The Standard Molar Enthalpies of Formation of Nano-ZnO Particles with Different Morphologies

The thermodynamic properties of nano-ZnO particles with different morphologies were studied. First, based on the measurements of XRD and SEM, the morphologies of the five nano-ZnO products were determined as sheet, block, sieve, column, and rod, and their particle size was about 40 nm. Then, based on a designed thermodynamic cycle, the standard molar enthalpies of formation (Δ fHm ) of the nano-ZnO particles were determined by the RD496-2000 microcalorimeter, and they are Δ fHm (sheet) = (−337.43±0.47) kJ⋅mol,Δ fHm (block) = (−336.57±0.43) kJ⋅mol,Δ fHm (sieve) = (−337.04±0.57) kJ⋅mol,Δ fHm (column) = (−343.56 ± 0.39) kJ⋅mol, and Δ fHm (rod) = (−341.43 ± 0.41) kJ⋅mol. The results show that the bigger difference of the morphologies of nano-ZnO, the bigger difference of the standard molar enthalpies of formation.


Introduction
Along with the development of nanotechnology, nano-ZnO becomes an important new species of material in the nanotechnology field [1][2][3].Nano-ZnO is a kind of particles whose size is among 1∼100 nm.Compared with conventional ZnO, nano-ZnO shows many special properties, such as surface effect and size effect.Because of the small size and large specific surface area, nano-ZnO exhibits unique properties of light, electricity, heat, and magnetic susceptibility.So it is widely used in many fields, such as ultraviolet absorption materials, piezoelectric materials, magnetic materials, pressure sensitive materials, and medicine [4][5][6].It has many morphologies, such as rod, sheet, and needle ones.In present work, the preparation of nano-ZnO with different morphologies has been studied.It is the base to research the different functions of nano-ZnO with different morphologies in different fields.The thermodynamic properties are inherent properties of nano material.To find out the change rule of the thermodynamic properties of nano-ZnO with different morphologies and different sizes is helpful for its application in different fields [7,8].The "word" nanothermodynamics appeared formally in 2000 [9].Then Giebultowicz classified nanothermodynamics to thermodynamics [10].He thought nanothermodynamics is a small-system thermodynamics, which created a new branch of thermodynamics.In 2001, Hill published a series of papers about nanothermodynamics in Nano Letters [11][12][13].The prospects of nanothermodynamics and the contribution of it to the nanosystem were described in those papers.In 2010, a report about the entropy and enthalpy change of nanoparticle with different sizes was published [14].But the study on the thermodynamic properties of nano-ZnO with different morphologies is absent.
In this work, we determined the standard molar enthalpies of solution of nano-ZnO particles with five different morphologies by RD496-2000 microcalorimeter and calculated the standard molar enthalpies of the nano-ZnO particles.The standard enthalpies of formation of nano-ZnO particles were determined according to a designed chemical cycle shown in Figure 1.

Experimental
In the experiment of calorimetry, 15 mL steel reservoir with two separable glass sleeves in it is used as sample cells, and the solid and liquid substances were put into the glass sleeves, respectively.Then the cells were put into calorimeter chamber at 298.15 K; when the baseline kept going steadily (the fluctuation is less than ±0.00001 mV) the firing pin was triggered to break the glass sleeve and to make the sample mix with the solution.Then the chamber was kept to be at 298.15 K until the thermodynamic curve kept going parallel to the baseline with very small fluctuation which shows the completion of reaction.In this experiment, the solids were products (0.0141 g ZnSO 4 ⋅7H 2 O and 0.0086 g K 2 SO 4 ) and reactants (0.0040 g ZnO and 0.0134 g KHSO 4 ) (there were 5 kinds of reactant systems by choosing different morphologies of nano-ZnO) showed in Figure 1.The liquid was 0.3000 g HCl solution (0.26 mol⋅L −1 ).The products and reactants were mixed with the HCl solution separately.Then the heat of the reaction ( 1 and  2 ) in the process was determined.

Particle Size and Morphology of Nano-ZnO.
The XRD and SEM results of 5 kinds of nano-ZnO particles with different morphologies were shown in Figures 2 and 3.The sharp XRD peaks suggest that the products are highly crystallized and the morphology of each nanoparticle is uniform, and the The same final state +2HCl +2HCl morphologies of the nano-ZnO can be seen clearly in the SEM pictures.

Standard Molar Enthalpy of Formation of Nano-ZnO.
Standard molar enthalpy of formation of nano-ZnO can be obtained according to the designed thermodynamic cycle shown in Figure 1 and Hess law, and the detailed calculation was performed as formula (1).The experimental and calculated data were shown in Tables 2, 3, and 4 According to (1) and the data in Table 2 to Table 4, the standard molar enthalpies of formation of the five kinds of

Table 1 :
The enthalpy of solution of KCl in water at 298.15 K.

Table 2 :
Thermochemical data of reactant system of sheet nano-ZnO with particle size of 45 nm.

Table 3 :
The Δ r  m1 } of reactants system of nano-ZnO with the morphology of sieve, block, column, and rod.

Table 4 :
Thermochemical data of system products.