Sonochemistry synthesis of zinc silicate ceramic nanoparticles and their characterization

This research aims to present a sonochemical synthesis method for high-purity willemite nanopowders. Initially, zinc silicate hydrate nanoparticles were created using a modi�ed sonochemistry method in which zinc salts and waterglass were used as starting materials to obtain zinc silicate precipitate under pH-controlled conditions (11-12) and Argon gas �ux. Following that, the precipitate was heat treated at various temperatures. TGA/DSC, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), dispersive X-ray spectrometry (EDX), and N2 gas adsorption characterizations were also used to assess phase changes, morphological properties, microstructures, and chemical composition. The formation temperature of a well-crystalline willemite monophase was determined to be 890°C, and this was supported by XRD analysis. The synthetized material had high homogeneity and excellent purity, according to EDX elemental mapping. Its nanometric nature was further con�rmed by microscopic observations (SEM, TEM). Notably, compared to previously reported methods, the aforementioned technique uses a moderate synthesizing temperature, making it economical for mass production and potentially useful in a variety of industrial �elds, including ceramics, paints, plastics, biomaterials, and composites, among others. EDX elemental mapping demonstrated high homogeneity and excellent purity of the material. Microscopic observations (SEM, TEM) further con�rmed its nanometric character. Notably, the aforementioned method employs a moderate synthesising temperature compared to previously reported methods, making it cost-effective for mass production with potential applications in various industrial �elds, such as ceramics, paints, plastics, biomaterials, and composites, among others.


Introduction
From construction to electronics, their transformative properties contribute signi cantly, and their presence in steel metallurgy, electrochemistry, radiochemistry, construction, electronic, glass manufacturing, ceramics, pottery, cosmetics, personal care …etc.[1][2][3][4][5][6], enhances functionality and appeal.They are unsung heroes, leaving an indelible mark on human progress.Once upon a time, in the enchanting world of minerals, a rare and captivating zinc silicate mineral known as "willemite" (Zn 2 SiO 4 ) took center stage [1,2].This remarkable gem could be found in the depths of the earth, gracefully forming as a natural wonder, or be carefully synthetized by human.What made willemite truly special was its abundant concentration of zinc, making it a coveted source for the production of valuable zinc metal.But its allure didn't end there, it possessed a magical luminescence that emitted a captivating glow, enchanting all who gazed upon it.[1,3].Its luminescence e ciency was unmatched, adding an ethereal charm to its already vibrant and captivating colors.Beyond its visual splendor, Willemite's physicochemical properties were equally exceptional, showcasing excellent stability and endurance, making it a reliable gem for various industrial applications [1,2].Moreover, its thermal stability ensured that it could withstand the test of high temperatures without losing any of its enchanting attributes.
As the tale of willemite unfolded, it became a prized gem in the realm of minerals, weaving a story of beauty, value, and fascination.In the enchanting realm of mineralogy, willemite took on three distinct polymorphs, named α-Zn 2 SiO 4 , β-Zn 2 SiO 4 , and γ-Zn 2 SiO 4 respectively, each possessing a unique crystalline structure and a mesmerizing array of physical properties [1,[4][5][6]].The rst one (α-Zn 2 SiO 4 ) is the most common stable polymorph, which crystallizes in the trigonal system [1].It is one of the few silicate minerals that have a trigonal-rhombohedral symmetry [7,8] with phenakite structure, that consists of complex tetrahedral framework [1,9] Where, each oxygen ion coordinated to one silicon (Si) and two zinc(Zn) atoms [10], and all atoms are generally in the overall position [9].With this kind of structure this polymorph provides excellent and special optical properties [7,11].On the other hand, the two additional polymorphs are considered metastable phases, able to transforming into the stable αphase at elevated temperatures [6].Among these, the beta-phase (β-Zn 2 SiO 4 ) is less prevalent and crystallizes in the tetragonal system, exhibiting a crystal structure with square symmetry.Typically, opaque and adorned with brown or yellow-brown coloration, β-Zn 2 SiO 4 is deemed metastable and is known to form under speci c conditions, such as high temperatures or pressures [1].In contrast, the γ polymorph is the rarest among the three, featuring a cubic crystal structure that is predominantly stabilized under extremely high temperatures or under speci c synthesis conditions [1].This particular polymorph has received less extensive research attention compared to its α-Zn 2 SiO 4 and β-Zn 2 SiO 4 counterparts [1].
Synthetic willemite has rapidly gained signi cance and widespread adoption as a phosphorescent agent in optoelectronic and lighting devices [1,12,13].Its historical importance is evident as it served as the foundation for the rst-generation uorescent tubes [5,14,15].Particularly, when manganese-doped willemite (Zn 2 SiO 4 :Mn 2+ ) is exposed to speci c wavelengths of light, a fascinating phosphorescence effect emerges.The emitted light's speci c wavelength and intensity are contingent upon the energy levels and electronic transitions within the dopant ions and the crystal structure of willemite [16].
In the present work, we report a new way to synthesize a pure zinc silicate nanopowders, through a modi ed sonochemistry route, using inexpensive and nontoxic raw materials as well as a simpli ed processing methodology.This nanopowder was synthesized at moderate temperature compared to what is reported by several authors [4,[53][54][55].It can be dedicated to numerous applications, particularly, if subjected to subsequent manganese (Mn) doping.The applied procedure is an attractive technique, due to its advantages such as low-processing cost, energy e ciency and high production rate and in particular the ability of this method to carry out doping during synthesis (Mn, Eu, Ce…etc.).After all, this study introduces a novel method for low-temperature synthesis of willemite nanostructures utilizing ultrasonic waves.The process involves ultrasonic treatment at temperatures below 60°C for less than 60 minutes.A similar sonochemical approach was previously employed for wollastonite nanoparticle synthesis [56].

Synthesis
The as-prepared zinc silicate nanopowders were synthetized using a new way which has been reported in our previous paper [56].The experimental protocol was conducted as indicated in Fig. 1 as following: rstly, three clear and stabilized solutions of zinc chloride (ZnCl 2 ), sodium hydroxide (NaOH) and sodium disilicate (Na 2 O.2SiO2) were preprepared using deionized and decarbonated water.This was done by dissolving zinc chloride and sodium disilicate, separately, in given quantity of water so that the ratio zinc to silicon is equal to tow.The rst two solutions (CaCl 2 , NaOH) were added dropwise into third one (including sodium disilicate) under continuous stirring, argon gas (Ar) ux and ultrasonic waves generated by a Hielscher's digital ultrasonic processor (UP400St-400w,24kHz).The pH of the reaction medium was adjusted to 11-12, through the control of the addition (dropwise) of both zinc chloride and 1N sodium hydroxide solutions.Subsequently, after 2 hours of stirring at 40°C, the white precipitates were collected, then vacuum ltered repeatedly, and nally washed (with a mixture of ethanol and distilled water) several times.The resulting precipitates were then washed with pure ethanol and dried at 90°C in a vacuum oven, for 48 hours.The nal product was then calcined rstly at 650°C and t 800°C respectively for 2 h, in an electric mu e furnace (in ambient air).

Characterization techniques
Phase analysis was performed with "Panalytical X'PERT PRO MPD" diffractometer, operating with a Cu Kα radiation (λ = 1.54056Å).The morphology and microstructures of the powders were studied rstly with a scanning electron microscope (SEM, JEOL-7500F) equipped with an energy dispersive X-ray Spectroscopy (EDS) and secondly with a transmission electron microscope (TEM, Philips Tecnai10).Surface area, pore diameter and pore volume were estimated at 77 K according to the Brunauer-Emmett-Teller (BET) method, with a Micromeritics "ASAP 2420" unit.While, thermal analyses were carried out with a "Mettler Toledo TGA/DSC 3+" device.Particle size distribution (PSD) were estimated by "ImageJ" software exploiting TEM or SEM photographs (by measurement of hundreds of particles size).

Thermal analysis
Thermal analysis curves for the main dried powder, are shown in Fig. 2. From the TGA curve, a total weight loss ratio of about 13%, is measured in the temperature range of 25-1100°C.The stepped shape of the TG curve is due to the successive removal of various forms of water present in the powder.In fact, this weight loss spans over three distinct stages.The rst one: 25-200°C, in which the weight loss is 4.0 wt.%, can be attributed to the dehydration process generated by the removal of adsorbed and free water.
The second stage 200-450°C, in which the weight loss is 4.0 wt.%, suggests a dehydration (removal of molecular and crystallized water).The third stage: 500-850°C, the weight loss is 4 wt.%; it is probably, related to the dehydroxylation of the substance.The process of water removal and dehydroxylation tacks place with the simultaneous formation of the crystalline zinc orthosilicate Zn 2 SiO 4 .The DSC curve shows two small exothermic peaks in the temperature range 700-900°C.
The rst at 760°C may be associated with the crystallization of β-Zn 2 SiO 4 ; and the second at about 890°C, may be linked to the formation of α-Zn 2 SiO 4 .

X-Ray diffraction analysis
In order to con rm the purity and crystalline structure of the main products, XRD analyses were carried out.XRD patterns of the main synthesized products and those calcined at different temperatures (650°C,800°C and 900°C) are shown in Fig. 3.We can see that the XRD patterns of the as-synthetized powder, we have initially a broad hump centered around 2θ ~ 28°; it may originate from an amorphous characteristic caused by the long-range disorder, due to a poorly formed crystal phase.
It is very instructive to point out that no XRD impurity peaks from other phases, were detected (zinc oxide, silicon oxide…Etc.);which con rms the high purity of the product.This behavior is completely consistent with Mu-Tsun Tsai ob ndings [6].

Electron microscopy (SEM/TEM) analysis
Powders morphology, before and after calcinations, were investigated using SEM &TEM microscopy.As we can see on the SEM image (Fig. 4-a), the sample before calcination presents a loose uffy cloud-like morphology.The use of microphotography reveals aggregate, characterized by a very rough surface, made of small particles (mainly in the range 50 ~ 100 nm) as can be seen on the corresponding TEM images in Fig. 5-a.These ne grains are elongated as tiny nano-sticks (50 nm length x 100 nm width) as revealed by ImageJ software.These observations are consistent, in terms of the morphology with a previous reports Samigullina [60] and Zhiying Ding [61].
As regards Fig. 4-b, it shows the SEM micrographs for the powders calcined at 650°C.It has a morphological structure similar to that of the as prepared powder.
On the other hand, TEM images (Fig. 5-b) show a mixed morphology; (a combination of the as prepared and that obtained after heating at 650°).However, TEM image (Fig. 5-b), reveals that some ne grains coalesce to give elliptical and larger ones as a result of heating.These particles are in general, not isolated, but clustered.Furthermore, Fig. 4-c presents the SEM micrograph of the powder heat treated at 900°.Inward, we can notice a new morphology (corresponding to the willemite phase).In all cases, SEM and TEM images (Fig. 5-c), reveal clearly, that the powder is less agglomerated, but characterized by a smooth particle surface (when compared to the precedent one).In addition, the particles are separated (distinguishable) and the surface presents fewer pores.In the meanwhile, grain size tends to increase with temperature (500 nm length x 800 nm width).It is worth noting that Quadri's TEM results are in good agreement with our observations [62].
3.4.Energy-Dispersive X-ray Spectroscopy (EDS) analysis Figure 6, and Fig. 7 shows both the Energy Dispersive X-Ray Spectroscopy (EDS) qualitative elemental spectrum evolution and quantitative elemental evolution respectively.First of all, EDS qualitative elemental spectrum (Fig. 6) shows evidently, that all samples (whether calcined or not), are basically made of three elements: O, Zn and Si (these are the major constituents of zinc silicate).Regarding gold (Au), its presence can be attributed to the applied metallization layer.It is crucial to underscore that only these elements are present, within the detection limit of the EDS analysis, and no other elements have been detected.This con rmation validates both the high purity and excellent quality of the synthesized material.
According to the data obtained from the EDS quantitative analysis (Fig. 7), the variations in the atomic percentages of Zn, O, and Si with temperature do not exhibit a consistent pattern.However, as illustrated in Fig. 8, a clear correlation is evident between the Zn/O atomic ratio percentage (Zn/O %) and the heat treatment process.It exhibits a direct proportional relationship with the calcination temperature.This behavior can be ascribed to the process of dehydration and/or dihydroxylation occurring within the powder during the calcination process, resulting in the removal of water molecules that contain an excess of oxygen atoms.
Furthermore, the observed Zn/Si ratio in the as-synthesized powder (Fig. 7) does not exhibit a scienti cally meaningful correlation.Where, the lack of the signi cant correlation in the Zn/Si can be attributed to the precision of the EDS analysis (semi-quantitative technique).As a result, the measured Zn/Si ratio may have some level of uncertainty, making it di cult to establish a precise correlation with other factors or properties.
Figure 9 presents the elemental mapping and chemical composition analysis of the nal synthesized willemite powder.The results demonstrate a uniform distribution of colors corresponding to each element (Zn, Si and O), indicating a homogeneous dispersion of Zn, Si, and O throughout the sample.It is necessary to note that, no im-purities are detected, which con rms both the purity and the quality of the samples.However, it's important to consider that the uniformity of the EDX mapping can also be in uenced by other factors, such as the detection limits of the instrument, the resolution of the mapping technique.Furthermore, it should be noted that this uniformity may also be in uenced by factors such as sample preparation techniques and the surface characteristics of the powder.
Anyway, EDS analysis unequivocally con rms the absence of impurities, such us chloride, carbonates, or nitrides anions, within the limits of the optimal EDS acquisition conditions.This nding provides compelling evidence for the high purity of the nal product "willemite" (Fig. 8).Although EDS may lack precision compared to quantitative analytical methods, it is still valuable and informative in providing qualitative and approximate quantitative information.

BET surface area and pore properties analysis
Figure 10 shows nitrogen adsorption-desorption isotherms of the as-synthetized and calcined powders (starting powder, powder annealed at 650°C and powder annealed at 900°C).
As expected, through these results, it appears that there is a complete agreement with the previously mentioned structural and morphological behaviors.First of all, the as-synthetized powder's physisorption isotherm, belongs to the IV th type (associated with capillary condensation in mesopore structures) [63]; it is also characterized by an H 3 hysteresis loop (IUPAC classi cation) [64-67].
The latter indicates the presence of excessive and slit shaped pores; this results from the coexistence of loosely coherent particles (as presented by TEM & SEM images previously).
Figure 11, shows the evolution of the Barret-Joyner-Halenda (BJH) pore size distribution (PSD) as a function of temperatures, obtained by BET nitrogen adsorption and desorption measurements.
As it can be seen, pore size distribution (PSD) curves do not follow a coherent and ordinary behavior.The curve corresponding to the as-synthesized powder, is characterized by a broad peak centered around 56nm (mesopores region) and a very small one at 3nm (micropores region).This is a family of slit-like pores or empty spaces created during the synthesis of the nanopowders.After the rst heat treatment (650°C), PSD maximum peak, moves towards smaller pore sizes values; getting a value of 17 nm.We have the same behavior with the peak intensity, the number of pores decreases after ring at 650°C.
However, the sharp peak belonging to the micropore zone (3nm), remains unchanged.This could be due to the presence of tiny spaces between the sheets because of the release of water during the dehydration process or free water by evaporation.
Moreover, when the temperature reaches 900°C, the PSD peak; shifts again to higher values (22 nm).while, the intensity of the peak increases, indicating an increase in the amount of mesopores.This can be explained by the stacking/fusing of previous nano-sheets in addition to a phase transformation (βwillemite to α willemite).
As shown in Fig. 12, the speci c surface area (SSA) decreases as the temperature is raised.This is because of the increase in particle size leading to agglomeration, as a result of the heating effect at high temperatures (650-900°C) [68].In fact, we are in the presence of a process consisting in dehydration and dehydroxylation, accompanied by pore clogging, coalescence or open porosity decline, taking place when the annealing temperature is increased [69].As examples, we have the following SAA measurements: 75.6 m 2 g − 1 , 6,1 m 2 g − 1 and 1.69 m 2 g − 1 corresponding to the as synthetized, heat treated at 650°C and heat treated at 900°C, respectively.The same behavior is also observed (Fig. 12) concerning the total pore volume (TPV); the maximum value (TPV ≈ 0,41 cm 3 .g− 1 ) was recorded for the as-synthetized powder, while a smaller one (≈ 4.10 − 5 cm 3 .g− 1 ) was obtained for the powder calcined at high temperature(T = 900°C).
These results have shown that the speci c surface area and pore volume are affected by the heat treatments of samples.

Particle size distributions
Figure 13 shows particle size distributions for the nanopowders heat treated at different temperatures.
Image J software was utilized to make measurements on several hundreds of particles using TEM images.It is found that the average particle size increases with the annealing temperature, due to the nucleation process.At higher temperatures, the latter becomes slow, thus allowing amorphous zinc silicate hydrated nanoparticles to grow.As it can be seen the particle size of willemite (194.03nm) is larger than that of amorphous zinc silicate annealed at 650°C(82.3nm), it is also greater than that of the prepared powder (30.5 nm).Another fact, the growth of nanoparticles depends on the surface reaction.All this means that the temperature helps the coalescence process.At elevated temperatures, the coalescence process slows down, facilitating the growth of amorphous zinc silicate hydrated nanoparticles.The particle size of willemite was determined to be approximately 194.03 nm, which is larger than that of the amorphous zinc silicate annealed at 650°C (estimated at 82.3 nm) and signi cantly greater than the initially prepared powder (estimated at 30.5 nm).This indicates that temperature plays a crucial role in in uencing the nanoparticle growth, and the process is governed by surface reactions, leading to the coalescence of nanoparticles.

Conclusion
A simple, cost effective, environment friendly and very e cient method has been used to synthesize high purity willemite (α-Zn 2 SiO 4 ) nanopowders, via a modi ed wet chemical route.Pure monophasic willemite (α-Zn 2 SiO 4 ) powder was obtained after heat treatment at a 900°C, this temperature remains moderate compared to other techniques.The method is characterized by its ability to inject other reagents, in particular dopants (Mn, Ce, Eu, etc.).Were, dopant salts (e.g., MnCl 2 , Ce(NO 3 ) 3 , Eu(NO 3 ) 3 ) can be added to the reaction medium along with the precursors.XRD and EDS analysis indicate the high purity and the good homogeneity of the nal willemite nanopowder.On the other hand, it is found that heat treatment has an important in uence on the morphology of the samples, indeed, as the temperature increases, assynthetized powders exhibit several micro-structural changes; this is con rmed by SEM and TEM microscopy.Furthermore, BET textural analysis reveals an inverse proportionality between the speci c surface area (SAA) and the calcination temperature.We can add that the particle size diameter (PSD) changes with temperature through the coalescence process.
Finally, we conclude that the method in question, can be used to synthesize pure massive zinc silicate materials on a large scale at a lower cost, which opens up an interesting perspective for use in several industrial elds such as ceramics, paints, plastics, biomaterials, composites, and sensors. Process overview for powder synthesis Page 16/25

Figure 10 EvolutionFigure 11 Evolution of the pore volume distribution with the calcination temperature Figure 12 Evolution
Figure 10