Role of Sodium Ion on TiO2 Photocatalyst - ACS Publications

Article pubs.acs.org/JPCC

Role of Sodium Ion on TiO2 Photocatalyst: Influencing Crystallographic Properties or Serving as the Recombination Center of Charge Carriers? Huan Xie, Neng Li,* Baoshun Liu, Jingjing Yang, and Xiujian Zhao* State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, Hubei Province People’s Republic of China S Supporting Information *

ABSTRACT: There have been continuing debates about the role of Na+ on TiO2 photocatalyst in the past decades. Most researchers accepted that Na+ served as the recombination center of photogenerated electrons and holes. Nevertheless, other opinions also existed, such as Na+ increased the crystallite size of TiO2, Na+ hampered the crystallization of anatase TiO2, and Na+ promoted the formation of brookite TiO2 or titanate sodium. In this research, we have systematically investigated the role of Na+ during the fabrication of TiO2 film and powder through the sol−gel method and studied the influences of crystallinity and the content of Na+ on the photocatalytic activities of TiO2 film and powder. It has been found that the existence of Na+ in TiO2 film and powder should influence their crystallographic properties, in detail, inhibiting the crystallization and growth of anatase phase in TiO2 film and powder, promoting the formation of brookite phase in TiO2 film, and increasing the transformation temperature of anatase to rutile phase in TiO2 powder. Even though the existence of Na+ forms the Ti−O−Na bond on the surface of TiO2, however, the widely adopted hypothesis of Na+ serving as the recombination center of photogenerated electrons and holes is not correct. small ionic radius and univalent state.26 Nevertheless, there have been continuing debates about the role of Na+ on TiO2. Most researchers accepted that Na+ was detrimental to the photocatalytic activity of TiO2; they proposed that Na+ served as the recombination center of photogenerated electrons and holes.15,26,29−31 However, Nam et al. doubted the correctness of the recombination center hypothesis; they forwarded that Na+ could increase the crystallite size of TiO2 leading to inferior photocatalytic activity.29,32 Additionally, other opinions also existed, such as Na+ promoting the formation of brookite TiO2 or titanate sodium instead of anatase TiO230−33 and Na+ preventing the crystallization of anatase TiO2 or disordering the crystallinity of TiO2.28,34,35 Why does the TiO2 film directly fabricated on soda-lime glass have much lower photocatalytic activity? Does Na+ really act as the recombination center of photogenerated electrons and holes? The answers to these questions are important to the fabrication and application of TiO2. In this work, we aim to find out the role of Na+ on TiO2. We have fabricated TiO2 films on various glass substrates through the sol−gel method, including soda-lime glass, quartz glass, borosilicate glass, and the sodalime glass precoated with a SiO2 layer. The differences of the

1. INTRODUCTION For the past decades, TiO2 photocatalyst has attracted much attention because it has various promising applications in environmental purification and solar energy conversion.1−3 It can be used for the photodegradation of multiple air and water pollutants (acetone,4,5 benzene,5 formaldehyde,6 methyl orange,7 methylene blue,8−10 etc.), solar cell,11 solar hydrogen production, and solar carbon dioxide reduction.12−14 During the use of TiO2, it is usually immobilized or directly fabricated on various rigid or soft substrates to form film, including sodalime glass (widely used in daily life and architectures),5 quartz glass,15 borosilicate glass,16 silicon wafer,17 stainless steel,18 ceramic tile,19 polyurethane,20 polypropylene,21 etc. Among them, soda-lime glass is considered as one of the ideal substrates for TiO2, owing to its cheap price, thermal and chemical stabilities, and wide applications. However, it has been reported that the TiO2 film directly fabricated on soda-lime glass has much lower photocatalytic activity than that of the TiO2 films fabricated on the other kinds of substrates, including quartz glass,15 borosilicate glass,16 the soda-lime glass precoated with a SiO2 or SiNx barrier layer,22,23 and silicon wafer.17 Many researchers have ascribed the reason to Na+, which can thermally diffuse from soda-lime glass into TiO2 film during calcination.15,16,22−28 In general, the atomic percentage of Na+ in commercial soda-lime glass is as high as about 15%,16,28 and Na+ has strong diffusion ability due to its © 2016 American Chemical Society

Received: February 19, 2016 Revised: May 3, 2016 Published: May 4, 2016 10390

DOI: 10.1021/acs.jpcc.6b01730 J. Phys. Chem. C 2016, 120, 10390−10399

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The Journal of Physical Chemistry C

temperatures (400, 450, and 500 °C) for 2 h. Besides, the TiO2 powders with different crystallinities were prepared by following procedures. 20 mL of TBOT was added dropwise into 300 mL of distilled water undergoing violent magnetic stirring. TBOT was quickly hydrolyzed, and white precipitate was formed. The obtained white precipitate was filtered and washed with ethanol and distilled water for several times. Then it was dried at 100 °C for 5 h, followed by calcination at 200 or 400 °C for 1 or 2 h. TiO2 powders with amount of 0.1 g were used for the measurement of photocatalytic activity. 2.3. Fabrication of TiO2 Film on Soda-Lime Glass through the PVP Modified Sol−Gel Method. To fabricate highly crystallized TiO2 film on soda-lime glass, polyvinylpyrrolidone (PVP K30, with content of 6 wt %) was added into titania sol according to our previous work.5 Other ingredients were identical with the titania sol described above. The TiO2 film fabricated on soda-lime glass through the PVP modified sol−gel method (named as PVP-SL) was dip-coated at a withdrawal speed of ca. 12 mm/s for once. It was also calcined at 450 °C for 1 h. To reduce the content of Na+, after calcination, it was ultrasonic washed in distilled water for ca. 30 min (named as PVP-SL-UC), or first it was immersed in 9 M H2SO4 aqueous solution for ca. 16 h and then ultrasonic washed in distilled water for ca. 30 min to remove the adsorbed H2SO4 (named as PVP-SL-HS). The samples were dried with a blower. 2.4. Measurements of the Photocatalytic Activities of TiO2 Powders and Films. The photocatalytic activities of TiO2 powders and films were estimated by the photodegradation of acetone. A xenon lamp (Trusttech, CHF-XM500 W) was used as light source. The intensity of generated ultraviolet light was detected using an UV radiometer (Beijing Normal University Factory) with a 365 nm detector, which could measure UV light in the range of 320−400 nm, and the measured value was 11.02 mW/cm2.5 A homemade stainless steel cylindrical container was used as photocatalytic reactor, which had a quartz window and with a volume of 415 mL.5 The distance between reactor and xenon lamp was ca. 16 cm. The changes of the concentrations of acetone and evolved CO2 were detected with a gas chromatograph (Huaai, GC9560), which was equipped with a methane converter, a Porapak R column, a flame ionization detector (FID), and a PEG20M column.5,36 The injected amount of acetone in reactor was 0.9 μL, and the detected initial concentration was 500 ± 40 ppm. 2.5. DFT Calculation. To examine whether Na+ serves as the recombination center of photogenerated charge carriers from theoretical point of view, the Vienna ab initio simulation package (VASP) was used for all the density function theory (DFT) computations.37−39 The plane-wave projector-augmented wave (PAW) was used to describe the ion−electron interaction, and the Perdew−Burke−Ernzerhof (PBE) from the generalized gradient approximation (GGA) was used to describe the exchange-correlation functional. The plane-wave energy cutoff was set to 370 eV. To model the Ti−O−Na bond on anatase TiO2 (110) surface, the 2 × 2 × 1 anatase TiO2 (110) surface is considered here, with a single Na ion on its surface. The Brillouin zone was represented by Monkhorst− Pack special k-point mesh of 3 × 3 × 1 for both geometry optimizations and electronic structure computations. For structure relaxation, all atoms were allowed to move until the energy was less than 10−5 eV/atom and the force on each atom was less than 0.05 eV/Å. For electronic structure calculations,

obtained TiO2 films in crystallographic properties (phase composition, crystallinity, and crystallite size), the content of Na+, and photocatalytic activity have been systematically investigated. In addition, we also have fabricated TiO2 film with high crystallinity on soda-lime glass through PVP (polyvinylpyrrolidone) modified sol−gel method as reported in our former work.5 Besides, TiO2 powders containing various amounts of Na+ and with different crystallinities also have been prepared. The influences of crystallinity and the content of Na+ on the photocatalytic activity of TiO2 have been studied. Additionally, we also have calculated the energy level of Ti−O− Na bond on the surface of TiO2 through density functional theory (DFT).

2. EXPERIMENTAL SECTION 2.1. Fabrications of TiO2 Films on Various Glass Substrates through the Sol−Gel Method. All chemicals used in this work were in analytical grade, and they were used as received without further purification. TiO2 films were fabricated on different glass substrates via the sol−gel method. For the preparation of titania sol, 0.1 M diethanolamine (DEA) and 0.2 M tetrabutyl titanate (TBOT) were dissolved in ethanol. After that, the mixture of ethanol and distilled water (0.4 M) was added dropwise into former solution while magnetically stirred. The obtained titania sol was aged at ambient conditions for 7 days before use. In addition, for the preparation of silica sol, 0.5 M tetraethyl orthosilicate (TEOS) and 2 M H2O were dissolved in ethanol. Then, HCl solution (37 wt %, molar ratio HCl:TEOS = 0.017:1) was added dropwise into former mixture. The obtained silica sol was aged at ambient conditions for 1 month before use. Soda-lime glass (8.5 cm × 8.0 cm × 1.0 mm), borosilicate glass (8.5 cm × 8.0 cm × 2.0 mm), and quartz glass (8.5 cm × 8.0 cm × 1.0 mm) were ultrasonic washed in distilled water and ethanol. The compositions of these three kinds of glass substrates were examined with X-ray fluorescence spectrometer (XRF, PANalytical, AXIOS), which was operated at 4 kW. The results are shown in Table S1 (Supporting Information). Sodalime glass coated with a SiO2 layer was prepared by the dipcoating method. In detail, soda-lime glass was immersed in silica sol for ca. 15 s, and then it was pulled out vertically at a withdrawal speed of ca. 2 mm/s. The obtained sample was heated at 300 °C for 1.5 h, and then it was cooled down to room temperature. The above-described procedures were repeated for a total of four times. TiO2 films were fabricated on different glass substrates through a similar process as the SiO2 layer. The withdrawal speeds were all at ca. 14.0 mm/s. The obtained samples were dried at 80 °C for ca. 15 min. After cooling down to room temperature, they were recoated with the same process. Afterward, they were calcined at 450 °C for 1 h. The heating rates in this work were all ca. 5 °C/min. TiO2 films fabricated on soda-lime glass, the soda-lime glass precoated with a SiO2 layer, borosilicate glass, and quartz glass were named as SL, SiO2-SL, BS, and QZ, respectively. The TiO2 films fabricated on soda-lime glass and quartz glass were also calcined at 500 °C for 1 h, which were named as SL-500 and QZ-500, respectively. 2.2. Fabrications of TiO2 Powders. The TiO2 powders containing different amounts of Na+ were prepared by the sol− gel method. NaNO3 (with molar ratios of Na:Ti = 0, 0.05, 0.1, and 0.15) was added and dissolved into 300 mL of titania sol. Then, the obtained titania sols were dried at 70 °C for 3 days. The obtained xerogel powders were calcined at various 10391


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The Journal of Physical Chemistry C the convergence of self-consistent field (SCF) computations were set to 10−6 eV/atom. 2.6. Characterizations. The surface and cross-sectional images of TiO2 films were taken with a field emission scanning electron microscope (FESEM, Hitachi, S-4800), which was operated at 5 kV. XRD patterns of TiO2 films were recorded with an X-ray diffractometer (XRD, PANalytical, Empyrean), using grazing incidence mode with an incident angle of 0.3° and with a scanning speed of 1.84°/min. XRD patterns of TiO2 powders were also tested with the same machine but using normal powder diffraction mode with a scanning speed of 7.5°/ min. The operation voltage and current were 40 kV and 40 mA, and Cu Kα radiation was used as X-ray source. HRTEM images were examined by transmission electron microscope (TEM, JEOL, JEM2100F) with accelerating voltage of 200 kV. The UV−vis absorbance spectra of TiO2 films were measured with a UV−vis spectrometer (Shimazu, UV-2600). The elemental compositions of TiO2 films were tested using an X-ray photoelectron spectrometer (XPS, Thermo Scientific, VG Multilab 2000), and Mg Kα radiation was used as the X-ray source. The obtained XPS spectra were calibrated by the binding energy of C 1s electrons of adventitious carbon, which was 284.6 eV. Some TiO2 films were bombarded with Ar ions with an etching rate of ca. 1 nm/min to expose their inner parts for XPS measurement.

3. RESULTS AND DISCUSSION 3.1. Photocatalytic Activities of the TiO2 Films Fabricated on Different Glass Substrates. Figure 1 shows the photocatalytic activities of the TiO2 films fabricated on different glass substrates through sol−gel method for the photodegradation of acetone. As can be seen in Figure 1a, the evolved CO2 concentrations of QZ, BS, SiO2-SL, and SL TiO2 films within 25 min are 1464, 1451, 1309, and 251 ppm, respectively. The former three ones are 5.83, 5.78, and 5.22 times higher than the last one. As shown in Figure 1b, acetone can be almost completely degraded by SiO2-SL, BS, and QZ TiO2 films after 25 min. However, for SL TiO2 film, the residual percentage of acetone is around 74.72% after 25 min, indicating that most acetone is not degraded. In addition, as given in Figure S1, the thicknesses of these TiO2 films are similar, and the calculated mean thickness is 265 nm with a standard deviation of 11.73 nm. The above obtained results illustrate that when thicknesses are similar, the photocatalytic activity of the TiO2 film fabricated on soda-lime glass is much lower than that of the TiO2 films fabricated on the other three kinds of glass substrates, which is consistent with the observations of many other reported literatures.15,22,25,28 To investigate the origin of this phenomenon, various characterizations have been carried out as follows. 3.2. Crystallographic Properties of the TiO2 Films Fabricated on Different Glass Substrates. Figure 2 shows the XRD patterns of the TiO2 films fabricated on different glass substrates through the sol−gel method. In this research, TiO2 films were tested using the grazing incidence X-ray diffraction method, which is a powerful tool to show more detailed information on the crystallographic properties of surface layers or films.40 As shown in Figure 2, most of the detected diffraction peaks can be ascribed to anatase TiO2 (JCPDS No. 21-1272). It also can be seen that the diffraction peaks of anatase TiO2 for SiO2-SL, BS, and QZ TiO2 films are much higher than that of SL TiO2 film; the diffraction intensities of the dominant (101) peak of the former three TiO2 films are

Figure 1. Evolved CO2 concentrations (a) and the degradation curves (b) of SL, SiO2-SL, BS, and QZ TiO2 films for the photodegradation of acetone.

Figure 2. XRD patterns of SL, SiO2-SL, BS, and QZ TiO2 films.

about 3.75, 3.88, and 4.38 times higher than that of the latter, respectively. This is caused by the poor crystallinity of SL TiO2 film, which will be further confirmed by HRTEM result. In addition, the average crystallite sizes of anatase TiO2 for SL, SiO2-SL, BS, and QZ TiO2 films are 11.6, 11.7, 11.3, and 14.0 nm, respectively, which are calculated by the Scherrer equation with anatase (101) peak. 10392


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Figure 3. HRTEM images of SL, SiO2-SL, BS, and QZ TiO2 films: red circles indicate the crystallized parts, and blue circles show the uncrystallized parts. The inset images are the SAED patterns of SL and QZ TiO2 films.

Besides, as also can be seen in Figure 2, for SL TiO2 film, a very small peak at around 30.80° also exists, as pointed out with arrow, which can be ascribed to the (121) plane of brookite TiO2 (JCPDS No. 29-1360). The diffraction peak of (121) plane is the second strong peak of brookite TiO2, the most intense peak is the (120) plane (2θ = 25.34°). However, this peak is overlapped with the intense peak of (101) plane for anatase TiO2 (2θ = 25.28°). The appearance of the weak peak of brookite (121) plane indicates that a very small amount of brookite TiO2 exists in SL TiO2 film. The weight percentages of brookite and anatase TiO2 in SL TiO2 film are 9.61% and 90.39%, respectively, which are calculated by the following equations: (101) (121) (120) R brookite = Ibrookite /(Ibrookite + Ianatase ) × 100%

(1)

R anatase = 100% − R brookite

(2)

titanate sodium can be found in the XRD pattern of SL TiO2 film, which is consistent with the observation of Paz et al.30 The FESEM surface images of the TiO2 films fabricated on different glass substrates through the sol−gel method are given in Figure S2. As can be seen, BS and SiO2-SL TiO2 films have very smooth and compact surfaces, and no nanoparticles can be observed on their surfaces. In contrast, numerous nanopores randomly existing in QZ and SL TiO2 films, and the pore size of the former is larger than the latter. Additionally, more wellshaped nanoparticles can be observed on the surface of QZ TiO2 film. The differences in the surface morphologies of these TiO2 films may arise from the variations of the thermal expansion rates of different glass substrates during heating and cooling processes. Besides, Figure 3 demonstrates the HRTEM images of the TiO2 films fabricated on different glass substrates through the sol−gel method. For SL TiO2 film, only few places distinctly show lattice fringes, which are indicated with red circles, illustrating that only few well-crystallized TiO2 nanocrystals exist and most parts are amorphous, which is accordance with its weak X-ray diffraction peaks in Figure 2. In contrast, for SiO2-SL, BS, and QZ TiO2 films, most parts of HRTEM images clearly show lattice fringes, indicating that more well-crystallized TiO2 nanocrystals are formed, and their crystallinities are greatly enhanced comparing to that of SL TiO2 film. Nevertheless, as also can be seen in Figure 3, few places in SiO2-SL, BS, and QZ TiO2 films show no lattice fringes, which are pointed out in blue circles, indicating that few parts are still uncrystallized. Additionally, higher magnification HRTEM images are shown in Figure S3. It can be seen that most of the exposed facets in these TiO2 films are the (101) plane of anatase TiO2 with a lattice fringe spacing of 0.34 nm. Besides,

in which Rbrookite and Ranatase represent the weight percentages of (120) (101) brookite and anatase TiO2, I(121) brookite, Ibrookite, and Ianatase indicate the areas of (121) and (120) peaks of brookite TiO2 as well as the area of (101) peak of anatase TiO2, respectively.41 The average crystallite size of brookite TiO2 is 11.7 nm, which is calculated by the Scherrer equation with (121) peak. Contrastingly, no peaks of brookite TiO2 can be observed in the XRD patterns of SiO2-SL, BS, and QZ TiO2 films. Ohara et al. also observed that the TiO2 film fabricated on soda-lime glass composed of anatase TiO2 and a small amount of brookite TiO2, and no brookite TiO2 was found in the TiO2 film fabricated on silicon substrate.42 They ascribed the reason to Na+ which thermally diffused from soda-lime glass into TiO2 film during calcination. In addition, no diffraction peaks of 10393


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1s electrons for SiO2-SL, BS, and QZ TiO2 films are about 529.6 eV; however, that for SL TiO2 film is 529.2 eV. The reported binding energies of Ti 2p1/2, Ti 2p3/2, and O 1s electrons for TiO2 are 464.1, 458.5, and 529.8 eV,43 respectively, which are accordance with the values of SiO2-SL, BS, and QZ TiO2 films. The left shift of the binding energies of Ti 2p and O 1s electrons for SL TiO2 film can be ascribed to the existence of Na+. Liu et al. reported that the binding energies of Ti 2p electrons in Na2Ti3O7 were 463.5 and 457.7 eV for Ti 2p1/2 and Ti 2p3/2, respectively, and that for O 1s electron was 529.5 eV,44 which are close to the values of SL TiO2 film. In Na2Ti3O7, Na+ is incorporated into lattice forming chemical bond; however, in our case, as have been discussed in Figure 2, no diffraction peaks of titanate sodium are found. Yaghoubi et al. proposed that the thermal diffused Na+ from soda-lime glass into TiO2 film would aggregate on the surface of TiO2 grain forming the Ti−O−Na bond, in which Ti and O bonded through stable covalent bond, but Na and O bonded via weaker ionic bond.45 The formation of Ti−O−Na bond on the surface of TiO2 nanoparticle causes the left shift of the binding energies of Ti 2p and O 1s electrons for SL TiO2 film. Figure 5 shows the absorbance spectra of TiO2 films from 200 to 800 nm. As can be seen, TiO2 films show high

we also have measured the selected area electron diffraction (SAED) patterns of SL and QZ TiO2 films, which are shown in the inset images of Figure 3. As can be seen, the SAED pattern of SL TiO2 film exhibits very ambiguous and incontinuous diffraction circles. Contrastingly, the diffraction circles in the SAED pattern of QZ TiO2 film are more clear and intense. These results further testify that SL TiO2 film possesses much lower crystallinity. 3.3. Surface Elemental Compositions and the UV−Vis Absorbance Spectra of the TiO2 Films Fabricated on Different Glass Substrates. Table 1 shows the surface Table 1. XPS Surface Elemental Compositions (%) of SL, SiO2-SL, BS, and QZ TiO2 Films sample

Ti

O

Na

Mg

Ca

Si

C

SL SiO2-SL BS QZ

17.56 19.29 18.70 17.84

44.91 49.80 49.63 49.49

13.04 3.95 3.69 1.90

9.53 11.16 11.40 13.78

0.13

1.28 0.53 0.82 0.76

13.55 15.28 15.42 16.23

0.34

elemental compositions of the TiO2 films fabricated on different glass substrates through the sol−gel method. The molar ratios of Na+ to Ti on the surfaces of SL, SiO2-SL, BS, and QZ TiO2 films are 0.74, 0.20, 0.20, and 0.11, respectively, indicating that the molar ratio of Na+ on the surface of SL TiO2 film is more than 3 times higher than that of the other three TiO2 films. Because the contents of Na+ in quartz glass and borosilicate glass are much lower than that in soda-lime glass (see Table S1), and SiO2 layer can block the thermal diffusion of Na+ during calcination, thus QZ, BS, and SiO2-SL TiO2 films have much lower amounts of Na+ than that in SL TiO2 film. These observations are accordant with many other reported literatures.15,22,25,28 Besides, Figure 4 shows the XPS spectra of Ti 2p and O 1s electrons of these TiO2 films. As can be seen, the binding

Figure 5. UV−vis absorbance spectra of SL, SiO2-SL, BS, and QZ TiO2 films within 200−800 nm; the inset graph is the amplified spectra within 250−450 nm.

absorption ability within UV light region. The absorptions appeared within visible light region are caused by the interference effect of visible light in TiO2 films. As can be seen in the inset graph of Figure 5, the absorption edges of SiO2-SL, BS, and QZ TiO2 films are 345 nm, and that for SL TiO2 film is 340 nm. The slightly blue-shift of the absorption edge of SL TiO2 film compared to that of the other three TiO2 films probably caused by its slightly distorted surface structure, since the existence of Na+ forming Ti−O−Na bond on the surface of TiO2 nanoparticle in SL TiO2 film. 3.4. Influences of Na+ on the Crystallographic Properties of TiO2 Powder. To further testify the influences of Na+ on the crystallographic properties of TiO2, we have added various amounts of NaNO3 into titania sol and calcined the obtained xerogel powders at different temperatures. The XRD patterns of the resulted TiO2 powders are shown in Figure S4, and the average crystallite sizes and the phase compositions are summarized in Table 2. As can be seen in Figure S4, at 400 °C, the obtained diffraction peaks can be ascribed to anatase TiO2 (JCPDS No. 21-1272). However, the diffraction intensity of (101) peak of the TiO2 powder prepared

Figure 4. XPS spectra of Ti and O elements for SL, SiO2-SL, BS, and QZ TiO2 films.

energies of Ti 2p1/2 and Ti 2p3/2 electrons of SiO2-SL, BS, and QZ TiO2 films are around 464.2 and 458.4 eV. Dramatically, the binding energies of Ti 2p electrons for SL TiO2 film are left shifted about 0.5 eV compared to that of the other three TiO2 films, which are 463.7 eV for Ti 2p1/2 and 457.9 eV for Ti 2p3/2, respectively. A similar phenomenon is also observed for O 1s electrons. As can be seen in Figure 4, the binding energies of O 10394


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hydrolysis and polycondensation reactions of Ti precursor; thus, larger TiO2 nanoparticles were obtained. In this work, we use DEA as stabilizer which is a chelating agent and a weak base, and NaNO3 is added as Na+ source, which is neutral and will not intensely influence the PH value of titania sol. In addition, as also can be seen in Figure S4 (the amplified image of (b)), the diffraction peak positions of (101) plane of anatase TiO2 for the TiO2 powders prepared with Na+ show no apparent left or right shift compared to that of the one prepared without Na+, indicating that Na+ is not incorporated into TiO2 lattice, which is consistent with XPS observation that Na+ aggregates on the surface of TiO2 forming the Ti−O−Na bond. Additionally, for the TiO2 powders prepared with various amounts of Na+ and calcined at different temperatures, no peaks of brookite TiO2 are observed, which can be probably ascribed to the ratio of Na+ to Ti in powder system (Na:Ti = 0, 0.05, 0.1, and 0.15) is not as high as that in the TiO2 film fabricated on soda-lime glass (Na:Ti = 0.74) because NaNO3 has limited dissolving amount in solvent ethanol. As to why the existence of Na+ in the TiO2 film and powder prepared through sol−gel method hinders the crystallization of anatase TiO2, hampers the growth of anatase TiO2, and increases the transformation temperature of anatase to rutile, these can be ascribed to two reasons. The first reason is the existence of smaller sized Ti−O−Ti networks in the titania sol prepared by the sol−gel method.5 The second one is the formation of the Ti−O−Na bond on the surface of TiO2 nanocrystal, hindering the further growth or transformation of the Ti−O−Ti bond. 3.5. Influences of Crystallinity and the Content of Na+ on the Photocatalytic Activity of TiO2. In summary, there are three main differences for the TiO2 films fabricated on different glass substrates through the sol−gel method: (1) the content of Na+ in SL TiO2 film is much higher than that in SiO2-SL, BS, and QZ TiO2 films, forming the Ti−O−Na bond on the surface of TiO2 nanoparticles; (2) the crystallinity of SL TiO2 film is much lower than that of SiO2-SL, BS, and QZ TiO2 films; (3) SL TiO2 film is composed of anatase TiO2 and a small amount of brookite TiO2, but SiO2-SL, BS, and QZ TiO2 films are only consisted of anatase TiO2. Tay et al. found that the photocatalytic hydrogen generation property of the mixed phases of anatase and brookite TiO2 was higher than that of pure anatase and brookite TiO2.47 Thus, the existence of a small amount of brookite TiO2 in anatase TiO2 will not drastically decrease the photocatalytic activity of SL TiO2 film. The dominant reason should exists between the first and second ones. Some researchers also noticed that besides the TiO2 film fabricated on soda-lime glass through sol−gel method had higher content of Na+, it also had poor crystallinity than the TiO2 films fabricated on quartz glass,15,25 borosilicate glass, and the soda-lime glass precoated with a SiO2 layer.15,33 But most of them omitted the influence of crystallinity on the photocatalytic activity of TiO2 and ascribed the reason for the much lower photocatalytic activity of the TiO2 film fabricated on soda-lime glass to the existence of higher amount of Na+.15,25 They proposed that Na+ served as the recombination center of photogenerated electrons and holes.15,25,26,29,30,33 However, crystallinity is an important factor that governs the photocatalytic activity of TiO2, which should also be taking into account. We have fabricated TiO2 powders with different crystallinities by adding TBOT into water and calcining the obtained white precipitate at various temperatures. Figure 6 shows the

Table 2. Crystallographic Properties of the TiO2 Powders Containing Various Amounts of Na+ and Calcined at Different Temperaturesa Na:Ti

400 °C

450 °C

0

A(9.3 nm)

0.05

A(9.0 nm)

A(12.3 nm, 95.42%), R(15.9 nm, 4.58%) A(9.9 nm)

0.1 0.15

A(8.5 nm) A(7.6 nm)

A(9.7 nm) A(9.5 nm)

500 °C A(18.6 nm, 71.43%), R(24.1 nm, 28.57%) A(10.9 nm, 97.18%), R(11.9 nm, 2.82%) A(9.9 nm) A(9.8 nm)

a

A denotes anatase phase, R designates rutile phase, and the average crystallite sizes and the phase compositions (wt %) are given in parentheses.

without Na+ is about 1.11, 2.82, and 3.44 times higher than that of the TiO2 powders prepared with the molar ratios of Na to Ti at 0.05, 0.1 and 0.15, respectively, indicating that the existence of Na+ hampers the crystallization of anatase TiO2. At 450 °C, a little amount of rutile TiO2 (JCPDS No. 21-1276) appears in the TiO2 powder prepared without Na+, while the other three ones prepared with Na+ are only consisted of anatase TiO2. The weight percentages of rutile and anatase TiO2 in the TiO2 powder prepared without Na+ and calcined at 450 °C are 4.58% and 95.42%, respectively, which are calculated by the following equations: 1 R anatase = × 100% (101) (110) 1 + 1.26(Irutile + Ianatase ) (3) R rutile = 100% − R anatase

(4)

where Ranatase and Rrutile are the weight percentages of anatase (101) and rutile TiO2 and I(110) rutile and Ianatase are the areas of rutile (110) peak and anatase (101) peak, respectively.46 Further rising calcination temperature to 500 °C, the amount of rutile TiO2 increases in the TiO2 powder prepared without Na+ reaching to 28.57%. Besides, about 2.82% rutile TiO2 appears in the TiO2 powder prepared with Na to Ti at 0.05, but the TiO2 powders prepared with Na to Ti at 0.1 and 0.15 are still only composed of anatase TiO2. These results imply that the existence of Na+ increases the phase transformation temperature of anatase to rutile in TiO2 powder, which is accordance with the observation of Nam et al.32 Besides, as can be observed in Table 2, the crystallite size of anatase TiO2 in the TiO2 powder prepared without Na+ increases substantially with the rise of calcination temperature; however, that of the other three TiO2 powders prepared with Na+ increase moderately, illustrating that the existence of Na+ hinders the crystal growth of anatase TiO2. A similar phenomenon has also been observed in TiO2 film. Figure S5 shows the XRD patterns of the TiO2 films fabricated on sodalime glass and quartz glass through the sol−gel method and calcined at 500 °C for 1 h. Table S2 summarizes the average crystallite sizes and phase compositions of them. As can be seen, when improving calcination temperature from 450 to 500 °C, the crystallite size of anatase phase in SL TiO2 film increases slightly from 11.6 to 12.2 nm; nevertheless, that for QZ TiO2 film is raised from 14.0 to 18.1 nm. However, Nam et al. declared that Na+ could increase the crystallite size of TiO2,29,32 which is opposite to our observation. Nevertheless, there was a drawback of their experimental design. They used HCl solution as the stabilizer of titania sol. However, they added NaOH as the source of Na+, which could react with HCl increasing the pH value of titania sol and promoting the 10395


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photocatalytic activity. When TiO2 nanoparticles are well crystallized, other factors will further influence its photocatalytic activity, such as surface area, crystallite size, defects, etc. In order to investigate the influences of crystallinity and the content of Na+ on the photocatalytic activity of TiO2 film, we have fabricated TiO2 film with high crystallinity on soda-lime glass through the PVP modified sol−gel method (PVP-SL), and the thickness of PVP-SL TiO2 film is about 270 nm.5 We have treated PVP-SL TiO2 film with ultrasonic wash in distilled water (PVP-SL-UC) or immersing in H2SO4 solution (PVP-SLHS) to reduce the content of Na+. Figure S6 compares the XRD patterns of SL and PVP-SL TiO2 films, as can be seen, both of them are composed of anatase TiO2 (JCPDS No. 21-1272) and a small amount of brookite TiO2 (JCPDS No. 29-1360). The calculated weight percentages of brookite and anatase TiO2 in PVP-SL TiO2 film are 3.37% and 96.63%, respectively. The calculated average crystallite sizes of anatase and brookite TiO2 for PVP-SL TiO2 film are 9.4 and 9.0 nm, respectively. However, the diffraction intensity of anatase (101) peak of PVP-SL TiO2 film is about 3.78 times higher than of SL TiO2 film, indicating that the former has higher crystallinity than the latter. Besides, Table S3 summarizes the molar ratios of Na to Ti on the surface and in the internal of PVP-SL, PVPSL-UC, and PVP-SL-HS TiO2 films. It can be seen that the ratio of Na to Ti on the surface of PVP-SL TiO2 film is 2.72 and 3.78 times higher than that of PVP-SL-UC and PVP-SL-HS TiO2 films. Besides, in the internal, the ratio of Na to Ti of PVP-SL TiO2 film is also about 2.61 and 3.35 times higher than that in PVP-SL-UC and PVP-SL-HS TiO2 films. These results indicate that the amount of Na+ in PVP-SL TiO2 film can be effectively reduced by these treatments. In addition, as given in Figure S7, we also have observed that the binding energies of Ti 2p and O 1s electrons on the surface of PVP-SL TiO2 film are left shifted about 0.5 eV than that of PVP-SL-UC and PVP-SLHS TiO2 films, which indicates that Ti−O−Na bonds are formed on the surface of PVP-SL TiO2 film as SL TiO2 film, and much fewer Ti−O−Na bonds exist in PVP-SL-UC and PVP-SL-HS TiO2 films. The photocatalytic activities of SL, PVP-SL, PVP-SL-UC, and PVP-SL-HS TiO2 films are shown in Figure 8; acetone can be almost completely degraded by PVP-SL, PVP-SL-UC, and PVP-SL-HS TiO2 films within 25 min. As can be seen, the

Figure 6. XRD patterns of the TiO2 powders with different crystallinities.

XRD patterns of the resulted TiO2 powders; all diffraction peaks can be attributed to anatase TiO2 (JCPDS No. 21-1272). As can be seen, the diffraction intensity of T1 TiO2 powder (without calcination) is very weak; only a small peak of (101) plane exists, indicating that its crystallinity is very poor. At 200 °C for 1 h, the crystallinity of T2 TiO2 powder is a little bit enhanced comparing to that of T1 TiO2 powder. At 400 °C for 1 (T3) and 2 h (T4), the diffraction peaks are sharp and intense, illustrating that T3 and T4 TiO2 powders are with high crystallinities. Besides, the calculated average crystallite sizes of T1, T2, T3, and T4 TiO2 powders are 5.6, 7.5, 11.2, and 11.4 nm, respectively. Figure 7 shows the photocatalytic activities of

Figure 7. Evolved CO2 concentrations of the TiO2 powders with different crystallinities for the photodegradation of acetone within 25 min.

these TiO2 powders, and acetone can be almost completely degraded by T3 and T4 TiO2 powders within 25 min. The evolved CO2 concentrations of T1, T2, T3, and T4 TiO2 powders within 25 min are 489, 1070, 1629, and 1653 ppm, respectively, indicating that the photocatalytic activity of T1 TiO2 powder is 2.12, 3.33, and 3.38 times inferior to that of T2, T3, and T4 TiO2 powders, respectively. These results point out that the crystallinity of TiO2 can greatly influence its photocatalytic activity. That is because higher crystallinity means that more well-crystallized TiO2 nanocrystals are formed, thus more electrons and holes can be generated than that of the one with poor crystallinity under same conditions; thus, the TiO2 powder with higher crystallinity exhibits superior

Figure 8. Evolved CO2 concentrations of SL, PVP-SL, PVP-SL-UC, and PVP-SL-HS TiO2 films for the photodegradation of acetone within 25 min. 10396


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Figure 9. (a) and (b) are the side views of the structures of bare anatase TiO2 and the anatase TiO2 with Ti−O−Na bond on its (110) surface; (c) and (d) are the band structures of (a) and (b); (e) is the TDOS and PDOS of (b).

evolved CO2 concentrations of PVP-SL and SL TiO2 films within 25 min are 1225 and 251 ppm, respectively; the former is 4.88 times higher than the latter. The main difference in them is that PVP-SL TiO2 film has higher crystallinity than SL TiO2 film. On the other hand, as also can be seen in Figure 8, even though the amounts of Na+ in PVP-SL-UC and PVP-SL-HS TiO2 films are greatly reduced, however, their evolved CO2 concentrations within 25 min are 1262 and 1235 ppm, respectively, which are very close to that of PVP-SL TiO2 film (1225 ppm). These results testify that crystallinity plays an important role on the photocatalytic activity of TiO2 film, and the photocatalytic activity of TiO2 film is not apparently related to the content of Na+, even though Ti−O−Na bonds are formed on the surface of TiO2 nanoparticles, proving that the hypothesis of Na+ serving as the recombination center of photogenerated electrons and holes is not correct. 3.6. DFT Calculation Results. The structures of bare anatase TiO2 and the anatase TiO2 with Ti−O−Na bond on its (110) surface are shown in Figure 9a,b, and the calculated band structures of them are shown in Figure 9c,d. We can see that comparing to bare anatase TiO2, the formation of Ti−O−Na bond on its surface introduces an impurity level which is close to the valence band minimum (VBM) of TiO2. From the TDOS of Ti−O−Na bond on anatase TiO2 and the PDOS of Na 2s electrons, we can see that Na have a main peak in conduction band, but its intensity is much lower than that of the PDOS of Ti 3d electrons, meaning that Na contributes a small part to conduction band, which is consistent with its band structure. Besides, it also can be seen that Na 2s electrons have two small peaks in valence band, compared to that of the O 2p electrons, indicating that it also contributes a very little part to

valence band. Additionally, we also can observe that the shape of the conduction band peak of the PDOS of Na 2s electrons is similar to that of the Ti 3d and O 2p electrons, indicating that Ti, O, and Na atoms forming chemical bond, which is in accordance with the result of XPS analysis. Since the energy levels of Ti−O−Na bond lie in conduction band and valence band, Na+ is not the recombination center of photogenerated charge carriers of TiO2, which is consistent with our experimental results. Choi et al. doped TiO2 with multiple metal ions, and they found that doping TiO2 with metal ions with a core−shell electronic configuration, such as Li+, Mg2+, Al3+, Zn2+, Ga3+, Zr4+, Nb5+, etc., had little impact on the photocatalytic activity of TiO2.48 Na+ has a similar electron structure as Li+; our observation is consistent with their discovery.

4. CONCLUSIONS In this work, during the fabrications of TiO2 film and powder through the sol−gel method, it has been found that the existence of Na+ influences their crystallographic properties, which are inhibiting the crystallization and growth of anatase phase in TiO2 film and powder, promoting the formation of brookite phase in TiO2 film, and increasing the transformation temperature of anatase to rutile phase in TiO2 powder. The poor crystallinity of the TiO2 film fabricated on soda-lime glass is the main reason for its much lower photocatalytic activity compared to that of the TiO2 films fabricated on borosilicate glass, quartz glass, and the soda-lime glass precoated with a SiO2 layer. Additionally, even though Na+ forms Ti−O−Na bond on the surface of TiO2 nanoparticle, however, it dose not 10397


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Byproduct Using Palladium Modified TiO2 Films Under UV254+185nm Irradiation. Appl. Catal., B 2011, 105, 220−228. (7) Xu, Y. J.; Zhuang, Y. B.; Fu, X. Z. New Insight for Enhanced Photocatalytic Activity of TiO2 by Doping Carbon Nanotubes: A Case Study on Degradation of Benzene and Methyl Orange. J. Phys. Chem. C 2010, 114, 2669−2676. (8) Breault, T. M.; Bartlett, B. M. Lowering the Band Gap of AnataseStructured TiO2 by Coalloying with Nb and N: Electronic Structure and Photocatalytic Degradation of Methylene Blue Dye. J. Phys. Chem. C 2012, 116, 5986−5994. (9) Xie, Y. J.; Zhang, X.; Ma, P. J.; Wu, Z. J.; Piao, L. Y. Hierarchical TiO2 Photocatalysts with A One-Dimensional Heterojunction for Improved Photocatalytic Activities. Nano Res. 2015, 8, 2092−2101. (10) Choi, C.; Hwang, K. J.; Kim, Y. J.; Kim, G. W.; Park, J. Y.; Jin, S. H. Rice-Straw-Derived Hybrid TiO2-SiO2 Structures with Enhanced Photocatalytic Properties for Removal of Hazardous Dye in Aqueous Solutions. Nano Energy 2016, 20, 76−83. (11) Li, Y.; Lu, P. F.; Jiang, M. L.; Dhakal, R.; Thapaliya, P.; Peng, Z. H.; Jha, B.; Yan, X. Z. Femtosecond Time-Resolved Fluorescence Study of TiO2-Coated ZnO Nanorods/P3HT Photovoltaic Films. J. Phys. Chem. C 2012, 116, 25248−25256. (12) Tang, J. W.; Durrant, J. R.; Klug, D. R. Mechanism of Photocatalytic Water Splitting in TiO2. Reaction of Water with Photoholes, Importance of Charge Carrier Dynamics, and Evidence for Four-Hole Chemistry. J. Am. Chem. Soc. 2008, 130, 13885−13891. (13) Ramesha, G. K.; Brennecke, J. F.; Kamat, P. V. Origin of Catalytic Effect in The Reduction of CO2 at Nanostructured TiO2 Films. ACS Catal. 2014, 4, 3249−3254. (14) Yu, X.; Zhang, J.; Zhao, Z. H.; Guo, W. B.; Qiu, J. C.; Mou, X. N.; Li, A. X.; Claverie, J. P.; Liu, H. NiO-TiO2 P-N Heterostructured Nanocables Bridged by Zero-Bandgap rGO for Highly Efficient Photocatalytic Water Splitting. Nano Energy 2015, 16, 207−217. (15) Fernandez, A.; Lassaletta, G.; Jimenez, V. M.; Justo, A.; GonzSlezelipe, A. R.; Herrmann, J. M.; Tahiri, H.; Aitichou, Y. Preparation and Characterization of TiO2 Photocatalysts Supported on Various Rigid Supports (Glass, Quartz and Stainless Steel). Comparative Studies of Photocatalytic Activity in Water Purification. Appl. Catal., B 1995, 7, 49−63. (16) Yu, J. G.; Zhao, X. J. Effect of Surface Treatment on The Photocatalytic Activity and Hydrophilic Property of The Sol-Gel Derived TiO2 Thin Films. Mater. Res. Bull. 2001, 36, 97−107. (17) Zarubica, A.; Vasic, M.; Antonijevic, M. D.; Randelovic, M.; Momcilovic, M.; Krstic, J.; Nedeljkovic, J. Design and Photocatalytic Ability of Ordered Mesoporous TiO2 Thin Films. Mater. Res. Bull. 2014, 57, 146−151. (18) Duminica, F. D.; Maury, F.; Hausbrand, R. N-Doped TiO2 Coatings Grown by Atmospheric Pressure MOCVD for Visible LightInduced Photocatalytic Activity. Surf. Coat. Technol. 2007, 201, 9349− 9353. (19) Xie, T. H.; Lin, J. Origin of Photocatalytic Deactivation of TiO2 Film Coated on Ceramic Substrate. J. Phys. Chem. C 2007, 111, 9968− 9974. (20) Mahesh, K. P. O.; Kuo, D. H.; Huang, B. R.; Ujihara, M.; Imae, T. Chemically Modified Polyurethane-SiO2/TiO2 Hybrid Composite Film and Its Reusability for Photocatalytic Degradation of Acid Black 1(AB 1) under UV Light. Appl. Catal., A 2014, 475, 235−241. (21) Mishra, M. K.; Chattopadhyay, S.; Mitra, A.; De, G. Low Temperature Fabrication of Photoactive Anatase TiO2 Coating and Phosphor from Water-Alcohol Dispersible Nanopowder. Ind. Eng. Chem. Res. 2015, 54, 928−937. (22) Yu, J. G.; Zhao, X. J. Effect of Substrates on The Photocatalytic Activity of Nanometer TiO2 Thin Films. Mater. Res. Bull. 2000, 35, 1293−1307. (23) Zita, J.; Maixner, J.; Krysa, J. Multilayer TiO2/SiO2 Thin Sol-Gel Films: Effect of Calcination Temperature and Na+ Diffusion. J. Photochem. Photobiol., A 2010, 216, 194−200. (24) Yu, J. G.; Zhao, X. J.; Zhao, Q. N. Effect of Film Thickness on The Grain Size and Photocatalytic Activity of The Sol-Gel Derived Nanometer TiO2 Thin Films. J. Mater. Sci. Lett. 2000, 19, 1015−1017.

serve as the recombination center of photogenerated electrons and holes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01730. XRF compositions of the adopted three kinds of glass substrates, FESEM surface and cross-sectional images of TiO2 films, higher magnification HRTEM images of TiO2 films, XRD patterns of the TiO2 powders containing various amounts of Na+ and TiO2 films, XPS spectra and the molar ratios of Na to Ti for TiO2 films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (N.L.). *E-mail [email protected]; Phone +86-027-87652553, Fax +86-027-87883743 (X.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of Hubei Province (No. 2015CFB227), the Fundamental Research Funds for the Central Universities (No. 20410686), the research borad of the State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology (No. 47152005), and the Natural Science Foundation of Shenzhen under Grant (No. JCYJ20130402113127530). The project sponsored by the scientific research foundation for the returned overseas chinese scholars. This work is also supported by the National Natural Science Foundation (No. 51461135004), the Natural Science Foundation of Hubei Province (No. 2013CFA008), the Doctoral Fund of Ministry of education priority development project (No. 20130143130002), and the key technology innovation project of Hubei Province (No. 2013AAA005).

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REFERENCES

(1) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J. L.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114, 9919−9986. (2) Ong, W. J.; Tan, L. L.; Chai, S. P.; Yong, S. T.; Mohamed, A. R. Facet-Dependent Photocatalytic Properties of TiO2-Based Composites for Energy Conversion and Environmental Remediation. ChemSusChem 2014, 7, 690−719. (3) Li, H. B.; Zhou, Q. Y.; Gao, Y. T.; Gui, X. C.; Yang, L.; Du, M. D.; Shi, E. Z.; Shi, J. D.; Cao, A. Y.; Fang, Y. Templated Synthesis of TiO2 Nanotube Macrostructures and Their Photocatalytic Properties. Nano Res. 2015, 8, 900−906. (4) Kitano, S.; Murakami, N.; Ohno, T.; Mitani, Y.; Nosaka, Y.; Asakura, H.; Teramura, K.; Tanaka, T.; Tada, H.; Hashimoto, K.; et al. Bifunctionality of Rh3+ Modifier on TiO2 and Working Mechanism of Rh3+/TiO2 Photocatalyst under Irradiation of Visible Light. J. Phys. Chem. C 2013, 117, 11008−11016. (5) Xie, H.; Liu, B. S.; Zhao, X. J. Facile Process to Greatly Improve The Photocatalytic Activity of The TiO2 Thin Film on Window Glass for The Photodegradation of Acetone and Benzene. Chem. Eng. J. 2016, 284, 1156−1164. (6) Fu, P. F.; Zhang, P. Y.; Li, J. Photocatalytic Degradation of Low Concentration Formaldehyde and Simultaneous Elimination of Ozone 10398


Article


Exposed {101} Anatase Facets and Enhanced Visible Light Photocatalytic Performance. Appl. Catal., B 2015, 170−171, 17−24. (45) Yaghoubi, H.; Taghavinia, N.; Alamdari, E. K.; Volinsky, A. A. Nanomechanical Properties of TiO2 Granular Thin Films. ACS Appl. Mater. Interfaces 2010, 2, 2629−2636. (46) Wang, C. C.; Ying, J. Y. Sol-Gel Synthesis and Hydrothermal Processing of Anatase and Rutile Titania Nanocrystals. Chem. Mater. 1999, 11, 3113−3120. (47) Tay, Q. L.; Liu, X. F.; Tang, Y. X.; Jiang, Z. L.; Sum, T. C.; Chen, Z. Enhanced Photocatalytic Hydrogen Production with Synergistic Two-Phase Anatase/Brookite TiO2 Nanostructures. J. Phys. Chem. C 2013, 117, 14973−14982. (48) Choi, W. Y.; Termin, A.; Hoffmann, M. R. The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics. J. Phys. Chem. 1994, 98, 13669−13679.

(25) Krysa, J.; Novotna, P.; Kment, S.; Mills, A. Effect of Glass Substrate and Deposition Technique on The Properties of Sol Gel TiO2 Thin Films. J. Photochem. Photobiol., A 2011, 222, 81−86. (26) Aubry, E.; Lambert, J.; Demange, V.; Billard, A. Effect of Na Diffusion from Glass Substrate on The Microstructural and Photocatalytic Properties of Post-Annealed TiO2 Films Synthesised by Reactive Sputtering. Surf. Coat. Technol. 2012, 206, 4999−5005. (27) Neti, N. R.; Parmar, G. R.; Bakardjieva, S.; Subrt, J. Thick Film Titania on Glass Supports for Vapour Phase Photocatalytic Degradation of Toluene, Acetone, and Ethanol. Chem. Eng. J. 2010, 163, 219−229. (28) Novotna, P.; Krysa, J.; Maixner, J.; Kluson, P.; Novak, P. Photocatalytic Activity of Sol-gel TiO2 Thin Films Deposited on Soda Lime Glass and Soda Lime Glass Precoated with a SiO2 Layer. Surf. Coat. Technol. 2010, 204, 2570−2575. (29) Nam, H. J.; Amemiya, T.; Murabayashi, M.; Itoh, K. Photocatalytic Activity of Sol-Gel TiO2 Thin Films on Various Kinds of Glass Substrates: The Effects of Na+ and Primary Particle Size. J. Phys. Chem. B 2004, 108, 8254−8259. (30) Paz, Y.; Heller, A. Photo-Oxidatively Self-Cleaning Transparent Titanium Dioxide Films on Soda Lime Glass: The Deleterious Effect of Sodium Contamination and Its Prevention. J. Mater. Res. 1997, 12, 2759−2766. (31) Watanabe, T.; Fukayama, S.; Miyauchi, M.; Fujishima, A. Photocatalytic Activity and Photo-Induced Wettability Conversion of TiO2 Thin Film Prepared by Sol-Gel Process on a Soda-Lime Glass. J. Sol-Gel Sci. Technol. 2000, 19, 71−76. (32) Nam, H. J.; Amemiya, T.; Murabayashi, M.; Itoh, K. The Influence of Na+ on The Crystallite Size of TiO2 and The Photocatalytic Activity. Res. Chem. Intermed. 2005, 31, 365−370. (33) Stangar, U. L.; Cernigoj, U.; Trebse, P.; Maver, K.; Gross, S. Photocatalytic TiO2 Coatings: Effect of Substrate and Template. Monatsh. Chem. 2006, 137, 647−655. (34) Guillard, C.; Beaugiraud, B.; Dutriez, C.; Herrmann, J. M.; Jaffrezic, H.; Renault, N. J.; Lacroix, M. Physicochemical Properties and Photocatalytic Activities of TiO2 Films Prepared by Sol-Gel Methods. Appl. Catal., B 2002, 39, 331−342. (35) Tada, H.; Tanaka, M. Dependence of TiO2 Photocatalytic Activity upon Its Film Thickness. Langmuir 1997, 13, 360−364. (36) Li, Y. Z.; Sun, Q.; Kong, M.; Shi, W. Q.; Huang, J. C.; Tang, J. W.; Zhao, X. J. Coupling Oxygen Ion Conduction to Photocatalysis in Mesoporous Nanorod-like Ceria Significantly Improves Photocatalytic Efficiency. J. Phys. Chem. C 2011, 115, 14050−14057. (37) Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (38) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using A Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (39) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (40) Eymery, J.; Rieutord, F.; Nicolin, V. F.; Robach, O.; Niquet, Y. M.; Froberg, L.; Martensson, T.; Samuelson, L. Strain and Shape of Epitaxial InAs/InP Nanowire Superlattice Measured by Grazing Incidence X-Ray Techniques. Nano Lett. 2007, 7, 2596−2601. (41) Zhao, Y. B.; Pan, F.; Li, H.; Zhao, D. X.; Liu, L.; Xu, G. Q.; Chen, W. Uniform Mesoporous Anatase-Brookite Biphase TiO2 Hollow Spheres with High Crystallinity via Ostwald Ripening. J. Phys. Chem. C 2013, 117, 21718−21723. (42) Ohara, C.; Hongo, T.; Yamazaki, A.; Nagoya, T. Synthesis and Characterization of Brookite/Anatase Complex Thin Film. Appl. Surf. Sci. 2008, 254, 6619−6622. (43) Gaikwad, P. N.; Hankare, P. P.; Wandre, T. M.; Garadkar, K. M.; Sasikala, R. Photocatalytic Performance of Magnetically Separable Fe, N Co-Doped TiO2-Cobalt Ferrite Nanocomposite. Mater. Sci. Eng., B 2016, 205, 40−45. (44) Liu, C.; Sun, T.; Wu, L.; Liang, J. Y.; Huang, Q. J.; Chen, J.; Hou, W. H. N-Doped Na2Ti6O13@TiO2 Core-Shell Nanobelts with 10399
