Metallization channels in ion-induced decomposition of molybdates and niobates

Volume 133, number 2

METALLIZATION OF MOLYBDATES

CHEMICAL PHYSICS LETTERS

CHANNELS IN ION-INDUCED AND NIOBATES

Shu Fang HO, Salvatore CONTARINI Department

9 January 1987

DECOMPOSITION

and J. Wayne RABAIAIS

of Chemrstty, lJniversi@ of Houston, University Park, Houston, TX 77004, USA

Received 2 October 1986; in final form 26 October 1986

X-ray photoelectron spectroscopic analysis of 4 keV Ar+ bombarded NaaMo04, NaNbO,, MoOa, and the formation of lower valence metal oxides in all cases but reduction to the metallic state only for the niobate samples. Steady-state concentrations for oxidation states of (VI), (V), (IV) and (0) for molybdate (II), and (0) for niobate are determined following ion bombardment. Results are discussed in terms of decomposition mechanism.

1. Introduction Energetic ions incident on a multicomponent target deposit large quantities of energy in localized regions near the surface, resulting in a variety of processes including preferential sputtering, dissociation, atomization, recombination, implantation, etc. In an attempt to understand the mechanism of multicomponent target sputtering, we have studied [l-3] ionbombardment-induced decomposition of salts of complex anions such as CO:- ,NO;, and SO:-. Such oxyanions whose central atoms (C, N, S) form volatile compounds with oxygen were found to become deficient in this central atom upon bombardment. Extension [4] of these studies to oxyanions with metal central atoms such as CrOi- ,MoOi- ,WO;- ,VO, , NbO; , and Tao, revealed that the central metal atom was reduced in oxidation state but not depleted from the damaged layer. Corresponding studies on metal oxides have shown that the instability towards ion-induced decomposition correlated with the oxide vapor pressures in the region of thermal-spike temperatures [5-71 and with their thermodynamic free energies of formation [8] . In our recent studies [4] of ion-induced decomposition of metal-containing oxyanions, we studied the corresponding metal oxides in order to determine whether decomposition of oxyanions proceeds through intermediate lower oxide phases. A dramatic differ0 009-26 14/87/$ 03.50 0 Elsevier Science Publishers B.V.

(North-Holland Physics Publishing Division)

Nb,O, reveals molybdate and and (V), (IV), an ion-induced

ence in the decomposition products for some of the metal combinations was noticed. Whereas reduction to lower metal oxidation states was ubiquitous for all compounds, reduction all the way to the metallic state was observed for some oxyanions and not for the corresponding oxides. Such preferential metallization observed from two compounds which are similar in their metal-oxygen content, albeit in different chemical bonding environments, has strong implications as to the nature of the mechanism of ion-induced damage in insulating materials. In this Letter we present the results of 4 keV Ar+ ion bombardment of Na2Mo04, Mo03/Mo02, NaNbO,, and Nb,O, as monitored by X-ray photoelectron spectroscopy (XPS) as examples of the above effects. The results are discussed in terms of their implications to the mechanisms of ion-induced decomposition.

2. Experimental methods XPS measurements were performed on a PerkinElmer PHI model 550 ESCA/SAM system using Mg Ko X-rays at 1253.6 eV as excitation source. Ion bombardment was carried out in situ with 4 keV Ar+ ions at a flux of 2.5 d/cm2, as measured on a stainless steel plate with area equivalent to the samples. Wide scan XPS surveys were performed to check for impurities; absorbed carbon and hydroxide were the on-

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ly impurities observed. Narrow high resolution scans were performed on all core-level lines before and at various stages of irradiation. XPS scans were taken after one hour bombardment intervals until changes in the line shapes and atomic concentrations were no longer visible. Saturation typically occurred at doses of w 1.5 X 1Ol7 ions/cm2. The impurity carbon 1s line at 285.0 eV was used as a reference. The binding energy values of the decomposed samples for which all impurity carbon was removed were referenced to representative peaks which were unchanged by the bombardment. Data collection and quantitative analysis were similar to that described in previous works ]1,21Research grade powder samples from Alfa Products were pressed into 1 cm wide by 0.2-0.5 mm thick disks that were positioned on the variable temperature probe of the spectrometer. The Na2M004*2H20 was heated in the pretreatment chamber to 150°C for 40 min to remove water of hydration. The other samples were not heated so that the native hydroxide on the surface could be monitored.

3. Results and discussion The MO 3d core level spectra of Na2Mo04 and MoO,/MoO, before and after 4 keV Ar+ bombard-

MO3dW. sh

BOMBARDED/

\

BOMBARDED

BINDING

ENERGY (eV)

Fig. 1. XPS MO 3d5,2,3,2 spectra of N%Mo04 and MOO,/ Moo2 before and after 4 keV Ar+ ion bombardment.

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9 January 1987

ment to saturation are shown in fig. 1. All of the spectra exhibit overlapping peaks which were deconvoluted through use of the multiplet intensities and splittings and the 3d binding energies (BE) as measured for the oxyanion before bombardment. Since no deviations were observed in these multiplet ratios and splittings, values of 1.5 were used for the 3d,/,/3d3/, peak area ratios and 3.0 eV for the 3d,/2-3d3/2 energy separations. Values for the 3d,i2 BE of various oxidation states were, as assigned previously [9,10], 232.5 eV Mo(VI), 230.6 eV MO(V), 229.2 Mo(IV), and 227.6 eV MO(O). The initial Na2Mo04 surface contains dominantly Mo(V1) with minor MO(V) impurity. The initial MOO,/ MOO, surface contains dominantly Mo(VI), i.e. MOO,, with small amounts of MO(V) and Mo(IV). This air oxidation of the MOO, surface to Moo3 and Mo20, has been observed previously [8,9]. The 0 Is peaks have small shoulders on the high BE sides due to atmospheric hydroxide contamination. The relative atomic concentrations, as determined from the peak areas, and the measured BE values are listed in table 1. Upon Ar+ bombardment the MO 3d region broadens into a structure characteristic of several overlapping multiplet peaks. Using the deconvolution method described above, the Na2Mo04 3d region can be shown (fig. 1) to consist of four overlapping multiplet peaks of MO which correspond to formal oxidation states (VI), (V), (IV), and (0). The MO(O) metal peak is particularly obvious. The Mo03/Mo02 3d region after bombardment can be shown (fig. 1) to consist of three overlapping multiplet peaks of MO corresponding to formal oxidation states of (VI), (V), and (IV). The MO(O) metal peak is notably lacking. The only change observed in the 0 1s peak is the appearance of a low BE shoulder in the oxyanion spectra which corresponds [2] to the formation of Na20, while the high BE shoulder remains, possibly due to the oxygen in interstitial sites [l 1,121 . BE shifts in the 0 1s level for the different reduced oxides are extremely small and not resolvable. The atomic concentrations of these species following bombardment are listed in table 1. Similar experiments were performed on NaNbO3 and Nb20, with results that paralleled those found for the MO compounds. The initial NaNbO, surface exhibited only Nb(V) which, after bombardment,

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CHEMICAL PHYSICS LETTERS

Table 1 Binding energies (in eV) and atomic concentrations Constituents

Na2 MOO, Mo 3dw2

for Na,MoO,

9 January 1987

and Moo2 before and after 4 keV Ar+ bombardment

Initial surface

a)

Ion bombarded

binding energy

atomic concentration

binding energy

232.2

1T.i ]

232.5

. 230.7

atomic concentration

8.7

229.2 14.5

230.6 227.6

58.6

0 1s

531.9 530.1

““,‘;]

530.3 532.1 528.7

Na 2s

63.0

26.9

63.0

21.2 i-6’ .J 5.0 47.8 53.3 2.1 3.4 1 25.4

MOO,

MO 3d

230.9 229.3 232.6

2.9 22.2 2.5 1 16.8

230.8 228.8 232.7

5.7 39.5 30.0 3.8 1

0 IS

532.4 530.4

61;:;}77.8

532.7 530.7

51.8 8.6 1 60*4

a) Dose 1.5 x 10” ions/cm2.

reduced to several overlapping peaks corresponding to four Nb oxidation states (V), (IV), (II), and (0).

The initial Nb205 surface also exhibited only NbQ, however bombardment produced only three reduced Nb species corresponding to (V), (IV), and (II) oxidation states. The Nb(0) or metal peak was notably absent in the decomposed oxide spectrum. This work is presently being extended to include NaTaO, , Na2W04, and their corresponding maximal valency oxides. Summarizing the atomic concentration changes upon bombardment from table 1, we find that in all cases the central metal concentration increases and the concentration of oxygen either stays constant or decreases slightly (< 15%). The dramatic effect of metallization upon decomposition of Na2Mo04 and NaNbO, and its absence upon decomposition of the oxides Mo03/MoO, and Nb205 implies that such ion-induced decomposition proceeds by non-statistical channels. There must be a channel for metal formation in the oxyanions that is absent in the oxides. Schematically such processes can be represented as:

Al+

Na,MoO,,

NaNbO,

lower valence oxides --C

metal,

(1)

Ar+

MoO,/MoO,,

Nb,O,-

lower valence oxides.

(2)

That is, reduction of complex ions to the metal does not proceed through lower oxide phases in a step-wise fashion. That this phenomenon has been observed in both the MO and Nb oxide systems, the former which has a high, temperature-dependent sputtering yield due to a combination of collisional and thermal sputtering and the latter which has a low, temperatureindependent sputtering yield [ 131, implies that it is not an isolated observation attributable to a specific form of sputtering. The result that the niobate and molybdate oxyanions have a reduction channel to the metal that is absent in the corresponding oxides will be considered in terms of the mechanism proposed [l-3] for ioninduced decomposition. This mechanism can be summarized as follows. Ion beams can deposit large amounts of energy into localized regions near a crys173

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talline surface, the energy density being sufficiently high to cause dissociation, atomization, and ionization of species within this region. Translational energy acquired by the constituents through collision cascades and attractive or repulsive electrostatic potentials results in ejection of some particles and random collisions between others. The high atomic density (of the order of the crystal density) within this activated region results in an extremely high collision rate such that recombination reactions between atoms and molecular fragments occur as thermalization takes place to form the thermodynamically most stable species. The resulting decomposition products remaining on the surface are typically non-volatile species with relatively high negative free energies of formation. Elements which are selectively lost from the surface are typically those which form products that are volatile or those which have relatively low free energies of formation in combination with other constituent elements. Considering our results in view of this decomposition mechanism, we can at present suggest the following interpretation. Some major differences in the oxyanion compounds and the corresponding oxides are (i) the volatility of the counter ion (Na+) and its oxides, (ii) the high affinity of sodium for oxygen, and (iii) the higher covalency of the oxyanions compared to the oxides. For case (i), it is known that sodium can be highly mobile under ion bombardment in insulators and that both sodium and its oxides are volatile and therefore could be lost efficiently. Removal of Na or Na,O from the lattice should leave a structure of marked instability which should disproportionate to lower valence states. The Na/Mo ratio decreases on ion bombardment from 1.86 to 1.20 (table l), as does the O/MO ratio consistent with loss of an Na-0 moiety. For case (ii), we note that sodium has a high affinity for oxygen. This fact, together with the high mobility of sodium, will result in efficient reaction of oxygen with sodium in the activated region. The resulting oxygen deficient condition is favorable for reduction to lower oxidation states and the metal. For case (iii), energy dissipation is slowest (or, similarly, energy is most highly localized) in covalently bonded rather than ionic or metallic systems. The high degree of localization of the deposited energy into covalent bonds results in more efficient bond rupture and atomization in covalent rather than in 174

9 January 1987

ionic or metallic systems. Xo calculations [14] on Li,MoO, and MOO, have found that the MO-O bond is more covalent in the molybdate than in the oxide. In fact, the calculated atomic charges of MO and 0 in the molybdate (4d”) are t2.38 and -1.09, respectively, while those in Mo02(4d2) are t2.71 to t2.75 and -1.80 to -1 Sl, respectively. Although the sensitivity of ion-induced decomposition to the degree of covalency of a system has not been previously studied, the results presented herein suggest that such a correlation may exist. It should be noted also that electron irradiation of very similar salts, Li,CrO, and Li,WO,, as reported by Sasaki et al. [ 151 did not produce zero-valent Cr or W. Presumably the shorter range of the ions, as compared to the electrons, results in an effective local temperature that is higher for ion bombardment, and should therefore lead to a larger amount of “thermal” desorption of products. This proposed interpretation lists several factors that may be important in the dissociation mechanism. These factors are now being tested by extending the studies to other metal oxyanion systems.

4. Conclusions The reduction to MO and Nb metal and lower oxides observed upon Ar+ ion bombardment of sodium molybdate and niobate salts contrasted to the reduction only as far as lower oxides for the MOO, and Nb20, systems indicates that the oxyanion reduction mechanism does not follow a step-wise reduction through lower oxides, but contains a different channel which leads to metallization. It is suggested that these differences in reduction behavior of the metal oxyanions and oxides may be dependent upon the affinity of sodium for oxygen, the mobility and volatility of sodium and its oxides, and the high degree of covalency of the oxyanions.

Acknowledgement This material is based upon work supported by the Robert A. Welch Foundation under Grant No. E-656 and the National Science Foundation under Grant No. CHE-85 13966. The photoelectron spectrometer was provided by National Science Foundation Grant No. CHE-8306122.

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CHEMICAL PHYSICS LETTERS

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[ 81 K.S. Kim, W.E. Baitinger, J.W. Amy and N. Winograd, J. Electron Spectry. 5 (1974) 351. [9] R.J. Colton, A.M. Guzman and J.W. RabaIais, J. Appl. Phys. 49 (1978) 409. [lo] A. Cimino and B.A. de Angelis, J. Catal. 36 (1975) 11. [ 111 S. Storp and R. Holm, J. Electron Spectry. 16 (1979) 183. [12] Z.C. Jiang, L.D. An and Y.G. Yin, Appl. Surface Sci. 24 (1985) 134. [13] R. Kelly and N.Q. Lam, Rad. Effect 19 (1973) 39. [ 141 T.A. Sasaki and K. Kiuchi, Chem. Phys. Letters 84’ (1981) 356. [15] T.A.Sasaki, R.S. Williams, J.S. WongandD.A.Shirley, J. Chem. Phys. 69 (1978) 4374.

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