Reactive Blue 21 Dye Degradation and Surface Modification of Cu and Ag/Cu Thin Films Prepared by Pulsed Laser Deposition

1. Introduction

With this paper we present the results of the study on the interactions on the surface of the copper thin films and silver/copper double thin films produced by pulsed laser deposition (PLD) with an aqueous solution containing sodium bicarbonate and Reactive Blue 21. The aim of the study is to test copper thin film and silver/copper double thin films catalytic ability and if the method is suitable for etching the copper thin films for further applications.

Copper thin films are a versatile and essential material in various industries, particularly in electronics and optoelectronics. They are known for their high purity, stability, and processing versatility, making them ideal for applications such as: electrical conductivity: Copper thin films offer precise control over electrical conductivity, which is crucial for high-performance electronics and data storage applications. These films are engineered for smooth surfaces and uniform layer thickness, which is vital for the production of stable and adherent layers. Copper thin films can be used to achieve transparency or spectral selectivity, making them suitable for applications like heat reflectors and window coatings [1]. Techniques such as electrochemical deposition (ECD) allow for the fabrication of smooth surface copper films, which are used in printed circuit boards and touchscreen technology [1,2].

Copper thin films produced on dielectric substrates by pulsed laser deposition showed different morphologies and optical absorption behaviors [3]. Based on the quality of the copper pulsed laser deposition, the method was investigated to produce copper-based photocathodes [4]. The conductive ink based on copper nanoparticles or copper complexes offers a cheap alternative with high conductivity for flexible electronics, with synthesis and sintering methods adapted to prevent oxidation and allow processing at low temperatures. Methods have been designed to produce copper ink, such as thermal sintering at low temperatures compatible with plastic or paper substrates, photonic or laser sintering, which selectively heats the copper nanoparticles without damaging the substrate, or chemical sintering, using reducing agents to facilitate particle coalescence at lower temperatures [5,6,7].

The use of copper thin films is not only limited to electronics but also extends to research and development, where they facilitate studies into novel interconnect materials and next-generation storage media. Their high purity and stability make them a cornerstone material for both industrial manufacturing and scientific advancement. Copper conductive ink, based on copper nanoparticles or copper complexes, offers a cheap alternative with high conductivity for flexible electronics, with synthesis and sintering methods adapted to prevent oxidation and allow processing at low temperatures. Methods have been designed to produce copper ink, such as thermal sintering at low temperatures compatible with plastic or paper substrates, photonic or laser sintering, which selectively heats the copper nanoparticles without damaging the substrate, or chemical sintering, using reducing agents to facilitate particle coalescence at lower temperatures [5,6,7]. Copper-based inks are gaining popularity in the electronics industry due to their high conductivity, low cost, and suitability for large-scale manufacturing. Recent advancements have focused on surface and interface designs to enhance the stability and conductivity of copper-based inks, making them suitable for a range of electronic applications, including transparent conductive electrodes, sensors, optoelectronic devices, and thin-film transistors [6].

The thin films obtained by the PLD method are actually composed of clusters of nano- and microparticles deposited successively. This structure gives them special properties, including a high surface-to-volume ratio. In this regard, copper thin films exhibit catalytic properties, antibacterial, antioxidant, and antifungal activities, as well as cytotoxicity and anticancer properties [8]. The use of PLD technology is very suitable for the production of complex and easily manageable oxides. For the development of copper-based electrocatalysts for biosensor applications, multiphase copper oxide thin films were fabricated on a Si (100) substrate using the PLD technique. The Cu-0 electrode showed a good sensitivity for glucose detection and the highest selectivity for glucose detection among various organic, inorganic, and biological molecules [9]. Also through pulsed laser deposition (PLD), a nanocomposite thin film of silver/copper oxide was fabricated and used in the reduction of nitrophenol, nitrophenols being known as the main pollutants in industrial and agricultural waste due to their water solubility. The Ag/CuO films were deposited on a quartz substrate. Nd:YAG was used as the laser source in the PLD technique with a wavelength of 532 nm to obtain a nanocomposite with high-performance catalytic activity [10]. Also through the PLD technique, thin films were fabricated by ablating a high-purity copper target, mounted on platinum in ultra-high vacuum (UHV). The samples demonstrated excellent photocatalytic degradation efficiency of dyes in wastewater [11].

Theoretical studies on copper complexes have focused on their molecular structure, properties, and potential applications in solar cells [12].

The interaction between copper and bicarbonate can lead to chemical reactions as a result of the process of copper dissolution, which can cause the material to be perforated, depending on the concentration and conditions of the solution [13,14]. These interactions have been studied over time, both to prevent damage to copper pipes [14] and for other types of processes, as well as for using the reaction in a beneficial way. Thus, the process of anodizing copper in sodium bicarbonate solutions was explored to form nanowires composed of crystalline Cu₂O, CuO, Cu(OH)₂, and malachite Cu₂CO₃(OH)₂ [14,15]. This process provides a method for nanostructuring copper oxides to be used in various applications, including CO₂ reduction reactions, photoelectrochemical water splitting, and improving the performance of direct methanol fuel cells [16].

When dissolved in a sodium bicarbonate solution, copper goes into its ionic state in which it can react with sodium bicarbonate, but in small amounts. The compound formed is copper (II) bicarbonate with the chemical formula Cu(HCO₃)₂, unstable, especially in the presence of heat or ultraviolet light. It decomposes to form carbon dioxide and Copper (II) Carbonate. Although copper (II) bicarbonate does not have special uses, the reaction itself is of interest for research due to its effects. In the study we present in this paper, the purpose of using NaHCO₃ is to test a new method for etching the copper thin films obtained by the pulsed laser deposition (PLD) method. Other properties will be also explored based on the results. The Reactive Blue 21 dye, a copper coordination compound, was used in the bicarbonate solution for a better evaluation of the results, based on a similar study previously conducted which showed the catalytic effect of the thin silver film in the reaction between the RB21 dye and bicarbonate [17].

2. Materials and Methods

2.1. Method of Work

The pulsed laser deposition technique (PLD) was used to produce the copper thin film and the silver/copper double thin film.

The PLD process was performed in the same conditions as per Cocean et al. 2024 [17] with the laser system part of the installation presented in Figure 1. The laser system is a Quantel laser system, YG 981E/IR-10 model which is a Q-switched Nd:YAG laser produced by producer Quantel, Les Ulis, France. The laser beam of λ=532 nm wavelength with spot dimension of d=336 μm, the pulsed width τ=10 ns with repetition rate ν=10 Hz, laser energy per pulse E = 180 mJ/pulse was set to hit the target at an angle α=45⁰. The pressure in the deposition chamber was brought to p=3·10^-2 Torr with the help of the vacuum pump. The distance d=2 cm was set between the target and the glass slab used as deposition support. During the deposition of t=30 min for each of the samples, the targets were moved along a spiral trajectory in accordance with their round shape to avoid piercing them and also so that the material resulting from ablation would not be affected or to reduce the effects produced by the redeposits resulting from the previous ablation process. The movement of the target is computer aided.

2.2. Materials:

The Cu thin film and the Ag/Cu double thin film prepared by PLD method were obtained from a copper target and the silver target with Ni and Fe impurities [17].

An aqueous solution of the CI RB21 dye (a copper coordination compound) with a concentration of 10 g/l was prepared. Separately, a saturated solution of sodium bicarbonate with a concentration of 10 g/l in warm water (40°C) was prepared, after which it was allowed to cool to room temperature (24°C). Equal amounts of 100 ml from the two solutions were mixed. Then, from the resulting RB21-NaHCO₃ solution, 0.5 ml were dripped onto each of Cu and Ag/Cu thin films and on a glass slab (same as the one used as deposition support). The resulted samples were noted as R-Cu and R-Ag/Cu and the resulted materials from solution leakage after interaction with thin film were noted as Q-Cu, Q-Ag/Cu.

The resulted aqueous solution of Reactive blue 21 (10 g/L) and NaHCO₃ (10 g/L) dripped on the glass slab, namely RB21-NaHCO₃ and the RB21 dye are the control samples.

The aqueous solutions of RB21 and NaHCO3 were prepared with deionized and distilled water (18.2 MΩcm) obtained through a purification and deionization system produced by Barnstead EasyPureII Milli-Q, Thermo Fisher Scientific, Waltham, USA.

2.3. Methods of Analysis

The interaction of the targets with the laser beam was evaluated by numerical simulation in COMSOL 5.6 - 01 software version (COMSOL AB, Stockholm, Sweden).

The morphology and the elemental composition of the surface of the thin films were analyzed with Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-Ray (EDS) performed with Vega Tescan LMH II, Brno, Czech. For the electron spectroscopy a SE detector was used at 30 kV filament supply and a working distance of 15.5 mm.

Fourier Transform Infrared Spectroscopy (FTIR) analysis of the functional groups was performed on the samples of dried material collected from each surface of the Cu and Ag/Cu thin films covered with the RB21-NaHCO₃ solution (R-Cu and R-Ag/Cu) and from the surface of the deposition support (glass slab) next to the thin film where the solution leaked from the thin film (Q-Cu and Q-Ag/Cu). The spectra of the studied samples were compared with the control samples RB21-NaHCO₃ and the RB21 dye. The Bomem MB154S FT-IR Spectrometer at an instrumental resolution of 4cm^-1 (Bomem, ABB Group, Saint-Laurent Canada) was used. Each sample was then incorporated into about 280 mg of potassium bromide (KBr) used as suspension media and it was pressed into a stainless-steel ring of 12mm inner diameter using a pneumatic pressing device at 100 atm pressure. The ring with the pellet was placed in the sample holder of the spectrometer.

Profilometry with DektakXT Stylus Profilometer: (Bruker, Bruker Nano Surfaces Division, 3400 East Britannia Drive, Suite 150, Tucson, AZ 85706) was carried out on the thin films after washing them with distilled water to remove the material resulted after treatment with the RB21-NaHCO₃ solution. The roughness (profile) of the thin films after treatment were compared with the roughness (profile) before treatment.

The optical microscopy (OM) was performed using Namicon ISMM1000 trinocular inverted metallographic microscope (INSIZE, Suzhou, China), equipped with a MotiCam camera (10+, 10.0 MP), specialized in microscopic analysis, and Motic Images Plus 3.0 (×86) software, version 3.0.12.41 (Motic, China-Group Co., Ltd. 2015, Hong Kong, China).

The UV-Vis absorption spectra of the analyzed solutions were recorded in the range of 200–800 nm using a Shimadzu UV-1800 spectrophotometer (Kyoto, Japan). The sample was introduced into a quartz cuvette with a volume of 3 mm and at a scan rate of [800 nm/min] with a slit width of [1 nm].

3. Results and Discussions

3.1. Numerical Simulation in COMSOL

For a better explanation of the deposition phenomena, it is also necessary to understand the thermal phenomena developed on the target surface in the spot of incidence of the laser beam in order to evaluate the magnitude of ablation as well as the fluid phase of the resulting material. Since the temperature developed during the interaction of the laser beam with the material cannot be measured due to the very short action time, it was estimated through the results of numerical simulation in the COMSOL software. The theoretical and mathematical model is according to Cocean et al. 2017 [18] and Cocean et al. 2024 [17]. Figure 2 shows the results of the numerical simulation in COMSOL that reflect the thermal effects in space and time produced on the Cu target and on the Ag target in space (Figure 2a) and in time (Figure 2b). The numerical simulation takes into account the laser parameters according to those used for experimental deposition and the material constants and parameters of the two materials used as targets (Cu and Ag). The optical parameters of Cu and silver materials were taken from the database [19,20], namely the refractive index n(Cu) = 1.1159 and n(Ag) = 0.054007, while the extinction coefficients k(Cu) = 2.5956 and k(Ag) = 3.4290, values to which correspond the absorption coefficients α(Cu) = x 10⁵ cm^-1 and α(Ag) = 8.0996 x 10⁵ cm^-1. The absorption coefficients are important both for calculating the heat source and the thermal effect resulting from the interaction of the laser beam with the target material, as well as for being able to optimize the meshing for finite element calculation so that no values of interest are lost, but also so that the computer's working memory is not overloaded. The other material parameters were taken from the COMSOL software library as functions of temperature, extrapolating with the closest function for temperature values higher than those provided in the functions supplied by COMSOL.

As a reference for evaluating the thermal effect, the melting points of the two materials were taken into account (M.P. Cu = 1357.77 K; M.P. Ag = 1234.95 K). The melting point is considered in this numerical study as the temperature above which the fluid phase results in the laser ablation process and which is important for the deposition, also influencing the amount of material deposited. For comparing the results obtained for the two materials, the dimensions on which the conditions for the formation of the liquid phase are met on each target were also measured at two time values (t=10 ns and t=20 ns) and which are indicated on the graph by Δx. Also, the results of the thermal effect were compared through the time during which the conditions for the existence of the fluid phase were met (temperature greater than or equal to the melting point), this parameter being indicated on the graph with Δt.

For the Cu target, the simulation results show a maximum of 9394 K at time t=11 ns and values of 9230 K during the pulse (t=10 ns) and 1458 K at the end of the studied interval (t=20 ns). The time interval during which the liquid phase occurs was calculated to be 17.5 ns, and the spot size where the solid phase threshold is reached ranges between Dx=9.9 mm at time t=10 ns and Dx=1.2 mm at time t=20 ns. Regarding the silver target, a maximum temperature resulting from the interaction with the laser radiation of 1811 K is observed at time t=11 ns, and a pulse temperature (t=10 ns) of 1797 K, which ensures the production of the liquid phase. At the end of the studied interval (t=20 ns), the temperature at the center of the laser-irradiated spot drops well below the melting point of silver (T=515 K). From the studied time interval, the time during which the liquid phase occurs is Δt=7 ns. The area of the spot affected by the laser irradiation that reaches temperatures greater than or equal to the melting point is at time t=10 ns equal to Δx=3.2 mm.

These values show an estimated ablation yield relative to the fluid phase much higher for copper compared to silver. Much higher temperatures resulting on the surface of the Cu target during laser irradiation lead to the production of a fluid with lower viscosity, which should be reflected in a deposit with lower amplitude roughness, and the obtained thin film should be more consistent, with a greater thickness than in the case of silver.

3.2. Scanning Electron Microscopy and Energy Dispersive X-Ray (SEM-EDS)

In the SEM images from Figure 3 a and b, the Cu thin film is smoother and more uniform in its morphology compared to the Ag/Cu double film. For the latter, the aspect of the surface is given by the top layer which is silver and it is similar to the thin film obtained in a previous study reported by Cocean et al., 2024 [17]. The Cu thin film [Figure 3a] presents round-shaped particles, similar to pearls of maximum 3 μm diameter which are in small number, and the large majority of the particles with their diameter less than 1 μm, meaning hundreds of nanometers as also Lorusso et al., 2023 reported for the thin films they obtained by PLD even though they used an UV laser beam and different parameters and conditions [4]. The silver thin film on top of the copper thin film in the Ag/Cu sample (Figure 3b), shows numerous structures in the range of 10-20 μm. These structures were explained as being the result of the fluid instabilities [17] and it can be seen that they are ring-shaped (crown-splash), pearl-shaped and aggregated structures. The most of the structures are around 5 μm diameter and less. The difference between the surface morphology of the thin Cu film and that of Ag obtained by laser ablation and deposition lies in the efficiency of producing the fluid phase as well as in the quality of the fluid phase (lower viscosity) of copper compared to silver, as shown by the COMSOL simulation results presented in Section 3.1. The Cu-RB21 sample presents crystalline structures on all the analyzed areas (Figure 2c,d) reflecting a good dispersion of the RB21-NaHCO₃ aqueous solution that could be influenced by the copper when compared with the lower dispersion that was obtained on the silver thin film reported by Cocean et al., 2024 [14]. The good spreading of RB21-NaHCO₃ aqueous solution can be explained by the copper dissolution due to NaHCO₃ resulting copper ions. The copper ions enter into reaction with the bicarbonate, replacing the sodium ions. However, the amount of copper that reacts is small due to the high reducing potential of copper compared to that of sodium [13]. Also, the morphology of the thin Cu layer shows fewer droplet and crown splash type structures and they are also smaller in size compared to those in the morphology of the thin Ag layer and the double Ag/Cu layer. The RB21-NaHCO₃ on the double thin film Ag/Cu (Figure 3e,f) appears to be less dispersed, similar to what was observed for the silver thin film [17].

The EDS spectrum of Cu thin film Figure 4a) and the elemental composition (Figure 4c) show a strong adsorption of oxygen on the thin film, the oxygen being in proportion of 95.5% weight on the thin film surface. For the double film Ag/Cu where silver is the top thin film, based on its EDS spectrum (Figure 4b) and its elemental composition (Figure 4d), the adsorbed oxygen is reduced to only 52% weight. That makes copper a better oxygen sorbent, but also more sensitive to oxidation. This property may play an important role if the sorption can be reversed by controlled mechanism.

3.3. FT-IR Spectroscopy

In the spectra presented in Figure 5 with the bands of vibration listed in Table 1, the bands assigned to O-H free stretching are noticed at 3861 cm^-1, 3810 cm^-1, 3727 cm^-1 (RB21), 3857 cm^-1, 3734 cm^-1 (RB21-NaHCO₃), 3850 cm^-1, 3760 cm^-1, 3647 cm^-1 (R-Cu), 3803 cm^-1, 3740 cm^-1 (Q-Cu), 3857 cm^-1, 3734 cm^-1 (R-Ag/Cu) and 3857 cm^-1, 3743 cm^-1, 3647 cm^-1 (Q-Ag/Cu). The O-H and N-H free and bonded strong stretching vibrations are observed at 3449 cm^-1 (RB21) and 3439 cm^-1 (RB21-NaHCO₃) in the control samples, while bands reduced in intensity are noticed at 3504 cm^-1 -3426 cm^-1 and 3510 cm^-1 for the Q-Cu and Q-Ag/Cu, respectively. They are missing for the R-Cu and R-Ag/Cu samples. These changes in the spectra indicate severe modification in both the chromophore and the chromogen structures. The vibrations assigned to N-H bending and to the pyrrole heteroatomic rings skeleton at 1594 cm^-1 and 1508 cm^-1 in RB21 are missing in RB21-NaHCO₃, R-Cu and Q-Cu, but there are bands at 1552 cm^-1 and 1526 cm^-1 for R-Ag/Cu and Q-Ag/Cu, respectively, which can be assigned to nitro groups (N-O) stretching.

The aromatic rings evidenced by the 3072 cm^-1 C-H aromatic stretching for RB21 and 3014 cm^-1 for RB21-NaHCO₃ are no longer observed to R-Cu, Q-Cu, R-Ag/Cu and Q-Ag/Cu. They may not be visible because they overlap with the broadband in that area, which extends to the 2900 cm^-1 -2887 cm^-1 aliphatic region in all spectra. This hypothesis is supported by the presence of vibrations at 1652 cm^-1, 1641 cm^-1, and 1625 cm^-1 in the Q-Cu, R-Ag/Cu, and Q-Ag/Cu spectra, respectively, these vibrations being attributed to C-H aromatic bending vibrations. In the R-Cu spectrum, the band corresponding to the C-H bending is missing.

The aliphatic C-H stretching denoted by the medium intensity doublet bands at 2930 cm^-1, 2855 cm^-1 in the RB21 spectrum and 2919 cm^-1, 2849 cm^-1 in the RB21-NaHCO₃ (Figure 5a), become very weak and single in the tested samples: 2944 cm^-1 (R-Cu), 2882 cm^-1 (Q-Cu) and 2887 cm^-1 (R-Ag/Cu and Q-Ag/Cu). In the fingerprint area, the corresponding bands for CH₃ asymmetric and CH₂ bending are indicated by the bands at 1493 cm^-1, 1463 cm^-1 in the RB21 spectrum, 1455 cm^-1 in the RB21-NaHCO₃; 1458 cm^-1 (R-Cu), 1422 cm^-1 (R-Ag/Cu) and 1494 cm^-1 (Q-Ag/Cu) and for CH₃ symmetric bending, they overlap with the sulfonic and sulfonamides groups for CH₃ symmetric and CH₂ bending are at 1301 cm^-1 (RB21), 1353 cm^-1 (RB21-NaHCO₃), 1390 cm^-1 (R-Cu) and 1396 cm^-1 (Q-Cu).

The Carbonate (CO₃^2-) groups are evidenced in the RB21-NaHCO₃ spectrum (Figure 5a) by the 1748 cm⁻¹ and 1462 cm⁻¹ bands; the latter also indicating the lattice modes [17]. The crystalline structure of the NaHCO₃ is denoted by the ascending spectrum baseline, which is also assigned to the carbonates as Mie scattering effect which is also observed for the R-Cu, Q-Cu, R-Ag/Cu and Q-Ag/Cu baselines of the spectra (Figure 5b,c). In the R-Cu, Q-Cu and R-Ag/Cu spectra, the C=O group in carbonates is denoted by the bands at 1861 cm^-1, 1761 cm^-1, 1730 cm^-1 and 1720 cm^-1, while in the Q-Ag/Cu spectrum, the band is missing.

The sulfonic groups (SO₃) in the chromogen and the sulfone groups (SO₂) in the sulfonamides of the chromogen are denoted by the 1301 cm^-1 in the RB21 spectrum and 1353 cm^-1 in the RB21-NaHCO₃ spectrum. In the R-Cu and Q-Cu spectra, the shifts of the bands at 1390 cm^-1 and 1396 cm^-1 (higher wavenumbers) can be assigned to transitions to sulfate groups (SO₄^2-). In the R-Ag/Cu and Q-Ag/Cu spectra, the vibrations in the range of sulfonic and sulfone groups are missing and no vibrations in the range of groups with sulfur and oxygen are observed.

The changes in the RB21 dye IR spectrum (Figure 5a) in its interaction with the NaHCO₃ denoted by the spectral bands for the RB21-NaHCO₃ are found as being highly enhanced when the aqueous solution of the two compounds enter in contact with the thin films of copper (R-Cu and Q-Cu samples) and with the Ag/Cu double layered thin films (R-Ag/Cu and Q-Ag/Cu). Both thin films induce chemical transformations of high aggressiveness on the RB21 dye, with decomposition results similar to each other and to those observed in the presence of the silver thin film presented earlier [17]. The catalytic effect of thin copper films and the double-layered Ag/Cu films is proven the same as it was for the thin silver film [17].

3.4. Profilometry

The topography of the surfaces of thin copper films and double silver/copper thin films produced by the PDL method was studied through the analysis of peak and valley profile measurements and the statistical calculation of characteristic parameters.The measurements were carried out under the following conditions:

Scan Duration:	30 s
Scan Length:	3000 µm
Scan Resolution:	0.333259 µm
Scan Type:	Standard Scan
Stylus Force:	10 mg
Stylus Scan Range:	65.5 µm
Stylus Type:	Radius: 12.5 µm

The thickness of the thin films was evaluated by measuring the profile starting from their edge, being able to observe the step denoting the change in the height of the baseline of the profile. Thus, the copper thin film on area denoted as Cu(1) shows a first step of 75.55 nm in height, and after 1 cm in length in a horizontal direction, the base line rises up to 312.58 nm height along an upward slanting direction (Figure 6a). This profile indicates that the deposition of the copper on the substrate is more concentrated to the middle of the substrate used for thin film deposition. On a different area, Cu(2), the copper thin film presents the same pattern of the baseline, only less difference between step1 (59.67 nm) and step2 (97.86 nm). Also, when compared the two areas on the copper thin film, Ra=150.78 nm which is the peaks’ arithmetical mean height of Cu(1) area is higher than Ra=60.48 nm of the Cu(2) area, despite the Rp=666.56 nm value of maximum peak height of Cu(1) area being less than Rp=806.30 nm of Cu(2) area (Figure 6a,c). Regarding Ag/Cu thin film, the profiles of the two analyzed areas show a step of 226.93 nm corresponding to the edge of the thin film, following a horizontal direction of the baseline. The highest peaks of the surface profile of the double thin film Ag/Cu have the values Rp = 6899.45 nm for Ag/Cu(1) and Rp = 5218.49 nm for the area Ag/Cu(2). The highest peaks of the surface profile of the double thin film Ag/Cu have values of Rp = 6899.45 nm for Ag/Cu(1) and Rp = 5218.49 nm for the area Ag/Cu(2), which are approximately 10 times higher than those observed on the surface of the copper thin film. Also, the values of the arithmetic mean on the two areas on the surface of the thin Ag/Cu film are approximately 10 times higher, namely 986.75 nm on Ag/Cu (1) and 817.49 nm Ag/Cu (2).

To better understand the aspect of the roughness of the studied surfaces, peaks from the profile obtained through the measurements performed were analyzed. This way, proportions between the width and height (Width/Height ratio) of the peaks with values of 28.19 for the peak A on the Cu(1), 19.42 for the peak B on the Cu(2), 54.21 for the peak C on the Cu(2), 4.38 for the peak D of the Ag/Cu (1) and 5.37 for the peak D of the Ag/Cu (2) (Figure 6b,d,e,g,i). The calculated Width/Height ratios are the smallest, because the selected peaks are the maximum peaks on the analyzed surface area. The information obtained consists of the fact that, in general, droplets have Width/Height ratios much smaller than these. The Width/Height ratio shows that the width of the droplets on the surface of the thin copper film is much greater than their height. Comparatively, the Ag/Cu thin films show droplets with Width/Height ratios 4 to 10 times smaller.

The parameters of the surface roughness of the measured areas on the two samples of thin films are presented in Table 2.

The roughness of all the analyzed areas of the samples – Cu(1), Cu(2), Ag/Cu(1) and Ag/Cu(2) – is characterized by a positive skewness (Rsk>0). In the right-skewed (positively skewed) distribution most data points cluster on the left, with a long tail to the right. The tails represent the probability or frequency of values that are extremely high or low compared to the mean. This means a few unusually high values pull the mean upward and it indicates that a larger number of peaks is below the mean line of the roughness.

Kurtoisis as a measure of the tailedness of a distribution shows how often outliers occur. The kurtosis variation for the Cu thin film surface roughness from Rku =2.76 to 13.48 on the two measured areas indicates non uniform distribution of the peaks and pits (extreme values) from a platykurtic (thin tails) distribution (Rku < 3) to a high leptokurtic (fat tails) distribution (Rku >> 3). That means that on some areas the Cu thin film presents fewer peaks and valleys than a normal distribution and more peaks and valleys than the normal distribution on other areas. The kurtosis of 6.08 and 5.34 of the two areas measured on the Ag/Cu thin film shows a leptokurtic (thin tails) distribution of the extreme values [25].

The profile analysis of the two thin films complements the analysis of surface morphology using the SEM technique, providing information about the surface relief and their ability to retain the aqueous solutions with which they are treated. The results of the roughness measurements are consistent with the estimates obtained by simulating the laser irradiation process of the two targets, the production of the fluid phase, and its quality in the ablation process.

Subsequently, the analysis of the profile of the two thin films after treatment aims to evaluate how the aqueous RB21-NaHCO₃ solution produces corrosion on its surface.

The profiles after treatement with RB21-NaHCO₃ (Figure 7) show a high degradation of the Cu thin film and of the Ag/Cu thin film. The corrosive effect of the treatment solution on the thin layers of Cu and the double-layer Ag/Cu is highlighted by the baseline that slopes downward to the right for the thin Cu film and by a deep valley in the case of Ag/Cu, where residual peaks are observed, probably from the thin layer prior to treatment with the solution. The changes in the height of the maximum peak after treatment at the value of Rp=2070.93 nm for Cu-RB21-NaHCO3, which shows an increase of approximately three times compared to that determined on the copper thin film, and at the value of Rp=1782.54 nm for Ag/Cu-RB21-NaHCO3, which shows a decrease of almost 4 times compared to the double thin film, are due to the different action of the RB21-NaHCO3 solution on the surface of copper and silver, differently modifying their roughness. Regarding the mean values, a significant change is observed for the Cu-RB21-NaHCO3 sample, for which the arithmetic mean of the height Ra=271.80 nm represents an increase of almost 2 times and more than 4 times compared to the initial thin film, while the mean value Ra=924.59 nm of the Ag/Cu-RB21-NaHCO3 sample remained almost the same as the initial thin film. It can thus be deduced that corrosion occurred in the pits (valleys) also due to changes in the size of the pits. In this regard, increases in the maximum pit height value were recorded, 35 times higher for the Cu-RB21-NaHCO3 sample, for which Rv=730.67 nm, compared to the initial thin copper film, for which Rv=20.61 nm, while for the Ag/Cu-RB21-NaHCO3 sample, the maximum pit height value Rv=2311.69 nm is 70 times higher than Rv=33 nm recorded for the initial thin double Ag/Cu film.

By calculating the Width/Height ratios for the highest peak in the roughness profile for the two thin layers after treatment with the aqueous RB21-NaHCO₃ solution, values of 10.50 are obtained for the Cu-RB21-NaHCO₃ sample and 12.75 for the Ag/Cu-RB21-NaHCO₃ sample. These results show a decrease of 2 to 5 times in the Width/Height ratio for Cu-RB21-NaHCO3 and 2 times increase for Ag/Cu-RB21-NaHCO3.

The left-skewed (negatively skewed) obtained for the roughness of the two films after treatment with RB21 – NaHCO₃ aqueous solution (Table 3) shows that most data points cluster on the right, with a long tail to the left. A few unusually low values pull the mean downward. The negative skewness (Rsk < 0) resulted for the Cu-RB21-NaHCO₃ sample and for the Ag/Cu-RB21-NaHCO₃ sample shows that both thin films were subjected to notable corrosive degradation, which led to the arrangement of several peaks below the median line, in contrast to the positive skewness noticed for the thin films before their treatment with the RB21-NaHCO₃ solution.

The kurtosis value of Rku =5.65 for the Cu-RB21-NaHCO₃ being greater than 3 indicates that the distribution of the extreme values is leptokurtic (fat tails) [19], while Rk=1.94 for the Ag/Cu-RB21-NaHCO₃ shows that the distribution of the extreme values is platykurtic (thin tails) [25].

Comparing the profilometric results of the surface roughness of the studied films for the initial thin films and after their treatment with the RB21-NaHCO3 solution, it can be observed that corrosion has reduced the extreme values, also leading to the lowering of the peaks to a level below the mean line. Also, the fact that the corrosion process led to the increase in the depth of the pits on the surface of the thin films is due to the accumulation and longer retention of the solution in the pits, which allows the chemical process to take place for a longer period of time.

3.5. Optical Microscopy (OM)

Optical microscopy (OM) highlighted high corrosion on the Cu-RB21-NaHCO3 sample resulting from the thin copper film after treatment with an aqueous RB21-NaHCO3 solution, as observed in Figure 8 (a), while the Ag/Cu-RB21-NaHCO3 sample resulting from the treatment of the Ag/Cu thin film with an aqueous RB21-NaHCO3 solution (Ag/Cu-RB21-NaHCO3), as seen in Figure 8 (b), shows a more compact structure that can be attributed to lower corrosion.

The OM images are consistent with the results of the SEM analysis and with the roughness measurements, which also show that the RB21-NaHCO3 solution produced more pronounced corrosion on the thin copper film than on the thin double Ag/Cu film.

3.6. UV-Vis Spectroscopy

Figure 9 shows the UV-Vis spectra which indicate an advanced decomposition of the dye compared to the RB21-NaHCO3 control sample. In the visible range, a shift of the peaks from 619 nm and 664 nm to 625 nm and 665 nm is observed in the spectra of the Cu-RB21-NaHCO₃ and respectively Ag/Cu-RB21-NaHCO₃ samples, as well as a reduction in intensity. These spectral changes in the visible range show a shift towards higher wavelength values (bathochromic effect), the color of the absorbing material (the solution resulting after the interaction of RB21-NaHCO₃ with the surface of thin films of Cu and Ag/Cu). The decrease in intensity reflects a hypochromic shift. Both effects indicate changes at the level of the chromophore –SO₂NH–Benz.–SO₂CH₂CH₂OH of the RB21 dye, which is consistent with the results of the FTIR analysis.

Changes in conjugation or the conjugation effect are observed in the UV domain by the shifting of the peaks from 261 nm and 337 nm in the spectrum of the RB21-NaHCO3 control sample to 272 nm and 342 nm in the spectra of the solutions resulting after interaction with the two thin films. As with the visible range, a drastic decrease in intensity is observed. The conjugation effect indicates changes within the chromogen of the RB21 dye.

The bathochromic effect observed for the peaks in the UV range indicates changes in the auxochromes of the dye molecule (methyl, hydroxyl, amino groups) regarding their interaction with the π electron conjugation system.

4. Conclusions

The study showed differences between the morphological aspect of the deposition with pulsed laser of copper and silver thin films when the same laser parameters and working conditions were used.

Treating both thin films (Cu thin film and Ag/Cu double layered thin film) with an aqueous solution of RB21 and NaHCO₃, chemical reactions between the two substances occurred in solution strongly enhanced by the Cu and Ag/Cu thin films with high decomposition of the RB21 dye, indicating catalytic activity of the copper thin film and silver thin film. This may be a good way to clean the residual dyeing baths directly in the vats and thus avoid the discharge of water excessively contaminated with organic dye into the sewer.. This finding is very important as an alternative to photocatalytic treatment on copper films of wastewater containing organic dyes.

The different degree of corrosion of thin films made from different materials when treated with the RB21-NaHCO3 solution could allow the development of layered materials that can be etched by printing for their functionalization. The long-term stability of the aqueous RB21-NaHCO3 solution makes it possible to use it as ink for corrosive printing. The highly corrosive effect of the solution of RB21 and NaHCO₃ on Cu thin films may be adapted to be used on printing machines and to become a possibility to develop a new etching method for electronic applications, including for the solar cells fabrication and also for solar windows which is a promising technology for the future.

The numerical simulation in COMSOL contributed to anticipating and then explaining the experimental results, the two study methods (simulation and experiment) being in agreement.