Effect of the tri-sodium citrate as a complexing agent in the deposition of ZnS by SILAR

Abstract

Using the SILAR method Zinc sulfide coatings were deposited on glass slices. The physical properties and the chemical mechanism throughout the variation in concentration of tri-sodium citrate (TSC) as a chelating agent in the synthesis of thin films were investigated. Results shows that ZnS thin films exhibit an average transmittance of 16% in visible light spectra region and a zinc blende structure. The ZnS films synthesized using TSC as a complexing agent, present a smaller average particle size, an average transmittance of 85%, and an adsorption edge at 300–340 nm. Based on our experimental data and analysis, we conclude that the contribution of the oxychloride species, a subproduct in the chemical deposition, is suggested to be related as an impurity level former in the synthesis of ZnS thin films. TSC as a complexing agent in the SILAR technique is a non-toxic option to reduce the generation of the oxychloride species and synthesize a wide band gap semiconductor. Moreover, the use of complexing agents could be extended to other types of semiconductors deposited by SILAR.

Highlights

• •Zinc sulfide with the variation of tri-sodium citrate thin films were deposited as a complex agent by SILAR method with homemade equipment.
• •We focus on the optical effect of ultraviolet and visible regions in the thin films showing a blue shift in the characteristic bandgap value for the ZnS n-type compound.
• •Grazing-incidence X-ray diffraction (GIXRD) shows a characteristic peak for zinc blende ZnS.

1. Introduction

Wide bandgap semiconductors materials have become a topic studied globally due to their attractive properties for optoelectronic-based devices, photocatalysis, and photovoltaic applications [[1], [2], [3]]. In this sense, the synthesis and characterization of coatings based on chalcogenides semiconductors have attracted a lot of attention due to their suitable properties [4,5], one example of these semiconductors is the zinc sulfide. The ZnS possess a direct bandgap of 3.68 eV or 3.91 eV, depending on the crystalline phase either cubic zinc blende or hexagonal wurtzite, respectively [6,7]. Besides, in its optoelectronics properties the ZnS presents an electron Hall mobility in the range between 161 and 165 cm²/Vs and optical transmission for the range of 75–85% in visible region [7]. These values make the ZnS suitable to be used in optoelectronic devices such as sensors or photodetectors and in solar cells as a buffer layer and photodetectors.

The deposition of ZnS thin films is possible by various techniques, such as vacuum evaporation [8], chemical bath deposition (CBD) [9], spray pyrolysis (CSP) [10], ion beam sputtering [11] and successive ionic layer absorption and reaction (SILAR) [12] Among them, SILAR possesses remarkable advantage due to its simplicity, scalability, low cost, and the variety of precursors reagents that can be used [12,13]. SILAR is a cyclic process, each cycle consists of 4 steps: adsorption over the substrate by immersion in the cationic species, continuing with the rinsing to remove excess of the ionic species, the reaction of the adsorbed species with the anionic precursor as third step, and finally rinsing to remove unreacted species [12]. For the deposition of ZnS by SILAR, ZnCl₂ is a non-toxic and abundant compound preferentially used as a Zn²⁺ ions precursor [14,15]. Nevertheless, an aqueous mixture of ZnCl₂ could promote the formation of unwanted species in the cationic source. A proper conception of the dissolution process demonstrates evidence that ZnCl₂ obtained by dissolving this salt in deionized water (DIW) is a mixture of ZnCl₂ plus HCl synthesizing a strikingly bulky precipitate forming oxychlorides in the process [16]. To our knowledge, the effects of oxychlorides in the SILAR deposition of ZnS have been mostly disregarded in previous works [17,18], but could influence the deposition of the films contributing to the formation of subproducts based on chloride that impact the film properties.

A strategy to prevent the existence of precipitation and to increase the solubility of chloride salts is to use complex agents [19,20]. In the literature have been reported use of complexing agents such as ethylenediamine (EN), triethanolamine (TEA), and ethylenediaminetetra-acetic acid (EDTA) to avoid the formation of subproducts and to control the precipitation of ZnS coatings deposited by CBD and SILAR with successful results [21,22]. We propose the use of economic and low toxicity complexing agent, the tri-sodium citrate (TSC). Through this complex agent, it is proposed to decrease the presence of oxychlorides in the cationic source and to study the effect on the morphology, optical and structural properties of ZnS thin films synthesized by the SILAR methodology.

2. Experimental details

2.1. Film deposition

ZnS films were deposited on glass substrates, prior to deposition Pearl® microscope glass slides (25.4 × 76.2 mm) were ultrasonically cleaned in acetone, isopropyl alcohol, and DIW for periods of 10 min then dried in hot air. The reagents used for the synthesis were ZnCl₂ (97.0% A.C.S. grade) and Na₂S•xH₂O (60–63% A.C.S. grade) both from Jalmek®, Na₃C₆H₅O₇·2H₂O (99.2% A.C.S. grade) from FAGA® Lab, and DIW with a conductivity of 1.5 μS/cm from an EcoDI ENVIROGEN system. To study the complexing effect of the tri-sodium citrate in the zinc chloride cationic precursor different molar relations were prepared. 30 mL of the cationic precursors was used as a final volume keeping the same ZnCl₂ concentration in the final product and varying the ratio of the TSC concentration. The studied molar ratios between the ZnCl₂ to TSC were 1:0, 1:0.5, 1:1, 1:2, and 1:4.

Solutions used in the SILAR process consisted of ZnCl₂/TSC (30 mL) as the cationic precursor, sodium sulfide as the anionic source (30 mL [0.05 M]), and 35 mL of DIW as rinsing between the ionic precursor. Deposition of the films was effectuated in a homemade SILAR deposition system at RT (≈27 °C) using 20 s immersion in the ionic source, 100 s immersion for the rinsing steps, 40 cycles, and finally dried the deposited films with hot air. The thickness of ZnS layer was calculated using gravimetric weight difference method, resulting in approximately 87.93, 103.01, 138.19, 173.36 and, 201.15 nm thick for the ZnS/TSC ratio 1:0.5, 1:1, 1:0, 1:2 and, 1:4 respectively.

2.2. Characterizations techniques

Optical transmittance spectra of the thin films were collected with a Shimadzu UV-3600 plus spectrophotometer in the spectral range from 300 to 2500 nm. Crystallographic characterization of the films was determined with a PANalytical, grazing-incidence X-Ray Diffractometer using Cu Ka (lambda 1.5405 Å radiation with 2Ɵ of 10–80°and a grazing-incidence angle $\omega =0.5°$ to adjust the beam penetration to the films.

3. Results and discussions

3.1. Cationic precursor

During the preparation of the cationic precursor, the mixture without the complexing agent (ZnCl₂/TSC ratio 1:0) exhibited an observable bulky white precipitate. A proper conception of the dissolution process demonstrates evidence that aqueous ZnCl₂ pH = 5 and 0.1 M is composed of a mixture of ZnCl₂ plus HCl synthesizing oxychlorides in the process [16]. For the cationic source with the presence of TSC, it was observed that the incorporation of this complex agent promotes the dissolution of the precipitates creating a clear and transparent product, even at the lower ratio concentration of 1:0.5. The formation of a Zn²⁺ complex in the presence of the citrate ion can be expressed with the following chemical reaction:

$${Zn}^{2+}+{ncitrate}^{2-}\to {\left[Zn\left(citrate\right)n\right]}^{2+}$$

In addition to the visible transparency, the aqueous ZnCl₂/TSC mixture presented changes in the chemical properties related to an increment of the pH values as it increases the TSC ratio. The pH was 5 for the 1:0, 1:0.5, and 1:1 ratio, 6 for the 1:2 ratio, and 7 for the 1:4 ratio. A possible explanation of this effect is the presence of complexed cationic in the final mixture. As the concentration of TSC increases, the position of equilibrium moves toward the right-hand side of the reaction to generate protonated citrate anions creating an increase in the pH value and the possibility to complex the metallic ions present in the product [22]. In addition, the pH influences the SILAR deposition because the relation of the zeta potential in glass with the pH. As the pH value becomes basic, the zeta potential decrease to negative values far from the isoelectric point, generating a negative potential in the glass surface [23,24]. This behavior of the pH and the zeta potential of the glass makes it relevant in SILAR to promote the cationic precursor absorption during the immersion in the cationic source.

To evaluate the bond stability of the complexed cationic precursor, a stability constant was calculated for all the complexed samples prepared considering the chemical equation:

$$pM+qL\leftrightarrow M_p{L}_q$$

and the equilibrium constant equation:

$$K_s=\frac{\left[M_p{L}_q\right]}{{\left[M\right]}^p{\left[L\right]}^q}$$

Where M is the metal, L is the ligand, p and q are the coefficients, and Ks is the stability constant. Table 1 shows the values of the stability constants, as can be seen, there is a decrease in the stability constant with the increment of the concentration in the complex agent. We can visualize the stability constant as quantification of ionic energy of the complexed cationic precursor. While the concentration of the complex agent increments the ionic energy of the complexed cationic compound decrease, therefore a favorable effect to minimize the presence of subproduct based on the complex agent will be increased when the stability constant decreases.

Table 1.

Stability constant for the aqueous complexed cationic source.

Ratio (ZnCl2 to TSC)	Ks
1:4	35.65
1:2	46.8
1:1	67.12
1:0.5	83.33

3.2. Optical characteristics

Fig. 1 shows transmittance spectra of the films. Beginning with the (UV) range, the absorption edge of the ZnS films evidences the fundamental transition. In the visible range, a change in transmittance is notably apparent for the films with TSC complex in comparison with the non-complexed film. This is evidence of the contribution of TSC complex in optical response of the samples. The transmittance spectrum of the ZnS film is consistent with the characteristics of an amorphous structure as reported in the literature [25] the absorption edge is observed to soften and be featureless. For the films deposited in mixture with the complex agent, it is observed a monotonically increase in the transmittance, which goes from ≈60% for the 2:1 film to ≈90% for the 4:1 film. Followed by an absorption edge and the fall down due to the bandgap of the transmitted light, which corresponds to a typical semiconductor behavior. On the other hand, from an optoelectronic perspective, the transmittance performance matches the requirements for its use as a solar window buffer [26]. Finally, in the near-infrared region, the transmittance reaches values of 80%–90% for the complexed film.

Fig. 1.

Transmittance spectra of the synthetized ZnS coatings in function of the variation in the TSC concentration.

Subsequently, optical analysis of the transmittance spectra, the bandgap values were determined by considering the Stern's relation $\alpha \left(hv\right)=k{\left(hv-E_g\right)}^{\frac{n}{2}}$, where $\alpha$ is the optical absorption coefficient, calculated from the optical transmittance (T) using the relation $\alpha =\left(1/d\right)\mathit{ln}\left(100/T\%\right)\text{,}$ where d is the thickness of the film specified above. The bandgap (E_g) characteristic values were calculated from the linear fitting in the ($\alpha$ hv)² vs. (hv) plots. From Fig. 2 could be observed that the bandgap energy increase (blue shift) with the use of complexing agents. For the as-deposited ZnS thin film, the calculated E_g was 3.62 eV. For the complexed films were: 3.68 eV for the 1:0.5 film, 3.66 eV for the 1:1 film, 3.65 eV for the 1:2 film, and 3.66 eV for the 1:4 film. These bandgap values are comparable with the previous experimental reported values [7,13]

Fig. 2.

Bandgap calculation in function of the variation in the TSC concentration for the ZnS thin films deposited by SILAR.

In the literature, the bandgap shift is attributed to the quantum size effect and impurity levels [[27], [28], [29], [30]]. While the two possible causes for the impurity levels could arise from structural defects or subproducts in the reaction. These observations, along with the chemical analysis could imply that in this work the presence of the complex agent minimizes the contribution of the oxychloride species, which are suggested to be related as impurity level formers. Therefore, a blue shift in the complexed films could be correlated with the absence of impurity levels.

3.3. Morphology characteristics

It is observed in Fig. 3(a–e) SEM images for synthetized pure and complexed ZnS thin films, all samples exhibit uniform morphology. As the TSC concentration increases the cumulative of grains takes places and exhibits a sphere-like morphology. It is seen from Fig. 3(a) that the coatings deposited with 87.93 nm thickness had low presences of cracks in the structure, as the film thickness increase to 138.19 nm the grain coalescence begins, similar nature in the number of imperfections was describe in the literature using complexing agents for ZnS nanoparticles [31]. Also, due to aggregation, the grain size decrease, and smallest grain size was obtained for the ZnS/TSC [1:4] sample ratio. It is concluded that the incorporation of TSC as a complex agent decreases the presence of impurity compounds in the structure as Zinc hydroxides or oxychlorides creating a significant effect on the ZnS matrix in the variation of grain size in agreement with the GIXRD result and accordingly with Trejo et al. in the purity control by modifying the pH [32].

Fig. 3.

SEM micrographs for thin films of ZnS/TSC ratio a) 1:0, b) 1:0.5, c) 1:1, d) 1:2, and e) 1:4.

3.4. Crystallographic characteristics

Fig. 4 shows the GIXRD diffractogram from the films. The crystallographic characteristic was assessed by analyzing all the samples in the range of 10–80 2 $\theta$ degrees. Using ICSD 98-006-7790 from the database in the Highscore Plus software with indexation in all the diffraction signals for a zinc blende [33]structure with a preferent orientation in (111) with 28.925, 28.915, 28.945, 28.905 and 28.465 $\theta$. A notable apparent from the results is the widening of the diffraction signals, a characteristic increment of Gibbs free energy causes an amorphization as the TSC concentration increases similar to the response in the UV–vis spectra. The increase of the Gibbs free energy is evidence in contribution of TSC in the decrease of crystallite size comparable to the non-complexed film. Table 2 shows the crystallite size and strain values of the lattice determined from the FWHM value and were calculated by Debye Scherrer equation and using the Bragg's law relationship between lattice strain and peak broadening.

$$D=\frac{0.9\lambda }{\beta \mathit{cos}\theta }$$

$$\epsilon =\frac{\beta }{4\mathit{tan}\theta }$$

where D, $\lambda =0.154056\mathrm{n}\mathrm{m},\beta \mathrm{a}\mathrm{n}\mathrm{d},\theta$ are the grain size, the X-Ray wavelength, FWHM and Bragg diffraction angle of the (111) diffraction signal for all the GIXRD diffractograms respectively.

Fig. 4.

GIXRD diffractograms for the film deposited (with different concentration of TSC) deposited by SILAR.

Table 2.

Parameters and results for the thin films evaluated.

Compound	$\mathit{\theta }$	$\mathit{\beta }$	D/nm	$\mathit{\epsilon }/$ Strain
ZnS 1:0	28.925	1.37 e−03	26.043	4.499e-04
ZnS 1:0.5	28.915	1.39 e−03	25.562	4.661e-04
ZnS 1:1	28.945	1.48 e−03	24.045	4.664e-04
ZnS 1:2	28.905	1.49 e−03	23.932	5.102e-04
ZnS 1:4	28.465	1.51 e−03	23.561	1.956e-03

As mentioned before, the effect of confinement in the exciton by the quantum size is notable through amorphization of samples [34], which are suggested to be related to the crystallite size of crystalline structure. In the other hand, strain lattice values increase as the crystallite size decreased, which is a particular behavior for compression of the structures. Similar behavior was presented by Cavusoglu et al. by the growth of thin film of CuO using similar chemical deposition technique with TEA as complexing agent [35]. These observations, along with the optical analysis of the films deposited, could be correlated with the absence of impurity level, causing a diminution in the crystallite size of the ZnS.

4. Conclusions

ZnS coatings were deposited on glass substrates via the SILAR methodology using tri-sodium citrate as a chelating agent. Effect of complexing agent and their concentrations on optical properties, morphology, structure, and chemical mechanism was investigated. The concentration of the oxychloride species decreases as the concentration of the TSC increase, which may be explained by the bond stability of the cationic precursor. GIXRD analysis of the ZnS thin film exhibit a pure zinc blende structure with a preferential diffraction plane in (111), an amorphization behavior was determined from FWHM values as the TSC concentration increased. The ZnS thin films prepared using TSC are composed by a large number of uniform particles with crystallite sizes in a range of 23.56–26.04 nm accordingly with SEM micrographs exhibit the increment in the grain size as the ZnS/TSC ratio vary from 1:4 to 1:0. Moreover, the optical transmission of thin film synthesized increased from 18 to 85% with a sharper adsorption edge at 300–340 nm in the UV light range. The bandgap increased from 3.62 eV to 3.66 eV with the variation of TSC concentration. A more detailed study to quantify the oxychloride species is still in progress. However, the present findings support the use of tri-sodium citrate as a complex agent in ZnS synthesis by SILAR as a via to avoid unwanted species. Moreover, the use of complexing agents could be extended to other types of semiconductors deposited by SILAR.

Author contribution statement

Jesús Octavio Sigala-Valdez, MSc: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Obed Yamil Ramirez-Ezquivel, PhD: Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Celina Lizeth Castañeda Miranda, PhD; Rocio García Rocha, MsC: Contributed reagents, materials, analysis tools or data.

Harumi Moreno-García, PhD: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Ismailia Leilani Escalante-García, PhD: Analyzed and interpreted the data and wrote the paper.

Antonio Del Rio, PhD.: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Data availability statement

Data will be made available on request.

Declaration of competing interest

The authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.