Qingshan Fu

Comparison of different models of melting transformation of nanoparticles

A reasonable melting model plays an important role in research into the melting trends and mechanism of nanoparticles for improved application. Herein, accurate equations between melting temperature and particle size corresponding to the three classical thermodynamic models, namely the Reiss, Pawlow, and Rie melting models, are derived. Then the differences and relations between the expressions of the three melting models and the corresponding accurate equations are discussed. The scope of application of each thermodynamic model and the effect of the thickness of the liquid shell on the melting temperature are discussed based on comparison of calculated melting temperatures with literature values for Au nanoparticles. The results show that the currently accepted thermodynamic melting equations for nanoparticles are just approximations to those derived here, which can describe the melting behavior of nanoparticles quantitatively. The Reiss and Pawlow melting models are applicable to the initial melting of nanoparticles, and the Rie melting model to the later stage. The initial melting temperature of nanoparticles is the maximum melting temperature, and as melting progresses, the melting temperature decreases as the thickness of the liquid shell increases.

Study on the size-dependent oxidation reaction kinetics of nanosized zinc sulfide

Numerous oxidation problems of nanoparticles are often involved during the preparation and application of nanomaterials. The oxidation rate of nanomaterials is much faster than bulk materials due to nanoeffect. Nanosized zinc sulfide (nano-ZnS) and oxygen were chosen as a reaction system. The influence regularities were discussed and the influence essence was elucidated theoretically. The results indicate that the particle size can remarkably influence the oxidation reaction kinetics. The rate constant and the reaction order increase, while the apparent activation energy and the preexponential factor decrease with the decreasing particle size. Furthermore, the logarithm of rate constant, the apparent activation energy and the logarithm of preexponential factor are linearly related to the reciprocal of particle diameter, respectively. The essence is that the rate constant is influenced by the combined effect of molar surface energy and molar surface entropy, the reaction order by the molar surface area, the apparent activation energy, by the molar surface energy, and the preexponential factor by the molar surface entropy. The influence regularities and essence can provide theoretical guidance to solve the oxidation problems involved in the process of preparation and application of nanomaterials.

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.

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.

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.