A Green Approach to Multifunctional Textiles: ZnO In Situ Synthesis Using Plant Waste Extracts and Gaseous Plasma

Abstract

Introducing green technologies and minimising the use of synthetic and toxic compounds are the most important steps to overcome the environmental obstacles in textile finishing. Another essential requirement for a better life is the prevention of waste, which negatively impacts the environment, economy and human health. This chapter presents a comprehensive study on the development of a novel and environmentally friendly synthesis of zinc oxide (ZnO) using aqueous extracts from plant waste and gaseous plasma technology, directly (in situ) on cotton and polyethylene terephthalate (PET), to obtain fabrics with ultraviolet (UV)-protective and hydrophobic or hydrophilic properties. Plant waste from the food processing industry and invasive alien plants were used as a source of phytochemicals in in situ ZnO synthesis. ZnO is an inorganic compound that is widely used in various industries due to its multifunctional properties. It can exhibit UV-protective, antimicrobial, self-cleaning, hydrophobic and other properties when applied to textiles. The chapter investigates different methods and parameters to achieve the most optimised synthesis procedure to enable textiles with functional protective properties. It also discusses the importance of the selection of a plant-based reducing agent in ZnO synthesis, the use of gaseous plasma and its effect on polymer modification and assistance in ZnO synthesis.

1. Introduction

The textile industry is one of the biggest polluters of the environment. With growing consumerism and expansion of the textile industry, consumption of chemicals is also increasing. In the process of converting raw materials into finished textile products, as many as 3500 different chemicals are used, and approximately a tenth of those chemicals is confirmed to be hazardous to human health [1]. At the same time, the environmental awareness of the general population, industry and government organisations is growing. European Commission has accepted a commitment to become the first climate-neutral continent by 2050 [2]. By accepting the European Green Deal and Net-Zero Industry Act, the European Union (EU) has committed to ensure climate neutrality, clean technologies and to enable clean, healthy and affordable water, environment and food [2]. Negative consequences of mass production and use of chemicals have been recognised and the recommendations listed above are being encouraged for applications in different industries including textile. In the past years, the textile industry has been developing environmentally friendly processes, especially in areas where chemicals that pose a threat to human health and the environment are used. In addition to the ecological aspect, an essential challenge for the textile industry is to ensure competitiveness. That is why companies are moving towards developing multifunctional textile products with high-added value [3]. The modification of textile materials to improve functional properties (quality, comfort and protective properties) while preserving basic textile properties is the aim of many research articles. One of the ways in which new properties of a material can be imparted is by the application of nanoparticles. Most often, pre-prepared nanoparticles are applied to textiles because they have a known size and shape. At the same time, they often agglomerate and have poor adsorption to textile fibres [4]. The two-step process of applying pre-synthesised nanoparticles to textiles is called ex situ synthesis, as the particles are pre-synthesised and then deposited on the textile in a separate step. The opposite of ex situ is in situ synthesis, i.e., synthesis directly on the textile substrate. In situ synthesis is a more environmentally friendly alternative because it is a one-step process, which reduces the use of chemicals, water, time and energy. In addition, previous research has shown that it also enables uniform distribution and good adsorption of particles on textile substrates [5, 6]. Zinc oxide (ZnO) is an inorganic compound that has, in the last decade, been extensively used and researched in different industries, including textile, due to its multifunctional properties. In addition to its photocatalytic and semiconducting properties, functionalisation of textiles with ZnO particles exhibits ultraviolet (UV)-protective, antimicrobial and self-cleaning properties of the textile material [7]. Some authors also reported increased hydrophobic, flame-retardant, electrically conductive, thermally insulative and moisture managing properties of the textile material (Figure 1) [7].

Figure 1.

Schematic presentation of the textile materials functionalities achieved with ZnO application. Reprinted with permission from [7].

While ZnO is found in nature in the form of the rare mineral zincite, it is mostly synthesised in laboratories. The chemical synthesis of ZnO in laboratories involves using various chemicals that act as reducing, stabilising, dispersing or binding agents in the process of synthesis or application to the textile substrate. Since the use of chemicals can be harmful to humans and the environment [8], the development of a new, environmentally friendly “green” process for the synthesis of ZnO is required.

In a typical chemical synthesis, ZnO is synthesised in the reaction between zinc salt (precursor) and reducing agent (usually NaOH or KOH). Hydroxyl (−OH) groups of the reducing agent are crucial for successful synthesis. They assist in the formation of intermediate zinc hydroxide, which is why a large amount of hydroxyl groups is required. During the drying process, zinc hydroxide is converted to ZnO [9, 10, 11, 12]. In “green” synthesis procedures, chemical reducing agents can be replaced by ecologically acceptable alternatives, such as plant extracts, since plants are a great source of polyphenols, containing large amounts of hydroxyl groups. Additionally, considering that cotton contains hydroxyl group in its structure, our study also examined if the zinc salt and OH groups of the cotton alone are able to produce ZnO.

When reviewing the literature regarding the green in situ synthesis of ZnO on textiles, it was observed that there are a lot of articles that describe its processes as green or in situ, but from the content it is clear that chemical reducing agents, which alone produce ZnO when combining them with zinc salt, are still included in the process or that harsh solvents were used to prepare plant extracts, or that the synthesis in fact does not take place directly on textiles. To our knowledge, there is only one research paper, where ZnO was synthesised directly on cotton using zinc salt and aqueous plant extract, without the addition of any other chemicals [13]. In the study by Aladpoosh et al., burnt Seidlitzia rosmarinus plant was used as a source of alkali. Authors reported good antimicrobial properties, tensile strength and crease recovery of the samples; however, the synthesis took place at an elevated temperature (90°C) for 90 minutes, meaning that the process was quite time- and energy-consuming. Furthermore, to achieve a truly environmentally friendly synthesis procedure, it has to be taken into account that the plant material used should not be grown specifically for the consumption as a plant extract for the synthesis. For the process to be considered as an example of a green circular economy, plant waste, which is usually discarded and burnt (such as food industry waste and invasive alien plant species), should be used to prepare the aqueous plant extracts, without the use of harsh solvents. It has also to be taken into account that the developed process should not be time- and energy-consuming.

Plasma treatment is an environmentally friendly technology in which an ionised gas is used to induce changes in the chemical and physical properties of textiles [3, 14]. Depending on the type of gas used and the plasma parameters applied, new reactive sites, cleaning, etching or polymerisation can be achieved. Compared to conventional processes, plasma treatment allows different effects to be achieved without using chemicals and water in shorter treatment times, affecting only the surface properties of the textile. Since the use of chemicals can have a negative impact on human health and the environment, the aim of this research was to develop an environmentally friendly process for the synthesis of ZnO directly (in situ) on cotton and polyethylene terephthalate (PET) fabrics, using oxygen plasma instead of chemical agents to increase the uptake of reagents, and using aqueous extracts of plant waste materials instead of conventional chemicals for ZnO synthesis, with the goal to achieve new functional properties of textiles.

Considering the existing gap of knowledge in the field of environmentally friendly in situ (direct) ZnO synthesis on textile substrates, this chapter presents a comprehensive study on the development of a novel, green procedure of ZnO synthesis directly on cotton and PET, using only zinc salt and plant waste aqueous extracts. Wood ash extract was used as alkali and aqueous extracts of food industry waste (avocado seed, avocado peel, green tea, pomegranate peel) or invasive alien plants (staghorn sumac leaves, staghorn sumac drupes, Japanese knotweed leaves) as reducing agents for the synthesis. The chapter addresses the importance of selecting an appropriate synthesis procedure, the order of synthesis solutions, different plant extracts as reducing agents and other parameters, which influence the successful formation of ZnO directly on textile substrates, with an emphasis on achieving the most optimised and environmentally friendly in situ ZnO synthesis procedure, resulting in textiles with functional protective properties. It also discusses the application of the optimised synthesis procedure from the cellulosic to synthetic textile material and the use of gaseous plasma and its effect on polymer modification and assistance in ZnO synthesis.

2. Experimental setup and methodology

2.1 Modification of textiles with in situ synthesised ZnO

2.1.1 Materials

Plain weaved, chemically bleached and mercerised cotton (Tekstina d.d., Ajdovščina, Slovenia) and de-lustered polyethylene terephthalate (PET) (Commerce, Ljubljana, Slovenia) were used as textile substrates. Zinc acetate dihydrate (Honeywell, Charlotte, NC, USA) and zinc nitrate hexahydrate (Sigma-Aldrich, Missouri, USA) were used as zinc precursors. Aqueous extracts of plant waste, i.e., food industry waste (avocado seed, avocado peel, green tea, pomegranate peel) or invasive alien plant species (staghorn sumac leaves, staghorn sumac drupes, Japanese knotweed leaves) were used as natural reducing agents. An aqueous extract of wood ash, a by-product of wood pellet heating system, was used as an alkaline medium. The extract preparation procedures are described in Verbič et al. 2021 [15].

2.1.2 Synthesis methods

Initially, four different synthesis methods were compared and examined to determine the optimal synthesis process [14]. All methods involved impregnation in three reaction solutions, i.e., alkali, zinc precursor and reducing agent. The first method (Method 1) involved only the successive impregnation of cotton in the reaction solutions. The second method (Method 2) involved padding the sample on two-cylinder foulard and drying in a continuous dryer after each impregnation. The third method (Method 3) involved ultrasonic shaking of the impregnation bath and drying of the samples in a continuous dryer after each impregnation. The fourth method (Method 4) involved drying the samples in a continuous oven after impregnation in each reaction solution (Table 1). Other synthesis parameters, such as concentration of reaction solutions, time and temperature of impregnation and drying, were standardised.

Method no.	Cotton processing conditions according to the method
1	Alkali – Reducing agent – Zinc precursor – Drying 30 min at 100°C – Drying 5 min at 150°C
2	Alkali – Padding and drying 2 min at 100°C – Reducing agent – Padding and drying 5 min at 100°C – Zinc precursor – Padding and drying 5 min at 100°C – Drying 30 min at 100°C – Drying 5 min at 150°C
3	Alkali (sonicated) – Drying 2 min at 100°C – Reducing agent (sonicated) – Drying 5 min at 100°C – Zinc precursor (sonicated) – Drying 5 min at 100°C – Drying 30 min at 100°C – Drying 5 min at 150°C
4	Alkali – Drying 2 min at 100°C – Reducing agent – Drying 5 min at 100°C – Zinc precursor – Drying 5 min at 100°C – Drying 30 min at 100°C – Drying 5 min at 150°C

Table 1.

Description of different methods of in situ ZnO synthesis on cotton fabric [15].

After determining the most optimal method, the order of the synthesis solutions was varied [16]. Two in situ synthesis procedures were compared. A procedure in which the impregnation in alkali was followed by a reducing agent and lastly zinc precursor, and a modified procedure in which the impregnation in alkali was followed by a zinc precursor and lastly a reducing agent. The experiments were carried out with different types and concentrations of zinc precursors and reducing agents [17]. The natural plant extracts used as reducing agents (avocado seed, avocado peel, green tea, pomegranate peel, staghorn sumac leaves, staghorn sumac drupes extract) were compared regarding their polyphenol content and antioxidant properties. The influence of using different reducing agents on the properties of in situ synthesised ZnO particles and fabric was examined.

2.2 Plasma modification of textiles

The procedure that proved to be optimal for the in situ synthesis of ZnO particles on cotton was then tested on PET fabric. Due to the hydrophobic properties of the PET fabric, the fabric was pre-treated with oxygen plasma for further investigations. We tested the effect of single (i), double (ii) and triple (iii) plasma treatments (before immersion in alkali (i), alkali and zinc precursor (ii), or alkali, zinc precursor and reducing agent (iii)) of the PET fabric [18] and lastly, compared the activation of the PET substrate by alkali and plasma [19].

2.3 Analysis of morphological, chemical and physical properties of textile samples

The morphological characteristics of the samples were examined by scanning electron microscopy (SEM) using JEOL JSM-6060 LV scanning electron microscope. Ultraviolet (UV) protection was determined by measuring the UV transmittance of the samples using a UV/visible (UV/Vis) spectrophotometer (Varian CARY 1E) following the AATCC test method 183-2000, from which the UV protection factor (UPF) was calculated. The samples’ colour values (CIELAB) were measured on a reflectance spectrophotometer (Datacolor Spectraflash 600 PLUS-CT). Quantitative analysis of the ZnO distribution on the samples was carried out by inductively coupled plasma mass spectrometry (ICP-MS), using Agilent technologies 7900 ICP-MS (mass spectrometry) instrument, equipped with MicroMist glass concentric nebuliser and Peltier-cooled, Scott-type spray chamber. Elemental analysis of the surface of the samples was carried out by energy-dispersive spectroscopy (EDS) on a Quanta 650 Scanning Electron Microscope. Chemical changes on the textiles were determined by X-ray photoelectron spectroscopy (XPS) with Physical Electronics XPS instrument, X-ray powder diffraction (XRD) using Phillips PW3710 type diffractometer, X-ray fluorescence (XRF) using Thermo Fisher Scientific portable XRF analyser Niton XL5+ and Fourier transform infrared spectroscopy (FTIR) with Bruker Optics Vertex-70LS FTIR spectrometer. The mechanical properties of the samples were determined by measuring the tensile strength and elongation in accordance with standard ISO 13934–1:2013 on Instron 5567 strength tester. The chemical composition of the aqueous plant extracts was analysed by the determination of total phenolic compounds content (TPC) values in extracts by the Folin-Ciocalteu method (according to the AOAC 2017.13-2017 method). The antioxidant activity of the extracts and cotton samples was determined spectrophotometrically (PerkinElmer Lambda 850+ UV/Vis spectrophotometer) using DPPH (2,2-diphenyl-1-picrylhydrazyl) radical decolorisation. An optical assessment of the surface porosity was made using stereomicroscope (Leica Microsystems GmbH). The hydrophilicity or hydrophobicity of the samples was determined by measuring the water contact angles (WCAs) using a drop shape analyser DSA 100E (Krüss) and water absorption test was performed according to DIN 53924:2020 testing standard.

3. Results and discussion

This comprehensive study examines the environmentally friendly modification of textiles, with the aim of achieving new functional properties. We focused on the in situ (direct) synthesis of ZnO on textile substrates. Our main objective was to use aqueous extracts of plant waste instead of conventional chemicals, required for the successful synthesis of ZnO.

In our previous research in the field of classical chemical in situ synthesis of ZnO [4, 20], the influence of different reducing agents and their concentration on the formation of ZnO directly on cotton fabric was tested. It was found that the use of a lower concentration leads to uneven distribution of ZnO and the formation of agglomerates. Using a higher concentration of reducing agent resulted in larger ZnO layers with visible cracks. Furthermore, the use of a reducing agent with OH groups included in its chemical structure resulted in a higher amount of synthesised ZnO, and an even distribution of particles on cotton fibres, ensuing in higher UV-blocking ability. The results showed that an alkaline medium is essential for successfully synthesising ZnO on cotton and that a medium with OH groups incorporated into its chemical structure performs better as a reducing agent. As explained in the Introduction, the theory of ZnO synthesis states that OH groups are required to successfully synthesise ZnO from zinc salt. Since cotton contains a number of OH functional groups in its structure, we investigated whether the application of a zinc precursor and drying of the cotton fabric could be sufficient for the formation of ZnO [15]. But the result showed that this treatment forms a net-like structured layer on the fibres, presumed to be the crystalline form of zinc salt used [21]. There were no individual particles that could be ZnO on the fibres, meaning that the combination of cotton and zinc salt alone does not produce ZnO (Figure 2).

Figure 2.

SEM image of a cotton fabric functionalised with only zinc acetate. Reprinted with permission from [15].

Furthermore, we investigated the formation of ZnO on cotton fabric using zinc acetate as a precursor and an aqueous extract of pomegranate peels as a reducing agent. Pomegranate peels contain high amounts of phenolic compounds, therefore, we were curious, if their numerous OH functional groups would be capable of forming ZnO. From the results [15], it is clear that an alkaline medium is necessary for successful synthesis of ZnO, as has been reported by others who have included NaOH or KOH in the synthesis reaction [20, 22, 23]. However, as the aim of our study was to exclude classical chemicals from the synthetic reaction, the alkaline medium was prepared from wood ash, a by-product of home heating system using wooden pellets as a fuel source. Phongarthit et al. [24] used wood ash in the ex situ synthesis of ZnO, and it proved to be an excellent alternative to NaOH when the calcination temperature of 900°C was used. An attempt of synthesising ZnO on cotton fabric was made by combining zinc precursor and alkali from wood ash and drying at 150°C, but the synthesis was unsuccessful. In our case, high calcination temperatures, such as 900°C, could not be used because the textile material would degrade. Thus, we attempted to achieve ZnO synthesis by using three reaction solutions, all prepared in water: an alkali prepared from wood ash (hereafter referred to as alkali), a reducing agent prepared from plant waste, i.e., plant food production waste or parts of invasive alien plant species, and a zinc precursor. The alkaline pre-treatment was used to achieve swelling of the cotton fibre, thereby increasing the number of free OH groups on the surface of the fibre and further boosting the binding possibility of various compounds. Initially, four synthesis methods were studied, which differed in the inclusion of drying the cotton fabric between impregnations in each reaction solution, padding the fabric on a two-cylinder foulard and drying, or ultrasonic shaking of the impregnation bath and drying (Table 1) [15].

All samples were prepared using the same concentrations of synthesis solutions (zinc precursor, pomegranate peel reducing agent and alkali). To maximise the energy efficiency of the process, the impregnation time for each reaction solution was limited to 1 minute at room temperature. Using Method 1 for the ZnO in situ synthesis, where drying the samples between the immersion into reaction solutions was not included, ZnO particles were successfully formed, and the fabric exhibited an extremely high UV protection factor (UPF 1000+). However, stiff, thick ZnO layers were formed, which peeled off the fabric’s surface when handling. Since the sample was unfit for end-use, it was excluded from the further study. When the samples were padded on a two-roller foulard after each impregnation (Method 2), a significant amount of reaction compounds was removed from the sample, resulting in the lowest ZnO concentration and UPF value (Figure 3). Nevertheless, the sample still exhibited excellent UV protection (UPF 50+). The ultrasonication of the reaction solutions during the immersion, following by drying of the sample (Method 3), allowed a uniform distribution of ZnO particles on the fabric and also a higher ZnO content and UPF value than Method 1. However, the highest ZnO content and UPF values were achieved with Method 4, where the in situ ZnO synthesis included only drying of the fabric between the immersion in synthesis solutions. Drying the fabric between impregnations in the individual reaction solutions showed to be a necessary step to ensure an even distribution of ZnO throughout the substrate.

Figure 3.

Ultraviolet protection factor (UPF) values and zinc concentration (c (Zn)) of the samples where ZnO was in situ synthesised on cotton using methods 2–4.

The cotton sample where ZnO was in situ synthesised using Method 4 was analysed with FTIR analysis and compared with the cotton sample, treated only with zinc acetate and sample, treated with zinc acetate and pomegranate peel extract as reducing agent. The results showed that adding natural alkali (wood ash aqueous extract) in the first step of the synthesis is a necessary step which provides better adsorption of the synthesis reagents (Figure 4).

Figure 4.

IR spectra of cotton samples: (a) in situ synthesis of ZnO using method 4, (b) treated with zinc acetate precursor, (c) treated with pomegranate peel extract reducing agent. Reprinted with permission from [15].

It was previously confirmed that plant extracts act as reducing agents and free radical scavengers due to their redox potential and high antioxidant ability caused by polyphenols [25, 26, 27, 28, 29]. Our results [15] showed a correlation between the antioxidant activity and values of total phenolic compounds content in the plant extracts used as reducing agents, while there was no correlation between antioxidant activity and the values of total flavonoid content. Nonetheless, the content of phenolic compounds varied between the extracts [30]. When comparing different plant extracts used as reducing agents (avocado seed and peel, green tea leaves and pomegranate peel), it was noticed that when using an extract with high content of phenolic compounds, a higher UPF value was achieved due to synthesised ZnO (very good or excellent UV blocking), and when using an extract with low values of phenolic compounds, lower UPF values were achieved (insufficient or minimal UV protection) (Figure 5). Furthermore, the antioxidant activity results showed that the samples functionalised with in situ synthesised ZnO using plant extracts as reducing agents have lower antioxidant activity than extract alone, because the phenolic compounds from the extract are consumed for binding on the fabric and in the reaction to form ZnO. Due to the consumption, a smaller amount of phenolic compounds is attached to the sample, influencing the measured value of the antioxidant activity. The samples with the highest ZnO content and highest UPF value had the lowest antioxidant activity due to the consumed phenolic compounds for the ZnO synthesis [15].

Figure 5.

Correlation between TPC value of the extract and UPF value of the cotton fabric with in situ synthesised ZnO using the same extract.

The cotton samples prepared with the extracts with the lowest TPC values had the lowest amount of synthesised ZnO particles (Figure 5). Those samples were also the least coated or had larger agglomerates of ZnO particles present, affecting the UV protection values (Figures 5 and 6) [30]. From the results, it was assumed that extracts with low concentrations of phenolic compounds could not react with the zinc precursor and further research was carried out to update the synthesis method.

Figure 6.

Scanning electron microscopy (SEM) images of cotton samples functionalised using method 4, using (a) avocado seed, (b) avocado peel, (c) green tea and (d) pomegranate peel extract [30].

Due to such different levels of polyphenols in the extracts (Figure 5), we had to modify the in situ synthesis method to allow the best possible reaction of polyphenols with zinc precursor on the textile substrate. The current synthesis method consisted of immersion in alkali, followed by a reducing agent and lastly, zinc precursor, meaning that polyphenols were first used to bind with the cellulose of the cotton fabric and then to form ZnO. That is why in the next step, the reaction solutions order was changed to alkali, zinc precursor, and lastly, reducing agent [16]. It was assumed that in this way, the reducing agents with low polyphenol content would have sufficient opportunity to react with the Zn precursor to form ZnO. The order of reaction solutions showed to have a significant effect on the formation of ZnO particles, reflecting in the protective properties of the functionalised fabric. With the modified method, the cotton fibres were entirely and uniformly coated with small ZnO particles that did not form agglomerates. The modified method enabled better reaction of the synthesis solutions, which resulted in higher zinc content (an increase from 3.2 to 8.4%) (Figure 7), as obtained by ICP-MS, offering improved fabric functionality. The morphology of the particles also changed since smaller, evenly distributed ZnO particles were produced. It is generally recognised that smaller particles with a higher specific surface area (surface-to-volume ratio) offer higher UV protection [22]. The modified procedure, where smaller and evenly distributed particles were synthesised, remarkably increased the UV-blocking ability of the samples (Figure 8). Since the colour of a sample can influence the ability to block UV radiation [31], we investigated for a possible correlation between the colour strength (K/S) and the achieved UPF values of the functionalised samples. The colour strength (K/S) results (Figure 8) showed that the samples have similar colour strength values (2.46 and 2.17) and at the same time very different UV-protective properties (UPF 18.33 and 46.57). A higher K/S value was measured for the sample that exhibited only minimal UV protection, while the K/S value was lower for the sample with very good UV protection. As a high K/S value means that the sample contains a high concentration of colourants [32], these results confirm that the UV protection achieved is a reflection of the synthesised ZnO particles and not due to the colour of the sample from the natural reducing agent.

Figure 7.

Concentration of zinc (c(Zn)) and antioxidant activity (AA) of cotton samples where ZnO was in situ synthesised using unmodified and modified methods.

Figure 8.

Ultraviolet protection factor (UPF) and K/S values of cotton samples where ZnO was in situ synthesised using unmodified and modified methods.

Several published studies have confirmed [27, 28, 29, 33, 34] that the content of phenolic compounds in the natural extracts used as reducing agents for the synthesis of metal nanoparticles is crucial for their reducing ability, mainly due to their antioxidant properties, since antioxidants have good reducing capability. High content of phenolic compounds is therefore assumed to be crucial for the extract to perform as a reducing agent [30, 35]. Our results showed [15, 16, 30] that the extract used as a reducing agent has high values of antioxidant activity. When the extract was used for the in situ ZnO synthesis on cotton, the antioxidant value decreased in comparison with the extract alone (Figure 9).

Figure 9.

Antioxidant activity of untreated cotton sample, liquid reducing agent and functionalised cotton samples using unmodified and modified synthesis methods [16].

Since phenolic compounds are consumed in the green synthesis process to form ZnO, based on the results of the UPF values, it would be expected that the sample prepared using the modified procedure (alkali → zinc precursor → reducing agent) would have a lower antioxidant activity value due to the lower content of polyphenolic compounds, which were consumed in the ZnO synthesis process. But in our study, the antioxidant activity values of the two samples were similar, or rather, using the unmodified procedure (alkali → reducing agent → zinc precursor), the antioxidant value was a little lower than when using the modified procedure (alkali → zinc precursor → reducing agent) [16]. This can be attributed to the fact that similar values of phenolic compounds were used for the synthesis, but in the case of the modified synthesis procedure, evenly distributed particles were formed (Figure 10), resulting in a higher UV protection (Figure 8). EDS analysis showed that the sample prepared with modified synthesis procedure contains a higher amount of zinc on the sample (Table 2) [16]. Furthermore, the colour measurements (K/S) of both samples were similar, indicating that a similar amount of phenolic compounds containing extract (reducing agent) was adsorbed on the samples (Table 2). Since better results were achieved with the modified ZnO in situ synthesis procedure, this procedure was used in further studies.

Figure 10.

SEM images of (a) untreated cotton sample, and cotton samples, functionalised with (b) unmodified ZnO in situ synthesis method and (c) modified ZnO in situ synthesis method. Reprinted with permission from [16].

Sample	Zn [%]	K/S (at 400 nm)
Untreated cotton	0	0.04
Unmodified ZnO in situ synthesis on cotton	3.2	2.45
Modified ZnO in situ synthesis on cotton	8.4	2.17

Table 2.

Zinc content (%) and K/S values of untreated cotton, and cotton samples functionalised with unmodified and modified ZnO in situ synthesis methods [16].

Next, we investigated how using different plant sources to prepare reducing agents can affect the successful ZnO synthesis [17]. As a source of phytochemicals for the preparation of reducing agents, we used green tea leaves (GT), pomegranate peels (PG) and staghorn sumac leaves (SSL) and drupes (SSD). The selected waste plant sources were from the food waste group (green tea, pomegranate) and invasive alien plants in our local environment (staghorn sumac leaves and drupes). Furthermore, two different zinc precursors were used for the synthesis, zinc acetate and zinc nitrate. When the functionalised samples were prepared for analysis, it was observed that the samples synthesised with zinc nitrate as a precursor were very brittle. The mechanical properties of the samples were examined first by determining the breaking strength and elongation. We found that regardless of the type of reducing agent used, the samples where zinc nitrate was used in the synthesis process had significantly lower values of breaking strength and elongation (Figure 11). It is assumed that the mechanical properties of the samples were worsened due to the low pH value of the zinc precursor solution and because zinc nitrate has a low boiling point and it decomposes at around 125°C [36]. This temperature was exceeded in the last step of sample drying (5 minutes at 150°C in the laboratory oven). Also, the pH of the zinc nitrate solution was 3.64. Cellulose fibres are sensitive to acidic conditions, which can cause material degradation [37]. Since textile functionalisation processes should not significantly deteriorate the basic properties of the material, zinc nitrate was not suitable to be used as a precursor for in situ ZnO synthesis process on cotton, and was therefore excluded from further studies.

Figure 11.

Mechanical properties (tensile strength and elongation) of the cotton samples where in situ ZnO synthesis was performed using zinc acetate or zinc nitrate precursor using green tea (GT), pomegranate peel (PG), staghorn sumac leaves (SsL) and staghorn sumac drupes (SsD) reducing agent. Reprinted with permission from [17].

The samples where zinc acetate was used as a precursor for in situ ZnO synthesis were further analysed for UV transmittance, and UPF values were determined. Although all the functionalised fabrics achieved excellent UV protection (UPF 50+ according to the Australian/New Zealand standard AS/NZS 4399: 2017), the values varied between samples depending on the reducing agent used. The UPF values decreased in the following order: staghorn sumac leaves > pomegranate peel > green tea > staghorn sumac drupes (Figure 12) [17]. When preparing the samples for further analysis, it was observed that some samples showed hydrophilic, while others showed hydrophobic properties, so the water contact angles of the samples were measured (Figure 12). It was observed that the samples with the highest UPF values (pomegranate peel, staghorn sumac leaves) had the lowest measured water contact angles and the samples with the lowest UPF values (green tea leaves, staghorn sumac drupes) had the highest water contact angles. In order to understand the connection between the extremely high UPF values and the different wettability of the fabrics, SEM, EDS, XPS, XRD, XRF and ICP-MS analyses were further performed [17].

Figure 12.

Ultraviolet protection factor (UPF) values and water contact angles (WCAs) of the untreated cotton sample and cotton samples where ZnO was in situ synthesised using staghorn sumac leaves, pomegranate peel, green tea and staghorn sumac drupes [17].

X-ray photoelectron spectroscopy (XPS) analysis confirmed that the identified compound on the spectrum is ZnO, with the presence of Zn 2p₁, Zn 2p₃ peaks, as well as the most intense Zn LMM Auger peak at 495 eV (Figure 13a,b) [17]. The XRD analysis results showed a weak intensity of the peaks characteristic of ZnO (Figure 13c), which could indicate that the synthesised ZnO is amorphous or that the ZnO content is too low to be detected. Although, various other researchers have confirmed that high temperature (above 150°C or even 300 or 600°C) is required for ZnO to the change from the amorphous to the crystalline form [38, 39, 40, 41].

Figure 13.

XPS spectra of the untreated and in situ ZnO synthesis functionalised cotton samples (a, b), XRD spectra of in situ ZnO synthesis functionalised cotton samples (c), XRD spectra of ex situ synthesised ZnO (d) and XRF spectra of ex situ synthesised ZnO (e) using green tea (GT), pomegranate peel (PG), staghorn sumac leaves (SsL) and staghorn sumac drupes (SsD) reducing agent. Reprinted with permission from [17].

As cotton fabric is sensitive to such high temperatures, drying at temperatures this high cannot be included in the ZnO synthesis process directly on cotton. In order to prove that the synthesised particles are indeed ZnO, ex situ synthesised ZnO particles, without textile substrate, were prepared by the same procedure, which could be calcined at such high temperatures. Analyses (Figure 13d) of the prepared powders showed that crystalline ZnO was synthesised with distinct characteristic peaks corresponding to the mineral form of ZnO according to ICSD-98-000-9346. The intensity of the peaks varied depending on the type of extract used as a reducing agent. Using pomegranate peel or staghorn sumac leaves and drupes extract as reducing agents resulted in higher and narrower peaks, meaning that higher crystallinity of ZnO was achieved [42]. Moreover, no other peaks were observed in the spectra that were not specifically characteristic of ZnO, which means that the synthesised powder is pure ZnO with no impurities present. ICP-MS, XRF, EDS and UV/Vis results (Figures 12 and 13d-e) showed that the use of different extracts as reducing agents also influences the concentration of synthesised ZnO, which was highest when pomegranate peel extract and staghorn sumac leaves extract were used as reducing agents.

Various studies have indicated that surface morphology has a significant influence on the wettability of the substrate. It is generally accepted that increased surface roughness exhibits a more hydrophobic character [12]. Thi and Lee [43] describe that increased surface roughness contributes to more trapped air between the rough surface and water droplet, increasing the contact angle. In addition, when using nanoparticles to increase the surface roughness, the thickness of the nanoparticle layer is believed to have a major influence. A thicker ZnO layer is thought to increase the polar interactions with water, resulting in a lower contact angle and increased wettability. Our study [17] showed that the cotton samples on which ZnO was synthesised with pomegranate peel and staghorn sumac leaves extracts had the thickest layer of ZnO particles on the surface and, at the same time, had the lowest water contact angle values (Figures 12 and 14). These two extracts also had the highest TPC values (Figure 15). As the OH groups of the reducing agents were not entirely consumed during the synthesis to form ZnO, they remained on the surface of the substrate, making these samples more hydrophilic. When green tea extract or staghorn sumac drupes extract, with lower TPC values, was used for ZnO synthesis, the OH groups were consumed during the synthesis process and the samples were more hydrophobic.

Figure 14.

SEM images of cotton fabric where in situ ZnO synthesis was performed using (a) green tea, (b) pomegranate peel, (c) staghorn sumac leaves and (d) staghorn sumac drupes extract. Reprinted with permission from [17].

Figure 15.

Correlation between TPC values of the staghorn sumac leaves, pomegranate peel, green tea and staghorn sumac drupes extract and UPF values of untreated cotton fabric and cotton samples, where ZnO was in situ synthesised using the same extracts.

Green tea leaves, pomegranate peel, staghorn sumac leaves and drupes extracts were measured for TPC values (Figure 15). The highest values were measured in the staghorn sumac leaves extract (14,348 mg of gallic acid (GA) equivalent/100 g of the sample) and pomegranate peel extract (11,509 mg GA/100 g). The cotton samples on which ZnO was synthesised with these two extracts achieved the highest UV protection values and the lowest water contact angles. Meanwhile, the two samples prepared with extracts with lower TPC values (6103.2 mg GA/100 g for green tea leaves extract and 1769.4 mg GA/100 g for staghorn sumac drupes extract) achieved lower (but still excellent according to the AS/NZS 4399: 2017 standard) UV protection properties and had hydrophobic nature (high values of water contact angles). When green tea or staghorn sumac drupes extract was used, the polyphenolic compounds from the extract were mainly consumed for ZnO synthesis. The results of EDS, XRF and ICP-MS analyses [17] showed that the extracts containing a higher amount of total phenolic compounds were able to synthesise more ZnO particles. At the same time, many OH groups of the polyphenols were still retained on the surface of the ZnO, which affected the wettability of the samples. Therefore, textiles with different functional properties and their applicability can be designed by using different plant extracts as reducing agents in the ZnO synthesis process directly on textiles.

Since cotton and polyester are the most commonly used textile substrates, green in situ ZnO synthesis was also carried out on PET fabric. For the synthesis on PET, the same procedure as on cotton was used, i.e., impregnation in alkali, zinc precursor and reducing agent, with drying included between the impregnations. Pomegranate peel aqueous extract was used as the reducing agent, as excellent results were obtained with it in previous studies. Using the existing synthesis method, ZnO particles formed on the PET fabric and the UPF value of PET increased from 16.5 (untreated PET) to 177.9 (PET with in situ synthesised ZnO). The SEM analysis [18] showed that ZnO particles were not uniformly distributed over the sample and there were parts of the sample where ZnO was absent. Due to the hydrophobic nature of the PET fabric, the used reagents were not adsorbed uniformly on the sample, resulting in uneven synthesis of ZnO particles. For successful and uniform in situ synthesis of ZnO on PET fabric, the reactivity of the substrate needed to be increased. The classical chemical modification of PET to increase hydrophilicity is treatment in alkali [44]. Instead of using classical chemical treatment, we chose to increase the reactivity of our substrate by treating it with low-pressure inductively coupled plasma system, using oxygen as working gas. The low-pressure oxygen plasma treatment of PET causes partial hydrolysis of the ester bond and the formation of oxygen functional groups (COOH and OH) on the surface of the PET polymer fibres [45, 46]. The peaks characteristic of –C=O groups also increase [46, 47, 48]. The newly formed oxygen groups increase the hydrophilicity of PET, i.e., the WCA is decreased. In order to determine the optimal plasma treatment conditions, PET fabric was treated with oxygen plasma for 1, 4, 8 and 16 seconds, and the effect of plasma treatment duration on the achieved WCA was measured. The results are presented in Figure 16.

Figure 16.

Measurements and pictures of water contact angles (WCAs) of untreated PET and PET samples modified with different durations of oxygen plasma treatment [30].

The untreated PET was poorly wettable with WCA of 72.1° ± 5.6°. The one-second plasma treatment lowered the sample’s WCA to 22.1 ± 5.9°. While the individual values of the contact angles of the samples with the one-second plasma treatment are satisfactory, the standard deviation between the measured values remains high. The PET sample treated with plasma for 4 seconds became the most hydrophilic with an average contact angle of 11.2 ± 2.2°. This sample also achieved the lowest standard deviation between measurements, meaning it is the most uniformly wettable. As the plasma treatment time was extended, the contact angle values started to increase (to 26.9 ± 8.7° and 47.5 ± 3.2°). At the beginning of plasma treatment, the functionalisation of PET with oxygen-rich groups takes place, but with prolonged treatment time, etching begins to prevail. In addition, the samples that were plasma modified for 8 and 16 seconds were dimensionally deformed during processing and not perfectly flat, which is attributed to the fact that the PET fabric used was raw. Thus, 8- and 16-second plasma treatments were not appropriate for our sample at selected parameters.

Based on these results, we continued our study on in situ ZnO synthesis, where PET was pre-activated by a four-second plasma treatment. We decided to take an additional step of plasma treatment of the sample prior to impregnation in each reaction solution, i.e., prior to impregnation in alkali, prior to impregnation in alkali and zinc precursor, and prior to impregnation in alkali, zinc precursor and reducing agent. Therefore, single, double and triple plasma treatments were performed to increase reaction solution adsorption onto the PET substrate [18]. Due to the increased hydrophilicity of PET with plasma treatment, the reaction solutions were adsorbed uniformly, allowing ZnO particles to be synthesised evenly throughout the fabric. An even distribution of ZnO also increased UPF value of the fabric, which increased with the number of plasma treatments (UPF 218.6 > 295.2 > 303.7) (Figure 17).

Figure 17.

Ultraviolet protection factor (UPF), K/S values and zinc concentration (c(Zn)) of the PET samples including single, double and triple plasma treatments.

Single, double and triple plasma-treated samples were analysed with EDS [18]. The analysis showed that the untreated PET sample contained high concentrations of carbon and oxygen, while no zinc was present on the sample. With single plasma activation before the in situ ZnO synthesis, the concentration of carbon decreased and the concentrations of oxygen and zinc increased. With increasing the number of plasma treatments (double and triple plasma activation), the values of zinc on the PET sample further increased, which confirms the activation of the PET substrate with oxygen plasma. The activation of PET substrate with oxygen plasma makes the sample more hydrophilic, further contributing to the formation of ZnO particles. As the number of plasma treatments increased, the ZnO particles were also more evenly distributed over the substrate, as confirmed by SEM analysis [18]. When PET is treated with oxygen plasma, a number of new reaction sites (oxygen functional groups) are created to which compounds from the reaction solutions can bind. This is reflected in the homogeneity of the coating and high UPF value, and also affects the colour values of the samples. As more compounds could be adsorbed on the sample, the K/S values of the samples increased with the number of plasma treatments (from 8.32 to 12.14). The highest K/S value was measured for the sample that was plasma-treated also before impregnation in reducing agent. CIELAB colour measurements [18] showed that the samples treated with plasma become less red or greener (CIE a*), and also yellower (CIE b*) with the increasing number of plasma treatments.

As the primary goal of this comprehensive study was to make the in situ synthesis of ZnO on textiles as environmentally friendly as possible, and as alkaline PET pre-treatment has shown a favourable effect on ZnO formation, we further investigated whether plasma treatment has a sufficiently strong effect on polymer activation for successful ZnO synthesis without the use of alkali in the first step of the synthesis [19]. The results showed [19] that with both activation processes, a successful formation of ZnO particles can be achieved. Although, the water uptake analysis (Figure 18) showed that the sample activated in oxygen plasma had a better water uptake than the alkali-activated sample, suggesting that the plasma treatment produced more oxygen-rich functional groups on the PET substrate surface, which increased the hydrophilicity. Consequently, more reactive compounds could be adsorbed on the plasma-activated sample, resulting in a higher content and more even distribution of ZnO particles, ensuing in higher UPF values. Optical assessment of the fabric porosity of the sample showed that the sample where plasma was used to activate the substrate had a higher percentage of open area (higher fabric porosity) than the sample activated with alkali, but still achieved better protection against UV radiation, indicating that the high UPF value is due to the presence of synthesised ZnO particles.

Figure 18.

Wicking height (mm) of the untreated, alkali-treated and plasma-treated PET samples, measured after 30, 60 and 300 seconds.

The results show that both processes of PET activation are suitable, as both alkali and oxygen plasma enable the formation of hydrophilic active sites (COOH, and OH), due to the partial hydrolysis of the PET ester bond [45, 46]. Those are new adsorption sites where Zn ions of the zinc precursor can bind to. These sites present nucleation sites for the formation of ZnO particles. ZnO is synthesised in the third step of the synthesis process with the impregnation in a reducing agent, high in phenolic compounds. From the point of view of achieving a process that is as environmentally friendly as possible, it is also necessary to consider the preparation time and the consumption of reaction solutions (environmentally friendly alkali prepared in water) and to achieve as uniform distribution of ZnO particles on the fabric as possible, which makes plasma more appropriate for PET polymer activation.

4. Conclusions

This comprehensive study presents an important contribution to understanding the green synthesis of ZnO directly (in situ) on textile substrates. It addresses the importance of choosing the appropriate synthesis method, the selection and order of reagents for successful synthesis, and the influence of other parameters, such as the type of zinc precursor, the type and concentration of the “green” reducing agent, the influence of the pH value, the choice of the substrate and the plasma treatment on the chemical, morphological, mechanical and physical properties of the functionalised textiles. The findings provide a significant contribution to the development of functional protective textiles without the use of conventional chemicals which can be harmful to human health and the environment. Instead, it replaces conventional harmful chemicals with environmentally friendly aqueous extracts prepared from plant wastes, and by using oxygen plasma pre-treatment, thus contributing to solving the problem of plant waste and the development of protective textiles at the same time. Such modified textiles could be used for technical, protective or medical purposes in the future. The synthesis process is optimised in a way that low concentrations of reagents, low temperatures and short processing times are used, and could be transferred from the laboratory level to industrial applications.

Acknowledgments

This work was financially supported by the Slovenian Research Agency (Project J2-1720, Programme P2-0213, Programme P1-0034 and a grant for a doctoral student A. V.)