Synthesis and Characterization of Ag 2 S Layers Formed on Polypropylene

Silver sulphide, Ag2S, layers on the surface of polypropylene (PP) �lm was formed by chemical bath deposition method (C�D). Film samples were characterised by X-ray photoelectron spectroscopy (XPS), attenuated total re�ection Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction analysis (XRD).e surface morphology, texture, and uniformity of the silver sulphide layers were formed on PP surface dependent on the number of polymer immersions in the precursor solution. XPS analysis con�rmed that on the surface of the polypropylene �lm, a layer of Ag2S was formed. ATR-FTIR and FTIR spectra analysis showed that the surface of Ag2S layers is slightly oxidized. All prepared layers gave multiple XRD re�ections, corresponding to monoclinic Ag2S (acanthite). e Ag2S layer on polypropylene was characterized as an Ag ion selective electrode in terms of potential response and detection limit.e electrode was also tested as an end-point electrode for argentometric titration of thiamine hydrochloride.


Introduction
Polymers modi�ed by thin electrically conductive or semiconductive �lms of binary inorganic compounds, particularly of metal chalcogenides [1][2][3][4] represent a new class of materials-composites. e application of these composites is determined by their properties, wheares the latter depend signi�cantly on the preparation method, even the chemical composition and structure of composites slightly change [5,6].
ese unique properties suggest potentially broad applications of Ag 2 S �lms in various devices such as solar cells, super ionic conductors and semiconductors, photo detectors, photo thermal conversion, electroconductive electrodes, microwave shielding coating, gas sensors functioning at temperatures tending to room temperature, polarizer's of infrared radiation, and active absorbents of radio waves [5,6,[13][14][15].e properties of Ag 2 S �lms can be successfully utilized by deposition its layers (either by in situ or ex situ methods) onto polymer surface, since a polymer can be easily designed into almost any shape required by a particular application.Additionally, the polymer can also act as the controlled environment for the growth of the �lm layers.Furthermore, the Ag 2 S/polymer composites may exhibit new properties that differ from those of single Ag 2 S �lms, arising from the spatial orientation and network arrangement.erefore, the ability to prepare such composites on a large scale is an important goal of materials science.e properties of Ag 2 S/polymer composites are very sensitive to preparation technique and signi�cantly depend on the nature of polymer on which Ag 2 S layers are formed.Recently, there have been a number of papers suggesting different methods for the incorporation of silver sulphide nanocrystals in different polymer matrices and their characterization.e well-dispersed Ag 2 S nanoparticles incorporated in polyvinylpyrrolidone (P�P)/�bre matrices using the electrospinning technique [16].Kumar et al. [17] have applied a sonochemical method for the preparation of Ag 2 S nanoparticles in the presence of polyvinyl alcohol.Composite thin �lms consisting of nanosized Ag and Ag 2 S particles dispersed in nylon 11 thin �lms have been prepared by using a thermal relaxation technique [18].Wang et al. developed a gas/solid reaction method based on surface initiated atom transfer radical polymerization (ATRP) for fabricating nanoparticles/polymer composite thin �lms, while preventing the aggregation of nanoparticles; thus the reaction may happen on the surface of composite thin �lms [19].
is present study focuses on the deposition of Ag 2 S layers on the polypropylene (PP) by the chemical bath deposition method (CBD).PP �lm is characterized by many excellent properties than other polymers.is is a low density, high thermal resistance, high strength at high �exibility, resistance to cracking under stress and a very good chemical stability [20].
e CBD method is relatively simple and inexpensive, and highly reproducible technique.e technology is based on slow controlled precipitation of the Ag 2 S through the reaction between Ag + and sulphur compounds as the sulphur source.
e sulphur precursors were usually thiourea [24], the elemental sulphur dissolved in a concentrated NaOH solution [26], thioacetamide [27], sodium sulphide [25], sodium thiosulphate [3], dimethylthiourea [6] and toxic carbon disulphide (CS 2 ) in ethanol [28].e most pronounced role in the formation of the Ag 2 S particles and consequently deposits has been played by the complexing agents.An ethylenediaminetetraacetate disodium salt (EDTA) [8] and ammonia solution [21] acting as a complexant for Ag ions or as a catalyst is usually employed to control the reaction rate.e commonly accepted underlying mechanism involves alkaline solution (pH 6−10) as ammonia for pH adjustment [8] and higher temperature 30−80 ∘ C [21].
However, the adding of complexing agent increases experimental parameters whereas the nonaqueous medium is expensive or toxic generally.
In this paper we report on the depositing of Ag 2 S layers by CBD from an acidic aqueous solution containing silver nitrate and an excess of sodium thiosulphate at 20 ∘ C temperature.e replacement of complexing agents with excess of sodium thiosulphate results to the simpler reaction mechanism.A reaction mechanism for depositing continuous Ag 2 S layer has been examined, and the optimal precursor's concentrations in solution are suggested.e chemical composition, structural, and morphological properties of Ag 2 S layer formed on polypropene surface have been investigated.e Ag 2 S layer formed on polypropene was also tested as an indicator electrode for argentometric titration of thiamine hydrochloride (vitamin B 1 ).Before the chemical analysis, samples of PP strips plated with Ag 2 S �lms have been mineralized.Samples were treated under heating with 6 mol/dm 3 nitric acid to oxidize sulphur compounds to sulphates.Heating with concentrated hydrochloric acid removed the excess of nitric acid.e resulting solution was poured into a 25 cm 3 �ask and made up to the mark with distilled water.en the 5 cm 3 of this solution was taken for the sulphur concentration determination, and remaining part was used to determine the silver concentration.

Experimental
Sulphur concentration in the silver sulphide layer was determined turbidimetrically [29].Sulphate ions in the range from 1 mg/dm 3 to 15 mg/dm 3 may be readily determined by utilising the reaction with barium chloride in a solution slightly acidi�ed with hydrochloric acid to give barium sulphate.A glycerolethanol solution helped to stabilise the turbidity of the barium sulphate suspension.e measurement of the intensity of the transmitted light as a function of the concentration of the suspension of BaSO 4 was carried out with a GENESYS 20 spectrophotometer (ermo Spectronic, UK), at a wavelength   4 nm.e standard deviation of the method with the photometric procedure in the range of concentrations from 5 mg SO 4 2− dm 3 to 10 mg SO 4 2− dm 3 was ±8%.
Silver concentration was determined with an atomic Perkin-Elmer absorption meter 503 (  32 nm).e sensitivity of the method is 0.06 g/cm 3 for 1% Ag absorption.
e content of silver and sulphur was expressed as mol/cm 2 .ree samples of each PP strip plated with Ag 2 S �lm have been used to determine the concentration of silver and sulphur.
e scanning electron microscope (SEM) Quantax 200 with a detector X Flash 4030 (Bruker AXS Microanalysis GmbH, Germany) was applied for the obtained Ag 2 S layers surface analysis.
e surface morphology of the �lms was investigated using an atomic force microscope (NANOTOP-206, Microtestmachines, Belarus).An ULTRASHARP Si cantilever (force constant 5.0 N/m, curvature radius less than 10 nm) was used.e measurements were performed in the contact mode.e AFM characteristics: maximum scan �eld area up to 15 × 15 microns, measurement matrix up to 512 × 512 points.e image processing and the analysis of scanning probe microscopy data were performed using a Windowsbased program (Surface View version 2.0).
XPS measurements were performed with a Vacuum Generator (VG) ESCALAB MKII spectrometer.e nonmonochromatic Al K X-ray radiation (ℎ  1 eV) was used for excitation.e Al twin anode was powered at 14 kV and 20 mA.e photoelectron takeoff angle with respect to the sample surface normal was 90 ∘ , and XPs spectra of Ag 3d 5/2 , S 2p 3/2 , and O 1s were taken at a constant analyzer energy mode (at 20 eV pass energy).e base pressure in the working chamber was kept bellow 5 × 10 −7 Pa.e spectrometer was calibrated relative to Ag 3d 5/2 at 30 ± 01 eV and Au 4f 7/2 at 3 ± 01 eV.e quantitative elemental analysis was done by estimating peak areas and taking into account empirical sensitivity factors for each element [30].A standard program was used for data processing (XPS spectra were treated by a Shirley-type background subtraction and �tted with mixed Gaussian-Lorentzian functions).Binding energies were referenced to the C 1s (284.5 eV) on unsputtered surfaces.All samples were scanned as received-without ion beam surface cleaning procedure.Assignation of the signals to speci�c structures or given oxidation state of elements analysed was done by comparison with data reported by NIST Standard Reference Database 20, Version 3.5 [31] and to the literature references.
e changes in chemical structure and binding con�guration of a thin layer were analyzed by attenuated total re�ectance (ATR) spectroscopy, since ATR spectroscopy is an oen chosen technique as it may be used to obtain the spectra of the surfaces of the adhesive sides of a sample.ATR-FTIR spectra were recorded in the wavenumber range 4000-600 cm −1 by the compensation method.In the 600-100 cm −1 spectral region, were recorded FT-IR spectra.Ag 2 S deposits were scraped with a nonmetallic scraper, and then scrapings were solid mixed with CsI, modulated into pellets and subjected to FT-IR spectroscopy.Spectra were recorded on a Perkin Elmer FT-IR Spectrum GX spectrophotometer (USA) by averaging 64 scans with a wave number resolution of 1 cm −1 (0.3 cm −1 ) at room temperature.
X-ray diffractometry was carried out under a Brag Brendan circuit on a diffractometer (Dron-6, Russia) using Cu K (  01517 nm) radiation, 30 kV voltage, and 30 A current.e scanning range was 2 = 22-60 ∘ .e scanning speed was 1 ∘ ⋅min −1 .Results were registered in in situ mode with a computer, and X-ray diffractograms of samples were treated using the Search Match, X�t, ConvX, Dplot95, and Photo Styler programs.Potentials were measured by using modi�ed microprocessor HI 2200 (Hanna Instruments USA) relative to the reference Ag, AgCl/KCl (sat.)electrode.e indicator electrodes were Ag 2 S/PP samples.e potentiometric measurements were made with the following electrochemical cell Ag/AgCl/KCl (sat.)//sample solution/Ag 2 S/PP/Ag.e Ag 2 S/PP electrode was soaked in 0.01 mol/dm 3 AgNO 3 solution for 2 h before each measurement.e 0.100 g amount of vitamin B 1 (thiamine hydrochloride CAS RN67-03-8) powder was dissolved in 20 cm 3 of water.e resulting solution was poured into a 100 cm 3 volumetric �ask and made up to the mark with distilled water.50 cm 3 of the sample solution containing 50 mg of thiamine was transferred into a beaker, 10 cm 3 of distilled water or 10 cm 3 of 1 mol/dm 3 NaOH was added, and resulting solution was titrated with 0.01 mol/dm 3 silver nitrate solution in 0.5 cm 3 increments using Ag 2 S/PP membrane electrode.e end point of the titration was determined from the Gran plots.[32]) is formed by reaction

Results and Discussion
Ag 2 S 2 O 3 is comparatively unstable and hydrolyzes to release Ag 2 S into the reaction solution: e initial experiments indicated that the usage of 0.2-0.1 mol/dm 3 AgNO 3 and 0.2-0.1 mol/dm 3 Na 2 S 2 O 3 solutions for Ag 2 S layers depositing on PP, results in quick settle of Ag 2 S precipitates on reactor bottom and layers on polypropylene could not be formed.erefore, the slow precipitation of silver sulphide particles is essential for the formation of the layers on polymer surface.
In order to diminish the rate of silver sulphide formation process, the silver ions were complexed by thiosulphate ions.When the sodium thiosulphate solution was added into the aqueous silver nitrate solution, silver ions were coordinating with some thiosulphate groups, resulting in the relatively high silver ions concentration around these groups.In the presence of thiosulphate excess, the soluble In nitric acid solution, these complexes decomposed, and they slowly released S 2− ions combined with the Ag + ions to form a very insoluble Ag 2 S (the solubility product for Ag 2 S is  sp  72 × 10 −50 [32]) nuclei on some special sites of PP.
Simultaneously, the side reactions are also possible.e thiosulphate can react with nitric acid forming thiosulphuric acid by reaction: e formed thiosulphuric acid may slowly decompose, and the new structural units such as elemental sulphur appear: Elemental sulphur can dissolve in nitric acid by reaction: Silver nitrate and sulphur acid can react to form silver sulphate: e incorporation of impurities such as a silver sulphate and sulphur is quite possible in the bulk Ag 2 S layer.
It was found that the 0.08 mol/dm 3 AgNO 3 and 0.20 mol/dm 3 Na 2 S 2 O 3 solutions are the most suitable for the formation of silver sulphide layers on polypropylene surface at pH 2.3 and 20 ∘ C temperature.

Chemical Analysis of Ag 2 S Layers.
As mentioned above, the preparation method in�uences the composition and properties of sulphide layers.erefore, the initial stage of the work was aimed to establish the chemical composition of the deposited silver sulphide.
e chemical composition of silver sulphide layer obtained for different PP �lm immersion time in 0.08 mol/dm 3 AgNO 3 and 0.20 mol/dm 3 Na 2 S 2 O 3 solutions at 20 ∘ C is presented in Table 1.e molar ratios of silver to sulphur show that the stoichiometric silver sulphide and nonsulphide sulphur can coexist in synthesised layers.

Morphology of Ag 2 S Layers.
Figure 1 shows the representative SEM images of an Ag 2 S layers on PP surface formed aer 5 immersions (40 min each) of polymer in precursor solution.e sulphide surface exhibits nonhomogeneous morphology.e irregularly shaped grains join to form different sized islands on PP surfaces.e maximum size of islands is 19.5 × 47 m and the smallest 7.8 × 8 m.e large number of micro bumps and irregularly spread clusters are also visible.
e AFM images of Ag 2 S layers on PP surface are shown in Figures 2 and 3. e Ag 2 S layer exhibits a graintype surface morphology; however, the size of grains, like the roughness of layer depends on the number of polymer immersion time in precursor solution.
When the polymer was immersed for 1 cycle in precursor solution, the irregular growth of different shaped grains is observed (Figure 2).e height of grains changed from 36 nm to 108 nm, and the average width was about 148 nm.In the local regions of layer surface, some of grains coalescence forming aggregates.e width of aggregates changed from 0.54 to 0.97 m.e maximum height of the aggregates was about 138 nm.e appreciable numbers of bumps is also observed on polymer surface (Figures 2(a) and 2(b)).e root mean square roughness (  ) was 25.9 nm, while an average roughness (  ) was about 20.2 nm.e   value is greater than   , which support the rough morphology of Ag 2 S layer surface.
e increase of polymer immersions times of in precursor solution leads to PP surface ful�llment.When the number of immersions was increased up to 5 (Figure 3), the rather large agglomerates composed of the different size particles are observed on surface.e width of aggregates changed from 0.56 to 1.6 m.e maximum height of the aggregates increased up to 162 nm.On the other hand, in the regions surrounding aggregates, the numerous grains are still observed.e width of grains varied from 28 to 200 nm, while the height of grains equaled to 46-48 nm.e parameters   and   are of higher magnitudes, namely, about 33.8 and 25.5 nm, respectively, suggesting a rather rough surface.[33,34].e sulphide layer on the PP surface is non-homogeneous (Figure 1) enabling an easy contact of the atmospheric oxygen with silver and sulphur ions.erefore, the composition of the Ag 2 S layer on the surface can differ from the chemical composition of the entire layer.

XPS Analysis of in Ag
It is necessary to emphasize that the XPS method investigates a very thin (up to few nm) surface layers.e analysis of experimental XPS data shows that the composition of Ag 2 S layers formed at different immersions time is rather similar.ey consist of silver, sulphur, and oxygen in various combinations (Table 2).
According to the NIST XPS date base [31], the S 2p 3/2 peak at 160.8 eV is assigned to Ag-S-Ag bonding of Ag 2 S, and the S 2p 3/2 peak at 163.0 eV is assigned to -S-S-bonding.e latter is about from 13.77 to 23.66% of S 2p 3/2 peak (Table 2), depending on the immersion time.is peak probably resulted from the chemisorbed sulphide nanoparticles, rather than the incorporation of elemental sulphur, since the S 2p 3/2 peak occurs above 164.0eV [35].A spectral line at higher energies of about 168 eV originates from oxidized sulphur [36].For all samples, the higher energy S 2p 3/2 peak position at 168.8 eV well coincides with the known Ag 2 SO 4 position (168.6 eV) [31]; therefore, we assign this peak to sulphur(VI) bond to oxygen.
According to the literature data, Ag 3d 5/2 core level, located at 368.3 eV, corresponds to binding energy of metallic silver [37], while the line at lower energies of 367.8 eV indicates the presence of Ag + [38,39].e peaks of Ag 3d 5/2 at 368 eV and S 2p 3/2 at 161.2 eV are the characteristic binding energies for the Ag 2 S [26], whereas the Ag 3d 5/2 line for Ag bounded to sulphur has been reported at 368.9 eV [40].e Ag 3d 5/2 component with binding energy of 367.9 eV was reported for Ag 2 O [41], while for AgO, it shied by 0.5 eV to value of 367.4 eV [42].Summarising analysis data of experimental Ag 3d 5/2 peak detected in the range between 368.5 to 368.0 eV (Table 2), it is difficult to distinguish between Ag-O-Ag and Ag-S-Ag bond position.However, concerning the S 2p 3/2 core levels (Table 2), it can be noted that the majority of the sulphur is in the S 2− state, con�rming that the obtained layers are indeed Ag 2 S. e XPS spectrum for O 1s can be deconvoluted into free different contributions with binding energies at around 531.0, 532.0, and 533.3 eV (Table 2).e peak at around 533.3 eV on the high binding energy side of the O 1s region was ascribed to the atmospheric oxygen adsorbed on the layer surface.e peak with binding energy of 529.5 eV was reported for Ag 2 O [41].Two peaks at 528.7 and 531.0 eV are observed in the O 1s spectrum of AgO [43].e presence of component at 531.0 eV in all studied O 1s spectra con�rms that the silver sulphide surface layer oxidized.Figure 4(a) shows ATR-FTIR spectra of untreated (curve 1), etched polypropylene �lm (curve 2), and PP samples, coated with a layer of silver sulphide (curve 3) in the range of 4000 to 600 cm −1 , and Figure 4(b) displays FTIR spectrum of PP samples, coated with a layer of silver sulphide in the range of 600 to 100 cm −1 .

ATR-FTIR Analysis of Ag
e PP sample ATR-FTIR spectrum (Figure 4(a), curve 1) displays absorption peaks which coincide well with the reported literature data [44].In polypropylene spectrum, moderate absorption peaks of deformation vibrations of the plane methylene group arise in the spectral range of 1445 to 1485 cm −1 , and methyl groups vibrations are registered in range of 1430-1470 cm −1 or 1365 to 1395 cm −1 .ese peaks in our spectrum appear at 1451 cm −1 and 1375 cm −1 , respectively.A broad and intense band at around 2915 cm −1 was attributed to vibrations of CH bands.e absorption features at 840, 1000, and 1170 cm −1 are characteristic vibrations of terminal unsaturated CH 2 groups present in isotactic PP [44].ese characteristic features in our experimental spectrum are observed at 840, 990, and 1165 cm −1 (Figure 4(a), curve 1).
ATR-FTIR spectrum of etched PP sample (Figure 4(a), curve 2) shows some changes.e characteristic peaks sharply diminish, and a new signal appears at 1719 cm −1 .is signal is probably due to superposition of signals at 1710 and 1724 cm −1 , characteristic of carboxylic and carboxylic stretching vibrations, respectively [45].e spectrum of etched PP con�rms that the oxidi�ing pretreatment of polypropylene increases surface hydrophility.Better hydrophilic characteristics of PP surface induce electrostatic interactions between the charged sites of the polymer and the sulphide particles, providing its adhesion to the polymeric surface.e ATR-FTIR spectrum of PP samples covered with Ag 2 S layers (Figure 4(a), curve 3) shows that the intensity of peaks, characterising etched PP, decreased, and two new broad vibration bands at 1137 cm −1 and 640 cm −1 appeared.On the grounds of the literature data [46][47][48], the bands in the range of the wavenumbers 1000-1250 cm −1 correspond to vibration frequencies of the S-O band: in the range of 1186-1224 cm −1 , it corresponds to the asymmetric valence S-O vibrations,  as (S-O), while the peaks corresponding to the symmetric valence S-O vibrations,   (S-O), appear in the interval of 1018-1095 cm −1 .According to the reported data [47], the sulphates vibration modes arise in the wavenumber range of 1080-1130 cm −1 and 610-680 cm −1 .e scissoring vibration mode of the O−S−O group occurs as a strong band between 400-650 cm −1 [49].
erefore, the peak of 640 cm 3.6.XRD Analysis of in Ag 2 S Film Layers.e deposited Ag 2 S layers were subjected to X-ray diffractometry to investigate the crystallographic structure of these layers.Structural studies of the Ag 2 S deposits on polypropylene are limited by the crystallinity of the PP �lm itself.e intensity of its peaks exceeds the intensity of silver sulphide peaks several times in 2  2 ∘ .erefore, XRD diffractograms of samples were recorded in the 2 range between 22 and 60 ∘ .
Figure 5 presents the X-ray diffraction pattern of the as-prepared Ag 2 S/PP composites.e phase composition was determined by comparing Ag 2 S/PP composites XRD diffraction patterns with those of known minerals.For analytical applications, the response time of a sensor is an important factor.e average time to reach a potential within ±1 mV of its �nal equilibrium value was measured aer successive immersion of the Ag 2 S/PP composite electrode in a series of AgNO 3 solutions of different concentration, each having a 10-fold increase in Ag + ion concentration from 1. × 1 −8 to 1. × 1 −2 mol/dm 3 .e ionic strength of solutions was 0.1 mol/dm 3 .e static response time for Ag 2 S/PP composite electrode obtained was 20 s for 1 × 1 −2 mol/dm 3 Ag + ion concentration, while at lower concentrations the response time increased up to 60 s.
A linear potential response was obtained in the concentration range from 1 × 1 −2 to 1 × 1 −5 mol/dm 3 (Figure 6).e change in slope per decade change in Ag + ion concentration was found to be slightly lower than that of 25.6 mV decade for a theoretical Nernstian response, which is the expected value for a univalent ion.A potential deviation from linear behaviour was observed in the concentration range below 1 × 1 −6 mol/dm 3 .
e Ag 2 S/PP composite electrode was also tested as an end-point indicator electrode in potentiometric titration involving Ag + ions.e method can be applied for the determination of thiamine hydrochloride.
In highly alkaline medium, thiamine undergoes chemical transformation and loses two protons through two distinct dissociation steps.e reaction occurs through intramolecular addition of the amino group of thiamine to the thiazolium ring with loss of �rst proton, accompanied by opening of the thiazole ring and loss of a second proton [51].ese protons can be replaceable by Ag + ions.In presence of sodium hydroxide, the silver ions do not react with free chloride but react with thiols only [52].
e solubility product of AgOH ( sp = 2 × 10 −8 ) is higher than that of AgCl ( sp = 1.78 × 10 −10 ); therefore, the precipitation of AgCl would be suppressed by the addition of NaOH excess.In alkaline medium, Ag 2 S/PP does not respond to HO − ions; hence, this electrode can be used as an indicator electrode for the determination of thiamine content in vitamin B 1 .
e potentiometric titration curves of 0.0024 mol/dm 3 thiamine with 0.01 mol/dm 3 silver nitrate in alkaline medium are shown in Figure 7. ere is a clear potential break (Figure 7) corresponding to the consumption of 0.78 mole of silver nitrate per mole of thiamine.

Conclusions
Ag 2 S layers were successfully prepared at 20 ∘ temperature on polypropylene (PP) immersed in a bath containing the aqueous solutions of 0.08 mol/dm 3 silver nitrate and 0.2 mol/dm 3 sodium thiosulphate solution.e surface morphology, texture, and uniformity of the silver sulphide layers formed on PP surface dependent on the number of polymer immersions in the precursor solution.Chemical and X-ray photoelectron spectroscopy analyses con�rmed a formation of silver sulphide on polypropylene surface.ATR-FTIR and FTIR spectra analysis showed that the surface of Ag 2 S layers is slightly oxidized.e XRD studies indicated that the deposited layers were polycrystalline with monoclinic (acanthite) crystal structure.e Ag 2 S/PP sensor exhibited low detection limit and therefore, are promising as indicating electrode in argentometric titration of vitamin B 1 as well as a sensor for determination of silver ions.

F 1 :
SEM image of Ag 2 S layer formed on etched PP surface which was immersed in AgNO 3 /Na 2 S 2 O 3 solution 5 times at 20 ∘ C.
2 S Layers.Seeking to investigate the changes of the chemical composition of the untreated PP, etched PP, and PP samples, coated with a layer of silver sulphide, ATR-FTIR spectra (Figure4) were recorded.
−1 to the symmetric deformation of O-S-O vibrations,   (O-S-O), and the peak of 1137 cm −1 corresponds to the asymmetric deformation of O−S−O vibrations,  as (O-S-O).ebroad absorption band maximum at 237 cm −1 observed in FTIR spectrum (Figure4(b)) corresponds to characteristic vibrations of Ag−S and is in good agreement with reported data for Ag 2 S[48,50].

F 7 :
Potentiometric curves for the titration of 0.0024 mol/dm 3 thiamine with 0.01 mol/dm 15 mm × 70 mm size samples of nonoriented isotactic polypropylene (PP) �lm of 150 m thicknesses (Proline X998, KWH Plast, Finland) were used for the experiments.Before Ag 2 S formation process, the hydrophobic PP sample requires a previous surface treatment process in order to facilitate its adhesion properties.Polymer was treated for 25 min at 90 ∘ C with etching solution (H 2 SO 4 /H 3 PO 4 (1 : 1), saturated with CrO 3 ).eformation of Ag 2 S layers on PP was carried out in the glass reactor.Etched PP �lms were immersed for 40 min in aqueous Na 2 S 2 O 3 (0.2 mol/dm 3 ) and AgNO 3 (0.08 mol/dm 3 ) solutions at 20 ∘ C with pH 2.3.enobtained samples were removed from reaction solution, rinsed with distilled water and dried at room temperature.eAg 2 S/PP samples were subjected for repetitive immersions in order to increase Ag 2 S amount.eAg 2 S/PP samples were repeatedly immersed in the freshly prepared reaction solution.eimmersions procedure was repeated for 5 times.Distilled water, and analytically pure Na 2 S 2 O 3 ⋅5H 2 O, AgNO 3 , reagents were used to prepare reaction solutions.
3.7.Application of Ag 2 S/PP Layers for Potentiometric Measurements.To assess the practical applicability of the prepared silver sulphide layer on polypropylene �lms, an attempt was made to test Ag 2 S/PP composite as an Ag + ion selective electrode.