Exploration of spray pyrolysis technique in preparation of absorber material CFATS: Unprecedented hydrophilic surface and antibacterial properties

2.1 Chemicals

Copper (II) chloride dihydrate (CuCl₂·2H₂O, 99.99%), aluminum (III) chloride hexahydrate (AlCl₃·6H₂O, 95%), iron (III) chloride hexahydrate (FeCl₃, 6H₂O, 0.004 M), tin (II) chloride dehydrate (SnCl₂, 2H₂O, 98%) and thiourea (CH₂N₂S, 98%) were purchased from Sigma Aldrich and Fluka. All chemicals products were used without any further purification for the elaboration of Cu₂Fe_1-x Al_xSnS₄ (CFATS) thin layers. The methanol (CH₄O, 99.99%) was purchased from Fisher Chemical society and used as a solvent.

2.2 Deposition of Cu₂Fe_1-x Al_xSnS₄ (CFATS) thin films

Cu₂Fe_1-xAl_xSnS₄ (CFATS) thin films deposited on glass substrates using chemical spray pyrolysis technique in ambient air. The glass substrates were cleaned in ultrasonic bath using double-distilled water (Bi-distiller water GFL), followed by isopropyl alcohol for 10 min. All cleaned substrates were dried in a furnace at 80 °C for 15 min. Typically, the sprayed solution was prepared by mixing CuCl₂ (0.01 M), AlCl₃ (0.004 M), FeCl₃ (0.004 M), SnCl₂ (0.004 M) and CH₂N₂S (0.04 M) in 300 ml of methanol. This process was repeated three times to obtain approximately good physical properties. Then, the solutions were varied with and without aluminum concentrations by maintaining ×  = 0, 0.25, 0.50, 0.75, and 1 for the growth of Cu₂Fe_1-x Al_xSnS₄ layers. The obtained solutions were stirred for 15 min until they became clear and transparent without impurities which can potentially block the nozzle of the sprayer. Therefore, this solution is sprayed with a flow rate of 25 ml/min onto preheated substrates (T_s = 240 °C) as fine droplets by means of compressed air as carrier gas. The substrates and nozzle distance is equal to 28 cm. The possible proposed growth mechanism may follow the reactions as follows (Orletskii et al., 2018): 2Cu²⁺ + Sn²⁺→2Cu⁺ + Sn⁴⁺ Cu⁺ + Cl^-→CuCl CuCl + 2SC(NH₂)₂ → [Cu(SC(NH₂)₂)₂]Cl Sn⁴⁺ + 2SC(NH₂)₂ → [Sn(SC(NH₂)₂)₂]⁴⁺ Al³⁺ + 2SC(NH₂)₂ → [Al(SC(NH₂)₂)₂]³⁺

The formation of CATS is the result of the following reaction: Cu(SC(NH₂)₂)₂]⁺ + [Sn(SC(NH₂)₂)₂]⁴⁺+[Al(SC(NH₂)₂)₂]²⁺→ Cu₂AlSnS₄ precursor solution.

2.3 Bacterial adhesion analysis

Basically, the Gram-negative bacterium Escherichia coli (E.coli) and the Gram-positive bacterium Staphylococcus aureus (S. aureus) have been utilized as the experimental strains by counting forming unity (CFU). Overnight the growth of E. coli and S. aureus were attained at 37 °C in 5 ml of Tryptic Soya Broth (TSB) medium under agitation. Spread 10 µl of each culture on the chemically coated thin films and other uncoated glass slides that will serve as a control (previously sterilized in glass petri dishes) make a series of dilutions of the two cultures to spread it and count it on Tryptic Soya Agar TSA (up to 10^-6 is sufficient). Incubate the slides at 37 °C for 48 h, taking care not to tip them over when moving to the oven. After 48 h, immerse each film in Eppendorf tube filled with 1 ml of physiological water (using sterile forceps without touching the surface on which you have deposited the culture) and vortex it well to detach the slide bacteria. Therefore, this is a stock solution. Carry out successive dilutions and spread on (TSA) for enumeration. Also, spread 100 µl of the stock solution (normally a dilution up to 10^-4 is sufficient). The decrease in CFU/mL is considered as the effect of thin films against the bacterial growth. Calculate the viability (%) with the following formula;

2.4 Characterization methods

The crystalline phases were studied using an XPERT-PRO diffractometer system with λ = 1.5406 Å and 2θ varied from 20° to 60° at room temperature. The purity of films was investigated by Raman spectrometer type Jobin Yvon technology T 64,000 with 488 nm argon in laser like excitation source. Elemental compositions were explored using Energy dispersive X-ray spectroscopy (EDX). Film morphology and thickness was examined using scanning electron microscopy (SEM) type Zeiss EVO MA10 microscope. Optical behavior was recorded using spectrophotometer type Perkin Elmer Lambda 950 in the wavelength range 250–2500 nm at room temperature. The surface wettability of Cu₂Fe_1-xAl_xSnS₄ (x = 0, 0.25, 0.50, 0.75 and 1) thin films were investigated by Water Contact angle θ (WCA) measurements using Drop Shape Analysis System type DSA100 (Kruss GmbH, Hamburg).

3.1 Chemical composition

The composition of synthesized Cu₂Fe_1-xAl_xSnS₄ thin layers (x = 0–0.25 – 0.50–0.75 and 1) has been determined using energy-dispersive X-ray (EDX) spectroscopy and the results are provided in Table 1. This table indicates the existence of copper, iron, aluminum, tin and sulfur comprising the Cu₂Fe_1-xAl_xSnS₄ molecular structure. As expected, the rise in aluminum concentration seems to change the elemental composition of Cu, Sn and S contents in the films. It is shown that copper and tin amounts increased with Al inclusion. Fig. 1 shows the elements composition of Cu₂Al_xSnS₄ (where x = 1) thin layer. Moreover, copper, aluminum, tin and sulfur ratios in Cu₂AlSnS₄ (where x = 1) are about 28.7: 5.92: 10.38: 54.99, which are nearly in accordance with the stoichiometric ratio of Cu₃Al_0.6Sn₁S₆ (CATS) composition.

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Fig. 1

EDAX spectrum of CATS (where x = 1) film.

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3.2 Structural characterization

The crystallographic structure of Cu₂Fe_1-xAl_xSnS₄ thin layers (x = 0–0.25 – 0.50–0.75 and 1) has been studied by XRD with Cu (kα) radiation source in the range of 2θ [10°-70°] as shown in Fig. 2. The major diffraction peaks 2θ = 18.20°, 28.77°, 47.86° and 56.87° were indexed respectively to (0 0 2), (1 1 2), (2 0 4), and (3 1 2) plans of stannite CFTS structure, coordinated with the standard (JCPDF card No. 74-1025). In stannite structure the space group is I $\overline{4}$ 2 m, each atom has four nearest neighbors and each anion S is surrounded by two Cu one Fe and one Sn atoms. On the other hand, each cation is tetrahedrally coordinated by four anions. After aluminum inclusion in Cu₂Fe_1-xAl_xSnS₄ (CFATS) matrix, the peak intensities are slightly decreased accompanied with broad full width at half-maximum (FWHM), designed a decline of the crystallinity. The poor crystalline quality at x = 0.25 indicates that some quantities of Al atoms prefer to situate in or near the grain boundary regions (Venkatachalam et al., 2008). Meanwhile, the large FWHM may be originated to a combination of grains and possible assembling of the micro-strains at several planes as crytallographic defects form (Das Bakshi et al., 2018). It is also evidenced that all peaks are shifted toward lower angles, which is due to an increment of lattice parameters for CFATS materials. This trend may be ascribed to the ionic radii of Al³⁺ (0.53 Å) that is lower to that of Fe³⁺ (0.64 Å). However, for x = 1, the patterns of CATS thin film revealed a polycrystalline nature with diffraction peaks 2θ = 28.34°, 47.43° and 55.93° that are identified respectively to (1 1 2), (2 0 4), and (3 1 2) plans. No sign of contamination CATS thin film.

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Fig. 2

XRD patterns of Cu₂Fe_1-xAl_xSnS₄ films (x = 0–0.25 – 0.50–0.75 and 1) films. Inset presents the Stannite Type Structure of CATS material.

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Crystallite size (D), dislocation density (δ_dis), crystallites number (Nc) and strain (ε) were calculated for (1 1 2) plane using the following relations (Monisha et al., 2021; Agawane et al., 2014) and tabulated in Table 2:

Where: k is the shape factor, λ is the wavelength of incident X-ray, β is the full-width at half maximum, e is the film thickness and θ is the Bragg angle. The average crystallite size decreases from 60 nm for CFTS film to 45 nm for CATS film. This trend suggests the high crystallinity at x = 1 than the other concentration. Consequently, this enhancement leads to lesser crystal defects and for small recombination centers at the grain boundaries. Additionally, micro-strain and dislocation densities calculated to investigate the film quality such as crystal defects, dislocations and the presence of lattice imperfections. Therefore, we can say that CATS deposited with high quality and lesser defects, where δ_dis and N_c were equal to 5*10¹¹ cm⁻² and 15*10¹² cm⁻² respectively.

XRD data used to evaluate Lattice parameters ‘a’ and ‘c’, Unit cell volume (V), Molecular weight (M), X- ray density (d_x) and Porosity (P) from the following expressions (Somvanshi et al., 2020): $$\mathrm{a}=\mathrm{d}\sqrt{h^2+k^2+\frac{a^2}{c^2}{l}^2}\mathrm{V}=a^2\times c$$

where, d, (h, k, l), M, Z, N_A and d_B are the inter-planar spacing, miller indices, molecular weight, atoms number per unit cell, Avogadro’s number and bulk density, respectively. The structural parameters are tabulated in Table 3. Generally, it is found that Al concentration promotes a remarkable increment of lattice parameters ‘a’ and ‘c’ that correlated with the shift of diffraction peaks toward lower angles. Consequently, ‘a’ and ‘c’ lead to increase in the unit cell volume that decreases the X- ray density. Notably, the highest film porosity found for CATS film may be ascribed to the presence of major void space.

3.3 Raman spectroscopy

XRD patterns of all chalcogenides Cu₂XSnS₄ (where X is a metal) are similar and close to that of FeS and Cu₃SnS₄ (Mokurala et al., 2016). Raman spectroscopy utilized 488 nm as excitation wavelength and shown in Fig. 3. As mentioned from our previous work and literature (Nefzi et al., 2018), quaternary chalcogenides show principally two Raman peaks. For example, CFTS is kknown to show two Raman peaks at 285 cm⁻¹ and 319 cm⁻¹, which are observed here. Fig. 3 demonstrates that CATS displays two principal peaks at 335 cm⁻¹ and 295 cm⁻¹ which may be identified as the A₁ and the B/E modes respectively. In our knowledge, there is no work, which identified the Raman peaks of CATS material before. As a consequence, Raman peak matched at 295 cm⁻¹ may correspond to S^2- around Cu²⁺ (Lee et al., 2012). Then, the principal peak at 335 cm⁻¹ may be attributed to the strongest asymmetric vibrational mode of S^2- around Sn (Lee et al., 2012).

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Fig. 3

Raman scattering results for CFTS and CATS films.

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3.4 Morphological analysis

Fig. 4 shows the morphological states of CFATS thin layers (x = 0 – 0.25 – 0.50–0.75 and 1) extracted from SEM, revealing a transformation in film morphology. It is evident that Cu₂FeSnS₄ film appears with smooth and uniform surface. At x = 0.25, it is clear that the surface morphology is uniform and dense with some cracks. A granulated structure with small grain size distribution was observed at x = 0.50. The micrographs at x = 0.75 reveal that the film surface is covered with large numbers of rod-shaped grains that are closely attached to each other. By contrast, the surface morphology of CATS appears as well as a nanofibers structure accompanied with voids and cavities. This trend is beneficial for photovoltaic applications as motioned by Hao Guan et al. (Guan et al., 2014). These obtained results are similar to those reported in other researches (Guan et al., 2014; Ozel et al., 2015). Fig. 5 indicates that film thickness of CATS slightly decreased to 723 nm compared to CFTS which mainly associates with the decrease of crystallite size.

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Fig. 4

Morphology of the as-synthesized Cu₂Fe_1-xAl_xSnS₄ (CFATS) films (x = 0 – 0.25 – 0.50–0.75 and 1).

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Fig. 5

Cross-section views of CFTS and CATS films for scale of 1 µm.

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3.5 Optical properties

The optical band gap (Eg) of sprayed CFATS thin films was deduced by utilizing Tauc’s relationship (Jrad et al., 2016):

Where A is a constant and n presents the nature of transition. After fitting, all Cu₂Fe_1-xAl_xSnS₄ layers were found to obey equation (3) with n = 2, which presents the direct transition type as shown in Fig. 6. Generally, the molarity of Al in the solution precursors affects the optical band gap values of elaborated CFATS thin films. The optical band gap of CFATS was varied with incorporating more Al atoms in the Fe lattices. This increment is due to the modification in composition and surface morphology (Fig. 4; Table 1). Eg is found to be increased from 1.47 eV to a maximum value about 2.1 eV with decrement of crystallite size, which may be attributed to the quantum confinement effect (Beraich et al., 2020). The films grown at x = 0.50 and 1 with optimum band gaps are well suited to be used as absorber layers in solar cell applications. In addition, the increase in Eg can also be originated to the less structural defects in samples that accompanied by the decrease in the density of defect states due to the decrease in film thickness of CFATS, as well as during the growth process. Hence, the optical band gap Eg = 1.52 eV of CATS is tuned to make it optically active as absorber layer for solar cell application.

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Fig. 6

Tauc-plot of the as-deposited Cu₂Fe_1-xAl_xSnS₄ (CFATS) films (x = 0–0.25 – 0.50–0.75 and 1).

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3.6 Wettability measurement

The surface wettability of materials is considered as a significant parameter to exploring their performance. It let to investigate the difference of energy between the molecules, which exist on the surface and those of material. Thomas Young described the contact angle θ_Y in 1805 using the following equation (Ben Ameur et al., 2020):

Where ${\mathit{\gamma }}_{\mathit{S}\mathit{V}},{{\mathit{\gamma }}_{\mathit{L}\mathit{V}}\mathrm{a}\mathrm{n}\mathrm{d}\mathit{\gamma }}_{\mathit{S}\mathit{L}}$ correspond to the interfacial tensions with solid/vapor, liquid/vapor and solid/liquid respectively. Fig. 7 presents water contact form on CFATS thin layers. It is clear that CFATS thin films exhibit a hydrophilic character with ${\mathrm{\theta }}_{\mathrm{Y}}<{90}^{°}$ . According to the curve, after inclusion of aluminum atoms, CATS (x = 1) material exhibits lowest contact angle near 44.6°, because CATS appears as nanofibers accompanied with voids and cavities. As stated by the figure, the droplets on materials surface exhibited elliptical form. The surface wetting behavior is controlled by various parameters such as micro/nanotexture and chemical composition. The variation of water contact angle θ_Y is related to the change of surface free energy and surface morphology. It can also be explained by the weak adhesive force accompanied with Van der Waals forces between the water and material surfaces (Geim et al., 2003). However, it has been demonstrated that films with hydrophilic character produce large surface area between solid and liquid (Mrabet et al., 2016). This surface type favors the physisorption processes, which are necessary for antibacterial and photocatalytic activities (Dridi et al., 2017).

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Fig. 7

Water contact angle of Cu₂Fe_1-xAl_xSnS₄ (CFATS) films deposited by spray pyrolysis (x = 0–0.25 – 0.50–0.75 and 1).

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3.7 Cyclic voltammetry (CV) analysis

See Fig. 8a represents the cyclic voltammogram plot for CATS thin layer.The electrochemical energy level of CATS thin film was extracted from the first oxidation onset potentials peak. It is shown that the valance-band edge (VB) equals to 0.29 eV. Conduction-band edge was given from the following expression: E_CB = E_VB – Eg, whereas E_VB = 0.29 eV and E_g = 1.52 eV so E_CB = -1.23 eV. From our previous work (Nefzi et al., 2020c), the valence and conduction band edges of SnO₂:F were located respectively at −3.96 eV and 0.1 eV.

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Fig. 8

a) Cyclic voltammogram of CATS thin film, b) Mechanism of visible light antibacterial activity of CATS/SnO₂:F heterojunction.

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3.8 Antibactetrial and hydrophilicity properties

Antibacterial tests were carried on Escherichia coli (E.coli) and Staphylococcus aureus (S. aureus) because they are culprits in various infections. The antibacterial activities of SnO₂:F, CATS films and CATS/SnO₂:F heterojunction against the degradation of E. coli and S. aureus were examined, and the results are presented in Fig. 9a. The evaluation of antibacterial activity has been performed by counting forming unity (CFU). SnO₂:F and CATS films exhibit stronger antibacterial activities for Gram positive S. aureus than that of Gram negative E. coli, while the opposite trend was found for CATS/SnO₂:F heterojunction. These behaviors are due to the antibacterial mechanisms of films and the different bacteria structures (Qi et al., 2019). Generally, a greater inhibition of both S. aureus and E. coli was attained by CATS/SnO₂:F heterojunction which may be attributed to its fast carriers generation (Ali et al., 2020). Deposition of p-type CATS on the surface of n-type SnO₂:F caused generation of the space charge layer (SCL) that affected the charge carriers diffusion. Therefore, it will increase the negative and positive carrier’s separation. The viability rates achieved against S. aureus was 40 %, 31 %, and 15% for SnO₂:F, CATS films and CATS/SnO₂:F heterojunction, indicating the excellent results of antibacterial activity of coupled CATS/SnO₂:F. A poor inhibition of E. coli bacterium implying just 90 % and 71% reductions to the surface of SnO₂:F and CATS films. However, CATS/SnO₂:F heterojunction has good effect in the antibacterial activity reaching 5 % as viability rate of E. coli. It has been proposed that the cation released from materials (in our case Cu²⁺_, Al³⁺ and Sn²⁺) could bind to thiol groups (SH) found in proteins and enzymes on the surface of cellule. Then, they can intervene in with cell division and conductive to bacterial cell death (Shao et al., 2015). Antibacterial activity assay of the membranes by CATS/SnO₂:F heterojunction against the degradation of E. coli and S. aureus are shown in Fig. 9b.

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Fig. 9

Percentage of bacterial inhibition of E. coli and S. aureus bacteria against SnO₂:F and CATS films and CATS/SnO₂:F heterojunction by monitoring the optical density (OD) at 600 nm. Antibacterial activity assay of CATS/SnO₂:F heterojunction against E. coli.

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See Fig. 8b reports the antibacterial activity mechanism of CATS/SnO2:F heterojunction. Beside, CATS presents narrow band gap, the electron can jumped from its valance-band (VB) to its conduction band (CB) by the generation of same contents of holes in its VB. The photogenerated electrons located in CB of CATS can migrate to the CB of SnO₂:F, because of the more positive CB of SnO₂:F than that of CATS. Therefore, the photogenerated holes located at the VB of SnO₂:F achieve the VB of CATS. Thus, the efficient charge separation enhances the lifetime of charge carriers, increasing the reduction and oxidation process that produce the reactive oxygen species (ROS). Meanwhile, the death of bacteria is mostly ascribed to the destruction of their outer membrane. The crust damage instigated by physical or chemical modification of the surface membrane because it consists of lipid along a polyunsaturated fatty acid (Ali et al., 2020). Then, a reaction of free radicals with polyunsaturated acids and initiation of lipid peroxide formation modifies the fluidity of membrane and interrupts protein bonds. This rapid augmentation of radicals induces oxidative stress that results in the degradation of polyunsaturated fatty acids in harmful products (Ali et al., 2020). CATS/SnO₂:F heterojunction is easy to penetrate the cell wall of E. coli than S. aureus, SnO₂:F and CATS films present lower antibacterial activity to E. coli.

3.9 Correlation between antibactetrial and hydrophilicity

It has been demonstrated here that CATS was characterized with hydrophilic surface. This surface type may be caused by the nanofiber structure accompanied with voids and cavities, which have been clearly observed in SEM images. It has been already established in the literature (De Falco et al., 2018; Qi et al., 2019; Karam et al., 2013), that materials with hydrophilic surfaces display higher antibacterial activity than those with hydrophobic surfaces. Contrary to the mechanism that conduct wettability, holes which exist in the valence band react with the adsorbed H₂O to generate hydroxyl radicals, whereas electrons contribute to reduction processes with oxygen molecules generating superoxide anions O²⁻ (Chen et al., 2012; Cho et al., 2004). Such responsible species for the inactivation of bacteria through oxidative damages (Cai et al., 2014) are also capable in damaging the cytoplasmic membrane and the outer membrane of microorganisms, consequently, deteriorate essential cellular components leading to cell death.