High Energy Pulsed Laser Beam to Produce a Thin Layer of Crystalline Silver without Heating the Deposition Substrate and Its Catalytic Effects

Abstract

Crystalline silver thin layers were obtained using high-energy pulsed laser ablation without the heating of the deposition substrate. The fluid Plateau–Rayleigh (PRI), Rayleigh–Taylor (RTI), and Richtmyer–Meshkov (RMI) instabilities, as well as the crown splash induced during the pulsed laser deposition (PLD) in the high energy regime, resulting in ring and pearl-shaped structures, offer the benefit of an increased sorption surface. These morphological structures obtained for the silver thin layers make them of interest for catalytic applications. This study addresses both fundamental and applied issues on the morphological structures obtained for the silver thin layers and their catalytic function in organic processes. In this sense, the catalytic action of the thin silver layer was highlighted by modifications of the Reactive Blue 21 dye (C.I.) in an aqueous solution with sodium bicarbonate. Specific investigations and analyses were carried out using electron microscopy and elemental analysis (SEM-EDX), atomic force microscopy (AFM) and profilometry, mass spectrometry, ablation plasma diagnosis, diffractograms (XRD), as well as IR spectroscopy (FTIR). In addition to the experimental investigation and analyses, the simulation of the ionization energy threshold was conducted in COMSOL for complementary evaluation on the involved processes and phenomena.

1. Introduction

Laser ablation has been intensively studied since the 1960s, with the beginnings of the development of laser techniques and its application in LIBS and LIF spectroscopy [1,2]. The investigation of the physico-chemical phenomena developed both at the level of the target in the interaction with the laser beam and of the ablation plume was carried out by developing techniques to analyze the composition of the target and capture the dynamics of the plume by using the ICCD camera, Langmuir probe, LIBS spectroscopy (laser-induced breakdown spectroscopy), LIF (laser-induced fluorescence), etc. [1,2,3,4,5,6,7,8,9,10]. Due to the very short laser pulse width (nanoseconds, femtoseconds), some parameters could not be measured, such as the temperature that is reached when the laser radiation interacts with the target. The development of numerical models and numerical simulations have replaced this technical impediment [11,12,13]. While for pure metal targets, the analysis of plasma components is reduced to the nature of charged, neutral, or aggregated species in the form of so-called clusters, for targets made of metals with impurities, such as alloys or chemicals of greater complexity, the chemical analysis of the ablation plume is harder to achieve. In the analysis using laser ablation, the laser ablation inductively coupled plasma mass spectrometry technique has been developed and used so far [14,15,16,17,18]. This technique only addresses the analysis of the target material. However, mass spectroscopy has not yet been used directly for ablation plume analysis because laser ablation is not sufficient to reach the ionization energy. As shown in previous studies, laser ablation produces liquid and gas phases, as well as a smaller amount of the ionized phase (plasma) [9,10,11,12]. The current study aims to increase the ionization rate in laser ablation using previous chemical treatments of the target.

Ablation plasma has been previously investigated using optical imaging techniques [8,19,20]. Silver thin films obtained using the PLD technique have been described as the result of nanoparticle deposition [9,11,12]. pulsed laser deposition (PLD) using a silver target has been treated from the perspective of the fluid model of ablation plasma. The Plateau–Rayleigh (PRI), Rayleigh–Taylor (RTI), and Richtmyer–Meshkov (RMI) instabilities, as well as the crown splash have been reported [12]. Therefore, the roughness of thin metallic films obtained due to the fluid instabilities can be interpreted as the result of the last micro and nanoparticles deposited during the PLD process, which are not sufficient in number/quantity to form a compact continuous layer [12]. Therefore, thin films with high roughness are potential materials to be used in sorption processes and catalytic processes.

The novelty of the study presented in this work consists in a complex analysis of the phenomena and physico-chemical processes that take place during the pulsed laser deposition of a thin layer of silver with impurities, as well as of those arising from the interaction of the thin layer with complex organic molecules in the sense of the catalytic effect. In the analysis of the ablation plasma plume, the novelty consists in obtaining information on the developed chemical species using mass spectroscopy and the pulsed laser as a source of the ionization energy of the chemical species.

2. Materials and Methods

2.1. Materials

The silver thin film (Ag-thin film) was obtained from the silver target containing iron and nickel impurities of jewelry provenience. The target was produced using thermal and chemical procedures as described in our previous papers [12,13]. Sodium tetraborate (Na₂B₄O₇·10H₂O) and sodium borohydride (NaBH₄) were used to convert metal cations from oxides and hydroxides into their atomic state [21,22], while the sulfur from silver sulfide (Ag₂S) was removed with sodium bicarbonate and aluminum foil. Specifications for the component elements of the target are shown in

Table 1. Characteristics of the target component elements [23,24,25,26,27].

Element	Melting Point (K)	Boiling Point (K)	Atomic Mass (a.m.u.)	Atomic Number	Heat of Vaporization (kJ/mol)	Ionization Energy (eV)
Ag	1234	2435	107.8682	47	254	7.5762
Fe	1811	3134	55.845	26	340	7.9024
Ni	1728	2002	58.6934	28	379	7.6398
B	2349	4200	10.811	5	508	8.29803

.

The aqueous solution of C.I. Reactive Blue 21 (10 g/L) and NaHCO₃ (10 g/L) (RB21) was prepared and used to test the Ag-thin film catalytic properties in the decomposition of the residual textile dyestuff from industrial wastewaters.

2.2. Method of Work

The target was used in the pulsed laser deposition (PLD) process for producing thin films and for plasma dynamics and composition investigation, using a YG 981E/IR-10 laser system, Q-switched Nd:YAG laser (producer Quantel, Les Ulis, France). The system working laser beam wavelength (λ) domains are visible (Vis) at 532 nm, ultraviolet (UV) at 355 nm and 266 nm, and infrared (IF) at 1064 nm, in pulses of 9–10 ns with a repetition frequency of 10 Hz and with energies of 1–820 mJ/pulse, 1–490 mJ/pulse, 1–150 mJ/pulse, and 1–1600 mJ/pulse, respectively. The silver target was ablated using this system at the following parameters and conditions: τ = 10 ns pulse width, λ = 532 nm wavelength, α = 45° incident angle, and ν = 10 Hz pulse repetition rate, laser fluence F = 50 J/cm², and power density P = 5 × 10¹³ W/m² (E = 180 mJ, r = 336 μm). The pressure in the deposition chamber was 3 × 10⁻² Torr and the deposition support was placed at a 2 cm distance from the target. The setup is schematically represented in Figure 1, and it includes the ICCD camera and the mass spectrometer.

The catalytic test was performed as follows: 0.5 mL of RB21 solution was poured on the silver thin film deposited on a glass slab. The same quantity of RB21 solution was poured on a glass slab as a control test. The dry resulting materials on the Ag-thin films are denoted as R-Ag, while the resulting materials from the solution leakage after the interaction with the thin film and after drying are denoted as Q-Ag. The control test consisting in a dried RB21 solution deposited on the glass slab is denoted as RB21-NaHCO₃.

2.3. Methods of Analysis

The morphology of the thin film surface was analyzed with scanning electron microscopy coupled with energy dispersive X-ray spectroscopy using the SEM-EDX Vega Tescan LMH II, Brno, Czech Republic. For the electron spectroscopy, an SE detector was used at a 30 kV filament supply and a working distance of 15.5 mm. A precise experiment was conducted using a Bruker detector X-Flash 6/30 with automatic mode detection for the EDX analysis.

The topography was studied using atomic force microscopy (AFM) with the Nanosurf Easy Scan 2 (Liestal, Switzerland) AFM contact mode cantilever n+—silicon with a resistivity of 0.01–0.02 Ωcm, thickness of 2 ± 1 μm, and force constant of 0.02–0.77 N/m.

The recorded images were resolved in space and time using a sequential imaging technique with variable exposure times (integration) and delays in the nanosecond range. The camera with CCD intensification (Roper Scientific ICCD 2ns Pi-Max 3-1024i camera) has an integration time of 2 ns and a resolution of 1024 × 1024 pixels. In the experiment, an integration time of 3 ns was chosen, while the images were recorded after the laser pulse, from 100 ns to 1200 ns; for each image, 10 acquisitions were made.

The elemental composition of the ablation plasma was studied with a HAL RC9 (2U) mass spectrometer equipped with a quadrupole mass spectrometer controller, designed by Hiden Analytical Ltd., UK, for the pulsed laser ablation direct monitoring of the resultant plasma plume. The types of analysis that can be performed using the electrostatic quadrupole plasma (EQP) analyzer are the mass and energy of ions generated in the ablation plasma using the ion extraction system, and neutrals and radicals using the twin-filament, fully adjustable electron-impact ion source for residual gas analysis (RGA). The detector used on the EQP probe is an ion counting secondary electron multiplier (SEM), which counts the number of ions striking it per second. The pressure in the EQP probe should be less than 10⁻⁶ mbar and the probe is pumped to operating pressure by its own turbomolecular pump.

A Fourier transform infrared spectroscopy analysis was performed with a Bomem MB154S FT-IR spectrometer at an instrumental resolution of 4 cm⁻¹ (Bomem, ABB group, Québec, QC, Canada) on the samples of the RB21 dried material collected from the surface of the thin film and of the solution that leaked from the thin film, as well as from the control test.

Profilometry was performed with a DektakXT stylus profilometer (Bruker, Bruker Nano Surfaces Division, 3400 East Britannia Drive, Suite 150, Tucson, AZ 85706), with a repeatability of 4 Å, on the Ag-thin film after washing the dried material that was obtained after treating the thin films with the RB21 solution, and the thinness and roughness of the thin films after treatment was compared with the thinness before treatment.

The crystalline structure of the Ag-thin film was analyzed with X-ray diffraction using a Shimadzu LabX XRD-6000 diffractometer with a Cu Kα radiation (λ = 1.54 Å). The diffraction patterns were recorded in the 10°–80° 2θ range with a 2 deg/min scanning speed.

A numerical simulation was conducted in COMSOL 5.6-01 software (COMSOL AB, Stockholm, Sweden).

3. Results and Discussion

Monitoring with the ICCD 2 ns Pi-Max 3 camera during pulsed laser deposition (Figure 2) allowed the calculation of the speed of the ablation plume, which was 6.5 × 10³ m/s. Based on the same measurements, a periodical plasma layered structure was highlighted at about 1 mm from the target, with a 447 ns delay, and with the distance between the two layers of particles being approximately the same, 0.06 mm. This phenomenon could be assigned to a perturbation materialized in the electric field of the diffusion current generated by the charged carriers in the plasma plume in motion and in the shockwave developed under the very high velocity plasma plume.

The number of species (atoms, ions, crystalline structures, etc.) developed in the ablation plasma during its movement, and the phenomena of recombination, crystal growth, and cooling, were also considered. In this sense, analyses were carried out with the mass spectrometer connected to the deposition installation. For the analysis of the ablation plume composition, a laser beam with an energy of 30 mJ of 532 nm was used in a pulsed regime of 10 ns and 100 Hz repetition frequency, this being at the same time the source of ionization of the component chemical species. The elements of silver, iron, nickel, and boron were identified in the results according to the spectral lines in the spectra presented in Figure 3. The spectral lines of Ni ions and Fe in the mass analysis (Figure 3a) are less visible due to the low number of ions of these two species in the ablation plasma. The energy analysis of the ions generated in the ablation plasma (Figure 3b–e) shows that, for similar energies, Fe, Ni, and B detected ions are 20 times less than Ag ions. The lines in the mass spectra at 63 and 65 amu, followed by less intense lines at 66 and 68 amu, may be assigned to copper, which could be the result of the laser beam’s penetration through the target support (made of copper), but they could also be lines of the cluster ions spectra.

Reaching an ionization level of silver atoms is located after a significant time interval, namely 100 µs, as it results from the 3D diagram obtained during the analysis with the mass spectrometer (Figure 4).

The composition of the ablation plume corresponds to the results of the analysis of the Ag target with Fe and Ni impurities, which was treated with boron-based compounds. The EDX analysis of the target was presented in our previous works on two analyzed areas of 64.56% silver, 27.01% nickel, and 8.43% iron, and of 77.20% silver, 17.56% nickel, and 5.20% iron, respectively. In addition, the mass spectrometry also highlighted the presence of boron, which was expected according to the treatments during the fabrication of the target.

Considering the data in Table 1, boron requires a higher ionization energy than the other components present in the Ag target with Ni and Fe impurities. Also, the melting and boiling temperatures are higher than those of the other components of the target. However, laser ablation and boron ionization were produced with considerable effectiveness (Figure 3) considering the impurity status of the target. One assumption concerns the efficiency of its interaction with laser radiation, given both its optical characteristics and thermal and heat transfer properties. In this case, boron could also influence the thermal effects on the other components, contributing to their intensification. Also, the thermal influence of the other components on the boron in the laser ablation process must be considered. In order to understand the phenomena that contributed to the ionization of the chemical species in the silver target with impurities that was used in the laser ablation process, a simulation was conducted in COMSOL 5.6. The mathematical model and its implementation in the heat transfer module were made according to [11,12,28], and the parameters in the simulation were the same as the laser parameters used during the mass spectrometer analysis.

The simulation results are presented as ionization energy threshold (IET) plots in Figure 5. According to Table 1 and the plots of Figure 5e,f, the ionization threshold energy is reached and exceeded even after a single laser pulse for the elements iron (maximum achieved of 77.9 eV compared to IET of 7.9024 eV), nickel (maximum achieved of 48.5 eV compared to IET of 7.98 eV), and boron (maximum achieved of 270 eV compared to IET of 8.298 eV). For silver, the simulation indicates that thermal energies of 0.62 eV (Figure 5d) developed under the action of one laser pulse, which is below the ionization energy threshold level of 7.5762 eV. Furthermore, the analysis of the simulation results was performed for a time of 2 μs starting from the laser pulse ignition, which is as much as the processing memory of the computer allowed in the context where the data acquisition is very large, i.e., where a large number of reading nodes given by extra fine mesh is required. From the extended diagrams for the duration of 2 μs from the ignition of the laser pulse, it appears that, in the presence of boron, each of the elements, including silver, iron, and nickel, respectively, changes its thermal behavior in the sense of extending the heating effect of the ablation plasma. In this sense, although the thermal energy decreases between two laser pulses, the residual thermal energy will improve the thermal effect of the next laser pulse. This phenomenon leads to the idea that the generation of silver ions under low pulsed laser energy is influenced by the presence of residual boron or boron compounds after being introduced during target processing.

The optical images of the layered structures obtained with the ICCD camera (Figure 2) could be in connection with the analysis of the ablation plume with mass spectrometry, which indicates a large number of neutral and charged chemical species (atoms and ions of silver, iron, nickel, boron, as well as different combinations of Ag-B, Fe-B, Ni-B, etc.).

The diffractogram obtained from the XRD analysis of the thin film deposited from the Ag target (Ag-thin film) shows its polycrystalline structure (Figure 6). The X-ray diffraction analysis on the Ag-thin film evidenced the crystalline specific sharp peaks and Miller indices as (111), (200), (220), and (311), as in the literature [28,29,30,31,32]. Since the deposition was performed on a glass substrate at an ambient temperature (without heating), this could indicate that crystalline structures were formed in the ablation plume during its cooling and deposited on the cold support by crystal growth. Silver crystalline states in different crystal growth stages could also contribute to the optical images of the ablation plume as layered structures in Figure 2.

In the images obtained from the SEM and AFM analysis (Figure 7), a morphology consisting of rings and droplets as well as their aggregates is noticeable. Formations with sizes ranging from 2.9 μm to 0.801 μm, and even smaller for rings, were measured, and in the case of droplets, the sizes range from 2.403 μm to 0.133 μm and even smaller. These images indicate the formation of nanoparticles, as well as micrometric structures, with some resulting from the aggregation of smaller particles, and others resulting from melt splashes.

This morphology of the thin metallic films indicates, as previously mentioned [12], the instabilities in the fluid phases during ablation and plasma plume motion from target to support, which consist in the following phenomena and processes that are schematically represented in Figure 8.

(a) One of the instabilities, a RTI type, occurs at the liquid–gas interface in the areas on the target with temperatures near the boiling point (Figure 8(a1)), when the gas phase starts to develop into the liquid phase in a bubbling phenomenon. The bubbles with a lower density than those in the melted phase will tend to develop in volume, being at the same time compressed by the liquid phase. Further, a pressure gradient will be generated, where p_gas > p_liquid while ρ_gas < ρ_liquid, fulfilling the condition (1): $\nabla \mathsf{\rho }·\nabla \mathrm{p} <0$ [26]. The pressure and density gradients will generate the perturbation responsible for the RTI and the gas bubbles will leave the liquid phase, also generating a splash of liquid (Figure 8(a4)). The liquid streams splashed under the PRI will break up into droplets for unstable modes (induced under the laser beam electromagnetic field and the electric field of diffusion current) when $\mathrm{k}·{\mathrm{R}}_0<1$, and where k is the wave number. The fastest growing modes occur for $\mathrm{k}·{\mathrm{R}}_0=0.697$, when the wavelength of the disturbance is ${\mathsf{\lambda }}_{\max }\cong 9.02 ·{\mathrm{R}}_0$ [33,34].

(b) The other situation, where an instability of the RTI kind is developed, is on the areas of the target where, on the irradiated surface, the temperature highly exceeds the boiling point, so that a plasma/gas layer develops on a melted layer (Figure 8(b1)). The plasma/gas will expend the volume in both directions, toward the support, but also toward the target, and, in the latter case, a deformation inside the melted material will be induced, continuing to “dig” as a spike-shaped hole filled with gas (Figure 8(b2)). The side melted material will be under a stress that will be released at the liquid–gas interface in the immediate vicinity, again generating (like situation “a”) a splash of liquid (Figure 8(b3)) that, crossing the plasma, will be under the perturbing effects of the electric field of the diffusion current induced by charged carriers, leading to the liquid streams breaking up into droplets (Figure 8(b4)), under the same conditions presented in paragraph (a).

(c) Figure 8(c1)–(c3) schematically represent the same type of RTI instability as in paragraph (b), which occurs in target areas where temperature values are close to boiling points only at the boundary between the liquid phase and the plasma/gas phase developed on the irradiated surface of the target. The pressure in the plasma/gas phase is increased by the thermal effect and phase change, with the plasma/gas phase tending to consume its volume and, with the pressure and density gradients under conditions from paragraph (a), develop an inward peak. The melt will be directed towards the neighboring molten zone; on the one hand, this generates flows and droplets in the gas/plasma zone (Figure 8(c1)) as a PRI-type instability, and, on the other hand, gas bubbles will form in the molten zone and they will continue to move into the liquid mass in the region of lowest pressure at the interface of the liquid phase with the ambient atmosphere, where, in the meantime, the ablation plasma “cloud” should have already expanded, lowering the pressure as well and providing the carrier charge to generate the diffusion electric field. The escape of the gas from the bubbles in the liquid mass will be accompanied by a kind of volcanic eruption, in a splash nozzle-less spray when the droplets are generated as a PRI under the electric field of the diffusion current disturbance. A fraction of the droplets generated under the conditions described in paragraphs (a), (b), and (c) will be re-deposited, while another fraction will be entrained in a nozzle with less of a spray effect in the ablation plasma plume. This contributes to the following phenomena and processes, including elastic collisions and a return on the target or deposition on the walls of the vacuum chamber, but also moving with the plasma plume to the deposition support and becoming seeds for a new nucleation process in the droplets in contact with the resulting liquid phase during plasma/gas condensation, before or in the support quarter. The complex phenomenon of droplet generation in the PLD process continues with the instabilities developed in coexisting fluids which are associated with the plasma plume and its movement from the target to the support; the effects of the instabilities already presented from the target in its immediate vicinity in the first nanoseconds after each pulsed laser ignition of the ablation are also acquired.

When the ablation plasma starts cooling while getting closer to the substrate, liquid streams are generated in a complex condensation process, from mixed gas and liquid state clouds to crystalline growth (Figure 8(c2,c3)). The droplets are grown on crystalline seeds that originate into the primary droplets developed under the RTI on the target during ablation in a nucleation process, in the first nanoseconds. The primary droplets are trained into plasma motion, cooling in time up to the solid state, as monocrystals.

(d) In the deposition chamber, the plasma travels from the target to the substrate on a path of 2 mm or 6 mm with a very high speed, of 6.3 × 10³ m/s. At such a value of speed, the plasma pushes the buffer air gases in its front and produces a pressure wave, which remains behind the plasma. The plasma plume is a fluid phase that will allow the pressure wave (shockwave) to propagate through its mass, leading to a harmonic constraint in the plasma (Figure 8(d1–d3)), in turn leading to RMI [35], followed by condensation into liquid streams and/or droplets (Figure 8(d4)). Also, the very high speed of the plasma will develop a very high temperature, based on the Mach’s number diagram with stagnation temperature [10]. The temperature achieved in this way is favorable to the existence of a plasma phase that will cool only when the plasma plume expands to such an extent that it arrives at a density value where the pressure wavelength cannot be produced anymore.

The droplets generated under the conditions described in the present paragraphs (a), (b), and (c) will be re-deposited and some will be entrained in the spray-less nozzle effect in the ablation plasma plume, contributing to the following phenomena and processes. These phenomena and processes include elastic collisions and a return on the target or deposition on the walls of the vacuum chamber. The breaking of the fluid thread is believed to explain the formation of the larger (main drops) and smaller (satellites) droplets observed in the SEM images in Figure 7a. This means that, when the flow of molten metal breaks in a main drop that will still be attached to the main stream for a period of time by a thinner mass of liquid, it will break up later, but into droplets of smaller sizes, called satellite drops.

The ring-shaped droplets from the SEM image (Figure 7a) are the result of a crown splash. The mechanism consists, on the one side, in a “kinetic crown splash” based on the high kinetic energy of the droplets at the impact with the substrate, and on the other side, in the existence of liquid state on the support from the previous layered particles and droplets; in the latter case, the crown splash is the effect of an instability of the RTI kind, or a “RTI crown splash”. The “kinetic crown” splash generates rings that are arranged on the surface, and the “RTI crown splash” generates craters in the first layer, which is still in a melted state. The heated substrate could enhance the crown splash effect because the film is kept in a melted phase on the support during the deposition.

The Ag-thin film morphology and deeply embossed topography observed from the SEM and 3D, 2D, and 1D images obtained during AFM analysis indicates a suitable surface for catalytic processes to take place.

In the SEM images on the silver thin film, the uniform dispersion of the RB21 solution can be noticed (Figure 9a). In Figure 9a, the shape of the droplets can be distinguished, just like in the SEM image of the thin silver film in Figure 7a, but the image is darker and matte because of the organic material that covers the thin silver layer. No differences in brightness are observed on the studied surface compared to the SEM image of the thin silver layer in Figure 7a, where the droplets are brighter than the background. A higher magnitude of the image evidenced large crystalline structures (Figure 9b). The crystal structure in Figure 9b belongs to the components resulting from the reaction between RB21 and NaHCO3 in the aqueous solution poured on the silver thin film.

In the FTIR spectra of Figure 10, the high decomposition of the RB21 dyestuff (chemical structure shown in Figure 11) in the RB21 solution is observed on the Q-Ag and R-Ag samples. The changes of the functional groups are identified in the FTIR spectra (Figure 10) using the previous literature [36,37,38,39] and compared to the initial structure of the RB21. The vibration band assignments are presented in

Table 2. Vibration bands of the functional groups.

RB21	RB21-NaCO3	R-Ag (Dried RB21-NaCO3 aq after Interaction with Ag-Thin Film)	Q-Ag (Dried Leakage of RB21-NaCO3)	Comments [36,37,38,39]
3861; 3810 v.w.	3857 v.w.	3813 v.w.		O-H free str.
3727 v.w.	3734 v.w.	3746 v.w.	3735 v.w.	O-H free str.
		3646 v.w.		O-H free str.
3439 s.	3449 s.	-	3483 v.w	O-H and N-H free and H-bonded str.
3232 sh.				N-H
	3014 v.w.	3075–2955 m.		C-H in aromatic/alkenes
2930 m.	2919 m.		2993 m.	C-H alifatic
2855 m.	2849 m.	2882 m.	2883 m.	C-H alifatic
2346 w.		2316 w.		O=C=O carbon dioxide
	1875 w.			C=O stretching
1737 w.	1748 m.	1720 m.	1720 m.	C=O stretching in carbonate CO32−
1630 s.	1636 s.			C-H aromatic; N-H bending; SO3
1594 s.	-	-	-	Ring skeleton in pyrrole group; N-H bending in sulfonamide group
1508 s.	-	-	-	N-H bending in sulfonamide group
1493 s.	-	-	-	C in heterocycles
1463 s.	1455 w.	1462 s.	1462 s.	C-C in heterocycles; C=O in (COO)−; C-H aliphatic bending; CO3−2 lattice vibrations
1301 s.	1353 w.	1390 m.	1390; 1397 m.	S=O stretching asymmetric in SO3 of sulfones in chromophore and sulfonamides in chromogen
1227 v.s.	1245 v.w.	1206 sh.	-	C-N stretching in amines; C-O stretching in alcohols; S=O stretching symmetric in SO3 in chromophore
1174 v.s.	1125 m.	1133 w.	1143 sh.	SO2 in chromogen
1021 v.s.	-	-	-	C-N bending
973 v.s.	992 v.w.	908 m., wide	-	C=C bending
894 s.	-	861 m.	-	C=C bending; S-O stretching
830 m.	-	-	841 m. sharp	N-H bending in sulfonamides; C=C bending; S-O stretching
729 s.	-	-	-	N-H bending; S-O stretching
681 m.	698 m.	694 w.	-	S-O stretching
612 m	601 m.	636 m. sharp	605 m. (multiple peaks	S-O stretching
-	554 m.	537 m. sharp	-	S-O stretching

.

Decomposition starts right after the RB21 solution interacts with the silver thin film, which is proved by the changes in the Q-Ag and R-Ag spectra. Carbonate (${\mathrm{CO}}_3^{-2}$) groups are evidenced by the 1748 cm⁻¹ and 1462 cm⁻¹ bands; the latter also indicates the lattice modes. The crystalline structure of the resulting Q-Ag compound, denoted by the ascending spectrum baseline, is also assigned to the carbonates as Mie scattering effects. Sulfonic (SO₃) groups are still intact (1390 cm⁻¹), while the aromatic rings are still present in R-Ag (3075 cm⁻¹), which is the same in the control test RB21-NaHCO₃ (3014 cm⁻¹). Aliphatic C-H is denoted in all samples by the bands at 2930 cm⁻¹, 2855 cm⁻¹ (RB21); 2919 cm⁻¹, 2849 cm⁻¹ (RB21-NaHCO₃); and 2955 cm⁻¹, 2882 cm⁻¹ (R-Ag and Q-Ag). The heteroaromatic rings with nitrogen and N-H in the sulfonamide groups were largely affected, which was noted by the severe transmission attenuation of the 3438 cm⁻¹ (initial dye RB21) and 3449 cm⁻¹ (RB21-NaHCO₃) vibration bands, becoming very weak at the 3483 cm⁻¹ peak of the Q-Ag dried leakage sample and missing in the R-Ag dried sample. Also, the skeleton vibrations at 1590 cm⁻¹, which were assigned to the pyrrole heteroatomic rings, are missing in all treated samples and noticed only in the initial RB21 dye. The same band is assigned to N-H bending in the sulfonamide group and is missing in RB21-NHCO₃, Q-Ag, and R-Ag, confirming the modifications. Since the vibration bands at 3438 cm⁻¹ (initial dye RB21), 3449 cm⁻¹ (RB21-NaHCO₃) and 3483 cm⁻¹ (Q-Ag), and the missing bands in the R-Ag sample also correspond to the OH functional groups, it turns out that they were also modified compared to the initial molecule of the RB21 dye.

Based on the chemical structure of the RB21 dye (Figure 11), the changes in the FTIR spectra of the studied samples (R-Ag and Q-Ag) in comparison with the reference samples (RB21 and RB21-NaHCO₃) indicate the following processes and reactions under the silver thin film catalytic effect:

1. A sulfonyl group reaction with sodium bicarbonate is indicated, with an increased rate under the catalytic effect of the Ag-thin film:

D-SO₃H + Na⁺H⁺CO₃²⁻--> D-SO_2--O⁻Na⁺ + CO₂ + H₂O.

(1)

The increased reaction rate is observed by the decrease in the intensity of the hydroxyl groups when comparing the FTIR spectra of the aqueous RB21-NaHCO₃ solution, dried on a glass plate with the R-Ag (aqueous RB21-NaHCO₃ solution dried on Ag-thin film) and Q-Ag (leakage of aqueous RB21-NaHCO₃ over thin-filmed Ag and dried on glass) samples. In the case of the R-Ag sample, the vibration of the hydroxyl groups is missing from the spectrum. This proves that they have been completely transformed.

2. The deprotonation reaction of the sulfonamide group (SO₂-NH) is indicated due to the “nucleophilic attack” of CO₂ by amino group, which requires a catalyst (usually Brønsted catalyst) [40].

D-SO₂-NH-Benz.-SO₂-CH₂-CH₂-OH + CO₂ --> R-SO₂-NCOOH-Benz.-SO₂-CH₂-CH₂-OH.

(2)

3. Dehydration at the level of the terminal hydroxyl group in the side chain led to the formation of the vinyl group. Usually, the reaction of sodium bicarbonate with alcoholic hydroxyl groups does not occur, because the first is a weak acid in this case (amphoteric character), while the second is a weak base. The lack of specific vibrations for the hydroxyl groups in the case of the R-Ag sample and their reduction in the case of the Q-Ag sample shows the damage of all OH groups in the dye molecule, resulting in the Ag-thin film catalyzing the chemical process.

R-SO₂-NCOOH-Benz.-SO₂-CH₂-CH₂-OH --> R-SO₂-NCOOH-Benz.-SO₂-CH = CH₂ + H₂O.

(3)

4. Also, the FTIR spectra indicate changes in the aromatic and heteroaromatic structures when the RB21 and NaHCO₃ dye solution comes into contact with the thin silver film.

The strong roughness of the silver layer due to ring-shaped particles is evidenced with profilometry measurements and the profiles are presented in Figure 12a,b; the reduced consistency of the thin films after the catalytic interaction is presented in Figure 12c. The density of the peaks is lower after the RB21 solution was added and there are cavities between peaks, proving that the RB21 solution had an effect of “etching” on the thin layer. The effect shows that some of the silver thin film material was consumed. This etching effect could be caused by the entrainment of submicrometer silver particles in the resulting leakage by pouring the aqueous solution of RB21 and NaHCO₃ and/or the binding of silver particles to modified and ionized functional groups such as sulfonate and carboxylate.

In the optical microscope image in Figure 13a, a compact structure is observed, while in the image of Figure 13b, darker areas are noticed, probably due to clogging with reaction products during the interaction with the RB21 solution. The size of the observed area is 1.4 × 1.1 mm and can be measured on the 0.1 × 0.1 mm grid attached with the image of the thin film.

4. Conclusions

The results of the study carried out in this work demonstrate the catalytic action of silver thin films with high roughness in the catalytic decomposition processes of some organic compounds with complex molecules. The fluid instabilities (PRI = Plateau–Rayleigh instability; RTI = Rayleigh–Taylor instability; RMI = Richtmyer–Meshkov instability) induced by the use of high energy laser radiation led to obtaining thin films with high roughness. Also, the study provides a fundamental approach to the phenomena and processes that led to obtaining the crystalline structure of the silver thin film produced by the PLD technique on a substrate at an ambient temperature (without heating the substrate). The mass spectrometry analysis using the pulsed laser with an energy of 30 mJ/pulse as the ionization source highlighted the role of boron in reaching the silver ionization energy threshold, which was also confirmed by the simulation results in COMSOL.

Further studies that we propose to carry out are aimed at improving the quality of the morphology of the thin films of this type to increase and better control their catalytic effects.