Visible light activated SnO2:Dy thin films for the photocatalytic degradation of methylene blue

This paper explores the impact of dysprosium (Dy) doping on structural, optical, and photocatalytic properties of tin oxide (SnO2) thin films fabricated via spray pyrolysis. Dysprosium doping levels ranged from 0 to 7 at%, and films were grown on glass substrates at 350 °C. X-ray diffraction (XRD) analysis revealed an increase in crystallite size with Dy doping, signifying improved crystalline quality. Simultaneously, dislocation density and strain decreased, indicating enhanced film quality. Texture coefficient (Tchkl) results showed a predominant crystal orientation along the (110) plane due to Dy doping. Optical band gap energy (Eg) decreased with Dy doping up to 5%. Urbach energy increased with Dy doping, suggesting atomic structural flaws and defects. Scanning electron microscopy (SEM) analysis revealed the presence of numerous micro-aggregates on the film's surface. Notably, the density of these micro-aggregates increased proportionally with higher Dy doping levels, particularly emphasizing the pronounced effect observed in SnO2:Dy 5% thin films. These findings underscore the potential of Dy-doped SnO2 thin films for advanced photocatalytic applications, with SnO2:Dy 5% exhibiting favorable properties and demonstrating a 90.99% degradation efficiency in three hours under solar irradiation.


Introduction
Dyes encompass a category of colored aromatic organic compounds known for their light-absorbing properties, which contribute to their ability to introduce color into the visible spectrum. 1,2Various industries, including textiles, food, rubber, printing, cosmetics, medicine, plastics, concrete, and the paper industry, employ a wide range of dyes for various purposes. 3It is estimated that there are over 100 000 commercially utilized dyes globally, with an annual production volume ranging from approximately 7 × 10 8 to 1 × 10 9 kilograms. 4Unfortunately, these industries produce substantial amounts of wastewater that oen contains carcinogenic and toxic dyes, thereby contaminating water sources and rendering them unsuitable for human consumption. 5Among these sectors, the textile industry stands out as the largest consumer of dyes, with textile dyes being particularly complex compounds featuring diverse structural groups. 107][8] MB is classied as an aromatic heterocyclic basic dye. 9 It is widely recognized as a cationic primary thiazine dye, characterized by its molecular formula C 16 H 18 N 3 ClS and an absorption maximum l max at 663 nm.1][12][13] Therefore, many efforts have been devoted to remove MB dye from waste water including adsorption, [14][15][16] chemical precipitation and some biological means but for their limitations.Photocatalysis, an advanced oxidation process, is highly efficient, cost effective; eco-friendly and stable photocatalysts have found their routes through semiconductors in nano forms.8][19] Among all these and other photocatalysts, tin oxide (SnO 2 ) has gained more attention due to its numerous properties including high charge carrier mobility, a wide band gap, excellent transparency, ease of tunability, as well as remarkable thermal and chemical stability. 20Enhancing the photocatalytic performance of SnO 2 has been a subject of study, with signicant improvements achieved through doping with transition metals or their oxides such as Cu, 21 Ni, Sb, 22 Co, Fe, and Mg. 235][26][27] Recently, rare earth (RE) metals have been explored as additives due to their interactive electron conguration, excellent catalytic properties and fast oxygen ion mobility, which tend to enhance the speed of degradation by making up for the shortcomings in the structure of the thin lms.These interesting materials involve a wide range of elements including Ce, 28 Y, 29 Tm, 30 Tb, 31 Nd 32 and Pr. 33umerous studies have investigated the impact of doping with rare earth elements on the structural and photocatalytic properties of thin lms.For instance, Loyola et al. 34 explored the synergistic effect of terbium (Tb) doping on SnO 2 thin lms, prepared through nebulized spray pyrolysis and demonstrated an impressive 85% degradation efficiency for 10 −5 M methylene blue (MB) dye aqueous solution.Similarly, Ayadi et al. 35 examined the inuence of cerium (Ce) doping on SnO 2 thin lms, also prepared using spray pyrolysis, and achieved a noteworthy 19.10% degradation of MB dye under UV irradiation.Their results highlighted that the photocatalytic performance was signicantly enhanced by the presence of a mixture of tetragonal and cubic phases, particularly with a 2 wt% Ce doping.These two exemplary studies illustrate the profound impact of rare earth element doping on thin lms' photocatalytic capabilities and provide valuable insights into the optimization of such materials for efficient dye degradation.Dysprosium (Dy), being among the most abundant rare earth elements, exhibits high surface reactivity, rapidly oxidizing in the presence of air and readily reacting with water to release dihydrogen.Introducing Dy-doping into TCOs has been observed to enhance visible light absorption and suppress the recombination of photo-generated electrons and holes, thus making it a favorable choice for photocatalytic applications. 36In this study, Dy-doped SnO 2 thin lms were prepared by means of spray pyrolysis.In contrast to many other lm deposition processes, this technique is a fairly straightforward and comparatively inexpensive processing approach (low-cost equipment used).It is an incredibly simple method for creating lms of any composition.Spray pyrolysis does not require premium chemicals or substrates, and can be used to deposit thick lms, porous lms, and to create powder.This adaptable method makes it simple to produce multi-layered lms.To the best of our knowledge, spray pyrolysis has never been used to develop Dy-doped SnO 2 thin lms for photocatalysis applications.The main goal of this study was to determine how dysprosium dopant affects the structural, morphological, optical, chemical and electrical properties of SnO 2 thin lms, specically by studying how the use of rare-earth dysprosium as a dopant helps improve the photocatalytic performance of thin lms made of SnO 2 by focusing on the effects of time, dye concentration, and sample composition on the degradation efficiency of MB.

Preparation of the thin lms
Several separate steps were performed prior to the spraying stage.The glass substrate was treated following a specic cleaning protocol to eliminate impurities using detergent, ionized water, acetone and 1/4 HNO 3 + 3/4 HCl.The substrate was then dried in an oven at a temperature of 120 °C.The dysprosium (Dy) doped and undoped SnO 2 thin layers were prepared using a chemical spray pyrolysis technique on a glass substrate heated to a xed temperature of 350 °C as presented in Fig. 1.Meanwhile, the SnO 2 solution was prepared by dissolving 0.05 M of tin(II) chloride SnCl 2 (99.9%,Sigma-Aldrich) in 40 mL of distilled water.For the doped thin lms, the dysprosium DyCl 3 (DyCl 3 $2H 2 O, Sigma-Aldrich) was mixed with the precursor before being dissolved in concentration ratios of 1, 3, 5 and 7 at%.Since SnO 2 is insoluble in water, it must undergo continuous magnetic stirring prior to and throughout the entire deposition period with heating to just 30 °C, and with the addition of a few drops of HCl.Tin oxide thin lm deposition on a glass substrate is anticipated to occur aer the chemical reaction as: The solution was sprayed for 10 min at a solution ow of 4 mL min −1 .All the parameters were controlled, including air compression carrying gas at a xed ow of 10 mL min −1 and substrate temperatures maintained at 350 °C through a thermocouple connected to the hotplate on which the substrates were placed.The space between the nozzle and the substrates was estimated to 20 cm.
All these conditions were designed to promote the formation of the thin layer by maintaining contact between droplets of the solution and the substrates.

Characterization of the thin lms
The thin lms obtained in the spraying process were then characterized.Their structural properties were determined using X-ray diffraction (XRD) with a Philips X'pert diffractometer with Cu Ka radiation (l = 1.5406Å) granting a diffraction angle (2q) ranging between 20°and 70°.Optical properties like transmittance and reectance were obtained using a spectrophotometer (Shimadzu-UV 1800) providing a scan range of between 300 and 1800 nm.
While the measurement of electrical properties such as resistance and resistivity were taken by means of the four-point technique using a Jandel four-point probe head, the factor shape of which is widely known, coupled to an HP 3458A multimeter (Agilent).Field emission scanning electron microscopy was used to examine the surface morphology of the synthesized thin lms (FE-SEM JEOL JSM-7800 F).

Photocatalytic tests
The photocatalytic efficiency of pure and Dy-doped (1, 3, 5, and 7% at.)SnO 2 layers was tested using methylene blue dye (MB).Degradation occurred under solar irradiation (temperature: 35  °C, latitude: 11.24°, northern Tunisia).The photodegradation of MB was conducted using an estimated power level in the range of 180 to 230 W m −2 .Each of the 5 thin layers were placed in a 40 mL glass container and totally immersed in methylene blue aqueous solution at different concentrations of 2, 3 and 4 mg L −1 .Next, pure and Dy-doped SnO 2 layers (1%, 3%, 5%, and 7% at.) were magnetically swirled for 30 minutes, and then le in the dark.This process ensured the balanced adsorption and desorption of the photocatalyst and the dye.Lastly, the samples and solution were exposed to solar irradiation for three hours, with a 3 mL sample taken each hour.
The residual content was determined by a degradation rate X dened as follows: 37 where A 0 is the initial absorbance value and A t is the Value of the real-time absorbance of measurement at maximum wavelength of the dye.
3 Results and discussion

Structural properties: XRD analysis
The XRD patterns of undoped and Dy-doped SnO 2 [1, 3, 5 and 7% at.] thin lms are shown in Fig. 2a.All of the XRD patterns are indexed in agreement with standard JCPDS 00-041-1445.The peaks are well identied at the positions 2q = 26.6,34.4,38 and 51.5°corresponding to the (hkl) planes ( 110), ( 101), (200), and (210) respectively, which conrms the tetragonal rutile structure of the SnO 2 and SnO 2 :Dy thin lms with a preferential direction of (110).In Fig. 2b, the 2q angle corresponding to the peak (110) is depicted.A slight shi towards higher angles is noticeable as the dopant concentration increases.This shi can be attributed to the integration of Dy ions into the SnO 2 crystal structure. 38All the peaks detected in the spectra are almost identical, except for thin layers based on SnO 2 :Dy 7%, where the two peaks at positions 2q = (101) and ( 200) disappear.This may be caused by the excess presence of the Dy 3+ ion triggering the incipient destruction of the lattice structures of the SnO 2 .According to Table 1, there is not a big difference between the lattice parameters, which increase slightly with increasing the rate of Dy-dopant.It is worth noting that the absence of any additional phases of Dy peaks like Dy 2 O 3 in any of the patterns indicates the successful incorporation of the dopant particles in the matrix of the SnO 2 . 39he effect of Dy doping is very clear.When the doping rate increase, the intensity of the peaks also increase, with the maximum effect detected in the SnO 2 :Dy 5% layers.In order to better observe the effect of Dy doping on the thin lm structures, several structural parameters should be described such as inter-lattice distances d hkl , lattice parameters (hkl), crystallite sizes (D) as well as other parameters, which are summarized in Table 1.
The crystallite size (D) of pure and Dy-doped SnO 2 thin lms are evaluated using the Debye Scherrer equation: 40 where l, b and q are the wavelength of the X-ray diffraction.The full width half maxima (FWHM) of the observed preferential peak are presented in Table 1.
It has been noted that D increased with the increase in doping rate up to the highest value of 14.86 nm recorded for the SnO 2 thin lms doped at 5% Dy, suggesting an enhancement in crystalline quality, as shown in Fig. 2.
The dislocation density (d) which provide information about the lm's quality and any aws in the material, 41 is determined using the eqn (5): A decrease with the increase of the dopant to reach minimum values of 4.53 × 10 −3 nm −2 for the 5% Dy dopant was observed. 42he strain is determined by the eqn (6) below 43 is tabulated in Table 1: The strain was decreased with the increase of Dy to attend 2.44 × 10 −3 rad for the 5% thin lm, then an increase was subsequently recorded for layers doped to 7%. 44he unit cell volume was determined using the eqn ( 7): The rise in SnO 2 crystallite size can be greatly suppressed by doping with Dy, and grain renement can be accelerated.In addition, the SnO 2 :Dy 5% thin layers yielded the maximum number of unit cells.
The number of unit cells is calculated using the eqn (8): The texture coefficient (T chkl ) which measures the relative degree of preferential orientation of a plane (hkl) is determined according to the following formula: 46,47 where n is the number of diffraction peaks, I 0 is the plane standard intensity (hkl) of JCPDS data, I is the measured plane intensity (hkl) observed in XRD diffractograms.
When T c = 1, the crystallites are randomly oriented while the abundance of crystallites in a given plane is indicated by T c > 1.If 0 < T c < 1, this can denotes the absence of crystallite orientation in this direction.
The texture coefficient for the non-doped and Dy-doped SnO 2 layers is greater than 1, indicating the predominant tendency of the crystals to be oriented along the (110) plane.The increase in   the texture coefficient indicates that the Sn particles have been replaced by the Dy 3+ ions. 48

Optical properties
Fig. 3 shows the variation of the transmittance and reection spectra in the 300-1800 nm wavelength range for the pure and Dy-doped SnO 2 thin lms.As the gure shows, all of the thin lms exhibited high transmittance levels, with transmittance reaching up to 80% in the NIR, in good agreement with what could be expected from a (TCO).Transparency decreased slightly as Dy doping increased.The high transparency of these samples would enable their use as an optical window in solar cell applications.
It is important to note the presence of several interference fringes (oscillations with minima and maxima) in the recorded reection spectra.This is clearly caused by a multiple reection phenomenon occurring between the free surface and the surface in contact with the substrate.The thickness of the SnO 2 :Dy thin lms can be estimated using a common technique, from the locations of the interference reectance minima and maxima. 49The values of the thickness of the SnO 2 :Dy lms are illustrated in Table 2, which show that Dy doping increased the thickness of the thin layers.The thickness of thin lms made by spray pyrolysis greatly depends on factors such as the concentration of the precursor, the substrate treatment and controlling the spray duration.Doping can generate molecular interactions, which leads to condensed lms.
The reection rates are between 15 and 20%.In addition, the optical absorption edge of Dy-doped SnO 2 lms is red-shied compared to pure samples, moving forward to the visible light region.This red shi may be due to the formation of shallow electronic levels within the band gap arising from dopant atoms present in the lattice. 50y plotting (ahn) 2 as a function of (hn) and based on the Tauc model 51 indicated in eqn (10), the gap energy was easily estimated the by extrapolating the linear portion of the plots of (ahn) 2 versus hn to hn = 0 and based on the Tauc model interpreted by the following equations: where n is the frequency of the incident photon, hn is photon energy, B is a constant, a is the absorption coefficient and d is the thickness of the layers considered.E g is the optical band gap energy presenting the minimum required energy to excite an electron from the valence band to the conduction band and n is a fraction whose values depend on the nature of the transition (1/2, 2, 3/2 and 3).In our case, n takes 1/2 as a value, which is consistent with direct-allowed transitions. 52,53s demonstrated in Fig. 4, the presence of a linear portion in the band gap versus doping level plot conrms that our material maintains its character as a direct gap semiconductor.The reduction in band gap energy from 3.91 eV to 3.56 eV, observed as the percentage of Dy doping increases up to 5%, can be primarily attributed to the incorporation of Dy ions into the SnO 2 crystal structure.Dy doping induces lattice distortion and disorder, which, in turn, affects the electronic structure of the material.This distortion results in a reduction of the band gap, as Dy contributes additional energy levels within the band structure.It's important to note that this reduction in the band gap is a characteristic effect of doping and is a well-documented  Paper RSC Advances phenomenon in the literature.Beyond the 5% doping level, a slight increase in the band gap is observed.This behavior may be related to the saturation effect of doping, where excessive dopant concentrations can lead to the formation of new phases, lattice defects, or other structural changes, all of which can inuence the band gap.These effects are in accordance XRD results.
Meanwhile the increase in the gap in the 7% doped lm can be explained by the Burstein-Moss effect, which is an increase in the absorption edge generated when the electrons require more energy to go from the valence band to the conduction band when the Fermi level, located within the conduction in the case of n type semiconductor, shis further in the conduction band, resulting in the widening of the band gap of the lm. 54Palanichamy et al. reported similar band gap variations for (Gd 3+ )doped SnO 2 thin lms prepared with a simplied spray pyrolysis technique using a nebulizer. 55We should note that the optical band gap of the lm is also known to be effectively changed by defects like impurities, interstitials, oxygen vacancies, and substitutional defects.
This displacement of the energy gap might be in part attributed to the implementation of defects and disorder in the SnO 2 lattice due to the incorporation of dopants, which has the effect of forming localized states and deep levels in the band gap, which allows the general structure of it to change. 56The spectral lines depicted in Fig. 3c show the values of the absorption coefficient of the synthesized thin lms as a function of the photon energy.Dysprosium doping caused an increase in optical absorption across all layers, which is consistent with the ndings of Gnanasekar et al., 57 who showed that such doping could give rise to changes in the optical properties of CuO, allowing them to be used for applications that involve radiations and chemical reactions.The values of the absorption coefficients were very high with an average value of 10 5 cm −1 for all thin lms, giving our thin lms the absorbent character they need for good photocatalytic performance.
In semiconducting materials, structural disorder produces localized states, also known as band tails, around the margins of prohibited bands.The Urbach Energy, also known as the Urbach tail, is the exponentially increasing absorption edge located close to the fundamental band gap. 58The Urbach Energy can be used to determine the lattice defect concentration because it characterizes the breadth of the localized states in the principal optical band gap.The following relation is used to assess the Urbach tail: where E u stands for the Urbach Energy, a is the absorption coefficient, a 0 is a constant and hn is the incident photon energy.Fig. 4b shows that E u increased as Dy doping increased, going from 232 to 526 meV, which demonstrates that atomic structural aws occurred.The increase in the Urbach Energy values with the increase in Dy doping concentration suggests that the band gap structure of the thin lms doped with Dy had more defects than before.Actually, the optical data might lead one to believe that Dy atoms were successfully incorporated into the SnO 2 lattice.The shrinkage of the optical band gap also explains the rise in the Urbach Energy, 59 as shown in Fig. 4b.

Morphological characterization
SEM is used to analyze the surface morphology of pure and Dydoped SnO 2 thin lms.The morphological modication in the SnO 2 thin lm samples at various doping rates (0, 1, 3, 5 and 7% at.) are shown in Fig. 5.These gures suggest that for pure SnO 2 , the grains do not seem to be tightly packed, leaving noticeable voids in the sample.Numerous microscopic grains multiplied and collected to create a grainy surface with the appearance of gaps and ssures as the rate of Dy increased at 1 and 3%.However, at a Dy rate equal to 5%, the surface of the chopped lm is covered with numerous microaggregates that together create a granular and dense surface.In addition, small grains occurred between the aggregates to ll in the voids and condense the surface of the thin layer, indicating that the layer is well crystallized at this rate, which is also supported by the XRD data.The SEM photos show that the aggregation of these particles is evenly distributed for the SnO 2 :Dy 5% layers.As a result, charge carrier dispersion is reduced, increasing the electrical conductivity of lms.The optoelectronic characteristics of our semiconductors may be enhanced by this behavior. 60he sample doped at Dy 5% exhibits the optimal stable properties; furthermore, the optical and structural results conrm this enhancement.The thickness of the thin lms is obtained by measuring the side surface width in the cross section of an SEM image.Fig. 6 presents the SEM cross-section images for pure and doped SnO 2 thin lms.Using this method, the average thickness of the different SnO 2 layers was determined to be 237, 317.5, 364, 378 and 339 nm for the 0, 1, 3, 5, and 7% Dy-doped thin lms, respectively.In addition, the images allowed us to clearly see the effect of Dy, as the thickness value increases as the doping percentage increases.These results conrm those calculated by means of the interference fringes method only for the 7% Dy- doped SnO 2 thin lm, whose error range is slightly greater at about ±30 nm.This slight difference does not detract from the effectiveness of the interference fringes method and is related to the inhomogeneous character of the porous surfaces produced via the spray method.

Chemical composition
Energy-dispersive X-ray spectroscopy (EDS) is used to determine the chemical composition of the pure and Dy-doped SnO 2 thin lms.The elemental compositions obtained using EDS are shown in Table 3. Analysis of this data reveals the presence of Dy, Sn, and O elements in accordance with atomic ratios obtained from the initial precursors for respective samples, as well as Si (with substrate contribution) with very low concentration traces of Mg, Al, Na, Cl, and Ca.
The correct levels of Dy are present in every layer.The high levels of O atoms are related to the experimental conditions, which included the use of H 2 O as a solvent.

Electrical properties
As illustrated in Fig. 7, a voltmeter was used to measure the voltage (V) across probes A and B while a high impedance (I) current source was utilized to deliver current through the two probes C and D. Eqn (13) was applied to the values of the observed voltages and sourced current to determine the bulk resistivity of the sample. 61 where t is the thickness in cm of the contacted thin layer (calculated from the fringes in the reection spectra) and R s is sheet resistance for the thin lms determined by the equation below: 62 where b is a geometric factor attributed to p ln 2 z 4:532 in the case of semi-innite thin lms.
The electrical resistance and resistivity values are listed in Table 4.
Fig. 8 shows the sheet resistivity for pure SnO 2 and Dy-doped thin lms.All samples show low sheet resistivity, due to the formation of oxygen vacancies, which function as electron donors and raise the concentration of free carriers.The undoped SnO 2 thin lms showed a low sheet resistivity of 102.07 ×    10 −4 U cm, which can be attributed to the divergence from stoichiometry. 63First, the sheet resistivity dropped until it reached a minimal value of 33.49 × 10 −4 U cm for the Dy 5% as the dopant concentration increased.This is due to the presence of Dy 3+ ions in the SnO 2 crystal lattice, leading to an addition in the numbers of the free electrons and causing an increase in the concentration of free carriers. 64The sheet resistivity of the thin lms increased aer the minimum point as the dopant concentration is raised above the ideal doping level.This is because the crystal structure of the thin lm began to degenerate when the dysprosium atoms inserted themselves into the interstitial spaces, which resulted in a decrease in the mobility of free charge carriers and an increase the sheet resistivity of the thin lms.Similar results were obtained by Patrick and al., who examined the effect of Pd doping on SnO 2 thin layers. 65

Photocatalytic activity
The photocatalytic activity of the undoped and Dy-doped SnO 2 thin lms is evaluated using the degradation of methylene blue (MB).This organic dye exhibits a strong absorbance band at 660 nm whose intensity decreases as the MB degrades.Variations in intensity were assessed at regular time intervals.Fig. 11 shows the spectra of the degradation rate of MB aer solar irradiation for three hours using the Dy-doped SnO 2 thin lms at various Dy concentrations under solar irradiation.The diagrams in Fig. 14a demonstrate the good photocatalytic responses of pure and Dy-doped SnO 2 thin lms.MB photo degradation rates were about 85% at three hours under sunlight for all doped thin lms.Furthermore, by calculating the degradation efficiency given by eqn (1), we were able to conrm the efficiency of the thin lms through the degradation  of the MB dye, which increased when the Dy doping level increased to reach 90.99% for 5% Dy, proving that the dopant caused the degradation mechanism.This effect is highest for the 5% doped thin lms, which can be attributed to a higher number of free electrons (these samples show the highest conductivity) to powerfully activate the degradation activity, as well as to the band gap reaching the visible range, which enhanced the absorption of the sunlight irradiation.This is comparable to other results, and this low-cost approach could offer a exible means of enhancing the photocatalytic activity of SnO 2 to high levels of efficiency.
Table 5 provides some examples of the photocatalytic efficiency of SnO 2 , either doped or coupled with other materials, that has attracted the attention of many researchers: To more clearly observe the effect of Dy doping on the photocatalytic activity of the thin lms, the kinetic rate (K) is calculated using this formula: where A 0 is the MB dye's starting absorbance, A is the dye's absorbance aer exposure for time t, and K is a pseudo-rstorder rate constant.The inset of Fig. 14b shows the analysis of the relationship between the degradation ratios ln(A/A 0 ) of MB dye and time for both pure and doped SnO 2 .
The slope of the ln(A/A 0 ) vs. irradiation time (t) plot yields the constant rate K (Fig. 14b).The results shows that the reaction rate constant increased as the doping concentration increased with a higher value of K for thin lms doped at 5%, demonstrating that the addition of Dy to SnO 2 at several levels improves photocatalytic activity under solar irradiation.
Study of stability and reusability Three consecutive cycles of methylene blue (MB) degradation were carried out under solar irradiation, as depicted in the Fig. 9, to evaluate the sustained effectiveness of SnO 2 :Dy samples with a dye concentration of 3 mg L −1 .Aer each cycle, the samples underwent a thorough washing process, involving multiple rinses with distilled water, followed by drying at 50 °C.Remarkably, only a minor decrease in photocatalytic efficiency, estimated at approximately 10%, is observed aer each cycle.This highlights the remarkable resilience of Dy-doped SnO 2 thin lms in the degradation of methylene blue.It should be noted that the effectiveness of SnO 2 in photocatalyzing methylene blue may vary depending on the specic experimental conditions or application.
Effect of MB dye concentration The inuence of methylene blue (MB) concentrations on the photocatalytic degradation process was investigated by varying MB concentrations from 2 mg L −1 to 4 mg L −1 , with a 1 mg L −1 increment, while exposing the system to solar light irradiation surface of the SnO 2 photocatalyst.At higher concentrations, the solution effectively screens a signicant portion of the incoming light, allowing fewer photons to reach the SnO 2 surface.Consequently, the production of electron-hole pairs, which are crucial for the degradation process, is signicantly reduced.This reduction in electron-hole pairs, in turn, leads to a decrease in the degradation of the dye. 72echanism of photocatalytic degradation Much research has focused on better understanding the process of photocatalysis.It involves a sequence of reduction and oxidation reactions, where the electrons move from the valence band (VB) to the conduction band (CB), simultaneously generating oxidative valence holes (h + ) and reductive conduction electrons (e), generally activated when the energy of the light irradiation is equal to or higher than the energy band gap of the catalyst (eqn ( 16)).In a redox reaction, which involves both electrons and holes, the excited conduction band electrons (e CB ) are captured by oxygen molecules, creating reactive peroxide radicals (cO 2

−
) eqn (17).Hydroxyl radicals are then created when (h VB + ) and the water (H 2 O) that has been adsorbed react on the surface of the thin lm producing (OHc), as shown in eqn (18).
Various dopants have been used to improve the effectiveness of photocatalytic reactions and decrease the process of hole and electron recombination.Rare earth dopants have been found to be one of the most effective materials for acting as trapping sites to slow down the recombination process and extend the mean lifetime. 73The doping procedure causes the trapped charges to be transported to the lm's surface where they interact with the dye molecules to boost photocatalytic efficiency. 74In our case, dysprosium also has a potent 4f orbital that is partially occupied, making it a Lewis acid that may quickly react with electrons from the conduction band. 75ccording to eqn (24), a small number of (OH) radicals rearrange to create H 2 O 2 , which then dissociates into (OH) radicals as shown in eqn (25).Finally, the breakdown of the organic contaminants is demonstrated in eqn (26) by the hydroxyl radicals produced by the photocatalysis processes (Fig. 13).The mechanism of the photodegradation of MB is simplied in Fig. 12.
Better photocatalytic activity can result from the overlap of a variety of factors like the higher electron mobility and charge carrier concentration of the samples and the right choice of doping materials at the right concentration levels.Other keys to achieving better results may include the synergistic effect on the adsorption phenomenon and the extended charge separation resulting from the interface of the photocatalyst species following charge transfer between two semiconducting materials, as well as intrinsic defects like oxygen vacancies that favor the interfacial charge transfer in their role as trapping centers of the photogenerated electrons and preventers of the electronhole recombination.

Conclusions
Pure and dysprosium-doped SnO 2 thin lms were successfully synthesized on glass substrate using the spray pyrolysis technique.In order to obtain high-quality thin lms, the impact of various dysprosium rates, which were examined by means of several different characterizations, was studied.XRD analyses revealed a tetragonal rutile structure for the SnO 2 :Dy thin lms, with a better crystallization for the SnO 2 :Dy 5% thin lms.The optical study showed that the materials used have high transparency levels and the optical gap decreases from 3.91 to 3.56 eV with an increase in the doping rate.SEM images showed an enhancement in the structure of the surface with an increase in the density of the grain to reach the maximum with SnO 2 :Dy 5%.In addition, sheet resistivity exhibited low values that decreased as the doping rates increased, reaching a minimum of 33.49 × 10 −2 U cm for the 5% doped thin layers, which increased considerably for the 7% doped thin lms.These results are conrmed by the destruction of the structure shown in the SEM images.Although methylene blue photodegradation showed high levels, reaching 90% in three hours under solar irradiation for the 5% Dy-doped SnO 2 thin lms, the cooperation of all these properties makes SnO 2 :Dy thin lms good candidates for use in the eld of photocatalytic applications such as wastewater treatment.

Fig. 4
Fig. 4 (a) Plot of (ahn) 2 versus (hn) of undoped and Dy-doped SnO 2 thin films, (b) variation of E g and E u versus variation of doping rates of Dy.

Fig. 7 A
Fig. 7 A diagram showing the four-point configuration used in taking the electrical measurements.

Fig. 8
Fig. 8 Variation of resistivity in sheet characteristics of SnO 2 :Dy by Dy levels.

Fig. 14 (
Fig. 14 (a) Degradation rate of MB in presence of the Dy-doped SnO 2 thin films at various Dy ratios (0, 1, 3, 5 and 7% at.) for 3 hours under solar irradiation, (b) graphic determination of the kinetic rate K as a function of Dy content.

Table 5
Comparison of dye efficiency in similar materials reported in the thin film literature Recyclability of Dy doped SnO 2 at 5%.