3382
J. Phys. Chem. 1992,96, 3382-3388
Photoluminescence from Zinc Oxide Powder To Probe Adsorption and Reaction of O,, CO, H,, HCOOH, and CH30H H.Idriss and M. A. Barteaul Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 (Received: July 8, 1991)
Two emissions were observed at room temperature from zinc oxide powder excited with photons of higher energy than its band gap (3.2 eV). The sharp UV emission at 383 nm corresponded to band gap emission, and a broad visible (green) emission at ca. 500 nm corresponded to sub-band gap emission. Adsorption of 02,H2, CO, methanol, and carboxylic acids (formic acid and acetic acid) changed the intensities of both emissions. At room temperature, 02,H2, and CO exposure decreased the emission intensities; after evacuation at 10” Torr the effect of O2adsorption was only partially reversible, while those of H2 and CO were virtually completely reversible. In contrast, adsorption of methanol and formic acid (and acetic acid) caused an irreversible increase of both emissions. Surface formate species were formed by dissociative adsorption of formic acid as well as by oxidation of methanol, as shown by temperature-programmed desorption and FT-IR. The decomposition of formate intermediates further increased the intensity of both emissions. Moreover, the increase of the visible emission intensity was more pronounced in the case of the decomposition of formates produced by methanol oxidation than of those resulting from formic acid dissociation. The increase of the visible emission intensity upon formate decomposition may be related to the creation of oxygen vacancies in the course of this reaction. These vacancies act as acceptor centers for deexcited electrons from the conduction band. This study demonstrates that photoluminescence can be used to probe adsorption, both reversible and irreversible, and reaction phenomena on the surface of zinc oxide.
Introduction The catalytic properties of zinc oxide have received a great deal of attention; its applications include olefin CO oxidation?^^ and its participation as a basic component of catalysts for syngas conversion to methan01,~~~ higher branched alcohol^,^ and higher linear alcohol^.*^^ Progress in surface spectroscopy has shed light on important characteristics of the reactivity of zinc oxide, including the requirement of different sites for adsorption of H2,IJ0CO,I1-l3and CO2;I4-I6the nature of surface acid-base and the structure, stability, and decomposition pathways of surface species resulting from adsorption of alcohols,1s-20al-
( I ) Dent, A . L.; Kokes, R. J. J . Phys. Chem. 1969, 73, 3773. (2) Kokes, R. J. Ace. Chem. Res. 1973, 6, 226 and references therein. (3) Stone, F. S. Adu. C u r d 1962, 13, I . (4) Kobayashi, M.; Kamo, T . J . Chem. Soc., Furud. Truns. I 1988, 84, 2099. (5) Klier, K. Adu. Curul. 1982, 31, 243. (6) Rogerson, P. L. The IC1 low-pressure methanol process. In Hundbook of Synfuels Technology; McGraw-Hill: New York, 1980; Chapter 2, p 2. Kochlcefl, K. In Preparation (7) Hofstadt, C. E.; Schneider, M.; Bock, 0.; of CafulysfsIII; Elsevier: Amsterdam, The Netherlands, 1983; p 709. (8) Courty, P.; Durand, D.; Freund, E.; Sugier, A . J . Mol. Curd. 1982, 17, 241. (9) Cao, R.; Pan, W. X.; Griffin, G. L. Langmuir 1988, 4, 1108. (10) Kokes, R. J.; Dent, A . L. Adu. Curul. 1972, 22, 1. (1 I ) Bowker, M.; Houghton, H.; Waugh, K. C . J . Chem. SOC.,Furuday Truns. I 1981, 77, 3023. (12) Boccuzzi. F.; Garrone, E.; Zecchina, A.; Bossi, A,; Camia, M. J . C a r d . 1978, 51, 160. (13) Bolis, V.; Fubini, B.; Giamello, E.; Reller, A . J . Chem. Sm., Faruduy Trans. I 1989, 85 (4), 855. (14) Saussey, J.; Lavalley, J. C.; Bovet, C . J . Chem. Soc., Furuduy Truns I 1982, 78, 1457. ( 1 5 ) Knozinger, H. Adu. Coral. 1976, 25, 184. (16) Hindermann, J . P.; Idriss, H.; Kiennemann, A . Muter. Chem. Phys. 1988, 18, 5 13 and references therein. (17) Spitz, R. N . ; Barton, J . E.; Barteau, M . A,; Staley, R. H.; Sleight, A . W. J . Phys. Chem. 1986, 90,4067. (18) Ueno, A.; Onishi, T.; Tamaru, K. J . Chem. Soc., Furuduy Truns. I 1971, I . 3585. (19) Cheng, W. H . ; Akhter, S.; Kung, H. H. J , Curul. 1983, 82, 341. (20) Vohs, J. M.; Barteau, M. A . Sur/. Sci. 1989, 221, 590.
d e h y d e ~ , ’carboxylic ~,~~ acids,20*21 and alkyne^.^^.^^ Zinc oxide is an n-type semiconductor, with a direct band gap of 3.2 eV at 294 K,making it potentially useful for optoelectronic devices that emit at short wavelength^.^^ For example, ZnObased, electron-beam-pumped semiconductor lasers have been developed that emit in the ultraviolet region.% Films of zinc oxide oriented along the c axis are useful for surface acoustic wave and acousto-optic devices.27 When ZnO is excited by radiation of higher energy than its band gap, it emits photons in two regions of the spectrum: in the ultraviolet region at ca. 370 nm and in the visible region at ca. 500 nm, as shown as early as 1948 by Nicoll.2* The emission a t ca. 370 nm is considered to be due to an electron-transfer mechanism; Le., the absorption of a photon corresponding to the fundamental absorption of the oxide leads to the formation of an electron/hole pair (exciton), and light is emitted (photoluminescence) by radiative decay29 Zn2+-02-.
hu‘= 3.2 cV
Minami et al.27found a correlation between the UV emission intensity and the crystallite size of zinc oxide. The authors27 proposed that photolumineqcence could be applied as a method to test the quality of zinc oxide thin films. (Higher quality is associated with stronger UV emission intensity.) On the other hand, the origin of the band at 500 nm (referred to as deep, green, visible or subband gap emission) is still not clear. van Craeynest et concluded that this visible emission is generated by the recombination of electrons from the conduction band with trapped holes. Anpo et al.30argued that the trapping of photoformed electrons is done by oxygen ion vacancies, since it is expected that (21) Vohs, J. M.; Barteau, M . A . Surf. Sei. 1986, 176, 91. (22) Vohs, J. M.; Barteau, M . A . SurJ Sci. 1988, 201, 481. (23) Vohs, J . M.; Barteau, M . A . J . Phys. Chem. 1987, 91, 4766. (24) Vohs, J. M.; Barteau, M . A. J . Phys. Chem. 1990, 94, 882. (25) Bethke, S.; Pan, H.; Wessels, B. Appl. Phys. Leu. 1988.52 (2). 138. (26) Nikitenko, V. A,; Tereschenko, A . 1.; Kuzmina, I . P.; Lobachev, A . N . Upr. Specr. (USSR)1981.50, 331. (27) Minami, T.; Nanto, H.; Takata, S. Thin Solid Films 1983, 109, 379 and references therein. (28) Nicoll, F. H . J . Upr. SOC.A m . 1948, 38, 817. (29) van Craeynest, F.; Maenhout-van der Vorst, W.; Dekeyser, W. Phys. Sforus Solidi 1965, 8, 841. (30) Anpo, M.; Kubokawa, Y . J . Phys. Chem. 1984, 88, 5556.
0022-3654 I92 12096-3382%03.00/0 0 1992 American Chemical Societv
Photoluminescence from Zinc Oxide Powder the anion vacancies should lie 2.7 eV below the conduction band. Calculations using a semicontinuum model showed that the energy level of the excited state of the oxygen vacancies is about 2.49 eV:’ in good agreement with the visible emission wavelength (2.49 eV corresponds to a A- of ca. 500 nm). Takata et al.32attributed the visible emission to a transition within self-activated (SA) centers (some associated centers are formed by doubly ionized zinc vacancy defects, V Z n ,and ionized interstitial zinc donors, ZrPin,at one and/or two nearest-neighbor interstitial sites); thus, the emission is ascribed to the transition between an electron in an ionized interstitial zinc donor and a hole trapped a t a doubly ionized zinc vacancy.33 Other a ~ t h o r s ~claim ~ - ~that ~ s the ~ ~deep emission is due to the presence of impurities such as Cu2+. A third band is observed by electroluminescence. (Zinc oxide is a common electrode material in semiconductor electrochemistry and ph~toelectrochemistry.~~*~’) Takata et observed a band at ca. 950 nm for the electroluminescence of zinc oxide singlecrystal metal-semiconductor diodes; the transition mechanism is not yet known. Addition of oxygen to zinc oxide decreases its photoluminescence intensity. Anpo et aL30 and Oster et attributed this phenomenon to trapping of electrons by 02,resulting in the formation of 02-anion radicals. Similar results have also been observed on CdS and ZnS39under UV irradiation. Zakharenko et alea studied the photocatalytic oxidation of CO on zinc oxide (pure and doped with Li and A1 oxides) and suggested that the only form of oxygen active in the oxidation of CO is a radical ion of the form 0-. These authors also showed that a decrease in the negative potential of the surface of zinc oxide leads to a reduction in the quantum yield for photocatalytic oxidation of CO. The effects of alcohols, carboxylic acids, amines, and olefins on the luminescence from other 11-VI semiconductors, e.g., CdS and CdSe, have been investigated by Ellii et aL4Id3 They observed in all cases that the effect on the luminescence was reversible. In general, molecules possessing a lone pair of nonbonding electrons, such as alcohols and amines, enhance luminescence, while those acting as electron acceptors, such as carboxylic acids, decrease the intensity of the band gap luminescence from CdS and CdSe. We report here an investigation of the effects of adsorption and decomposition of C, oxygenates as well as of 02,CO, and H2 on the photoluminescence behavior of zinc oxide powder. Unlike CdS and CdSe, adsorption of these molecules on ZnO is generally irreversible. Thus, this study is more complicated but may also probe a greater variety of surface processes, since surface modification occurs via surface redox reactions that produce Zn desorptionI8 (leaving VZn,a zinc vacancy) and water and C 0 2 desorption (leaving Vo, an oxygen vacancy).
Experimental Section The zinc oxide powder (Fischer Scientific Co.) had a surface area of 4 m2/g (B.E.T.); elemental analysis (Schwarzkopf Microanalytical Laboratory) showed that levels of Na, K, and C1 impurities were < 5 ,
