Zirconium Nanostructures Obtained from Anodic Synthesis By-Products and Their Potential Use in PVA-Based Coatings

Abstract

Nanostructures obtained as a by-product of the electrochemical synthesis of ZrO₂ nanotube membranes have scarcely received any attention despite their enormous potential. This is mainly due to their size properties, morphology, and composition. In the present work, these nanostructures are characterized, and their potential application as an additive in PVA-based coatings is analyzed. The characterization was performed by X-ray fluorescence, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and X-ray diffraction. The results showed that the nanostructures consist of tubular fragments generated during the formation of the ZrO₂ membrane, with a dimension of 626.74 nm in width, a length of 1906.39 nm, and a clear cubic structure. The ZrO₂-PVA coating, which is prepared by using the spin coating technique, presented a uniform and homogenous particle distribution, which was later confirmed by Fourier transform infrared spectroscopy, scanning electron microscopy, and atomic force microscopy. The optical transparency and thermal resistance were evaluated through UV-Vis spectroscopy and thermogravimetric analysis, showing that the incorporation of ZrO₂ as an additive improved its UV absorption properties and thermal stability during the pyrolysis stage. The results suggest that the ZrO₂ nanostructures enhance the thermal and protective properties of the PVA-based coatings by acting as physical barriers and stabilizers within the polymer matrix.

1. Introduction

During the Zr anodic synthesis process to generate nanostructured ZrO₂ surfaces (nanotubes, nanopeaks, nanopores), nanostructured by-products are also obtained, which accumulate and/or precipitate in the electrolyte solution. These by-products, which are generally ignored and thrown away, have received little to no research attention, mainly due to the predominant focus on the characteristics of synthesized nanostructures. The composition of such nanostructures has been deduced from the formation mechanisms of ZrO₂ nanostructures, suggesting they might have valuable applications [1,2,3].

The physicochemical and morphological properties of these nanostructures could significantly impact fields such as catalysis, biomaterials and coatings. In the coating field, the use of ZrO₂ as an additive is known to improve its anti-corrosive properties abrasion resistance, and thermal stability [4,5,6]. Therefore, the use of these nanostructures could open up more opportunities for improving materials protection.

In this context, one of the materials that could most notably benefit from ZrO₂ as an additive is polyvinyl alcohol (PVA) coating. The use of PVA as a coating material has great advantages, such as being labeled a green material, as it is environmentally friendly and has a low ecological impact [7,8]. However, when used as a coating, PVA shows some limitations, such as low thermal resistance and a tendency to wear out in extreme conditions [9,10]. To address these limitations, the incorporation of ZrO₂ into PVA has been shown to improve mechanical properties and thermal resistance, while reducing polymer degradation, as reported in previous studies [11,12,13].

In the present work, a detailed characterization of the composition of the generated nanostructures, as a by-product of the anodic synthesis of Zr surfaces, to produce ZrO₂ nanotube membranes (ZrO₂ NTs) was carried out, as well as an evaluation of their potential as an additive to PVA coatings. Through the characterization, we propose an innovative and sustainable application for these often-overlooked Zr nanostructures, which could offer a significant improvement in the thermal and protective properties of the coating, thus contributing to the development of more efficient and longer-lasting materials.

2. Materials and Methods

2.1. Synthesis and Zr Nanostructure Recovery

The obtention of Zr nanostructures was collected from a removable membrane of a previously published ZrO₂ NTs [3]. For its synthesis, a high-purity Zr sheet (99.1%, Baoji Hanz Metal Material Co., Ltd., Baoji, China) with a 0.05 mm thickness and a mirror-like finish was used. The sheet was cut into 3 × 5 cm rectangles. Furthermore, the samples underwent a cleaning process with distilled water and ethanol for 15 min each, followed by a 24 h drying period.

For the anodic synthesis, the electrolyte consisted of a mixture of 1 M (NH₄)₂SO₄ and 0.75 M NH₄F in distilled water. The process was performed in a two-electrode cell, where the Zr sheet was used as a working electrode and the Pt electrode acted as a counter electrode. Then, 40 mL of the electrolyte solution was added into the cell and the process was performed using a power source (DC power supply 305D, XPOWER, Singapore, Singapore), which supplied a constant voltage of 25 V for 20 min at room temperature.

After this time, the electrodes were unmounted from the cell, and the electrolyte along with its by-products from the synthesis were recovered. To separate the by-products from the electrolyte synthesis, a vacuum filtration system was performed by using a Whatmann No. 1 filter (Canterbury, UK). The by-products resting in the filter were then washed with deionized water until neutralized. A total of 0.1907 ± 0.008 g of dried, filtered by-products were obtained and stored in a desiccator until its characterization and application.

2.2. Coating Preparation

The PVA coating was prepared using the drop-casting and spinning technique [14,15,16]. In order to do so, 10 g of PVA were dissolved in 100 mL of deionized water at 80 °C, leaving it to cool to room temperature. In an Eppendorf tube, a ZrO₂ suspension was prepared, adding 0.5 g of the ZrO₂ nanostructures with 1 mL of deionized water. The suspension underwent a sonication process with a probe (OMNI SONIC RUPTOR 400, Kennesaw, GA, USA; 50% power, 50 Hz) for 10 min to secure a homogenous dispersion of the ZrO₂ nanostructures. Lastly, 9 mL of the PVA solution and 1 mL of the ZrO₂ suspension were mixed for 5 min using a vortex until a colloidal ZrO₂-PVA solution was obtained.

In total, 0.04 g of the colloidal ZrO₂-PVA solution was deposited onto a 436 stainless steel sheet with × 1 cm dimensions (Table S1) using a micropipette. To achieve a uniform coating, the spin coating technique was used at 1800 rpm for 30 s. Finally, the coated sheets were left in a desiccator for 24 h until its characterization.

2.3. Elemental Composition of ZrO₂ Nanostructure Analysis

The elemental composition and oxides of the ZrO₂ nanostructures were determined by X-ray fluorescence spectroscopy (XRF). The XRF analysis was performed using a Shimadzu spectrometer (Kyoto, Japan) equipped with a Rh X-ray tube at a voltage of 30 kV. Prior to analysis, 0.5 g of the ZrO₂ nanostructures were placed in a 32 mm sample holder for subsequent measurements.

2.4. Morphological Characterization and Chemical Composition

The morphology, distribution, and chemical composition of the ZrO₂ nanostructures were characterized using scanning electron microscopy (SEM) coupled to an energy dispersive spectroscopy (EDS) system (JEOL-TESCAN-BRUCKER; Brno, Czech Republic). The samples were prepared by manually depositing the ZrO₂ nanostructures on a carbon ribbon with a lab spatula. For capturing the micrographs, an accelerating voltage of 15 kV was used, with a working distance of 10 mm, and a secondary electron detector (BES). Chemical composition analysis was carried out with an acquisition time of 1.0 s, and the results were processed through Brucker qualitative and quantitative component classification software (Esprit 3.0, Brucker, Billerica, MA, USA).

2.5. Morphological Characterization and Crystallinity

The morphology and size of the ZrO₂ nanostructures were characterized through transmission electronic microscopy (TEM) by using a JEOL 2010 (Tokyo, Japan) microscope operating at an accelerating voltage of 200 kV. The samples were prepared by depositing a drop of ZrO₂ colloid into a carbon-coated Cu cell, followed by a drying process at room temperature in a desiccator. The ZrO₂ nanostructures’ crystalline structure was studied through selected area electron diffraction (SAED), obtained during the TEM characterization.

The crystalline structure of ZrO₂ nanostructures was determined through X-ray diffraction (XRD). The measurements were performed using a BRUKER D8 ADVANCE (Billerica, MA, USA) diffractometer equipped with a Cu-Kα (λ₁ = 1.54059 y λ₂ = 1.54441) radiation source operating at 40 kV and 30 mA. The diffraction patterns were registered at a 2θ range of 5° to 100° with a step size of 0.01° and a time measurement of 1 s per step. The crystallographic phases were identified by comparing diffraction patterns obtained by the information provided by the Joint Committee on Powder Diffraction Standards (JCPDS), using the JCPDS 98-005-3998 crystallographic chart.

2.6. Transparency Evaluation

In order to evaluate the ZrO₂-PVA coating transparency, the coating was manually removed with a spatula. The obtained film was characterized through UV-Vis spectroscopy using an integrating sphere with a 6 mm opening in a Shimadzu 2600 (Kyoto, Japan) spectrophotometer in transmittance mode. The scan was performed with a wavelength between 200 nm and 800 nm with a scannThe speed of 0.5 nm/s. the results were compared with a film made purely with PVA.

2.7. ZrO₂ Incorporation in the Polymeric Matrix

The presence of ZrO₂ in the polymeric matrix and the functional groups of PVA were analyzed through Fourier Transform Infrared (FTIR) spectroscopy, using PerkinElmer Frontier (Waltham, MA, USA) equipment. To run the analysis, a drop of ZrO₂-PVA solution was poured into the equipment’s attenuated total reflection (ATR) accessory, and the water was left to evaporate. Furthermore, an analysis with a range of 4000 to 400 cm⁻¹ and a scanning speed of 0.1 cm⁻¹ was performed.

2.8. Quality and Surface Roughness of the Coating

The topography of the ZrO₂ coating was compared to that of the uncoated stainless steel sample through atomic force microscopy (AFM) by using a Park Systems (Suwon, Republic of Korea) NX10 microscope. The pictures were taken using the no contact mode (NCM) at a scanning frequency of 0.8 Hz, c covering an area of 5 µm × 5 µm.

2.9. Thermic Resistance

The thermic resistance of ZrO₂-PVA was compared to a pure PVA coating by thermogravimetric analysis (TGA), using a PerkinElmer (Waltham, MA, USA) STA 6000 instrument. Approximately 5 mg of the coating was placed in an Al₂O₃ crucible. The analysis was performed at a flow of 19.5 mL/min, with a temperature range of 30 °C to 800 °C, with a ramp of 10 °C/min.

3. Results and Discussion

3.1. Characterization of the ZrO₂ Nanostructures

The chemical composition of the recovered ZrO₂ nanostructures was characterized by XRF before and after undergoing a filtration process. The intention behind this study is to determine the origin of these by-products and to confirm their cleanliness after the filtration process.

In the elemental analysis (Figure 1a), before being filtered, the composition of the ZrO₂ nanostructures consists mainly of S, Zr, and P, aside from minor remnants of the alloy (Hf, Ca, and Cs). Likewise, the oxide analysis showed mainly SO₃, ZrO_2, and HfO₂ (Figure 1b). This composition was expected and originates from complexes such as [ZrF₆]²⁻, which are created during the formation of pores or tubes in Zr. As was explained in our previous work, ammonium sulfate and ammonium fluoride are crucial in the catchment and precipitation of Zr, thus the creation of nanotubes [3].

On the other hand, the composition after being filtered (Figure 1c,d), shows the absence of S in the ZrO₂ nanostructures. This is due to the S originating from the remaining ammonium sulfate dissolved in water during the washing process, leaving particles in the main Zr composition.

By the SEM characterization, the shape and distribution of the ZrO₂ nanostructures were determined (Figure 2a–c). In the same way, due to the mechanism described in our previous work, the ZrO₂ nanostructure consists mainly of bars. These bars are tube walls that have been fragmented during the anodic process (rupture and growth of the oxide layer).

Based on what was captured in the micrograph (Figure 2a), the bar distribution tends to be uniform, with a width of 626.74 nm and a length of 1906.39 nm (Figure 2b,c). The size obtained solely from the synthesis is within the range of the formed structures found in processes such as chemical conversion coating [17,18,19]. The EDS characterization determined the composition by area and by mapping a zone with ZrO₂ bars (Figure 2d–g). The bars show a Zr, O, and F composition, conjugating with the XRF analysis.

The TEM characterization confirmed that the by-products of the ZrO₂ membrane synthesis are fragmented tubes, showing with greater detail the shape of the tube and wall thickness (Figure 3a). The micrographs captured by HR-TEM, SAED, and XRF (Figure 3b–f) all show that the nanostructures are made up of a cubic structure and detected an interplanar spacing of 0.23 nm in correspondence to the plane (200) [20]. Interestingly, this is the crystallinity of ZrO₂ that requires the most energy in order to be obtained. However, due to the electrolyte and electrochemical parameters, the obtaining of this crystallinity was already shown in previous research [3,21].

3.2. Coating Characterization

3.2.1. Distribution and Topography

The incorporation of ZrO₂ nanostructures into the polymeric PVA matrix was characterized by SEM-EDS. When comparing the coated and uncoated micrographs, the incorporation of the ZrO₂-PVA coating is noteworthy (Figure 3a,b,d,e). Interestingly, the ZrO₂ distribution is uniform throughout the surface, and when seen up close (Figure 3e,f), the union is appreciated in different tube fragments, achieving a flower form. This correct ZrO₂ distribution has the advantage of improving its mechanic properties, energy storage, and dielectric response [22,23]. It is important to mention that the coating quality is maintained throughout the stainless steel area, which confirms that the quantity used and spin parameters were the correct ones for its application.

The AFM topographic characterization also highlights the differences in uncoated (Figure 4g,h) and coated (Figure 4i,j) stainless steel. At a macroscopic level, the coated surface appears visually smoother due to the homogeneous distribution of the polymeric matrix. However, the average surface roughness (Ra) values were 106.25 nm and 182.37 nm for the uncoated and coated surfaces, respectively. This apparent contradiction is explained by the influence of nanoscale surface features introduced by the incorporation of ZrO₂ structures. Although the polymer provides a uniform covering, the dispersed ZrO₂ structures increase the vertical distance between peaks and valleys on the nanoscale, thereby elevating the Ra value [24]. This behavior is consistent with the presence of ZrO₂ observed in SEM images (Figure 4d,e), where the particles are distributed across the coated surface and contribute to a more complex topography within the polymeric matrix [25].

3.2.2. Coating Properties

A visual inspection of the coatings reveals that both pure PVA and ZrO₂-PVA coatings exhibit transparency (Figure 5a). The transmittance of these coatings was evaluated by UV-Vis spectroscopy (Figure 5b). In the UV region (200-400 nm), both coatings show low transmittance, indicating a high UV radiation absorption by the polymeric matrix. Notably, the ZrO₂-PVA coating shows a lower increase in transmittance between 300 and 400 nm, suggesting that the incorporation of ZrO₂ into the PVA matrix enhances UV absorption [26,27]. This is further supported by the increase in the optical band gap (Eg) to 5.60 eV with the addition of ZrO₂ (Figure 5b).

In the visible region (400–700 nm), the transmittance of both coatings increases significantly; however, the ZrO₂-PVA coating shows a slightly lower transmittance, indicating that the addition of ZrO₂ reduces the transparency of the coating [28].

In the region closer to the infrared (700–800 nm), both coatings achieve approximately 100% transmittance. These observations demonstrate that the addition of ZrO₂ to the PVA matrix primarily improves UV absorption. This behavior is consistent with the expected performance of metal oxides, such as ZnO₂, which is commonly used as a UV blocker. In particular, nanoparticulate aggregates in sunscreens generate a protective layer against UV radiation, remaining invisible to the human eye [29]. Other potential applications for these coatings, where UV absorption and transparency are critical, include optical devices and packaging materials [30,31].

FTIR characterization confirmed the integration of ZrO₂ into the PVA matrix (Figure 5c). The spectrum of ZrO₂ shows a band at 404 cm⁻¹, corresponding to Zr-O stretching. In the case of PVA, the characteristic bands of this polymer were observed, such as those located at 3310 cm⁻¹, 2940 cm^−1, and 2908 cm⁻¹, corresponding to O-H and C-H stretching, respectively [32,33]. Additionally, bands were identified at 1832 cm⁻¹, assigned to C=O stretches; at 1424 cm⁻¹, assigned to -CH₂ buckling; and at 1086 cm⁻¹, corresponding to C-O-C stretches. In the spectrum of the ZrO₂-PVA compound, a merging of the characteristic bands of the individual spectra of PVA and ZrO₂ was observed.

3.2.3. Thermal Stability

The thermal stability and decomposition behavior of PVA and ZrO₂-PVA coatings were studied by thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). The TGA and DTG curves of pure PVA coating (Figure 5d) and ZrO₂-PVA composite coating (Figure 5e) reveal distinctive thermal degradation patterns.

The TGA curve of pure PVA shows an initial weight loss of around 100 °C, attributed to the evaporation of absorbed moisture. Between 250 °C and 400 °C, a significant weight loss is observed, corresponding to the decomposition of the PVA main chain. This process involves the elimination of hydroxyl groups and depolymerization of the PVA chains. The maximum rate of decomposition, as evidenced by the DTG peak, occurs at around 340 °C [34,35].

In contrast, the TGA curve of the ZrO₂-PVA composite coating shows a similar initial weight loss, also due to moisture evaporation. However, the main thermal degradation point shifts towards slightly lower temperatures (315 °C), indicating a significant influence of the ZrO₂ particles on the initial decomposition of the PVA. Despite this initial shift in the pyrolysis stage, the presence of ZrO₂ shifts the final pyrolysis stage (445 °C) and decreases the magnitude of weight loss in the final stages of thermal degradation, thus suggesting that these nanoparticles act as thermal barriers, providing additional stability to the PVA matrix [36,37].

Furthermore, the residual mass of the ZrO₂-PVA compound at 800 °C is significantly higher (~20%) compared to pure PVA, which is due to the presence of thermally stable ZrO₂ particles remaining after complete degradation of the organic matrix. These observations are in agreement with the expected behavior of inorganic nanomaterials, which improve the thermal performance of polymeric matrices [38]. The incorporation of ZrO₂ into the PVA matrix not only improves the thermal stability of the coating but also modifies its decomposition mechanism, as observed in the broader and more complex DTG profile. These findings suggest that ZrO₂ nanoparticles act as physical barriers and stabilizers in the polymer matrix, which could be beneficial for applications requiring a high-temperature performance.

The improved thermal stability of the ZrO₂-PVA compound coating highlights its potential for advanced applications, especially in high-temperature environments where pure PVA may not be enough. The ceramic nature of ZrO₂ contributes to a higher residual weight, providing structural integrity even after the degradation of the polymeric component. This property could improve the durability and longevity of the coating, making it suitable for use in extreme thermal conditions.

4. Conclusions

In this study, we successfully incorporated ZrO₂ nanostructures, specifically nanotube fragments with cubic crystallinity, into PVA-based coatings for application on 436 stainless steel. These nanostructures, obtained via anodic synthesis, exhibited average dimensions of 626.74 nm in width and 1906.39 nm in length. The resulting composite coating preserved the optical transparency of pure PVA while enhancing its UV absorption properties (Eg = 5.60 eV) and improving thermal stability, particularly in the final pyrolysis stage. SEM and AFM analyses confirmed a homogeneous dispersion of the nanostructures, correlating with the observed enhancements. The novelty of this work lies in the use of ZrO₂ nanotube fragments as an additive to PVA, demonstrating for the first time their dual role in improving UV shielding and thermal resistance without compromising transparency. These findings position ZrO₂ nanostructures as promising functional additives for developing advanced polymer coatings for stainless steel protection.