Synthesis and application of WO3/PVA nano composite for antibacterial and thermal insulation

Muhammad Amir; Muhammad Sajid Ali Asghar

doi:10.1088/2053-1591/ad8a1f

1. Introduction

Tungsten trioxide, denoted as WO₃, stands as a wide band gap transition-metal oxide semiconductor, exhibiting a range of distinctive properties that hold significance in advanced technological applications [1]. WO₃ nanomaterials are promising for smart windows, allowing customized control of light and thermal transmission to reduce energy consumption [2]. WO₃ nanmaterails used to develop cost-effective, durable, multifunctional, and environmentally friendly smart window materials [3]. The history of WO₃ film development, inspired by major corporations, has evolved to include applications like automobile mirrors and gained popularity in the late 1970s [4].

This naturally occurring oxide, known as WO₃, boasts enhanced chemical stability over an extended temperature range, is less harmful to living organisms, and aligns with eco-friendly principles [4, 5]. WO₃ is among the few oxides capable of forming crystals with diverse crystallographic structures [6]. The crystal structure of pure tungsten trioxide is influenced by temperature, with each structure possessing unique characteristics. The introduction of other ions, such as Na, K, or Fe, through doping, can stabilize different crystal structures [7, 8]. WO₃ is a notable inorganic photochromic material [9], though challenges like slight color variations, prolonged reaction times, and extended bleaching periods exist. Various strategies have been proposed to enhance WO₃ photochromic films [5]. WO₃ offers advantages as a wide band gap transition metallic oxide semiconductor, including environmental friendliness, thermal stability, and unique optical and electrical properties [10, 11]. WO₃ nanoparticles also countered environmental pollution, especially water contamination, is a growing concern, affecting 55% of the global population. CO gas poses health risks, and WO₃ nanomaterial's can detect CO concentrations to prevent health issues [12]. Inorganic nanoparticles, known for their small size and high reactivity, hold promise for smart material development. Materials like tungsten oxide, zinc oxide, aluminum oxide, cerium oxide, and copper oxide in nano sizes exhibit novel properties due to their increased surface areas [13]. The field of 'smart clothing' integrates technology into everyday wear, with smart color-changing fibers finding applications in wearable displays and visual sensors. Nanoparticles like silver is toxic, titanium dioxide stands out for its biocompatibility and stability, finding applications in antibacterial and self-cleaning materials [14]. Metal and metal oxide nanoparticles, such as WO₃, have antibacterial properties with potential applications in healthcare and food preparation [15]. WO₃ nanomaterials have diverse applications, from energy-efficient windows to healthcare and environmental solutions, making them a valuable area of research and development. Modifications in its crystal structures and morphologies have been scientifically proven to enhance its light responsiveness. As an illustrative example, two-dimensional WO₃ PVP Nano sheets were employed to create a composite film [16]. When exposed to ultraviolet light for 125 s, this film exhibits a striking light blue hue. The successful application of photochromic materials, like WO₃ nanoparticles-coated glass, in smart windows has tremendous potential for energy conservation and customer satisfaction [17]. In the context of the model house depicted, this technology significantly lowered indoor temperatures by approximately 15 °C, resulting in a 20% reduction in electricity consumption for everyday household electrical and electronic devices [2–4]. Inorganic antibacterial materials, like monoclinic tungsten oxide Nano rods (WO_2.72) with diameters between 1.1nm and 1.3nm, are promising for eradicating bacteria in human cells. WO₃ treatment alters bacterial cell structures [18]. Researchers have explored various methods, such as chemical processes and sol–gel techniques, to synthesize and characterize tungsten trioxide nanoparticles for their antibacterial and thermal insulation properties [19]. Photochromic fibers are gaining attention for military safety gear and textiles. Inorganic tungsten trioxide (WO₃) is a cost-effective and stable photochromic material. A simple dip-coating method is used to make continuous photochromic fibers with WO₃ and polyvinyl alcohol (PVA) as a matrix (WO₃/PVA composites) [20]. These fibers change color rapidly and reversibly from light yellowish-green to dark blue when exposed to UV and infrared light. They can be produced at various scales, maintain mechanical strength, and are washable, making them promising for photochromic textiles [21]. In nanoparticle production, Ultraviolet-visible Spectroscopy is crucial for understanding optical characteristics. Nanoparticles absorb visible light, elevating electrons to higher energy levels, and emit photons when these electrons return to their base energy level [22].The size, shape, and production environment affect the nanoparticles' surface plasmon resonance. Tungsten nanoparticles show strong absorption at 350nm and remain stable for 24 h, as confirmed by UV–vis spectra [23]. It was established that synthesis methods have influenced on the characteristics of metal oxide nanoparticles, including WO₃. The Chemical Precipitation method has advantageous over other methods as high purity products can be synthesized with narrow particle size distribution, chemical homogeneity and low environmental pollution and it is an easy process and economical to do [24].

To fill the gap, this research is focused on chemical precipitation technique for synthesis and characteristic of WO₃ nanoparticles and investigates antibacterial and thermal insulation properties implication on developed different concentration of nano particles glass coated samples.

2. Materials and method

2.1. Raw materials for WO₃ nanoparticles synthesis

Sodium Tungstate di-hydrate ( ${{\rm{Na}}}_{2}{{\rm{WO}}}_{4}.$ $2{{\rm{H}}}_{2}$ O), (99% pure) used as a precursor salt product of Sigma Aldrich and Hydrochloric Acid (HCL 37%) as a reducing agent. De-ionized water used for preparation of the aqueous solution and for washing of powders and equipment to remove unwanted materials from it.

2.2. Methodology

Methods for the production of ${{\rm{WO}}}_{3}$ nanoparticles produces different morphologies and phase due change in ions concentration, pH value and change of parameters [2]. Chemical precipitation method used for the synthesis of ${{\rm{WO}}}_{3}$ nanoparticles by using precursor salt of tungsten and HCL as a reducing agent.

2.2.1. Chemical precipitation method

In this procedure, a magnetic stirrer operated at 1000 rpm at 70 °C temperature, using a molar ratio of 1:1 for sodium tungstate di-hydrate and hydrochloric acid, respectively. One Mole aqueous solution of sodium tungstate di-hydrate (Na₂WO₄·2H₂O) was prepared and add 10 ml of 1Mole hydrochloric acid drop by drop into the Na₂WO₄·2H₂O solution at 70 °C and continuous stirring is crucial throughout the titration reaction. It was shown in figure 1 that after 6 min, the solution's color changes from white to light green, and after 10 min, it turns into a light yellowish-green hue. At the 60 min mark, it transforms into a dark yellowish-green color, which persists until the 70 min indicating the formation of nano particles. This is confirmed by the presence of yellowish-green powder settled at the bottom of the solution. The powders are then washed with deionized water three times to eliminate unwanted materials.

Figure 1. Refer to the following caption and surrounding text. — **Figure 1.** Chemical precipitation reaction.
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Sodium tungstate di-hydrate (Na₂WO₄·2H₂O) is reduced by HCl, yielding hydrated WO₃ nanoparticles as depicted in equation [25]:

$\begin{eqnarray}&&{{\rm{Na}}}_{2}{{\rm{WO}}}_{4}\cdot 2{{\rm{H}}}_{2}{\rm{O}}\,+\,2{\rm{HCl}}\,+\,{{\rm{H}}}_{2}{\rm{O}}\,\to \,{{\rm{H}}}_{2}{{\rm{WO}}}_{4}\cdot 3{{\rm{H}}}_{2}{\rm{O}}\,+\,2{\rm{NaCl}}\end{eqnarray} \tag{ 1 }$

$\begin{eqnarray}&&{{\rm{H}}}_{2}{{\rm{WO}}}_{4}\cdot 3{{\rm{H}}}_{2}{\rm{O}}\,\to \,4{{\rm{H}}}_{2}{\rm{O}}\,+\,{{\rm{WO}}}_{3}\end{eqnarray} \tag{ 2 }$

After washing, filter the solution using filter paper; see figure 2(b). It was shown in figure 3 that material dried in an oven at 70 °C to remove the 24 percent water content during chemical reactions calculated by stoichiometric technique. This drying process takes 6 h to complete. After drying, use a mortar and pestle (see figure 2(c)) to break the agglomerated powders into fine powders (see figure 2(d)). Perform calcination of the powders at 400 °C holding temperature for 2 h at 10 °C min⁻¹.

Figure 2. Refer to the following caption and surrounding text. — **Figure 2.** Filtration of WO₃ nanoparticles.
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Figure 3. Refer to the following caption and surrounding text. — **Figure 3.** Drying and calcination cycles.
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2.2.2. Preparation Of $W{O}_{3}$ /PVA Coating

A 50 ml solution of 5g or 10% of PVA was prepared in de-ionized water at 80 °C by using strong magnetic stirrer for 3 h. Prepared four different concentration of ${{\rm{WO}}}_{3}$ /PVA solution by adding 1%, 3%, 4% and 7% ${{\rm{WO}}}_{3}$ nanoparticles again strongly stirrer for an hour to get a homogenous solution, see figure 4.

Figure 4. Refer to the following caption and surrounding text. — **Figure 4.** $W{O}_{3}$ /PVA solution (a) 1% $W{O}_{3},$ (b) 3% $W{O}_{3},$ (c) 5% $W{O}_{3}$ and (d) 7% $W{O}_{3}$ .
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**Figure 4.** $W{O}_{3}$ /PVA solution (a) 1% $W{O}_{3},$ (b) 3% $W{O}_{3},$ (c) 5% $W{O}_{3}$ and (d) 7% $W{O}_{3}$ .
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2.2.3. Surface cleaning

A square shape sodium silicate glasses of 4 × 4 inches in cross section and 2 mm in thickness used as a coating substrate. Glass surface cleaned with ethanol, acetone and distilled water to contamination from its surface such as oil, grease etc. Coating of prepared different concentrations of ${{\rm{WO}}}_{3}$ /PVA solution was done by using doctor blade of thickness 50 μm, see figure 5. Prepared samples had coating thickness of 50 μm but different concentration of ${{\rm{WO}}}_{3}$ nanoparticles; 1%, 3%, 5% and 7%.

Figure 5. Refer to the following caption and surrounding text. — **Figure 5.** Procedure of coating on glass substrate (a) Applicator, (b) Glass Substrate, (c) Glass after coating and d) rack of coated samples.
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2.3. Methods of characterization

To find the characteristics of WO₃ nanoparticles, performs XRD, SEM, DSC and TGA. Thermal insulation and antibacterial test were conducted on WO₃ nano (1%, 3%, 4% and 7%) coated samples.

2.3.1. X-ray diffraction analysis (XRD)

X-ray diffraction (x pert PRO, Panalytical Company, Netherlands) is a powerful analytical technique based on Bragg's law to determine the structure of crystalline materials. X-rays are produced by bombarding a copper metal with high-energy electrons in an x-ray tube. When electrons collide with the copper metal, they decelerate rapidly, resulting in the emission of x-rays. The generated x-rays are directed toward the crystalline sample. When x-rays hit the WO₃ sample, they can be scattered by the electrons in the atoms of the crystal lattice. The diffracted x-rays are collected by a detector, typically a scintillation counter or a charge-coupled device (CCD). As the angle θ changes, the detector measures the intensity of the diffracted x-rays.

2.3.2. Scanning electron microscopy (SEM)

SEM (Jeol, JSM-6380A, Japan) was used a focused beam of electrons, typically generated by a tungsten filament electron gun. These electrons are accelerated towards the sample. When the electron beam hits the sample surface, secondary electrons are emitted from the surface and are primarily used for high resolution imaging, providing topographic information. The emitted signals are collected by the ETD detectors for imaging and analysis of size, shape and morphology of WO₃ NPs. For SEM sample preparation by placing a small amount of WO₃ powder into a micro centrifuge tube. Fill the centrifuge tube with ethanol. Proper dispersion of the WO₃ powder by employed ultra-sonication. Take an aluminum SEM stub and apply double-sided conductive carbon tape to it. Apply 2-3 drops of the prepared sample onto the carbon tape, allowing it to dry, and subsequently capture SEM images.

2.3.3. DSC and TGA

Thermal Analyzer (TA Instrument, Model STD-Q600, USA) was used to measure the thermal properties of WO₃ sample as a function of temperature and time. DSC focuses on measuring heat flow related to thermal transitions, while TGA measures changes in weight as a function of temperature. Both techniques are complementary and used together to provide a comprehensive understanding of a WO₃ thermal properties.

2.3.4. Thermal insulation

Hot plate technique (ACE Model MS7-H550-Pro, Germany) was employed to measure the thermal insulation in terms of temperature difference of a coating on a glass samples. A digital thermometer was captured the upper surface temperature of the glass from three different points P-1, P-2 and P-3 and then takes an average, while the bottom temperature remained unchanged at 40 °C.

2.3.5. Antibacterial testing

The antibacterial effectiveness of ${{\rm{WO}}}_{3}$ nanoparticles was assessed using the agar well diffusion method. Gram-positive (staphylococcus aureus (Sa)) and Gram-negative bacteria (Escherichia coli (Ec)) were put in a petri dish with varied amounts of the powder and cultured for one day at 38 °C to test the antibacterial effect of the newly generated ${{\rm{WO}}}_{3}\,$ powder throughout the course of a 24 h incubation period. The minimum inhibitory concentration (MIC) of WO₃ nano particles were calculated by the help of most common disc diffusion method. For the test the WO₃ nano powder was first weight and mix in a PVA solution and sonicated for 30 min for better separation. Measure the required amounts of nano solution and agar for aseptic testing. In this testing, the available local agar containing 95 CFU/ml of bacterial cells as a medium to culture used. The Petri dishes having the both WO₃ nano solution and microbes were incubated under optimized conditions. For antibacterial activities, WO₃ nano solution have optimized the one-day incubation period at 38 °C for the growth of bacteria in an incubator. After incubation period the zone diameter around each tested nano solution shows the inhibition of growth of the bacterial strain evaluated by the adjoining millimeter.

3. Results and discussion

3.1. X-ray diffraction analysis (XRD)

XRD diffraction pattern, peaks [22, 26] manifest as curves along the axis of their respective characteristic wavelengths on the x-axis, while the y-axis represents the number of counts.

The diffraction peaks will be compared with those of the standard JCPDS No. 830–950 for WO₃ [27], as shown in figure 6. The mean size of the crystalline WO₃ nanoparticles was calculated by using the Debye Scherer formula [28, 29].

Figure 6. Refer to the following caption and surrounding text. — **Figure 6.** XRD spectrum of WO₃ nanoparticles.
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XRD spectrum of nanomaterials confirm the formation of the ${{\rm{WO}}}_{3}$ compound but some stoichiometry compound of ${{\rm{WO}}}_{3}$ form such as ${{\rm{W}}}_{25}{{\rm{O}}}_{73}$ which is confirm by XRD spectrum with previous studies [4, 6] and matching with standard spectrum of these compounds, both the compounds have different crystal structures such ${{\rm{WO}}}_{3}$ consists of tetragonal crystal structure while ${{\rm{W}}}_{25}{{\rm{O}}}_{73}$ consists of monoclinic crystal structure. These changes in crystal structure are due to the doping of Na ions into ${{\rm{W}}}_{25}{{\rm{O}}}_{73}$ which alters the crystal structure. This behavior was studied previously when excel Na ions will be present and it will be doped in ${{\rm{WO}}}_{3}$ to form its stoichiometry compounds depends upon the concentration of Na ions without Na ions. It will remains as tetragonal crystal structure but increasing in Na ions concentration could be monoclinic, hexagonal and triclinic crystal structure in this experiment. Sodium tungstate di hydrates precursor salt to form ${{\rm{WO}}}_{3}$ nanoparticles so the sodium is present in this solution so that small amount of ${{\rm{W}}}_{25}{{\rm{O}}}_{73}$ compound formed.

3.2. Scanning electron microscopy (SEM)

It was revealed from Scanning electron microscopy (SEM) image figures 7(a)–(d) that the morphology of synthesis ${{\rm{WO}}}_{3}$ nanoparticles at different magnification; black portion reflects carbon tap and grey portion is ${{\rm{WO}}}_{3}$ and showed that round particle morphology formed but the particles is in agglomerated form so that the size is very large 10–100 nm due to agglomerated form. The crystalline size was measured from peak list data which were confirmed the formation of nanoparticles of ${{\rm{WO}}}_{3}$ because the average crystalline size is 20.672 nm.

Figure 7. Refer to the following caption and surrounding text. — **Figure 7.** SEM image of $W{O}_{3}$ .
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**Figure 7.** SEM image of $W{O}_{3}$ .
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The laser particle analyzer was used for size distribution of WO₃ nano particles which is shown in figure 8. From figure 8, the significant coefficient of determination (R² = 0.7874) evident the figure 7 findings.

Figure 8. Refer to the following caption and surrounding text. — **Figure 8.** Particle size distribution.
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3.3. DSC and TGA

Figure 9 shows DSC and TGA curve of $W{O}_{3}.$ $W{O}_{3}$ TGA curve is continuously losing its weight due to removal of surface water and bonded water up to 500 °C with a heating rate of 10 °C min⁻¹ similar to previous studies [26, 30] and from DCS curve calculated the enthalpies value of fusion and crystallization –ΔHf = 117.82J and ΔHc = 78.33J respectively by integration of areas of deap and peak by using origin lab software.

Figure 9. Refer to the following caption and surrounding text. — **Figure 9.** DSC and TGA curve for ${\boldsymbol{W}}{{\boldsymbol{O}}}_{3}$ nanoparticles.
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**Figure 9.** DSC and TGA curve for ${\boldsymbol{W}}{{\boldsymbol{O}}}_{3}$ nanoparticles.
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It was also revealed that the exothermic peak near 200 °C and indicated the removal of available water molecules and indicating the presence of crystalline WO₃. Thermogram also indicating the presence of amorphous WO₃ at around 200 °C–250 °C and after that the crystallization occurs near temperature of 300 °C with a diverse exothermic peak. This behavior also specified that the most suitable calcination temperature is starting after 300 °C for the formation of crystalline WO₃ nano structures.

3.4. Thermal insulation

In figure 10, a digital thermometer shows the upper surface temperature of the glass from three different points P-1, P-2 and P-3 and then takes an average, while the bottom temperature remained unchanged at 40 °C.

Figure 10. Refer to the following caption and surrounding text. — **Figure 10.** Temperature difference measurements by using a hot plate.
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Temperature difference test measure which shows that both the nano WO₃particles and PVA enhance the thermal insulation properties of the glass as shown in figure 11. Mentioned Regression equation exercised with different concentration of ${{\rm{WO}}}_{3}$ nanoparticles and different thickness of PVA with same concentration to evaluate the thermal insulation behavior of coating on the glass substrate at 50 μm with 1%, 3%, 5% and 7% in weight % concentration of ${{\rm{WO}}}_{3}$ in PVA. Thermal insulation test done by using a hot plate (see figure 10) for constant bottom/outer surface temperature T₁ and a digital thermometer attached on the top surface of glass to measure the top surface temperature T₂ with the help of a plunger pump to avoid air distraction during temperature stability. Uncoated sample was only 2 mm glass without coated resist 0.5 °C temperature when 40 °C was constant bottom temperature T₁ and 39.5 °C was top surface temperature T₂. As shown in figure 11 that thermal insulation of coated glass samples were increased with % of WO₃ NPs concentration in PVA and validate by mentioned substantial significant coefficient of determination (R² = 0.985).

Figure 11. Refer to the following caption and surrounding text. — **Figure 11.** Upper surface temperature and $W{O}_{3}$ concentrations.
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**Figure 11.** Upper surface temperature and $W{O}_{3}$ concentrations.
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3.5. Antibacterial testing

In figure 12, WO₃/PVA solution showed the antibacterial properties measure by agar disc diffusion method on gram positive and negative bacterial culture S. coli and E. bacterial culture and the obtained results after incubation were showing very minor zone of incubation as compare to previous studies [3, 5, 6, 17].

Figure 12. Refer to the following caption and surrounding text. — **Figure 12.** (a) Positive (b) and negative antibacterial testing.
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4. Conclusion

Synthesized WO₃ nanoparticles through chemical precipitation method characterized by sizes ranging from 40 to 100nm and confirmed to have a spherical shape through SEM imaging at 3500, 6000, 10000 and 15000 magnifications. The average crystalline size 20.672 nm was measured from peak list data which were confirmed the formation of ${{\rm{WO}}}_{3}$ nanoparticles. Coating of prepared different concentrations of ${{\rm{WO}}}_{3}$ /PVA solution was done by using doctor blade of thickness 50 μm. It was established that thermal insulation of coated glass samples were increased with % of WO₃ NPs concentration in PVA and validate by mentioned significant coefficient of determination (R² = 0.985). In addition, WO₃/PVA solution showed the antibacterial properties by agar disc diffusion method on gram positive and negative bacterial culture S. coli and E. bacterial culture and the obtained results after incubation were showing very minor zone of incubation as compare to previous studies.

Acknowledgments

Present sincere thanks to NED UET for an experimental bench work support.

Data availability statement

All data that support the findings of this study are included within the article (and any supplementary files).

Conflict of interest

No conflict of interest.