Yongqiang Xue

Size- and shape-dependent melting enthalpy and entropy of nanoparticles

A theoretical model free of any adjustable parameter was derived based on the relation between Gibbs energy change and size to describe the size- and shape-dependent behavior of the melting enthalpy and entropy of nanoparticles. For the melting enthalpy and entropy of vanadium (V), silver (Ag), and copper (Cu) nanoparticles, the results of pure theoretical calculation are in good agreement with available molecular dynamic results. The effect of size on the melting enthalpy and entropy of nanoparticles is greater compared to that of shape effect. The melting enthalpy and entropy decrease with particle size decreasing and the smaller the particle size, the greater the size and shape effects. Furthermore, at the same equivalent diameter, the more the shape of nanoparticles deviates from that of the sphere, the smaller the melting enthalpy and entropy. The thermodynamic relations derived herein can quantitatively describe the influence regularities of size and shape on the melting thermodynamic properties of nanoparticles.

Effect of calcination atmospheres on the catalytic performance of nano-CeO2 in direct synthesis of DMC from methanol and CO2

Nano-CeO2 was prepared through the calcination of Ce(OH)3 precursor in different atmospheres (H2, Ar, air, O2), which was prepared by a hydrothermal method, and then used as catalysts in the direct synthesis of dimethyl carbonate (DMC) from methanol and CO2. The results indicated that the catalyst calcined in O2 (CeO2-O2) showed an optimum catalytic performance, and the yield of DMC reached to 1.304 mmol/mmolcat. In addition, reaction temperature and weight of catalyst were optimized. Based on characterizations of the catalysts, the ratio of Ce(IV)/Ce(III) and Lewis acid-base property of nano-CeO2 catalyst could be adjusted through different calcination atmosphere treatment. It was determined that the higher activity of CeO2-O2 catalyst is mainly attributed to its higher ratio of Ce(IV)/Ce(III) as well as abundant and moderate intensity Lewis acid base sites.

The size dependence of dissolution thermodynamics of nanoparticles

Dissolution of nanoparticles is involved in the preparation, research and application of nanomaterials, but there is a surprising difference in dissolution thermodynamics between nanoparticles and the corresponding bulk materials. In the paper, the relations of dissolution thermodynamic properties, equilibrium constant of nanoparticles, respectively, and particle size were derived by introducing interface variables and the surface chemical potential. Experimentally, the solubility of nano-barium sulfate with different average particle sizes at different temperatures were determined by the method of electrical conductivity, obtaining the influencing regularities of particle size on the dissolution thermodynamic properties and the equilibrium constant. The regularities are in accordance with the theory. The results show that there are remarkable effects of particle size of nanoparticles on the dissolution thermodynamic properties and the equilibrium constant; with the decreasing of the size of nanoparticles...

Size-Dependent Thermodynamic Properties of the Reaction of Nano-ZnO with Benzoic Acid

Studying the thermodynamic properties of the reaction of nanoparticle with organic substance is significant for the application of nanomaterial in organic fields. In this work, by using the reaction of nano-ZnO with different particle sizes with benzoic acid as a research system, the size-dependent standard equilibrium constant and reaction thermodynamic properties were analyzed theoretically, and the influence regularities of the particle size on the standard equilibrium constant and the reaction thermodynamic properties were studied. The excellent agreement between the experimental results and theoretical analysis shows that the particle size has remarkable influence on the standard equilibrium constant and the thermodynamic properties of the reaction; with the decrease of the particle size, the standard equilibrium constant increases, while the standard molar reaction Gibbs energy, the standard molar reaction enthalpy and the standard molar reaction entropy decrease. Furthermore, the logarithm of the standard equilibrium constant and the thermodynamic properties present linear relations with the reciprocal of the particle diameter, respectively. The theory and the influence regularities will provide useful tools to lead better and broader application of nanomaterial in organic fields.

Research into the rationality and the application scopes of different melting models of nanoparticles

AbstractA rational melting model is indispensable to address the fundamental issue regarding the melting of nanoparticles. To ascertain the rationality and the application scopes of the three classical thermodynamic models, namely Pawlow, Rie, and Reiss melting models, corresponding accurate equations for size-dependent melting temperature of nanoparticles were derived. Comparison of the melting temperatures of Au, Al, and Sn nanoparticles calculated by the accurate equations with available experimental results demonstrates that both Reiss and Rie melting models are rational and capable of accurately describing the melting behaviors of nanoparticles at different melting stages. The former (surface pre-melting) is applicable to the stage from initial melting to critical thickness of liquid shell, while the latter (solid particles surrounded by a great deal of liquid) from the critical thickness to complete melting. The melting temperatures calculated by the accurate equation based on Reiss melting model are in good agreement with experimental results within the whole size range of calculation compared with those by other theoretical models. In addition, the critical thickness of liquid shell is found to decrease with particle size decreasing and presents a linear variation with particle size. The accurate thermodynamic equations based on Reiss and Rie melting models enable us to quantitatively and conveniently predict and explain the melting behaviors of nanoparticles at all size range in the whole melting process. Graphical abstractBoth Reiss and Rie melting models are rational and capable of accurately describing the melting behaviors of nanoparticles at different melting stages. The former is applicable to the stage from initial melting to critical thickness of liquid shell, while the latter from the critical thickness to complete melting. The critical thickness of liquid shell decreases with decreasing particle size and a linear relationship between them is observed. This paper provides us an effective and convenient method to address the fundamental issue regarding the melting temperature of nanoparticles.

Size-dependent Thermodynamics and Kinetics of Adsorption on Nanoparticles: a Theoretical and Experimental Study

Owing to their excellent adsorption properties compared with those of the corresponding bulk materials, nanoparticles have been widely applied in many fields. Their properties depend on the thermodynamics and kinetics of adsorption, which depend on the particle size. In this paper, we present universal theories of the thermodynamics and kinetics for nanoadsorption that have been developed over the past few years. Theoretically, we have derived relationships between the adsorption thermodynamic properties and the particle size, as well as those between the adsorption kinetic parameters and the particle size. Moreover, we discuss the regularities and mechanisms of influence of the particle size on the thermodynamics and kinetics of adsorption. Experimentally, taking the adsorption of methyl orange on nano-CeO2 in aqueous solution as a system, we have studied the size-dependent thermodynamics and kinetics of the system, and the size dependences were confirmed to be consistent with the theoretical relationships. The results indicate that particle size has a significant effect on the thermodynamic properties and kinetic parameters of adsorption: with decreasing particle size of nano-CeO2, the adsorption equilibrium constant K⊖ and the adsorption rate constant k increase, while the molar Gibbs free energy of adsorption Δads Gm⊖, the molar adsorption entropy Δads Sm⊖, the molar adsorption enthalpy Δads Hm⊖, the adsorption activation energy Ea, and the adsorption pre-exponential factor A all decrease. Indeed, ln K⊖, Δads Gm⊖, Δads Sm⊖, Δads Hm⊖, ln  k, Ea, and ln  A are each linearly related to the reciprocal of particle size. Furthermore, thermodynamically, Δads Gm⊖ and ln  K⊖ are influenced by the molar surface area and the difference in surface tensions, Δads Sm⊖ is influenced by the molar surface area and the difference in temperature coefficients of surface tension, and Δads Hm⊖ is influenced by the molar surface area, the difference in surface tensions, and the difference in temperature coefficients of surface tension. Kinetically, Ea is influenced by the partial molar surface enthalpy of the nanoadsorbent, ln  A is influenced by the partial molar surface entropy, and ln  k is influenced by the partial molar surface Gibbs energy. The theories can quantitatively describe adsorption behavior on nanoparticles, explain the regularities and mechanisms of influence of particle size, and provide guidance for the research and application of nanoadsorption.

Controlled synthesis of t-Se nanomaterials with various morphologies via a precursor conversion method

Trigonal selenium (t-Se) nanomaterials with different morphologies present distinct properties and great potential applications in electric devices. However, controlled synthesis of t-Se nanomaterials with various morphologies is difficult in a typical preparation process. Therefore, it is imperative to develop an easily controlled and high-efficiency method to prepare t-Se with various morphologies. Herein, a precursor conversion method was proposed to prepare t-Se nanomaterials with different morphologies. That is, uniform amorphous selenium (a-Se) nanospheres were prepared by reducing sodium selenite with glucose, and then t-Se nanomaterials with morphologies of spheres, tubes, rods, belts and wires were obtained by different subsequent treatments for the conversion of a-Se into t-Se. The results demonstrate that the t-Se nanospheres were obtained by hydrothermal treatment at 150 °C, t-Se nanorods and nanotubes by ultrasonication of a-Se in water and with the addition of PVP K30 for nanotubes, t-Se nanowires by the aging of a-Se in ethanol and in a dark environment, and t-Se nanobelts by increasing the concentration of a-Se in ethanol. The conversion processes from a-Se nanospheres into t-Se 1D nanostructures comply with a “solid–solution–solid” formation mechanism, while the conversion from a-Se nanospheres into t-Se nanospheres complies with the mechanism of crystalline phase transformation. The method provides us a mild and easily controlled route for the preparation of t-Se nanomaterials with desired morphologies.

Determination Method and Size Dependence of Interfacial Tension between Nanoparticles and a Solution

Interfacial tension plays an important role in the processes of preparation, research, and application of nanomaterials. Because the interfacial tension is fairly difficult to be determined by experiments, it is still unclear about the effect of particle size on interfacial tension. In this paper, we proposed a method to determine the interfacial tensions and its temperature coefficients by determining the electrode potential of the nanoparticle electrode. Nano-Au with different radii (from 0.9 to 37.4 nm) in an aqueous solution was taken as a research system; we determined the interfacial tension and its temperature coefficient of the interface and discussed the size dependence. At the same time, we found surprisingly that this method can also be applied to determine the Tolman length and the atomic radius. The results show that the particle size of nano-Au has remarkable influences on the interfacial tension and its temperature coefficient. As the particle size decreases, the interfacial tension and the absolute value of its temperature coefficient increase. With the decrease of radius, the influences of the particle size on the interfacial tension and its temperature coefficient become more significant, whereas the influences can be neglected when the radius exceeds 10 nm. In addition, the results also show that the Tolman length is a negative value, and temperature has little effect on the Tolman length. This research can provide a new method to conveniently and reliably determine the interfacial tension on interfaces between nanoparticles and solutions, the temperature coefficients, the Tolman lengths, and the atomic radii; and the size dependences can provide important references for preparation, research, and application of nanomaterials.