Fabrication and characterization of well-ordered PbS nanowires in aluminum oxide template by sulfurization and vacuum injection molding processes

AAO membranes can possess two different morphologies: nonporous-type and porous-type. Which one will be formed depends on the chemical nature of electrolytes. Generally, nonporous AAO membranes are formed in neutral electrolytes (pH 5–7). However, in acidic electrolytes, such as sulfuric, oxalic, phosphoric, and etc, porous AAO membranes are formed. In this paper, we chose sulfuric, oxalic, phosphoric acids as our electrolytes. First, the surface roughness of aluminum foil was flattened by mechanical polishing. Then, it was annealed in an air furnace at 550 °C for 1 h. In order to obtain a smooth, uniform and defect-free surface, the Al substrate was electropolished in a bath consisting of 15 vol% HClO₄, 15 vol% C₂H₆O, and 70 vol% CH₃(CH₂)₃OCH₂CH₂OH with 40 V DC for 30 min. Due to pore diameter is proportional to the applied voltage and base on our findings. After the above pretreatments, the first anodization process was conducted by applying a voltage of 20 V in 10 vol% sulfuric acid solution (H₂SO₄, 98%) at 5 °C for 1 h. The obtained substrate was soaked in a solution of 1.8 wt% CrO₃ and 6 vol% H₃PO₄ at 70 °C for 1 h to remove the resulting porous AAO layer with disordered pores in its top part to obtain textured aluminum with array patterns of almost hemispherical concaves. The second anodization was carried out under the same conditions, but for a longer time (usually more than 24 h) to form AAO films of varying thickness. In addition, the as-prepared substrate was soaked in a solution of 8 wt% CuCl₂ and 5 vol% HCl to remove the residual Al to get the AAO template. Finally, to widen the pore diameter of the AAO template, it was immersed in a 6 vol% H₃PO₄ solution for 10 min. In the steps above, if the voltage and electrolyte are altered to 50 V and 3 wt% C₂H₂O₄ during the anodization process, and the pore widening time is changed to 30 min, an AAO template with a pore size of 100 nm is formed. Furthermore, if the voltage and electrolyte are altered to 170 V and 1 vol% H₃PO₄, and the pore widening time is changed to 60 min, an AAO template with a pore size of 300 nm can be obtained.

The PbS NWs were fabricated by applying vacuum injection molding process and sulfurization process on AAO templates. First, Pb pellets were placed above the AAO template inside a vacuum chamber (figure 1(a)). After the chamber was heated to 360 °C (the melting point of Pb is 327.5 °C) for 10 min, 50 kgf cm⁻² hydraulic force was applied to inject the molten Pb into the AAO template. Second, the Pb/AAO composite material was put in a quartz tube together with sulfur powder and sulfurized in a furnace (figure 1(b)) for different time spans (3 h, 5 h and 10 h) at various temperatures (300 °C, 400 °C, 450 °C, 500 °C and 550 °C) to form the PbS/AAO composite. Then, the PbS NWs were released from the AAO by a chemical etching method. Finally, high purity homogeneous crystalline PbS NWs were obtained. Their morphologies were observed with scanning electron microscopy (SEM, JEOL JSM-7900F). The compositions and crystallization of NWs were examined with energy dispersive x-ray spectroscopy (EDS, EDAX) and x-ray diffraction (XRD, Bruker, D2 PHASER).

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The morphologies and compositions of the as-prepared AAO template and Pb/AAO were identified with the scanning electron microscopic and the energy dispersive spectrometer (EDS). Figure 2(a) is the SEM image of top view of an AAO template in which pore diameter was 30 nm. Figure 2(b) shows that the Pb NWs formed as hydraulic pressure enforced the molten Pb into the AAO nanomold. From figure 2(c), the cross-section view reveals that the sufficient time and temperature of Pb in vacuum injection molding allowed molten Pb to occupy deeper into the nanochannels. The EDS data in figure 2(d) shows that Pb, O and Al elements were detected, with higher signals of Al and O, because most of the Pb was embedded in the AAO template.

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The influences of different sulfurization time spans and temperatures on Pb NWs were investigated. In the thermodynamics of equilibrium, the state function of Gibbs free energy (G) is a simple way to determine whether a reaction can react spontaneously under a specific temperature and pressure. The following equation can be used to calculate the difference of Gibbs free energy (${{\rm{\Delta }}}_{r}{G}_{m}^{\varnothing }\left.\left(T\right)\right)$ of a reaction in any temperature if ignoring the influence of temperature on ${{\rm{\Delta }}}_{r}{H}_{m}$ and ${{\rm{\Delta }}}_{r}{S}_{m}:$

$$\begin{eqnarray*}&&{{\rm{\Delta }}}_{r}{G}_{m}^{\varnothing }\left(T\right)={{\rm{\Delta }}}_{r}{H}_{m}^{\varnothing }\left(298.15\right)-T{{\rm{\Delta }}}_{r}{S}_{m}^{\varnothing }(298.15).\end{eqnarray*}$$

The top right symbol 'ø' means it stays in standard state. The bottom right symbol 'm' means advancement of reaction is 1 mol. The bottom right symbol 'γ' means reaction. From the thermodynamics aspect, the Standard Gibbs Free Energy Changes for PbS formation can be written as [42]:

$$\begin{eqnarray*}&&{\mathrm{Pb}}_{\left(s,l\right)}+\frac{1}{2}{S}_{2\left(g\right)}\to {\mathrm{PbS}}_{\left(s\right)},\end{eqnarray*}$$

$$\begin{eqnarray*}&&{{\rm{\Delta }}}_{r}{G}_{m}^{\varnothing }=-163,200+88.03T\,({\rm{J}}).\end{eqnarray*}$$

Based on the thermodynamic calculation, the Gibbs free energy G are −112.76 kJ mol⁻¹, −103.96 kJ mol⁻¹, −99.55 kJ mol⁻¹, −95.15 kJ mol⁻¹, −90.75 kJ mol⁻¹, −81.95 kJ mol⁻¹ as the temperature reaches 300 °C, 400 °C, 450 °C, 500 °C, 550 °C and 650 °C respectively. Since the values of reaction Gibbs free energy are all negative, the reaction is spontaneous at these specific temperatures. All of the Pb NWs would be transformed into PbS crystals if the reaction time is long enough. The growth rate of PbS NWs in the template was significantly affected by the sulfurization parameters. The XRD spectrum of Pb NWs of different diameters is shown in figure 3(a). As shown in figure 3(b), the temperature was fixed at 300 °C, which is lower than the melting point of Pb, for different sulfurization time spans (3 h, 5 h and 10 h), and the XRD spectrum shows that the PbS phase appears under 300 °C for 3 h, but the diffraction intensities are lower than Pb. Furthermore, it also reveals that there is no obvious difference in the intensities of PbS and Pb from 3 h to 10 h. According to the thermodynamic theory, the sulfurization rate can be accelerated by increasing the temperature. As the temperature is increased, the material's thermal vibrational energy, reaction ability and diffusion rate are enhanced. For chemical reaction, the rate constant can be estimated by the equation: ${K}={A}\exp \left(-\tfrac{{\rm{\Delta }}{G}_{R}}{RT}\right).$ As indicated in this equation, the activation energy is ($\left.{\rm{\Delta }}{G}_{R}\right),$ and A is a pre-exponential factor that is related to the activation mechanism. For diffusion, the diffusion coefficient can be estimated by the equation: ${D}={{D}}_{0}\exp \left(-\tfrac{Q}{RT}\right).$ As mentioned, the values of rate constant (${K}$) and diffusion coefficient (D) are all increased by increasing the reaction temperature. Temperature causes a higher effect on chemical reaction rate than diffusion because diffusion activation energy (Q) is usually lower than activation energy (${\rm{\Delta }}{G}_{R}$). Thus, as shown in figure 3(c), the intensity of PbS is already higher than Pb because the state of Pb is turned solid into liquid when the sulfurization temperature is 400 °C, and the convection effect between Pb and sulfur gas increased, improving the chemical reaction. Besides, the diffraction peaks which were assigned to (111), (200), (220), (311), (222), (400) and (331) planes of a face-centered-cubic structured PbS were observed. The diffraction intensity difference between PbS and Pb is more obvious and the FWHM of diffraction peaks of (111) and (200) decreased when the temperature reached 450 °C, which is higher than the boiling point of sulfur, because it occurred much sulfur gas to facilitate the fabrication of PbS. However, the diffraction intensity of PbS is higher than Pb under 450 °C for 5 h but the Pb still could not be transferred into PbS completely, probably because the temperature was not high enough. Thus, when the temperature was increased to 500 °C, no other peaks of Pb are observed, suggesting pure PbS is obtained. The sharp and strong intensity of XRD peaks suggest that the samples have high crystallinity, as displayed in figure 3(d). The FWHM of diffraction peaks of (111) decreased from 0.1623° to 0.1472° gradually as the temperature was increased from 400 °C to 650 °C. This is because the increase in temperature allows the atoms to have enough energy to release stress through migration and diffusion, thereby improving the crystallinity.

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Figure 4(a) shows that the morphology of PbS/AAO composite with 5 h sulfurization time at 500 °C. There are a lot of PbS nanocrystals grow outside the pores of the AAO templates, as shown in figure 4(b). Then, the PbS/AAO composite was immersed in an acid solution containing 20 vol% of H₃PO₄ for various time spans at room temperature to remove the AAO film on the surface. As shown in figure 4(c), the tips of PbS NWs are clearer than figure 4(a) after immersing in the acid solution for 30 min. And figure 4(d) shows that the PbS NWs are more obvious after immersing in the acid solution for 6 h. It also proves that the PbS NWs can be obtained from the PbS/AAO composite by a chemical etching method.

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To further confirm whether the whole nanowires are of the same structure, the morphology and structure of the PbS nanowire which was treated in 500 °C for 5 h was observed by TEM images. As shown in figure 5, there are light and dark contrast regions in the TEM image of the nanowire. The Zone axis of the light-colored region is [01$\bar{1}$] in which there are several planes in this direction, including (111), (0$\bar{2}\bar{2}$) and (3$\bar{1}\bar{1}$) planes were confirmed through selected area electron diffraction. From the HRTEM image (figure 6(a)), it can be seen that the fringe spacing are about 0.3483, 0.2128 and 0.1828 nm which corresponding to the interplanar spacing of (111), (0$\bar{2}\bar{2}$) and (3$\bar{1}\bar{1}$) planes of the face-centered-cubic structured PbS. As shown in figure 6(b), the Zone axis of the dark-colored region is also [01$\bar{1}$], and in this direction also contains same planes as the light-colored region. The fringe spacing are about 0.3537, 0.2179 and 0.1859 nm which corresponding to the interplanar spacing of (111), (0$\bar{2}\bar{2}$) and (3$\bar{1}\bar{1}$) planes. Therefore, the results further confirms that both the light and dark contrast regions are face-centered-cubic structured PbS, indicating that the NWs prepared by vacuum injection molding process and sulfurization are of the same structure.

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The plausible sulfurization mechanism of the formation of PbS on Pb/AAO composites, which complies with the thermodynamic theory, is illustrated as figure 7. We assume that the sulfurization reaction began from the top (marked in red) of the Pb/AAO composite and then proceeded to the bottom (marked in blue), because the bottom of the AAO structure is a condense barrier layer which can almost keep the sulfur gas from flowing in. When the temperature reached 95 °C, the sulfur powders started to transfer into gas. Then the sulfur gas was adsorbed by the Pb and occurred reaction with each other to form nucleus of PbS crystal on the surface. As time goes on, the film of PbS is forming gradually. Because this film prevented the molecules of sulfur gas from reaching the inner Pb, it needed to depend on the interface reaction not only between Pb and PbS but also between PbS and the molecules of sulfur gas to do further sulfurization inside. On the other hand, it depended on the diffusion between the molecules of sulfur gas and PbS film. Thus, PbS phase appears under 300 °C, but the diffraction degree is low. When the temperature was increased to 450 °C, the state of Pb changed from solid to liquid, stimulating the convection between Pb and sulfur gas and, as a result, the PbS formed faster and deeper in the AAO pore. Finally, when the temperature was increased over 500 °C, the Pb was sulfurized into PbS.

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To confirm that the sulfurization parameters could be used to fabricate the PbS NWs with uniform composition, a chemical etching process using 20 vol% H₃PO₄ solution was applied to release the PbS NWs from the AAO. After the AAO was dissolved, the solution containing PbS NWs was centrifuged to make the PbS NWs settle in the bottom. Then, elemental mapping analysis and EDS analysis was conducted on the as-prepared PbS NWs, now free of AAO contaminants. Si signals were detected because we put the solution containing clusters of PbS NWs on a silicon substrate. The atomic number ratio of Pb to S is 1.65 when the sulfurization temperature is 300 °C, indicating that the material has not been completely transformed into PbS. This result can also be seen from figure 3(b) that the XRD patterns at this temperature still has Pb diffraction peaks. When the temperature was increased from below the melting point of lead metal to the over-melting point (from 300 °C to 500 °C), it can be found that the ratio of Pb to S becomes more and more close to 1 as the temperature rises. Moreover, according to the XRD analysis in figure 3(d), the diffraction peaks of Pb cannot be observed at the temperature of 500 °C. The ratio of Pb to S is approximately 1:1 at 500 °C for 5 h, and the elements of Pb and S are distributed evenly, as shown in figure 8, to further verify the Pb NWs were sulfurized completely to form PbS NWs. The EDS results of different sulfurization parameters were listed in table 1.

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PbS is an excellent semiconductor material because of its narrow band gap ∼0.41 eV and a large excitation Bohr radius of 18 nm. Quantum confinement of both charge carriers can be easily achieved by fabricating PbS with nanosize radius. The ultraviolet–visible (UV–Vis) absorption spectra of PbS NWs of 30 and 100 nm in diameters, shown in figure 9, were measured from 600 to 900 nm at the room temperature. Figure 9 indicates that the two different diameters of PbS NWs have their peaks at about 750 and 720 nm. The probable explanation for the energy gap of PbS NWs shift from 0.41 to 1.65 eV or 1.72 eV is indicative of the presence of the quantum confinement effect.

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