Topics in Catalysis

, Volume 52, Issue 12, pp 1651–1659

Application of Highly Functional Ti-Oxide-Based Photocatalysts in Clean Technologies

Authors

  • Masato Takeuchi
    • Department of Applied Chemistry, Graduate School of EngineeringOsaka Prefecture University
  • Shirou Sakai
    • Department of Applied Chemistry, Graduate School of EngineeringOsaka Prefecture University
  • Afshin Ebrahimi
    • Department of Applied Chemistry, Graduate School of EngineeringOsaka Prefecture University
  • Masaya Matsuoka
    • Department of Applied Chemistry, Graduate School of EngineeringOsaka Prefecture University
    • Department of Applied Chemistry, Graduate School of EngineeringOsaka Prefecture University
Original Paper

DOI: 10.1007/s11244-009-9300-7

Cite this article as:
Takeuchi, M., Sakai, S., Ebrahimi, A. et al. Top Catal (2009) 52: 1651. doi:10.1007/s11244-009-9300-7

Abstract

Various Ti-oxide based photocatalysts such as the highly dispersed Ti-oxide species within zeolite frameworks, TiO2 nano-particles hybridized with hydrophobic zeolite adsorbents as well as visible light responsive TiO2 thin films have been successfully prepared. Characterization studies at the molecular level, such as X-ray absorption fine structure (XAFS) and photoluminescence (PL), revealed that the highly dispersed Ti-oxide species within the nano-spaces of zeolites possess a tetrahedral coordination and that they demonstrate unique and high performance for the photocatalytic decomposition of NOx and the photocatalytic reduction of CO2 with H2O. A high photocatalytic reactivity for the TiO2 semiconducting photocatalysts could be achieved by blending them with hydrophobic siliceous zeolites which was equal to the performance of TiO2 deposited with expensive Pt particles. The role of the siliceous zeolites can be described as a so-called “catch and release effect of organic compounds”, i.e., (i) the condensation of the reactants within the hydrophobic cavities of zeolites and; (ii) the efficient diffusion of the reactant onto the TiO2 photocatalytic sites. Furthermore, a novel photocatalytic system which can convert abundant solar energy into renewable H2 energy by the decomposition of H2O into H2 and O2 can also be achieved by using visible light responsive TiO2 thin film photocatalysts prepared by a RF-magnetron sputtering deposition method. The conversion efficiency of solar energy into H2 energy may be estimated at ca. 0.1% from the initial rate of H2 evolution.

Keywords

PhotocatalystsSingle-siteZeolitesVisible light

1 Introduction

Environmental pollution on a global scale as well as the lack of natural energy resources have drawn much attention to the vital need for ecologically clean chemical technologies—one of the most urgent challenges facing chemical scientists today. Since the photosensitization effect of a TiO2 electrode on water electrolysis was discovered by Honda and Fujishima [1], pollution-free photocatalysis by TiO2 semiconductors has been widely studied with the final goal of the efficient conversion of clean solar energy into useful chemical energy such as hydrogen [29]. The effective use of clean solar energy will lead to many new promising solutions not only for energy issues caused by the exhaustion of fossil fuels but also for the abatement of environmental toxins. Along these lines, photocatalysts which can operate under visible and/or solar light irradiation are strongly desired for applications in the purification and sustenance of our living environment.

Studies have also been carried out on TiO2 nano-particles as well as on various Ti-oxide based binary oxides such as TiO2/SiO2, TiO2/Al2O3 and TiO2/B2O3 [1013]. In particular, we have found that TiO2 nano-particles of less than 10 nm show significant enhancement in photocatalytic reactivity. This phenomenon is due to an electronic modification of the TiO2 semiconductors as well as the close existence of the photo-formed electron and hole pairs and their efficient contribution to the photoreaction. These findings have provided new insights into the development of highly dispersed transition-metal oxide species as single-site catalysts. Moreover, the application of an anchoring method enabled the preparation of molecular or cluster-sized photocatalysts on various supports such as SiO2, Al2O3, various zeolites and mesoporous materials. As shown in Fig. 1, highly dispersed Ti oxide species incorporated within the cavities or frameworks of zeolites are especially interesting due to their unique local structures such as their four-fold coordinated species and efficient photocatalytic properties for the reduction of CO2 with H2O, NO decomposition as well as the selective photoepoxidation of alkene with O2, as compared to semiconducting photocatalysts [1424].
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Fig. 1

Schematic diagram of the highly dispersed TiO4 species of tetrahedral coordination incorporated within the zeolite framework as a single site photocatalyst

In fact, various air-cleaning systems equipped with TiO2 photocatalysts and UV light sources that reduce volatile organic compounds (VOCs) which cause the so-called “sick house syndrome” are now commercially available. However, the removal efficiency of air-cleaning systems still needs to be improved to be as simple and low-cost as possible for widespread use. Although the deposition of small amounts of Pt on TiO2 catalysts is known to enhance their photocatalytic reactivity [2529], Pt is too costly for common use in home electrical appliances. Meanwhile, the hybridization of adsorbents such as zeolites or mesoporous materials [1419, 3034] with TiO2 particles has been reported to show elevated photocatalytic reactivity. In a previous report [35], TiO2 nano-particles hybridized with siliceous zeolites prepared by impregnation as well as a simple mechanical blending method showed higher photocatalytic reactivity for the complete oxidation of gaseous acetaldehyde than TiO2 catalysts since such siliceous zeolites can efficiently condense acetaldehyde diffused in the gas phase and smoothly supply them onto TiO2 photocatalyst surfaces.

The conversion of solar light energy into renewable clean energy is also one of the most challenging research topics in science. Sunlight including near-infrared, visible and ultraviolet light provide tremendous energy of ca. 87–308 kJ mol−1 so that solar energy should be utilized as efficiently as possible [3642]. It will, thus, be of great importance to develop effective systems able to convert abundant solar light energy into applicable and sustainable energy resources. At least two systems have been considered for the conversion of sunlight into other renewable energy sources: one is the design of solar cells to convert sunlight into electricity and the other is artificial photosynthesis for the conversion and storage of solar energy into safe and useful chemical energy. Although hydrogen is also the focus of much attention as a renewable clean energy alternative, at the moment, we do not have any highly efficient systems to produce hydrogen in an environmentally harmonious way without producing CO2. From this viewpoint, the photocatalytic or photoelectrochemical decomposition of water to produce hydrogen under solar light irradiation is now of utmost importance.

In this review, the development of highly functional Ti-oxide based photocatalysts, i.e., (i) tetrahedral Ti-oxide species incorporated within the framework of zeolites and mesoporous materials as single-site photocatalysts; (ii) TiO2 nano-particles hybridized with hydrophobic zeolite adsorbents in practical applications for photocatalytic air-cleaning systems; and (iii) the photocatalytic decomposition of H2O into H2 and O2 under solar light irradiation using visible light-responsive TiO2 thin films prepared by a RF-magnetron sputtering deposition method will be summarized.

2 The Design of Highly Dispersed Molecular-Sized Ti-Oxide Species as a Single-Site Photocatalyst Incorporated Within the Framework of Zeolites and Mesoporous Materials

2.1 Photocatalytic Reduction of NOx

The removal of NOx from exhaust emission gases of internal combustion engines or industrial boilers is one of the most urgent issues we face today since NOx is an atmospheric pollutant which causes acid rain and photochemical smog. In fact, a great challenge has been the direct decomposition of NO into harmless N2 and O2. To address such concerns, highly dispersed titanium oxides incorporated within the framework of zeolites or mesoporous materials can be considered as one of the most promising candidates in the design of effective photocatalysts for the decomposition of NOx directly into N2 and O2 [4348].

Highly dispersed Ti-oxide species within Al2O3 or SiO2 matrices can be easily prepared by the sol-gel method or a precipitation method [1013]. Figure 2 shows the XAFS spectra of Ti/Si binary oxides. The Ti K-edge XANES (left) of binary oxides with low Ti content of less than 10 wt% exhibit an intense preedge peak in the XANES region. The curve fitting analysis of the EXAFS oscillation (right) indicates that these catalysts mainly consist of isolated tetrahedral TiO4 species having a Ti–O bond distance of ca. 1.83 Å within the SiO4 matrices. On the other hand, the samples having Ti content of higher than 50 wt% showed three small preedge peaks in the XANES region which is typical of an anatase structure for TiO2 and showing the existence of aggregated TiO2 nano-particles.
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Fig. 2

Ti K-edge XANES (a–d) and Fourier transforms of EXAFS oscillation (A–D) of the Ti/Si binary oxide photocatalysts prepared by a sol–gel method. Ti content (vol.%): (a, A) 1; (b, B) 20; (c, C) 50; (d, D) 100

These highly dispersed tetrahedral TiO4 species can also be incorporated within the framework of zeolites or mesoporous materials and were observed to show unique photocatalytic performance, especially for the hydrogenolysis reaction of unsaturated hydrocarbons with H2O, the direct decomposition of NO, and the photoreduction of CO2 with H2O. UV light irradiation of these TiO2/SiO2 catalysts in the presence of NO was found to lead to the effective decomposition of NO to produce N2 with high selectivity, while the TiO2 semiconductor photocatalyst decomposed NO into mainly N2O.

As shown in Fig. 3, as the coordination number of the Ti-oxide species in the binary oxide catalysts as determined by XAFS measurements decreased, the selectivity for N2 formation in the photocatalytic decomposition of NO increased. The highly dispersed Ti-oxide species showed a unique photoluminescence at around 480 nm upon excitation by UV light irradiation at around 250 nm. As shown in Fig. 1, the absorption and photoluminescence can be attributed to the charge transfer process and its reverse radiative deactivation from the excited state of the tetrahedral Ti-oxide species, respectively. Since a good relationship between the photoluminescence intensities and the yield of the photocatalytic reaction could be observed, the highly dispersed tetrahedral Ti-oxide species was, thus, found to play an important role in the direct photocatalytic decomposition of NO into N2 with high selectivity. Furthermore, the unique photoluminescence of the tetrahedral Ti-oxide species was effectively quenched by the addition of NO. These findings indicate that the added NO molecules easily interact with the excited state of the Ti-oxide species.
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Fig. 3

Relationship between the coordination number of the Ti-oxide of the photocatalysts and selectivity for the formation of N2 in the photocatalytic decomposition of NO on various Ti-containing catalysts

As illustrated in Fig. 4, we have proposed the reaction mechanism for the decomposition of NO over the tetrahedral Ti-oxide species as a single-site photocatalyst, i.e., two NO molecules are able to adsorb onto the Ti-oxide species as weak ligands to form the reaction precursors. Under UV-irradiation, the charge-transfer excited complexes of the oxides [Ti3+–O]* are formed. Within their lifetimes, the electron transfers from the Ti3+ site at which the photo-formed electrons are trapped into the anti-π*-bonding orbital of NO molecule while the electron simultaneously transfers from the π-bonding orbital of another NO molecule into the O site at which the photo-formed holes are trapped. These simultaneous electron transfers lead to the direct decomposition of two sets of NO into N2 and O2 over the tetrahedral Ti-oxide photocatalyst under UV light irradiation. These results clearly demonstrate that zeolites and mesoporous materials used as supports enable the anchoring of the tetrahedral TiO4 species within their frameworks and/or cavities in a highly dispersed state. Such tetrahedral Ti-oxide photocatalysts are, thus, promising candidates for unique applications in the reduction of toxic NOx.
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Fig. 4

The reaction mechanism of the photocatalytic decomposition of NO into N2 and O2 on the tetrahedrally coordinated Ti-oxide species under UV light irradiation

2.2 Photocatalytic Reduction of CO2 by H2O

The development of photocatalytic reduction systems of CO2 with H2O into valuable chemicals such as CH3OH or CH4 is a challenging goal in research on environmentally friendly catalysts. We have found that the highly dispersed Ti-oxide species within zeolite frameworks show unique photocatalytic reactivity for the photoreduction of CO2 with H2O as compared with bulk semiconducting TiO2 photocatalysts [4956]. In fact, UV-irradiation of the Ti/zeolite catalysts in the presence of CO2 and H2O catalyzed the photocatalytic reduction of CO2 to form CH3OH and CH4 as major products as well as CO, O2, C2H4, and C2H6 as minor products.

The photoreduction of CO2 with H2O over the Ti-β zeolites, which were synthesized under hydrothermal conditions in the presence of OH or F anions of structure-directing agents (SDA) [53], have also been investigated. Ti-β(OH) and Ti-β(F) catalysts showed an absorption band at around 220–250 nm and photoluminescence spectra at around 470–500 nm, attributed to the tetrahedral TiO4 species. The photoluminescence yield of Ti-β(OH), which is related to the concentration of the tetrahedral TiO4 species in the excited state, is much higher than that of Ti-β(F). As shown in Fig. 5, the photoluminescence spectra due to the highly dispersed Ti-oxide species in the Ti-β zeolites were quenched by the addition of H2O or CO2 molecules. Moreover, the lifetime of the charge-transfer excited state was observed to be shortened by the addition of CO2 and H2O molecules. The lifetime of the charge transfer excited state of such Ti-oxide species was also shortened. Such a quenching of the photoluminescence suggests that the added CO2 and H2O easily interact with the Ti-oxide species of the Ti-β zeolite in its excited state. In the photocatalytic reduction of CO2 with H2O over these Ti-containing zeolites, Tiβ(OH) showed higher reactivity as compared to other catalysts. However, the Tiβ(F) catalyst showed the highest selectivity for CH3OH formation among these Ti-containing zeolites. The higher reactivity of Ti-β(OH) over Ti-β(F) can be explained by the higher concentration of charge-transfer excited complexes, as observed by photoluminescence measurements. These results clearly indicate that the highly dispersed tetrahedral Ti-oxide species exhibit higher selectivity as well as efficiency for CH3OH formation as compared with bulk TiO2 semiconductors. Figure 6 shows the relationship between the coordination numbers of the Ti-oxide species obtained from XAFS measurements and the selectivity for CH3OH formation in the photocatalytic reduction of CO2 with H2O on various Ti-containing zeolites. A clear dependence of the selectivity for CH3OH formation on the coordination numbers of the Ti-oxide species can be observed, i.e., the tetrahedral Ti-oxide species showed higher selectivity for CH3OH formation. However, the TiO2 semiconductor did not show any reactivity in the photocatalytic reduction of CO2 and H2O. From these results, it can be proposed that the highly efficient and selective photocatalytic reduction of CO2 with H2O can be achieved by using Ti/zeolites involving a highly dispersed tetrahedral Ti-oxide species in their frameworks.
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Fig. 5

Effect of the addition of (a, b) CO2 or (A, B) H2O molecules on the photoluminescence spectra of: (a, A) Ti-β(OH) and (b, B) Ti-β(F) catalysts at 298 K. Excitation: 260 nm. Amount of added (a, b) CO2 or (A, B) H2O: (a) 0, 0.05, 0.21 and 1.1 mmol/g; (b) 0, 5.3, 10.5 and 15.8 mmol/g; (A) 0, 0.11, 0.27 and 0.53 mmol/g; (B) 0, 0.27, 0.52 and 1.04 mmol/g (top to bottom, respectively)

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Fig. 6

Relationship between the coordination number of the Ti-oxide species of the Ti/zeolite catalysts and selectivity for CH3OH formation in the photocatalytic reduction of CO2 with H2O on various Ti/zeolite catalysts

2.3 Enhancement of the Photocatalytic Reactivity of TiO2 Nano-Particles Hybridized with Hydrophobic Zeolite Adsorbents

The highly dispersed tetrahedral Ti-oxide species incorporated within the framework of zeolite or mesoporous materials was observed to show unique photocatalytic reactivity for the reduction of NOx or CO2 [4356]. However, such highly dispersed Ti-oxide species are less effective for the complete oxidation of organic compounds into harmless CO2 and H2O. For the purification of polluted air, water and soil, improvement in the photocatalytic reactivity of TiO2 photocatalysts is still strongly desired. In this section, the effective enhancement of the photocatalytic reactivity of TiO2 nano-particles by mixing with hydrophobic zeolite adsorbents has been summarized [3032, 35, 57].

The photocatalytic reactivity of TiO2 nano-particles hybridized with a hydrophobic ZSM-5 zeolite for the oxidation of acetaldehyde with O2 as well as the effects of H2O vapor addition on their photocatalytic reactivity are shown in Fig. 7. The reactivity was found to strongly depend on the SiO2/Al2O3 ratios of the ZSM-5 zeolites. In the absence of the H2O vapor, the TiO2 hybridized with H-ZSM-5(1880) showed the highest photocatalytic reactivity. As the Al2O3 content of the ZSM-5 zeolites increased, the reactivity decreased, whereas TiO2 prepared on Na-ZSM-5(23.8) did not show any reactivity. Adsorption isotherm measurements of acetaldehyde molecules on these ZSM-5 zeolites revealed that the amount of saturated acetaldehyde on the ZSM-5 zeolites could be estimated at ca. 2.0–2.5 mmol/g, despite their different SiO2/Al2O3 ratios. However, TPD measurements revealed that the interaction of the acetaldehyde molecules with the ZSM-5 zeolite surfaces depend on the different SiO2/Al2O3 ratios, indicating that the amount of saturated adsorbed acetaldehyde depends on the surface area of the ZSM-5 zeolites (ca. 300–350 m2/g) and not on the surface chemical properties originating from changes in the SiO2/Al2O3 ratios. The silanol groups in the H-ZSM-5(1880) zeolite can condense acetaldehyde molecules and then efficiently provide them onto the TiO2 photocatalytic sites, resulting in higher photocatalytic reactivity. On the other hand, the H+ sites on H-ZSM-5(220 and 68) did not work as efficient adsorption sites for the acetaldehyde molecules, resulting in lower photocatalytic reactivity. Moreover, the acetaldehyde molecules largely trapped on the Na+ sites of the Na-ZSM-5 zeolite could not diffuse onto the TiO2 sites, resulting in no photocatalytic reactivity. The H2O vapor was observed to promote effectively the photocatalytic oxidation of acetaldehyde with O2 on the TiO2 photocatalysts hybridized with ZSM-5 zeolites. The role of the H2O molecules in the photocatalytic oxidation of acetaldehyde is considered to be significant as the source of the active OH radicals. Moreover, since H2O molecules strongly adsorb on the Ti4+ sites of the TiO2 surfaces, they may inhibit the strong adsorption of acetaldehyde as a reactant or acetic acid molecules as the intermediate species on the TiO2 photocatalytic surfaces, followed by a decrease in photocatalytic reactivity.
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Fig. 7

Photocatalytic oxidation of acetaldehyde in the absence and presence of H2O vapor over TiO2 prepared on various ZSM-5 zeolites under UV light irradiation

Interestingly, the photocatalytic reactivity of commercial TiO2 nano-particles could easily be enhanced by simple mechanical blending with a high-silica zeolite. The diffuse reflectance UV–Vis absorption spectra of the TiO2 particles mechanically blended with the high-silica mordenite (MOR) zeolite are shown in Fig. 8A. When smaller amounts of TiO2 powders than 5 wt% were blended with the siliceous zeolite, some of the incident light was found to pass through powder samples of several millimeters thickness due to the high transparency of the siliceous zeolite. When the amount of TiO2 nano-powders blended with the zeolite reached about 10–20 wt%, the incident light could not penetrate the mixed powder samples, suggesting efficient irradiation of UV light onto the entire TiO2 particles, as shown in the inserted illustration. However, since the absorption coefficient of the TiO2 powder in UV light regions is known to be high, as the fraction of the TiO2 powders to the zeolite powders increases in the blended catalysts, the fraction of the incident light that penetrates into the catalysts decreases and then it becomes impossible to irradiate most of the TiO2 particles in the catalysts. These results clearly indicate that one of the important properties of the siliceous zeolite powders is the efficient irradiation of incident UV light onto all of the TiO2 particles without any loss of light intensity.
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Fig. 8

A Diffuse reflectance UV–Vis absorption spectra of TiO2 particles mechanically blended with the MOR zeolite. TiO2 contents (wt%): (a) 100, (b) 50, (c) 20, (d) 10, (e) 5, (f) 1, (g) 0. B Photocatalytic reactivity (UV light irradiation: 3 h) of the TiO2 particles mechanically blended with MOR zeolite samples

Figure 8B shows the effects of the TiO2 content on the photocatalytic reactivity of the TiO2/MOR samples for the oxidation of acetaldehyde with O2 under UV light irradiation. TiO2 particles of ca. 5–15 wt% mechanically blended with the hydrophobic zeolite showed almost twice as high photocatalytic reactivity as the commercial TiO2 sample. Since the high-silica zeolite does not have Brönsted acid sites, which strongly adsorb various polar molecules, the acetaldehyde concentrated within the zeolite cavities could smoothly diffuse on the blended TiO2 surfaces. In addition, as seen from the results of UV–Vis absorption measurements, the incident UV light was efficiently irradiated on the entire TiO2 particles of ca. 5–15 wt% blended with the zeolite due to the high transparency of the zeolite powder in UV–Vis light regions. From these results, the major factors for the hydrophobic zeolite in enhancing the photocatalytic reactivity of the TiO2 particles can be concluded as: (i) the condensation effect for acetaldehyde nearby the TiO2 photocatalytic sites; and (ii) the appropriate diluent effect of the TiO2 photocatalysts as an intense absorber of UV light with highly transparent zeolite powders.

3 Separate Evolution of H2O into H2 and O2 Using Visible Light-Responsive TiO2 Thin Film Photocatalysts Prepared by a RF-Magnetron Sputtering Deposition Method [3642]

TiO2 thin films were prepared by a RF magnetron sputtering (RF-MS) deposition method, as shown in Fig. 9A. The various procedures for the deposition method are detailed in previous literature, however, here, the TiO2 thin films were prepared by sputtering the TiO2 target (grade: 99.99%) with only Ar gas plasma in the absence of O2 atmosphere as a reactive gas. Since the thin films were deposited under high vacuum condition, the risk of contamination of the TiO2 thin films with impurities could be avoided. The UV–Vis absorption (transmittance) spectra of the TiO2 thin films prepared at different preparation temperatures are shown in Fig. 9B. The TiO2 thin films prepared at temperatures lower than 473 K showed high transparency and clear interference fringes in the visible light region, similar to the TiO2 thin films prepared by sol–gel [5861] or ionized cluster beam (ICB) deposition [62, 63]. These results clearly show that stoichiometric TiO2 thin films can be prepared using a TiO2 plate as a sputtering target and Ar as the sputtering gas without the coexistence of O2 as the reactive gas. When the preparation temperatures were increased, the TiO2 thin films were found to show effective absorption in visible light regions with a maximum for the thin film deposited at 873 K. Since the amount of impurities included in the TiO2 target material is quite low, hardly any impurities were included in the deposited films. The TiO2 thin films prepared by sputtering the TiO2 target in the presence of O2 at 873 K did not show any significant absorption in the visible light region (spectrum not shown). These results indicate that visible light-responsive TiO2 thin films can be successfully prepared only when the TiO2 target as an ion source is sputtered with Ar gas without a mixture of O2 gas at temperatures higher than 773 K.
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Fig. 9

A Schematic diagram of the RF magnetron sputtering deposition method. B UV–Vis absorption (transmittance) spectra of the TiO2 thin films prepared at different preparation temperatures. (Sputtering gas: Ar) Preparation temperatures: (a) 373, (b) 473, (c) 673, (d) 873 and (e) 973 K

In order to discuss the mechanism for the absorption of visible light, SEM images of the TiO2 films were observed (Fig. 10). Differences between the UV and visible light-responsive TiO2 thin films could be observed in their cross-sectional views. The TiO2 film prepared at 473 K has a structure in which nano-sized TiO2 particles are randomly sintered with each other. On the other hand, the thin films prepared at 873 K show a unique structure in which TiO2 single crystals with a columnar structure (diameter: ca. 100 nm) are orderly aligned. The depth profiles of the O/Ti atomic ratio were also investigated by AES measurements (data not shown). The O/Ti ratio of the UV-type TiO2 film prepared at 473 K was 2.0 at the surface and the ratio from the surface to deep bulk was constant, suggesting a stoichiometric TiO2 composition. On the other hand, the O/Ti ratio of the Vis-type TiO2 film prepared at 873 K was found to decrease gradually from 2.00 at the surface to 1.933 in the deep bulk. It has already been reported that small amounts of oxygen vacancies in the TiO2 lattice give rise to a distortion of the TiO2 octahedral unit and weaken the Ti–O bonds, resulting in a reduction of the splitting between the bonding and nonbonding levels [64]. Taking these results into consideration, such a characteristic declined structure in the O/Ti atomic ratio as well as the orderly aligned columnar TiO2 crystals of the Vis-type TiO2 thin films may be related to the modification of the electronic properties of the TiO2 semiconductors, enabling the efficient absorption of visible light.
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Fig. 10

Cross-sectional SEM images of: A the UV-type TiO2 thin film prepared at 473 K and B the Vis-type TiO2 thin film prepared at 873 K

Vis-type TiO2 thin films were, thus, applied for the separate evolution of H2 and O2 from the decomposition of water under solar light irradiation. The Vis-type TiO2 film was prepared on one side of the Ti foil while the opposite side was deposited with small amounts of Pt. The prepared photocatalytic device was mounted on an H-type cell, as shown in Fig. 11a. The TiO2 side of the photocatalyst was immersed in 1.0 M NaOH solution and the Pt side was immersed in 0.5 M H2SO4 solution in order to add a chemical bias (ca. 0.8 V). As shown in Fig. 11b, water could be separately decomposed into H2 and O2 with irradiation of natural solar light from the sunlight-gathering system, while no reaction proceeded on the UV-type TiO2 film under the same reaction conditions. The experiment was performed on a clear sunny day in March and the changes in the relative intensity of sunlight along with the irradiation times are also shown in Fig. 11b. The efficiency of solar energy conversion could be estimated at ca. 0.1% from the initial rate of H2 evolution. The decline observed in the evolution rates of H2 and O2 in the late afternoon (2:00 PM) can be attributed to a decline in sunlight intensity. A novel photocatalytic system which can produce H2 and O2 separately from water under solar light irradiation could, thus, be achieved with the visible light-responsive TiO2 thin film photocatalysts we have designed and investigated.
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Fig. 11

A Scheme of the H-type photoreactor. B Separate evolution of H2 and O2 from pure H2O by the visible light- responsive TiO2 thin film in an H-type photoreactor under sunlight irradiation. Solar light irradiated for 7 h from 9:30 AM to 4:30 PM

4 Conclusion

The design and preparation of a tetrahedrally coordinated Ti-oxide species incorporated within zeolite frameworks, TiO2 nano-particles hybridized with hydrophobic zeolite adsorbents as well as visible light-responsive TiO2 thin film photocatalysts by a RF-magnetron sputtering deposition method have been summarized. The tetrahedral Ti-oxide species demonstrated unique and high performance for the photocatalytic decomposition of NOx and photocatalytic reduction of CO2 with H2O. The high photocatalytic reactivity of TiO2 semiconducting photocatalysts could be achieved by blending them with hydrophobic siliceous zeolites, its reactivity equal to the high reactivity of TiO2 deposited with Pt particles. Significantly, a more economical method could, thus, be devised with these zeolite photocatalysts. Hydrophobic zeolites can efficiently adsorb and condense the organic compounds within their cavities and provide them onto the hybridized TiO2 surfaces, thus, an important role of such hydrophobic zeolites is the “catch and release” of organic compounds onto the photocatalyst surfaces. In addition, visible light-responsive TiO2 thin film photocatalysts could be prepared by using a RF-MS method as a single-step process. Novel Vis-type TiO2 photocatalysts were also successfully applied for the separate evolution of H2 and O2 from water under solar light irradiation. These Ti-oxide based photocatalytic systems which can effectively operate under solar light irradiation will be one of the most desirable candidates for applications that address environmental and energy issues in the future.

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