Dimensionally stable cellulosic aerogels functionalized by titania

Introduction

Cellulose is a ubiquitous polymer of biological origin that belongs to polysaccharides. Its primary sources are wood and cotton, but it can be separated also from plants, algae, some bacteria and the tunicate Ascidiacea [1], [2]. They produce annually 100–300 billion tons of cellulose that can be considered as an inexhaustible raw biomass 2], [3]. Abundance and availability in combination with excellent mechanical properties and low disposal cost make it widely employed material in daily life. Traditional cellulosic products, textile and paper, are made from natural fibers. Man’s earliest fragments of dyed flax date to as early as 30000 years ago 4]. Paper manufacturing was invented in China at approximately 100 AD [5]. Nowadays novel forms, such as microfibers and nanofibers, nanopaper, liquid crystals and aerogels are available [6], [7], [8], [9]. They open up new avenues of applications of cellulose. However, there are serious limitations. Cellulosic materials like the most polysaccharides are highly hygroscopic. When being wetted, they are susceptible to fungal and bacterial decay 10], [11]. Absorbed water brings about also a sharp decrease in the mechanical strength 12], [13]. Wetted materials are not dimensionally stable 13], [14]. Another factor restricting the application is hydrophilicity of cellulose. Owing to this property, the polysaccharide is incompatible with thermoplastic and thermoset hydrophobic polymers like polypropylene and polyethylene. In particular, they cannot be mixed properly together that restricts the application of cellulose as a reinforcing, biodegradable filler and the preparation of composites 15], [16].

To extend practicality, cellulose is the subject of various chemical modifications including esterification and etherification, oxidation, grafting and crosslinking [15], [17], [18], [19], [20], [21], [22], [23]. Chemical treatments were started soon after its discovery in 1838 3], [18], [24]. At the present time, cellulose serves as a convenient organic synthon of biological origin, template in syntheses, preparation of bionanocomposites and precursor for carbon. Its modification by nanosized metals and metal oxides is considered as a very fruitful new avenue of research. Inorganics have great variety of properties and various hi-tech applications. Combination of nanosized metals and metal oxides with cellulose, which is called the mineralization, results in composites that can be referred to bionanocomposites because of constituents of biological origin and inorganic nanoparticles 25]. Inorganics can increase thermal, chemical and biodegradation stabilities, mechanical strength, but what is most significant is to impart new properties and functionalities, such as UV protection, self-cleaning and bacteriostatic abilities, water proofing, superhydrophobicity and electric conductivity [11], [26], [27], [28], [29], [30], [31], [32], [33], [34]. The cellulose mineralization is an emerging field of engineering materials of novel types for innovative textiles and paper, electronic devices, bioengineering, biotechnology and ecology 1], [8], [23], [35], [36], [37], [38], [39], [40], [41], [42], [43].

Cellulose as substrate and template has much potential for coating by inorganics. It is explained by abundant surface hydroxyl groups, high porosity and hierarchical structure. Metal and metal oxides have an opportunity to condense on these groups, while their structural organization from nano- to the macroscopic level is determined by the template. Cellulosic aerogels possessing dimensional stability, elasticity and improved mechanical strength in the wetted state, which have been developed recently, have potential as substrate for newly emerging applications.

When biopolymers are combined with metals and metal oxides, account is taken of the fact that mineralized tissues, cells and microorganism are almost ubiquitous in living nature [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]. Primary sources of cellulose, cotton and wood, are exceptions. Polysaccharide content in the former accounts for 95%, whereas it is shelled in the latter by organics, lignin (15–35%) and hemicelluloses (25–40%) 2], [24]. Cellulose is mainly associated with biominerals in plants. It was mentioned yet in 1815 54] that ash of some plant leaves contained up to 70% of silicon. Such plants as rice, cereals and cucumber are considered to be silica accumulator [52], [55]. The fact worth mentioning is a limited number of biominerals in living nature. There are mainly silica, calcium carbonate and some phosphates. Chemists can prepare bionanocomposites of cellulose in principle with any metal and metal oxide. Silica and titania are used frequently for the mineralization. Because of the vast scope of the subject this review is restricted by consideration of titania. This nontoxic metal oxide has a reasonable price. The interest is caused by its remarkable photocatalytic activity [56], [57], [58], [59], [60], [61], [62]. The mineralization of cellulosic materials provides multi-functional modification and novel consumer properties, such as self-cleaning, antimicrobial, superhydrophobic, deodorizing and ultraviolet protecting 11], [23], [28], [35], [63], [64], [65].

Cellulose

The discovery of this polysaccharide is dated by 1838 when Anselme Payen described results of examination of the chemical composition of what is called pulp now after crushing plant and wood [66]. The term cellulose consisting of Latin cellula (cell)+French -ose was coined in a report of the French Academy of Sciences prepared on the Payen’s publication by Jean-Baptiste Dumas in 1839 [67]. It was also mentioned that there is a similarity of the empirical formulas of cellulose and dextrin (starch). They both consist of D-glucose or D-glucopyranose.

Two D-glucose residues are linked in cellulose by a covalent glycosidic C–O–C bond between the C4 and C1 carbon atoms, forming a cellobiose that is a repeating unite of macromolecule. Structural formula is shown in Fig. 1. One can see three hydroxyl groups in each D-glucose residue that make the biopolymer hydrophilic. Owing to their presence, polysaccharides are usually hygroscopic and water soluble, but it is not valid for cellulose. Although it is hygroscopic, the solubilization is possible only in harsh conditions or in some ionic liquids because of tight association of macromolecules [68], [69]. It happens in the course of biosynthesis. The association occurs in the extracellular compartment in which a microfibril is formed from up to 36 macromolecules synthesized simultaneously in a supercomplex of cellulosesynthases [70], [71], [72]. The microfibrils are elemental units of macrofibrils which form fibers. Hierarchical structure of cellulose is shown schematically in Fig. 1.

Fig. 1:

Schematic presentation of hierarchical structure of cellulose. Picture of cellulose fibers was taken by scanning electron microscope. Cellulose I is presented by two parallel aligned cellobiose units consisting of two D-glucose (β-(1–4)-D-glucopyranose) residues. Macromolecules are linked numerous inter- and intramolecular hydrogen bonds shown by doted red lines.

It is believed that the association of macromolecules occurs through numerous hydrogen bondings. Chains are aligned parallel to one another, forming a crystalline lattice known as the cellulose I [3], [8], [69], [70]. This structure is presented in Fig. 1. The crystalline domains alternate with amorphous regions. They are arranged into a microfibril of which diameter is varied from 3 to 5 nm in wood up to 40–60 nm in cottonseed linters 73]. Crystallinity or the percentage of crystalline regions reaches ca. 45% in ramp and flax, 56–65% in cotton linters, 65–79% in bacterial and >80% algal cellulose 1], [8], [9], [74], [75]. Crystalline domains are available in the form of nanofibrils termed frequently nanocellulose (Fig. 1) 1], [6], [7], [74], [76], [77], [78], [79].

Cellulosic materials. Aerogels

Cellulose fibers were used throughout human history. Today, mankind is on the threshold of widening the scope of applications of cellulose into areas in which it has not been used. The expansion is related mainly to its basic structural units, nanocellulose and microfibrils (Fig. 1) [6], [7], [8], [9]. Their separation has been developed in the last 20 years after the seminal work published in 1983 80], [81]. Turbak et al. observed a transformation of chopped pulp fibers into a translucent jelled mass after exposing it to the action of high shear and pressure up to 8000 psi at temperatures of approximately 80°C in the industrial milk homogenizer. Their product consisted of micro/nanofibrils, which were obtained after the disintegration of initial fibers. Turbak et al. coined it as microfibrillated cellulose (MFC), but further researchers called it as nanofibrillated cellulose and nanocellulose. The reason is that there are fibrils of which diameter is of the nanometer range.

Effects of cellulose microfibrillation on the properties and strength of paper was mentioned in the thirties of last century [82], [83], [84], [85]. Mechanical (beaten) treatment of pulp fibers was introduced to strengthen the paper. Nowadays refining process is common in the paper industry 85]. Fiber suspension is repeatedly passed through a gap between disks the surfaces of which contain bars and grooves. These passages result in the microfibrillation that was clearly demonstrated first by Bell in his PhD thesis of 1933, presenting a set of microphotographs of initial and beaten fibers [82].

The refiner does not allow reaching the well homogenization. For example, Kim et al. used 40 grinding cycles [86]. Turbak et al. could increase a degree of microfibrillation when they introduced additional treatment [80], [81]. Dilute slurry of cellulosic fibers after refining was exposed to a large pressure drop with shearing as a valve in industrial milk homogenizer was opened. Even this overwhelmingly greater mechanical treatment needs to be repeated over and over again.

Microfibrillation is simplified if cellulose is initially pretreated. Preliminary oxidation is in common practice. At present it is made frequently by using sulfuric acid. It serves as an agent hydrolyzing preferentially amorphous parts of fibers. After Dong et al. performing thorough study [87], 64% (w/v) sulfuric acid at 45°C is applied. Cellulose is also oxidized by means of hydrochloric acid [88], [89], sodium periodate 90], ammonium persulfate [91] hydrogen peroxide [65], but a widely accepted oxidizing mixture is TEMPO (2,2,6,6-tetramethyl-1-piperidine oxoammonium salt) with sodium hypochlorite and sodium bromide introduced first by Chang and Robyt [92]. They showed that TEMPO provided selective oxidation of primary hydroxyls to carboxylic groups that makes polysaccharide soluble in water.

The MFC preparation can be facilitated also by the digestion with endoglucanases [93]. Enzymes reduce the degree of polymerization of cellulosic macromolecules, similar to the chemical oxidation [94]. It is pertinent to note that the decrease of molecular weight and the oxidation, which introduces functional groups, modify notably the properties of cellulose and sometimes can deteriorate the performance characteristics [95], [96], [97].

Chemical oxidants and enzymes act mainly on amorphous regions of microfibrils [1], [10], [79], [94], [98], [99], [100], [101]. Their disintegration results in a release of the nanosized crystalline regions (Fig. 1) as rod-shaped nanoparticles which are rather chemically resistant. They were separated first by Wiesner in 1886 after the treatment of fibers with acids at elevated temperatures (50–70°C) [102]. There are first images in the form of drawings of fine particles termed “dermatosomes”. Ranby treated wood and cotton cellulose in boiling 2.5 N sulfuric acid as suggested earlier by Nickerson and Habrle 99], preparing dispersions in water in 1949 [[103], [104]. These crystalline nanofibrils are called nanocellulose after Klemm et al. 78]. Oxidants and endoglucanases mentioned above were applied successfully to prepare nanocellulose by various teams [20], [74], [78], [79], [100], [105], [106], [107].

MFC and nanocellulose have attracted increasing attention over the past decade because they are thought to be the main innovative cellulosic products. In particular, it is evident from patent activity the total number of which had exceeded 10000 by the beginning of 2012 [76]. Battista was among the first who perceived the innovative potential of nanocellulose. He started simultaneously with Ranby in 1950 [88]. Thorough studies of cellulose degradation by the action of hydrochloric acid at various concentrations and temperatures led him to the commercialization of product in 50 s called microcrystalline cellulose under the trademark Avicel. Battista demonstrated in the later article that Avicel can be put to a number of new uses as a tablet binder and carrier of antibiotics in pharmaceutical industry, a texturizing and jellifying agent, a fat replacer and edible substrate for vitamins and essential oils in food, a strengthening additive in paper and composites, sound-proofing and flame-retardant material in construction [108].

Innovative potential of MFC and nanocellulose is explained by properties not found in cellulose. It is practically impossible to make a homogeneous dispersion of fibers in solutions, whereas microfibrils and nanosized fibrils form stable dispersion in water [[108]. One can find a birefringent solution of dispersed nanocellulose after its preparation in a case the critical concentration (3–7 wt.%) has been achieved [109]. Picture of brilliant interference colors can be seen for a sample placed between crossed polarizers (Fig. 2). Formation of optically active mesophase is caused by uniform orientation of nanofibrils self-organizing into a chiral nematic phase [109], [110], [111]. This liquid-crystalline organization is retained in films after the solvent evaporation [112]. They can be used as photonic crystals of which selective diffraction of light wavelength is regulated by means of helical pitch of the chiral nematic structure. High sensitivity of screw-like structural organization of nematics to the sample dryness [113], temperature [114], ionic strength [114], organic substances [114] and magnetic field [115] make them useful in sensors, optical filters and displays 1], [6], [106], [110], [116].

Fig. 2:

Picture of nanocellulose dispersion in water shown in a separating funnel between crossed polarizers at a final stage of separation. It was prepared from cotton wool oxidized by 64% (w/v) sulfuric acid at 45°C, washed, dialyzed and finally ultrasonically treated.

Mann et al. used films with chiral nematic long-range order as a template to synthesize initially a birefringent nanocellulose-silica bionanocomposites and then to prepare birefringent mesoporous silica by the calcination at 400°C [117]. Systematic studies performed by MacLachlan’s team have revealed main features of the mineralization structurally ordered nanocellulose and come up with chiral bionanocomposites and silica with tunable mesoporosity, structure and photonic properties [118], [119], [120]. Inorganic mesoporous matrix was further functionalized by entrapped noble metal nanoparticles and quantum dots [121], [122]. Chiral silica was used by Zhang et al. to prepare a stationary phase for the high performance liquid chromatographic separation of positional isomers [123]. Furthermore, approach was extended for synthesis of titania with helically arranged mesopores [124], [125]. Nanocellulose formed chiral nematic mesophase not only in water, but also in liquid nitrogen in the presence of ammonium thiocyanate that was used to prepare titanium and vanadium nitrides [126].

These preceding examples illustrate novel functional materials which are appropriate for many industrial product sectors as reflective filters, photonic materials, supercapacitors, enantioselective sensors, membranes and chromatographic columns separating enantiomers [[110], [111], [116], [120], [123], [127]. However, nanocellulose and MFC have wider innovative potential. High-performance cellulosic materials are engineered on the basis of free-standing films, (nano)paper, membranes, coatings, fillers, bionanocomposites and aerogels 7], [38], [41], [76], [77], [78], 110], [111], [116], [128], [129], [130], [131], [132], [133], [134], [135]. The latter are very promising for various applications. Aerogels advantages include low or ultra-low (0.004 g/cm³) density, large specific surface area and a pore volume that sometimes reaches 99.8% 60], [136], [137], [138], [139].

Kistler made the first attempt to prepare cellulosic aerogels from regenerated cellulose in 1931 [[140]. He mentioned that they were not mechanically strong, ductile and dimensionally stable. Wetted aerogels shrunk up to a film when being dried. The problem is caused by the formation of an inhomogeneous dispersion of fibers in water and other solvents. They can only swell. Agitation of solution results in the association of fibers into clots 13]. If it is stirred, fibers are coiled around the stirring rod. Their separation is possible only by hands, but then they are clotting again in solutions. Heterogeneity retains in a prepared aerogel. It is illustrated by a picture of sample from cotton wool in Fig. 3a. As seen in the next picture, this aerogel is dimensionally unstable after wetting (Fig. 3b,c).

Fig. 3:

Aerogels made from cotton wool (a) and MFC (d) in water (b, e) and wetted state (c, f), respectively. Both samples were prepared from cotton wool that was chopped up preliminary into small pieces of 5–7 mm in length.

The situation is reversed if cotton wool is exchanged for MFC. Such a sample is shown in Fig. 3d. Aerogels made from MFC are homogeneous. The most notable difference from the cotton wool sample (Fig. 3a) consists in the dimensional stability [13]. Wetted aerogel does not shrink up and sag when it was retrieved from water (Fig. 3e,f).

Another important issue of the cellulose fibrillation is in the improvement of mechanical properties. Figure 4a shows a plot constructed in the coordinates of compressive stress upon the deformation (strain) for dried MFC aerogel after its preparation (curve 1). Straight line 2 is a continuation of the initial linear region. It serves to determine the Young’s modulus or modulus of elasticity. Curve 3 presents the best fitting result in accordance with the BST (Blatz, Sharda and Tschoegl) equation [141]:

Fig. 4:

(a) Compressive stress vs. strain diagram for MFC aerogel (1), the tangent to the initial linear region (2) and the best fit to the BST eq. (3). (b) Young’s moduli of aerogels made of cotton wool and dry and wet MFC.

σ = 2E/(3n) (λn– λ(−2n))

where σ is the stress, λ the strain, E the Young’s modulus, n the elasticity parameter. Blatz et al. came to this phenomenological equation when examined behavior of rubber-like materials at large mechanical deformations [141]. It consists in two constituents including linear and non-linear parts at small and large elastic deformations, respectively (Fig. 4a). The former is characterized by the Young’s modulus, the latter, by the elasticity parameter n. The Young’s modulus obtained from the plot in Fig. 4 is equal to 61 kPa. Value of E for cotton wool aerogels varied from 1.2 to 11 kPa for dense parts and from sample to sample. Notable variation is explained by their heterogeneity (Fig. 3a).

Elasticity parameter n found by means of the fitting procedure (Fig. 4a) is equal to 3.8 for aerogels from both MFC and cotton wool. This result deviates significantly from the n determined for rubbers by Blatz et al. In their case the value of n was obtained around 1.5 [141]. Cellulosic aerogels can be considered as quasi-elastic materials. Difference from rubbers shows up most vividly not in the elasticity parameter, but after the compression. Aerogels were not restored to their initial state.

Effect of water on the mechanical properties of MFC aerogel is obvious from diagram in Fig. 4b. Young’s modulus drops for about an order of magnitude. Much the same or even larger decrease of mechanical strength on cellulosic materials is mentioned by another authors (see, e.g. [96], [97], [142], [143]). Large loss in the mechanical strength and an increase of softness is caused by considerable swelling. Water absorbed by fibers/fibrils weakens interactions between them [144], [145] and occur “plasticizing” effect.

It should be mentioned that the Young’s modulus of wetted MFC aerogel is at the level of E for dried aerogels of cotton wool. The latter becomes too weak and soft in water to measure its Young’s modulus. Wetted MFC aerogels are rather mechanically strong to held their shape and be dimensionally stable. Furthermore, dried samples are somewhat tough and elastic (Fig. 4a). This is serious advantage of MFC against inorganics like silica. Siliceous aerogels are mechanically strong but they are very fragile and brittle to be handled [60], [136], [146]. Fragility is serious disadvantage, limiting application of inorganic aerogels. There are good prospects for MFC in this area.

Substantial improvement of mechanical properties and dimensional stability of aerogels after the cellulose microfibrillation is explained by partial disintegration of fibers. It was provided by a combination of sequential mechanical treatment and freeze-thawing. Initial and treated fibers can be seen in Fig. 5a and b, respectively. The former have smooth surface. The number of contacts between them is not so much. After the microfibrillation, partially disintegrated fibers are surrounded by numerous nano-/micro fibrils (Fig. 5b). Their entanglement and contacts serve as cross-links in the three-dimensional network. They are weak in comparison with covalent bonds but their drastically increased number enhances the mechanical strength and results in the dimensional stability of aerogels from MFC. Chakraborty et al. showed in their experiments that microfibers added in a polyvinyl alcohol matrix consisting of linear macromolecules like fibers in cellulosic aerogel caused a significant increase of stiffness [147]. Tejado et al. mentioned an opposite effect of a dramatic decrease of strength when number of entanglements was decreased after the addition of stiff glass rods into paper [148]. Therefore, the increased number of contacts between entangled nano-/micro fibrils after the microfibrillation should play a major role in the stabilization of cellulosic aerogels.

Fig. 5:

Scanning electron microscope images of initial and flaked fibers of cotton wool (a) and MFC (b) aerogels, respectively.

It is worthy of mention that our dimensionally stable aerogels (Fig. 3d–f) were prepared from cellulose only mechanically treated. Preliminary oxidation, which is usually practiced, has not been used. This additional procedure simplifies the dispersion of microfibrils in water (see, e.g. [21], [149], [150], [151]), but it brings about their chemical modification as well. In particular, functional groups like carboxylic are introduced. Such cellulose possesses properties of polyelectrolytes. Electrostatic repulsion between charged macromolecules prevents usually their association, while they form polyelectrolyte complexes in the presence of oppositely charged polymers [152]. Complexation of oxidized cellulose with positively charged polyelectrolytes is used to make films and coating by the layer-by-layer technique (see, e.g. [153]). Electrostatic repulsion can weaken aerogels and cause their disintegration in water. Therefore, we did away with the chemical oxidation. To ease the microfibrillation, freeze-thawing procedure was introduced. Its combination with common grinding allowed us to reduce the number of cycles 13]. It is our belief that the good dimensional stability and improved mechanical properties of aerogels in water (Fig. 4) is explained by the absence of charged functional groups.

Mineralization. Titania

Cellulose is functionalized by compounding, chemical modification and mineralization. Compounding is mainly applied to mix it with another polymer(s). Chemical modification serves to introduce functional groups like carboxylic or covalently attach residues of organic molecules, oligomers and polymers. Introduction of metal and metal oxides refers to the mineralization that is performed by infiltration or chemical modification by using methods of inorganic chemistry. The latter is receiving a great deal of attention because of its broad potential for nontraditional emerging applications of cellulose in the textile, paper and food industry, electronics, bioengineering, biotechnology and ecology [1], [8], [23], [35], [36], [37], [38], [39], [40], [41], [42], [43].

MFC aerogels consisting of a three-dimensional network of entangled micro-nanofibrils (Figs. 1 and 5b) covered by numerous hydroxyl groups are very convenient for the mineralization. They have high porosity and surface area of hierarchically structural organization of fibers/fibrils and pores. Any point inside aerogels is readily accessible because of the minimal hydrodynamic restrictions in macropores. Mineralizing components can diffuse or can be entrapped easy in macropore, thus performing functionality. These accessible diffusion pathways are also helpful for reuse and recovery of aerogels [23], [60], [136], [154], [155], [156].

Mineralization in contrast to the chemical treatments like common oxidation [1], [87], [100], [149] does not cause a significant change of the crystallinity and morphology of fibers and fibrils 11], [27], [28], [157]. It is frequently performed at ambient conditions by methods of green chemistry. Inorganics is usually dimensionally, chemically and thermally stable. Therefore, coating of cellulose by metal and metal oxide nanoparticles can increase the dimensional and thermal stability, the mechanical strength, wear and abrasion resistance, decrease the hygroscopicity, preventing the fungal and bacterial decay, impart the electrical conductivity, catalytic and bacteriostatic activity, provide the self-cleaning ability and the hydrophobic character 1], [11], [26], [27], [28], [29], [30], 36], [40], [41], [158]. This great variability in properties and functionalities meets any common and specific requirements for engineering and applications 34], [154], [156], [159], [160], [161].

Titanium dioxide TiO₂, which is further referred to in concise form as a titania, is one of the main mineralizing agents [34], [162]. Titania is a chemically stable, nontoxic and reasonably priced metal oxide. It possesses very remarkable photocatalytic activity [56], [57], [58], [59], [60], [62], [163]. This property presents itself interest but the mineralization by TiO₂ imparts some new functionalities to cellulosic materials, such as self-cleaning, antimicrobial, deodorizing and ultraviolet protecting, thus resulting in multi-functional modification and significant improvement in consumer properties 11], [23], [28], [35], [63], [64], [65].

Photocatalytic activity is caused by the semiconducting nature of titania. Its irradiation with light can trigger jumping an electron (e⁻) from valence band into the conduction band, leaving an electronic vacancy called a “hole” (h⁺):

TiO2+hν → e−+h+

where hν denotes a photon with energy larger than the gap between the two bands.

Electron-hole pair thus generated determines the photocatalytic properties of TiO₂. The former transfers to an oxygen reducing it, the latter captures an electron from a water molecule or hydroxyl adsorbed on the titania surface, oxidizing them [58], [163], [164], [165], [166]. These reactions provide generation of highly reactive oxygen species, including a hydroxyl radical OH˙ with the oxidation potential equal to 2.8 V. This value is exceeded only in the case of fluorine [163], [164]. Owing to a very high chemical reactivity, generated oxygen species trigger an oxidative degradation of organic substances that proceeds up to carbon dioxide and water through a sequence of reactions. This harmful action extends over bacteria, viruses and fungi because of the indiscriminating destruction of organic materials of biological origin as well 29], [62], [164], [167], [168], [169], [170].

Titania can be combined with cellulose, thus mineralizing it by means of one of three main methods: chemical vapor deposition, dip-pad-dry-cure and sol-gel techniques. The main disadvantage of the former is an inhomogeneous distribution of titania in the aerogel matric because of the phenomenon called “shadowing” or “step coverage” [34], [139], [171], [172]. The reason is in the evaporation made mostly from a single direction. Dip-pad-dry-cure treatment includes stages which are mentioned in the term. Fibers are dipped into a solution of TiO₂ nanoparticles, pressed, dried and finally cured. The technique, which is rather simple, holds much favor 65], [173], [174], [175], [176], [177]. However, embedded nanoparticles do not well adhere to the cellulosic surface 29], [167], [178], [179], [180]. It refers to the main disadvantage. To improve their adhesion, cellulose is modified by binders or oxidized by chemical oxidants, UV irradiation or various plasma techniques to introduce carboxylic groups 65], [149], [175], [181], [182], [183]. The latter bear a negative charge that conduces to the adsorption of positively charged TiO₂ nanoparticles. Schutz et al. could prepare homogeneous, transparent and highly mechanically strong films when combining carboxyl-containing nanocellulose with commercial titania nanoparticles [183]. However, oxidants, UV irradiation and plasma treatment cause the chemical destruction of macromolecules and accordingly fibers that decreases the performance characteristics.

Sol-gel method is among the most used techniques for the direct mineralization of cellulosic materials, but this is not true in a case of titania [34]. The reason lies in the absence of adequate control on the processing. Reactions of hydrolysis of precursor Ti(–OR)₄:

and following condensation of the hydrolysis products:

or

where RO– is frequently the isopropanol or butanol residues and n≤4, proceed very fast [56], [57]. The TiO₂ starts forming immediately when the hydrolysis has begun after the contact of precursor with water. It happens before reaching cellulosic fibers/fibrils. Therefore, titania precipitates rather than mineralizing [184], [185].

We could turn this inconvenience of the sol-gel chemistry of titania to a novel mineralizing method [[186], [187]. The hydrolysis-condensation reactions (1)–(3) were directed toward the surface of cellulosic materials. To provide the certain localization of processing, the high hygroscopicity of cellulose was taken into account. Syntheses were made in nonaqueous media in which a very restricted amount of water was admixed. The added H₂O was mainly absorbed by hygroscopic cellulose. In case precursor was introduced in a solution, titania formation proceeded on the surface of hydrated cellulosic fibers/fibrils because of the instant reactions of hydrolysis (1) and condensation (2–3) triggered by its contact with water. This method was referred to as directional sol-gel processing 34].

It should be pointed out that titania can be involved not only into the condensation reactions (2) and (3), but also in similar reactions with hydroxyl groups of cellulosic macromolecules [16], [174], [188]:

Their consequence is an attachment of TiO₂ to the cellulose via covalent bonds that result in strong adhesion of coating thus prepared. Covalent attachment is a decisive advantage of directional sol-gel processing over the dip-pad-dry-cure method.

Images of two aerogels mineralized by titania are presented in Fig. 6. Titania synthesized by the directional sol-gel processing forms dense coating. There is a difference in morphology that is determined by conditions of TiO₂ preparation.

Fig. 6:

Scanning electron microscope images of two samples of cellulose aerogel mineralized by titania that were taken at two various magnifications. Precursor concentration is increased in four times in the synthesis of sample c, d in comparison with a, b.

It should be mentioned that samples shown in Fig. 6 were treated at 500°C in the inert atmosphere after the synthesis. This posttreatment was made to transfer TiO₂ into a crystalline state. During the process cellulose fibrils were carbonized. Owing to the covalent bonding of TiO₂ with macromolecules, the strongly adhered coating did not break away from fibrils in the course of carbonization although transition of amorphous titania into the crystalline state is attended by its shrinkage [164], [165], [189], [190].

Formation of crystalline TiO₂ is obvious from images taken by transmission electron microscope at high resolution (Fig. 7). One can see crystalline clusters of which dimension is about few nanometers (Fig. 7b). Their presence means that titania microparticles formed in the synthesis and then thermally treated (Fig. 7a) have nanocrystalline structure. Nanocrystal dimension is between 2 and 5 nm (Fig. 7b). Analysis of sets of pictures and various samples revealed also large perfect nanocrystals. An image is presented in Fig. 7c. One can see clearly recognizing lattice planes, which are marked off by the yellow, with indexed spacing d=0.352 nm. They bear on the <101> lattice plane of anatase [164], [165], [189], [190].

Fig. 7:

Scanning (a) and transmission electron microscope images (b, c) of crystalline titania precipitated on cellulosic fibrils in the course of synthesis at ambient conditions and then treated at 500°C in inert atmosphere.

Cellulosic aerogel can serve as an useful template for synthesis of titania and its composites with carbon after the oxidative degradation or carbonization of polysaccharide, respectively. Mineralization is considered as a fresh opportunity for synthesizing metal oxide of certain morphology and shape [34], [162], [191]. Some examples for titania are presented in Fig. 8. Tubular TiO₂ (Fig. 8a,b) and nanoparticles (Fig. 8c) were prepared by calcination at 500°C.

Fig. 8:

Transmission electron microscope pictures of tubular titania (a, b) and nanoparticles (c) prepared by calcination of initially mineralized cellulosic aerogels.

The conversion of titania into crystalline form imparts various functionalities [29], [35], [58], [62], [164], [165], [189], [192], [193], [194]. We used cellulosic aerogel to develop efficient photocatalysts [186], [187]. Because TiO₂ is photocatalytically most active in the anatase form, posttreatment of mineralized aerogels was performed at 300–500°C. This is a common temperature range of its corresponding transformation 58], [62], [163], [164]. Cellulose is out of its thermosstability region at these temperatures 2], [195]. There is either degradation or carbonization, respectively, in an air or inert gas atmospheres. It was shown previously that polysaccharides could reduce notably the temperature of transition of amorphous titania into the crystalline state [184]. Perhaps, this is typical of processes in the presence of cellulose. Ruiz-Hitzky with collaborators observed a reduction of nickel oxide at very low temperature in the course of bacterial cellulose carbonization [196]. Notable effect of cellulose was valid for its bionanocomposites with TiO₂ as well [187]. Decreased temperature of the transformation made possible photocatalytically active mineralized aerogels at temperature not exceeding 100°C. A sample thus prepared is shown in Fig. 9 in which fast degradation of dye contaminating cellulosic fibrils was observed within 10 min under the outdoor sunlight irradiation. This kind of cellulose coated with titania is very promising for developing self-cleaning, antibacterial and UV radiation protected materials like textile 11], [28], [29], [33], [34], [35], [63], [170], [197].

Fig. 9:

Methylene Blue degradation under the sun irradiation. A drop of water solution with solubilized dye was placed on aerogels from cotton wool (control) and MFC mineralized by titania at ambient conditions and then post-treated at 80°C.

Conclusions

Cellulose is considered as highly promising structure-directing and -regulating scaffold for engineering high-performance materials. Its application is complicated by the polysaccharide hygroscopicity. Water absorption causes its swelling, structural and functional instability during storage, sharp decrease of mechanical strength. The latter does not allow treating the cellulose for improving properties by methods of wet chemistry. To solve the problem, it is suggested here to fabricate dimensionally stable aerogels. Although the mechanical strength is decreased sharply after placing in an aqueous solution, they are still strong enough in the wetted state to held the shape and geometry. Their dimensional stability opens up fresh opportunities for modification of cellulose. We apply such aerogels to mineralize them by titania using the method of directional sol-gel processing. Interest in this nontoxic metal oxide is explained by a reasonable price, multi-functional modification and novel consumer properties. Titania can provide photocatalytic, self-cleaning, antimicrobial, superhydrophobic, deodorizing properties and ultraviolet protection. As shown, its transition into a photocatalytically active crystalline anatase form in the presence of cellulose proceeds at a reduced temperature. Another advantage is a strong adhesion to macromolecules owing to covalent bonds formed with hydroxyl groups via the condensation reactions. Titania thus synthesized demonstrates high photocatalytic activity resulting in self-cleaning of cellulose aerogels under outdoor sunlight irradiation.