A Study On Novel Visible-light Photocatalysts Based On Layered Compounds Containing Titanium/niobium

Photocatalysis, using semiconductors, has become an area of great research interest due to its potential applications in solving energy supply and environmental pollution problems. TiO₂ is the most widely investigated photocatalyst due to its high photoactivity, exceptional optical and electronic properties, low cost and toxicity, and good chemical and thermal stability. However, its practical applications are greatly restricted due to the narrow lightresponse range and low separation probability of the photoinduced electron-hole pairs. Thus, much effort has been devoted to exploit new visible light-driven photocatalysts.Layered inorganic solids have been extensively investigated with the purpose of producing advanced materials through the intercalation of different guest species into the two-dimensional host interlayer region or hybridization with other semiconductors.The resulting hybrid materials are receiving increasing attention because the synergism between host and guest often gives rise to properties that are superior to the sum of those of the individual components.In this work, novel nanocomposite photocatalysts were successfully synthesized by intercataltion, non-metal doping and hybridization using KTiNbO₅ as the starting materials. The resulted samples were characterized in detail by X-ray diffraction （XRD）, Fourier Transform Infrared Spectrometer （FT-IR） high resolution transmission electron microscopy （HRTEM）, transmission electron microscopy （TEM）, scanning electron microscopy （SEM）, UV-vis spectroscopy, N₂ adsorption-desorption measurements, X-ray photoelectron spectroscopy （XPS） and photoluminescence spectra （PL）.The catalytic activities of the obtained samples were evaluated by the photodegradation of dyes solution under visible light irradiation. The photocatalytic mechanism was also investigated and discussed in detail.The main results are summarized as follows:1. Nanocomposite of polyaniline and nitrogen-doped layered HTiNbO₅ with an excellent visible-light photocatalytic performanceNovel PANI/N-HTiNbO₅ nanocomposite was prepared by in situ polymerization using aniline intercalation compound ANI/N-HTiNbO₅ as the intermediate. An effective approach has been used to synthesize N-doped HTiNbO₅ with a better intercalation property. Because N-doping could affect the surface charge distribution of [TiNbO₅]- layers, the resulted PANI/N-HTiNbO₅ nanocomposite showed a high thermal stability, and the intercalated monolayer of PANI molecules had a much narrower molecular weight distribution. FT-IR and UV-vis results also confirmed that there was a strong interaction between N-HTiNbO₅ and PANI in PANI/N-HTiNbO₅, giving rise to the enhanced visible-light response compared with N-HTiNbO₅. PANI/N-HTiNbO₅ nanocomposite had a good redox activity and an electrochemical cycling stability in acidic solution. Moreover, PANI/N-HTiNbO₅ nanocomposite had a high photocatalytic activity and photocatalytic stability for the degradation of methylene blue （MB） under visible light irradiation and is promising for practical application in water purification.The advantages of the present method can be viewed in three respects. Firstly, N-doping tuned the electronic structure of KTiNbO₅, and enhanced the visible-light response. Secondly, the intercalation of N-HTiNbO₅ with PANI led to the fact that the excited state electrons in PANI could migrate to the CB of N-HTiNbO₅ and the photogenerated holes in the VB of N-HTiNbO₅ could directly transfer to the HOMO of PANI, thus effectively preventing a direct recombination of electrons and holes. In addition, since the intercalation is a kind of hybridization at a molecular level, the resulted nanocomposite had a higher photodegradation stability. The present work provides a new way for the design of novel efficient visible-light photocatalysts.2. N-doped Na₂Ti6O₁3@TiO₂ core-shell nanobelts with exposed （101） anatase facets and enhanced visible light photocatalytic performanceA novel N-doped Na₂TieO₁3@TiO₂ core-shell nanobelts have been successfully synthesized by first mixing Na₂Ti₃O₇ with titanium isopropoxide, and then calcinating with urea in air. A structural evolution from the phase Na₂Ti₃O₇ to Na₂Ti6O₁3 occurred as the TiO₆ octahedra in Na₂Ti6O₁3 are more regular. Anatase TiO₂ nanoparticles with specifically exposed （101） facets were deposited on the external surface of Na2TieO13, leading to the maximized interfacial contact between anatase TiO₂ and Na2TieO13 due to common structural features of TiO₆ octahedra in two components. It was found that both the loading amount of TiO₂ and the content of doped N have a great effect on the physicochemical properties and photocatalytic activity of the resulted composites. The doped N atoms were mainly located in the crystal lattices of TiO₂ as titanate core is fully covered with TiO₂ shell and N doping proceeds from the exterior to the interior. The resulted composite displayed an excellent photocatalytic performance in degrading MB under visible light irradiation owing to the following reasons. Firstly, the nanobelts possessed a higher charge carrier mobility and provided the pathway for transport of charge carriers throughout the longitudinal direction to different reaction sites for subsequent reactions. Secondly, the exposed （101） anatase facets acted as a possible reservoir of the photogenerated electrons, yielding a highly reactive surface for the reduction of O₂ to O₂^·-. Thirdly, the heterojunction structure achieved between Na₂Ti6O₁3 and anatase TiO₂ further greatly decreased the recombination probability of photogenerated electron-hole pairs and increased the lifetime of charge carries. Finally, the band gap became narrowed through nitrogen doping, and thus the light response was expanded to the visible region. The present work may provide an insight for the fabrication of novel visible-light photocatalysts with an excellent performance.3. S-doped Na₂Ti6O₁3@TiO₂ core-shell nanobelts with an enhanced visible light photocatalytic performanceS-doped Na₂Ti6O₁3@TiO₂ core-shell nanobelts have been successfully synthesized by first mixing Na₂Ti₃O₇ with titanium isopropoxide, and then calcinating with thiourea in air. Anatase TiO₂ NPs with exposed（101） facets are deposited on the external surface of titanate core, leading to the formation of heterojunction structure between two components due to the common structural features of TiO₆ octahedra in anatase TiO₂ and Na₂Ti6O₁3.The resulted composite displayed an excellent visible-light photocatalytic performance in degrading MB solution. The improved photoactivity is attributed to the following reasons. Firstly, the substitution of Ti⁴⁺ by S⁶⁺ in the S-TTO lattice enhances the visible-light absorption and leads to the formation of considerable active groups typically as hydroxyl radical adsorbed on the surface of catalyst. With the formation of Ti-O-S bond, partial electrons can be transferred from S to O atoms. The electron-deficient S atoms might capture e- and thus inhibits the recombination of h+ and e-, resulting in a high quantum efficiency. Secondly, the exposed（101） facets favor the formation of active species of O₂^·- radicals through the reduction of O₂ by e-, and thus the effective separation of photogenerated electron-hole pairs. Thirdly, the synergistic effects of nano-heterojunctions between Na₂Ti6O₁3 and anatase TiO₂ can significantly speed up the separation rate of photogenerated charge carries.Fourthly, the anisotropically shaped titanate nanobelts provide a pathway for the quick transport of charge carriers throughout the longitudinal direction to different reaction sites for subsequent photocatalytic reactions. The present work may provide an important indication of how to construct novel visible-light photocatalysts through an integration of hybridization, doping and morphology engineering.4. S-doped nanocomposite of TiO₂ nanoparticles and HTiNbO₅ nanosheets with an enhanced visible light photocatalytic activityS-doped nanocomposite of TiO₂ NPs and HTiNbO₅ nanosheets has been successfully synthesized by first mixing freeze-dried HTiNbO₅ nanosheets with titanium isopropoxide, and then calcinating with thiourea in air. Anatase TiO₂ NPs with exposed （101） facets are deposited on the external surface of HTiNbO₅ nanosheets, leading to the formation of nano-heterojunction between two components due to the common structural features of TiO6 octahedra in anatase TiO₂ and HTiNbO₅. The resulted nanocomposite showed an excellent visible-light photocatalytic performance in degrading RhB solution. The improved photoactivity is attributed to the following reasons. Firstly, the resulted composite is mesoporous with a large specific surface area, leading to the more reactive sites and thus the increased photocatalytic activity. Secondly, the nano-heterojunction anatase formed between TiO₂ and HTiNbO₅ can significantly speed up the separation rate of photogenerated charge carries. Thirdly, the substitution of Ti⁴⁺by S⁶⁺ in the lattice enhances the visible-light absorption. With the formation of Ti-O-S bond, partial electrons can be transferred from S to O atoms. The electron-deficient S atoms might capture e- and thus inhibits the recombination of h+ and e-, resulting in a high quantum efficiency. Finally, the exposed（101） facets favor the formation of active species of O₂^·- radicals through the reduction of O₂ by e-, and thus the effective separation of photogenerated electron-hole pairs.