PHOTOCHEMISTRY OF RUTILE W. A. Weyl and Tormod Farland The Pennsylvania State College, State College, Pa.
T h e color, phototropy, and photoelectric oxidation phenomena of rutile are discussed on the basis of crystal chemistry of defect structures. Foreign ions, especially cations with a charge higher or lower than that of Ti4+ as well as the surface itself, represent the defects of the crystals. An explanation is presented for the strong coloring effect of traces of Fea+, Cb6+, W6+ and other impurities in rutile. The reversible color change of titanium dioxide containing some ferric oxide is interpreted by a concept based on the electroneutrality of the lattice which has been used previously for explaining the electronic conductivity of the Nernst glower and the color of cerium dioxide contaminated with praseodymia. Photoelectric phenomena are typical of a combination of an easily polarizable anion with a strongly polarizing cation which,
for oxides, usually has to be of the nonnoble gas type. The probability of electrons being released from anions increases with their polarization. The extreme effects resulting from one-sided polarization cause even silicon dioxide, or sulfates, to transfer electrons from surface oxygen ions to their cations. This photoelectric process leads to the formation of atomic oxygen which can be detected by its characteristic reactions, formation of silver peroxide, or oxidation of certain organic amines to dyestuffs. An especially sensitive test for atomic oxygen has been developed which detects photo-oxidation within very short expdsure times. This test is based on the oxidation of 4,4’,4”-hexamethyltriaminotriphenyl methane to the carbinol, which is the leuco base of the intensely colored crystal violet.
I
certain optical properties, formation of mixed crystals, and the phase equilibria as affected by temperature, but it does not help the chemist who wants to understand the chemical reactions involving crystal surfaces. The ions in the surface film of the crystal do not have the symmetry and the coordination of the average building units of the crystal. The properties of the surface ions differ from those inside the crystal because the former are exposed to the asymmetrical force field in the surface which causes a different degree of deformation of the electron orbits of these ions. The degree of deformation or polarization in a given field varies with the nature of the ions. It is greater for anions than for cations of the same type, charge, and size. The deformation of small ions having the octet shell (noble gas-type ions) is practically negligible, but the deformation phenomena are very important when large ions and those of the nonnoble gas type are involved. In spite of the extensive material on the polarization properties of ions which Fajans (8) and his students have accumulated over a period of 25 years, the phenomena of polarization and deformation are not yet sufficiently appreciated or applied to crystal chemistry. As a result, surface phenomena such as the “Receding contact angle” or the “Helmholtz double layer” are not understood. Simple crystal structures such as mercurous chloride involving strongly polarizable ions which have been known for 20 years and have been redetermined many times are not mentioned in textbooks on crystal chemistry, apparently because their significance cannot be understood if one ignores polarization phenomena. Goldschmidt (8),the father of crystal chemistry, taught that the atomic structure of solids is governed by the stoichiometric ratio of the ions and by their sizes, charges, and polarizabilities. The polarization properties are governed by the electronic configurations of the ions, especially of the outer shells. The polarizability of an ion-that is, its ability to adjust its force field to suit its environment-can be determined from optical properties. The molar refractivity represents the response of an ion or molecule t o the alternating electromagnetic field of light but can serve also as a measure of the deformability of their outer orbitals in the static electric fields of surrounding ions. These phenomena are of primary interest to the chemist. Foreign Cations with Charge Lower than Four. Now the response of ions in external electrial fields can be applied t o the
T IS the object of this paper to discuss photochemical reactions
involving rutile and to explain the effect of minute impurities on its photochemical and optical properties. For this purpose it is necessary to explain the behavior of foreign elements in the crystal lattice of an oxide such as titanium dioxide. The optical properties of titanium dioxide show three features, which are not only of interest to the industrial chemist engaged in the manufacture of titania products and their application in the fields of pigments, ceramic materials, dielectrics, and semiconductors but which also contribute to a better understanding of the chemistry of solids and of photo-oxidation phenomena in general. First, the presence of traces of ions usually considered colorless -namely, Cb6+, Ta6+, Sb6+, W6+-produces strong changes in the light absorption and in other electronic properties of the titanium dioxide. Colors produced by impurities such as Fe*+ have intensities which are out of proportion to their low concentration. Secondly, the presence of traces of iron in combination with columbium causes phototropy-that is, the titanium dioxide darkens when exposed to light and the tan-to-brown color fades in the dark. The phototropy of rutile is imparted to ceramic glazes containing rutile as an opacifying agent. The third feature, which is of particular interest t o a chemist, is the strong oxidizing effect which titanium dioxide may exert on organic and inorganic substances, when it is irradiated with ultraviolet light.
Light Absorption of Im nrity Ions in Rutile and Crystal Ehemistry Crystal chemistry should be the natural basis for the interpretation of chemical reactions involving solids either as reactants, reaction products, or as catalysts. Unfortunately the branch of chemistry of solids cal1e.d crystal chemistry has not yet been developed sufficiently to be of direct use to workers in many fields of chemistry such as catalysis, synthesis of fluorescent materials, pigments, and semiconductors. A little refleetion shows that there are several reasons for the reluctance of industrial chemists to use crystal chemistry as a tool in their research. Crystal structures are treated in our textbooks aa perfect arrays of rigid ions extending infinitely into space. Such an idealized model is well suited for understanding density,
25’2
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INDUSTRIAL AND ENGINEERING CHEMISTRY
000000000 000O0D000 00 00 0 0 0 0 00000 Figure 1. Schematic Picture of a Flaw on Partly Reduced Titanium Dioxide
problem of light absorption resulting from impurities in the rutile crystal. Light absorption-the process of raising an optical electron into a higher energy level-is a rare event for most gaseous ions. Light absorption becomes more probable if an ion is brought into a condensed system. Because of thermal motion, the ion-dipole interaction in aqueous solutions produces a slight deformation of the outer orbitals and increases the probability of absorbing a photon as a result of this deformation. This effect is not very pronounced if the ions are studied in dilute solutions where, in spite of thermal motion, the force field around the ion is fairly symmetrical. The molar coefficient of extinction of Goa+, Cuzf, Fe", and Ti3+ in diluted aqueous solutions is very small in the visible region as compared with the values for certain organic dyes. The Tia+ ion when surrounded symmetrically by water molecules is practically colorless, but in concentrated solution-for example, of the chloride-where, owing to cationanion interaction, these ions are exposed to less symmetrical force fields, they assume a purplish tint. The molar coefficient of extinction for the Tis+ ion increases with decreasing symmetry of the surrounding electric field. The intensive bluish gray color of partly reduced titanium dioxide can be considered to be due to Ti3+ ions which are strongly deformed in the asymmetrical field of the defective titanium dioxide crystal. Figure 1 shows schematically what happens when a crystal of Ti4+Oi- loses one oxygen atom and the two electrons of the 0 2 - ion change two Ti4+ into Ti3+ ions. These Tia+ ions are strongly polarized; they assume a state which in its extreme can be described as two Ti4+ ions and two additional electrons. The latter may assume the position of the missing 02ion. The intensive light absorption in such partly reduced crystals is the result of the strong distortion of the outer orbitals of the Tia+ ions which in this state absorb light stronger than undeformed Ti3+ ions. The same applies to other impurity ions such as Fe3f which, taking the place of a tetravalent cation, may produce pink hues, Strong pink colors have been observed if Fe3+ ions take the place of some Si4+ ions. Certain varieties of feldspar are deep red in spite of containing only minute amounts of ferric oxide. The observation of Hedvall and Sjoman ( I d ) that a rosecolored product can be obtained if ferric oxide and quartz are
Vol. 42, No. 2
allowed to react in the temperature region of the inversion of quartz has found technical application (11). A salmon-colored silica containing a small amount of iron is synthesized and marketed as a pigment for ceramic materials where it replaces a natural pigment, the Japanese ochre. This rose color is well known to those engaged in the synthesis of titanium compounds for dielectrics and is the result of Fe3+ ions occupying the positions of Ti4+ ions in compounds such as barium titanate. Summarizing the effect of trivalent impurities, such as iron, on the color of rutile, one may say that the intensity of their light absorption is out of proportion to their low concentration. For each two X 3 +ions replacing two Ti4+ ions, one oxygen ion is released from the lattice. The color centers are the two X3 ions which are polarized in a fashion which may be described as cores with their outer electrons being pulled toward the place of the missing oxygen ion. Foreign Cations with Charge Higher than Four. The replacement of a few T i 4 +ions by trivalent cations results in the formation of a structure where 02-ions are missing, precisely one for each two X 3 + ions. Such'a crystal has the formula: +
The formation of mixed crystals usually imposes rather severe conditions on the ions; they must have identical charge, comparable sizes, and similar polarizing properties. Hon-ever, these rules governing mixed crystal formation do not hold true for the minute concentrations of impurities, especially if the host lattice contains ions of great polarizability. Foreign ions not only may have a different size but even a different charge than the cation of the host lattice. For these impurities there exists only one supreme law which has to be obeyed, that of electroneutrality of the crystal. The lattice reacts towards an impurity in such a way that the sum of positive and negative charges tends to become zero within a minimum volume. In replacing an occasional T i 4 +ion by a cation having a charge higher than four, electroneutrality requires that additional electrons be incorporated into the lattice. From a chemical point of view this means that an equivalent number of T i 4 +ions have to be reduced to Ti3+ ions. However, these additional electrons may move in orbitals which are much larger than those of the normal Ti3+ ions. Traces of columbium, tantalum, or pentavalent antimony incorporated into titanium dioxide produce a rutile crystal which may be described by the formula:
or
Tif:sX:+(e);O;-
where (e); are the additional electrons. Tungsten W 6 + and molybdenum M05+ ions cause a similar change, but their reducing effect on Ti4+ions is t,.iuice that of a pentavalent ion, as can be seen from: or
Ti4+W6+ (ek0;-
The effect of these impurities on the color of rutile is not unlike that of partial reduction. A rutile containing traces of columbium is bluish gray due to the presence of Ti$+ions. Color and electronic conductivity of a rutile containing Cb5 and of one which has been partly reduced are similar. However, the bluegray titanium dioxide contaminated by Cb5+ cannot be bleached by heating in oxygen because it does not have the oxygen vacancies characteristic for the partly reduced material. Recently this principle has been used (24, 66) to synthesize "controlled semiconductors"-that is, crystals having electronic conductivity where the number of conducting electrons can be controlled precisely by incorporating into the lattice a +
February 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
predetermined concentration of impurities, instead of attempting to control the degree of reduction for the N-type (reduction semiconductors such as titanium dioxide or zinc oxide) or of oxidation for the P-type semiconductors (nickel oxide). In summary, foreign ions of a valency higher than four which take the place of an occasional Ti'+ ion cause the rutile to assume the bluish-gray color which is characteristic of the partly reduced oxide. In contrast to the latter, the color produced by impurities cannot be removed by oxidation. The same color change can also be produced by Be2+ ions, because these ions are sufficiently small to enter interstitial positions, thus adding their positive charges to those of the normal cations.
259
require increased temperature or additional energy in the from of radiation which leads to their phototropy. The phototropy of titanium dioxide contaminated with ferric oxide may consist of an excitation process which transforms the group FeS+-oxygen vacancy-Fe3 + into the chromophore Fe4+2(e)--Fe4+. This process is reversed in the dark.
tx
m
a
OXYGEN ION MISSING
"
@
2 ELECTRONS TAKING THE PLACE OF 0 '- I O N
A
Phototropy of Rntile a
Figure 2.
Phototropy is the property of a crystal to change its color on exposure to light and to return to its original color when left in the dark. Phototropy has been studied primarily with organic substances but it can also be found in the mineral kingdom. Hackmanite, for example, a mineral belonging to the sodalite group assumes a deep-red color when irradiated with ultraviolet light (16). This color fades to the original gray in daylight. T h e phototropy of rutile was observed by Parmelee and Badger (17) who found that certain glazes which contain rutile as the opacifying phase darken when exposed to sunlight. Some of these ivory-colored glazes were so sensitive t o light that less than 1minute exposure produced a dark-brown color which faded in the dark. Williamson (28) carried out a more comprehensive study of the photosensitivity of natural and synthetic rutile, and he found t h a t it is caused by minute amounts of iron. The presence of Fe3+ in the rutile lattice has been described by the scheme shown in Figure 2A, where two strongly deformed Fe3+ ions are separated by an oxygen vacancy. If sufficient energy is put into such a group one electron may become excited and be removed from the Fe3+ ion and may assume the position of the missing 0 2 - ion. This dissociation process leading t o an Fe4+ion satisfies the force field of the crystal which calls for tetravalent ions instead of the Fe8+ ions and for two negative charges where the 02-is missing. For very low concentrations of Fe8+ an electron transfer from an Fe3f ion to a Ti4+ ion and from a Ti3f ion t o an Fez+ ion can be considered as a possible cause of phototropy, because the probability of having two Fe3+ions as neighbors is small. The deep-brown color produced by Fe3+ and interpreted by the grouping shown in Figure 2B can be the result of a broadening of the absorption band of the titanium dioxide which, for the perfect crystal, has a rather sharp cutoff in the near ultraviolet. Strongly phototropic titanium dioxide has been obtained by incorporating into the rutile a combination of the oxides of iron and columbium (or tantalum), the latter causing the formation of Ti3+ ions. As pointed out previously (24), color centers of the type (Fe3+ e) are very common. The chrome-tin pink, a com+Fe4+ mercial pigment obtained by incorporating Cr3+ ions into stannic oxide or calcium stannate, is one of the best known examples. Its intensive color can be attributed to (Cr4+ e) centers. Manganese dioxide and lead dioxide do not exist as compounds of exact stoichiometric composition. Their formulas should be written M n O p 2 and PbOl-z where x may reach values of the order of 0.01. Cadmium oxide might owe its brown color to an oxygen deficiency. It is also possible that a compound of this type (CdO, PbS, PbSe) has the exact stoichiometric composition but an equal number of cation and anion deficiencies. Lead dioxide, for example, may owe its dark color to the groups P b 4+2(e)-Pb4+. I t s composition has been described recently by Thomas (20)as corresponding to the formula PbOl.DBI. For cadmium oxide ahd the defect structures mentioned, the thermal vibrations of the lattice are sufficient to produce the chromophore groups a t room temperature. Other materials
+
+
@
0 t
B
Schematic Picture of Two Extreme Forma of Color Centers
Such a configuration change is by no means unique or characteristic for the rutile lattice. Cerium oxide, zirconium oxide, or thorium oxide assumes, a similar brown color if an occasional tetravalent cation is replaced by a trivalent praseodymium ion. The force field of these crystals causes the loss of an electron per Prs+ ion, and these electrons assume the place of the missing oxygens. Cerium dioxide contaminated with praseodymium is deep brown and can be described by the formula: Ce:t.Pr:+
(e); 0 ;
I
=
Incorporating some Y 8+ ions into zirconium dioxide or thorium dioxide does not lead to the formation of such a chromophore group, a t least not a t room temperature. However, a t red glow the thermal energy is sufficient to produce electronic conduc ivity which can be explained by the change of following groups: Th:?.Y:+O;:,
,
into Thf?sY:+(e);O;:o,
5r
According t o Wiegand (27) the light emission of these substances above 1000° C. increases with temperature much more rapidly than that of other materials. Because of its electronic conductivity and high light emission, these anomalous mixed crystals between zirconium oxide or thorium oxide and yttrium oxide have been suggested as heating elements and light sources and are known as Nernst glowers. The combination of cerium dioxide with traces of trivalent terbium s e e m to be another example of a phototropic substance. According to Renz (18) traces of either terbium or praseodymium cause a cerium oxide crystal t o color when exposed to light and to fade in the dark.
Photo-Oxidation at Snrfaoe of Rutile The Phenomena. Renz (18) discovered that a number of oxides including titanium dioxide darken when exposed t o sunlight under conditions where they may give off oxygen to an oxidizable medium such as glycerol, tartaric or citric acid, or mannose. The fact that titanium dioxide can become an oxidant in sunlight is of great industrial significance. A pigment having this property may destroy the organic binder of a paint. Photo-oxidation leading to carbon dioxide as a reaction product produces a gas film between the pigment grains and the binder. This phenomenon is known as "chalking" because after having lost its adhesion the pigment comes off the paint like chalk. Recently Jacobsen (16) gave a complete description of this phenomenon and discussed the earlier literature on this subject. He correlated the chalking of various pigments with their photochemical activities: Chalking of titania-containing pigments is due to photooxidation. Photo-oxidation lowers the reflectivity of the pigment and this loss of reflectivity can be used to follow the rate of photo-oxidation. As far as the pigment is concerned, the end product of the photo-oxidation is alpha titanium sesquioxide. This final step, however, is rarely reached.
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The rate of oxidation is different for different vehicles; it is relatively low for alkyd resins but high for aqueous solutions of stannous chloride, glycerol, or mandelic acid. On the basis of a 0.5 M aqueous solution of mandelic acid a method has been developed for estimating the photo-oxidizing properties of various titania pigments. A mixture of the titanium dioxide with mandelic acid is exposed to a standard ultraviolet source and the loss of reflectancy is measured as a function of the exposure time. The photo-oxidation properties of different pigments vary over a wide range and so do their chalking qualities-for example, a certain loss of reflectivity required a 2000-minute exposure for the most inert but only 4 minutes for the most sensitive pigment.
As far as the phenomena are concerned, the paper of Jacobsen (16)presents the complete story and corroborates the qualitative observations of Renz (19). From the earlier work on this subject, that of Goodeve (9) and that of Goetz and Inn ( 7 ) should be mentioned. The last two authors studied the darkening oi silver salts adsorbed at various finely subdivided metal oxides, in particular titanium dioxide. Unfortunately, they interpreted the darkening of the silver compounds as a reduction process leading from the colorless adsorbed Ag+ to black metallic silver. However, the authors found that the silver compound formed is .4g202. This mistake does not decrease the value of their experimental work and the authors did not attempt to give an explanation for the phenomenon. Goodeve (9) studied the bleaching of organic dyestuffs which had been adsorbed on various pigments. Gion (6) found that titanium dioxide accelerates the photo-oxidation of ammonia. Interpretation of Oxidation Phenomena. The photo-oxidation of organic compounds or of silver to AgzOz is the result of atomic oxygen liberated a t the titanium dioxide surface under the influence of irradiation. Absorption of light is known to produce electronic conductivity in a number of crystals. This phenomenon was discovered by Gudden and Pohl (IO) and is called “photoconductivity.” The absorption of a light quantum releases an electron which can carry a current if its random motion in the crystal lattice is directed by an external electrical field. The removal of two electrons from an 02-ion produces atomic oxygen. The electrons are trapped in the lattice-that is, they will be found close to some Ti4+ions which thus are reduced to Ti$+ions. The process can be described by the equation:
+ hv = 0 + 2 ( e ) 2Ti*+ + 2(e)- = 2Ti3+ 2Ti02 + hv = TizO, + 0
0 2 -
or
The atomic oxygen may cause oxidation phenomena which resemble those of ozone. One of the most characteristic reactions of atomic oxygen is the formation of black AgzO2. This compound is called “silver peroxide” in the literature, but it seems doubtful to the authors that it is a peroxide. Divalent silver such as argentic perchlorate and fluoride are known to exist and the latter produces ozone when allowed to react with water. This compound forms readily if silver is exposed to ozone a t elevated temperatures; it is also observed when water is electrolyzed using silver metal as the anode. Snother typical reaction of atomic oxygen will be discussed later. This interpretation of photo-oxidation as the result of absorption of a light quantum sufficient to produce electrons and free positive holes (neutral oxygen atoms) raises several questions. Why do other oxides not show the same phenomenon and why is there such a difference in the photochemical properties of different preparations of titanium dioxide? The first question shall be answered more fully in a later part of this paper, but there is no basic difference between titanium dioxide and other oxides containing cations which may trap the. free electrons and thus prevent their recombination with the oxygen atoms. Even those oxides and compounds which usually
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are considered inert to chemical changes, such as silica in its various forms, aluminum oxide, or barium sulfate, exhibit photooxidation phenomena. -4 suspension of finely ground quartz in a diluted silver nitrate solution darkens in sunlight because of the formation of Agz02. The work of Renz (18)shows that the oxides of cerium, columbium, antimony, tungsten, vanadium, and bismuth darken in contact with oxidizable substances when exposed to light because they form a lower, colored oxide. In order to answer the second question it is necessary to realize that the electronic properties of a crystal are greatly affected by its defects. Defects in a lattice are the result of geometrical imperfections, missing atoms, interstitial atoms, foreign atoms, in short, of all those places where the symmetry of the force field is disturbed. Even a perfectly ordered titanium dioxide crystal is defective a t its surface, because its surface ions are exposed to strongly asymmetrical force fields. Before going further into the details concerning the role of the surface in photochemical reactions, some of the basic principles governing photolysis should be discussed briefly. Some Fundamental Aspects of Photolysis. ROLEOF FOREIGN A T O X S . Besides the photoconductivity there are a number of other phenomena which involve the absorption of a light quantum and the release of an electron. Fluorescence refers to this process under conditions where the electron remains closely attached to the absorbing atom and where it can return immediately with the emission of light. If the electron is trapped near its center of excitation and requires some energy (heat or infrared radiation) before it can return to its source, the process of light emission continues after excitation ceases and is then called phosphorescence. If the electron does not return to its original source but is trapped by another atom, there is a chemical change. This process may be accompanied by a color change and is then called solarization and when readily reversible in the dark i t is called phototropy. If the reaction is not reversible because one of the reaction products has escaped, one speaks of photolysis. There is no basic difference between these processes and often it is arbitrary to assign to a phenomenon one of these terms. Silver bromide exposed to light darkens in an irreversible fashion if the surrounding medium removes the bromine atoms by a chemical reaction. If the elemental bromine is available for recombination with the silver, this darkening becomes a reversible process and may be called phototropy or solarization. The same applies to titanium dioxide. Titania may undergo photolysis or solarization depending on the medium which surrounds the crystals. The parallelism between the behavior of titanium dioxide and silver bromide can be seen from the following reactions involving an electron transfer from the anion to the cation :
+ + hv = 2 ~ i 3 ++ o + Br- + hv = Ag + Br
2 ~ i 4 0~2 Ag+
The authors ( 2 4 ) have shown that phenomena which result from the excitation of an atom or the removal of an electron become more probable if the electron donor is exposed to asymmetrical force fields. The photoconductivity of binary compounds (Table I), according to Gudden and Pohl (IO), brings out very clearly that weakly polarizable anions such as F-, NO3-, SO:- and C0;- are not suitable for releasing electrons when irradiated with visible light. Their electron configuration requires larger light quanta for excitation-that is, their absorption bands are in the ultraviolet region of the spectrum and the removal of an electron is a rare event. Oxides and sulfides, on the other hand, absorb in the near ultraviolet or even in the visible region, especially if they are deformed by cations with incomplete outer electronic shells. C1- and Br- assume an intermediate position. In combination with the noble gas-type cations of low charge, for example, N a + and K+, the probability of an electron being released within the crystal is low, a t least for pure alkali
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
February 1950
halides. However, when polarized by nonnoble gas-type cations like Pb2+ or TI + (18 2 outer electrons), C1- and Br- do release electrons on irradiation.
+
Table I Cationa
m
Anions in order of increasing polarizability, Fajana ( 4 ) F - NO*- SO42 COO’- C1BrI0 2 S2-
The paramount importance of asymmetrical force fields t o the probability of an electron being released is well established experimentally. The sensitivity of silver halides can be increased greatly if sulfur atoms are attached to their surfaces. The asymmetrical grouping Br--Ag +-S2- causes a disturbance which enhances photolysis. By incorporating impurities into metal halides or by merely mixing different compounds a number of apparently stable crystals can become photosensitive (22, 23). The effect of asymmetrical groups on photoconductivity is known from the work of Case ( 1 ) and Hintenberger (15). According to these authors, the photoconductor properties of thallium sulfide and of lead sulfide are greatly enhanced if these compounds are partly oxidized-that is, if they contain the asymmetrical units 02--TI+-S2- and 02--Pb2+-S2- instead of the symmetrical group of the perfect crystal. Briefly, impurity atoms in a crystal produce asymmetrical force fields which deform the electron donor and, thus, increase the probability of an electron being released on irradiation. ROLEOF SURFACE.The release of electrons in crystals, on irradiation, can be explained satisfactorily on the basis of Fajans’s ( 2 ) ideas on the deformation of ions. The polarized or the deformed anion is the one most likely to release an electron. The state of polarization of an ion is determined by its polarizability (size, charge, electron configuration) and by its surroundingsthat is, the nature of the neighboring cations and the crystal symmetry. Imperfections, vacancies, and foreign atoms play a decisive role because they produce the same effect as the crystal surface-namely, a strongly asymmetrical force field. Earlier in this paper it was pointed out that the surface of a crystal cannot be treated on the basis of crystal chemistry because the classical crystal chemical concepts were derived for ideal and infinitely extending crystals. The chemistry of the surface is one of defect structures and is being developed by the authors (84). This “pulling over” of the electrons of an anion is not merely a convenient way of describing polarization but it is a reality as can be seen from the cupric halides (24): CUFZ White
CuClz Yellow
CuBrz Brown
CuIz Nonexistent
The deformation increases from the small F- to the larger and, therefore, more polarizable halogen ions, C1- and Br-. The increased mutual deformation increases the probability of light absorption. Attempts to synthesize cupric iodide fail because in this case the Cuz+ actually succeeds in pulling an electron away from the most polarizable I- ion. As a result, the Cu2+ completes its outer orbit to the stable 18-shell of the Cuf. Reactions which should lead to Cut+ IS give Cu+ I- and free iodine.
261
A similar situation arises in the surface of crystals containing
02-ions which are exposed to a highly charged cation on one side only. Another way of describing such a polarized 02-would be by giving its electron distribution curve. The radial probability distribution function characteristic for the ion in a symmetrical force field becomes asymmetrical, with a higher electron density towards the cation and a lower electron density towards the free space. The pulling over of two electrons of a surface anion by the cation cleaves off atomic oxygen which can be identified by its typical reactions. Because the number of surface oxygens which undergo this process is small, one has either to use very sensitive methods for detecting the atomic oxygen or to study materials with extreme surface areas (silica gel, clay). The experimental methods for studying these oxidation phenomena are described later. When comparing the oxidizing effect of silicon dioxide with that of titanium dioxide, an additional factor has to be taken into consideration. A Tisf ion in the rutile lattice can be treated as a Ti4+ ion plus an additional electron. These “loose electrons’’ travel from one Ti4+ to the next in a random fashion. In an external electrical field they are responsible for the electronic conductivity of the partly reduced titanium dioxide. For rutile, this electron transfer from 02to Ti*+may lead to a complete reduction of Ti02 to Ti20aas has been shown by Jacobsen (15) who irradiated titanium dioxide suspended in glycerol and identified the reaction products as carbon dioxide and titanium sesquioxide. The relative stability of the defective titanium dioxide lattice makes it probable that most of these photochemical reactions will not go to completion or lead to a mechanical mixture of Ti02 and TizO$ but to a rutile lattice with oxygen deficiency.
Rednetion Phenomena at Sarface of Rmtile The defective rutile lattice can absorb oxygen atoms and, thus, be oxidized to the original stoichiometric compound. Renz (18) has observed that the darkening of certain oxides becomes a reversible process especially if citric acid solution is used for the reaction. The reaction rate of titanium sesquioxide which has formed from titanium dioxide by photolysis is relatively high,, but this can be explained on the basis of the “memory effect.” Barium oxide, for example, obtained from barium peroxide by reduction, reacts much faster with oxygen to form barium peroxide than barium oxide prepared from barium hydroxide by dehydration. The latter, in turn, reacts faster with mater to form the hydroxide than barium oxide obtained from barium peroxide. This phenomenon which was called “memory effect” by Huttig (14) is probably the result of certain areas of the crystal still retaining a force field which resembles the original compound. This memory effect has been observed also for AgzOz. Whereas a new piece of silver metal reacts with ozone only a t elevated temperature to form the black AgzOZ,this reaction proceeds rapidly at room temperature if the metal has been oxidized previously. It is not surprising, therefore, that TizOs obtained from TiOz by removal of oxygen atoms a t a low temperature “remembers” its original structure and reacts as a reducing agent in contact with substances which provide oxygen atoms for filling out its lattice vacancies.
Methods for Studying Photochemical Changes Jacobsen (16) developed the qualitative observations of Renz (18) into a quantitative test. He measured the reflectivity of titania pigments which were mixed with a 0.5 M mandelic acid solution and then exposed to the radiation of a standard ultraviolet light source. The loss of reflectivity plotted against the time of exposure made it possible to compare different pigments and to estimate their photochemical reactivity.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Goodeve (9) studied the photochemical oxidation of pigments by following the bleaching of adsorbed organic dyestuffs, and Goetz and Inn (7) studied the darkening of adsorbed silver compounds. Both types of reaction can be developed into a quantitative or a t least semiquantitative method. For a quantitative determination of the photo-oxidizing effect of titanium dioxide or similar oxides one has to keep in mind the fact that some of these reactions are reversible and that one measures the difference between the rate of oxidation and reduction. In connection with work on the catalytic properties of silica and of silicates a sensitive method for detecting atomic oxygen had to be developed. This test is based on the oxidation of 4,4’,4”hexamethyltriaminotriphenyl methane to the carbinol, the base of the intensely colored crystal violet. This compound which is not oxidized by hydrogen peroxide is easily oxidized by atomic oxygen-for example, if the aqueous solution of its acetate is electrolyzed using platinum electrodes: [(CH3)z.N.C6H4]3 j CH
+ atomic 0 =
[(CH3)2.N.C6H4]3 i CO-H
This reagent can be applied in the form of its benzene solution. I t has a vapor pressure which permits its oxidation in the vapor phase a t approximately 150” C. Most tests were carried out in spot test plates with small amounts of the pigments exposed to sunlight or ultraviolet radiation for 1 to 10 minutes. The colorless compound changes into the deep purple dye which can be extracted with diluted acetic acid.
Photochemical Behavior of Titanium Dioxide Compared with That of Other Oxides The probability of an electron being released from an anion increases with its increasing deformation. Photoelectric phenomena are favored by the combination of strongly polarizing cations with large and polarizable anions. Photolysis requires, in addition to these factors, that the cations be able to permanently trap the electrons which are released from the anions. I n other words, photolysis requires that the cations may be easily reduced to a lower valency or to the neutral atoms. I n comparing different oxides with respect to their photoelectrical and photochemical properties, it is important to realize that photoelectric effects lead to photolysis only if the reduction product is energetically stable and if the rate of recombination is lower than that of photolytic dissociation. Goodeve ( 9 ) made the first quantitative investigation of the photochemistry of white pigments. He emphasizes the role which the absorption threshold of a compound plays in determining its photochemical properties. Only those wave lengths which are absorbed by a medium can produce chemical changes. However, the absorption threshold of a compound is a value which usually refers to the bulk of the crystal but not to its defects nor to its surface. Silica, for example, has “no visible absorption” and the absorption threshold of quartz is near 200 mp. The meaning of this numerical value is that the average Si04 unit in a quartz crystal cannot be excited by visible or near ultraviolet radiation, It requires larger light quanta to bring an electron of a Si04 group into a higher energy level. It would be a mistake, however, to conclude from this absorption threshold of quartz, that photochemical reactions are impossible if a system containing quartz is irradiated with sunlight. The oxygen ions in the surface of a quartz crystal are exposed to the one-sided attraction of Si4+ and, consequently, they find themselves in strongly asymmetrical force fields. Much less energy is required to remove an electron of such a polarized 0 2 - ion than from an ion which is symmetrically surrounded by Si4+ions. An atom strongly polarized by an asymmetrical field of the surrounding medium has its energy levels crowded more closely together than the same atom in its gaseous state. The absorption threshold of the surface lies at a much longer wave length than that of the bulk and the absorption spectrum assumes a noticeable “tail” if the surface to volume ratio is increased. Under the influence of sunlight, the various forms of silica show such an electron transfer in their surface layer which leads to atomic oxygen. The
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quantity of atomic oxygen is small but its oxidation potential is high. The difference between the photochemistry of titanium dioxide and silicon dioxide is one of degree. For SiOz photoelectric phenomena occur predominantly in the surface; for Ti02 the process of electron transfer may well take place in the interior, especially near flaws. Germanium dioxide, in its physicochemical aspects, resembles silicon dioxide closely. However, this similarity is restricted to conditions where the Si4+and Ge4+ are exposed to symmetrical force fields and where their own force fields can be treated as the result of charge and size. In symmetrical environments polarization is reduced to a minimum and, consequently, the properties of Ge4+ and Si*+are not very different. However, Ge*+has eighteen outer electrons and is more polarizable than the Si*+with its octet shell (neon-structure). In the asymmetrical force field of the surface where the polarizability dominates the behavior of an ion, Ge4+ and Si4+ are no longer alike. As compared with Si02 and TiOn,the GeO2 has very weak photo-oxidizing properties. Its surface anions are less deformed than those of silicon dioxide and it cannot easily trap an electron and form stable cations of lower valency. The chemical similarity between Si02 and GeOt does not extend into surface chemistry. Whereas Cr3+ and Coz+ ions adsorbed a t silica gel are oxidized to Cr6+ and Co3+ on gentle heating (200” C), no such reaction takes place a t GeO2.
It would far exceed the scope of this paper to extend this discussion to other oxides, but it shall be emphasized that the surface and the flaws in crystals are the spots where photochemical processes start. As a result, all combinations, additions, or surface treatments which balance the forces in the surface and lessen the strain on the surface anions decrease the photochemical properties of titanium dioxide. The effect which additions of other oxides exert on the photoactivity of titanium dioxide cannot be predicted from the behavior of these oxides alone. It has been shown (gf, 9 6 ) that a combination of several ions of different size and polarizability leads to the formation of surface active groups which lower the surface forces of glasses and fused salts. Potassium chloride or sodium chloride, for example, will adhere firmly to the wall of a platinum or porcelain crucible in which they had been fused and so will potassium ,odide. However, if a small percentage of an iodide is added to either potassium chloride or sodium chloride, the ions will arrange themselves in such a manner that the surface forces will become a minimum and, after fusing and cooling of the mixture, the salt cake falls out of the crucible and shows no signs of adherence. The reasoning which led to the discovery of this startling behavior of mixed salts is applicable also to the photochemistry of solids. On this basis one can understand that certain polarizable cations may decrease the photochemical reactivity of titanium dioxide. It is not possible here to discuss the numerous patents covering additions to titania in order to minimize the chalking of the paint. They can be divided into two groups. One type of addition leads to the formation of surface active groups and, thus, decreases the free energy of the surface film. Another type, such as alkali, alkaline earths, or silica, forms a film around each particle (alkali titanate) which prevents atomic oxygen from diffusing from the interior of the crystal into the pigment-resin interface. The first type represents a typical catalytic poison, a molecule which modifies the force field and, with it, the surface properties of the substance. The second type may be compared with a phenomenon which is known in metallurgy. Certain metals and alloys allow hydrogen atoms to migrate through their structures. These hydrogen atoms, however, combine to form hydrogen molecules and build up local gas pressures as soon as they hit an interface with a medium (enamel coating) which does not allow atomic hydrogen to exist in solution. I n a similar fashion oxygen atoms migrate through the titanium dioxide until they reach the pigment-resin interface where they exert their destructive oxidation. However, if the titanium dioxide particle has been coated by a film of silica, alkali titanate, or other substances which do not stabilize oxygen atoms in their structures, molecular oxygen is formed which does not have the oxidation power of the atomic form.
February 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
conclusions
e
II
Classical crystal chemistry is based on the concept that solids consist of an array of ions where each ion is surrounded by other ions of the opposite charge and, thus, exposed to a more or less symmetrical force field. This approach has been fairly successful so long as one was primarily concerned with crystals or glasses consisting of noble gas-like ions. Pauling’s rules for the crystal structures of complex ionic crystals were wisely limited to ions which exclude major polarization effects. By excluding the cations with incomplete electron.shells, such as Hg+, TI+, or Pb2+, and the polarizable large anions, such as Sz- or Se*-, it was possible to use ion charges and sizes as the parameters which chiefly determine their force fields and, with those, their roles in a crystal structure. This first approximation, however, becomes unsatisfactory as soon as polarization becomes a major factor. Fajans and Kreidl(6) showed that the atomic structure of lead silicate glasses cannot be understood on the basis of those rules of glass formation which apply to glasses containing only ions with a complete octet shell. I n order to understand the participation of lead oxide in the glass structure, the polarization phenomena connected with the presence of Pb2+ ions had t o be taken into consideration. I n crystal structures containing ions of the nonnoble gas type, polarization must be considered. For flaws and surfaces, polarization becomes a major factor, even in crystals consisting of noble gas-type ions. The strong effects of polarization resulting from asymmetrical force fields have been demonstrated by Fajans (3) in his interpretation of the internuclear distances of alkali and thallium halides in their crystalline and vapor states, respectively. Each surface and each flaw represents the seat of an asymmetrical force field. As a result, surface chemistry of solids is much more affected by the polarization properties of the ions than one might expect from the chemistry of aqueous solutions. The surfaces of silica, silicates, sulfates, and other apparently “inert” compounds have unexpected chemical properties. They become obvious, however, if the surface is treated as a defect structure. Surface oxygens are strongly polarized and their state of deformation allows optical excitation by light quanta which are much smaller than those necessary to raise an electron of the oxygen ion in the interior of the crystal into a higher energy level. In spite of the fact that quartz has an absorption threshold near 200 mp, its surface shows photoelectric sensitivity when irradiated with visible light. This is characteristic for all crystal surfaces which contain polarizable anions and cations with fairly strong force fields. Photolysis which is practically nonexistent for SrO or BaO becomes noticeable for ZrOl and T h o zand more so for SiO2. Ti01 undergoes complete photolysis, into Ti2O3 and oxygen, especially if a reducing agent removes the oxygen atoms from the crystal surface. It should be emphasized that the process of photo-oxidation has little or no connection with the oxidation-reduction properties of the substance in the usual chemical sense. It is a phenomenon characteristic for an interface and has been observed not only for “inert” materials, such as silicon dioxide or barium sulfate, but even for crystals which, in the usual chemical sense, are reducing agents. Oxalic acid, for example, could be used t o photooxidize the hexabase indicator used. SrO, BaO, NiO, GeO, and Inn08 are examples of oxides which showed little or no photooxidation. This phenomenon should be of considerable interest to industrial chemists who deal with interfaces between oxidizable organic substances and inorganic fillers, like rubber or filled plastics. Numerous photo-oxidation phenomena, such as the bleaching of dyestuff on cellulose, probably involve this photoelectric surface effect as the first step. The magnitude of the photochemical activity varies not only from one oxide to another, but is susceptible to strong fluctuations for one and the same chemical compound. The reasons for the fluctuation found for different titanium dioxide pigments are:
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1. Different specific surface areas. 2. Flaws and imperfections within the crystal as the result of: impurities, especially those accumulated in the crystal surface; lattice imperfections resulting from the phase transition, anataserutile; and deviation of composition from the stoichiometric ratio.
A picture of the role played by foreign atoms with an excess charge lower or higher than four has been derived on the basis of light absorption. It also was pointed out how foreign atoms entering the surface structure of the titanium dioxide may increase the symmetry of the force field and, thus, decrease photosensitivity. I n connection with this work, it became necessary to develop a reagent very sensitive to atomic oxygen which could be applied in the form of its solution in an organic solvent. This reagent does not respond to hydrogen peroxide or to the nitrogen oxides formed under ultraviolet radiation. The 4,4’,4”-hexamethyltriaminetriphenyl methane meets this requirement. Its solution in benzene is colorless. The oxidation product can be extracted with diluted acetic acid and the dye, crystal violet, can be determined colorimetrically. The sensitivity of this test made i t possible to detect photoelectric phenomena and photolysis within exposure times of the order of 1 minute.
Acknowledgment The authors wish to express their gratitude to J. A. Leermakers of the Eastman Kodak Research Laboratory, Rochester, N. Y., through whose cooperation the reagent used in this work was secured. The help given by Mrs. T. T. Forland and Dorothy Enright in carrying out numerous experiments also is acknowledged.
Literatare Cited (1) Case, T. W., Phys. Rev., 15, 289 (1920). (2) Fajans, K., “Chemical Forces and Optical Properties of Substances,” New York, McGraw-Hill Book Co., 1931. (3) Fajans, K., J . Chem. Phys., 9, 378 (1941). (4) Fajans, K., 2. Krasl., 66, 321 (1928). (5) Fajans, K., and Kreidl, N. J., Glass Sci. Bull., V, 147 (1947); J . A m , Ceram. SOC.,31, 105 (1948). (6) Gion. L., Compt. rend., 195, 421 (1932). (7) Goetz, A., and Inn, E. C. Y., Rev. Modern Phys., 20, 131 (1948). (8) Goldschmidt, V. M., Norske Vadenskaps.-Akad. M a t . Naturv. KL, Oslo, 1926, No. 8. (9) Goodeve, C. F., Trans. Faraday SOC.,33, 340 (1937). (10) Gudden, B., and Pohl, R. W., 2.Physzk, 16, 42 (1923). (11) Harbert, C. J., U. S. Patent 2,347,630 (1944). (12) Hedvall, J. A., and Sjoman, P., 2.Elektrochemie, 37, 130 (1931). (13) Hintenberger, H. Z., Naturforschung, 1, 13 (1946). (14) Hbttlg, G. F., Kollotd-Z., 106, 166 (1944). (15) Jacobsen, A. E., IND. ENG.CHEM.,41, 523 (1949). (16) Lee, 0. I., Am. Mzneralogzst, 21, 764 (1936). (17) Parmelee, C. W., and Badger, A. E., J . Am. Ceram. SOC.,17, 1 (1934). (18) Renz, C., Helv. Chim. Acta, 4, 961 (1921). (19) Renz, C., 2.anorg. Chem., 110, 104 (1920). (20) Thomas, U. B., Trans. Electrochem. SOC.,94, 42 (1948). (21) Weyl, W. A,, Glass Ind.. 23, 135 (1942). (22) Weyl, W. A., and Cramer, A. I., J . Optacal SOC.Am., 39 (1949). (23) Weyl, W. A,, and Enright, D. P., Ibid., 39 (1949). (24) Weyl, W. A., and Forland, T., Office Naval Research, Tech. Rept. 2, contract No. N6 onr 269 task order 11 NR 032-265 (1949). (25) Weyl, W. A., and Johnson, G., J . Am. Ceram. SOC.,32, 398 (1949). (26) Weyl, W. A., Marboe, E. C., and Smiley, W. D., J . 8 o c . Glass Technol., 32, 264 (1948). (27) Wiegand, E., 2. Physik, 30,40 (1924). (28) Williamson, A. 0.. J . Mzneralog. SOC.,London, 25, 513 (1940). RECEIVED August 31,1949
