Synthesis and characterization of silica–lead sulfide core–shell nanospheres for applications in optoelectronic devices

Nanoscale miniaturization of chalcogenide semiconductors such as lead sulfide (galena) can generate interesting quantum confinement effects in the field of optoelectronic applications. In this work, we developed a process in order to obtain SiO2 nanospheres coated with Galena, as the denominated core–shell system; this process is based on Stöber’s method, where the magnetic stirring was replaced by an ultrasonic bath to achieve well rounded and highly stable silica nanoparticles with diameters average of 70 nm. The PbS shell cover presents a thickness of 10 nm around. The nanostructures’ chemical composition, morphology, and optical properties were determined by transmission electron microscopy and UV–Vis spectroscopy. As a result, the nanoshells correspond to cubic PbS, presenting some interplanar distances of 2.95 Å and 3.41 Å; this nanoshell also shown an optical spectrum shift toward blue and a remarkable increase of 3.75 eV in its band gap, compared with the PbS bulk value. The chemical composition is studied by energy scattering spectroscopy and X-ray photoelectron spectroscopy analysis.


Introduction
Heavy metal sulfides, like PbS, are semiconductor materials used in different electronic devices as thermal detectors [1], photoresistors [2], and thin-film solar cells [3]. Due to their size scales, from nanometric to macroscopic, it is possible to observe changes in their physicochemical properties. Some semiconductor nanoparticles are comparable in size to their excitons, that lead to significantly affect their optical, electronic, luminescent, and material oxidation-reduction properties [4,5]. This way, new applications are found by reducing their size. For instance, there are reports that PbS nanoparticles have interesting nonlinear optical properties, with applications in gates and as optical signal processors [6]. Some semiconductor nanoparticles are synthesized by different techniques like solvothermal synthesis [7], sonochemistry [8,9] sol-gel [10,11], colloidal microemulsion [12], and chemical precipitation of aqueous solutions [13]. The sonochemical synthesis technique is used in order to obtain very small particles through acoustic cavitation [8,9] and also with the intention of building concentric multilayer semiconductor nanoparticles' core-shell type [14].
The relevance of core-shell nanoparticles has increased exponentially in the last 20 years since these nanostructures allowed the exploration of a new range of applications that link the chemistry of materials with other areas such as electronics [15], biomedicine [16], and optics [12]. The potential applications of these nanostructures in electronics are particularly interesting: enhancing photoluminescence and the manufacture of photonic crystals [17][18][19][20]. We used the Stö ber process to achieve a colloidal dispersion containing monodisperse spheres on a nanometric scale. This process consists of the preparation of controllable and uniform-sized silica particles through hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in a water/alcohol solution and using ammonium hydroxide as the catalyst system [21]. Other metal oxide nanocomposite materials obtained through green synthesis are copper nanoparticles supported on titanium dioxide, used in the reduction of various pollutants [22]; cerium oxide, with applications in electronic/semiconducting devices [23]; and zinc oxide and copper oxide used as anti-bacterial agents [24]. There are reports of green synthesis of nanospheres with PbS star-like coatings [25]; however, the main advantage is the simplicity of the procedure implemented and the use of manageable reagents of low toxicity.
In the case of complexing agents, to improve the adhesion and deposit control of the compound, triethanolamine was combined with citric acid, that is commonly found and easily biodegraded. Then, both solutions are mixed and the silica nanospheres are covered with PbS using the sonochemical method, and finally, the core-shell nanometric structures are formed. Through the results obtained in the characterizations made to the samples, the success of the synthesis of nanoparticles with the desired size, morphology, and composition was achieved. The above confirms the validity of the new formulation used, with which is possible to considerably reduce the reaction times without affecting the stability of the final product.

Experimental
At first, monodisperse silica nanoparticles were produced by the sol-gel process through a modified Stö ber's method. To reduce the reaction time, we used the sonochemical technique, submerging the solution in an ultrasonic bath instead of the traditional magnetic stirring. The following compounds were used: tetraethyl orthosilicate [Si(OC 2 H 5 ) 4 ] (TEOS, 1 ml) as a precursor of silica, deionized water (1 ml) and ethanol (C 2 H 5 OH, 27 ml) as solvent liquids, and ammonium hydroxide (NH 4 OH, 1 ml) as a catalyst. TEOS and deionized water were mixed into a clear 50 ml plastic vial, and later, it was deposited into a flask and was shaken for 15 s observing a transparent mixture. Subsequently, we added ammonium hydroxide and alcohol and applied 5 min of ultrasonic vibrations in a Cole-Parmer 8891 Ultrasonic Cleaner to form a homogeneous whitish solution.
We synthesized SiO 2 -PbS structures in aqueous phase by an organic compound of sulfur (thiourea), and at the same time, we prepared lead acetate as precursors, PbS particles by placing 2.5 ml of lead acetate [Pb(C 2 H 3 O 2 ) 2 ] 0.5 M solution in a 100 ml beaker, followed by adding 2.5 ml of sodium hydroxide (NaOH) 2 M, 3 ml of thiourea [SC(NH 2 ) 2 ] 1 M, 1 ml of triethanolamine TEA (C 6 H 15 NO 3 ) 1 M, 0.5 ml of citric acid (C 6 H 8 O 7 ) 1 M (after 15 min), and then the total volume of the solution was completed to 50 ml by adding deionized water. We used triethanolamine and citric acid as complexing agents. We added the solution of colloidal spheres to the preparation and the container was placed in the ultrasonic bath for 4 h until the colloidal suspension acquired a cloudy and dark appearance. The entire reaction proceeded at room temperature. We separated a sample of uncoated silica nanoparticles for comparison. The obtained samples of silica nanoparticles and of the ones coated with PbS were centrifuged using a model 59 A Fisher Scientific microcentrifuge during three sessions of 3 min at 7500 RPM and then washed with ethanol to remove remnant compounds in between sessions. The chemical composition and morphology of the nanostructures were determined by the techniques of energy-dispersive spectroscopy and transmission electron microscopy, respectively.

Results and discussion
We prepared two samples for characterization: one is the uncoated SiO 2 nanoparticles and the other coated with PbS. Both were analyzed and compared with TEM and EDS techniques. Figure 1 shows the TEM images at different magnifications of the pure silica sample. A homogeneous and spheroidal pattern in size and shape is remarkable in nanoparticles, which exhibited a narrow size distribution (* 70 ± 10 nm). 60 days after we accomplished the synthesis, the TEM measures, which demonstrates the stability of the nanoparticles.
Spectroscopy studies (EDS) in Fig. 1d revealed the presence of oxygen and silicon with atomic percentages of 59.79 and 40.21 %, respectively, something that is consistent with the SiO 2 stoichiometry. Traces of carbon, copper, and gold were also detected corresponding to the grating where the sample was deposited and are not significant. Figure 3 represents the evidence of the core-shell nanostructures where the formation of a uniform outer layer of PbS (* 10 nm) surrounds the silica.
In the sample corresponding to silica-lead sulfide core-shell, that had more time of ultrasonic bath were formed some agglomerates, this can be attributed high surface energy of each particle, causing them to attract each other to form these agglomerates [26], where the coalescence of the nanoparticles is not observable, rather they seem to be embedded within what could be the protective agent and lead residues (dark areas). In the obtained EDS spectrum, Fig. 3d, it can be observed the presence of oxygen, silicon, lead, and sulfur, with atomic percentages of 46.75 %, 36.06 %, 14.94 %, and 2.27 %, respectively. In the case of lead and sulfur, they overlap in the same peak, and where it is deconvoluted, there clearly appears two individual peaks of 2.34 keV (lead) and 2.46 keV (sulfur). There were also identified traces of carbon, copper, and gold due to the grating as explained above. The surface layer of the core-shell corresponds to cubic PbS (JCPDS # 01-078-1897) with interplanar distances of 2.95 Å and 3.41 Å with (2 0 0) and (1 1 1) Miller indexes, respectively (see Fig. 4).
For the analysis, we used high-resolution images obtained in the TEM, which were processed in the Digital Micrograph software to find the interplanar distances.  Figure 5 shows the XPS spectra of SiO 2 -PbS coreshell and uncoated silica nanospheres, as high-resolution XPS spectra for C 1 s, O 1 s, Si 2p, and Pb 4f of the silica-lead sulfide system. The raw XPS data were corrected by shifting all peaks to the standard value of C 1 s spectral component (C-C, C-H), with binding energy set to 284.85 eV. Figure 5a shows the typical results of XPS spectra for silica nanospheres before (A) and after coating with lead sulfide shell (B). The peaks at bond energy (BE) of 103, 285, and 532 eV correspond to the Si 2p, C 1 s, and O 1 s spectra, respectively, which exist in both samples. After the silica nanoparticles are coated with PbS, it appears new doublet peaks with binding energies of 21, 138, 413, and 645 eV, which could be assigned to the Pb 5d 3 , 4 f 7 , 4 d 5 , and 4 p 3 orbitals, respectively.
The deconvoluted C 1 s spectrum shows peaks at 284.03, 284.85 (reference peak), and 285.75 eV, corresponding to C-C, C-H, and C-N bonds, which can be explained due to used precursors. Furthermore, the O 1 s spectrum was deconvoluted into three peaks assigned to O=C (529.40 eV), C (530.65 eV), and SiO 2 The Pb 4f 7/2 and Pb 4f 5/2 peaks are observed at 138.7 and 143.7 eV, presenting a shift with respect to the expected values for Pb bound with sulfur. This XPS size shift can be explained by the relation of the growth of surface with respect to the volume atoms and correspondingly relative increase of dangling bonds on the surface while particles' size decreases, providing a nonlinear growth of XPS energy shift to higher values [27]. These results illustrate that the nanoparticles have a SiO 2 core and a lead sulfide surface layer. According to the results of EDS, TEM, and XPS, we can affirm that the PbS is bonded to the surface of SiO 2 nanoparticles. Figure 6 presents the optical absorption of SiO 2 -PbS core-shell nanospheres suspended in ethanol at the wavelengths ranging from 250 nm to 800 nm, which shows a low and constant absorption in visible region, resulting in this material with desirable features for use in window layers in solar cells [28]. The inset in Fig. 6 exhibits the Tauc diagram used to determine the band gap of nanoparticles. The band gap obtained is about 3.75 eV, which is higher than the PbS bulk band gap value, due to quantum confinement effect in the nanocrystalline PbS shell, with thickness of approximately 10 nm, as was referred previously [9,29]. The increased effective band gap causes that nanocrystalline PbS structures to be a more suitable window material in solar cell applications.

Conclusion
We successfully synthesized silica nanospheres with well-formed PbS shells adding citric acid as a second complexing agent to a typical formulation with triethanolamine. Monodisperse silica nanoparticles are  obtained and simplified through the modified Stöber's method, where ultrasonic bath is used instead of magnetic stirring. Both routes are remarkable for low cost and simplicity to prepare a chalcogenide core-shell with perspectives in photonic crystals, solar cells, electronics, and biomedical sensors.