Ultraviolet Visible Light Diffuse Reflectance Spectra Research Articles

Construction of Co-doped TiO2/rGO nanocomposites for high-performance photoreduction of CO2 with H2O: Comparison of theoretical binding energies and exploration of surface chemistry

We report the enhancement of the photocatalytic performance of CO2 reduction in H2O by introducing Co and reduced graphene (rGO) into TiO2 photocatalyst. The synthesized nanocomposites were characterized by a range of analyses including X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), ultraviolet–visible light diffuse reflectance spectra (UV–vis DRS), Fourier transform infra-red spectroscopy (FT-IR), thermogravimetry-differential scanning calorimetry (TGA-DSC), energy dispersive X-ray analysis (EDX), Brunauer-Emmett-Teller (BET) surface area and X-ray photoelectron spectroscopy (XPS). It was found that wrapping Co-doped TiO2 with rGO expanded the light absorption ability of Co-doped TiO2/rGO, thus, improving the photoreduction efficiency of CO2 and selectivity for methanol. The conversion yields of CO2 to methanol with TiO2, Co-doped TiO2 and Co-doped TiO2/rGO reached 32.3 μmol/gcat, 730 μmol/gcat and 936 μmol/gcat, respectively, after 7 h of irradiation. Theoretical studies via Density functional theory (DFT) revealed that doping TiO2 with Co ions facilitated the formation of adsorbed carbonate or CO2•- species, as CO2 adsorbs onto Co-doped TiO2 surface with binding energy (BE) of −18.12 kJ/mol. The research will deepen the understanding of mole ratio (Ti:Co) content on the nanocomposites performance and provide new ideas for designing efficient photocatalysts.

Microstructures and optical performances of nitrogen-vanadium co-doped TiO2 with enhanced purification efficiency to vehicle exhaust

Titanium dioxide (TiO2) is widely used to purify air pollutants in environmental engineering, but it is only activated by ultraviolet (UV) light. The metal or nonmetal single doping of TiO2 cannot observably improve the purification efficiency of TiO2 under visible light. To further increase the photocatalytic activity and purification efficiency of TiO2 on vehicle exhaust under visible light, nitrogen (N)-vanadium (V) co-doped TiO2 was first prepared. The influences of N–V co-doping on phase structures, morphology, microstructures, electronic structures, and photo-absorption performances were then observed and examined using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV–visible light diffuse reflectance spectra. Purification efficiency and reaction rates of N–V co-doped TiO2 on NOx, HC, CO and CO2 in vehicle exhaust were studied using a purification test system under UV and visible light irradiations, respectively. Results indicate that N and V are synchronously doped into the crystal structures of TiO2 to replace O and Ti, respectively. N and V show the synergistic co-doping effect to suppress the grain growth of TiO2 and improve the dispersity and specific surface area of TiO2. Also, the N–V co-doping introduces more lattice distortions and defects in the crystal lattices of TiO2. Further, N presents in the form of Ti–O–N and O–Ti–N bonds, and V exists in the form of V5+ and V4+. These form the impurity energy level in the band gap to narrow the energy band of TiO2. Additionally, the N–V co-doping broadens the photoabsorption threshold of TiO2 from 387 nm to 611 nm. These results show that N–V co-doping increases the photocatalytic activity of TiO2. Finally, the N–V co-doped TiO2 shows higher catalytic purification efficiency on NOx and HC under UV and visible light. The N–V co-doping obviously increases the purification efficiency of TiO2 on CO and CO2 when exposed to visible light, and their reversible reactions are not found. The N–V co-doping of TiO2 is a feasible approach to purify vehicle exhaust under visible light irradiations.

Effect of coordination interaction between water and zinc on photochemistry property of Zn3(PO4)2·2H2O

Although widely existing in nature, crystal water compounds are still ignored by photochemistry scientists so far. In this work, we mainly investigated the influence of structure water on light absorption, charge transfer and photochemical property of Zn3(PO4)2·2H2O. The X-ray diffraction (XRD) results confirm the formation of phase-pure Zn3(PO4)2 and Zn3(PO4)2·2H2O, respectively; the ultraviolet–visible light diffuse reflectance spectra (UV-DRS) results show that although as a weak electron donor, the coordination of H2O with zinc(II) increases the electron density of zinc(II), and a ligand-to-metal charge transfer (LMCT) occurs from H2O to zinc(II), obviously increasing light absorption of Zn3(PO4)2·2H2O; the theory calculation demonstrates that structure water obviously contributes to the energy bands and electronic states; the electrochemical impedance spectroscopy (EIS) results illustrate that Zn3(PO4)2.2H2O has a higher electron transfer ability. This work suggests that crystal water chemicals could become a new category of photochemical materials, which may greatly enrich the photochemical science.

Microstructures, Optical Properties and Degradation Efficiency of Nitrogen-doped Titanium Dioxide to Automobile Exhaust

To understand the influence of nitrogen (N) dopant on phase structure, morphology, optical property, photocatalytic degradation of TiO2 to automobile exhaust (AE) under visible light irradiation, the N-doped TiO2 was first prepared, and then X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible light diffuse reflectance spectra (UV-vis DRS) and photocatalytic degradation tests were conducted. Results indicated that only anatase phase was found in both pure TiO2 and N-doped TiO2, and the N doping improved the dispersity of TiO2. Also, the N doping increased the absorbance of TiO2 under UV and visible light, respectively. An obvious red shift was observed on UV-Vis DRS of pure and N-doped TiO2 samples, indicating that the spectral response range of TiO2 was expanded to visible light region, and the utilization efficiency of solar irradiance was increased. The prepared N-doped TiO2 nanoparticles with an N dopant content of 3.0% showed better optical properties. Finally, the N doping improved the photocatalytic activity of TiO2 to AE in the visible light region. Furthermore, the N-doped TiO2 presented lower degradation efficiency to CO than that to CO2 under visible light radiation. Thus, it is found that the degradation efficiency of TiO2 to AE can be increased by the N doping under visible light radiation.

Unpredictable adsorption and visible light induced decolorization of nano rutile for the treatment of crystal violet

Photocatalysts containing different ratios of anatase and rutile are prepared via heat treatment of Degussa P-25 titania. X-ray diffraction (XRD), Bruuauer-Emmett-Teller (BET), ultraviolet–visible light diffuse reflectance spectra (DRS), Raman spectra (Raman), positron annihilation lifetime spectra (PAL) and temperature-programmed desorption (TPD) are applied to investigate the phase composition of the synthesized catalysts. Using crystal violet (CV) as the target pollutant, the unexpected visible light decolorization of rutile is observed. Despite the decreased specific surface area, the as-synthesized rutile samples exhibit much higher adsorption capability of CV than P-25 does, which in turn leads to improved photoreaction efficiency. Since the rutile samples can't absorb the visible light, the degradation under visible light irradiation is attributed to self-sensitization of CV on the surface of rutile.

Effects of vanadium doping on microstructures and optical properties of TiO2

To improve the photocatalytic activity of titanium dioxide (TiO2), the vanadium (V) was selected to modify TiO2. Pure and V-doped TiO2 samples were first synthesized, then effects of the V doping on microstructures and optical properties of TiO2 were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible light diffuse reflectance spectra (UV–vis DRS). Results indicated that the prepared TiO2 samples only contained anatase phase. The V dopant was uniformly dispersed in TiO2 lattices, not affecting the crystal phase structures of TiO2. The crystallite sizes of TiO2 were decreased first and then increased with the increase in V dopant content within the diameter range from 12 to 16nm. The V dopant inhibited the increase in TiO2 crystallite size. Also, the dispersity of TiO2 nanoparticles was improved with the increase in V dopant content. The V doping brought more defects in TiO2 lattices to capture charge carriers, and to decrease the recombination of electron–hole pairs. Additionally, the prepared TiO2 included such elements as Ti, V and oxygen (O). V consisted of two chemical states of V4+ and V5+, which were served as trapping centers of photo-generated electrons to promote the photocatalytic oxidation process. Ti existed in the form of Ti4+, and O presented in the form of such chemical bonds as Ti-O, Ti-OH and Ti-O-V in V-doped TiO2. Finally, the optical absorption edges of V-doped TiO2 show obvious red shift. The optimal V dopant content is 1.0% for pure TiO2 sample. The V dopant could introduce the doping energy level and narrow the band gap of TiO2, extending the absorption spectral range of TiO2. The V doping was an effective method to improve optical absorption properties of TiO2 in visible light region.

Band gap manipulation of cerium doping TiO2 nanopowders by hydrothermal method

Cerium doped TiO2 nanopowders were prepared by hydrothermal method with cerium nitrate. The samples were characterized by high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), ultraviolet–visible light diffuse reflectance spectra. Results show that the crystal of cerium dioxide can be observed in the grains of TiO2. The morphology of TiO2 prepared by hydrothermal methods changed from nanosheets to nanopowders by cerium doping. Meanwhile cerium doping can significantly reinforce the absorption of visible light, and adjust the absorption cut-off wavelength, from 373 nm to 499 nm for the undoped and the 25% doped 499 nm samples respectively. With the increase in doping ratio of cerium, the band gaps also decreased from 3.32 to 2.48 eV. Based on our observations, it can be concluded that cerium oxide can improve the absorptivity of TiO2 and may have potential applications in the areas of solar cells and photocatalysis.

Enhanced photocatalytic activity of titania nanotube array films supported with highly dispersed Pt nanoparticles

Self-organized Pt–TiO2 nanotube array films were prepared on Ti sheets by anodizing in ethylene glycol solution containing a low concentration of NH4F and subsequent photoreduction process. Pt–TiO2 nanotube array films were characterized by means of X-ray diffraction, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy, and ultraviolet–visible light diffuse reflectance spectra. SEM images show the highly dispersed Pt nanoparticles were deposited on the surface of TiO2 nanotube array films. The effect of reduction treatment on the photocatalytic degradation activity of methyl orange for Pt–TiO2 nanotube array films was investigated. The photocatalytic results showed that an optimum calcination temperature of methyl orange photocatalytic degradation activity was at 473 K and the further increased temperature could be detrimental to the photocatalytic activity due to the decrease of surface Pt concentration.

A facile one-pot synthesis of gadolinium doped TiO2-based nanosheets with efficient visible light-driven photocatalytic performance

Gadolinium doped TiO2-based nanosheets (Gd-TNSs) with different Gd/Ti ratios are prepared by a facile one-pot hydrothermal method using gadolinium nitrate as the gadolinium precursor. The photocatalysts are characterized by high-resolution transmission electron microscopy (HRTEM), Raman spectra, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet–visible light diffuse reflectance spectra (DRS), fluorescence spectra (FL), and N2 adsorption–desorption measurement. The as-synthesized samples show clearly sheet-like structure, with the width of about 30–100 nm, length greater than 100 nm, and high specific surface area (200–300 m2/g). The patterns of XRD and Raman analyses indicate that doped gadolinium has obvious influence on the structure of samples. The band-gap energy (E g) of photocatalysts reduce from 3.17 to 3.02 eV with the content of gadolinium increasing from 0.1 to 1.5 at. %. The FL emission intensity initially decreases with the content of Gd increasing and then increases after minimizing at 0.5 %-Gd-TNSs, which reveals that appropriate amount of gadolinium doping can effectively inhibit the recombination of photo-generated electrons and holes. The adsorption capacity and photocatalytic activity are evaluated by Rhodamine B (RhB). The best photocatalytic performance is obtained when the doping ratio of gadolinium is 0.5 at. %. The visible reaction rate (K app) of 0.5 %-Gd-TNSs was 2.0-fold as compared to TNSs, and 4.7-fold as compared to P25. The possible mechanisms for enhanced visible-light photocatalytic activities are discussed in detail.

Effect of holmium doping on the structure and photocatalytic behavior of TiO2-based nanosheets

A series of holmium-doped TiO2-based nanosheet (Ho-TNS) photocatalysts with different Ho/Ti molar ratios were prepared via a simple hydrothermal method. The photocatalysts were characterized by field-emission scanning electron microscope, high-resolution transmission electron microscope, X-ray diffraction (XRD), Raman Spectroscopy, X-ray photoemission spectroscopy, nitrogen adsorption–desorption isotherm measurement, ultraviolet–visible light diffuse reflectance spectra (DRS), and fluorescence spectra (FL). The TEM images of Ho-TNS exhibit clearly sheet-like structures and existence of the (201) plane of H2Ti3O7. The increased (101) peak intensity in XRD spectrum and the enhanced Eg mode (141 cm−1) in Raman spectrum indicate that Ho doping has obvious influence on the formation and structure of Ho-TNS, which can be explained by the formation of Ho–O–Ti bonds. With the doping ratio (Ho/Ti molar ratio) increasing from 0 to 2.0 %, the absorption edge shifts to a longer wavelength, and the band-gap of photocatalysts reduces from 3.18 to 3.07 eV. The decline of FL intensity implies that the photocatalysts with higher Ho content have higher electron–hole separation efficiency. However, after Ho doping, the surface structure of specimens is significantly altered, which will lead to the specific surface areas and adsorption capacity decrease. Under the combined effect of the variables, the photocatalyst with a doping ratio of 1.0 % reaches the best photoactivity, which is 1.90-fold and 12.38-fold higher compared with undoped TNS and P25 under visible light, respectively. The high photoactivity of the prepared Ho-TNS indicates that it may be useful for dealing with wastewater.

Photocatalytic Reduction of CO2to Methane on Pt/TiO2Nanosheet Porous Film

Anatase TiO2nanosheet porous films were prepared by calcination of the orthorhombic titanic acid films at 400°C. They showed an excellent photocatalytic activity for CO2photoreduction to methane, which should be related to their special porous structure and large Brunauer-Emmett-Teller (BET) surface area. In order to further improve the photocatalytic activity, Pt nanoparticles were loaded uniformly with the average size of 3-4 nm on TiO2porous films by the photoreduction method. It was found that the loading of Pt expanded the light absorption ability of the porous film and improved the transformation efficiency of CO2to methane. The conversion yield of CO2to methane on Pt/TiO2film reached 20.51 ppm/h·cm2. The Pt/TiO2nanosheet porous film was characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and ultraviolet-visible light diffuse reflectance spectra (UV-vis DRS). Moreover, the transient photocurrent-time curves showed that the Pt/TiO2nanosheet porous film exhibited higher photocurrent, indicating that the higher separation efficiency of the photogenerated charge carriers was achieved.

High efficient photocatalytic selective oxidation of benzyl alcohol to benzaldehyde by solvothermal-synthesized ZnIn2S4 microspheres under visible light irradiation

Abstract Hexagonal ZnIn2S4 samples have been synthesized by a solvothermal method. Their properties have been determined by X-ray diffraction, ultraviolet–visible-light diffuse reflectance spectra, field emission scanning electron microscopy, nitrogen adsorption–desorption and X-ray photoelectron spectra. These results demonstrate that ethanol solvent has significant influence on the morphology, optical and electronic nature for such marigold-like ZnIn2S4 microspheres. The visible light photocatalytic activities of the ZnIn2S4 have been evaluated by selective oxidation of benzyl alcohol to benzaldehyde using molecular oxygen as oxidant. The results show that 100% conversion along with >99% selectivity are reached over ZnIn2S4 prepared in ethanol solvent under visible light irradiation (λ>420 nm) of 2 h, but only 58% conversion and 57% yield are reached over ZnIn2S4 prepared in aqueous solvent. A possible mechanism of the high photocatalytic activity for selective oxidation of benzyl alcohol over ZnIn2S4 is proposed and discussed.

Study on enhanced photocatalytic performance of cerium doped TiO2-based nanosheets

Cerium doped TiO2-based nanosheets (Ce-TNSs) photocatalysts were prepared by a one-pot hydrothermal method using cerium nitrate as the cerium precursor. The photocatalysts were characterized by high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), Raman spectra, ultraviolet–visible light diffuse reflectance spectra (DRS), X-ray photoemission spectroscopy (XPS) and fluorescence spectra (FL). The results show that Ce3+ and Ce4+ co-exist in Ce-TNS. The doping leads to changes in binding energies of Ti and O. The concentration of Ti3+ increases gradually when the cerium doping amount keep increasing. Appropriate amount of cerium doping can significantly inhibit the recombination of electron–hole pairs, which is proved by the decline in the intensity of the fluorescence spectra. Ce-TNS with doping ratio of 0.5% (molar ratio) possesses the highest photocatalytic activity when degrading Rhodamine B (RhB), which is 5 times and 1.6 times of that of P25 and the undoped TNS, respectively. It is suggested that cerium ions are efficient electron trappers to improve the separation efficiency of electrons and holes.

Synthesis and photocatalytic activity of nanocrystalline TiO2 co-doped with nitrogen and cobalt(II)

Nitrogen-modified cobalt-doped TiO2 materials were successfully prepared via a modified sol–gel method. The structure and properties of the catalysts were characterized via X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM, ultraviolet–visible light diffuse reflectance spectra (UV–Vis DRS), N2 adsorption–desorption isotherms, and energy-dispersive X-ray spectroscopy. The XRD patterns of the pure and co-doped TiO2 samples indicate that the predominant phase was anatase. The average grain size obtained from TEM was approximately 10 nm. The Brunauer–Emmett–Teller analysis results indicate that the specific surface area was 77.7 m2 g−1. The UV–Vis DRS results for the co-doped sample reveal an absorption edge that had been red-shifted to 500 nm. The photocatalytic activities of the samples were evaluated through photodegradation of papermaking wastewater under UV and visible light irradiation. Compared with the cobalt-doped TiO2 sample and Degussa P25, the 3 mol% N-doped mesoporous N/Co-TiO2 photocatalyst exhibited the highest photocatalytic activity, which can be ascribed to the synergistic effect of the N and Co co-doping.

Study on mechanism of photocatalytic performance of La-doped TiO 2/Ti photoelectrodes by theoretical and experimental methods

TiO 2 photoelectrodes with various nanostructures have been successfully prepared by the anodization method. The morphology, microstructure and optical properties of as-prepared photoelectrodes were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet/visible light diffuse reflectance spectra (UV/vis/DRS), surface photovoltage spectroscopy (SPS) and photocurrent. The electronic structure and optical properties of La doped/undoped TiO 2 photoelectrodes with different crystal structures were calculated by the density function theory. The photocatalytic and photoelectrocatalytic activities of as-prepared photoelectrodes were evaluated. The results showed that the anodization potentials played a crucial role in the surface morphology and microstructure. Both results of theoretical calculations and experimental tests demonstrated that La-doped photoelectrodes were more sensitive to light than undoped one. The difference of photoelectrodes performance was ascribed to the crystal configuration, impurity energy levels and long-range orientation moving of photogenerated carriers.

Characterization and activity of visible-light-driven TiO 2 photocatalyst codoped with nitrogen and cerium

Nitrogen and cerium codoped TiO 2 photocatalysts were prepared by a modified sol–gel process with doping precursors of cerium nitrate and urea, and characterized by X-ray diffraction (XRD), thermogravimetry–differential scanning calorimetry (TG–DSC), X-ray photoelectron spectra (XPS) and ultraviolet–visible light diffuse reflectance spectra (UV–vis DRS). Results indicate that anatase TiO 2 is the dominant crystalline type in as-prepared samples, and CeO 2 crystallites appear as the doping ratio of Ce/Ti reaches to 3.0 at%. The TiO 2 starts to transform from amorphous phase to anatase at 987.1 K during calcination, according to the TG–DSC curves. The XPS show that three major metal ions of Ce 3+, Ce 4+, Ti 4+ and one minor metal ion of Ti 3+ coexist on the surface. The codoped TiO 2 exhibits significant absorption within the range of 400–500 nm compared to the non-doped and only nitrogen-doped TiO 2. The enhanced photocatalytic activity of the codoped TiO 2 is demonstrated through degradation of methyl orange under visible light irradiation.