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A comparative study of partial reduction of ceria via laser ablation in air and soft chemical route
Applied Surface Science 109r110 Ž1997. 249–252
A comparative study of partial reduction of ceria via laser ablation in air and soft chemical route L. Filotti a , A. Bensalem a , F. Bozon-Verduraz a , G.A. Shafeev a
b,)
, V.V. Voronov
b
Laboratoire de Chimie des Materiaux DiÕises ´ et Catalyse, UniÕersite´ Paris-7 (Denis Diderot), 75251 Paris Cedex 05, France b General Physics Institute of Russian Academy of Sciences, 38, VaÕiloÕ str., 117942 Moscow, Russia Received 4 June 1996; accepted 15 August 1996
Abstract Experimental results are presented on the reduction of CeO 2 Žbandgap of 3.1 eV. either via irradiation in air with UV excimer lasers ŽKrF, XeCl. or via treatment with hydrazine at 1008C. Both kinds of treatment result in similar modifications of the diffuse reflectance spectra ŽDRS. of samples: the appearance of an absorption band in the visible which is assigned to oxygen vacancies and to charge transfer transitions between Ce 3q and Ce 4q ions. The influence of laser fluence on the DRS of ceria has been studied below and above the ablation threshold of ; 1 Jrcm2. Electron spin resonance ŽESR. spectra of ceria samples obtained from both kinds of treatment confirm the presence of paramagnetic species. The reduced ceria promotes the electroless deposition of Pd from a plating solution containing hydrazine. This finding has been applied to the selective metallization of composite oxides, that is to the selective deposition of Pd on ceria particle size of 2–8 nm supported on insulating carriers, such as alumina or silica.
1. Introduction Oxygen atoms in CeO 2 units are very mobile and easily leave the ceria lattice, giving rise to a large variety of non-stoichiometric oxides with two limiting cases being CeO 2 and Ce 2 O 3 . These nonstoichiometric oxides can be produced by chemical reduction at temperatures much higher than ambient, e.g. in hydrogen at temperaturesG 620 K w1x. They have also been obtained through laser irradiation in air w2x. In the present paper, the laser-induced partial reduction of ceria produced by nanosecond laser radiation is compared with its reduction via low-temperature reaction with hydrazine, a soft chemical route. )
Corresponding author. Fax: q7-95-1350376.
2. Experimental Ceria powder with high surface area Ž120 grm2 ., average particle diameter of 20 nm was used as received from Rhone-Poulenc. The same powder was used for preparing sintered ceria pellets: the powder pressed at 8 kbar was then annealed in air at 12008C for several hours. The sintering process results in narrow X-ray diffraction peaks indicating an increase in the average size of the ceria particles in the pellets compared to the virgin powder Žfrom 20 nm to several micrometers.. The irradiation of ceria samples was carried out in air using either a XeCl Ž308 nm. or a KrF Ž248 nm. excimer lasers, or a N2 laser Ž337 nm.. Diffuse reflectance spectra ŽDRS. of ceria were recorded using a Beckman spectrometer equipped
0169-4332r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 6 . 0 0 6 6 5 - 4
250
L. Filotti et al.r Applied Surface Science 109 r 110 (1997) 249–252
with the integrating sphere coated with BaSO4 , BaSO4 being also used as a reference sample if not specified otherwise. To avoid the oxidation of prereduced ceria upon exposure in air, the DRS spectra of the oxide powder moistened with hydrazine were taken in a special cell allowing atmosphere control Žhydrazine has no absorption bands in the visible where the main modifications of the DRS of ceria are observed.. Electron spin resonance ŽESR. spectra of partially reduced ceria were registered at 77 K or at 4 K on a Bruker ESP 300E spectrometer operating at 9.2 MHz. Prior to this, the ceria reacted with hydrazine was outgassed at 10y4 Torr for 48 h. The samples of laser-irradiated ceria were analyzed using an X-ray diffractometer ŽCu K a radiation.. The X-radiation was monochromatized with a focusing monochromator mounted on the diffracted beam. The diffractograms were taken in the 2Q range from 208 to 1508. Electroless deposition of Pd on the treated ceria was carried out with the aid of a standard solution for Pd electroless plating w3x using hydrazine as a reducing agent.
3. Results and discussion Untreated ceria shows a sharp increase of absorption near 400 nm, which corresponds to the interband electronic transition. Both reducing treatments of ceria lead to the modification of its DR spectrum in the visible ŽFig. 1.: a new band appears in the 400–500 nm range, that is accompanied by the coloration of the samples, either as a powder or a pellet. On the other hand, the absorption edge near 400 nm is not affected Žnot shown., which suggests that the modification of ceria takes place in a thin surface layer of the sample. This interpretation is supported by the absence of observable modifications of X-rays diffractograms of the ceria powder after reaction with hydrazine. Moreover, the stability of hydrazine- and laser-reduced samples is different. Indeed, the DRS of hydrazine-treated ceria is stable only in the presence of hydrazine and the spectrum of pure ceria is restored upon drying the powder in air. On the contrary, laser-induced changes of the DRS of ceria are relatively stable though the inten-
Fig. 1. Diffuse reflectance spectra of ceria samples submitted to various treatments: irradiation in air with an XeCl laser beam at laser fluence of 2 Jrcm2 Žspectrum taken 30 min after irradiation. Ž1.; after reaction with hydrazine at 1008C for 30 min Ž2.; after heating in vacuum at 8008C for 15 h Ž3.. All spectra were recorded with untreated ceria as a reference.
sity of irradiation plays a role: below the ablation threshold of about 1 Jrcm2 , the induced absorption band in the visible decays at ambient temperature within a few days while above the ablation threshold the coloration persists for many months. For comparison, Fig. 1 shows also the DR spectrum of ceria after heating in vacuum at elevated temperature Ž8008C.; in this case the band maximum is shifted to about 650 nm. One should note that the laser irradiation of the as-received ceria powder is impossible due to its sputtering under the laser pulse at laser fluence exceeding ; 0.5 Jrcm2 . In turn, the treatment of the sintered pellet of ceria in hydrazine at 1008C does not lead to observable changes of its DRS. The ‘mild’ reduction of ceria can be observed because of the high specific surface of the powdered sample, as will be shown below. Hence, the two methods of ceria reduction are complementary. Fig. 2 shows the ESR spectra of ceria treated in N2 H 4 Ža. and by laser radiation Žb.. The virgin ceria powder also shows a small signal corresponding to g f 2, but on the scale of Fig. 2 this signal is negligible. Hydrazine-treated ceria ŽFig. 2a. shows signals with g s 1.979, 1.998 and 2.013. Lasertreated ceria shows signals with g H s 2.014 and g 5 s 2.033. Neither spectrum contains the components corresponding to Ce 3q ions Ž g f 0.8, 2.2 and 3.7 w4x. which could be due to the short relaxation
L. Filotti et al.r Applied Surface Science 109 r 110 (1997) 249–252
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time of that species at 77 K. However, no signal from Ce 3q was also found at 4 K either. The diffractogram of the sintered ceria contains 11 peaks whose position correspond to those of CeO 2 ŽRef.: JCPDS card 34-394.. The peaks become broader upon laser irradiation in air, which indicates a decrease in size of the scattering domains and, probably, the appearance of microdeformation of ceria particles during solidification. Calculations show, however, that the contribution of the microdeformations to the peak broadening is only of the order of 0.05%. Hence, the broadening comes essentially from the decrease in average size of the ceria particles in the laser-treated ceria pellets; this mean particle size may be estimated to about 100 nm against 1 m m in the virgin sample.
Fig. 3. XRD of ceria pellets. Modification of the Ž531. peak: virgin sample Ža., irradiated in air with an excimer KrF laser at laser fluence of 1.2 Jrcm2 Žb. and at laser fluence of 6 Jrcm2 Žc.. The less intense peak is the Ž600..
Fig. 2. ESR spectra of ceria reduced with hydrazine Ža. and ablated by N2 laser Žb.. Dotted line in Žb. ESR signal of ceria reduced in H 2 at 5008C for 2 h.
Fig. 3 shows a portion of the diffractogram of ceria pellet before and after the laser treatment. Computer-assisted analysis of the diffractograms show that the peaks of the laser-treated ceria are slightly shifted to smaller diffraction angles which indicates an increase in the lattice parameter. This observation is in a good agreement with the increase of the lattice parameter of ceria reduced in H 2 w1x. On the contrary, no changes are observed in the diffractogram of ceria treated with hydrazine at 1008C within the accuracy of measurements. The ceria pellets irradiated with a XeCl laser beam at a laser fluence above the ablation threshold show catalytic activity towards Pd reduction from the electroless plating solution w2x. The irradiated parts of the pellet are covered by metallic Pd after dipping into the solution. The metal deposition is restricted to the areas where the laser-induced modifications of DRS are observed. The ability to reduce metals from electroless solutions strongly correlates with DRS changes: upon heating in air the absorption band in the visible ŽFig. 1. disappears along with the catalytic activity of the pellet for metal deposition. The observed modifications of DRS in ceria treated with N2 H 4 and by UV laser radiation can be assigned either to charge transfer transitions between Ce 3q and Ce 4q ions or to the presence of color centers Žoxygen vacancies. in the ceria. However,
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the ESR data imply the formation of color centers in ceria as the result of both kinds of treatment rather than charge transfer because of the absence of Ce 3q ions in the treated ceria. The soft reduction of ceria with hydrazine Žat moderate temperature. is especially remarkable since the reduction of ceria with hydrogen starts only at 640 K w1x. Laser-assisted reduction is due to the evolution of the lattice oxygen at elevated temperatures. However, during fast cooling, all oxygen atoms do not recombine with cerium ions, so that the CerO ratio corresponding to the high temperature attained during the laser pulse is ‘frozen’. In turn, this may lead to the increase of the lattice parameter detected in the diffractogram due to the change of the ceria particle environment. The cooling rate is much lower for ceria particles in powder or, on ceria nanoparticles supported on insulating oxides w5x. The partial reduction of ceria under laser ablation observed in the present study is similar to that reported earlier for TiO 2 where the formation of Ti 3q ions has been observed w6x and to laser-assisted formation of color centers in laser-ablated alumina w7x. The ability of partially reduced ceria to reduce metal ions from an electroless plating solution is due to the introduction of filled states in the band gap, which facilitates the electron transfer from the reducing agent to the metal ion. The laser activation of nanometer-sized ceria particles supported on SiO 2 has been successfully applied for the selective electroless deposition of Pd
onto ceria w8x. The metal is deposited preferentially on ceria and not on the supporting silica. The selective electroless deposition of Pt on nanosized ceria supported on alumina upon its activation via reaction with N2 H 4 is now under study. Acknowledgements This work was supported in part by INTAS, contract No. 93-1589. We are grateful to Dr. G. Bugli for X-ray measurements and to Dr. S.M. Pimenov for his assistance in laser treatment.
References w1x V. Perrichon, A. Laachir, G. Bergeret, R. Frety, ´ L. Tournayan and O. Touret, J. Chem. Faraday Trans. 90 Ž1994. 773. w2x A. Bensalem, G.A. Shafeev and F. Bozon-Verduraz, Catal. Lett. 18 Ž1993. 165. w3x G. Gutzeit, E.B. Saubestre and D.R. Turner, in: Electroplating Engineering Handbook, Ed. A.K. Graham, 3rd ed. ŽVan Nostrand Reinhold, New York, 1971.. w4x K. Okada, Y. Kaizu, H. Kobayashi, K. Tanaka and F. Marumo, Mol. Phys. 54 Ž1985. 1293. w5x A. Bensalem, F. Bozon-Verduraz, M. Delamar and G. Bugli, Appl. Catal. A 121 Ž1994. 81. w6x T. Le Mercier, J.-M. Mariot, P. Parent, M.-F. Fontaine, C.F. Hague and M. Quarton, Appl. Surf. Sci. 86 Ž1995. 382. w7x G.A. Shafeev, Adv. Mater. Opt. Electron. 2 Ž1993. 183. w8x A. Bensalem, G.A. Shafeev and F. Bozon-Verduraz, Catal. Lett. 37 Ž1996. 63.
A comparative study of partial reduction of ceria via laser ablation in air and soft chemical route L. Filotti a , A. Bensalem a , F. Bozon-Verduraz a , G.A. Shafeev a
b,)
, V.V. Voronov
b
Laboratoire de Chimie des Materiaux DiÕises ´ et Catalyse, UniÕersite´ Paris-7 (Denis Diderot), 75251 Paris Cedex 05, France b General Physics Institute of Russian Academy of Sciences, 38, VaÕiloÕ str., 117942 Moscow, Russia Received 4 June 1996; accepted 15 August 1996
Abstract Experimental results are presented on the reduction of CeO 2 Žbandgap of 3.1 eV. either via irradiation in air with UV excimer lasers ŽKrF, XeCl. or via treatment with hydrazine at 1008C. Both kinds of treatment result in similar modifications of the diffuse reflectance spectra ŽDRS. of samples: the appearance of an absorption band in the visible which is assigned to oxygen vacancies and to charge transfer transitions between Ce 3q and Ce 4q ions. The influence of laser fluence on the DRS of ceria has been studied below and above the ablation threshold of ; 1 Jrcm2. Electron spin resonance ŽESR. spectra of ceria samples obtained from both kinds of treatment confirm the presence of paramagnetic species. The reduced ceria promotes the electroless deposition of Pd from a plating solution containing hydrazine. This finding has been applied to the selective metallization of composite oxides, that is to the selective deposition of Pd on ceria particle size of 2–8 nm supported on insulating carriers, such as alumina or silica.
1. Introduction Oxygen atoms in CeO 2 units are very mobile and easily leave the ceria lattice, giving rise to a large variety of non-stoichiometric oxides with two limiting cases being CeO 2 and Ce 2 O 3 . These nonstoichiometric oxides can be produced by chemical reduction at temperatures much higher than ambient, e.g. in hydrogen at temperaturesG 620 K w1x. They have also been obtained through laser irradiation in air w2x. In the present paper, the laser-induced partial reduction of ceria produced by nanosecond laser radiation is compared with its reduction via low-temperature reaction with hydrazine, a soft chemical route. )
Corresponding author. Fax: q7-95-1350376.
2. Experimental Ceria powder with high surface area Ž120 grm2 ., average particle diameter of 20 nm was used as received from Rhone-Poulenc. The same powder was used for preparing sintered ceria pellets: the powder pressed at 8 kbar was then annealed in air at 12008C for several hours. The sintering process results in narrow X-ray diffraction peaks indicating an increase in the average size of the ceria particles in the pellets compared to the virgin powder Žfrom 20 nm to several micrometers.. The irradiation of ceria samples was carried out in air using either a XeCl Ž308 nm. or a KrF Ž248 nm. excimer lasers, or a N2 laser Ž337 nm.. Diffuse reflectance spectra ŽDRS. of ceria were recorded using a Beckman spectrometer equipped
0169-4332r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 6 . 0 0 6 6 5 - 4
250
L. Filotti et al.r Applied Surface Science 109 r 110 (1997) 249–252
with the integrating sphere coated with BaSO4 , BaSO4 being also used as a reference sample if not specified otherwise. To avoid the oxidation of prereduced ceria upon exposure in air, the DRS spectra of the oxide powder moistened with hydrazine were taken in a special cell allowing atmosphere control Žhydrazine has no absorption bands in the visible where the main modifications of the DRS of ceria are observed.. Electron spin resonance ŽESR. spectra of partially reduced ceria were registered at 77 K or at 4 K on a Bruker ESP 300E spectrometer operating at 9.2 MHz. Prior to this, the ceria reacted with hydrazine was outgassed at 10y4 Torr for 48 h. The samples of laser-irradiated ceria were analyzed using an X-ray diffractometer ŽCu K a radiation.. The X-radiation was monochromatized with a focusing monochromator mounted on the diffracted beam. The diffractograms were taken in the 2Q range from 208 to 1508. Electroless deposition of Pd on the treated ceria was carried out with the aid of a standard solution for Pd electroless plating w3x using hydrazine as a reducing agent.
3. Results and discussion Untreated ceria shows a sharp increase of absorption near 400 nm, which corresponds to the interband electronic transition. Both reducing treatments of ceria lead to the modification of its DR spectrum in the visible ŽFig. 1.: a new band appears in the 400–500 nm range, that is accompanied by the coloration of the samples, either as a powder or a pellet. On the other hand, the absorption edge near 400 nm is not affected Žnot shown., which suggests that the modification of ceria takes place in a thin surface layer of the sample. This interpretation is supported by the absence of observable modifications of X-rays diffractograms of the ceria powder after reaction with hydrazine. Moreover, the stability of hydrazine- and laser-reduced samples is different. Indeed, the DRS of hydrazine-treated ceria is stable only in the presence of hydrazine and the spectrum of pure ceria is restored upon drying the powder in air. On the contrary, laser-induced changes of the DRS of ceria are relatively stable though the inten-
Fig. 1. Diffuse reflectance spectra of ceria samples submitted to various treatments: irradiation in air with an XeCl laser beam at laser fluence of 2 Jrcm2 Žspectrum taken 30 min after irradiation. Ž1.; after reaction with hydrazine at 1008C for 30 min Ž2.; after heating in vacuum at 8008C for 15 h Ž3.. All spectra were recorded with untreated ceria as a reference.
sity of irradiation plays a role: below the ablation threshold of about 1 Jrcm2 , the induced absorption band in the visible decays at ambient temperature within a few days while above the ablation threshold the coloration persists for many months. For comparison, Fig. 1 shows also the DR spectrum of ceria after heating in vacuum at elevated temperature Ž8008C.; in this case the band maximum is shifted to about 650 nm. One should note that the laser irradiation of the as-received ceria powder is impossible due to its sputtering under the laser pulse at laser fluence exceeding ; 0.5 Jrcm2 . In turn, the treatment of the sintered pellet of ceria in hydrazine at 1008C does not lead to observable changes of its DRS. The ‘mild’ reduction of ceria can be observed because of the high specific surface of the powdered sample, as will be shown below. Hence, the two methods of ceria reduction are complementary. Fig. 2 shows the ESR spectra of ceria treated in N2 H 4 Ža. and by laser radiation Žb.. The virgin ceria powder also shows a small signal corresponding to g f 2, but on the scale of Fig. 2 this signal is negligible. Hydrazine-treated ceria ŽFig. 2a. shows signals with g s 1.979, 1.998 and 2.013. Lasertreated ceria shows signals with g H s 2.014 and g 5 s 2.033. Neither spectrum contains the components corresponding to Ce 3q ions Ž g f 0.8, 2.2 and 3.7 w4x. which could be due to the short relaxation
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time of that species at 77 K. However, no signal from Ce 3q was also found at 4 K either. The diffractogram of the sintered ceria contains 11 peaks whose position correspond to those of CeO 2 ŽRef.: JCPDS card 34-394.. The peaks become broader upon laser irradiation in air, which indicates a decrease in size of the scattering domains and, probably, the appearance of microdeformation of ceria particles during solidification. Calculations show, however, that the contribution of the microdeformations to the peak broadening is only of the order of 0.05%. Hence, the broadening comes essentially from the decrease in average size of the ceria particles in the laser-treated ceria pellets; this mean particle size may be estimated to about 100 nm against 1 m m in the virgin sample.
Fig. 3. XRD of ceria pellets. Modification of the Ž531. peak: virgin sample Ža., irradiated in air with an excimer KrF laser at laser fluence of 1.2 Jrcm2 Žb. and at laser fluence of 6 Jrcm2 Žc.. The less intense peak is the Ž600..
Fig. 2. ESR spectra of ceria reduced with hydrazine Ža. and ablated by N2 laser Žb.. Dotted line in Žb. ESR signal of ceria reduced in H 2 at 5008C for 2 h.
Fig. 3 shows a portion of the diffractogram of ceria pellet before and after the laser treatment. Computer-assisted analysis of the diffractograms show that the peaks of the laser-treated ceria are slightly shifted to smaller diffraction angles which indicates an increase in the lattice parameter. This observation is in a good agreement with the increase of the lattice parameter of ceria reduced in H 2 w1x. On the contrary, no changes are observed in the diffractogram of ceria treated with hydrazine at 1008C within the accuracy of measurements. The ceria pellets irradiated with a XeCl laser beam at a laser fluence above the ablation threshold show catalytic activity towards Pd reduction from the electroless plating solution w2x. The irradiated parts of the pellet are covered by metallic Pd after dipping into the solution. The metal deposition is restricted to the areas where the laser-induced modifications of DRS are observed. The ability to reduce metals from electroless solutions strongly correlates with DRS changes: upon heating in air the absorption band in the visible ŽFig. 1. disappears along with the catalytic activity of the pellet for metal deposition. The observed modifications of DRS in ceria treated with N2 H 4 and by UV laser radiation can be assigned either to charge transfer transitions between Ce 3q and Ce 4q ions or to the presence of color centers Žoxygen vacancies. in the ceria. However,
252
L. Filotti et al.r Applied Surface Science 109 r 110 (1997) 249–252
the ESR data imply the formation of color centers in ceria as the result of both kinds of treatment rather than charge transfer because of the absence of Ce 3q ions in the treated ceria. The soft reduction of ceria with hydrazine Žat moderate temperature. is especially remarkable since the reduction of ceria with hydrogen starts only at 640 K w1x. Laser-assisted reduction is due to the evolution of the lattice oxygen at elevated temperatures. However, during fast cooling, all oxygen atoms do not recombine with cerium ions, so that the CerO ratio corresponding to the high temperature attained during the laser pulse is ‘frozen’. In turn, this may lead to the increase of the lattice parameter detected in the diffractogram due to the change of the ceria particle environment. The cooling rate is much lower for ceria particles in powder or, on ceria nanoparticles supported on insulating oxides w5x. The partial reduction of ceria under laser ablation observed in the present study is similar to that reported earlier for TiO 2 where the formation of Ti 3q ions has been observed w6x and to laser-assisted formation of color centers in laser-ablated alumina w7x. The ability of partially reduced ceria to reduce metal ions from an electroless plating solution is due to the introduction of filled states in the band gap, which facilitates the electron transfer from the reducing agent to the metal ion. The laser activation of nanometer-sized ceria particles supported on SiO 2 has been successfully applied for the selective electroless deposition of Pd
onto ceria w8x. The metal is deposited preferentially on ceria and not on the supporting silica. The selective electroless deposition of Pt on nanosized ceria supported on alumina upon its activation via reaction with N2 H 4 is now under study. Acknowledgements This work was supported in part by INTAS, contract No. 93-1589. We are grateful to Dr. G. Bugli for X-ray measurements and to Dr. S.M. Pimenov for his assistance in laser treatment.
References w1x V. Perrichon, A. Laachir, G. Bergeret, R. Frety, ´ L. Tournayan and O. Touret, J. Chem. Faraday Trans. 90 Ž1994. 773. w2x A. Bensalem, G.A. Shafeev and F. Bozon-Verduraz, Catal. Lett. 18 Ž1993. 165. w3x G. Gutzeit, E.B. Saubestre and D.R. Turner, in: Electroplating Engineering Handbook, Ed. A.K. Graham, 3rd ed. ŽVan Nostrand Reinhold, New York, 1971.. w4x K. Okada, Y. Kaizu, H. Kobayashi, K. Tanaka and F. Marumo, Mol. Phys. 54 Ž1985. 1293. w5x A. Bensalem, F. Bozon-Verduraz, M. Delamar and G. Bugli, Appl. Catal. A 121 Ž1994. 81. w6x T. Le Mercier, J.-M. Mariot, P. Parent, M.-F. Fontaine, C.F. Hague and M. Quarton, Appl. Surf. Sci. 86 Ž1995. 382. w7x G.A. Shafeev, Adv. Mater. Opt. Electron. 2 Ž1993. 183. w8x A. Bensalem, G.A. Shafeev and F. Bozon-Verduraz, Catal. Lett. 37 Ž1996. 63.