Keep your powder dry!

Photoactivity of TiO2 appears closely related to its hydrophilicity

Tomáš Dinkov *
Jaroslav Stabryn

Photochemical reactions of the surface of titanium dioxide and the consequent chalking processes have been investigated in many reports for more than half a century, e.g. [1-11]. Coating oxidation processes, catalysed by a photochemically active pigment such as TiO2, are largely responsible for degradation of coating films.

Although it is known that the photochemical reaction and associated chalking can only proceed in the presence of water, the role of the water has not yet been fully explained.
Investigations are reported below into this problem (determining the source of hydrophilicity) in particular, and a way is proposed to suppress the photochemical activity of titanium dioxide in the coating and thereby increase its resistance against weathering.

Because the photochemical reaction can only proceed if water is present, attention was paid to water adsorption and to the influence of surface-bound steady-state water (hydrophilicity) on the intensity of the photochemical reaction.
On the one hand, the surface-bound water on TiO2 is necessary for the hydration of photoexcited electrons. Since a photochemical reaction can only proceed if an acceptor of photoexcited electrons (holes) is present, the question of whether the surface-bound water also has a role as an acceptor was studied.
Since the photocurrent depends on acceptor concentration, any possible dependence between the amount of the surface-bound steady-state water on TiO2 and the intensity of the photochemical reaction was examined.
The specific behaviour of dried TiO2 during adsorption of water was also investigated and compared to the behaviour of dried TiO2 mixed with anatase gel mixture during adsorption of water.

Five samples of commercial, non-surface treated, non-micronised anatase titanium dioxide pigment were used for the experiments. Anatase gel, an intermediate in anatase titania manufacturing, was also used.
A key property considered in this work is the adsorption period, which is defined as the time elapsed from the beginning of adsorption until an absorption equilibrium is reached under given conditions. The test conditions used in this work were:
TiO2 weight 1 g
Relative air humidity 40 %
Air temperature 25 °C
1± 0.0001 g of sample was weighed into a weighing pot and dried in a drying chamber at 150 °C for 2 hours. Then the weighing pot was sealed with a lid and left in a desiccator to cool down. The lid was removed and a stop-watch was started as soon as the pot with sample had been chilled.
The weighing pot and lid were placed on an analytical balance and instantaneous time and weight were read periodically. The measurement was stopped as soon as three consecutive weighings gave the same result.
As a measure of the degree of intensity of the photochemical reaction (degree of photoactivity) the difference between the remission of a sample before and after UV irradiation was taken. Remission was obtained by spectrophotometer at a wavelength of 522 nm and a measuring geometry of 0/d.
A mercury vapour lamp was used as a source of UV-radiation. The lamp produces UV-radiation at wavelengths between 180 and 400 nm. The irradiation period was determined experimentally as the time when no further decrease in remission occurred.

Water content at 105, 150, 175 and 197 °C was determined on five samples of anatase titania. The degree of photoactivity was also determined in these samples. The values obtained were ordered into Table 1 and are shown diagrammatically in Figure 1.
The results in Table 1 and the diagram in Figure 1 show clearly that the photoactivity of samples 15 (surface untreated, non-micronised anatase titanium dioxide pigment) is directly dependent on (i.e. directly proportional to) the water content in these samples. From this it is possible to conclude that the steady-state surface-bound water has a function as an acceptor of photoexcited electrons (holes).
The air humidity adsorption period was measured on five samples of titania dried at 150 °C. Anatase gel was added to these samples at levels corresponding to an increase of water content of 0.20, 0.10, 0.15, 0.115 and 0.25 % respectively. Samples of TiO2 were homogenised with the additions of gel and the air humidity adsorption period was measured again. The results are summarised in Table 2.
Results in Table 2 show that the adsorption rate of samples 15 did not change after the gel was added. Since the adsorption period is a significant attribute that characterises an adsorbent or a chemical substance selectively, it can be concluded that the same substance is adsorbing in both cases. This means that the added gel is, as regards adsorption properties, equivalent to the water-adsorbing component of anatase titanium dioxide pigment.

The particles responsible for water adsorption and photoactivity are probably formed only in industrial manufacturing (calcination), because the photoactivity and hydrophilicity of laboratory prepared titania, when the whole bulk is almost perfectly calcined, is completely insignificant, as was shown in an earlier report [12].
The results obtained suggest a hypothesis: the particles responsible for the photochemical reaction are those that have not fully transformed into a stoichiometric TiO2 and that have retained an inner gel structure during calcination, so that they react with air humidity and form the gel particles immediately after drying is concluded.
Those particles which have lost their entire surface energy due to water adsorption may not react with incident light directly. However, the particles contain OH-groups that work as an acceptor of holes (electrons) photoexcited from other particles and thereby make the photochemical reaction possible. Furthermore, the amount of the particles (acceptor concentration) determines the photocurrent intensity (see Table 1 and Figure 1).
The photocurrent is then determined by the water content, which corresponds to the level of the particles that are able to form a gel again by means of retroreaction with (adsorption of) water. It follows from the principles so far mentioned that the water content in surface-untreated anatase titanium dioxide pigment is determined by the calcination conditions.

It is also obvious that differences between anatase and rutile TiO2 in relation to photochemical activity might also be explained by the lower content of gel particles in rutile TiO2, which is calcined at a markedly higher temperature.
It is evident from what has been described so far that the use of TiO2 dried immediately before adding it to a mixture during paint manufacture may decrease its photoactivity in a coating film made of this paint and thereby increase the weathering resistance of the coating.
The residual photoactivity of the TiO2 in the coating will be dependent on the amount of water not fully removed from the pigment. The difficulties keeping TiO2 in a dried state come from the powerful affinity of the adsorbing particles on the TiO2 surface to water. The measurements reported here show that almost 20 % of the adsorption sites of dried TiO2 (anatase) are occupied after the first minute of adsorption when the relative humidity of the air is 40%.

A direct dependence between the amount of steady-state surface-bound water on TiO2 surface (hydrophilicity) and intensity of the photochemical reaction was observed. A hypothesis about the particles responsible for the photochemical reaction on TiO2 has been developed.
The results clearly suggest the possibility of modification of TiO2 for paint manufacture. One possible way is to use TiO2 dried immediately before application in paint production.
The difference in photoactivity between anatase and rutile TiO2 may be caused by the lower content of gel particles in rutile TiO2, which is calcined at a markedly higher temperature. í

[1] Renz G., Helv.Chim.Acta 4, 1921, p 961. [2] Keidel E., Farben Z. 34, 1929. p 1242. [3] Jacobsen A. E., Ind. Eng. Chem. 41, 1939, p 523. [4] Lund E., Weider C. F., FATIPEC Kongreßbuch, 1953, p 286. [5] Clay H. F., JOCCA 40, 1957, No 11. [6] Sbrolli W., Bertotti E., Ann. Chimica, 1959, No 41, p 1143. [7] Völz H. G., Kämpf G., Fitzky H. G., X. FATIPEC Kongreßbuch, Verlag Chemie Weinheim, 1970, p 107. [8] Völz H. G., Kämpf G., Klaeren A., FARBE UND LACK, 1976, Vol 82, No 805, p 9. [9] Mackor A., Koster Th. P. M., Kui-Wen L., FARBE UND LACK, 1988, Vol 94, No 954, p 11. [10] Gesenhues U., FARBE UND LACK, 1994, Vol 100, No 244, p 4. [11] Gesenhues U., FARBE UND LACK, 1995, Vol 101, No 7, p 1. [12] ?Cešpiva K., Laboratory preparation of the pure anatase from TiO2-hydrolysate, unpublished work, HCHZ, 1954.

Degradation of coatings is largely driven by oxidation processes, which can be catalysed by photoactive pigments such as titanium dioxide. This photochemical reaction occurs only in the presence of moisture. It was found that water on the surface of TiO2 particles acts as acceptors of excited electrons and the photocurrent is proportional to the level of surface-bound water, which in turn is governed by the hydrophilicity. There also appears to be a relationship between the degree of calcination and the photoactivity; fully calcined laboratory samples were found to have negligible photoactivity. It is proposed that the higher calcination temperatures used in rutile production may be responsible for its lower photoactivity compared with anatase, and that coating durability may be enhanced by pre-drying the TiO2 before paint manufacture.

Ovachem
T+ 420 724 764 441
tomas.dinkov@
ovachem.cz