Dongmei Dai

The efficient adsorption removal of Cr(VI) by using Fe3O4 nanoparticles hybridized with carbonaceous materials

Fe3O4 nanoparticles hybridized with carbonaceous materials, such as pinecone and graphene, were successfully synthesized by a facile hydrothermal method, which could be applied for Cr(VI) removal in aqueous solution. The nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and N2 adsorption–desorption isotherms. Due to the combination of pinecone and graphene, both the surface properties and morphologies of Fe3O4 were modified. Fe3O4 spherical particles were distributed and firmly anchored on the loose surface of pinecone or wrinkled graphene layers. The specific surface area increased from 23.85 to 27.86 and 121.17 m2 g−1 for Fe3O4/P and Fe3O4/G, respectively. It enhanced the adsorption capacity for Cr(VI) of Fe3O4/P (62.5 mg g−1) and Fe3O4/G (78.5 mg g−1). Study of the kinetics and isotherms showed that the pseudo-second-order kinetic and Langmuir isotherm models fitted the adsorption data well. There were three steps in the adsorption process, namely an instantaneous adsorption step, intraparticle diffusion and a final equilibrium stage. The reaction rate decreased along with temperature increasing, which indicated that Cr(VI) adsorption was an exothermic process. The Ea values were 34.39, 25.77 and 34.92 kJ mol−1 for Fe3O4, Fe3O4/P and Fe3O4/G, respectively, which illustrated that the adsorption of Cr(VI) onto the surface of the nanocomposites was a physical process. In no more than 5 h, about 92.6% and 94% Cr(VI) were desorbed from the surface of Fe3O4/P and Fe3O4/G, respectively, which indicated that the adsorption–desorption process for Cr(VI) was reversible. The results demonstrated that Fe3O4/P and Fe3O4/G exhibited excellent adsorption performance in the removal of Cr(VI). It was proved that carbonaceous materials, such as pinecone or graphene, could enhance the adsorption performance of Fe3O4, and could be used as adsorbents to remove heavy metals in industrial effluents.

Photoactivity and electronic properties of graphene-like materials and TiO2 composites using first-principles calculations

Graphene-like materials (GLM) have attracted intense attention in the research on photocatalytic technology due to their superior properties. First-principles calculations based on DFT were used to explore the photoactivity and electronic properties of graphene-like materials and TiO2 composites in this work. It was found that TiO2/GLM composites might be demonstrated for high thermodynamic stability. The electronic properties of TiO2/GLM, such as geometric structure, density of states, charge density, charge density difference and optical properties, were characterized. There were interactions between GLM sheets and TiO2, which caused charge accumulation on the GLM surface and charge depletion on the other side of TiO2 in the heterojunction. Electrons in the highest occupied molecular orbital (HOMO) consisted of the O2p orbital from TiO2 could be directly excited or dispersed to the lowest unoccupied molecular orbital (LUMO) composed of the hybridized orbital from GLM and TiO2 under irradiation. The produced well-separated electron–hole pairs induced an enhanced photocatalytic performance of TiO2/GLM. The theoretical results pointed out the electron migration and transfer path at the interface, which might illustrate the mechanism of enhanced photocatalytic activity of TiO2/GLM.

Preparation, characterization and photocatalytic activity of N-doped TiO2 nanocrystals

Abstract N-doped TiO 2 nanocrystals were prepared using titanium alkoxide as precipitant with different proportional materials. The products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV-vis diffuse reflectance spectra. It is confirmed experimentally that the photocatalytic activity of N-doped TiO 2 is much higher than that of Degussa P25, when used for the degradation of crystal violet. The degradation kinetics follows an apparent first-order reaction, which is consistent with a generally observed Langmuir-Hinshelwood mechanism. The doping of TiO 2 with nitrogen significantly increases the absorption in the region of visible light. The energy of the band gap of N-doped TiO 2 is 2.92 eV. The better performance of N-doped TiO 2 can be explained by the fact that it is also excited with longer-wavelength light.

Improved Adsorption Performance of α-Fe2O3 Modified with Carbon Spheres for Cr(VI) Removal from Aqueous Solution

Carbons spheres, easily fabricated by glucose hydrolysis, were integrated with α-Fe2O3 for removing heavy metal from contaminated water. The α-Fe2O3 particles were anchored on the surface of carbon spheres and the combination of two components provided more rough surface area, enhancing the adsorption performance of α-Fe2O3. The removal efficiency of Cr(VI) on α-Fe2O3/carbon spheres was 88% in 240 min, which was 1.93 times higher than that of pristine α-Fe2O3. The investigation on adsorption kinetics and isotherm showed that the pseudo-first-order kinetic and Langmuir isotherm models could well fit the experimental data. The adsorption rate was mainly controlled by both exterior and interior surface diffusion steps. Adsorption thermodynamics investigation proved that the Cr(VI) adsorption on α-Fe2O3/carbon spheres was an endothermic (93.32 kJ · mol-1) and spontaneous (-3.96 kJ · mol-1) physical process. The adsorption capacity was 18.7 mg · g-1 and after recycling five times, the decline of adsorption capacity of α-Fe2O3/carbon spheres was 7.8%, which indicated that the adsorbents could be recycled in the removal of Cr(VI). It indicated that the hybridization with carbon spheres could enhance the adsorption performance of α-Fe2O3, which might be used as convenient adsorbent to remove heavy metal in industry.

Characterization and photocatalytical analysis of N, Cd-codoped TiO 2 synthesized by modified sol-gel method

N, Cd-codoped TiO2 have been synthesized by sol-gel method coupling with thermolysis. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), UV-visible diffuse reflectance spectra (DRS), and X-ray photoelectron spectroscopy (XPS) respectively. The photocatalytic activities of samples were studied on the degradation of methyl orange (MO). XRD results showed that photacatalysts possessed an anatase crystalline framework having particle sizes of 10–15 nm. XPS analysis indicated that N atoms were incorporated into the TiO2 crystallattice, while Cd atoms existed on the crystal surface. Doping resulted in the observation of optical absorption edges shifting to the red light. The optical absorption edge of N-doped TiO2 and N, Cd-codoped TiO2 was 425 nm and 435 nm, respectively, which illustrated that the products represented photoactivity in the visible region. The products represented good performance in degradation on MO, and the degradation rate corresponding to the N, Cd-codoped TiO2 catalyst was the highest. It indicated a strong N-Cd synergistic interaction appeared to play a decisive role in driving the excellent photoactivity performance of N,Cd-codoped TiO2.