1 Introduction

Population increase leads to greater industrial production and, consequently, greater waste generation [1, 2]. Waste from textile industries are difficult to be processed using conventional methods due to the presence of –N=N– groups linked to aromatic rings, requiring specific methods for their processing [3]. Therefore, it is necessary to use less conventional methods, such as heterogeneous photocatalysis [4, 5]. Heterogeneous photocatalysis is characterized by the use of semiconductor to generate electron/hole (e−/h+) pairs that mineralize the organic molecules of dyes [6, 7]. Reactive oxygen species (ROS) generated by e−/h+ pairs also have antimicrobial action, enabling semiconductor materials to be used in the treatment of microorganisms resistant to antibiotic treatments [8, 9].

Semiconductor materials have been widely studied due to their high optical and electronic properties [10]. Moreover, their properties can be easily altered by the manipulation of defects in their crystalline lattice [11, 12]. Several semiconductor materials such as TiO2 [13], AgCl [14], SnO2 [15], ZnO [16], and others, have been widely studied. Lead-based materials have gained attention due to their range of possibilities and applications, such as supercapacitors [17], photocatalytic activity [18], lithium storage [19], electrodes [20], and others. Among lead-based materials, PbS is an important semiconductor of the IV-VI group, which has excellent optical and optoelectronic properties [21].

However, to use PbS in photovoltaic, photocatalytic, photodetection and antimicrobial applications, it is crucial to improve its separation efficiency of photogenerated electrons and holes, which can be achieved through doping [22]. Silver is an attractive noble metal with high electrical conductivity to facilitate fast electron transfer [23]. Hu et al. [24] obtained significant improvements in the performance and stability of solar cells based on PbS films by the addition of silver, where carrier concentration and mobility and the extreme band, as well as Fermi energy levels, can be adapted by adjusting the Ag/Pb ratio.

In this work, silver-doped PbS powders (PbS:xAg with x = 0, 1, 2, 4 and 8 mol%) were obtained by the sonochemical method. Powders were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM) and diffuse reflectance spectroscopy. The photocatalytic activity was measured with methylene blue dye when submitted to UV radiation. The antimicrobial activity was estimated against S. aureus and E. coli bacteria and C. albicans yeast.

2 Materials and methods

PbS powders were obtained by the sonochemical method, using lead nitrate (Pb(NO3)2, Sigma–Aldrich, 99%), thiourea (H2NCSNH2, Synth, 99%), silver nitrate (AgNO3, Strem Chemicals, 99.9%), sodium hydroxide (NaOH, Synth, 98%) and deionized water as received.

Initially, lead nitrate and thiourea were weighed, maintaining the molar ratio of 1:1 (1.383 g/0.315 g), and kept under constant stirring in 50 mL of deionized water. After solution homogenization, sodium hydroxide (approximately 1 g) was added to control pH, fixing it at 12, keeping under stirring. The solution was then taken to Branson 102C ultrasonic tip, operating at 20 kHz frequency, for 30 min. The solution was then washed and centrifuged to obtain the powder (PbS), which was oven dried for 24 h. Ag+ doped samples were obtained in a similar way, always respecting the ratio of 1, 2, 4 and 8 mol% in relation to lead. Samples were named P1A, P2A, P4A and P8A, according to the molar Ag+ percentage added.

The phases obtained for pure and Ag-doped PbS powders were investigated using the Shimadzu diffractometer (XRD-6000) with CuKα radiation (1.5418 Å). In order to better verify the changes promoted by doping, refinement was performed using the General Structure Analysis System (GSAS) software with graphic interface EXPGUI [25]. Field emission scanning electron microscopy (FE-SEM) was used to observe the morphology. Elemental X-ray fluorescence analysis (XRF) was performed on the Shimadzu EDX-720 spectrometer to analyze the silver doping efficiency in PbS.

UV–Vis spectroscopy was performed on Shimadzu (UV-2550) equipment, with wavelength range of 200–900 nm and programmed for the diffuse reflectance mode, and based on these results, the optical gap band (Egap) energy of these materials was determined using the Wood and Tauc Equation [26]. The sample was placed in a quartz tube for the EPR measurements. The analysis occurred in room temperature using the EMM micro-9.5/12/S/W spectrometer, operated in 9.833 GHz frequency and 25 dB alternation.

The photocatalytic properties of powders were tested against methylene blue (MB) dye of molecular formula [C16H18ClN3S] (Mallinckrodt, with 99.5% purity), under UV–Vis radiation. About 0.05 g of powder were placed in beaker containing 50 mL of MB (1 × 10−5 mol/L−1) and kept under constant stirring, illuminated by six UVC lamps (TUV Philips, 15W, with maximum intensity of 254 nm = 4.9 eV). At intervals of 30 min, 2 mL aliquot was collected and the absorbance spectrum variation was analyzed using Shimadzu spectrometer (model UV-2600). By these values, the dye concentration variation was determined by the test time.

The antimicrobial properties of powders were evaluated by in vitro tests using agar diffusion method against gram negative E. coli (ATCC-25922) and gram positive S. aureus bacteria (ATCC-25923) and C. albicans yeast (CA14), according to standard procedures stipulated by the Institute of Clinical and Laboratory Standards (CLSI), similarly to study carried out by Araújo et al. [27]. The inoculum was prepared by direct suspension of colonies in phosphate-buffered saline (PBS) in water plate and maintained for 24 h at 35 °C. The solution turbidity was then adjusted according to the McFarland scale (bacterial density of 1 × 108 CFU/mL). The test was performed using the well diffusion technique with 6 mm wells and adding 50 µL sample aliquots.

3 Results and discussion

Figure 1 shows the XRD patterns of pure and silver-doped PbS samples. The patterns indicate the formation of PbS, with cubic crystalline system and Fm-3m space group (no. 225), characterized by JCPDS card 65-2935. The characteristic PbS peaks in 2θ = 26.02, 30.06, 43.14 and 51.13° refer to (111), (200), (220) and (311) planes, respectively. The presence of defined peaks and with considerable intensity indicates the high crystallinity of pure PbS samples. Tshemese et al. [28] observed, by increasing the intensity of XRD patterns, the increase in the crystallinity of PbS nanoparticles by raising the synthesis temperature from 150 to 200 °C. The reduction in the intensity of characteristic peaks with the increase in the amount of dopant occurs due to defects generated in the crystalline lattice as a result of differences between ionic rays and the valence of Pb2+ (1.19 Å) and Ag+ ions (1.15 Å). El-kemary et al. [29] observed that the addition of Ag+ in the crystalline lattice of CdO generates a reduction in the intensity of diffraction peaks due to the generation of defects. The reduction in the intensity of characteristic PbS peaks and the non-appearance of the secondary phase indicate that the sonochemical method is efficient in obtaining silver-doped PbS.

Fig. 1
figure 1

XRD patterns for (a) PbS, (b) P1A, (c) P2A, (d) P4A and (e) P8A samples

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For analysis of possible differences in structural arrangements induced by the addition of Ag+ in the PbS lattice, the Rietveld refinement was used [30]. The General Structure Analysis System (GSAS) software with EXPGUI graphic interface was used to perform the refinement [25]. JCPDS card 65-2935 was used for refinement. The parameters used in refining were: scale factor and phase fraction; background, which was modeled using a displaced Chebyshev polynomial function; peak shape, which was modeled using Thomson-Cox-Hasting pseudo-Voigt; change in lattice constants; fractional atomic coordinates; and isotropic thermal parameters. The Rietveld refinement results are shown in Fig. 2 and Table 1.

Fig. 2
figure 2

Rietveld refinement for a PbS, b P1A, c P2A, d P4A and e P8A samples

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Table 1 Structural parameters of the Rietveld refinement for pure and silver-doped PbS samples
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By the small difference obtained between observed (obs) and theoretically calculated (calc) curves, it was concluded that the diffractogram patterns of samples are well adapted to the JCPDS card 65-2935. In addition, the χ2, Rwp and Rp reliability parameters exhibit low values, indicating good quality of structural refinements and numerical results. These results confirm that samples are isostructural and crystals are well adapted to the cubic structure and Fm-3m spatial group (No. 225). The values obtained from the occupation of Pb2+ and Ag+ cations in the unit cell are close to the synthesis stoichiometric values, confirming that doping was successfully performed.

Through the results shown in Table 1, the volume of unit cells is reduced with the addition of Ag+. This occurs due to the substitution of Pb2+ cations for Ag+, which have ionic radius of 1.19 Å and 1.15 Å, respectively. The increase in the amount of dopant causes more defects in the crystal lattice of PbS, yielding smaller crystallite sizes. This behavior is in accordance with Vergard’s law, where for solid metal solutions, the unit cell dimensions tend to decrease with increasing dopant concentration [31, 32].

There were no changes in the x, y and z positions occupied by Pb2+/Ag+ cations in the unit cell, being x, y and z = 0.50. S2− anions maintained their positions in the x, y and z axes, which value is 0.00. The cubic crystalline structure of PbS is formed by Pb2+/Ag+ cations arranged in the centered cube surrounded by six S2− anions. Figure 3 shows the unit cells of samples obtained from the Rietveld refinement data.

Fig. 3
figure 3

Model of hexagonal structure for a PbS, b P1A, c P2A, d P4A and e P8A samples

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Figure 4 shows the FE-SEM images obtained for pure and silver-doped PbS samples. Figure 4 shows that pure and silver-doped PbS particles are agglomerated and tend to form a rectangular morphology, with a non-uniform size distribution. As the silver concentration increases (Fig. 4a–e), a gradual reduction in particle size occurs, in addition to the appearance of nanoparticles with semi-spherical morphology, as can be seen in Fig. 4e. The reduction in mean particle size is in accordance with Fig. 1, where, as the silver concentration increases, the width of characteristic peaks also increases, indicating smaller crystallite size, which is usually accompanied by a reduction in particle size [33]. This relationship between width of diffraction peaks and PbS particle size was also observed in the study of Tshemese et al. [28].

Fig. 4
figure 4

Micrographs obtained by FE-SEM for a PbS, b P1A, c P2A, d P4A and e P8A samples

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The real reason for particle agglomeration is difficult to explain, mainly because nanometer-scale particles have high surface energy. The Ostwald ripening growth mechanism can be considered the most appropriate because it relates the critical core size and the surface energy of the particle without excess surfactant [34, 35]. This mechanism shows that even when several nanoparticles arise simultaneously, over time, larger particles tend to consume smaller ones to reduce their surface energy [35]. The use of surfactants during synthesis promotes alteration in the surface energy of particles, being able to avoid their agglomeration and alter their morphology [36]. However, morphological alteration resulting from the use of surfactants may be detrimental to the surface area dependent properties of materials, where a morphological alteration may promote the increase of its surface area and, consequently, a reduction of the reactive area of particles. Dong et al. [37] observed that the use of different anionic, cationic and non-ionic surfactants generated an increase in the surface area of CuO particles when compared to particles obtained without surfactant.

Figure 5 shows the XRF spectra obtained for the PbS and PA1 samples. All measurements were performed under vacuum atmosphere. Figure 5a shows the appearance of the peaks referring to Pb and S. As the analysis was performed from 0 to 25 keV, the formation of the peak referring to the Pb-Kα transition was not noticed, the Lα, Lβ and Lγ transitions appear in 10.552, 12.614 and 14.765 keV, respectively. There is an overlap in S-kα and S-kβ transitions, due to the energetic proximity (2.308 and 2.464 keV, respectively). According to Fig. 5b, the occurrence of the silver transition characteristic kα, at approximately 22.105 keV, confirming that doping occurred.

Fig. 5
figure 5

XRF spectra for the a PbS and b P1A samples

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The diffuse reflectance spectroscopy technique in the visible ultraviolet region was used to estimate the gap band (Egap) of pure and doped PbS powders. The Kubelka–Munk function [38] was used to convert reflectance data into absorbance. The Wood and Tauc method [26] was used to estimate the optical gap band value (Egap). In the Wood and Tauc method, the optical gap energy is given by:

$${\text{F}}({\text{R}}){\text{hv}} \propto {\left( {{\text{hv}} - {{\text{E}}_{{\text{gap}}}}} \right)^K}$$
(1)

Where: h is the Planck constant, v is the frequency, F(R) is the absorbance and k is indicated for different transitions (k = 1/2, 2, 3/2 or 3 for permissible direct, permissible indirect, direct prohibited and indirect prohibited, respectively). For pure and silver-doped PbS powders, k = ½ was admitted, that is, permissible direct. The absorbance versus photon energy values (eV) were plotted, extrapolating the linear portion of the curve to zero absorption to estimate Egap. Figure 6 shows the graph obtained by extrapolating the UV–Visible absorption spectrum to obtain direct Egap. Table 2 shows the direct Egap values for all samples.

Fig. 6
figure 6

Extrapolation of UV–Visible Absorbance Spectrum for direct Egap estimation for a PbS, b P1A, c P2A, d P4A and e P8A

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Table 2 Direct Egap values obtained by extrapolating the linear portion of the absorbance spectrum
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Through values shown in Table 2, it could be observed that PbS has a direct Egap of 0.79 eV, such values are in accordance with values found in literature [39, 40]. According to Table 2, the increase in the amount of Ag+ generates a gradual reduction in the direct Egap values. The reduction of Egap values is related to the increase in the structural disorder of PbS due to differences in ionic radius (1.19 Å e 1.15 Å) and valence between Pb2+ and Ag+, the latter being the most effective. The difference between valences of cations generates an increase in the number of anion vacancies, generating intermediate levels between valence and conduction bands, reducing the Egap value of the material [41]. Ali et al. [42] observed that the addition of silver atoms in TiO2 lattice generates, in addition to structural disorder, a displacement of the absorption band to the red region, resulting in lower Egap.

The photocatalytic activity of pure and silver-doped PbS samples was estimated against methylene blue dye (MB), under UV irradiation, with total test time of 180 min, and an aliquot was collected every 30 min. Figure 7a shows the variation of methylene blue concentration by the test time. The photocatalytic process may be described by a first-order kinetic model with relation to the absorbance of methylene blue [43], and the results obtained are shown in Fig. 7b. The graph shows the linear relationship of ln (C/Co) by the irradiation time. From this linear relationship, Eq. 2 was use and the kinetic constant (K) and the correlation coefficient (R2) were obtained, which are shown in Table 3.

Fig. 7
figure 7

a Variation of the Methylene Blue degradation as a function of time and b Evolution of photodegradation kinetic for PbS, P1A, P2A, P4A and P8A samples

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Table 3 Apparent first-order rate constant K of photocatalytic degradation and correlation coefficient R2
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$$\frac{{ - \ln ct}}{{Co}}=k \cdot t$$
(2)

where Ct is absorbance of methylene blue at time t; Co is initial absorbance; t is irradiation time; k is kinetic constant.

Through the results obtained from varying the MB concentration over time (Fig. 7a), pure PbS reduces approximately 25% the methylene blue concentration after 180 min. As the Ag+ concentration increases in the PbS lattice, an increase in the photocatalytic activity is observed, where, for P8A sample, the MB concentration was reduced by approximately 68% after 180 min. The increase in photocatalytic activity can be accompanied by the constant K values shown in Table 3. The kinetic constant relates the efficiency of the semiconductor in the degradation of the dye and was used for a better monitoring of the photocatalytic behavior during the course of the test, accompanied by the adjustment R2. Higher K values indicate that photocatalysis is more efficient. The photocatalytic activity of semiconductor materials is directly related to the impedance of the electron/hole pair recombination (e−/h+), because they act in the generation of hydroxyl radicals (OH•), which are strong oxidizing agents of organic materials, as dyes [44, 45]. Higher amounts of dopant present the best photocatalytic activities because they generate more anion vacancies, which act to prevent the recombination of e−/h+ pairs [44, 46]. As previously described, Ali et al. [42] showed that the addition of silver atoms in the TiO2 lattice displaces the absorption band towards the red region, such a function acts in order to prevent the recombination of e−/h+ pairs, increasing the photocatalytic activity of the material.

Figure 8 shows the EPR spectra obtained for samples of pure PbS and doped with 1 and 8%Ag. The signals were obtained with the presence of a standard band, referring to the sample port, characterized by the letter a. The appearance of the band b is related to the Ag0 ion (4d105s1). The presence of Ag0 indicates that some Ag+ ions adsorbed on the surface of PbS, where they were subsequently reduced, as reported by Scaini et al. [47]. The bands c refer to the Ag2+ paramagnetic centers (4d9). The simultaneous appearance of Ag0 and Ag2+ confirm that Ag+ ions have replaced the Pb2+ ions in the PbS lattice, where both are required for system neutrality [48]. In addition, Ag+ ions have a smaller size than Pb2+, 0.126 and 1.33 nm, respectively. Thus, silver also occupies interstitial sites, generating internal stresses in the lattice [49]. The different sizes and ionic rays between silver and lead promote the generation of oxygen vacancies and, consequently, act as centers that prevent recombination of the e−/h+ pairs, generating more reactive oxygen species to act on the photocatalysis [44, 50].

Fig. 8
figure 8

Room temperature EPR spectra of the PbS, P1A and P8A samples

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The antimicrobial activity of pure and silver-doped PbS samples was evaluated against E. coli and S. aureus bacteria and C. albicans yeast by measuring the inhibition halos formed. Tests were performed in triplicate and the mean value was used. Figure 9 shows the inhibition halos formed against S. aureus (Fig. 9a), E. coli (Fig. 9b) and C. albicans (Fig. 9c). Table 4 shows the mean values of inhibition halos formed as well as the standard deviation. Figure 9a shows the way of obtaining the diameters of inhibition halos.

Fig. 9
figure 9

Antimicrobial activity of PbS:xAg (x = 0, 1, 2, 4 and 8 mol%) against a S. aureus, b E. coli and c C. albicans

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Table 4 Mean values and standard deviation of inhibition halos obtained for pure and silver-doped PbS powders
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By means of values obtained for the inhibition halos, it was observed that the substitution of Pb2+ cations by Ag+ favors the antimicrobial activity against S. aureus, E. and C. albicans. As shown in Fig. 4, the increase in silver concentration leads to loss of the rectangular morphology of PbS and favors the appearance of smaller particles of irregular morphology, increasing the surface area available to contact the tested microbes. Zare et al. [51] varied the particle size of ZnO through different surfactants and showed that smaller particles present better antimicrobial activities against E. coli and S. aureus bacteria because they have more active sites for generation of reactive oxygen species (ROS).

In addition, the substitution of Pb2+ cations by Ag+ leads to defects in the crystal structure of PbS, which act to prevent the recombination of generated e−/h+ pairs in contact with microbes, providing more ROS in contact with bacteria [52, 53]. The main mechanism of action of metallic nanoparticles is still unknown; however, the literature reports that direct or electrostatic interaction between nanoparticles and the cell surface, the production of reactive oxygen species (ROS) and the cellular internalization of ZnO nanoparticles are the main mechanisms [54,55,56]. ROS and metal ions act by deforming the cytoplasmic membrane of bacteria, leading to the leakage of intracellular substances and, consequently, the death of bacteria [57, 58].

4 Conclusion

The sonochemical method is efficient to obtain pure and silver-doped PbS powders without the necessity of several steps. XRD patterns show that increasing the amount of silver generates deformations in the crystal lattice of PbS and provides smaller crystallite size. Silver doping also leads to a reduction in particle size and loss of the cubic aspect of PbS. The defects resulting from the addition of silver generate a decrease in the Egap values of the material and act as sites that prevent the recombination of electron/hole pairs, increasing the generation of oxidizing radicals that act favoring photocatalytic and antimicrobial activities. P8A sample showed more than twice the photocatalytic efficiency in reducing the methylene blue dye concentration compared to PbS. The antimicrobial analysis showed that silver-doped PbS is efficient against S. aureus (gram positive) and E. coli bacteria (gram negative) and C. albicans yeast.