PREPARATION OF A SELF-CLEANING GLASS USING SOLUTIONS OF TITANIUM FLUOR COMPLEXES

Об’єктом дослідження є силікатне скло з наноструктурованним покриттям з оксиду титану (в модифікації анатаза), яке відзначається фотокаталітичною активністю і, як наслідок, набуває здатність до самоочищення в умовах ультрафіолетового опромінювання.Існуючий промисловий метод нанесення такого покриття здійснюється піролітичним способом, але він ефективний для великомасштабного виробництва, і здійснюється для великогабаритних виробів з листового флоат-скла. Для виробництва малосерійних, або поштучних виробів, він не виправданий. Це стосується виробів складної конфігурації, і, особливо, порожнистих. У нагоді постають методи нанесення покриттів з рідкої фази. В першу чергу – золь-гель методом. Класичний такий метод потребує в якості прекурсорів алкоксиди титану, які мають високу вартість. Більш дешевим і гнучким є метод нанесення покриття з розчинів титанфторкомплексних сполук. Як прекурсор використовується гексафтортитанат амонію,але він дорогий. Запропоновано видозмінити ланцюг хімічних перетворень, а саме: в якості первинних прекурсорів використовувати такі, що є більш дешеві і доступні. Запропоновано його одержувати штучним шляхом, – біфторидним способом. В ході дослідження використовувалися біфторид амонію NH4HF2 і оксид титану TiО2, які за запропонованим методом синтезу утворюють (NH4)2TiF6, його поява підтверджена рентгенофазовим аналізом. Фторування оксиду титану біфторидом амонію відбувалося при температурі, що не перевищує 200 °С. Фторування супроводжувалося виділенням тільки парів води й аміаку. Отримано фотокаталітичне покриття на зразках флоат-скла шляхом осадження кристалічної фази анатаза з водного розчину (NH4)2TiF6. Наявність анатаза підтверджена рентгенофазовим аналізом. Розмір кристалічних утворень не перевищує 15–20 нм. Здатність до самоочищення оцінюється тестом на гідрофільність скла і спектральними характеристиками покриття в ультрафіолетовому діапазоні. Завдяки цьому забезпечується можливість отримання самоочисного покриття на склі, яке, у порівнянні з аналогічними відомими, не поступається за якістю і має такі переваги: дешевизна і доступність, відсутність шкідливих викидів, що відповідає принципам «зеленої хімії». Ключові слова:силікатне скло, наноструктуроване покриття, самоочисне покриття, (NH4)2TiF6, рентгенофазовий аналіз, біфторидний спосіб. Plemyannikov M., Kornilovych B.


Introduction
Glass, due to its transparency, is a unique material of modern materials science. However, not all indicators of the properties of glass and products from it remain at the desired level. They can be quite easily improved by surface modification. In this case, the surface layer itself is modified either chemically or physically to a very shallow depth from a few nanometers (nanotechnology) to se veral micrometers. The incorporation of certain substances nanoclusters into the surface layer of glass causes the emergence of new extreme properties of the glass product. First of all, the appearance of properties associated with quantum optical phenomena. One such extreme modifier is titanium dioxide.
A contemporary urgent scientific problem is the creation of selfcleaning coatings on glass [1]. Glass is selfcleaning or easy to clean with minimal human intervention, which significantly reduces maintenance costs. Currently, several methods for the manufacture of such coatings are known [2].
The main cause of glazing contamination is atmospheric aerosols. They get into the atmosphere as a result of natural processes (erosion, volcanic eruptions, fires), or as a result of human activities (motor vehicles, industrial fuel plants, bitumen, asphalt preparation, garbage burning, cigarette smoke, cooking, especially frying meat). The phenomenon of photoinduced hydrophilicity, discovered in 1997, marked the beginning of the creation of nanostructured transpa rent coatings for glasses based on titanium dioxide, which exhibit selfcleaning properties under the influence of ultra violet radiation (UV radiation).

The object of research and its technological audit
The object of research is silicate glass with a nanostruc tured coating of titanium oxide (in the anatase modifica tion), which is noted for photocatalytic activity and, as a result, acquires the ability to selfclean under ultraviolet irradiation.

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Titanium dioxide (TiО 2 ), in the modification of anatase, has been used as a highly active photocatalytic material and has received great attention in the scientific com munity over the past two decades [3].
Nanostructured titanium dioxide has a unique photo catalytic property. Its essence lies in the fact that in the volume of a semiconductor particle under the influence of electromagnetic radiation from the UV range of the generated electron -hole pairs, which, when TiO 2 par ticles reach the surface, enter into redox reactions with molecules adsorbed on it.
Titanium dioxide is an ntype semiconductor. The band gap for anatase and rutile is 3.2 eV and 3.0 eV, respec tively [4]. Photocatalytic reactions are initiated when pho tons are absorbed with energy equal to or greater than the band gap. This leads to the transition of an electron from the semiconductor valence band to the conduction band with the formation of an electronhole pair. The highest catalytic activity in such processes is expressed by the anatase modification. The optimal particle size of TiO 2 for catalytic processes lies in the range from 15 to 110 nm.
Titanium dioxide is used to create gas sensors [5], in medicine as biocompatible and antibacterial coatings [6]. It acts as a catalyst in the processes of oxidation of halogen organic compounds [7] and reduction of nitrogen oxides [8].
The wellknown method of applying a selfcleaning coat ing based on anatase by pyrolysis from a vaporgas phase is effective for largetonnage production of flat glass by the float method. For the production of smallscale, or piece products, products of complex configuration, and especially hollow, it is not advisable. In the case there are methods of coating from the liquid phase. First of all, the solgel method. The classical such method requires titanium alk oxides, which are of high cost, as precursors. Cheaper and more flexible is the method of coating from solutions of titanium fluorine complex compounds. Ammonium hexafluo rotitanate is used as a precursor, but it is expensive. The paper proposes to modify the chain of chemical transforma tions, namely: to use the cheaper and more affordable ones as primary precursors. It is proposed to use anatasebased coatings. Since the known method in which (NH 4 ) 2 TiF 6 is used as a precursor for liquidphase deposition is not rational due to the high cost of this substance.

The aim and objectives of research
The aim of research is development of a new method for preparing precursors for applying an anatasebased nano structured selfcleaning coating on glass.
To achieve this aim,it is necessary to complete the following objectives: 1. To propose other, less valuable and more affordable precursors for the implementation of the technological process, in the chain of which (NH 4 ) 2 TiF 6 becomes an intermediate substance, as well as appropriate laboratory equipment and set the synthesis parameters of (NH 4 ) 2 TiF 6 .
2. To assess the perfection of the synthesis reactions by the method of Xray phase analysis of the synthesized substance.
3. To coat silicate glass and make sure that the coat ing is nanostructured anatase.
4. To ascertain the presence of photocatalytic activity of the glass surface by spectrophotometry of the sample in the UV range and by measuring the contact angle.

Research of existing solutions of the problem
Today, there are many methods and technologies for pro ducing TiO 2 nanoparticles with various morphologies [9]. They obey two main directions: hightemperature synthesis from the gas phase and lowtemperature synthesis from the liquid phase.
According to the first direction, the most common is the method of pyrolysis in the gas phase. To obtain TiO 2 by hightemperature decomposition, gaseous halides [10] or titanium alcoholates that are volatile at elevated tempera tures are used as gaseous precursors. In the presence of water vapor, titanium tetrachloride vapor can be hydrolyzed to form finely dispersed TiO 2 nanoparticles [11]. Depend ing on the processes of formation of TiO 2 by chemical reactions, the methods of chemical and physical deposition are distinguished.
An example of chemical vapor deposition is the prepa ration of TiO 2 nanocrystalline films of pyrolysis of tita nium (IV) tetraisopropoxide in a gas mixture of helium and oxygen [12]. By physical deposition of titanium metal upon heating in vacuum followed by oxidation with gaseous oxygen, TiO 2 crystals can be obtained on the surface of various materials [13].
Lowtemperature synthesis from the liquid phase is carried out by various methods, among which the fol lowing are most common.
The hydrothermal synthesis method is based on the ability of water and aqueous solutions to dissolve substances that are practically insoluble under normal conditions at high temperature (up to 500 °C) and pressure (10-80 MPa). Using the hydrothermal synthesis method, various modi fications of TiO 2 can be obtained [14].
Solvothermal synthesis is carried out in organic solvents having a higher boiling point. Such a synthesis of TiO 2 nanoparticles in nonaqueous media at a higher tempera ture makes it possible to obtain more dispersed titanium dioxide particles [15].
One of the most popular methods for the preparation of TiO 2 nanosized particles is the hydrolysis of titanium bearing precursors, for example, TiCl 4 , titanium alkoxides, or titanyl sulfate.
The solgel method, widely used for the synthesis of glass and ceramics, has also been adapted to produce vari ous coatings based on nanostructured TiO 2 . Nanosized TiO 2 particles are synthesized by the solgel method using hydrolysis of titanium precursors. For the synthesis of TiO 2 with titanium alkoxides, the most effective titanium tetraisopropoxide and titanium tetrabutoxide.
The solgel method has been successfully used for the manufacture of silica, titanium, or composite (hybrid) coatings [16]. The synthesis technique of TiO 2 allows the formation of the anatase phase even at low tempera tures [17], especially when titanium tetraisopropoxide is used as alkoxides, and the acid acts as an electrostatic stabilizer and hydrolysis catalyst [18].
The technologies for producing TiO 2 nanoparticles obey two principal directions.
The first direction is the production of nanopowders, as such. In the state of lyosols or aerosols, they are used as highly effective cleaners of water or gas environments. The vast majority of scientific papers are devoted to this particular area.

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The second direction is the production of nanocoatings on various materials.
The selfpurification phenomenon, which is considered in this work, occurs on the basis of a combination of two effects: the photoinduced properties of a thin film of TiO 2 , namely: photocatalytic activity leads to degradation of organic substances, superhydrophilicity provides high sur face wetness.
The selfcleaning effect of glass occurs with the parti cipation of water (rain, service flushing) [19].
However, atmospheric aerosol pollution is not the only problem. An additional problem is the condensation of moisture, which settles on the cold surface of the glass. Condensation on glass surfaces can be minimized to a cer tain extent [20]. The combined effect of these two effects eliminates fogging of the glass. A thin transparent film of photoactive TiO 2 under the influence of light destroys organic pollutants, the surface is well wetted, and water falling on such a surface does not collect into droplets, but spreads over the surface. And then it evaporates without the appearance of spots and stripes.
On glass, water has a contact angle of wetting, which ranges from 30 to 90 degrees. Nowadays, unknown ma terials on the surface of which this angle would be less than 10 degrees. And only a titanium dioxide film can achieve such results. On the surface of the film, when it is subjected to ultraviolet radiation, the angle gradually decreases and after 4...24 hours of exposure reaches almost zero degrees. At this stage, a hydrophobic surface becomes completely hydrophilic, and such a surface is called a «super hydrophilic».
Under the influence of light, not only organic mole cules are destroyed on the surface of titanium dioxide, but harmful microorganisms, even those that are resistant to ultraviolet light, also die.

Methods of research
Spectrophotometry of glass samples is carried out using a Unico 2100 UV spectrophotometer (Russia). The ope rating range is 200 to 1000 nm.
The phases are identified using a JCPDSICDD file cabinet.
The calculation of crystallite sizes is carried out ac cording to the Scherrer formula: where А -the dimensionless form factor (usually taken equal to 0.9); l -the wavelength of Xrays; β -the peak broa dening at half height (2θ); θ -the Bragg angle. The device for measuring the contact angle -DSA 25 (Germany). Range of measurement is 1-180°. Accuracy 0.1°.

Research results
The process of liquid phasedeposition is used. It is car ried out in an aqueous solution. The method of deposition of thin films of titanium oxide from mixed solutions of (NH 4 ) 2 TiF 6 and H 3 BO 3 is known in the literature [21,22].
The deposition reaction consists in the equilibrium ligand exchange (hydrolysis) of ions of titanium fluorine complex: [TiF 6 ] 2-+nH 2 O↔[TiF 6-n (OH) n ] 2-n +nHF. (1) Reaction (1) According to the wellknown scheme for the imple mentation of chemical transformations, the precursor is ammonium hexafluorotitanate, the substance is quite ex pensive and scarce.
It is proposed in the work to obtain it artificially, using the bifluoride method.
The physicochemical basis of the process of fluorina tion of ammonium bifluoride is that TiO 2 , when reacted with NH 4 HF 2 , (NH 4 ) 2 TiF 6 , which becomes a precursor in the reagentphase deposition scheme.
Processing of titanium oxide with ammonium bifluo ride is easy to do: fluorination occurs at a temperature not exceeding 200 °C, byproducts (water and ammonia vapors) do not contain fluoride, ensures the environmental safety of the process.
The interaction of titanium oxide with ammonium bi fluoride occurs with the formation of ammonium fluorine or oxofluorotitanates. Fluorination is accompanied by the release of only water vapor and ammonia by reaction: The output reagents were solved in an equimolar ra tio (1:3), ground and mixed in a porcelain mortar.
To carry out such synthesis, a laboratory unit is assembled (Fig. 1). The unit is a vertical muffle electric furnace, in which there is a fluoroplastic glass with a twisting lid, in the opening of which a return air cooler is inserted, prevents premature removal of water. Ammonia is predominantly released, shifts the equilibrium to the right (work under the hood). The temperature in the furnace cavity is controlled by a chromelalumel thermocouple (CАthermocouple). The temperature is maintained at 170 ± 10 °C. Processing time is 5-6 hours.
After the end of the process, the reaction products are a semidry pastelike mass, well soluble in water. The

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resulting solution is filtered. The filtrate is evaporated and hung. The resulting powder is subsequently used to carry out reaction (3). For this, a solution of 0.1 М (NH 4 ) 2 TiF 6 and 0.3 M H 3 BO 3 is prepared. A glass product (plate) is immersed in the glass and an exposure time of 10 hours is carried out [23][24][25]. As a result, the glass is covered with a film of titanium oxide. The results convincingly show the feasibility of the proposed method for producing ammonium hexafluoroti tanate (Fig. 3, diffraction patterns 2). There is an assumption that adverse reactions also oc cur with the formation of ammonium oxofluorotitanate: Precipitation obtained on glass is also subjected to Xray phase analysis. The crystalline phase of anatase is already observed during lowtemperature precipitation from the mother liquor. Glass with this coating already produces photocatalytic activity.
As follows from the diagram in Fig. 4, the intensity of reflexes increases with increasing temperature. However, at temperatures above 800 °C, anatase turns into rutile.
The criterion for the presence of a photocatalytic coating of anatase on glass is the interaction of this coating with ultraviolet radiation. To state this fact, spectral studies are performed (Fig. 5).
As a prototype, a quartz glass plate 2 mm thick is used. The use of quartz glass is dictated by the necessary condition for transparency in the UV range. For uncoated glass (Fig. 5, curve 1), a fairly high transparency is ob served in the range 200-400 nm. For coated glass (Fig. 5, curve 2), there is a com plete absorption of radiation in the range 200-300 nm and a significant decrease in transparency in the range 300-400 nm. For clarity, the interdependence of the band gap on the wavelength is shown (Fig. 5, curve 3). As follows from Fig. 5, the high transparency of the coated glass is restored after about 380 nm, which corresponds to an anatase gap of 3.2 eV.

SWOT analysis of research results
Strengths.An alternative technology for applying a self cleaning coating to glass is proposed. At that time, it was known that instead of the expensive precursor (NH 4 ) 2 TiF 6 , much cheaper reagents were used as precursors in the technological scheme of transformations: TiO 2 and NH 4 HF 2 .
The synthesis of (NH 4 ) 2 TiF 6 , which becomes an interme diate product, is carried out at relatively low temperatures (not more than 200 °C) and without the release of toxic fluorinated substances. This complies with the principles of energy conservation and environmental safety of the process.
The yield of the reaction for producing (NH 4 ) 2 TiF 6 is quite high (not less than 95 %). This product is readily soluble in water. The precipitate obtained after filtering its solution and drying is subjected to xray phase analysis, which confirmed the presence of this particular product with a possible admixture of oxofluo rotitanate, which is not an obstacle to coating the glass.
The photocatalytic coating is applied to glass by immersion of a glass sample in a solution of (NH 4 ) 2 TiF 6 and H 3 BO 3 at room temperature and a holding time of several hours. Nanostruc tured anatase coatings are formed on the glass. Further heat treatment of the glass samples is advisable. Xray phase analysis showed an in crease in the content of nanostructured anatase to temperatures of about 750 °C. The size of the crystallites calculated by the Scherrer formula is 15-20 nm. At higher temperatures, there is a risk of the conversion of anatase to rutile, which is not desirable.
The presence of the photocatalytic properties of the coating and, as a consequence, the self cleaning properties of the glass are investigated indirectly: by the spectral method and by mea suring the contact angle of water wetting on the glass. A spectral study shows the disappearance of the transparency of quartz glass with this coating in the UV range of 200÷400 nm. The contact angle of glass wetting decreased to 2÷3°.
Weaknesses. However, the main characteris tic, namely, the ability to selfcleanse is not investigated in the work. Since the work speaks of selfcleaning glazing from atmospheric pol lutants, such an experiment will require a long time, modeling the effects of various pollutants, various degrees of moisture of the glass, and the like. The authors proceed from the follow ing axiomatic statement: «superhydrophilicity» is a mandatory attribute of the selfcleaning ability of glass. Opportunities. In recent years, fluoride technologies have begun to be intensively used in various sectors of the chemi cal industry. They allowto get high purity substances. Am monium bifluoride NH 4 HF 2 is used as the main reagent for fluorination. Of relatively great importance is its relatively low cost (5 USD per 1 kg). Important is the fact that am monium bifluoride is a byproduct of many fluoride plants. Titanium dioxide is also widely used in industry due to its relatively low cost (8 USD per 1 kg), chemical stability, and nontoxicity to living organisms. Given that the estimated market value of 1 kg of ammonium hexafluorotitanate is 800 USD, it is possible to predict the reduction in the cost of applying this coating by several tens of times.
Threats. Selfcleaning glass has an effective function only in conditions of periodic wetting by atmospheric precipita tion. Otherwise, for example, indoors, in car tunnels, such glass will require additional periodic maintenance (washing), greatly simplified compared to ordinary glass.

Conclusions
1. Instead of the expensive precursor (NH 4 ) 2 TiF 6 , sig nificantly cheaper reagents TiO 2 and NH 4 HF 2 are proposed as precursors in the technological scheme of transforma tions. To carry out such synthesis, a laboratory setup is proposed, which is a vertical muffle electric furnace with a fluoroplastic vessel in which the synthesis of (NH 4 ) 2 TiF 6 takes place. The temperature is maintained at 170 ± 10 °C. Processing time 5-6 hours.
2. It is found that the yield of the desired product of the synthesis reaction is at least 95 %. As a result of the synthesis, it is precisely (NH 4 ) 2 TiF 6 that is formed, which is confirmed by Xray phase analysis. In addition, ammonium oxofluorotitanate is also likely to form, which is not an obstacle to coating with Anatase.
3. Coated silicate glass from an aqueous solution of (NH 4 ) 2 TiF 6 and H 3 BO 3 . It is found that the crystalline phase of anatase forms immediately after precipitation at room temperature. Further heating of the coated glass to temperatures of about 800 °C is advisable, since the crys talline phase of anatase accumulates. The size of crystalline clusters is estimated by the Scherrer formula at 15÷20 nm.
4. It is shown that the ability of the coating to pho tocatalytic activity is ascertained by spectrophotometric method and measurement of the contact angle. So, in the first case, the complete opacity of the coating in the UV range is detected, and in the second case, the «super hydrophilicity» of the glass is detected. The contact angle with water decreases to 2÷3°.