ACTION OF LIGHT ON THE GAS ADSORPTION BY SOLIDS BY A. TERENIN AND Yu. SOLONITZIN Physical Institute, The University, Leningrad B-164, U.S.S.R. Received 22nd May, 1959 Specific photodesorption effects, not due to heating by light, have been previously observed for CO adsorbed on Ni and for H20 on Cd and Zn, but not on Bi and Sb (Valnev). Oxygen is photodesorbed from ZnO with Zn excess. Photosorption of 0 2 and of CO takes place on ZnO with oxygen excess. A strong photosorption of oxygen by well- degassed silica gel was found, and interpreted as due to the splitting-off of surface hydroxyls by the ultra-violet light. In a previous review of this field,l four kinds of processes occurring at the boundary gas/solid under illumination were described, viz., (a) a photodesorption of unchanged gas molecules, (b) a photosorption of gases under the influence of light, absorbed by the solid, (c) a photo-decomposition of the adsorbed gas molecules, or the surface compounds, (d) a photoreaction of the illuminated surface with the adsorbed molecules.In some cases it is impossible to separate the elementary processes here enumerated, which can take place simultaneously, or consecutively. We limit this contribution to the presentation of the results obtained for the photodesorption and the photosorption of gases at low pressures of the order of 10-2 to lO-5mm Hg, which have been partially described in previous papers.1-5 EXPERIMENTAL From the start of this line of research in the thirties,l we used in our laboratory various set-ups, the principle of which is shown in fig.1. The cell 1 was of Uviol glass, or quartz, joined to the rest of the apparatus by graded seals ; the Pirani gauge 2 had a tungsten filament 200 mm long and ca. 10 p thick, which formed a branch of the resistance bridge, and was immersed during the measurements into a constant temperature bath. Various cut-offs 3 have been tried, viz., with mercury,* melted tin and, in the last model, a metal valve. In the control experiments the cell with the gauge has been sealed-off from the vacuum line. The trap 4 was used to test the condensability of the desorbed gas, that at 5 was cooled by liquid-nitrogen or -air for a final elimination of condensable gases and vapours. For the solids in the form of powders the cell was shaped as shown in fig.1, the powder being kept between the semi-spherical surfaces and the light incident from the concave side. For sublimed metal layers a rectangular or cylindrical cell was used, the metal being sublimed on one side of the wall. The closed volume of the cell, the trap 4 and the gauge was, in the latter experiments, 10-15 cm3. Samples of the powdered solids, after being heated in air at 600°C in order to burn out organic contaminations, were subjected to a degassing in the cell for many hours at 300-400°C until there was no further gas desorption. The layers of the volatile metals Zn, Cd, Sb, Bi, investigated by Valnev,3 were obtained by evaporation of a small bead of the metal from an appendix of the cell into it, after careful degassing of both ; the metal layer was sublimed several times from one wall to another in a vacuum of 10-6 mm Hg.The pressures in the gauge, previously calibrated for various gases, were recorded on photosensitive paper using the spot of the galvanometer, and later with a pen recorder. The barograms in the figures are reproductions of the actually recorded curves. The * Control experiments showed that the presence of mercury vapour did not influence the results. 28A. TERENIN AND YU. SOLONITZIN 29 sensitivity of the gauge was 1.5 x 10-5 mm Hg per mm of the scale on the paper ; in the latest set-up the sensitivity was 6 x 10-7 mm Hg per mm. The sources of light were: (a) a condensed 200-W spark with Al, Zn, Cd or Fe-Ni electrodes, (b) a quartz high-pressure mercury 250-W lamp, (c) an incandescent projection 300-W lamp. The small magnitude of the effects observed did not permit the use of monochromators, therefore the active range of the ultra-violet light was roughly deter- mined with the following light-filters having absorption limits at the wavelengths given in brackets : glass 1-5 mm thick (330 mp), mica (310 mp), organic film (290 mp), gelatine film (240 mp), calcite 9 mm thick (225 mp), Cellophane film (210 mp).FIG. 1 .-Experimental set-up. 1, the cell ; 2, the Pirani gauge; 3, a mercury cut-off; 4 and 5 , traps. 4 2 5 The gases required were obtained in small quantities by the following procedures : oxygen by heating KMn04 crystals in vacuo; CO by heating a mixture of CaC03 and Zn powder; 6 hydrogen by electrolysis ; nitrogen from NaN3. These gases were passed through traps cooled by liquid-air.Water vapour was obtained at its saturation pressure from copper sulphate hydrate crystals. Ammonia was produced by heating in vacuo its complex salt with silver chloride : before admission the gas was purified by freezing it at - 78°C. For photodesorption experiments, the preliminary adsorption was carried out at pressures of 10-1 to 10-2 mm Hg, but in some cases a pressure of 10 mm was used. After introducing the gas to the solid, the latter was pumped off and the evacuation repeated until the thermal desorption of the gas stopped or was substantially reduced in the dark. For the photosorption experiments the degassed sample was put into contact with the gas studied at a pressure of 10-2-10-3 mm Hg. RESULTS AND DISCUSSION PHOTODESORPTION FROM METALS Fig.2, taken from Valnev’s paper,3 shows a typical barogram for the action of u.-v. light on Ni which has adsorbed CO.* From the experiments, it can be * The disperse nickel used in the experiments was obtained by thermal decomposition of NiN03 to the oxide state and subsequent reduction with hydrogen at 250-300°C. The degassing in vacuo of the metal layer was carried out at 300°C for many hours.30 ACTION OF LIGHT ON ADSORPTION inferred that an irreversible desorption of a gas non-condensable at - 18O"C, takes place under the action of wavelengths shorter than ca. 240mp. Water and ammonia vapours, adsorbed on a similar Ni sample, exhibit likewise an enhanced degassing under illumination, but in contrast to CO, the rate of this desorption does not depend on the light-filters used ; moreover, a slight desorption is observed under illumination by low intensity infra-red radiation.The gas desorbed in these cases condensed at - 180°C. This points, for H20 and NH3, to a purely thermal action of the light, absorbed by the disperse Ni. 0 3 rA 2 a FIG. 2.-Photodesorption of CO from disperse nickel. 1, unfiltered light of a zinc spark ; sX104 rnm "9 I I I 2, light filtered through a gelatin film 111 I I transmitting only wavelengths longer I 1 than 240 mp. I I < I I I I I I I I 0 10 min In contrast to this behaviour on Ni, water vapour adsorbed on sublimed laycrs of Cd and Zn, gives photodesorption of a gas which is 50 % condensable at - 180°C, but only under the action of wavelengths shorter than ca.250 mp. Moreover, the spectral threshold for Cd was obtained somewhat to the red as compared with Zn. No photodesorption from sublimed layers of Bi and Sb was observed after H20 adsorption. Likewise, no enhancing action of light could be found on the slow thermal desorption at 20°C of CO, CO2, 02, H2 from sublimed Cd, Sb and Bi layers, although the latter was black and absorbed practically all the incident energy. The presence of a sufficient concentration of the adsorbed gases with the sublimed layers was proved by their appearance on raising the temperature. A photodesorption of hydrogen from the Ni sample was also absent. From the desorption rate from the sublimed layers it could be inferred that the coverage was approximately monomolecular on the geometric area of the layer.For H20 adsorbed on Cd, Valnev2~ 3 ascertained that the photoelectronic emission from the metal produced by the u.-v. light simultaneously with the desorption, had nothing to do with it. With a potential difference of 15 V applied between the metal and a wire, the sign of the potential on the metal did not affect the photo- desorption rate, whereas the photocurrent increases 5-10 times when the metal is made negative. The specificity of the photodesorption by short u.-v. light limited to CO on Ni and to H20 on Cd and Zn, for which the purely thermal desorption in the dark is relatively slow, indicates a primary quantum origin of the phenomenon. For desorption caused by the thermal action of the light, the liberated gas is re- adsorbed.1 The presence in the case of H20 on Cd, or Zn of a high percentage of a non- condensable gas at - 1 80"C, indicates that a photodissociation of chemisorbed water molecules is taking place simultaneously.The active wavelengths are considerably shifted (by about 2 eV) towards the red as compared with the threshold of photodissociation of water vapour. It is strange that a photo-A. TERENIN AND YU. SOLONITZIN 31 dissociation of ammonia adsorbed on Cd could not be obtained, although such a process has been observed for NH3 adsorbed on alumina, MgO and Cu sulphate.7~ 8 There are no grounds for ascribing the photodesorption of CO from Ni to a similar photodissociation of nickel tetracarbonyl formed at the surface of the disperse metal upon CO adsorption.First, nickel tetracarbonyl is not known to be produced by simple contact of Ni with CO gas at room temperature. Secondly, Tagantzev 1 has shown that when nickel tetracarbonyl is formed on the surface of the metal by heating a layer of Ni in CO, the u.-v. irradiation produces a gas which is condensable at - 180°C, in contrast to the experiment described above. In this case, evidently nickel tetracarbonyl molecules were photodesorbed. The experiments on photodesorption described above indicate that the electronic energy of excitation imparted to adsorbed molecules by light is dissipated by the solid at a slower rate, than the rupture of a covalent, or adsorption bond. It can be presumed for metals that the electronic excitation energy of a chemisorbed species on them will be dissipated by the metal in a time of the order of 10-15 sec. The transition of the electronic excitation energy into the kinetic energy of the disrupted bond with a co-ordinate perpendicular to the surface would require ca.10-12 sec. This gives approximately 10-3 for the probability that a sufficient energy will remain in the bond, to disrupt it. This estimation, if correct, means that a photodesorption of an entire molecule or a photodissociation of an ad- sorbed molecule should be of observable magnitude, as is the case. In the paper,l potential curves were drawn for the photodesorption process of CO on the as- sumption that CO is chemisorbed as a carbonyl diradical =C=O. It was ten- tatively presumed that the absorption of a large u.-v. quantum would lead from the deep potential minimum of the chemisorbed state directly to the repulsion branch of the potential curve depicting the physical adsorption of the CO molecule.Thus, for the possibility of a photodesorption, the minimum on the chemisorption curve should not lie too deep, otherwise much shorter wavelengths would be required. Besides, a strong chemisorption should lead to a more efficient dis- sipation of any excess of vibrational, or kinetic energy in an adsorption bond. The chemisorption of H20 molecules on metals occurs probably through the oxygen atom.9 An alternative explanation of the relatively slow dissipation of the excitation energy of the adsorbed molecules on metals would be the assumption that the surface is in our samples a very irregular one, and therefore the centres on which the adsorbed molecules are attached, are feebly interacting with the bulk of the metal.PHOTODESORPTION AND PHOTOSORPTION OF GASES ON SEMICONDUCTORS The desorption of oxygen from ZnO under the action of light absorbed by the semiconductor, has been observed by Miassnikov,lol 11 Heiland 12 and Melnick 13 by the changes of the electrical conduction, by Putzeiko and Terenin14 by the changes of the e.m.f. under intermittent illumination, and by Tagantzev and Terenin 15 by the luminescence of ZnO. Direct manometric measurements have been carried out by one of the authors.4~ 1 * One of the barograms obtained by him is shown in fig. 3 ~ . The first experiments were unsuccessful ; this is explained by the fact that the effect is clearly observed only for ZnO with excess metal.The photodesorption can be observed even with metallic Zn, evidently covered with a thin oxide coating. If Zn metal remains in contact with oxygen several hours at 20°C, the photodesorption disappears and neither admission of fresh oxygen, nor a heating of the sample up to complete oxidation, renewed the photo- desorption. This explains why in the experiments of Valnev 3 a photodesorption * In a recent paper by Medved 11 the photodesorption of 0 2 from ZnO has been meas- ured with an ionization gauge'in a flow system. Such dynamic conditions do not always allow unambiguous conclusions.32 ACTION OF LIGHT ON ADSORPTION of 0 2 from Zn has not been observed, since in all these experiments the adsorption of oxygen took a long time.In the experiments with ZnO, samples of different origin have been used. The preliminary thermal degassing was such, that before the oxygen adsorption, no gas was evolved on illumination. 0 5 10 0 3 6 A B min FIG. 3.-A photodesorption of oxygen from ZnO under u.-v. light in the range 366 mp; (B) photosorption at 90°C of oxygen on ZnO, enriched by 0 2 . Some oxygen desorption from ZnO could be produced not only by light of wavelengths shorter than 400 mp, but to a much smaller extent by infra-red radi- ation in the range from 1 to 2p. The non-thermal origin of the photodesorption by the near u.-v. is also shown by the behaviour of CO in contact with ZnO : the illumination in the near u.-v. gives a photosorption, whereas the infra-red radiation always leads to a pressure increase (fig.4), the desorbed gas being adsorbed again after the end of the E? a k ) I 1.0 I 1 I. 0 5 10 I 5 time, min FIG. 4.-(a) Thermal desorption of CO from ZnO under infra-red illumination (filter 1 to 2 p) ; (b) photosorption of CO by ZnO under near u.-v. irradiation by 366 mp. illumination. It may be that the irreversible photosorption of CO on ZnO is in reality due to its oxidation, sensitized by the semiconductor. This effect is being studied further. As shown in fig. 3A, the continuation of the illumination leads to a pressure decrease of the initially photodesorbed gas. This photosorption cannot be ex- plained by the adsorption of the gas on sites of the surface kept in the dark, sinceA . TERENIN AND YU. SOLONITZIN 33 on raising the oxygen pressure to 1 x 10-2 mm Hg the photosorption increases.On ZnO enriched by oxygen, the photosorption is very striking (fig. 3 ~ ) . The wavelengths most effective for the photosorption are shorter than 400 mp, but a decreasing activity can be followed down to 500mp. The rate of photo- sorption sharply decreases on rise of temperature; * at 150°C, no photosorption of 0 2 on ZnO could be observed. We have, first, supposed that this photosorption is due to traces of water, since it became larger after ZnO has been in contact with water vapour. Under such conditions, a well-known oxidation of water by oxygen can occur, photosensitized by ZnO, with the formation of hydrogen peroxide. However, Fujita and Kwan 17 have recently found an irreversible photosorption of oxygen on oxidized ZnO; the active wavelengths were likewise shorter than 450mp.If, under their con- ditions, traces of water were definitely excluded, this would mean that in the experi- ment of one of US,^ reproduced in fig. 3 ~ , the same true photosorption was present. Kobajashi and Kawaji 18 have recently found a photosorption of oxygen on ZnS containing a trace of Cu (10-2mole %) and followed it by measurements of the contact potential of the surface. Some cases of photodesorption and photosorption of hydrogen on Tho3 have been mentioned by Luycks, Bodart and Rens (cf. ref. (1)) and by Duval (cf. ref. (2)), but the limitation of the active range to the Hg line 254 mp raises some questions about the possible participation of the excited mercury atoms present as traces in the gas phase.An explanation of the photosorption of oxygen on the electronic semi- conductors ZnO and ZnS should be sought in the accumulation of electrons in surface traps, according to the theoretical presentation of Kobayashi and Kawaji,lg or the theory of Volkenstein,lg implying an adsorption of molecular radicals on electrons at the surface. Oxygen can be regarded as a biradical. The photodesorption of oxygen from ZnO does not present theoretical diffi- culties. It is now generally admitted20~21 that 0 2 molecules, adsorbed on an electron-excess semiconductor, like ZnO, do act as electron traps producing a negative surface layer. Strongly held 02- molecules cannot leave the surface, unless they lose their electron when an exciton reaches the surface of ZnO.The direct experimental proof of the existence of such a negatively-charged layer has been given recently in this laboratory by measurements of the exit work of photo- electrons from ZnO.22 Moreover it has been shown in this laboratory 23 that for ZnO, and also for Ti02 and WO3, specific wide unselective absorption ranges are present in the infra-red between the wavelengths 4 and 14 ,u which are to be ascribed to electrons in donor surface levels.? In fact, this absorption spectrum dis- appears when oxygen, or NO, or quinone vapour, is in contact with the semi- conductor in powdered form. Under illumination in vacuo by near u.-v. wave- lengths in the range of the absorption of the semiconductor, the infra-red absorp- tion spectrum re-appears and the experiment can be reversibly repeated many times.A thermal degassing is less effective. We thus have here another inde- pendent evidence of a photodesorption process. The results for Ti02 and WO3 are corroborated by measurements of changes of the conductivity under illumin- ation.25 The fluorescence of ZnO equally experiences changes attributed to photo- desorption of NO and quinone.15 PHOTOSORPTION OF OXYGEN ON SILICA GEL One of the authors 5 has found that silica gel, or aerogel thoroughly degassed (5 h a . 600°C in vacuo) brought in contact with dry oxygen at a pressure of 10-2 mm Hg, exhibit a rapid irreversible photosorption on illumination by u.-v. * The decrease is exponential with a thermal deactivation energy of 0-7 eV. f For ZnO the same unselective absorption in the infra-red has been independently found in the work.24 D34 ACTION OF LIaHT ON ADSORPTION light in the range of wavelengths shorter than 250mp.A typical barogram is reproduced in fig. 5. No such effect is observed with nitrogen, hydrogen, CO and 4 liqht k- E 0 5 10 I 5 2 0 min FIG. 5.-Sorption of oxygen by silica gel : (a) under illumination by short u.-v. (Fe spark, wavelengths shorter than 250mp active); (b) after preliminary illumination. C02. The capacity of rapid sorption of oxygen is retained by the gel even after a preliminary illumination in vacuo (cf. fig. 5). In the presence of water vapour (5 x 10-2 mm Hg), the photosorption of 0 2 disappears, but after the removal of adsorbed water by evacuation at 20°C, the photosorption increases in magnitude somewhat.Although the gel was subjected, before de- gassing, to a heat treatment in air at 600°C to burn out possible organic contaminations, to disprove the possibility of a photo-oxidation of traces of organic compounds the following experiment has been performed. Acetone vapour (20mmHg) was adsorbed on the gel, then pumped off to a pressure of 10-2 to lO-3mmHg and the gel illuminated in the presence of oxygen. It was found that the adsorption of acetone entirely suppressed the oxygen photosorption. Evidently H20 and acetone molecules are blocking the surface centres of the gel which are responsible for the photosorption. It proves, in addition, that no photo-oxidation of acetone is taking place at the surface. The phenomen cannot be explained by a thermal action of the u.-v.light since : (a) heating of the gel for a long time in oxygen does not suppress the photo- sorption, (b) on samples exhibiting a marked photosorption, a comparable thermal sorption in the dark begins at 400-450°C only, (c) the photosorption rate'is pro- portional to the light intensity. On crushed and degassed crystalline quartz, photosorption of oxygen is also observed, although, however, to a much lesser The most plausible explanation of the photosorption of oxygen on silica gel is that the u.-v. light splits off OH radicals of the silanol Si-OH groups from the surface. These latter are easily observed and their behaviour studied with the help of infra-red spectra.26-29 The free valencies freed at the Si atoms can function as adsorption centres for 0 2 molecules, leading to the formation of peroxide radicals of the kind Si--O-O*.It has been shown recently that silica gel heated under high vacuum acquires the property of strong oxygen sorption.30 The OH radicals split off by the u.-v. light give subsequently H202 on the surface. This follows from the fact that, after the irreversible photosorption, a gas can be desorbed thermally at 100°C which partially condenses at - 180°C. A slow pressure increase observed during long periods of u.-v. illumination can be ascribed to the photolysis of H202. extent. pq 1 Terenin, Problems of Kinetics mtd Catalysis (ed. Acad. Sci. U.S.S.R.), 1955, 8, 17 ; J. Physic. Chem. (U.S.S.R.), 1935, 6, 189 ; 1940, 14, 1362 ; Heterogeneous Catalysis in the Chemical Industry (Moscow, 1956), p.197. 2 Terenin, J. 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U.S.S.R., 1955, 101, 645 ; Problems of Kinetics and Catalysis (ed. Acad. Sc. U.S.S.R.), 1955, 8, 53 ; J. Physique Rad., 1956, 17, 650. 15 Tagantzev and Terenin, Doklady Acad. Sci. U.S.S.R., 1957, 112, 251 ; Optika Spektroskopia, 1957, 2, 356 ; J. Physique Rad., 1956, 17, 650. 16 Medved, J. Chem. Physics, 1958, 28, 870. 17 Fujita and Kwan, Bull. Chem. SOC. Japan, 1958, 31, 379. 18 Kobajashi and Kawaji, J. Physic. SOC. Japan, 1955, 10, 270 ; J. Chem. Physics, 1956, 19 Volkenstein and Kogan, J. Chim. physique, 1958,55,483. 20 Bevan and Anderson, Faraday SOC. Discussions, 1950,8,246. 21 Hauffe, Angew. Chem., 1955,67, 189. 22 Wilessov and Terenin, Naturwiss., 1959, 46, 167 ; Doklady Acad. Sci. U.S.S.R., 23 Filimonov, Optika Spektroskopia, 1958, 5, 709. 24 Milosslavsky and Kovalenko, Optika Spektroskopia, 1958,5, 61 3. 25 Korssunovsky, Problems of Kinetics and Catalysis (ed. Acad. Sci. U.S.S.R.), 1960, 26 Yarosslavsky and Terenin, Doklady Acad. Sci. 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