Mössbauer studies on laser evaporated iron atoms and their reactions with oxygen in argon matrices

Applied Radiation and Isotopes 52 (2000) 157±164

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MoÈssbauer studies on laser evaporated iron atoms and their reactions with oxygen in argon matrices Yasuhiro Yamada a,*, Hirochika Sumino b, Yukako Okamura a, Hideo Shimasaki a, Takeshi Tominaga b a

Department of Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjyuku-ku, Tokyo, Japan b Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan Received 13 May 1999; received in revised form 1 July 1999; accepted 16 July 1999

Abstract Laser-evaporated iron atoms were isolated in low-temperature Ar matrices and their chemical reactions with oxygen were investigated by means of MoÈssbauer spectroscopy. Reactions of iron atoms with oxygen produce FeO, Fe(O2), FeO3, (O2)FeO2 and OFeO isolated in the Ar matrices and their yields vary depending on the concentration of oxygen. Similarly, FeO and Fe(O2) were obtained by the reaction of iron atoms with N2O. Infrared spectroscopy and molecular orbital calculations were applied to support their assignments. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: MoÈssbauer spectroscopy; Matrix isolation; Iron oxide; Laser ablation

1. Introduction Laser ablation is a very useful and convenient technique to vaporize materials which are hardly vaporized using resistive heating. Furthermore, laser ablation provides new chemical interest as it produces highly energetic atoms, which react with a variety of molecules and form novel compounds unavailable under normal conditions. Laser-vaporized iron atoms were isolated in low-temperature Ar matrices and were studied by means of MoÈssbauer spectroscopy to provide useful information on the production of iron-based ®ne particles as well as on the basic mechanisms of catalytic reactions. While most of the matrix-isolation studies have been performed using IR spectroscopy (Abramowitz et al., 1977; Chang et al., 1981; Andrews

* Corresponding author. Fax: +81-3-3235-2214. E-mail address: [email protected] (Y. Yamada).

et al., 1996a,b; Chertihin et al., 1996), MoÈssbauer spectroscopy provides direct information regarding the electronic properties of iron atoms in unstable novel species (Yamada and Tominaga, 1998a,b). In addition, information on the yields of iron based compounds are provided from a comparison of absorption areas in the MoÈssbauer spectra. Though MoÈssbauer studies on iron atoms vaporized resistively isolated in low temperature matrices have been reported (Barrett and McNab, 1970; McNab et al., 1971; Micklitz and Barrett, 1972a,b; Micklitz and Litterst, 1974; Nagarathna et al., 1983), laser evaporated iron atoms are yet to be studied by means of MoÈssbauer spectroscopy. Oxidation of iron is of the most fundamentally and chemically interesting subject because it is related to corrosion of materials, catalytic reactions and atmospheric reactions. The simplest oxidation reaction of iron is the reaction of an iron atom with oxidizing gas. Thus, we investigated the reactions of iron atoms with oxygen and nitrous oxide molecules.

0969-8043/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 9 ) 0 0 1 2 8 - 1

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2. Experimental Pulsed laser lights (248 nm, 200 mJ/pulse, 20 ns) from a KrF excimer laser (Lambda Physik EMG101MSC) were focused onto a 57Fe iron block. Laser evaporated iron atoms were mixed with Ar gas introduced by a magnetic pulse valve and condensed on an aluminum plate which had been cooled down to 17 K by a closed cycle helium refrigerator. A typical amount of Fe atoms and Ar matrix gas in a pulse was 1±2  10ÿ9 and 3±4  10ÿ7 mol/pulse, respectively and 5000 pulses were accumulated to produce a matrix sample. In order to examine the reactions of Fe atoms with reactant gases (O2 and N2O), the reactant gases were diluted in Ar gas before their introduction. In order to investigate the e€ects of annealing, the temperature of a sample was raised up to 32 K and then cooled down to 17 K again. All the MoÈssbauer spectra were measured at 17 K in transmission geometry with 57 Co/Rh source. In order to measure infrared spectra of the species produced in the same way, the aluminum plate was replaced by CsI plate for a low temperature substrate. 3. Molecular orbital calculations Ab-initio molecular orbital calculations were performed using the Gaussian 94 program (HF/6-311G and B3LYP/6-311+G) in order to check the assignments of the products (FeO, Fe(O2), FeO3, (O2)FeO2 and OFeO) (Gaussian 94 (Revision D.1), 1995). Previously, we studied the correlation between calculated electronic properties (r(0) and EFG) and observed MoÈssbauer parameters (d and DEq) for various iron compounds isolated in low temperature matrices (Yamada and Tominaga, 1998a,b): the correlation between observed MoÈssbauer parameters and calculated electronic properties of iron carbonyl species and iron halides show the linear relation r(0)=ÿ3.39 d+15092.99 and EFG=0.906 DEq using HF/6-311G, where r(0) and EFG are in atomic units and d and DEq are in mm/s. We performed molecular orbital calculations for the new species obtained in this study and con®rmed the assignment. Though we optimized the geometries of the species using B3LYP/6-311+G, we recalculated UHF/6-311G//B3LYP/6-311+G in order to compare the MoÈssbauer parameters and calculated electronic properties using correlations obtained in our previous works; the correlations depend on the selection of the calculation methods. The studies based on the IR spectroscopy of similar species are reported in the literature (Andrews et al., 1996a,b). We also performed the frequency analysis using B3LYP/6-311+G to compare the IR spectra and to con®rm the assignment.

Fig. 1. MoÈssbauer spectra at 17 K of laser-ablated Fe atoms condensed in Ar matrices varying pulse intervals. Pulse intervals are indicated in the ®gure. Molar ratios are 57Fe/Ar=1/ 330 (a), 1/320(b) and 1/210(c).

4. Results and discussion 4.1. Iron atoms and clusters We investigated the chemical states and bonding of the laser-vaporized iron atoms isolated in pure Ar matrices under various conditions such as mixture ratio, interval of gas-pules introduction and laserintensity. Several species were found in the spectra and their relative yields changed depending on the conditions of the sample preparation. The most dominant condition having in¯uence on the yield of the species is the pulse interval, while the mixture ratio (Fe/Ar) was found to have the least in¯uence on the relative yield of the products under our experimental conditions. MoÈssbauer spectra of iron atoms and small particles produced by laser evaporation and trapped in pure Ar matrices (17 K) are demonstrated in Fig. 1: iron monomer Fe, dimer Fe2 and small clusters Fen(n = 3, 4) as well as larger metallic iron particles which display sextet peaks are observed. MoÈssbauer spectra of iron

Y. Yamada et al. / Applied Radiation and Isotopes 52 (2000) 157±164

Fig. 2. MoÈssbauer spectra (17 K) of

159

57

Fe/Ar=1/330 (Fig. 1a) obtained after annealing at 32 K for 21 h.

atoms vaporized using a resistive heater and trapped in a matrix were reported previously; the isomer shift d of iron atom was reported to be very small (d=ÿ0.75 mm/s) (Barrett and McNab, 1970; McNab et al., 1971; Micklitz and Barrett, 1972a,b; Micklitz and Litterst, 1974; Nagarathna et al., 1983). The Fe3 and Fe4 appear together as one broad doublet because of their similar MoÈssbauer parameters. Yields of monomer, dimer and small clusters were a€ected mostly by the time lapsed between gas-pulses of the introduction. In Fig. 1, the matrix isolated samples were prepared with 0.5, 2.0 and 5.0 s pulse intervals while the mixture ratio was kept almost unchanged (Fe/Ar=1/210±1/ 330). The amount of Fe atom is most abundant in the sample prepared with a long pulse interval (Fig. 1a), while the amount for the larger particles is most abundant in samples prepared with short pulse intervals (Fig. 1c). The observed relative yields may be achieved by the migration and aggregation of iron atoms in a matrix with deposition. If we assume that the abundance of a species in a matrix re¯ects that in the gas phase just after laser-vaporization, it should not depend on the pulse interval of gas introductions. When the interval between the pulses becomes shorter, the temperature of the matrix may rise and the species may migrate in the matrix, thus the yields of clusters or larger particles may increase. On annealing the sample (Fig. 1a) at 32 K for 21 h, the intensity of the clusters increase at the cost of the intensity of the iron monomer: iron monomers migrate and aggregate to form clusters in a matrix (Fig. 2). The sextet peak corresponding to large iron particles did not increase on annealing, thus it can be suggested that larger particles hardly move in the matrix even on

annealing and that aggregation occurs mostly with Fe atoms to form small clusters. 4.2. Reactions with O2 Oxygen gas was diluted in an Ar matrix gas at various concentrations before its introduction in order to investigate reactions of laser-evaporated iron atoms with oxygen molecules (Fig. 3) and the MoÈssbauer spectra of the species trapped in the low temperature matrices (17 K) were observed. Figure 3a shows the iron species condensed in a pure O2 matrix without Ar gas. The sextet peaks having the same MoÈssbauer parameters with the bulk iron metal appears at the lowest concentration (Fig. 3e) and are assigned to small iron particles produced by laser ablations or by aggregation of Fe atoms during the matrix formation. On increasing the O2 concentration in the matrix (Fig. 3a±d), the sextet peaks having a large magnetic splitting appears and are assigned to iron oxide particles produced by the reactions of oxygen. Besides these sextet peaks (iron metal or iron oxide), ®ve sets of doublet peaks appear in the MoÈssbauer spectra. At the lowest O2 concentration, species A is observed as well as an unreacted iron monomer (Fig. 3e). The yields of species A decreases and species B increases (Fig. 3c±e) on increasing O2 concentrations in the matrix. Species C±E appear at higher O2 concentrations (Fig. 3a and b). On annealing the sample, the stable species increases at the cost of the unstable species because of the migration in the matrices. When the sample 57Fe/ O2/Ar=1/3.4/360 (Fig. 3d) was annealed at 30 K for 48 h, the intensity of A increased at the cost of the intensity of B (Fig. 4). On annealing at 30 K for

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Fig. 4. MoÈssbauer spectra (17 K) of 57Fe/O2/Ar=1/3.4/360 (Fig. 3d) obtained after annealing at 32 K for 48 h.

DEq) obtained in previous work (Yamada and Tominaga, 1998a,b) are demonstrated in Table 2. We assigned species A and B to Fe(O2) and FeO, respectively, because species A should contain more O atoms than species B: FeO forms Fe(O2) on annealing the matrix samples. Electronical structures of FeO have been studied extensively (Barrow and Senior, 1969; Bagus and Preston, 1973; West and Broida, 1975; Engelking and Lineberger, 1977; DeVore and Gallaher, 1979; Krauss and Stevens, 1985; Fan and Wang, 1995), which should be 5D. We performed molecular orbital calculations for FeO to con®rm our assignment; electronic properties estimated by molecular orbital calculation (UHF/6-311G//B3LYP/6311+G) are in agreement with the observed MoÈssbauer parameters (Table 2) as well as IR absorption reported in the literature (Chertihin et al., 1996) (Table 3). For Fe(O2), assuming various geometries and electronic structures, the molecular orbital calculations gave a structure of Fe(Z 2±O2), 3B2 (Lyne et al., 1993) which is in good agreement with the observed MoÈssbauer parameters (Table 2) and IR absorption Fig. 3. MoÈssbauer spectra at 17 K of laser-ablated Fe cocondensed with O2 in Ar matrices (b±e) and isolated in pure O2 (a). Molar ratios are indicated in the ®gure.

samples with di€erent mixture ratios (Fig. 3c), a similar behavior was seen: the intensity of A increased at the cost of the intensity of the intensity of B. The intensities of species C±E slightly increased and that of species B decreased on annealing samples with higher O2 concentrations (Fig. 3b). MoÈssbauer parameters (d and DEq) obtained in this work are summarized in Table 1. Estimated electronic properties (r(0) and EFG) using observed MoÈssbauer parameters and correlation (r(0)=ÿ3.39 d+15092.99 and EFG=0.906

Table 1 MoÈssbauer parameters of the species at 17 K isolated in Ar matrices Species

d (mm/s)

DEq(mm/s)

Fe Fe2 Fe3 or Fe4 A Fe(O2) B FeO C (O2)FeO2 D FeO3 E OFeO

ÿ0.77 20.03 ÿ0.12 20.03 0.54 20.05 0.35 20.07 0.32 20.11 ÿ0.59 20.09 ÿ0.62 20.01 ÿ0.03 20.05

± 4.052 0.10 1.362 0.10 0.962 0.07 2.422 0.15 2.812 0.14 0.752 0.02 1.652 0.15

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Table 2 Calculated electronic properties of the species using HF/6-311G Species

Calculated from observed MoÈssbauer parameters r(0)-15000 (a.u.) |EFG|a (a.u.)

Calculated by HF/6-311G//B3LYP6-311+G r(0)-15000 (a.u.) EFG (a.u.)

A Fe(O2) B FeO C (O2)FeO2 D FeO3 E OFeO

91.8 91.9 95.0 95.1 93.1

93.7 93.6 94.5 98.2 94.4

a

0.87 2.19 2.55 0.68 1.49

ÿ0.67 ÿ1.77 ÿ3.77 1.40 1.25

The sign of EFG cannot be determined from experimental data.

(Table 3) data. Geometries obtained by B3LYP/6311+G of FeO and Fe(O2) are indicated in Fig. 5. At lower O2 concentrations, though the total number of O atoms is small, the major reaction product is Fe(O2) rather than FeO. Molecular orbital calculations using B3LYP/6-311+G suggests that the reaction of Fe atom with O2 molecule to produce Fe(O2) is exothermic while the reaction to produce FeO and O atom is endothermic. Fe …S ˆ 2† ‡ O2 …S ˆ 1† 4 Fe…O2 † …S ˆ 1†

DH ˆ ÿ26

kcal=mol: Fe …S ˆ 2† ‡ O2 …S ˆ 1† 4 FeO …S ˆ 2† ‡ O …S ˆ 1† DH ˆ ‡26 kcal=mol: Thus the formation of Fe(O2) is preferable in the reaction of the Fe atom with the O2 molecule in view of the ground state energies. The relative yield of FeO increases with a higher O2 concentration, therefore the formation of FeO may be enhanced by the O2 molecule associated by the formation of O3: The formation of FeO becomes exthothermic with the association of O3 formation. Spin multiplicity of the species is conserved in this reaction. Fe …S ˆ 2† ‡ 2 O2 …S ˆ 1† 4 FeO …S ˆ 2† ‡ O3 …S ˆ 0† DH ˆ ÿ1 kcal=mol:

The energies of the laser evaporated iron atoms are released on collision with Ar or O2 in the gas-phase and the reaction products are stabilized before the formation of low temperature solid samples. With low O2 concentrations, the highly energetic iron atoms have more chance to collide with Ar atoms and thus Fe atoms with smaller energies react with the O2 molecule to form Fe(O2). On increasing the O2 concentration, the laser evaporated iron atoms have a greater chance to collide with O2 and the products (Fe(O2)) need to release excess energy to be stabilized, thus the energetic Fe(O2) may further react with O2 to form FeO. As the species become trapped in the matrix before reaching equilibrium, the unstable species can be trapped in the matrix. On annealing the sample, Fe(O2) increases at the cost of FeO (Fig. 4). This reaction may be interpreted by the reaction of FeO with O3 to form Fe(O2) and O2, which is exothermic. FeO …S ˆ 2† ‡ O3 …S ˆ 0† 4 Fe…O2 † …S ˆ 1† ‡ O2 …S ˆ 1† DH ˆ ÿ25 kcal=mol: This reaction is thought to have a very low activation energy because the reaction occurs even at very low temperature (32 K) in a matrix; only di€usion within the matrix restricts this reaction. The species C±E are observed only in the sample with the highest O2 concentration (Fig. 3a and b). Based on the MoÈssbauer isomer shift d, species C and D may have high electron densities almost comparable with free iron atom. We performed molecular orbital

Table 3 Calculated vibrational frequencies and IR intensities using B3LYP/6-311+G Species

Frequency (IR intensities)/cmÿ1 (KM/mol)

(A) Fe(O2) (B) FeO (C) (O2)FeO2 (D) FeO3 (E) OFeO

979 (68) 506 (1) 896 (172) 1069 (199) 1035 1052 (133) 1052 888 (47) 738 (2)

439 (20) (191) 1024 (0) 617 (1) 602 (8) 335 (4) 324 (0) 269 (1) 269 (6) (133) 971 (0) 345 (0) 345 (0) 179 (18) 233 (16)

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Fig. 5. Optimized geometries using B3LYP/6-311+G. Bond length and angles are indicated in the ®gure.

calculations assuming various structures and electronic states of iron-oxides and observed MoÈssbauer parameters (d and DEq) for C±E that are in agreement

with calculated electronic properties (r(0) and EFG) of (O2)FeO2 (S = 0), FeO3 (S = 0) and OFeO (S = 1), respectively: The structures of these species are indicated in Fig. 5. All the formation reactions are calculated to be exothermic. Though our calculations using B3LYP/6-311+G and other studies (Helmer and Plane, 1994a,b) estimate that the 7A1 state of OFeO and Fe(O2) is more stable than the 3B2 states, other calculations based on DFT (Chertihin et al., 1996) resulted in the 3B2 being more stable and well describes frequencies observed in IR spectra. Here, we employed 3 B2 for molecular orbital calculations of OFeO and its result well describes the observed MoÈssbauer parameters and IR frequencies. Structures of FeO2 and FeO3 are reported based on the DFT method (Lyne et al., 1993) and our calculations B3LYP/6-311+G are in agreement with these results. The structures of (O2)MO2 and MO3 are discussed (Huber et al., 1973; Almond and Downs, 1988) for other metals and these results show similar structures as with our results for (O2)FeO2 and FeO3. At high O2 concentrations, laser evaporated Fe atoms having enough energy to overcome the activation energy to break the O±O bonds have more chance to collide with O2 molecules and thus the yields of (O2)FeO2, FeO3 and OFeO, which have Fe±O bonds may increase. Though the calculated heats of formation suggest that OFeO (DH=ÿ57 kcal/mol) is more stable than Fe(O2) (DH=ÿ26 kcal/mol), the

Fig. 6. Infrared spectra at 17 K of laser-ablated Fe cocondensed with O2 in Ar matrix. Molar ratio is Fe/O2/Ar=1/40/400.

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163

results based on MoÈssbauer spectroscopy. These consideration con®rms that FeO, Fe(O2), (O2)FeO2, FeO3 and Fe(O2) are obtained in our experimental conditions and that our assignments based on MoÈssbauer parameters and molecular orbital calculations are correct. 4.3. Reactions with N2O Reactions of laser-ablated Fe with N2O were investigated in order to con®rm the assignments of the products obtained by reactions with O2 (Fig. 7); only products A and B were found and the intensity of B being strong. Molecular orbital calculations suggest that the reaction of Fe with N2O to form FeO is exothermic and preferable in this case, while the reaction of Fe with O2 to form FeO is endothermic. Fe …S ˆ 2† ‡ N2 O …S ˆ 0† 4 FeO …S ˆ 2† ‡ N2 …S ˆ 0† DH ˆ ÿ63 kcal=mol The reaction of Fe atom with N2O to form Fe(O2) and N2 is also exothermic, though this reaction is not simple because of a forbidden singlet±triplet transition in which the electron spin is not conserved. It is well known that N2O is thermodynamically unstable and it dissociates (2 N2O 4 2 N2+O2) at high temperatures, the O2 in this case is thought to be formed on laser ablation. The species (O2)FeO2, FeO3 and OFeO are not produced on reaction with N2O gas. Fig. 7. MoÈssbauer spectra at 17 K of laser-ablated Fe isolated in pure N2O (a) and cocondensed with N2O in Ar matrices (b). Molar ratios are indicated in the ®gure.

yields of Fe(O2) is larger than that of OFeO in matrices with lower O2 concentrations. This fact may be interpreted that the reaction of OFeO formation requires a greater activation energy than Fe(O2) formation. Though studies based on IR spectroscopy are well reported in the literature (Andrews et al., 1996a,b; Chertihin et al., 1996), the experimental conditions (such as temperature, gas introduction methods) may in fact be di€erent from our experiments. Therefore, in the present study, we undertook IR measurements using the same instruments as for the MoÈssbauer measurements (Fig. 6). We also performed frequency analysis of the species using the Gaussian 94 program in order to make a comparison with the IR spectra; the calculated results using B3LYP/6-311+G are in good agreement with the observed IR spectra (Table 3). The IR measurements show that FeO and O3 decrease and that Fe(O2) increases on annealing the matrix samples, which is in good agreement with the

5. Conclusion MoÈssbauer spectra of species produced by laser evaporation of iron metal were observed in low temperature Ar matrices; the isolated species were found to be iron atoms, dimers and small particles. It was demonstrated that the highly energetic iron atoms produced by laser evaporation react with oxygen molecules to form novel compounds which are not available under normal conditions. Newly found products in MoÈssbauer spectra are FeO, OFeO, (O2)FeO2, FeO3 and Fe(O2); their assignment was con®rmed by molecular orbital calculations and infrared spectroscopy. Their yields were in¯uenced by the O2 concentration in the matrix gas. The reaction of laser evaporated iron with N2O produced FeO and Fe(O2). References Abramowitz, S., Acquista, N., Levin, I.W., 1977. Infrared spectra of matrixisolated FeO2: evidence for a cyclic iron± oxygen. Chem. Phys. Lett. 50, 423±426. Almond, M.J., Downs, A.J., 1988. Production of binary ox-

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