Synthesis and characterization of nanoparticles and thin films of PbS by a high-performance procedure using CBD

The present work is devoted to find a simple, inexpensive, and efficient synthesis for PbS. Nanoparticles and nanostructured thin films of lead sulfide are synthesized in short reaction times by using physicochemical methods at room conditions or similar ones. Also, a simple laboratory equipment and a basic experimental configuration is used. PbS was synthesized without ammonium or ammonia. The main precursors used in both syntheses are lead acetate, as lead source, polyethyleneimine, as a complexing agent, and thioacetamide, as a sulfur source. Acetylacetone has been employed as a dispersant agent to elaborate PbS nanoparticles, while the sodium hydroxide maintains a high pH, to elaborate PbS thin films. Characterizations reveal the existence of PbS nanoparticles whose size is smaller than 10 nm and with a bandgap of 3.35 eV. On the other hand, the resulting thin film is composed of PbS little crystallites of 22 nm with a homogeneous profile and its bandgap is 1.85 eV.

 PbS nanoparticles synthesized at room conditions were made in a few minutes.
 Homogeneous nanoparticles less than 10 nm and a bandgap of 3.35 eV were obtained.
 PbS nanostructured thin films were elaborated by chemical bath deposition at 50°C in one hour.
 A homogeneous thin film around 160 nm of thickness is formed of small PbS crystallites of 22 nm approximately and bandgap of 1.85 eV was obtained.
 Sodium hydroxide was used to maintain the pH, instead of ammonium or ammonia hydroxide to make thin films of PbS.

Abstract.
In this research, nanoparticles and nanostructured thin films of lead sulfide are synthesized by using physicochemical methods at room conditions or suchlike conditions, at short reaction times; furthermore, is used simple laboratory equipment and a simplified experimental configuration. Both syntheses do not employ, ammonium or ammonia to elaborate PbS. The main used precursors for both processes are lead acetate, as the lead source, polyethyleneimine as the complexing agent, and thioacetamide, as the sulfur source. Acetylacetone has been employed as a dispersant agent to elaborate PbS nanoparticles. While the sodium hydroxide maintains a high pH, to elaborate PbS thin films. The characterizations reveal the existence of PbS nanoparticles whose size is smaller than 10nm and with

Introduction
The semiconductors are materials used in different electronic devices, according to their electronic properties, like bandgap or conductivity, for example. Particularly, the PbS is a chalcogenide material that can change these properties by reducing its size to nanometric scales. Specifically, it is possible to change its bandgap from the infrared region to the ultraviolet region, in contrast, the bulk value is 0.37 eV.
Consequently, the PbS can be employed in devices like solar cells, LEDs, photocatalysis, photoluminescence, or infrared photodetectors. Despite, many kinds of research that have been performed, from their synthesis processes, their characterizations, until the applications, however, it is still competing to find an effective and low-cost synthesis.
There are reported applications for PbS on photocell as patent since 1948 by R. J.
There are works where detectors are produced with PbS colloidal quantum dot photoconductors [4]. In 2017 Priyanka realized the synthesis biologically of PbS nanoparticles and applied it to detect arsenic(III) in water, see [5]. Also, PbS quantum dots have been used as contrast materials in laboratory studies in tumors, checking their low cytotoxicity, there are researches related to this [6]. Moreover, there are different methods of synthesis, for example, Khan et al. reported the deposition of PbS thin films via aerosol assisted chemical vapor deposition [7]. In recent times, mechanochemical synthesis of PbS nanocrystals from lead oxide [8] and, a chemical synthesis for PbS nanoparticles [9] were reported.
This research is focused on developing chemical processes for growing PbS nanoparticles, as well as PbS nanostructured thin films, and realize their fundamental characterization to verify the identity of the obtained compounds and their optical properties. The work proposes a first-principles technique to elaborate PbS nanoparticles and PbS nanostructured thin films, with room conditions or close to them, a few reagents, ammonia, and ammonium free, short reaction times, and being affordable.

Material and methods
In this article, a synthesis method to elaborate both nanoparticles and thin films of PbS is presented. This method was realized at room temperature or close to it, in short times. This method mainly consists of an aqueous solution containing a diluent, a complexing agent, a source of lead, and a source of sulfur, for both materials. For this method, different agents can be used but, in this work, particularly those in table 1 were used. Table 1. Reagents employed to both synthesis, PbS nanoparticles and PbS thin film.

Reagents
Molar concentration Volume Deionized water Continuing, are explained the employed formulations for each system, the used precursors, and the used equipment for the characterization.

PbS nanoparticles
The used reactor was a 30 ml test tube, the first reagent employed was 100 ml of acetylacetone (0.025 M), next 1 ml of lead acetate (0.5 M) were added, then 1.5 ml of polyethyleneimine (x M), and finally 1 ml of thioacetamide (0.1 M). Once added reagents, the test tube was collocated into an ultrasonic bath for 20 minutes, the reaction temperature was of the laboratory (24°C), after that, some color changes were observed until they were stable around 30 minutes. The used polyethyleneimine is a kind of branched compound, that makes it difficult to calculate its molarity, but we dissolve 3.5 milliliters of the pure polyethyleneimine compound in 50 ml of deionized water and this is denominated "x M". Acetylacetone has a double purpose, is a complexing agent and a dispersant; the lead acetate is the lead source, the polyethyleneimine is the complexing agent, and the sulfur source is the thioacetamide.

PbS thin films
The PbS thin films were synthesized by chemical bath deposition on a soda-lime glass substrate, corning brand. The used reactor was a 100 ml beaker, first 5 ml of lead acetate (0.5 M) was poured, second 5 ml of polyethyleneimine (x M), then 1 ml sodium hydroxide (2 M), next 5 ml thioacetamide (0.1 M) and finally 40 ml deionized water. In this system, sodium hydroxide has the purpose to increase the pH, while the other reagents have the same functionality as for nanoparticles system. The deposition temperature was 50°C. The glass substrates were located in a vertical position for 60 minutes. The deposited PbS thin film has a dark color, almost black, and presents a well adhered to substrate and reflective behavior.

Equipment
The

PbS nanoparticles
This section begins with characterizations of nanoparticles leading to the formal identification of the synthesized chemical compound, Figure 1 displays a set of micrograph images coming from transmission electron microscopy, the images 1a-1c correspond to three different work scale as is indicated, a kind of branched behavior adopted for the small cluster is observed at this sequence. In Figure 1c, the size of the particles is almost homogeneous.     From the data absorption is possible to apply the well-known Tauc process [10] [11] [12] to obtain the energy band gap value, Figure 3 illustrates the direct bandgap estimated of 3.35 eV with a good approach correlation coefficient, and located at the intersection of the linear fitting, with the energies axis.

PbS thin films.
The PbS thin film characterizations include SEM images, XRD pattern, the optical responses of UV-vis, and Raman spectroscopy.     From Raman dispersion spectroscopy, the signals shown in Figure 10 were obtained, this shape corresponds with those reported by [17] for PbS thin film. The

Conclusion.
In this article, we have presented a successful process to elaborate PbS nanoparticles with an aqueous solution reaction at room conditions in 10 minutes. In this reaction, the main precursors are lead acetate as the lead source, thioacetamide as the sulfur source, and polyethyleneimine as the complexing agent. The TEM micrographs images showed the presence of small clusters, while the HRTEM images showed PbS nanoparticles smaller than 10 nm. The obtained Fast Fourier Transform identified the crystallographic planes corresponding to the database PDF 77-0244. These nanoparticles were also characterized by a UV-vis spectrometer to obtain the corresponding absorption and transmission curves. From these results and using the Tauc method, the analysis was made to calculate the energy gap, obtaining a result of Eg = 3.35 eV.

Declarations.
The authors received no financial support for the research, authorship, and/or publication of this article.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The data that support the findings of this study are available from the corresponding author, B. G. Zaragoza-Palacios, on request.