Layer formation, morphology and photoluminescent properties of ultrasonic spray of pre-formed YVO4:Eu and SiO2 coated ZnS:Mn nanoparticles

Research article Layer formation, morphology and photoluminescent properties of ultrasonic spray of pre-formed YVO 4 :Eu and SiO 2 coated ZnS: Mn nanoparticles Eya Hergli a,b,* , Houssem Eddine Sekrafi a,b , Stanislav Ferdov a,b , Andr´ e Mota c , Joni Lopes c , Paulo J.G. Coutinho a,b , Luis Rebouta a,b a Physics Centre of Minho and Porto Universities (CF-UM-UP), University of Minho, 4710-057, Braga, Portugal b Laboratory of Physics for Materials and Emergent Technologies, LaPMET, University of Minho, 4710-057, Braga, Portugal c Sonae Arauco Portugal,SA, Lugar do Espido - Via Norte, Apartado 1129, 4470-177, Maia, Porto, Portugal ARTICLE INFO Keywords: Photoluminescence DLS Zeta potential Europium doped yttrium orthovanadate YVO 4 :Eu ZnS:Mn SiO 2 coated ZnS:Mn Surfactant Spraying ABSTRACT In this work, we study the coating process and coating quality of photoluminescent particles deposited on a glass surface in terms of particle distribution and associated film continuity. The dispersion process of commercial ZnS:Mn particles and YVO 4 :Eu particles synthesized by microwave reactor in aqueous solutions onto solid surface was performed using an ultrasonic atomizer. Two methods of particle deposition were used, one by moving the substrates while spraying and in the second the substrates were not moved. The measured zeta potential values of 42.9 mV and 45.3 mV, respectively, show that the dispersion of YVO 4 :Eu nanoparticles, in water, is stable at concentrations of 1.2 wt% and 2.4 wt% without the addition of capping agents. It was also found that the dispersion area increases as the flow rate increases from 1 ml/min to 3 ml/min at constant suspension concentration and spraying time. To stabilize the ZnS:Mn particles in solution, either surfactants or a silica capping layer prepared by a sol-gel process were used. The dispersion area encompassing the major amount of dispersed particles was evaluated, The SEM images demonstrate the effectiveness of the ultrasonic atomizer spraying procedure in the disaggregation nanoparticles, as seen by the reduced mean particle size in the sprayed layer compared to the as prepared powder. Finally, this study suggest that the dispersion process of photoluminescent nanoparticles on a substrate holds the potential for optical traceability. 1. Introduction Ultrasonic atomization is a technological process by which a thin layer of liquid while spreading on a vibrating surface disintegrates into fine droplets [1–4] forming a spray. Deposition of nanoparticles using sprays has been reported in several studies in which the spraying process is induced by pressurized nitrogen gas flow [5–7], by an electrostatic field [8], or by ultrasonication [9]. Another possibility is spraying appropriate precursors into hot surfaces resulting in various types of inorganic coatings with technological applications such as transparent conducting oxides (TCO) [10]. In this technique the spray characteristics are of utmost importance and ultrasonic atomization has emerged as a valid option [11], and additionally it is highly desirable for large-scale and cost-effective * Corresponding author. Physics Centre of Minho and Porto Universities (CF-UM-UP), University of Minho, 4710-057, Braga, Portugal E-mail address: [email protected] (E. Hergli). Contents lists available at ScienceDirect Heliyon journal homepage: www.cell.com/heliyon https://doi.org/10.1016/j.heliyon.2024.e38037 Received 11 February 2024; Received in revised form 9 September 2024; Accepted 16 September 2024 Heliyon 10 (2024) e38037 Available online 20 September 2024 2405-8440/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license ( http://creativecommons.org/licenses/by-nc/4.0/ ). practical applications. When using sprays of pre-formed nanoparticles the process of film growth and the structure of the resulting thin film was also addressed to deposit ZnO nanoparticles for dye-sensitized solar cells (DSSCs) [12], tungsten molybdenum oxide nanoparticles for electrochromic applications [13], silica, TiO 2 and Ag nanoparticles to produce superhydrophobic and antibacterial/self-cleaning surfaces [9,14], Ba 0.6 Sr 0.4 TiO 3 nanoparticles for High Capacitance Density Capacitors [7], SiC nanoparticles to fabricate laminate composites [15], ZnS for thin film solar cells [8] and Sn nanoparticles for Na-ion battery applications [16], among others. Various phosphor materials doped with rare earth ions have been extensively investigated due to their interesting magneto-optical properties such as high luminescence efficiency, and flexible emission colors with different activators [17]. Among these inorganic compounds, rare earth ion-doped yttrium orthovanadate phosphors, such as YVO 4 :Eu which is an efficient red-emission phosphor have been extensively used in cathode ray tubes (CRT) and in plasma display panels (PDP) [18,19]. In addition, the ZnS activated with Mn ion is the most efficient cathodoluminescence phosphor extensively used in other displays such as field emission display (FED), PDP and electron luminescence (EL) [20]. These properties make ZnS and YVO 4 :Eu appealing materials for deposition studies by ultrasonic atomization. Although the deposition of ZnS particles by ultrasonic atomization has been previously studied [21], the same method to the best of our knowledge has never been applied to YVO 4 nanoparticles. Ultrafine sprays can be obtained with aqueous dispersions of particles up to 30 % weight, however, it is difficult to disperse ZnS powders in water. In order to stabilize the particles in suspension, different surfactants [22–24] were tested due to their use as agents that inhibit the over-growth of particles and prevent their aggregation. The choice of a suitable capping layer is a key in stabilizing colloidal solutions. Another frequent approach to enhance the stability of ZnS is by covering its surface with a shell such as SiO 2 [25, 26]. Although pulverization is well known technique for deposition of nanoparticles there are systems of particles that are still not explored. As far as we know there is knowledge gap in the pulverization of photoluminescent nanoparticles as YVO 4 :Eu and SiO 2 coated ZnS:Mn. Due to the facile preparation of relatively uniform nanoparticles and highly efficient emission [27], YVO 4 :Eu is promising material for the pulverization by ultrasonic atomization and optical detection on surfaces. Consequently, the goal of this study is to evaluate the potential of the spraying process by maximizing the deposition of the particles only in a narrow track with the substrate in continuous motion and in one pass, minimizing the consumption of suspension to reduce costs, but enough to detect their presence by photoluminescence measurement. In this evaluation, a static spraying was mainly used, but with parameters that could be used in the above mentioned conditions, such as low probe-substrate distance, high concentration of particles, high flow rate. For example, we would like to evaluate the potential to deposit nanoparticles on a surface area 50 times higher compared to static deposition and still have enough particles to be able to measure photoluminescence. For this reason, we test suspensions with a low and a high particles concentrations and use a spray probe that enables relatively high flow rates. The high particle concentration also has the purpose of reducing the sprayed liquid component. The small distance between probe and substrate would allow not only a narrow track, but also reduce the use of particles to reduce costs. The obtained results support the potential for development of films of photoluminescent nanoparticles on mobile substrate. Considering that studies on spraying particles on solid substrates provide fundamental knowledge about surface phenomena, we investigate the dispersion of nano-to micron-sized photoluminescent particles on glass slides. With the help of an ultrasonic atomizer, we deposited commercial ZnS and laboratory-made YVO 4 :Eu particles and evaluated the dispersion area in terms of amount and distribution of particles as well as the morphology of the obtained coatings. 2. Experimental section 2.1. Materials 2.1.1. ZnS:Mn microparticles and reagents Experiments were mainly carried out using ZnS:Mn (fine powder) kindly supplied by Nemoto Portugal. Sodium dodecyl sulfate (SDS, 99+%) was purchased from Sigma Aldrich and used as a dispersion stabilizer to get more stable dispersion of nanoparticles. Tetraethylorthosilicate (TEOS, puriss.; ≥99.0 %, Fluka), ammonium hydroxide solution (NH 4 OH, puriss. ~25 % NH 3 basis, Fluka) and 2 propanol (≥99.9 %, Riedel-de-Ha¨ en) were used for the coating of ZnS:Mn. Sodium orthovanadate (Na 3 VO 4 , 99.98 %, trace metals basis, Sigma Aldrich), yttrium nitrate hexahydrate (Y(NO 3 ) 3 .6H 2 O, purity of 99.9 %, Sigma Aldrich), and europium (III) nitrate pentahydrate (Eu(NO 3 ) 3 .5H 2 O, purity of 99.9 %, Sigma Aldrich) were used for the synthesis of YVO4:Eu. Ultra-pure water was used throughout the experiments. 2.1.2. YVO 4 :Eu nanoparticles YVO 4 :Eu nanoparticles were synthesized by microwave-assisted hydrothermal method. The preparation method consisted in the following: 1 mmol of Na 3 VO 4 was dissolved in 3 ml of distilled water and 1 mmol of Y(NO 3 ) 3 •6H 2 O was dissolved in 0.5 ml of distilled water. Then, 0.1 mmol of Eu(NO 3 ) 3 •5H 2 O were dissolved in 0.5 ml of water. After stirring the mixture of the three solutions at room temperature for 10 min, it was transferred to a 10 ml borosilicate vial and placed in the Monowave 400 reactor from Anton-Parr. The mixture was heated at 120 ◦C in 10 min and the temperature was held for 60 min before cooling it at 60 ◦C for about 3 min. Finally, the sample obtained was separated by filtration using qualitative filter paper (Fisherbrand), washed for several times with distilled water, and dried in air at 50 ◦C for 10 h. E. Hergli et al. Heliyon 10 (2024) e38037 2 2.1.3. SiO 2 encapsulated ZnS:Mn microparticles The ZnS/SiO 2 core/shell spheres were prepared by the hydrolysis of TEOS. Typically, an amount of 4.4 g of ZnS particles was dispersed in 95 ml of 2-propanol. Ultra-pure water (5.5 ml) and a little amount of ammonia solution (0.75 ml) were added in order to motivate the hydrolysis of TEOS. The mixture was then heated to 40 ◦C, and kept stirring for 30 min. Finally, 10.1 ml of TEOS was added. The mixture was then kept for about 24 h at 40 ◦C under vigorous stirring. When the reaction completed, the sample was centrifuged. The remaining precipitate was washed several times with ultra-pure water by centrifugation and then dried [25,26]. 2.2. Spraying process An ultrasonic atomizer, whose experimental setup is schematized in Fig. 1, was used in the present study with frequency 40 kHz (Model QA40A-110 procured from Qsonica Sonicators, USA) connected to a probe with a circular irradiating surface. This probe (Part No 4630 from QSonica) allowing flow rates up to 50 ml/min has a section with 8 mm diameter and 19 mm length followed another one with 5 mm diameter and 31 mm long and ending with a flat tip. A low pressure Minipuls 3 Peristaltic pump (procured from Gilson, USA) was used to deliver the liquid sample to the nozzle. The tip of the nozzle was placed at a given distance from the surface of a preheated glass slide using a hotplate. With water, the median drop size at 40 kHz is 50 μ m. To prevent the formation of coarse agglomerates in ZnS:Mn suspensions, we used first a water bath sonicator and then added dispersion stabilizers [22], as described above. During the spraying process, the suspension was kept under magnetic stirring. Suspension concentrations of 1.2–30 wt% of ZnS:Mn and 1.2–16.6 wt% of YVO 4 :Eu particles were used. In all cases, the substrates were held at a temperature of 100 ◦C throughout the spraying in order to quickly evaporate the solvent in the drops and avoid their coalescence. The wide range of suspension concentrations associated to high flow rates (1 and 3 ml/min) has the purpose of, using the static spraying, to evaluate the potential of the spraying particles only in a narrow track with the substrate in continuous motion and in one pass, but enough to detect their presence by photoluminescence measurement. The small distance between probe and substrate (1.5 cm) would allow the deposition in track with a width of about 1 cm. Sprayed layers of PL particles were also prepared on glass slides with moving substrates during the spray for 15 s, 30 and 60 s. The distance between the probe and substrates was 1.5 cm and the flow rate was 1 ml/min. The spraying was performed all over the entire surface of the glass slide in a homogeneous way by moving it manually forwards, backwards and laterally. 2.3. Characterization The zeta potential of the suspensions was measured using Dynamic and Electrophoretic Light Scattering (Litesizer 500 from AntonPaar) at 25 ◦C, using a He-Ne laser of λ =658 nm. For dynamic light scattering (DLS) the detector was at an angle of 175◦, while for electrophoretic light scattering (ELS) the detector was at an angle of 15◦. Three measurements were performed for each sample. The photoluminescence was measured with two different equipment: a laboratory fluorometer (SPEX 1680B 0.22m Double Spectrometer), which uses a bifurcated optical fiber to irradiate the glass slide with 320 nm LED light from the equipment excitation channel and collect the sample photoluminescence into the emission channel, and a homemade portable setup (LED M310L1 from Thorlabs, with a peak wavelength of 310 nm) using a CCS200 spectrometer from Thorlabs. The scanning electron microscopy (SEM) micrographs of the sprayed layers were obtained using a FEI NOVA NanoSEM 200 microscope, which was used to obtain the cross-sectional and surface morphologies. The backside of the sample was marked with a diamond stylus and glass slide was fractured with one end fixed, while the other was pressed with a laboratory spatula. The cross sections were coated by a 2 nm gold-palladium layer to prevent charging of the surface and to promote the emission of secondary electrons. At the bottom of the SEM images some of the experimental conditions under which they were obtained are indicated: Magnification (mag); High voltage (HV), which gives the energy of the primary electrons; Secondary electrons mode (SE); Through-lens detector (TLD); working distance (WD) and the actual scan range horizontal field of view (HFW). The surface of the sprayed layers was also characterized by Atomic Force Microscopy (AFM), using a CSI – Nano-Observer Atomic Force Microscope in tapping mode. The powder X-ray diffraction patterns were recorded on a Bruker D8 Discover diffractometer operating with CuK α radiation (λ =1.5406 Å) in the Bragg-Brentano (θ/2θ) geometry in the range of 10◦–70◦with a step size of 0.04◦ Fig. 1. Schematic representation of the experimental setup. E. Hergli et al. Heliyon 10 (2024) e38037 3 and an integration time of 1 s. Soller slits were used to limit the divergence of the incident and diffracted X-rays. All characterizations were performed at ambient temperature. 3. Results and discussion The choice of conditions for particle deposition was based on the existing state-of-the-art [15] and the specific objectives of the present work. Namely, nozzle type, short nozzle to substrate surface distance (1.5 cm), variable particle concentrations (1.2–30 wt%), flow rate (1 ml/min to 3 ml/min), and time (1 and 5 s) for particle deposition were selected aiming to assure narrow but optically detectable area of deposited nanoparticles. 3.1. Sprayed layers prepared from YVO 4 :Eu suspension The YVO 4 :Eu nano powder was analyzed in terms of its crystallographic structure and the XRD pattern is shown in Fig. 2. The diffractogram indicates that this compound has crystallized in tetragonal structure of YVO 4 with space group I4 1 /amd (no. 141). All reflections are well consistent with JCPDS 00-016-0250. The crystallite sizes of prepared nanoparticles determined by using Debye Scherrer [28] equation was 11 nm. The Z-average particle size was measured by DLS and a value of 87 nm was obtained. The particle size was also calculated from SEM images of as prepared YVO 4 :Eu nano powder. The SEM image is illustrated in Fig. 3a, and 3b shows the particle distribution and the mean size. The particle size distribution was determined using “ImageJ” software, resulting in a mean size of 32 nm. From the SEM image is clearly depicted that the synthesized nanoparticles have been agglomerated. As shown later, these synthesized nanoparticles can be separated after the ultrasonication and atomization process. Suspensions with different YVO 4 :Eu nanoparticle concentrations (1.2 wt% - 16.7 wt%) were used, which are indicated in Table 1. In the table are also indicated the zeta potential values, measured by Dynamic and Electrophoretic Light Scattering. According to the zeta potential value of suspensions with 1.2 wt% and 2.4 wt% concentrations, we noted that the dispersion is stable without adding any capping agents. In case of 16.67 wt% suspension, the light transmittance though the suspension was very low, which did not allow the measurement of the Z potential. Fig. 4a exhibits photoluminescence spectra obtained with an excitation radiation with an average wavelength of 310 nm in three different positions of sprayed layer for the spraying time of 1 s. The emission spectrum exhibits six emission peaks, being the two most intense peaks at 619 nm and 622 nm, which were followed to identify the photoluminescence intensity, and the remaining four at about 594, 652, 702 and 707 nm [18]. Fig. 4b shows an image of photoluminescence emission when irradiated by a LED with a wavelength of 310 nm and Fig. 4c presents the intensity of the emission peak at 619 nm obtained in the scan performed along the sprayed layer, being the distance D measured from the center of the sprayed layer (insert of Fig. 4a). The irradiated spot during this scan had a diameter lower than 1 mm. The brighter area in Fig. 4a is about 9 mm, which corresponds to the region with the highest photoluminescence intensity in Fig. 4c (about from −4 mm to +4 mm). From the results shown in Fig. 4b and c it is clear that the sprayed layer formed a ring. The formation of this type of structure, a ringlike stain, known as the coffee-ring effect [29,30], is due to the migration of the dispersed particles to the edge of the drop during the liquid drying [31–33]. The rate of evaporation of the solvent varies along the droplet surface and is increased near the contact line, leading to a convective flow. Thus, during the water evaporation process, the edges of the droplets become attached to the substrate, and capillary flow from the center to the edges of the droplet carries the suspended particles to the edges as evaporation proceeds [34]. This effect is particularly evident in case of spherical particles and can be suppressed by using particles with anisotropic shape [35]. The spray-deposition method involves a complex behavior governed by aggregation, agglomeration, and the formation of clusters of particles during the deposition process. Fig. 2. Indexed powder XRD pattern of YVO 4 :Eu nanocrystals. E. Hergli et al. Heliyon 10 (2024) e38037 4 Fig. 3. SEM image of as prepared YVO 4 :Eu phosphors. The insert shows the particle size distribution and the mean size calculated from SEM images. Table 1 Concentrations of YVO 4 :Eu suspensions used in this work, and the respective zeta potential. Suspension YVO 4 :Eu concentration Zeta potential (mV) C 1 0.0125 g/ml (1.2 wt%) 42.9 C 2 0.025 g/ml (2.4 wt%) 45.3 C 3 0.19 g/ml (16.67 wt%) – Fig. 4. a) Photoluminescence spectra obtained in three different positions of sprayed layer prepared from YVO4:Eu suspension; b) Image of photoluminescence emission when irradiated by a LED with a wavelength of 310 nm. c) Intensity of the emission peak at 619 nm obtained in the scan performed along the sprayed layer. The excitation radiation had an average wavelength of 310 nm. The dimensions of the scale in the x-axis were measured in the sample showed in Fig. 4b, with the origin in the center of the area with the sprayed particles. E. Hergli et al. Heliyon 10 (2024) e38037 5 The surface morphology of the deposited structure is shown in Fig. 5a, where the SEM image reveals a uniform structure. The average particle size (Fig. 5b), calculated using the ImageJ software, is about 14 nm, which is significantly lower than the average particle size of 32 nm in as prepared YVO 4 :Eu powder and shown in Fig. 3. This shows that the spraying process using an ultrasonic atomizer is effective in the disaggregation of YVO 4 :Eu nanoparticles. From the cross section image of the sprayed layer (Fig. 5c), taken from a position deviated from the center of the circle, a well-defined granular shape with irregularities that must come from empty spaces is observed. The large particles lead to decreased relative density due to reduced grain boundary mobility and the increase of voids. From this image, a thickness of about 1.5 μ m was extracted. In the case of spraying time of 5 s, the increase of sprayed particles amount resulted in nanoparticles that are now dispersed in a larger circle with a diameter of about 13 mm. The ring formation is still evident, as shown in Fig. 6, presenting an image of the sprayed layer (Fig. 6a), an image of photoluminescence emission when irradiated by a LED with a wavelength of 310 nm (Fig. 6b) and the intensity of the emission peak at 619 nm obtained in the scan performed along the sprayed layer (Fig. 6c). The surface morphology is similar to that of the previous sample and shown in Fig. 5a. The SEM cross-section image taken in an intermediate region of the circle, 3 mm from the center (Fig. 7 a), shows a thickness with small fluctuations, of about 2.7 μ m (Fig. 7b). AFM images, on a square area of 1 ×1 μ m 2 , of the as prepared YVO 4 :Eu sprayed layer surface are shown in Fig. 8. Three surface profiles taken along the lines indicated in Fig. 8a are presented in Fig. 8c. They show a variation in height with an amplitude of about 140 nm, and with a rms surface roughness value of 18.3 nm for the region investigated. The observed range of grain size, or grain agglomerate size, determined by measuring the diameters in the AFM images, has a larger fraction in the range 40–70 nm, which is Fig. 5. a) SEM image of the surface of the YVO 4 :Eu sprayed layer; b) particle size distribution and the mean size, and c) SEM image of the cross section of the sprayed layer (spraying time =1 s). Fig. 6. a) Image of the suspension sprayed onto glass slide with substrate-probe distance of 1.5 cm (t spray =5 s); b) Image of photoluminescence emission when irradiated by a LED with a wavelength of 310 nm. For the images, the glass slide was placed on a black background to increase contrast; c) Intensity of the emission peak at 619 nm obtained in the scan performed along the sprayed layer. E. Hergli et al. Heliyon 10 (2024) e38037 6 slightly higher than mean particle size value seen by SEM (Fig. 5). We tested the feasibility of spraying a suspension of synthesized YVO4:Eu with a high particle concentration of 0.19 g/ml (16.66 wt %). At a spray time of 1 s and a flow rate of 1 ml/min, the nanoparticles were found to be dispersed in a circle with a diameter of about 13 mm, similar to a concentration of 1.2 wt%, a spray time of 5 s and the same flow rate. The ring formation is still present, but less obvious, as shown in Fig. 9, where an image of the photoluminescence emission is shown when irradiated with an LED with a wavelength of 310 nm. Fig. 10 shows the SEM cross-sectional images of the sprayed layers, taken from a position 3 mm away from the center of the circle. The thickness can change from 3.5 μ m (on the right side of Fig. 10a) to 6.4 μ m (center of Fig. 10a). The image with higher magnification (Fig. 10b) presents an average thickness of 6.2 μ m. In a second step, two concentrations of a synthesized YVO 4 :Eu were prepared (C 1 =0.0125 g/ml and C 2 =0.025 g/ml, which correspond to 1.2 wt% and 2.4 wt%, respectively). Fig. 11 shows the photoluminescence spectra of the sprayed layers using 320 nm excitation wavelength for C 1 and for 15, 30, 60 s spraying times and with moving substrates. The photoluminescence intensity increases as the spraying time increases. Fig. 7. SEM images of cross section of the sprayed layer (spraying time =5 s) in the intermediate region of the circle, 3 mm from the center of the circle. Fig. 8. AFM images (a and b) and three surface profiles (c), obtained along the lines indicated in AFM image (b), of the as prepared YVO 4 :Eu sprayed layer surface (spraying time =5 s and particle concentration of 1.2 wt%). E. Hergli et al. Heliyon 10 (2024) e38037 7 Fig. 9. Image of photoluminescence emission of the sprayed layer (spraying time of 1 s) prepared from YVO 4 :Eu suspension (0.19 g/ml) when irradiated by a LED with a wavelength of 310 nm. Fig. 10. SEM cross section images of the sprayed layer prepared from YVO 4 :Eu suspension (0.19 g/ml) and for a spraying time of 1 s, taken from a position deviated 3 mm from the center of the deposited area with low magnification (a) and with higher magnification (b). Fig. 11. a) Photoluminescence spectra of the sprayed layers prepared from YVO 4 :Eu suspensions at 320 nm excitation wavelength for C 1 =0.0125 g/ml and for spraying times of 15 s, 30 s and 60 s, respectively, with moving substrates. E. Hergli et al. Heliyon 10 (2024) e38037 8 However, the surface morphology of the sprayed layer has some defects, superficial cracks, when seen with low magnification (identified with circles in Fig. 12a), but fully covering the substrate. These defects may due to uneven evaporation rates during the drying process. Given the high spray rate, part of the liquid component may remain inside the coating, leading to the subsequent formation of gas bubbles. With the substrate heated, an increase in the amount evaporated increases the pressure exerted on the external layers, leading to their fracture and consequent release of gas. This layer structure is related with the different processes occurring during the deposition. The dynamics of structural evolution was studied by Sarkar et al. [12] with grazing incidence small-angle X-ray scattering during in situ spraying. According to these authors, during the initial spraying time the main occurrence is the formation of clusters that result from the coalescence of the incoming droplets [12]. Due to the substrate temperature, the droplets evaporate rapidly when they land onto the substrate. As a result of this rapid evaporation (during the initial spraying stage), the distances between those clusters stay fixed on the substrate. The structure will increase with the deposition of more droplets, and after this stage, the material arriving at the substrate bounds to the nanoparticle layer underneath, rather than clustering with each other to form larger structures [12]. The clusters touch each other and thereby form a closed film and the substrate reaches a considerable coverage, like those shown in SEM cross section (Fig. 13), where the clearly evident surface waviness probably results from the complex thin film growth [12]. The thickness varies from 1.3 to 2.7 μ m (Figs. 13a) and 1.6–2.8 μ m (Fig. 13b). Fig. 13b suggests that higher particle concentrations can lead to increased Fig. 12. SEM image of the surface of the sprayed layer prepared from YVO 4 :Eu suspension (C 1 =0.0125 g/ml) with moving substrate, and for different magnifications: a) 1000×; b) 200000×. The circles in a) identify the surface defects, which may be due to uneven evaporation rates during the drying process. c) particle size distribution and mean size calculated from b). Fig. 13. SEM image of cross section of the sprayed layers prepared from YVO 4 :Eu suspensions for a flow rate of 1 ml/min and a: a) concentration C 1 =0.0125 g/ml and spraying time of 60 s; b) concentration C 2 =0.025 g/ml and spraying time of 30 s, with moving substrates. E. Hergli et al. Heliyon 10 (2024) e38037 9 [6] C. Tso, C. Zhung, Y. Shih, Y. Tseng, S. Wu, R. Doong, Stability of metal oxide nanoparticles in aqueous solutions, Water Sci. Technol. 61 (2010) 127–133, https://doi.org/10.2166/wst.2010.787. [7] E. Tetsi, G. Philippot, I.B. Majek, C. Aymonier, J. Audet, R. Lemire, L. Bechou, D. Drouin, Ba 0.6 Sr 0.4 TiO 3 thin films deposited by spray coating for high capacitance density Capacitors, Phys. Status Solidi A 215 (2018) 1800478, https://doi.org/10.1002/pssa.201800478. [8] M. Karimi, S.M. Mirkazemi, Y. Vahidshad, J. Javadpour, Preparation and characterization of zinc sulfide thin film by electrostatic spray deposition of nanocolloid, Thin Solid Films 737 (2021) 138929, https://doi.org/10.1016/j.tsf.2021.138929. [9] Y. Xiao, W. Huang, C.P. Tsui, G. Wang, C.Y. Tang, L. Zhong, Ultrasonic atomization based fabrication of bio-inspired micro-nano-binary particles for superhydrophobic composite coatings with lotus/petal effect, Composites, Part B 121 (2017) 92–98, https://doi.org/10.1016/j.compositesb.2017.03.029. [10] S. Karakaya, O. Ozbas, Boron doped nanostructure ZnO films deposited by ultrasonic spray pyrolysis”, Appl. Surf. Sci. 328 (2015) 177–182, https://doi.org/ 10.1016/j.apsusc.2014.11.084. [11] P.S. Patil, Versatility of chemical spray pyrolysis technique, Mater. Chem. Phys. 59 (1999) 185–198, https://doi.org/10.1016/S0254-0584(99)00049-8. [12] K. Sarkar, Erik V. Braden, Stefan Pogorzalek, Shun Yu, Stephan V. Roth, Peter Muller-Buschbaum, Monitoring structural dynamics of in situ spray-deposited zinc oxide films for application in dye-sensitized solar cells, ChemSusChem 7 (2014) 2140–2147, https://doi.org/10.1002/cssc.201402049. [13] H. Li, J. Chen, M. Cui, G. Cai, A.L. Eh, P.S. Lee, H. Wang, Q. Zhang, Y. Li, Spray coated ultrathin films from aqueous tungsten molybdenum oxide nanoparticle ink for high contrast electrochromic applications, J. Mater. Chem. C 4 (2016) 33–38, https://doi.org/10.1039/C5TC02802G. [14] S. Trabucco, S. Ortelli, B. Del Secco, I. Zanoni, F. Belosi, F. Ravegnani, A. Nicosia, M. Blosi, A.L. Costa, Monitoring and optimisation of Ag nanoparticle spraycoating on textiles, Nanomaterials 11 (2021) 3165, https://doi.org/10.3390/nano11123165. [15] M. Bastwros, G. Kim, Ultrasonic spray deposition of SiC nanoparticles for laminate metal composite fabrication, Powder Technol. 288 (2016) 279–285, https:// doi.org/10.1016/j.powtec.2015.10.039. [16] J.-H. Park, Y.-S. Choi, C. Kim, Y.-W. Byeon, Y. Kim, B.-J. Lee, J.-P. Ahn, H. Ahn, J.-C. Lee, Self-Assembly of pulverized nanoparticles: an approach to realize large-capacity, long-lasting, and ultra-fast-chargeable Na-ion batteries, Nano Lett. 21 (2021) 9044–9051, https://doi.org/10.1021/acs.nanolett.1c02518. [17] M. Ayvacikli, A. Ege, N. Can, Radioluminescence of SrAl 2 O 4 :ln 3+ (Ln =Eu, Sm, Dy) phosphor ceramic, Opt. Mater. 34 (2011) 138–142, https://doi.org/ 10.1016/j.optmat.2011.07.023. [18] G. Jia, Y. Song, M. Yang, Y. Huang, L. Zhang, H. You, Uniform YVO 4 :Ln 3+ (Ln =Eu, Dy, and Sm) nanocrystals: Solvothermal synthesis and luminescence properties, Opt. Mater. 31 (2009) 1032–1037, https://doi.org/10.1016/j.optmat.2008.11.012. [19] H. Zhang, X. Fu, S. Niu, G. Sun, Q. Xin, Low temperature synthesis of nanocrystalline YVO 4 :Eu via polyacrylamide gel method, J. Solid State Chem. 177 (2004) 2649–2654, https://doi.org/10.1016/j.jssc.2004.04.037. [20] N. Karar, F. Singh, B.R. Mehta, Structure and photoluminescence studies on ZnS:Mn nanoparticles, J. Appl. Phys. 95 (2004) 656–660, https://doi.org/10.1063/ 1.1633347. [21] H. Benamra, H. Saidi, A. Attaf, M.S. Aida, A. Derbali, N. Attaf, Physical properties of Al-doped ZnS thin films prepared by ultrasonic spray technique, Surface. Interfac. 21 (2020) 100645–100652, https://doi.org/10.1016/j.surfin.2020.100645. [22] P. Bihari, M. Vippola, S. Schultes, M. Praetner, A.G. Khandoga, C.A. Reichel, C. Coester, T. Tuomi, M. Rehberg, F. Krombach, Optimized dispersion of nanoparticles for biological in vitro and in vivo studies, Part. Fibre Toxicol. 5 (2008) 1–14, https://doi.org/10.1186/1743-8977-5-14. [23] R. Javed, M. Zia, S. Naz, S.O. Aisida, N. Ain, Q. Ao, Role of capping agents in the application of nanoparticles in biomedicine and environmental remediation: recent trends and future prospects, J. Nanobiotechnol. 18 (2020) 172, https://doi.org/10.1186/s12951-020-00704-4. [24] J. Hasanzadeh, Optical and structural properties of Cu doped ZnS nanocrystals: effect of temperature and concentration of capping agent, Acta Phys. Pol. A 129 (2016) 1147–1150, https://doi.org/10.12693/APhysPolA.129.1147. [25] H. Song, Y.M. Leem, B.G. Kim, Y.T. Yu, SiO 2 -coated ZnS submicrospheres with enhanced thermal stability and photoluminescence, Mater. Sci. Eng. B 143 (2007) 70–75, https://doi.org/10.1016/j.mseb.2007.07.008. [26] Y. Zhang, Y.D. Li, Synthesis and characterization of monodisperse doped ZnS nanospheres with enhanced thermal stability, Phys. Chem. B 108 (2004) 17805–17811, https://doi.org/10.1021/jp047446c. [27] H.E. Sekrafi, E. Hergli, S. Ferdov, A. Mota, J. Lopes, P.J.G. Coutinho, L. Rebouta, Optimization of photoluminescence of Eu 3+ doped YVO 4 nanoparticles by green microwave synthesis, Mater. Chem. Phys. 312 (2024) 128592, https://doi.org/10.1016/j.matchemphys.2023.128592. [28] A.L. Patterson, The scherrer formula for X-ray particle size determination, Phys. Rev. 56 (10) (1939) 978–982, https://doi.org/10.1103/PhysRev.56.978. [29] R.D. Deegan, O. Bakajin, T.F. Dupont, G. Huber, S.R. Nagel, T.A. Witten, Contact line deposits in an evaporating drop, Phys. Rev. E 62 (2000) 756–765, https:// doi.org/10.1103/PhysRevE.62.756. [30] R.D. Deegan, Pattern formation in drying drops, Phys. Rev. E 61 (2000) 475–485, https://doi.org/10.1103/PhysRevE.61.475. [31] T.P. Bigioni, Xiao-Min Lin, T.T. Nguyen, E.I. Corwin, T.A. Witten, H.M. Jaeger, Kinetically driven self-assembly of highly ordered nanoparticle monolayers, Nat. Mater. 5 (2006) 265–270, https://doi.org/10.1038/nmat1611. [32] J. Park, J. Moon, Control of colloidal particle deposit patterns within picoliter droplets ejected by ink-jet printing, Langmuir 22 (2006) 3506–3513, https://doi. org/10.1021/la053450j. [33] N. Tsapis, E.R. Dufresne, S.S. Sinha, C.S. Riera, J.W. Hutchinson, L. Mahadevan, D.A. Weitz, Onset of buckling in drying droplets of colloidal suspensions, Phys. Rev. Lett. 94 (2005) 018302, https://doi.org/10.1103/PhysRevLett.94.018302. [34] H. Hu, R.G. Larson, Evaporation of a sessile droplet on a substrate, J. Phys. Chem. B 106 (2002) 1334–1344, https://doi.org/10.1021/jp0118322. [35] P.J. Yunker, T. Still, M.A. Lohr, A.G. Yodh, Suppression of the coffee-ring effect by shape-dependent capillary interactions, Nature 476 (2011) 308–311, https:// doi.org/10.1038/nature10344. [36] P. Zhang, G. Santoro, S. Yu, S.K. Vayalil, S. Bommel, S.V. Roth, Manipulating the assembly of spray-deposited nanocolloids: in situ study and monolayer film preparation, Langmuir 32 (17) (2016) 4251–4258, https://doi.org/10.1021/acs.langmuir.6b00892. E. Hergli et al. Heliyon 10 (2024) e38037 16