacancies on the TiO2 surface. However, limited by its detection depth limits, the XPS technique cannot be used to detect the chemical environment at the inner surface of this bilayer film. To make sure whether the defects can be introduced at the interface of two TiO2 layers, the powder ESR technique was further used to analyze the defective sites (paramagnetic species) in the Id/ TiO2 sample. The TiO2 powders used for ESR tests were carefully scratched off the TiO2 coated quartz slides, and the ESR spectra in Figure 3 were collected in air at 77 K. ESR

Figure 3. ESR spectra analyzed in air at 77 K for N/TiO2 and Id/ TiO2.

signals at g = 2.004 attributed to the electrons traps on oxygen vacancies are observed for both TiO2 samples.28,29 Although a broad and weak form of this resonance exists in the N/TiO2, it becomes very strong after the CPT treatment (Id/TiO2), indicating that the defects have been introduced at the interface of Id/TiO2 bilayer film. Moreover, according to the UV−vis adsorption spectra of TiO2 thin films (Figure S3 of the Supporting Information), the Id/TiO2 spectrum shows an obvious red-shift as compared to the spectrum of N/TiO2 film, further confirming our hypothesis about the existence of interface defect sites.30 Photoinduced Properties Measurements. To evaluate the role of interface defects in the photocatalysis of TiO2 films, a comparison of the photoactivity of the Id/TiO2 films and N/ TiO2 films was conducted. The photocatalytic activities were examined by decomposition of MO in aqueous solution under UV-365 nm irradiation. No appreciable degradation of MO is observed after 7 h in the absence of photocatalysts (data not 25357

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Figure 4. (a) Degradation profile of MO over different TiO2 bilayer films. Inset: Temporal changes in the UV−vis adsorption spectra of MO as a function of irradiation time. (b) Water contact angle of TiO2 films as a function of time under UV illumination at 365 nm.

Figure 5. DMPO spin-trapping ESR spectra of TiO2 scrapings (a) in aqueous dispersion for DMPO-•OH and (b) in methanol dispersion for DMPO-•O2−.

−0.96 V vs Ag/AgCl at pH = 14 for N/TiO2 and Id/TiO2, respectively. The anodic shifts (∼0.14 V) in the Ufb observed for Id/TiO2 samples compared to N/TiO2 reflect the changes in the conduction band edge due to the existence of defect sites. It is proposed that the defects can add electric states within the band gap and then affect the conduction band edge of TiO2. Promisingly, the TiO2 bilayer films are indeed able to generate photocurrents (Iph) under UV irradiation. From curve A of Figure 7a, it can be seen that the induced Iph of the N/ TiO2 film increases rapidly and reaches a plateau within 10 s when the light is switched on. When the light is switched off, a dramatic decrease in the Iph is observed, followed by a mild decay. Likewise, a similar trend is observed in the Iph of the Id/ TiO2 film (curve B) but with a lower Iph intensity. In photoelectric measurements, the film electrode is used and the production of photocurrent relies on the amount of photoelectrons transferred from semiconductor to FTO glass substrate. As shown in Figure 7b, the photoelectrons are trapped in interface defect states, thereby diminishing the chance to contribute to the photocurrent generation. Moreover, it should be noted that when the light is switched off, the

Figure 6. Mott−Schotty plots of N-TiO2 and Id-TiO2 films.

C−2−E plot suggests the expected n-type semiconductor of TiO2. Another important parameter derived from the measurements is the flat-band potential (Ufb), which obtained by extrapolation of the Mott−Schottky plots is roughly −1.10 and 25358

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Figure 7. (a) Photocurrent action spectra of the TiO2 films and FTO glass. (b) Scheme diagram for interface defect state roles on the transfer of the photoelectron in the photoelectric process.

about 2.5 times higher than that of N/TiO2. This result indicates that in the irradiated Id/TiO2 film the photogenerated electrons, which are shallow trapped at the interface defect sites, can stay separated from their complementary holes and then thermally excited to the conduction band, resulting in a higher quantum efficiency of the Id/TiO2 film. On the basis of the experimental results and analysis, a possible mechanism of the promoted photoinduced properties of Id/TiO2 films was proposed and is illustrated in Scheme 1. Obviously, the promoted effects are the results of the synergetic action of the semiconductor and defects. It is believed that one specific defect species could play a unique role in the physicochemical properties of semiconductor.37,38 The storage of electrons in TiO2, which results from trapping at defect sites (oxygen vacancies or Ti3+ sites), dictates overall photocatalytic properties. In our study, the defects were created by CPT treatment and located at the interface (inner layer surface) of two TiO2 layers. Under the UV irradiation, the defect sites are proposed to act as electron trappers to shallow trap the photogenerated electrons and enhance the quantum efficiency of TiO2 films. Subsequently, more photogenerated carriers can transfer to the outer layer surface and dramatically enhance the subsequent reactions, such as the formation of surface defect sites (O− and Ti3+ ions, Figure 9) and/or the degradation of the adsorbed organic on the TiO2 surface. Therefore, compared with normal TiO2 films, the TiO2 films with interface defects is of higher quantum efficiency and shows an enhanced photoinduced catalysis and hydrophilicity under UV irradiation. Stability of the Defects. Defects have long been thought to be a promotional factor in the catalytic reaction process. According to previous studies,10,20−22 the surface defects are metastable surface states and may be readily removed. Herein, with a protective TiO2 layer to prevent the direct outside contact with O2 or H2O, the defects located at the inner layer surface (interface defects) are proposed to be more stable than those located at the outer layer surface of the TiO2 film (surface defects). To further investigate the stability of surface defects and interface defects, the photodegradation of RhB over TiO2 was examined under UV-365 nm irradiation. Since there are two types of spectral changes (hypsochromic shift and intensity loss) in the absorption bands of RhB, the oxidation efficiency of dye cannot be directly examined for the changes in maximal absorption of RhB at 553 nm. Hence, for quantitative analysis

photocurrent of Id/TiO2 decreases more slowly than that of N/ TiO2. We presume that the photoelectrons trapped at the defect states can escape from their traps to reform mobile electrons, prolonging the decay time of the response photocurrent. The interface defects are proposed to act as electron trappers to shallow trap the photogenerated electrons and enhance the separating efficiency of photogenerated electron−hole subsequently. In fact, the electrons trapped in the shallow midgap states can also provide a continuum of electronic excitations, resulting in an increase in the featureless background of the infrared spectrum. Several authors14,35,36 have reported this type of broad and unstructured IR absorption for TiO2 after UV excitation. According to Yamakata et al.,36 the direct optical transition of shallow-trapped photogenerated electrons from the trap state to the conduction band is one of the sources of the transient IR absorption. Figure 8 shows the IR absorption spectra of Id/TiO2 and N/TiO2 before and after UV irradiation. Compared with N/TiO2, a more pronounced increase in the background IR absorbance of Id/TiO2 is observed after UV exposure, and the increase amplitude is

Figure 8. FT-IR absorbance spectra for TiO2 thin films. (a) N/TiO2, (b) Id/TiO2, (c) N/TiO2 after 15 min UV irradiation and (d) Id/TiO2 after 15 min UV irradiation. Trapped electrons are measured by the increase in the background absorbance measured at 2000 cm−1. Δ(d− b):Δ(c−a) = 2.5:1. 25359

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Scheme 1. Schematic Illustration of the Interface Defect State Role on the Photocatalytic Process of TiO2 Bilayer Films

degradation of RhB. However, when oxygen is bubbled into the photocatalytic system, the photo-oxidation efficiency of RhB over Id/TiO2 films is slightly promoted (from 95 to 97%), while that over Sd/TiO2 films is remarkably inhibited (from 83 to 63%). Obviously, the outer surface defects of Sd/TiO2 are healed by oxygen, leading to the decline of photocatalytic activity. Figure 10b displays the durability of the photocatalytic activity of different TiO2 samples toward the degradation of RhB. Because of the surface carbon deposit on the photocatalysts, a continuous decline in catalytic activity seems to be inevitable. It should be noted that, in the second cycle, the photo-oxidation activities of Sd/TiO2 and N/TiO2 are very close to each other, meaning that the surface defects of Sd/ TiO2 have been removed in the first photocatalytic cycle. After three cycles, sample Id/TiO2 remains a high photocatalytic activity location for RhB oxidation, while the activity of sample Sd/TiO2 is further declined. The results demonstrate the stability of interface defects in the Id/TiO2 is indeed superior to that of surface defects on the Sd/TiO2.

Figure 9. ESR spectrum analyzed in air at room temperature for Id/ TiO2 powder with UV light (254 nm) irradiation.

in our case, the area of the main RhB absorption-peak was integrated in the range of 415−615 nm to judge the actual concentration of dye. As shown in Figure 10a, both Sd/TiO2 and Id/TiO2 samples show efficient photoactivity for the

Figure 10. (a) Degradation profile of RhB over different TiO2 films in air that has been equilibrated or oxygen saturated. (b) Cyclic photodegradation of RhB over different TiO2 bilayer films. The concentration of RhB is monitored by examining the integrated area of the main RhB adsorption peak (415−615 nm). 25360

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CONCLUSION In summary, all analytical methods suggest that the defects, created by the CPT technique, are located at the interface between two TiO2 layers and operate effectively under light irradiation. The photoinduced catalysis and hydrophilicity of TiO2 bilayer films were found to be dramatically enhanced by the introduction of interface defect sites. The defect sites are proposed to add electronic states within the band gap of semiconductor TiO2 and act as photoelectron trappers to effectively enhance the charge separation of electrons and holes, resulting in marked improvement in photoinduced catalytic and hydrophilic performances of TiO2 films. As compared with the outmost surface defects of TiO2 films, the interface defects between two TiO2 layer show higher stability in the heterogeneous photocatalysis. The fabrication of the interface defective TiO2 multilayer film is “green”, convenient, and of low cost, making it a promising technique to enhance the photon efficiency of TiO2 film. Moreover, this finding is also of great significance for the design of effective photoelectric and photocatalytic material in the field of solar energy conversion.

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ASSOCIATED CONTENT

* Supporting Information S

A scheme diagram of the preparation procedure, UV−vis adsorption spectra and photoluminescence spectra of TiO2 films. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Z.); [email protected] (P.L.). Phone/Fax: +86-21-52412404 (J.Z.); +86-59183779239. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by National Natural Science Foundation of China (No. 21173046), National Basic Research Program of China (973 Program, 2010CB234604), Technological Innovation Fund of Shanghai Institute of Ceramic (No. Y21ZC8180G), and Open Fund of Fujian Provincial Key Laboratory of Photocatalysis−State Key Laboratory Breeding Base in Fuzhou University.

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dx.doi.org/10.1021/jp307871y | J. Phys. Chem. C 2012, 116, 25354−25361