Search

The reasons for the difference of visible light activity between the direct contact g-C3N4/TiO2 Z-scheme composites obtained by one-step or two-step calcination process were investigated in detail by XRD, TEM, XPS, Raman and ESR technologies. TEM results showed that only g-C3N4 obtained by two-step calcination (denoted as CN-TSC) presented the porous structure which was in favor of the light absorption. XPS analysis indicated that interstitial doped N species formed in one-step calcination composite (denoted as CT-OSC) while substitutional N would appear in two-step calcination composite (denoted as CT-TSC). Nevertheless, interstitial N located at the higher position in the band gap of TiO2 and usually acted as the strong capture center of holes which was unfavorable to charge transfer. ESR and Raman results indicated that CT-TSC with some concentration of surface Vo and lower concentration of bulk Vo had excellent charge separation efficiency, according to the surface-enhanced Raman scattering (SERS) results of photo-induced charge-transfer (PICT) enhancement mechanism. And as a result, the visible-light activity for propylene oxidation of CT-TSC was twice higher than that of CT-OSC.

On the basis of density functional theory calculations and experimental results, we propose a spin-flip effect in a novel single-electron-trapped oxygen vacancy (SETOV; Vo ) and Fe co-doped TiO2 photocatalyst which improves significantly the propylene photo-oxidation under visible light illumination through the two spin channels photo-absorption and one spin channel recombination congestion. The presence of SETOV can make strongly localized Fe 3d states within the band gap get more delocalized, which not only increases the carrier mobility but also has an effect on restraining carrier recombination, thus resulting in the remarkably improved photocatalytic activity. Here the proposed photocatalytic process referring to spin-flip effect might afford a way for us about how to design a material owning the expected photoactivity in terms of spin selection rules about interband transition, opening the way to ground breaking applications in energy conversion.

An oxygen-vacancy-rich Z-scheme g-C3N4/Pd/TiO2 ternary nanocomposite was fabricated using nanotubular titanic acid as precursors via a simple photo-deposition of Pd nanoparticles and calcination process. The prepared nanocomposites were investigated by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-visible diffuse reflectance spectroscopy, respectively. For g-C3N4/TiO2 binary nanocomposites, at the optimal content of g-C3N4 (2%), the apparent photocatalytic activity of 2%g-C3N4/TiO2 was 9 times higher than that of pure TiO2 under visible-light illumination. After deposition of Pd (1 wt%) at the contact interface between g-C3N4 and TiO2, the 2%g-C3N4/Pd/TiO2 ternary nanocomposites demonstrated the highest visible-light-driven photocatalytic activity for the degradation of gaseous propylene, which was 16- and 2-fold higher activities than pure TiO2 and 2%g-C3N4/TiO2, respectively. The mechanism for the enhanced photocatalytic performance of the g-C3N4/Pd/TiO2 photo-catalyst is proposed to be based on the efficient separation of photo-generated electron-hole pairs through Z-scheme system, in which uniform dispersity of Pd nanoparticles at contact interface between g-C3N4 and TiO2 and oxygen vacancies promote charge separation.

The surface and bulk oxygen vacancy have a prominent effect on the photocatalytic performance of TiO 2 . In this study, TiO 2 possessing different types and concentration of oxygen vacancies were prepared by annealing nanotube titanic acid (NTA) at various temperatures in air or vacuum atmosphere. TiO 2 with the unitary bulk single-electron-trapped oxygen vacancies (SETOVs) formed when NTA were calcined in air. Whereas, TiO 2 with both bulk and surface oxygen vacancies were obtained when NTA were annealed in vacuum. The series of TiO 2 with different oxygen vacancies were systematically characterized by TEM, XRD, PL, XPS, ESR, and TGA. The PL and ESR analysis verified that surface oxygen vacancies and more bulk oxygen vacancies could form in vacuum atmosphere. Surface oxygen vacancies can trap electron and hinder the recombination of photo-generated charges, while bulk SETOVs act as the recombination center. The surface or bulk oxygen vacancies attributed different roles on the photo-absorbance and activity, leading that the sample of NTA-A400 displayed higher hydrogen evolution rate under UV light, whereas NTA-V400 displayed higher hydrogen evolution rate under visible light because bulk SETOVs can improve visible light absorption because sub-band formed by bulk SETOVs prompted the secondary transition of electron excited.

Molybdenum-doped akaganeite nano-rods were prepared by one step facile synthesis. The formation of akaganeite depends on the molar ratio of hexaammonium molybdate to ferric chloride hexahydrate. The nano-rods have an average particle size of ca. 30 nm and disperse uniformly. As a catalyst, the nano-rods can effectively increase the viscosity reduction rate of Shengli Oilfield extra heavy crude oil (175000 mPa s at 50 °C), especially combined with tetralin (a hydrogen donor). The results of SARA analysis, elemental analysis, and 1H NMR spectra indicate that the increase of light component caused by the denitrification and desulfuration of the catalyst plus tetralin as well as the loosening of heavy component molecules can be responsible for the viscosity reduction of the heavy oil.

Z-scheme heterostructure of nitrogen-vacancy rich g-C3N4 nanoparticles (VN-CN) confined in oxygen-vacancy rich TiO2 nanotube arrays (VO-TiO2) were fabricated to enhance CO2 photoreduction under visible light irradiation. Dual vacancies mediated VN-CN/VO-TiO2 heterostructure exhibits excellent photocatalytic CO2 reduction performance, and the CO and CH4 evolution rate are about 1.5 times and 3 times higher than that of pristine CN/TiO2, respectively. Possible reasons for the improvement of photocatalytic activity are as follows: (1) the adsorption and activation capacity of CO2 on the surface of CN/TiO2 are obviously improved due to the construction of nitrogen and oxygen dual vacancies; (2) the construction of dual vacancies are beneficial for enhancing the visible light absorption, promoting separation and transfer of photogenerated carriers due to intimate contact interface between VN-CN and VO-TiO2; (3) Z-scheme heterostructure could boost oxidation ability of VN-CN/VO-TiO2 and promote the production of more hydroxyl radical, which can react with CO2− to form HCO3−, leading to a new pathway to CO2 photoreduction compared to pristine CN/TiO2.

Anatase TiO2 nanocrystals exposed with {001} facets were fabricated by solvothermal strategy in HF-C4H9OH mixed solution, using titanic acid nanobelts (TAN) as a precursor. The shape of TAN is a long flat plane with a high aspect ratio, and F− is easily adsorbed on the surface of the nanobelts, inducing a higher exposure of {001} facet of TiO2 nanoparticles during the structure reorganization. The exposed percentage of {001} facets could vary from 40 to 77% by adjusting the amount of HF. The as-prepared samples were characterized by transmission electron microscopy, N2 adsorption-desorption isotherms, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscope. The photocatalytic measurement showed that TiO2 nanocrystals with 77% {001} facets exhibited much superior photocatalytic activity for photodegradation of methyl orange, methylene blue, and rhodamine B. And what’s more, the mineralization rate of methyl orange was as high as 96% within 60 min. The photocatalytic enhancement is due to a large amount of the high energetic {001} facets exposing, the special truncated octahedral morphology and a stronger ability for dyes adsorption.

Direct contact Z-scheme g-C 3 N 4 -TiO 2 nanocomposites without an electron mediator are prepared via simple annealing the mixture of bulk g-C 3 N 4 and nanotube titanic acid (NTA) in air at 600 °C for 2 h. In the process of annealing, the bulk g-C 3 N 4 transformed to ultra-thin g-C 3 N 4 nanosheets, and NTA converted to a novel anatase TiO 2 , then the two components formed a close interaction. The XPS result reveals that some amount of nitrogen is doped into this novel-TiO 2 , and g-C 3 N 4 nanosheets exist in the composites. The results of XRD, TEM and TG indicate that the thickness of g-C 3 N 4 nanosheets is very thin. The ESR spectrum shows the existence of Ti 3+ and single-electron-trapped oxygen vacancy in the 30%g-C 3 N 4 -TiO 2 composites. In photocatalytic activity test, the 30%g-C 3 N 4 -TiO 2 nanocomposites showed an excellent photo-oxidation activity of propylene under visible light irradiation (λ≥ 420 nm), and the removal efficiency of propylene reached as high as 56.6%, and the activity kept nearly 82% after four consecutive recycles. Photoluminescence (PL) result using terephthalic acid (TA) as a probe molecule indicated that the g-C 3 N 4 -TiO 2 nanocomposites displayed a Z-sheme photocatalytic reaction system and this should be the main reason for the high photocatalytic activity. A possible photocatalytic mechanism was proposed on the basis of PL result and transient photocurrent-time curves.

A facile development of highly efficient MgO/TiO 2 nanotubes network (MgO/TNTs) films is described. These MgO/TNTs films have a unique one-dimensional (1D) network nanotubular structure, and the different contents of MgO existed as amorphous thin layers located on the surfaces of TiO 2 nanotubes. They exhibited excellent photoreduction efficiency of CO 2 to methane compared with the bare TiO 2 film. MgO plays a critical role in CO 2 methanation, because it has the strong adsorption ability of CO 2 and initiates the reaction by binding a CO 2 molecule, forming a magnesium carbonate species on the surface. For further improve the photocatalytic activity, Pt nanoparticles were loaded on MgO/TNTs films by the photo-reduction method. It was found that the loading of Pt notably improved the transformation efficiency of CO 2 to methane, and the highest evolution rate of methane reached 100.22 ppm/h cm 2 . The fast electron-transfer rate in MgO/TNTs film and the efficient electron–hole separation by the Pt nanoparticals were the main reasons for the enhancement of the photoreduction activity. The synergy effect of Pt nanoparticles and MgO in the nanocomposites played an important role in CO 2 photoreduction.

A facile development of highly efficient MgO/TiO 2 nanotubes network (MgO/TNTs) films is described. These MgO/TNTs films have a unique one-dimensional (1D) network nanotubular structure, and the different contents of MgO existed as amorphous thin layers located on the surfaces of TiO 2 nanotubes. They exhibited excellent photoreduction efficiency of CO 2 to methane compared with the bare TiO 2 film. MgO plays a critical role in CO 2 methanation, because it has the strong adsorption ability of CO 2 and initiates the reaction by binding a CO 2 molecule, forming a magnesium carbonate species on the surface. For further improve the photocatalytic activity, Pt nanoparticles were loaded on MgO/TNTs films by the photo-reduction method. It was found that the loading of Pt notably improved the transformation efficiency of CO 2 to methane, and the highest evolution rate of methane reached 100.22 ppm/h cm 2 . The fast electron-transfer rate in MgO/TNTs film and the efficient electron–hole separation by the Pt nanoparticals were the main reasons for the enhancement of the photoreduction activity. The synergy effect of Pt nanoparticles and MgO in the nanocomposites played an important role in CO 2 photoreduction.