The microstructure and properties of PbS nanocrysatls via the reaction between Pb nanowires with 20 nm in diameter and H 2 S gas

Figure 5-15(a) shows the surface morphology of nanoporous structure in the alumina membrane. As can be seen, the alumina membrane has a relative uniform pore diameter distribution with a mean pore size about 20 nm. The porosity of this membrane is counted as 16.49%. Using pressure casting process, Pb nanowires produce in the nanopores of the alumina membrane as shown in Fig. 5-15(b). The diameter of the nanowire is the same as that of the nanopore and the filling ratio of Pb wires is nearly 100%.

After the Pb nanowire arrays were fabricated, a sulfization procedure was proceeded to obtain PbS nanocubes. PbS nanocubes produced on/in the porous alumina membranes after the Pb nanowires exposed to H2S gas at 573

K for 1 hour (Fig. 5-15 (c)), 3 hours (Fig. 5-15 (d)), 6 hours (Fig. 5-15 (e)), and 8 hours (Fig. 5-15 (f)), respectively. As the reaction time increases, the volumes of the PbS nanocubes on the alumina membranes increase obviously.

SEM and image analysis have been used in order to characterize the mean size and size distribution of PbS crystalline cubes. The SEM images of the PbS nanocubes (Fig. 5-15) are further analyzed with the Image-Pro Plus (IPP) software. The size distribution displays a single statistical ensemble of the PbS cubes as illustrated by the cube size histograms (Fig. 5-16). The average edge lengths in Figs. 5-16 (a)-(d) are 16.11 nm, 26.23 nm, 43.32 nm, and 55.64 nm respectively. An experiment formula for growth size of the PbS cubes has been work out. The edge length of the cube (L) and the sulfization time (t) are related according to L=10.57+5.44t as indicated in Fig. 5-17.

The edge length of PbS cube grows at the rate 5.44 nm an hour. Therefore, the PbS cube size desired can be produced by controlling the sulfization time.

XPS spectrum can provide information about the element’s chemical characteristics or oxidation state in a material and it measures the composition of the outermost 20~100 Å of a sample. Figure 5-18 shows the XPS elemental survey spectra of the as-prepared PbS sample surface. It is seen from Figs. 5-18 (a) and (b) that the PbS nanocubes show that the Pb4f spectrum is composed of two narrow peaks (Pb4f7/2= 137.5eV, Pb4f5/2= 142.3eV) and the binding energy of S2p peak is at 160.6 eV. Furthermore, the spectra in Figs. 5-18 (c) and (d) indicate an Al2p peak at 74.1 eV and an O1s peak at 531.5 eV. The results in Fig. 5-18 are in agreement with the values reported in literatures [105] and can be concluded that the as prepared sample is composed of PbS and Al2O3.

Fig. 5-15 (a) SEM image of a porous alumina membrane produced in 10% sulfuric acid solution. (b) SEM image of Pb nanowire arrays fabricated by vacuum pressure injection process. The morphologies of the PbS nanocrystals prepared at various time: (c) 1hour; (d) 3 hours; (e) 6 hours and (f) 8 hours.

(b)

(c) (d)

(e) (f)

(a)

Fig. 5-16 The size distribution of the PbS cubes with different sulfization time: (a) 1

0 2 4 6 8 10 0

20 40 60 80 100

Sulfization time (hours) Average edge length (nm)

L= 10.57+ 5.44t

Fig. 5-17 Relationship between average edge length of PbS nanocubes and sulfization time.

Fig. 5-18 Representative XPS spectra of the Al, O, S and Pb in the as-prepared PbS sample.

148 146 144 142 140 138 136 134 132

Count

Pb4f

Binding Energy (eV)

(a)

166 164 162 160 158 156

Count

S2p

Binding Energy (eV)

(b)

85 80 75 70 65 60

Binding Energy (eV)

Al2p

Count

(c)

536 534 532 530 528 526

Binding Energy (eV)

O1s

Count

(d)

20 30 40 50 60

Fig. 5-19 X-ray diffraction profiles of Pb nanowires and PbS nanocrystals with different sulfization time.

The results of the X-ray diffraction analysis of PbS nanocubes with four different sulfization time are shown in Fig. 5-19. Five peaks of the PbS nanocubes identified as (111), (200), (220), (311) and (222) are the same as the peaks of Pb bulk with a face-centered cubic structure. It also can be seen that the intensity of PbS increases with the sulfization time exposed to H2S gas.

Figure 5-20 (a) presents a TEM micrograph of a single Pb nanowire. The bright field image suggests a uniform and uninterrupted wire structure. Figure 5-20 (b) shows the corresponding selective area electron diffraction (SAED) of the Pb nanowire. According to index of the SAED, the nanowire is a single-crystal structure with a zone axis [001]. The low-magnification cross-sectional structure of PbS sample whose reaction time is 1 hour with

H2S gas is presented in the bright field TEM image given in Fig. 5-20 (c).

EDS analysis was performed with field emission TEM (FE-TEM) with nano-probe (~1nm) to determine the composition of the Pb-S nanocube marked in Fig. 5-20 (c). From the EDS spectrum (Fig. 5-20(d)), the nanocube is judged to consist of Pb and S in the ratio of 1:1 with a statistical error of 5%. The copper and gold signals are come from the TEM copper grid and surface conductive coating, respectively. The result is consistent with the XPS and XRD analyses which are discussed previously. High-resolution TEM (HRTEM) analyses were performed to determine the structure of PbS nanocube. The result of HRTEM imaging analysis (Fig. 5-20 (e)) indicates that the PbS nanocrystal has a cubic shape with about 8 nm edge length. The lattice fringes (d = 3.42Å) observed in the high-resolution TEM image are identical with the distance between the (111) lattice planes, confirming that the nanocrystal is composed of PbS. Figure 5-20 (f) shows the cross-sectional TEM image of the PbS nanocube with 3 hours sulfization process. It can be known that the PbS cube growing outside the pore is comprised several crystals.

The low-magnification cross-sectional structures of PbS cubes with 6 hours reaction time is presented in the bright field TEM image shown in Fig.

5-21(a). A particle growing out of the pore of the alumina membrane is marked by a circle. According to the selective-area electron diffraction pattern of this PbS nanoparticle (marked area), the particle growing outside the pore is a polycrystal structure and comprised several crystals as shown in the insert of Fig. 5-21 (a). The EDS analysis performed with TEM presents the composition of PbS nanocrystals marked in Fig. 5-21 (a). From the EDS

in the ratio of 1:1. To further confirm this result, high-resolution TEM images are performed to determine the structure of PbS nanocrystal. The HRTEM images (Figs. 5-22 (b-d)) of the boxed areas in Fig. 5-22 (a) further support the nanocrystal nature of PbS. The lattice fringes (d = 2.97Å) observed in those HRTEM images are identical with the distance between the (200) lattice planes.

When the sulfization time reaches 8 hours, the size of the nanocube outside the pore increases to 100 nm as displayed in Fig. 5-23(a). The HRTEM image (Fig. 5-23 (b)) was taken from a single crystal in a nanocube outside the pore of alumina membrane. In Fig. 5-23(c), Fast Fourier-transformation (FFT) analysis is performed on the lattice fringes from Fig. 5-23 (b) and the spots with a zone axis [001] match well with the rock salt structure of PbS. The PbS phase is identified to crystallize in cubic crystalline lattice (Fm3m, space group=225) with the lattice constant a = 0.59362 nm. Lattice-fringes image (Fig. 5-23 (d)) is generated by inverse Fourier-transformation with the (220) spots by Digital Micrograph Software [106]. A large amount of edge dislocations appear in the Fig. 5-23 (d). The edge dislocations should be created during sample preparation in the sulfization process. With increasing reaction time, the dislocations or other defects like vacancies are generated gradually.

Fig. 5-20 (a) TEM image of a single Pb nanowire and (b) the selective area electron