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1 Introduction

In the last few years, many research groups worldwide have focused their attention on zinc oxide (ZnO) and natural polysaccharides (PSH) as components for designing complex functional nanomaterials. This scientific interest is due to the fundamental characteristics of these materials as well as to their wide-ranging utility in areas such as optoelectronics, sensing, biomedicine, cosmetics, fiber engineering, pollution remediation, superhydrophobic surfaces etc. [1–22].

Zinc oxide , semiconductor characterized by a direct band gap (3.37 eV), a large exciton binding energy (60 meV) and a high transmittance of visible light [4, 21–23], can be easily engineered to yield functional materials with outstanding optical and electrical properties [3–5]. Also, among all the other metal oxides, ZnO occupies an exceptional role due to its unique combination of interesting properties such as low-price, many active sites with high surface reactivity and excellent mechanical, thermal and chemical stability. Furthermore, as a result of the fact that ZnO is an inorganic biocompatible material with antibacterial activity [24, 25], it can be used for a wide range of biomedical applications, ZnO being regarded as a bio-safe material for human beings and animals. Finally, ZnO has probably the richest family of structures such as whiskers, wires, rods, tubes, belts, cages, rings, combs, prisms, etc. [26–31], being a technological key material. All these morphologies featured by properties for different applications lead to the development of research regarding the ZnO synthesis by various physical (chemical vapor deposition, molecular beam epitaxy, pulsed laser deposition, magnetron sputtering, thermal evaporation) and chemical (chemical bath deposition, electrochemical deposition, hydrothermal, solvothermal, sol-gel, precipitation) methods [32–37]. Compared to the physical route where harsh conditions such as high temperature or special equipments are usually required generating a very high cost, the solution-based chemical approach presents the following advantages: easily accessible raw materials, relatively low temperature (below 100 ºC), the use of inexpensive equipment, control of the morphologies and properties of the final products by changing the experimental parameters, in other words a cheap preparation technique which is more attractive for mass production.

Polysaccharides , polymers of monosaccharides (sugars), are special raw materials because they are: natural polymers (called also biopolymers, they derive from natural source, like acacia trees (gum arabic), brown algae (alginate) or shelfish and fungi cell wall (chitosan)), inexpensive (low-cost polymers) and renewable resources [6, 7]. Also, they have biological and chemical properties such as ecologically safe biodegradability (degradation products are non-toxic, non-immunogenic and non-carcinogenic), biocompatibility (hydrophilic character due to hydroxyl groups of glucose units) and high chemical reactivity (large number of reactive hydroxyl and/or acetamido groups). Possessing numerous functional chemical groups on the macromolecular chains, PSH can serve for the preparation of multifunctional systems through the addition of other reactive groups which further augments the range of applications [12, 38, 39]. Last but not least PSH are extremely common in nature, the cellulose being the most abundant organic compound. As it is known, cotton and hemp are the most investigated cellulose-based fibers, cotton being the principal natural clothing fibers due to its outstanding properties e.g. softness, comfort, etc. [14–20].

In this context, the main point of this chapter is to highlight the combination of the ZnO, PSH and a simple wet chemical synthesis route for obtaining functional nanomaterials with improved features due to a synergistic effect. The focus has been set on two wet chemical preparation methods of semiconductor particles: polymer-mediated crystallization and electroless deposition. The most important advantages of these versatile techniques are the simplicity of the required facilities, low-fabrication cost, low process temperature (below 90 ºC), ambient pressure processing, high yields of pure products, scalability and the fact that they are less hazardous and more environmentally friendly. The polysaccharide-mediated crystallization of ZnO (covered in Sect. 5.2) consists in PSH addition during the ZnO nanostructures’ synthesis. In this way, a dramatic change in the morphology and the size of the semiconductor particles induces modifications in their optical properties. The ZnO electroless deposition (covered in Sect. 5.3) is a successfully surface functionalization technique which allows the coverage of non-planar substrates, such as fabrics, with good adhesion of the resulting deposit to the substrate. Due to the larger number of hydroxyl groups on cellulose-based fabrics surface, the natural fibers are highly water absorbent and therefore easily stained by liquids. Additional finishes, like functionalization with ZnO nanostructures transform them into water-repellent and easy-to-clean textiles. Another goal of this work is the investigation of different properties such as structure, morphology, optics and wettability of the selected examples.

Finally, taking into account two important aspects involved in the preparation of functional nanomaterials based on the ZnO and PSH, i.e. the green characteristic of the wet chemical methods and the eco-friendly properties of the both materials, this chapter aims to convince researchers to find and develop new materials and new technologies in order to be further used in solving the environmental and public health problems that the world is currently facing.

2 Polysachharides-Mediated Crystallization of ZnO Nanostructures

The formation of ZnO in solution is a widely used synthesis method consisting in the reaction of one solution containing a zinc salt (nitrate, acetate, chloride, etc.) with another containing a reducing agent (sodium or potassium or ammonium /hydroxide or nitrate or carbonate, triethanolamine, hexamethylenetetramine, dimethylamineborane, etc.) [36, 37]. Several solution based methods, such as precipitation, solvothermal, hydrothermal, sonochemical, etc. are used for the ZnO nanoparticles synthesis [35–37]. Among them, the precipitation carried out in water as reaction medium is the most simple yet powerful preparation technique due to the fact that by modifying the reaction parameters (reducing agent type, reactants’ concentrations, reaction temperature, reaction time) the morphological characteristics (shape, size, size distribution and width-to-length ratio) of the semiconducting particles can be tuned, features which govern their optical and electrical properties. Practically, a precipitation reaction takes place as following: when the specific reaction temperature is reached by the precursor solution placed in a thermostatic bath under continuous stirring the precipitating agent solution is added. The appearance of the white colour is the first evidence of the precipitate’s formation. After a specific reaction time, the white powder is collected through centrifugation, washed several times with distilled water and dried on filter paper at room temperature.

Usually, the synthesis of ZnO colloid crystallites involves two processes: nucleation and growth, these being responsible for the overall structure and morphology. According to [37], when zinc nitrate and different reducing agent were used as raw materials, the possible reactions which can take place during the ZnO synthesis are the following:

  1. (i)

    using strong base (KOH, NaOH)

    $$ {\text{Zn}}\left( {{\text{NO}}_{ 3} } \right)_{ 2} \to {\text{ Zn}}^{ 2+ } + {\text{ 2NO}}_{ 3}^{ - } $$
    (5.1)
    $$ {\text{Zn}}^{ 2+ } + {\text{ 2HO}}^{ - } \to {\text{ Zn}}\left( {\text{OH}} \right)_{ 2} $$
    (5.2)
    $$ {\text{Zn}}\left( {\text{OH}} \right)_{ 2} + {\text{ 2HO}}^{ - } \to \, \left[ {{\text{Zn}}\left( {\text{OH}} \right)_{ 4} } \right]^{ 2- } $$
    (5.3)
    $$ \left[ {{\text{Zn}}\left( {\text{OH}} \right)_{ 4} } \right]^{ 2- } \to {\text{ ZnO }} \downarrow \, + {\text{ 2HO}}^{ - } + {\text{ H}}_{ 2} {\text{O}} $$
    (5.4)
  2. (ii)

    using weak base ((CH2)6N4)

    $$ {\text{Zn}}\left( {{\text{NO}}_{ 3} } \right)_{ 2} \to {\text{ Zn}}^{ 2+ } + {\text{ 2NO}}_{ 3}^{ - } $$
    (5.5)
    $$ \left( {{\text{CH}}_{ 2} } \right)_{ 6} {\text{N}}_{ 4} + {\text{ 6H}}_{ 2} {\text{O }} \to {\text{ 6HCHO }} + {\text{ 4NH}}_{ 3} $$
    (5.6)
    $$ {\text{NH}}_{ 3} + {\text{ H}}_{ 2} {\text{O }} \to {\text{ NH}}_{ 4}^{ + } + {\text{ HO}}^{ - } $$
    (5.7)
    $$ {\text{Zn}}^{ 2+ } + {\text{ 3NH}}_{ 4}^{ + } \to \, \left[ {{\text{Zn}}\left( {{\text{NH}}_{ 3} } \right)_{ 4} } \right]^{ 2+ } $$
    (5.8)
    $$ \left[ {{\text{Zn}}\left( {{\text{NH}}_{ 3} } \right)_{ 4} } \right]^{ 2+ } + {\text{ 2HO}}^{ - } \to {\text{ Zn}}\left( {\text{OH}} \right)_{ 2} + 4 {\text{NH}}_{ 3} $$
    (5.9)
    $$ {\text{Zn}}\left( {\text{OH}} \right)_{ 2} \to {\text{ ZnO }} \downarrow \, + {\text{ H}}_{ 2} {\text{O}} $$
    (5.10)

In the strong alkaline aqueous solutions, the first step consists in the hydrolysis of the Zn(NO3)2 and the base (KOH or NaOH) giving rise to Zn2+ ions and HO− ions, which latter form Zn(OH)2. In the presence of the alkali excess, Zn(OH)2 react further with HO− producing [Zn(OH)4]2− complexes, species which decompose, under heating, to ZnO nuclei. By successively growth and aggregation processes ZnO crystallites are obtained, leading finally to the formation of ZnO complex structures. The reaction process can be modified by the different [Zn(OH)4]2− precursors influencing further the competition between thermodynamics and kinetics during the reduction of precursors, nucleation and growth of ZnO crystals.

In the weak alkaline aqueous solutions, the exact function of (CH2)6N4 in the ZnO synthesis is not completely understood. As non-ionic cyclic tertiary amine, (CH2)6N4 can act as a bidentate Lewis ligand capable of bridging two Zn2+ ions in solution [40]. In addition, it can influence the growth rates of different ZnO crystal facets by its preferential attaching to the non-polar facets of the crystallite leaving only the polar (001) face for growth [41]. Known as weak base, (CH2)6N4 is also a pH buffer [42]. So, when (CH2)6N4 is used, initially ammonia and formaldehyde are produced by its thermal decomposition. From the zinc nitrate hydrolysis are generated zinc ions which interacts with ammonia forming [Zn(NH3)4]2+ complexes. Under heating, these complexes are decomposed and released Zn2+ and HO− ions into solution, which subsequently lead to the formation of Zn(OH)2 which further are thermally dehydrated to ZnO.

The shape of a crystal is determined by the relative specific surface energies associated with the facets of this crystal, so if these energies are changed the facets growth rates can be modified and the crystal properties, such as size, shape and even structure can be controlled [43]. This modification can be achieved by adding appropriate organic additive agents, like surfactants [44, 45] or polymers [46–48] during the ZnO precipitation reaction. Because they are difficult to be washed completely, the surfactants cause inevitably contamination of the particles affecting the material properties and its applications. In the case of polymers, two aspects are synergetically combined: first, as surface capping agents, polymers can exert a direct influence on the semiconducting crystallite morphology by selective adsorption on some specific crystalline facets of the crystal (for this reason they can be called surface modifiers) and second, as complexing agents, polymers can control the crystallites’ growth step. Therefore, polymer-mediated crystallization is a promising and economic technique for crystal morphology design at ambient conditions with a great potential for synthesis of semiconductor nanoparticles. Among polymers, the biopolymers such as polysaccharides (gelatin, starch, gum arabic, sodium alginate or chitosan) are particularly interesting in ZnO synthesis [12, 49–62]. As surface-modifiers, their chains contain a high number of functional groups that are able to coordinate with the metal ions leading to a homogeneous dispersion of the cations in the PSH. In the case of ZnO synthesis by precipitation reaction [54–56], when a precipitation agent is added, the metal-occupied positions become incipient sites of nucleation and initial growth of crystallites which lately will aggregate and finally form the semiconducting particles. In this way, the PSH matrix containing these binding sites provides a size-limiting effect for the ZnO particles. Also, the PSH can be self-assembled into well-defined aggregating entities which restrict and direct the growth of the ZnO particles with interesting and useful physical properties. Generally, in order to investigate the properties of the ZnO nanostructures, the final product (as powder) is washed and centrifuged several times for PSH dissolution.

Choosing three natural low-cost PSH, gum arabic—GA, sodium alginate—SA and chitosan—CA, the influence of their charge (anionic—GA and SA or cationic—CA) and structure (branch—GA or linear—SA and CA) on the ZnO properties was evidenced. According to [63, 64], the following reactions parameters are involved in the ZnO preparation: 0.01 M Zn(NO3)2 solution, 0.01 M precipitating agent solution, 0.01 % PSH concentration, 90 ºC reaction temperature and 3 h reaction time.

The formation of ZnO crystallites is confirmed by the main peaks presented in the X-ray diffraction (XRD) pattern samples’ diffractograms (Fig. 5.1). These correspond to the Miller index of the reflecting planes for (100), (002), (101), (102), (110), (103), (200), (112) and (201) assigned to ZnO hexagonal wurtzite phase (JCPDS file no. 36-1451). In the diffractograms samples, the intense and sharp peaks suggest that the as-obtained products are well crystallized. Also, no other characteristic peaks for other segregate phases, such as Zn(OH)2, were observed. Using the Scherrer equation: D = Kλ/βcosθ, where K is a constant equal with 0.9, λ = 0.154 nm, θ = Bragg angle, β = full width at half maximum (FWHM), mean crystallite size (D) was estimated from the FWHM of the (101) diffraction peak of the samples. The ZnO crystallite sizes were calculated to be around: 30 nm (curve a), 28 nm (curve b), 18 nm (curve c), 16 nm (curve d), 65 nm (curve e), 20 nm (curve f), 77 nm (curve g) and 15 nm (curve h).

Fig. 5.1
figure 1

Powder XRD patterns of ZnO crystallites prepared from Zn(NO3)2 and KOH, NaOH, (CH2)6N4: in the absence of PSH (a, c, e) and in the presence of PSH: GA (b, d, f), SA (g), and CA (h)

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The changes induced by the different reaction parameters on the optical properties of the ZnO structures are evidenced in Fig. 5.2. For all the investigated samples, a strong decrease of reflectance can be observed below 400 nm, associated with the band-to-band transition in ZnO. Based on the reflectance data, the band gap of all samples is estimated by plotting [F(R)·E]2 versus photon energy (E), where F(R) is the Kubelka-Munk function with F(R) = (1 − R)1/2/2R and R is the observed diffuse reflectance. Thus, from the Kubelka-Munk representation (Fig. 5.2. insets), depending on the reducing agent and PSH types, band gap values are about: 3.30 eV (curve a), 3.32 eV (curve b), 3.27 eV (curve c), 3.33 eV (curve d), 3.28 eV (curve e), 3.31 eV (curve f), 3.25 eV (curve g) and 3.43 eV (curve h), confirming that the optical band gap of ZnO can be tuned by using polysaccharides-mediated crystallization technique.

Fig. 5.2
figure 2

Reflectance spectra of ZnO crystallites prepared from Zn(NO3)2 and KOH, NaOH, (CH2)6N4: in the absence of PSH (a, c, e) and in the presence of PSH: GA (b, d, f), SA (g), and CA (h). In the inset is shown the normalized representation of Kubelka-Munk function employed to estimate the band gap values of ZnO crystallites

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The photoluminescence (PL) spectra of ZnO structures are presented in Fig. 5.3 Typically, a ZnO PL spectrum reveals emission bands in the UV and visible regions [3]. Depending on the growth conditions, an UV emission centred at ~3.27 eV, a violet emission at ~2.90 eV and a green emission at ~2.20 eV are observed in the PL spectra of the samples. The origin of these emissions is intensely discussed among the scientific community.

Fig. 5.3
figure 3

Photoluminescence spectra of ZnO crystallites prepared from Zn(NO3)2 and KOH, NaOH, (CH2)6N4: in the absence of PSH (a, c, e) and in the presence of PSH: GA (b, d, f), SA (g), and CA (h)

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If the UV emission is accepted to have an excitonic origin, the other two emission bands are linked to the defect emissions, whose mechanisms have not been conclusively established, but models related to various defects which can provide recombinations centers within the band gap have been proposed and reviewed in [65]. Thus, zinc vacancies are probably linked to the violet luminescence [66]. It has to be mentioned that the presence of a polymer during the ZnO chemical synthesis process [67–69] favors the appearance of this emission band. The oxygen vacancies and the incorporation of hydroxyl groups in the crystal lattice during solution growth [70, 71] are usually related to the green emission.

Generally, a ZnO material with high defect concentration has a stronger green emission, the defects being responsible for a quenching process resulting in a weak UV emission [72]. Due to the fact that the intensity of the green emission is higher in the case of samples featured by a small crystallite size, means that the concentration of the oxygen vacancies are in inverse ratio relationship with the crystallite size, it can be said that through a higher surface to volume ratio parameter, the crystallite size parameter favors such emission.

The morphologies of all synthesized samples were investigated by scanning electron microscopy (SEM) (Figs. 5.4 and 5.5) in order to explore the influence of the reaction parameters (reducing agent and PSH types) on the ZnO particles’ shape. In the absence of PSH, particles like snow-flakes (Fig. 5.4a), flowers (Fig. 5.4c) and stars (Fig. 5.5e) with sizes in the range of 1–3 μm were prepared.

Fig. 5.4
figure 4

SEM images (at two magnifications) of ZnO crystallites prepared from Zn(NO3)2 and KOH, NaOH: in the absence of PSH (a, a′, c, c′) and in the presence of GA (b, b′, d, d′)

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Fig. 5.5
figure 5

SEM images (two magnifications) of ZnO crystallites prepared from Zn(NO3)2 and (CH2)6N4: in the absence of PSH (e, e′) and in the presence of PSH: GA (f, f′), SA (g, g′), and CA (h, h′)

Full size image

In the alkali solutions, the formation of different structures (snow-flakes or flowers) depends significantly on the nucleation frequency of ZnO. Hydroxyl ions, as predominant ions in the solution, play a crucial role in controlling the synthesis of the ZnO different crystalline facets through the considerable amount of formed [Zn(OH)4]2− complexes. These complex species are negatively charged and have the tendency to be preferably adsorbed on the surface of ZnO nuclei positively charged.

The influence of the GA on the shape of the synthesized ZnO crystallites when different reducing agents were used is observed in Fig. 5.4b, d and f. Thus, the particles shape did not change when GA was added into the KOH or NaOH solutions (Fig. 5.4b and d, respectively), only their size is decreasing at about 500 nm–1.5 μm. From the higher magnification SEM images (Fig. 5.4) it can be noticed that the ZnO particles are self-assembled in complex agglomerated structures: the snow-flakes contain platelets (Fig. 5.4a′ and 5.4b′) and the flowers have many (Fig. 5.4c′) or few petals (Fig. 5.4d′).

Particles synthesized when (CH2)6N4 is used as reducing agent , in the absence of PSH, are star-shape structures (~2–3 μm in diameter) formed by hexagonal prismatic rods (~100–200 nm in diameter, ~1 μm in length) (Fig. 5.5e). As it was mentioned previously, the exact function of (CH2)6N4 in the ZnO synthesis is still unclear [73]. It has been suggested [71] that by a preferentially attachment to the non-polar facets of the ZnO crystallite the (CH2)6N4 cut off the access of zinc ions to them and, in this way, leave for growth only the polar (001) face. In this way, initially are formed rods and later stars, as those seen in the Fig. 5.5e′.

The influence of the different PSH on the shape of the ZnO crystallites prepared in the presence of (CH2)6N4 is observed in Fig. 5.5f–h. All samples show complex agglomerated structures like double-raspberry (with GA—Fig. 5.5f (~200–300 nm in diameter, ~500 nm in total length)), double-jellyfish (with SA—Fig. 5.5g (trunk: ~1 μm in diameter, ~1.5–4.5 μm in total length; end-rod: ~100 nm in diameter, ~400 nm in length)) and edelweiss-flower (with CA—Fig. 5.5h (flowers size in the range of 300–500 nm)). Taking into account two PSH features: first, the PSH complexation nature with bivalent zinc ions [56, 58, 60–62] and second, the PSH high number of reactive functional groups (–COOH, –OH and –NH2), the formation of such different morphologies can be explained. Usually, the zinc ions are closely associated with the PSH chains, these zinc-occupied sites becoming incipient centers of nucleation and initial growth of crystallites. Later, the crystallites will aggregate and form the ZnO particles. If the information regarding the ZnO crystallites size is computed and compared with those obtained from the XRD diffraction patterns a noticeable difference is noted. This difference is explicable due to the fact that XRD data give sizes for ZnO crystallites which are, in fact, the building units of the ZnO structures observed by SEM.

When GA is used, depending on the precipitating agent type, its presence can affect more or less the ZnO particles formation mechanism. In strong alkaline agents (KOH and NaOH), when HO− ions are release very fast in the aqueous solutions, both the nucleation and growth steps being also fast, the GA influences only slightly the particles shape. On the other hand, if a weak base like (CH2)6N4 is used as steady source for slowly release of HO− ions, the GA changes significantly the particles shape, its presence correlated with the homogeneous release of HO− ions leading to the formation of uniform size ZnO structures like double-raspberry structures. The possible mechanism can be described as follows: the zinc ions which are coordinated by the carboxyl and hydroxyl groups from GA are the sites where the nucleation of ZnO begins, further the crystallites grow having the tendency to aggregate into organized structures. Also, the aggregation process of ZnO crystallites into double-raspberry structures (Fig. 5.5f′) is due to the hydrogen bonds’ network formed between the hydroxyl groups present on the ZnO crystallites’ surface and the GA functional groups [56]. Additionally, another aspect regarding the highly branched structure of GA must be taken into consideration. The growth of the ZnO crystallites can take place in the free space of this PSH with numerous interior cavities, as well as inward and outward oriented functional groups with a direct consequence on the uniform particle size. Summarizing, the possible reactions implied in the ZnO synthesis using GA are presented below:

  1. (i)

    using strong base (KOH, NaOH)

    $$ {\text{Zn}}\left( {{\text{NO}}_{ 3} } \right)_{ 2} \to {\text{ Zn}}^{ 2+ } + {\text{ 2NO}}_{ 3}^{ - } $$
    (5.11)
    $$ {\text{yZn}}^{ 2+ } + {\text{ GA }} \to \, \left[ {{\text{Zn}}\left( {\text{GA}} \right)} \right]^{{ 2 {\text{y}} + }} $$
    (5.12)
    $$ \left[ {{\text{Zn}}\left( {\text{GA}} \right)} \right]^{{ 2 {\text{y}} + }} + {\text{ 2yHO}}^{ - } \to \, \left\{ {\left[ {{\text{yZn}}\left( {\text{OH}} \right)_{ 2} } \right]{\text{GA}}} \right\} $$
    (5.13)
    $$ \left\{ {\left[ {{\text{yZn}}\left( {\text{OH}} \right)_{ 2} } \right]{\text{GA}}} \right\} + {\text{ 2yHO}}^{ - } \to \, \left\{ {\left\{ {\left[ {{\text{Zn}}\left( {\text{OH}} \right)_{ 4} } \right]^{{ 2 {\text{y}} - }} } \right\}{\text{GA}}} \right\} $$
    (5.14)
    $$ \left\{ {\left\{ {\left[ {{\text{Zn}}\left( {\text{OH}} \right)_{ 4} } \right]^{{ 2 {\text{y}} - }} } \right\}{\text{GA}}} \right\} \, \to \, \left[ {\left( {\text{yZnO}} \right){\text{GA}}} \right] \, \downarrow + {\text{ 2yHO}}^{ - } + {\text{ yH}}_{ 2} {\text{O}} $$
    (5.15)
  2. (ii)

    using weak base ((CH2)6N4)

    $$ {\text{Zn}}\left( {{\text{NO}}_{ 3} } \right)_{ 2} \to {\text{ Zn}}^{ 2+ } + {\text{ 2NO}}_{ 3}^{ - } $$
    (5.16)
    $$ {\text{yZn}}^{ 2+ } + {\text{ GA }} \to \, \left[ {{\text{Zn}}\left( {\text{GA}} \right)} \right]^{{ 2 {\text{y}} + }} $$
    (5.17)
    $$ \left( {{\text{CH}}_{ 2} } \right)_{ 6} {\text{N}}_{ 4} + {\text{ 6H}}_{ 2} {\text{O }} \to {\text{ 6HCHO }} + {\text{ 4NH}}_{ 3} $$
    (5.18)
    $$ {\text{NH}}_{ 3} + {\text{ H}}_{ 2} {\text{O }} \to {\text{ NH}}_{ 4}^{ + } + {\text{ HO}}^{ - } $$
    (5.19)
    $$ \left[ {{\text{Zn}}\left( {\text{GA}} \right)} \right]^{{ 2 {\text{y}} + }} + {\text{ 2yHO}}^{ - } \to \, \left\{ {\left[ {{\text{yZn}}\left( {\text{OH}} \right)_{ 2} } \right]{\text{GA}}} \right\} $$
    (5.20)
    $$ \left\{ {\left[ {{\text{yZn}}\left( {\text{OH}} \right)_{ 2} } \right]{\text{GA}}} \right\} \, \to \, \left[ {\left( {\text{yZnO}} \right){\text{GA}}} \right] \, \downarrow + {\text{ yH}}_{ 2} {\text{O}} $$
    (5.21)

When SA is used, the particles are more agglomerated comparatively with those synthesized in the presence of GA. This aggregation can be explained through the high affinity shown by the divalent metals ions to alginates rich in polyguluronate units which leads to the formation of egg-box structures [74]. Thus, the sodium ions are substituted by the zinc ions which then fit into electronegative cavities formed by the stack of polyguluronate units, like eggs in egg-box, finally a linked alginate strand being formed. Inside of this network, the interaction between the bivalent zinc ions coordinated to carboxyl groups in the alginate and the HO− ions released slowly in the aqueous solution by the (CH2)6N4 starts the nucleation of semiconductor crystallites of which further growth leads to the formation of double-jellyfish aggregates (Fig. 5.5g′).

When CA is used, the hydroxyl and amino groups presented in polymer are regarded as the reactive sites for the zinc ions’ complexation, through these interactions the metal ions connecting one or more polymer chains like a bridge [39]. The hydrogen bonds (inter- and intra- molecular) between the polymer chains leads to the formation of small and uniform edelweiss-flower ZnO structures (Fig. 5.5h′).

These investigations confirm that by using PSH with different charge and structure the ZnO morphology can be controlled, but the whole formation mechanism and the growth of ZnO crystallites is very complex. Therefore, in Fig. 5.6 is presented a simple illustration of these processes.

Fig. 5.6
figure 6

Schematic illustration of the possible formation of ZnO structures with different morphologies

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All these results suggest that the optical properties are directly influenced by the crystal qualities, sizes and structure defects, in agreement with those reported in [75, 76]. Consequently, each ZnO morphology being featured by different such parameters it can be concluded that the optical properties are sensitive to the particle morphology.

3 Functionalization by Electroless Deposition of Cellulose—Based Natural Fabrics with ZnO Nanostructures

Electroless deposition is a well-established technique for metal plating on non-conducting substrates. The approach is based on the activation of the surface which is intended to be covered with a layer of catalyst material, usually palladium or gold. The ZnO electroless deposition was reported for the first time in 1997 [77] and according to [78–80] the three typical steps (sensitization, activation and catalytic deposition) involved in the ZnO synthesis from an aqueous solution containing zinc nitrate and dimethylamineborane could be described as following:

Sensitization:

$$ {\text{SnCl}}_{ 2} \to {\text{ Sn}}^{ 2+ } + {\text{ 2Cl}}^{ - } $$
(5.22)

Activation:

$$ {\text{Sn}}^{ 2+ } + {\text{ Pd}}^{ 2+ } \to {\text{ Sn}}^{ 4+ } + {\text{ Pd}}^{0} $$
(5.23)

Electroless deposition:

$$ {\text{Zn}}\left( {{\text{NO}}_{ 3} } \right)_{ 2} \to {\text{ Zn}}^{ 2+ } + {\text{ 2NO}}_{ 3}^{ - } $$
(5.24)
$$ \left( {{\text{CH}}_{ 3} } \right)_{ 2} {\text{NHBH}}_{ 3} + {\text{ HO}}^{ - }\mathop{\longrightarrow}^{\text{Pd}}\left({{\text{CH}}_{ 3} } \right)_{ 2} {\text{NH }} + \, \cdot{\text{BH}}_{ 2} {\text{HO}}^{ - } + {\text{ H}} \cdot $$
(5.25)
$$ \cdot {\text{BH}}_{ 2} {\text{HO}}^{ - } + {\text{ HO}}^{ - } \to {\text{BH}}_{ 2} \left( {\text{HO}} \right)_{ 2}^{ - } + {\text{ e}}^{ - } $$
(5.26)
$$ {\text{BH}}_{ 2} \left( {\text{HO}} \right)_{ 2}^{ - } \mathop{\longrightarrow}^{\text{Pd}} \cdot {\text{BH}}\left( {\text{HO}} \right)_{ 2}^{ - } + {\text{ H}} \cdot $$
(5.27)
$$ \cdot {\text{BH}}\left( {\text{HO}} \right)_{ 2}^{ - } + {\text{ HO}}^{ - } \to {\text{BH}}\left( {\text{HO}} \right)_{ 3}^{ - } + {\text{ e}}^{ - } $$
(5.28)
$$ {\text{BH}}\left( {\text{HO}} \right)_{ 3}^{ - } \mathop{\longrightarrow}^{\text{Pd}} \cdot {\text{B}}\left( {\text{HO}} \right)_{ 3}^{ - } + {\text{ H}} \cdot $$
(5.29)
$$ \cdot {\text{B}}\left( {\text{HO}} \right)_{ 3}^{ - } + {\text{ HO}}^{ - } \to {\text{B}}\left( {\text{HO}} \right)_{ 4}^{ - } + {\text{ e}}^{ - } $$
(5.30)
$$ {\text{B}}\left( {\text{HO}} \right)_{ 4}^{ - } \leftrightarrow {\text{ BO}}_{ 2}^{ - } + {\text{ 2H}}_{ 2} {\text{O}} $$
(5.31)
$$ 3{\text{H}}{\cdot} + 3{\text{HO}}^{-} \to 3\text{H}_{ 2}{\text{O}} + {\text{ 3 e}}^{-} $$
(5.32)
$$ \left( {{\text{CH}}_{ 3} } \right)_{ 2} {\text{NHBH}}_{ 3} + {\text{ 7HO}}^{ - } \to {\text{BO}}_{ 2}^{ - } + \, \left( {{\text{CH}}_{ 3} } \right)_{ 2} {\text{NH }} + {\text{ 5H}}_{ 2} {\text{O }} + {\text{ 6 e}}^{ - } ; $$
(5.33)

reaction can be written as

$$ \left( {{\text{CH}}_{ 3} } \right)_{ 2} {\text{NHBH}}_{ 3} + {\text{ 2H}}_{ 2} {\text{O}} \to {\text{BO}}_{ 2}^{ - } + \, \left( {{\text{CH}}_{ 3} } \right)_{ 2} {\text{NH }} + {\text{ 7H}}^{ + } + {\text{ 6 e}}^{ - } ; $$
(5.34)

trapping 2H+ reaction became

$$ \left( {{\text{CH}}_{ 3} } \right)_{ 2} {\text{NHBH}}_{ 3} + {\text{ 2H}}_{ 2} {\text{O }} \to {\text{ HBO}}_{ 2} + \, \left( {{\text{CH}}_{ 3} } \right)_{ 2} {\text{NH}}_{ 2}^{ + } + {\text{ 5H}}^{ + } + {\text{ 6e}}^{ - } $$
(5.35)
$$ {\text{NO}}_{ 3}^{ - } + {\text{ H}}_{ 2} {\text{O }} + {\text{ 2e}}^{ - } \to {\text{ NO}}_{ 2}^{ - } + {\text{ 2 HO}}^{ - } $$
(5.36)
$$ {\text{Zn}}^{ 2+ } + {\text{ 2HO}}^{ - } \to {\text{ Zn}}\left( {\text{OH}} \right)_{ 2} $$
(5.37)
$$ {\text{Zn}}\left( {\text{OH}} \right)_{ 2} \to {\text{ ZnO }} + {\text{ H}}_{ 2} {\text{O}} $$
(5.38)

So, during the sensitization step, the tin chloride is hydrolyzed releasing Sn2+ ions which thought electrostatic attraction modifies the substrate. The activation step is based on the redox reaction which takes place on the modified surface: by adding Pd2+ ions the oxidation of Sn2+ ions into Sn4+ ions is triggered and simultaneously Pd2+ ions are reduced to metallic Pd forming Pdº colloids on the surface.

In the electroless deposition solution, the Zn(NO3)2 is initially hydrolyzed. The oxidation process of (CH3)2NHBH3 is very well explained in [80]. Shortly, the (CH3)2NHBH3 is oxidized in the presence of catalyst to BO2 − and (CH3)2NH and generates 7H+ and 6e−, two of the H+ ions being furthermore trapped to form HBO2 and (CH3)2NH2 +. The released free electrons will reduce NO3 − to NO2 −, inducing an increase of HO− concentration in the particular area. The HO− ions will further combine with Zn2+ ions leading to the appearance of Zn(OH)2, an intermediate product which is not stable and decomposes, at higher temperature, forming ZnO by spontaneous dehydration. The most important characteristic of the electroless process is that the ZnO growth takes place on the metal-catalyzed surface being triggered by the increase of the pH only in the vicinity of this surface via the redox reactions.

A priori, implying only simple subsequently dipping of the substrate in the appropriate solution, the ZnO electroless deposition can be regarded as an industrial technique very suitable for functionalization of complex shape, non-conductive substrates, i.e. cellulose-based natural fabrics with large-area ZnO uniform layers. Today, textile industry is one of the first areas where nanotechnology can be applied at an industrial scale [81–92]. Generally, by implying different nanostructures in a textile-finishing method their specific optical, chemical or electronic properties can be exploited. Thus, the covering of the fabrics with nanostructures such as silver, titanium dioxide or zinc oxide leads to an improvement of their characteristics such as antibacterial [86, 93–95], high UV-blocking [85, 90, 96], self-cleaning [97–99] or superhydrophobicity (water contact angle higher than 150º) [90, 92, 100] properties. Moreover, a chemical fabric functionalization can lead to an increase in durability of the additional textile characteristic due to a better affinity between the fabrics and nanostructures comparatively with the “classic” textile-finishing methods where fabrics lose their functions after laundering or wearing. Additionally, the fibers are optimal substrates for deploying nanostructures featured by a large surface area characteristic for a given weight or volume of fabric, the synergy between nanotechnology and the textile industry being practically based on this large interfacial area.

One of the most important natural fabrics’ property changed by their functionalization with ZnO is the wettability . As it is well known the abundant water-absorbing hydroxyl groups presented on the cellulose-based fabrics surface are responsible for their highly water absorbent behaviour. In order to make such surfaces water repellent, the fabrics are functionalized with ZnO using different techniques such as by layer by layer assembly [95], sol-gel [99, 101], hydrothermal [90, 99, 100], ultrasound irradiation [102] and pulsed layer deposition [103]. But as it was mentioned above, the electroless deposition seems to be an ideal method for achieving a high degree of functionalization of the natural fabrics being cheap, scalable and providing a uniform result on large areas and a good adhesion to the substrate.

Choosing two of the most used cellulose-based fabrics, i.e. cotton and hemp, the changes induced in their properties by the ZnO electroless deposition on the natural fabrics surfaces were put in evidence. It has to be mentioned that in the case of cotton, fabrics with three different mesh densities (net-like open appearances and spaces between the yarns), namely sparse, medium and dense were investigated. (Figs. 5.7 and 5.8). According to [104, 105], the ZnO synthesis involved the following steps: the cotton or hemp fabrics were immersed, at room temperature, 60 min in an aqueous sensitization solution (40 g/l SnCl2 and 20 ml/l HCl (37 % vol.)) and 30 min in activation solution (0.1 g/l PdCl2 and 20 ml/l HCl (37 % vol.)) further being immersed, at 70 ºC or 80 ºC, 2 h into aqueous solution which contained 0.07 M Zn(NO3)2 and 0.01 M (CH3)2NHBH3. For comparison purposes another metal which can be used as catalyst, gold, was sputtered as thin layer (ca. 3–4 nm) onto cotton fabric surface and the growth of ZnO low-dimensional particles was also tested on such fibers.

Fig. 5.7
figure 7

Optical images of cellulose-based fabrics: cotton with different mesh density (sparse (a), medium (b) and dense (c)) and hemp (d)

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Fig. 5.8
figure 8

SEM images of cellulose-based fabrics: cotton with different mesh density (sparse (a), medium (b) and dense (c)) and hemp (d)

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The XRD patterns of pure and ZnO-coated natural fabrics (cotton sparse mesh density and hemp) are presented in Fig. 5.9. For the native cellulose-based textiles, the XRD patterns (curves a and c) show two narrow peaks at 15º and 17º and one sharper intense peak at 23º related to the crystalline phases of cellulose fibers [106, 107]. For all functionalized fabrics the diffractograms show sharp, narrow and intense diffraction peaks corresponding to the Miller index of the reflecting planes for (100), (002), (101), (102), (110), (103) and (112) assigned to ZnO hexagonal wurtzite phase (JCPDS file no. 36-1451), confirming the deposition of ZnO crystallites on cellulose-based fabrics surface. In the XRD patterns of the samples no peaks characteristics of catalysts (gold or palladium) or other segregate phases, such as Zn(OH)2 were observed.

Fig. 5.9
figure 9

The XRD patterns of: pristine hemp (a) and cotton (c) fabrics; ZnO-coated cotton fabrics using a thin gold layer as catalyst (d); ZnO-coated natural fabrics using palladium as catalyst: hemp (b) and cotton at two reaction temperatures: 80 ºC (e) and 70 ºC (f). (Peaks attributed only to cellulose-based fabrics)

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The morphology of all synthesized samples was investigated by SEM. In Fig. 5.10 the influence of the catalyst type (gold or palladium) and fabric type (cotton or hemp) on the deposition of ZnO low-dimensional particles was evidenced. It had to be noted that in these cases, the same deposition conditions (reactants concentrations, deposition time and reaction temperature) were used. For two cotton samples, when gold is used (Fig. 5.10a), most of the fibers were not covered by semiconductor particles, while when palladium is used (Fig. 5.10b) the fibers were covered by a continuous and homogeneous film formed by ZnO crystallites. Responsible for this difference in fibers covering with ZnO could be the catalyst nature but also the different deposition techniques used for the two metals. Regarding the catalyst’s nature, according to [108], palladium is more active for dimethylamineborane oxidation (higher rate) than other noble metals (Au or Pt). Additionally, the two catalyst metals were deposited by different methods: palladium by a chemical approach and gold by a sputtering technique. The chemically deposited palladium has a result the formation of a continuous ZnO coating of the fibers surface. Instead, the gold layer catalyst leads to the appearance of only some sparse ZnO crystallites. Being made by interlacing yarns, a fabric presents many convolutions. Thus, when a physical method based on the application of non-chemical forces for deposition is used, for the metal deposition only parts of fibers are covered with catalyst metal. The fibers areas not coated with metal could not participate in the ZnO deposition and most of the surface remained deposit free. Because the applicability of gold as a catalyst for ZnO deposition from zinc nitrate solution containing dimethylamineborane were reported in the [109, 110], it can be concluded that the low efficiency of sputtered gold as a catalyst for ZnO electroless deposition is a consequence of both catalyst nature and method chosen for deposition i.e. sputtering on a rough substrate (fabrics). For hemp fabrics (Fig. 5.10c), where palladium is used as catalyst, a uniform covering similar with that observed for cotton is obtained.

Fig. 5.10
figure 10

SEM images of ZnO-coated natural fabrics: cotton with medium mesh density deposited using as catalyst gold (a) or palladium (b); hemp deposited using as catalyst palladium (c). All samples were deposited at 70 ºC

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Since the palladium colloids catalyst leads to the deposition of the most uniform ZnO films, further the investigations were focused on the properties of the samples prepared only with this metal.

Figure 5.11 presents the morphology of the ZnO crystallites deposit on the fabrics with different mesh density for two reaction temperatures 70 ºC and 80 ºC. In all cases, a continuous, homogeneous and densely packed array of ZnO crystallites covered the surface of the fibers. As a result of the important role played by the diffusion processes which are involved in each of the three steps, the fibers closest to the fabric surface show the highest deposit density. As it was already mentioned, the hydrolysis of tin chloride in the sensitization step gives rise to the Sn2+ ions, these being adsorbed on the material surface. The uniformity of coverage with Sn2+ ions of the entire sample is determined by the length of the adsorption process, this being important for fabrics, substrates with complex shapes. Based on the reduction of the Pd2+ ions by the Sn2+ ions, in the activation step, palladium metal colloids are formed by being attached on the material surface. According to [111] densities of 1012–1013colloids/cm2 are obtained. Finally, in the deposition step, the Zn(NO3)2 is hydrolysed, dimethylamineborane being the reducing agent for the NO3 − ions. The overall process is complex and implies many sub-steps very well explained in [80]. Referring to diffusion processes, all these reactions will take place faster on the exterior layer of fibers and slower on the fibers which are deeper in the textile material, where diffusion is limited. Also, it should be mentioned that the layer covering the fibers is relatively thick, the palladium catalyst being most probably completely covered by ZnO in the initial part of the deposition process. So, the reaction is catalyzed by the palladium colloids but also by the ZnO, the process could be described as being autocatalytic. Initially, due to the catalytic reaction , ZnO crystallites are formed on the fabric surface, their nucleation beginning from the palladium colloids. As the reactions continue, ZnO crystallites continue their growth and cover completely the activated surface. Besides the oxidation reduction processes, the formation of Zn(OH)2 and its subsequent decomposition are also essential in the growth process of the ZnO particles. Especially in the case of complex shaped surfaces, all these process parameters increase the role of both reaction constants and diffusion processes. These must be adjusted in such a way that the process becomes reaction limited in order to obtain a uniform layer of ZnO and not layers with increased thickness on the exterior surfaces of the fabric and thinner on the inner surfaces of the material.

Fig. 5.11
figure 11

SEM images of ZnO-coated cotton fabrics deposited at two reaction temperatures 70 ºC and 80 ºC: sparse (a, d), medium (b, e) and dense (c, f) mesh density

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Using a higher reaction temperature (80 ºC) a similar result was obatined, the cotton fibers being covered with a compact, highly uniform and densely packed array of ZnO crystallites . However the deposit surface presents a rougher aspect. In addition, above the compact layer, aggregates of ZnO particles with irregular shape are visible in this case.

SEM images at higher magnification (Fig. 5.12) show that regardless the reaction temperature, the ZnO low-dimensional particles present a characteristic hexagonal prism shape. The particles are well faceted, with the base of 400–500 nm and the height of approximately 1 μm (Fig. 5.12a′). The particular shape of the ZnO crystallites is linked to the fact that the c-plane corresponds to the densest packed plane and the growth is governed by Bravais empirical law for crystal growth [112]. As it can be seen, the morphology of the semiconductor particles is not strongly influenced by the reaction temperature, this affecting only the deposition rate and volume. A higher reaction temperature could most likely lead to a supplementary reduction/oxidation process besides of those catalyzed by the substrate, a higher density of loose aggregates of ZnO prisms being observed on the samples deposited in these conditions (Fig. 5.12b′). In other words, the aggregates appear as a result of a reaction taking place in the volume of the solution, instead of growning on the fiber surface. Nevertheless, the reaction is still limited to the proximity of the functionalized surface. Most probably, the reaction is still possible in this area due to a local increase in pH. It has to be noted that no precipitate was observed in the volume of the solution or later decanted on the bottom of the beaker. The same densely packed hexagonal prism with the base size ranging from 20 to 500 nm and the length scale of 0.7–1 μm is also observed in the case of hemp fabrics (Fig. 5.12c′).

Fig. 5.12
figure 12

Higher magnification SEM images of ZnO-coated natural fabrics: cotton with medium mesh density at two reaction temperatures 70 ºC (a, a′) and 80 ºC (b, b′) and hemp at 70 ºC reaction temperature (c, c′)

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A typical atomic force microscopy (AFM) image for the ZnO-coated cotton fabrics (Fig. 5.13) reveals the same ZnO hexagonal structure and a small roughness (Ra = 81 nm) in comparison with the prism dimensions, indicating also a densely packing of the ZnO prisms on the cotton fibers.

Fig. 5.13
figure 13

AFM image of ZnO-coated cotton fabric with medium mesh density

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The modifications induced by the ZnO-functionalization of cotton fabrics in their optical properties are presented in Fig. 5.14. In the ZnO covered samples spectra (curves d–f), a strong decrease of reflectance can be noticed at about 380 nm, due to the band-to-band transition in ZnO. A band gap of around 3.2 eV is estimated from the Kubelka-Munk representation (Fig. 5.14 inset). From Fig. 5.15 (curves a–c) it can be seen that the native cotton fabrics with different mesh densities have high transmittance for UV–VIS light with an increasing tendency from the VIS to the UV spectral region. This transmittance is dramatically reduces when the ZnO prisms are deposited on the cotton fibers surface (Fig. 5.15, curves d–f). Through the higher density coated textile nearly no UV light is transmitted in the 280–375 nm wavelength regions. Even the sparsest material presents a drop of transmittance from 60–70 to 30–40 %. Responsible for such excellent UV-blocking property of the coated cotton fabrics is the strong scattering of ZnO crystallites arrays [92]. Interestingly, the ZnO band-to-band absorption is not present in the transmission spectra. This can be linked to the strongly scattering by ZnO prisms of the incident light, which is further absorbed in the fabric due to an increased length of the optical path.

Fig. 5.14
figure 14

Reflectance spectra of pristine cotton fabrics (dense—a, medium—b, sparse—c) and ZnO-coated cotton fabrics (dense—d, medium—e, sparse—f). In the inset is shown the representation of Kubelka-Munk function employed to estimate the band gap values of ZnO-coated cotton fabrics (sparse—d, medium—e, dense—f)

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Fig. 5.15
figure 15

Transmission spectra of pristine cotton fabrics (sparse—a, medium—b, dense—c) and ZnO-coated cotton fabrics (sparse—d, medium—e, dense—f)

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In order to observe if the hydrophilic character of pristine cotton fabric was changed after the ZnO deposition, simple water droplet test were done on coated fabrics. Photographs of these tests carried on samples with three different mesh densities are presented in Fig. 5.16. In all cases the behaviour is similar, the surfaces being water repellent in contrast to pristine cotton fabrics which absorb immediately the water droplets. An important experimental detail must be emphasized: if the ZnO-coated fabrics were washed, the washing water remains clean, suggesting that they have a good adhesion to the fibers surface.

Fig. 5.16
figure 16

Optical photographs of a water repellent effect on ZnO-coated cotton fabrics with different mesh density: sparse (a); medium (b) and dense (c)

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The wetting properties of the ZnO-coated fabrics were further investigated by contact angle (CA) measurements, in Fig. 5.17 being presented the photographs of water droplets and CA values for different samples synthesized at two reaction temperatures along with the associated SEM images.

Fig. 5.17
figure 17

SEM images of ZnO-coated cotton fabrics with different mesh density at two reaction temperatures 70 ºC (sparse (a), medium (b) and dense (c)) and 80 ºC (sparse (d), medium (e) and dense (f)). In the inset are shown the optical photographs of the water droplets shape on the hydrophobic surface of ZnO-coated cotton fabrics and the water contact angles for the corresponding samples

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As it is seen, hydrophobic and even superhydrophobic behaviour is exhibited by the textile surface after the covering with ZnO. All samples are characterized by the CA values higher than 130º, the cotton fabrics with sparse mesh density having CA values even beyond 150º (Fig. 5.17a, d). This implies that a superhydrophobic behaviour is achieved when the cotton fibers are uniformly coated with ZnO crystallites arrays. As it was presented above, due to the diffusion to reaction ratios, lower mesh densities fabrics are better covered with ZnO than denser ones. In all cases, the outer surfaces are more exposed to reactants, while for deeper into the fabric sites the transport of reactants and therefore the growth process it is diffusion limited. In this way, the fibers with larger spaces between the yarns are better coated with ZnO comparatively with those with narrower spaces. The ZnO-coated hemp textile presents also a superhydrophobic effect (154º being the CA value).

Superhydrophobicity is typically a consequence of two factors, roughness and low surface energy. Usually this interesting and useful wetting property can be reached by a high roughness or by a chemical modification of the surface. In the present case, the surface covered with the ZnO prisms is already superhydrophobic and the chemical treatment is not necessary. Thus, the results can be explained based on the Cassie-Baxter model [113]. According to this model, when a liquid droplet is positioned on a rough surface it sits on a surface composed from both the solid substrate and the air entrapped. Taking into account that the deposited ZnO consists of a large number of small prisms is featured by a high roughness, so the apparent CA is higher than that obtained for a similar smooth surface [114, 115]. Practically, the numerous gaps between these ZnO crystallites are filled with air, acting as a support “buffer” for the water droplet which is in contact to the surface only in few small nanometric sites. A similar behaviour was observed for cotton fabrics coated with ZnO by pulsed laser deposition technique [103], the low-dimensional structures transforming the hydrophilic cotton fabric into a superhydrophobic one.

4 Conclusions

The preparation of functional nanomaterials based on ZnO and PSH (both eco-friendly materials ) is an exciting research field. In the present chapter has been shown the possibility of developing such materials using two simple and low-cost solution preparation methods which are less hazardous and more environmentally friendly and do not require complex apparatus favouring the scaling-up of the techniques. Using polysaccharide-mediated crystallization ZnO crystallites with complex morphology can be synthesized. It is found that both, the structure (linear or branch) and the charge (anionic or cationic) of the PSH play an important role in the morphological, structural and optical properties of the ZnO crystallites. Cellulose-based fabrics, such as cotton and hemp were functionalized by ZnO electroless deposition. It is demonstrated that the ZnO crystallites grown onto cotton fabrics strongly enhanced the protection provided by the cotton fabrics against UV radiation in comparison with the untreated material. A switch in the wetting behaviour of the cotton fabrics from hydrophilic to superhydrophobic was observed after their ZnO-functionalization. The full potential of such functional materials containing ZnO and PSH will be realized only when they will be synthesized in large quantities using green, environmentally responsible technologies.