Shape dependence of thermodynamics of adsorption on nanoparticles: a theoretical and experimental study.

Nanomaterials have excellent adsorption performance, which mainly depends on the adsorption thermodynamics that is related to the shape of the nanoparticles that make up the nanomaterial, but the effects of shape on the thermodynamics of adsorption are not fully clear. In this paper, theoretically, the general formulae of adsorption thermodynamic properties for nanoparticles with different shapes and different sizes were derived, and the influencing regularities and mechanisms on adsorption thermodynamic properties were discussed. Experimentally, the influences of the shape and size of nano-CeO2 on the thermodynamics of adsorption were studied in aqueous solution. The experiment results showed that the shape has significant influences on the thermodynamics of adsorption, and the smaller the particle size, the more significant the effects of shape on the thermodynamics. For the adsorption of nano-CeO2 with different shapes and the same equivalent particle size, compared with the sphere, the equilibrium constant of adsorption for the octahedron is larger, while the molar Gibbs free energy of adsorption , the molar adsorption enthalpy of adsorption and the molar adsorption entropy of adsorption are smaller. For the adsorption of nano-CeO2 with the same shape, with the decreasing particle size, increases, while , and decrease; and , , and are each linearly related to the reciprocal of particle size. The experimental results are consistent with the theoretical relations. The theories can quantitatively describe the adsorption behavior on nanoparticles, explain the regularities and mechanisms of influence of shape, and provide guidance for the research and application of nanoadsorption.

Synthesis of TiO2/Fly Ash Cenospheres by Sol-Gel Method and Their Photacatalytic Activities Under Visible Light.

Fly ash is a solid waste discharged from thermal power plant. Specific surface area of floating fly ash cenospheres (FACs) would increase after it was modified. The photocatalytic composite of TiO2/FACs was successfully synthesized by sol-gel method using the carrier of modified FACs and tetrabutyl titanate as starting materials. The different influence factors on the photocatalytic performance of TiO2/FACs composites were characterized through SEM, EDS, XRD, UV-vis DRS and BET surface measurements. The UV-vis DRS spectra revealed that the absorption edge of TiO2 is 387 nm while that of TiO2/FACs photocatalysts red-shifts to 500 nm. Photocatalytic activity of TiO2/FACs was evaluated by the photocatalytic depigmentation of methyl orange solution (MO, 20 mg L-1, pH = 6.3) under visible light irradiation. It was found that the specific surface area, surface roughness and activity of FACs were increased by NaOH solution activation. The degradation rate of MO reaches 52% in 180 min under the visible light illumination. But too much FACs could decrease its photocatalytic activity and degradation rate. And the recovery test indicated that TiO2/FACs photocatalyst was rather stable, easy to recover from the treated wastewater.

Persistent Methyl Orange Degradation Ability of MgAl2O4:(Pr3+,Dy3+)/M-TiO2 Luminescent Photocatalyst.

Metal ions (Cr, Ni, Co) doped titania (M-TiO2) coupled with the long after glow phosphor MgAl2O4:(Pr3+, Dy3+) particles were synthesized by the sol-gel method, with the best mass ratio of MgAl2O4:(Pr3+, Dy3+) to M-TiO2 as 4:6. MgAl2O4:(Pr3+, Dy3+)/M-TiO2 had the persistent methyl orange (MO) photocatalytic degradation ability and the photocatalytic degradation went on reacting more than 90 min in dark after turning off the light. MgAl2O4:(Pr3+, Dy3+) emitted the light as a light source in dark which was absorbed by M-TiO2. The differences of MgAl2O4:(Pr3+, Dy3+)/Cr-TiO2, MgAl2O4:(Pr3+, Dy3+)/Ni-TiO2 and MgAl2O4:(Pr3+, Dy3+)/Co-TiO2 might be attributed to the difference in the metal ions doping. The composite MgAl2O4:(Pr3+, Dy3+)/Cr-TiO2 revealed the highest ability of persistent photocatalytic degradation methyl orange. Different metal ions doping made the TiO2 with different band gap.

Size Dependence of the Integral Melting Enthalpy and Entropy of Nanoparticles.

Precise thermodynamic relations to describe the size-dependent integral melting enthalpy and entropy of nanoparticles were deduced by virtue of designing a thermochemical cycle. The differences between integral and differential melting enthalpy and integral and differential melting entropy of nanoparticles were discussed. Nano-Sn of different sizes was prepared by means of chemical reduction, and differential scanning calorimetry (DSC) was utilized to obtain the melting temperature, melting enthalpy, and melting entropy. The experimental results agree with the theoretical predictions and literature results, demonstrating that the melting temperature, enthalpy, and entropy decrease with decreasing particle size and linearly vary with the reciprocal of particle size within the experimental size range. The variations of melting enthalpy and entropy with particle size mainly depend on the molar surface area, the interfacial tension, and the temperature coefficient of interfacial tension. These findings offer a better understanding of the effect of particle size on the melting thermodynamic behaviors of nanoparticles at different melting stages.

The effect of microdroplet size on the surface tension and Tolman length.

A monomolecular layer model of the surface phase of microdroplets was proposed, and the exact expression for Tolman length was derived in this paper. The Tolman lengths of water, n-pentane, and n-heptane were calculated by the expression, and the values are quite in agreement with the experimental values. By use of the Gibbs-Tolman-Kening-Buff equation, the exact relationship between the microdroplet surface tension and the radius is obtained, and the predicted values agree well with the simulated values. The results show that there is an obvious effect of the size of microdroplets (or nanoparticles) on the surface tension, and the surface tension decreases with decreasing droplet size. For the microdroplets of general liquid, only if their radius approaches or reaches 10(-9) m does the effect become significant.