Formation and characterization of thin film vanadium oxides: Auger electron spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, and optical reflectance studies

Thin Solid Films, 198 (1991) 251 268

251

PREPARATION AND CHARACTERIZATION

FORMATION AND CHARACTERIZATION OF THIN FILM V A N A D I U M OXIDES: A U G E R E L E C T R O N SPECTROSCOPY, X-RAY P H O T O E L E C T R O N SPECTROSCOPY, X-RAY D I F F R A C T I O N , S C A N N I N G E L E C T R O N MICROSCOPY, A N D OPTICAL R E F L E C T A N C E STUDIES A. Z. MOSHFEGH AND A. IGNATIEV

Department of Physics, University of Houston, Houston, TX 77204-5504 (U.S.A.) (Received June 4, 1990; accepted September 18, 1990)

The growth of thin film vanadium oxides on a vanadium substrate under oxygen partial pressure has been investigated over a range of temperatures and times. A variety of oxide structures in the range VO2-V205 have been grown. Chemical composition, phase identification and structure of the oxides have been characterized using several analytical techniques including Auger electron spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction (XRD), scanning electron microscopy, and optical reflectance and have been supported by thermodynamic calculations. Small grain Crystalline V20 5 was successfully grown under a reproducible recipe. A visible band gap energy Eg .~ 2.38eV and an X R D morphological f a c t o r f ~ 0.94 were measured for the grown V20 5 film. Ultrahigh vacuum thermally induced reduction o f V 2 0 5 into a lower oxide has been observed. The various properties (structural, electronic, optical, and electrical) of V20 5 have also been reviewed.

1. INTRODUCTION Vanadium forms a large number of oxides each of which is stable over a certain composition range. Vanadium oxides, especially in the V2Os-V20 3 system, are complex and within this compositional range many stable intermediate oxide phases have been reported1-3:

VnO2n- 1

where 2 ~ n ~< 8

VnO2n

where n = 2

V,O2,+l

where n = 2, 4, 6

Among these oxides, there is a family of six (between V20 3 and VO2) forming a homologous series V, O2n-1 that is referred to as the Magneli (first investigator) phases. Higher oxides of the Magneli series (n > 8) have also been studied, but recently the stability of the oxide phases V9017 and V~ oO 19 has been questioned 4' s. Magnetic susceptibility measurements and phase diagrams of the V2Os-V20 3 system have been investigated by Kosugell who determined at least nine different 0040-6090/91/$3.50

© ElsevierSequoia/Printedin The Netherlands

2 5 "-)

A. Z. M O S H F E G l t , A. I G N A T I E V

phases within this range. The phase transition between V205 and V203 has been well explained by the insertion and elimination of the oxygen planes (shear planes) through the crystal from one side to the other ~ s Generally, the transition of pure and stoichiometric V205 into most of the lower oxides involves a rearrangement of the oxygen polyhedra surrounding vanadium atoms in the crystal. The removal of the oxygen from the V205 lattice may cause formation of point defects or formation of Wadsley's phases 9 11 described by the general formula V2,O5, 2- Derivation of reduced phases (V6013, V307) from the V205 parent oxide has been proposed and it has been suggested that there exists a structural relation between these oxides 2"6. Electron b o m b a r d m e n t induced transformation and ultrahigh vacuum thermally induced reduction of V205 into lower oxides have also been observed 2"12"13. The presence of these reduced phases is believed to influence the catalytic properties of V20 ~. However, it is well established that, during the reduction state of vanadium pentoxide, a vanadyl oxygen vacancy is formed and is considered to be the basic point defect in the crystal as a consequence of this process. The important role of this vacancy has been stressed during catalytic oxidation of hydrocarbons 2' 14. Vanadium-based alloys and oxides have been widely used in variety of scientific and technological applications, namely in industrial catalysis 1~ 17, in the electronic industrylS 20 and for the wall material in fusion reactors 21"22. From the catalytic point of view, the presence of the lower oxides in V20 5 is essential for the activity and selectivity of vanadium pentoxide in a variety of heterogeneous chemical reactions. As well as their catalytic properties, many developments in electronic applications of vanadium oxides (VO2, V203, and VO) have also been established. These applications originate from the semiconductor metal phase transition in vanadium oxides that results in an increase in electrical conductivity of the oxides. Most of the vanadium oxides are available commercially (in powder formt. However, a large number of these oxides can be prepared in the laboratory under certain oxidation reduction processes. Thc conditions under which thin films of the vanadium oxides are formed are however, still controversial. In this study, we have reproducibly grown various thin films of vanadium oxides on a metallic vanadium substrate under specific conditions. In the past 23 26 Auger electron spectroscopy IAES) and X-ray photoelectron spectroscopy (XPS) have been employed to investigate surface chemical composition and the oxidation states of some vanadium oxides with known phase composition, namely VO, V203, VzO 4, and V205. Nevertheless, among these studies, there exist some differences in the oxidation conditions for formation ofV 4 + and V 5 + species on the vanadium oxide surf0ce. In this paper we focus on formation of thin film vanadium oxides as well as their characterization (the nature and degree of surface oxidation of vanadium in the films) by a number of analytical techniques. It is organized in the following manner. The preparation of thin film vanadium oxides is dealt with in Section 2, all the results that have been obtained by the various characterization techniques are described in Section 3, together with a discussion for each subsection, the properties (structural, electronic, optical, and electrical) of V2Os are focused on in Section 4, ultrahigh vacuum thermally induced reduction of V205 is stressed in Section 5, the

THIN FILM V OXIDES

253

thermodynamic properties of vanadium oxides are described in Section 6, and the results that have been obtained are summarized in Section 7. 2.

PREPARATION

OF THIN FILM VANADIUM

OXIDES

Polycrystalline vanadium substrates were cut in the form of small disks (diameter, about 12mm; thickness, about 0.7mm) from a high purity (99.99~o) vanadium rod. They were highly polished using A120 3 polishing paste (down to 0.05 ~tm size grit), rinsed in distilled water and degreased in acetone and methanol solutions. They were subsequently placed into an evacuated quartz vacuum tube furnace (volume, about 340 ml) for direct oxidation in ultrahigh purity (99.999~o) oxygen (Po2 = l atm) under continuous flow (1.5-2.01min-~). Each film was maintained at the desired oxidation temperature for a certain time. The oxidation temperatures of the films were controlled by a thermocouple localized at the center of the furnace, and were kept within _ 1 °C. To obtain surface homogeneity during the oxide formation, all the films were cooled down to room temperature at a rate of about 2 °C min 1 under oxygen flow. Thin films of vanadium oxides have been grown in the composition range VO2-V205 under various oxidation conditions. Table I shows the temperature, time, and flow rate conditions under which six different types of films have been obtained. In addition, thickness as well as observed color of the films are listed in Table I. The vanadium oxide films in the range VO2-V20 5 have been reproducibly formed under the above conditions. Various experimental techniques have been employed in order to characterize and to enhance the understanding of the properties of these thin films, leading to potential applications in a number of technological processes. TABLE I THE GROWTH CONDITIONS FOR THE FORMATION OF THIN FILM VANADIUM OXIDES ON THE VANADIUM SUBSTRATE

Sample

T (°C)

t (h)

F (1min 1)

Color

Thickness

(rtm) V-12 V- 17 V-20 V-26 V-31 V-23

440 24 2.0 600 24 2.0 600 3 1.5 480 3 1.5 550 8 1.5 Ground, pressed commercial VzO 5 powder (99.8%, Alfa Products)

Blue-black Brown Orange-yellow Blue-purple Brown Orange-yellow

2.7 4.8 4.1 0.2 3.6 oo

The prepared films were transferred (ex situ) from the furnace to the analysis chamber for determination of surface composition. Because of the stability of these oxidesL1L27, room temperature exposure of the oxide films prior to installation in the ultrahigh vacuum chamber did" not change composition of the films under investigation.

254 3.

A. Z. MOSHFEGH, A. I G N A T I E V

RESULTS A N D DISCUSSION

Information on chemical bonding, chemical composition, surface structure and other related parameters of thin film materials can be provided by implementation of ultrahigh vacuum surface-sensitive analytical techniques 2~ 30. In this work AES, XPS, X-ray diffraction (XRD), scanning electron microscopy (SEM), optical reflectance measurement and thermodynamic calculations have been employed to investigate the valance state(s) of vanadium, surface chemical composition, structure, surface homogeneity and other related phenomena of the thin film vanadium oxides formed under the specific conditions.

3.1. Auger electron spectroscopy AES studies of the thin film vanadium oxides have been performed in a stainless steel ultrahigh vacuum chamber with a base pressure of about 2 × 10- so Torr using a PHI model 550 electron spectroscopy for chemical analysis-scanning Auger microprobe system. The system contains a double-pass cylindrical mirror analyzer with coaxial electron gun. Auger spectra were obtained in the derivative (dN(E)/dE) mode using a 4 V peak-to-peak modulation signal. Generally, surface oxidation states of metals by AES have been traditionally studied by employing the chemical shifts and/or the relative intensities of the metal Auger transitions with respect to the O(KLL) transition. Auger spectra ofVO, V20 3, and VO2 have been previously examined 23. It was found that the oxygen O(KLL, 509 eV) to vanadium V(LaMzaM23, 433 eV) intensity ratio as well as the Auger line shifts depend on the oxidation states of the vanadium. A chemical shift of about 0.6 eV per oxidation number for the vanadium inner shell Auger transition (L3M23M23) was obtained. In this study, we shall determine the surface chemical composition of several vanadium oxides from the 0(509 eV):V(469 eV) Auger intensity ratio. Because of the possibility of oxygen loss and decomposition of the vanadium oxides under the influence of an electron beam, 1000eV primary electrons and a current density of about 1 mA cm 2 have been used unless stated otherwise. Figure I shows the Auger spectrum of the V-20 thin film in the energy range 200-700 eV. The following peaks are identified: V(509 eV), 0(509 eV), V(495 eV), V(489 eV), V(469 eV), V(465 eV), V(449eV), V(433 eV), V(426eV), V(418 eV), V(407 eV), V(395 eV) and V(368 eV). The V(465 eV) and V(426 eV) structures are shoulders on the low energy side of the 469 eV and 433 eV peaks respectively. It is important to note that the above AES peaks are in good agreement with Auger spectra ofV2Os(010 ) surface 31, except that we found two additional peaks at 449 eV and 368 eV. The small peak at 272 eV corresponds to a carbon surface impurity. Surface chemical composition of the thin film vanadium oxides V- ! 2, V- 17, and V-20 as determined by the measure of the relative Auger intensity ratio O:V are shown in Table II. The O:V ratios have been obtained under the following considerations. (i) Sensitivity factors of Auger electron emission under the 1000 eV incident electron beam were taken to be 0.22 and 0.26 for vanadium and oxygen respectively. It is worth noting that these factors are in good agreement with both calculated

THIN FILM V OXIDES

255

V-20

I,I "D z

395

[

489

455

469 509

200

30o

40o

soo

700

Electron Energy (eV) Fig. 1. Auger spectrum of the V-20 thin film in the electron energy range 200-700 eV indicating formation of V20 s phase. TABLE II O(509eV):V(469eV) AND O(ls):V(2p) RATIOS FOR THE GROWN OXIDES OF VANADIUM

COMPARISON OF AUGER ELECTRON SPECTROSCOPY SPECTROSCOPY

Sample

AES 0: V

XPS O(ls) . V(ep)

v-12 v-17 v.20 v-23

2.16___0.07 2.31 +0.06 2.45 ___0.05 --

2.12+0.08 2.26+0.07 2.41 ___0.05 2.46 + 0.04

X-RAY PHOTOELECTRON

values of Mroczkowski and Lichtman 32 and extrapolated values from the Handbook of Auger Electron Spectroscopy 33. (ii) Because of the contribution of the vanadium LaVV Auger transition to the oxygen peak intensity, all the relative intensity ratios O:V have been reduced by a value of 0.20. According to both the Auger intensity ratio O:V listed in Table II and the observed characteristic Auger peaks for the V-20 film shown in Fig. 1, the V-20 film has been assigned the phase composition of V 2 0 5. AES analysis for the V-23 sample (pressed V 2 0 5 powder) could not be obtained owing to severe charging under the incident electron beam. The V-23 sample will be described in more detail in Section 3.2 where the charging effects in XPS are not as severe. 3.2. X-ray photoelectron spectroscopy XPS measurements were carried out in the surface analysis system that was described in the previous section using Mg K0t X-rays (1253.6 eV) as the excitation source. Core level binding energies (BEs) of the V(2p3n) and the O(ls) peaks have been used to characterize the chemical state (oxidation state) of the vanadium in the grown films.

256

A. Z. MOSHFEGH, A. IGNATIEV

Generally, a well-established relationship exists between core level BE and the charge state associated with an individual atom. An increase in the positive charge on the a t o m results in an increase in the BEn of the core levels. Therefore, in this particular study one could expect that, as the oxidation state decreases, the core level V(2p3/2 ) emission would shift to a lower BE. In addition, it is expected that its corresponding full width at half-maximum ( F W H M ) will broaden as has been observed previously 34' 35. All measured BEn are given relative to the position of the C(1 s) core level at 285.0 eV. Charging effects were minimal and could be controlled by flooding the sample surface with low energy electrons during the analysis. Figure 2 shows XPS spectra of the films V-12 and V-20 in the BE range 510-540eV. For the metallic vanadium, it is well established 3'~ that the binding BE of the V(2p3/z) level is at about 512.4 eV. In the case of the V-12 film, the V{2p3/2) core level shifts to a BE of about 516.0eV with an F W H M o f about 3.2 eV. As a result, a chemical shift AE = 3.6eV is obtained for the V(2p3/2) level of the V-12 film. O n the basis of the XPS analysis, the phase composition for the V- 12 film was designated as VO2. This is in agreement with a chemical shift AE = 3.8 eV that has been obtained 3~ for bulk VO2 oxide.

o (~ s)

v ,'2>_ ,~ } i

i

/ i\

Z

,/ /

L

/!'

I i

V- 2 0

,' 'i /

N~

540

535

S50

525

Binding E n e r g y

520 (eV)

515

5i0

Fig. 2. XPS spectra of the V- 12 and the V-20 grown lilms. The dashed lines, !. indicate chemical shifts of the V(2p3,2) core level BE.

The XPS spectrum of the V-20 film illustrated in Fig. 2 shows that the V(2p3/2 ) core level BE is shifted to a value o f a b o u t 516.8 eV with a chemical shift A E = 4.4 eV and a narrow F W H M (about 2.1 eV). This indicates that the v a n a d i u m ion on the surface of the film is in its highest oxidation state (V s+). This result is supported by recent observations 36 that shows the V(2p3/2) core level BE to be a b o u t 517.0 eV for V 2 0 5 film. It is worth noting that the V(2p3!2 ) core level for the V- 17 film was shifted to a b o u t 516.5 eV with a slightly broader F W H M as c o m p a r e d with the V-20 film, indicating that the v a n a d i u m in this film is not in a single oxidation state, but is in the range from V 4+ to V s+. This m a y be due to partial reduction o f V 2 0 s to lower oxides V307 a n d / o r V 6 0 j 3 on the V 2 0 s surtace.

THIN FILM V OXIDES

257

In XPS analysis of the V-23 sample (V205 pressed powder), the V(2p3/2) BE was shifted to a value of about 517.0 eV with an F W H M of 2.3 eV. A slight increase in both the BE and the F W H M of the V(2p3/2 ) was obtained for the V-23 sample as compared with the V-20 thin film with V20 5 composition. A summary of all the observed BEs and F W H M s of the V(2p3/2) core levels and the O(ls) levels for the oxide films V-12, V-17, V-20 and V-23 is shown in Table III. A typical spin-orbit splitting between the V(2p3/2 ) and V(2pt/2) peaks for all the films was about 7.5 eV. T A B L E III COMPARISON OF BINDING ENERGY AND FULL WIDTH AT HALF-MAXIMUM OF THE V(2p3/2 ) AND THE O ( l s ) CORE LEVELS OF THE GROWN VANADIUM OXIDE FILMS

Sample

V- 12 V-17 V-20 V-23

V ( 2p 3/e)

O ( l s)

B E (eV)

F W H M (eV)

B E (eV)

F W H M (eV)

516.0 516.5 516.8 517.0

3.2 2.4 2.1 2.3

529.3 529,5 529,6 529,7

2.9 2.2 2.3 2.6

The O(ls) and V(2p) core level area ratio O(ls):V(2p) has also been measured to determine surface chemical composition of the oxide films. The composition of the oxide films V-12, V-17, V-20, and V-23 as obtained by this method is shown in Table II. The data have been obtained after normalization of all the O(ls):V(2p) area ratios by a value of 0.40, regarded as the ratio of atomic sensitivity factors for O(ls) and V(2p). This value is in agreement with both the photoelectron crosssection calculation of 0.45 by Scofield 37 as well as the ratio of atomic sensitivity factors from the Handbook of X-ray Photoelectron Spectroscopy 3a. XPS as a surfacesensitive technique has also been utilized to study thermally induced reduction of V20 5 (V-20 film) under ultrahigh vacuum conditions. Details of this investigation will be discussed in Section 5.

3.3. X-ray diffraction XRD analysis of the thin film vanadium oxides has been undertaken using a Rigaku (D/MAX) system under the following operating conditions: 42.5 kV, 40 mA, Cu K s monochromatized radiation. All peak positions and diffraction intensities have been compared with standard patterns from the Joint Committee on Powder Diffraction Standards39. 20 scans have been used to obtain all the diffraction measurements. In this study, XRD spectra of the V-17, V-20, and V-26 vanadium oxide films have been obtained. Figure 3 shows the diffraction spectrum of the V-17 film. Three main peaks at 20 = 15.32 °, 20 = 20.20 °, and 20 = 26.12 ° are observed. On the basis of X R D analysis, the V-17 film has been assigned as principally V20 5 with, however, small contributions of V6013. Figure 4 illustrates the XRD spectrum of the V-26 film. As well as the three main peaks at 20 = 20.20 °, 20 = 27.80 °, and 20 = 40.96 ° which can be identified as due to

258

A. Z. MOSHFEGH, A. IGNATIEV

69 C

V-17

010)

C E 0

(ool) 1

~d 0

d~

10.0

20.0

30.0

40.0

50.0

60.0

2e Fig. 3. XRD analysis of the V-17 oxide film.

(ool)

vb~o)

V-26

U3

7__ 0 0 ,+__

tz] 10.0

70.0

30.0

40.0

50.0

60.0

20 Fig. 4. XRD analysis of the V-26 oxide film. The arrows designate the presence of V40,~ and/orVO2 phases.

oxides of vanadium, a very intense peak at 20 = 42.08 ° has also been found. This peak is a vanadium substrate peak, V(110). The appearance of the vanadium peak can be explained from the conditions under which this film was grown (relatively low temperature and short time). This resulted in formation of a very thin oxide layer on the vanadium substrate and thus a "shining through" of the substrate through the oxide. According to the XRD analysis, the observed distribution of the oxide phase components for this film was found to be V 2 0 5 + V,,09 + VO2. The XRD spectrum of the V-20 sample is shown in Fig. 5. The observed characteristic peaks at 20 = 15.30 °, 20 = 20.20 ° and 20 = 26.05 ° as well as the diffraction intensity ratios indicate that V205 is the only phase composition that exists in this film. The lattice parameters for the V-20 film, on the basis of measured d spacings, are a = 11.527/~, b = 3.567/~, and c = 4.374/~. These values are in agreement with bulk V205 XRD data and are also supported by low energy electron diffraction observations 4° of single-crystal V205(010). It is worth noting that in comparing the XRD spectrum of the V-20 sample with that of the V- 17 film (Fig. 3), it was found that diffraction intensities of the V-17 film along (h00) planes for h = even are reduced (probably because of preferred orientation) to about 50% as

THIN FILM V OXIDES

259

>, ~J

V-20

(OOl) C q)

(110)

C

(200)

C 0 0

(4oo) 123

I 0.0

20.0

50.0

40.0

50.0

60.0

2e Fig. 5. XRD analysis of the V-20 oxide film (see the text).

compared with the V-20 film. Therefore, it seems the longer oxidation time for the V-17 (see Table I) favors preferential orientation along a particular plane, possibly through an oxygen diffusion-segregation process.

3.4. Scanning electron microscopy SEM measurements have been obtained using a JEOL (JSM-T300) instrument at magnifications from 1000× to 50000x, at a 20 kV accelerating voltage with the electron beam at 45 ° to the sample surface. Figure 6 shows the grain morphology of the V-12 and V-17 samples. For the V-12 film, it is clear that the grains are closely compact and have a nearly rectangular shape. However, in the case of the V-17 sample, the grain morphology is one of individual and separated grains. In addition, the average grain size for the V- 17 film is much larger than for the V-12 film and the grain shapes vary from thin elongated bars to thin rectangular plates. The observed larger grain size for the V-17 sample is expected from the oxidation conditions (higher temperature) under which the film was grown. It is worth noting that the surface morphology of the V-20 film with composition V20 5 was found to be in the form of thick elongated bars. According to both the XRD and SEM data, the basic differences between the films under investigation are as follows: (i) variation in diffracted intensity of the

|'

"4

(a) (b) Fig. 6. SEM micrographs of the oxide films (a) V-12 and (b) V-17.

260

A . z . MOSHFEGH, A. IGNATIEV

(00t) V205 reflection and the (110) V2Os reflection from film to film, and (ii) change in the grain size and grain distribution, probably as a result of the contribution of these planes to the external surface of the grains. The XRD intensities of the thin film vanadium oxides under investigation can be uniquely defined in terms of the morphological factor f introduced by Ziolkowski and Janas41: 1110 f

-

(1)

Iool

f represents a semiquantitative measure of the contribution of the two planes (110) and (001) to the external surface of the grains, where I denotes the intensity of the X-ray reflection for the designated planes given by the index. The obtained f value of about 0.94 for the V-20 film (a film defined as V20 5 by AES and XPS analyses) is in good agreement with about 0.90 for single-crystal V20 5 39 Table IV summarizes the morphological factor f and typical grain sizes for the oxide films V-12, V-17, V-20, and V-26. The average grain size for a specified film is obtained by measuring the grain sizes of a given film for at least five different magnifications. According to these data, it seems that there exists a relationship between grain size and morphological factor: as grain size increases the J" tends to increase as well. TABLE IV COMPARISON OF AVERAGE GRAIN SIZ|i AND ~,I()RPHOLOfiI('AL ] A ( ' r O R / - FILMS

Sample

Grain size

l I lO,'/t)l}l ()F I-HI (IRO~2VN OXIDL

f

(pm) V- 12 V-26 V-20 V-17

0.21 0.24 0.31 0.54

0.03 0.07 0.94 1.92

On the basis of SEM and profilometer measurements (Tencor Instruments, a-step 250), the film thicknesses for the all vanadium oxides have been measured (see Table I) to be in the range 3 5 I.tm except sample V-26 with an oxide layer of thickness about 1500 &. According to both the SEM and the XRD data, the following qualitative statements can be made for the oxide films under investigation: (i) the average grain size increases with increasing growth temperature, (ii) compaction and agglomeration of the grains is observed at low growth temperature, and (iii) the grain shape varies from thick elongated bars at high growth temperature to small nearly rectangular shape at low growth temperature.

3.5. Opticalreflectance Optical reflectance measurements have been performed using a Beckman DK-2A spectrophotometer system. All reflectance data were calibrated relative to an MgO standard. Figure 7 shows the typical hemispherical reflectance for the films

261

THIN FILM V OXIDES

V-12, V-20, V-23, V-26, and V-31. Two optical absorption bands are observed for the oxide films V-20, V-23, and V-31: a primary edge in the 460-550 nm region and a secondary edge in the 350-390 nm. The observation of these two bands has been identified previously for a V20 5 sample 42. Because of our experimental limitations, we could not locate the lower end of the secondary band. For the films V-20, V-23, and V-31 an optical band gap Eg = 2.38 +0.07 eV has been determined from the inflection point of the sigmoid region of their spectra. This result is in agreement with both the chemical composition and the electronic properties of these films. An observed decreasing trend in the near-IR reflectance of the films from sample V-23 (highest reflectance) to V-26 (lowest reflectance) is believed to be due to an increase in oxygen vacancies in the lattice from V-23 (pure V205) to V-26 (lower oxides of V205). 50 ~ 4 0

/

~" 300 eu "*-

/

1. . . . . . . . . .

V-23

/ ,,/' / /,

." ....

~ .....

V-20

20-

,// ........................ V-51 --~ ~ ~ " = ~ - ~ ' ~7 < ~ "- -"""' "". -. . . . . -. . . . . . . . . . . . . . . 10~'" ~ ........... V I I 2 ....

0

550

450

,

i~ F g = 2 . 3 5 , 550

Wavelength

eV

650

750

(nm)

Fig. 7. Optical reflectance spectra for the oxide films V-12, V-20, V-23, V-26, and V-3I in the region 350-750 nm.

Figure 8 shows the reflectance spectra of the films in the IR region between 750 and 2500 nm. For the films V-23, V-20 and V-31, a decrease in reflectance intensity with increasing wavelength was observed. This decrease is consistent with the chemical composition (see Table II) and color of these films as listed in Table I, 50

~-

40 ¸

,.V-23

"~" \ , a

V-20

V-26 "'~-,~

20 q)

--._

10. 0 700

1000

1300

1600

Wavelength

1900

2200

2500

(nm)

Fig. 8. Optical reflectance spectra for the oxide films V-20, V-23, V-26, and V-31 in the region 7 5 0 - 2 5 0 0 nm.

262

A. Z. MOSHFEGH, A. IGNATIEV

which follows the decrease observed in the visible portion of the spectrum (Fig. 7). For the V-26 film, two interference maxima at the wavelengths 1565 nm and 2320 nm were found. This is due to the small thickness of the film (about 1500/~ as supported by XRD measurements (see Fig. 4)), and resultant interference with the IR-reflecting vanadium substrate. 4. SOME PROPERTIES OF V 2 0 5 Because of its potential applicability in many technological processes, the most important being in the catalytic industry, it is appropriate to discuss some of the fundamental properties of V2Os. In the past, the various aspects of V20 5 including structural2,43, electronic 2,~3.44, optical4.~ 4s and electrical 2°'4<5° have been investigated. The aim of this section is to review these properties in order to understand the nature and the chemistry associated with catalytic activity and selectivity of V 2 0 5 and its lower oxides (V409 and V6013) in many heterogeneous chemical reactions, in particular, the catalytic decomposition of isopropyl alcohol that has been discussed recently s 1,52 4.1. A t o m i c structure

Vanadium pentoxide, V20 5, is the highest oxide in the v a n a d i u m - o x y g e n system, and is often prepared in bulk powder form by decomposition of a m m o n i u m metavanadate, i.e. 2NH4VO3

~VzOs+2NH3+H20

(2)

at a temperature of about 600"C. It is established that V20 s crystallizes with an orthorhombic unit cell, and belongs to the low symmetry P m n m space group 43 with lattice parameters a = 11.510/~, b = 3.563/~, and c = 4.369/~, where the b and c axes are often interchanged. The structure of V 2 0 s is given in Fig. 9. The vanadium atoms form five bonds with oxygen (V-O bond lengths from 1.58 to 2.02/~): one with the O1 atoms, one with the 02 atom, and three with the 0 3 atoms. Figure 9(a) illustrates projections of the structure on the (010) plane of V 2 0 s as well as various V - O bond lengths. The shortest V - O bond length corresponds to a double vanadyl bond (V=O). Projection of the structure of the (001) plane of V 2 0 s is shown in Fig. 9(b). The stereochemistry of V 2 0 s may be considered to be either (i) the structure built up from distorted VOs trigonal bipyramids around the vanadium atom in which they are linked together via corner sharing, or (ii) a deformed octahedron VO 6 (through the consideration of the longest bond V--OL* (2.81/~) in the coordination) which serves as the building block of the V2Os structure. The deformed VO6 octahedra form warped layers in which oxide anions are shared by adjacent octahedra. The second model is however, more widely acceptable. From this structure it is assumed that only a weak Van der Waals type interaction exists between the layers. Thus, the crystal structure o f V 2 0 s is a layered structure 2 with an easy cleavage along the (001) plane. The oxygen in the crystalline V 2 0 s is divided into three types. According to IR and Raman spectroscopic studies s3, a band at about 992 cm - 1 was characterized as

THIN FILM V OXIDES

263

01

1.54A~2.02 Am

1.77 A L_V.3 J ~ ' 5

i281A ( ~

©

---

~

?

~

~

I

©

,,(3

(a)

I ©-0 L (b)

~CI

Fig. 9. Structure of V20~: (a) projection of the structure on the (010)plane; (b) projection of the structure on the (001) plane of V2Os; O, vanadium atom; O, oxygenatom. a stretching vibration mode of the V - - O 1 bond, a second vibrational mode at about 768 c m - 1 has been assigned to the V - - O 2 - - V bond, and the third type at about 702 cm - 1 was characteristic of the stretching vibration of the V

/ V--O3

bond.

\ V

4.2. Electronic structure A number of experimental studiesZ,13,44,4s on the electronic structure o f V 2 0 5 exist. Unfortunately, little work has focused on the theoretical investigation (i.e. band structure calculation) of VEO 5. Study of the electronic structure of V 2 0 ~ can, however, be undertaken using the layered aspects of VzO 5 and the weak Van der Waals type interaction between layers. As a result, several approximation methods have been applied in order to investigate the electronic structure of vanadium

264

A. Z. MOSHFEGH, A. I G N A T I E V

oxides. Lambrecht e t al. 54'55 applied a tight-binding method and perturbation theory to study the energy band structure of V20 5. It has been found that the bottom of the conduction band is formed by V 3d orbitals separated by a pseudogap from the main band states. Non-stoichiometric V 2 0 5 always exhibits loss of oxygen in the lattice leading to formation of vanadyl oxygen vacancies. The presence of these vacancies in the lattice is primarily important in defining the catalytic properties of V20~ as has been noticed recently 51. 4.3. Optical Several works have been devoted to the study of the optical properties of VzO 5 in the visible and IR regions 45"4~'48. It has been established that the fundamental absorption edge occurs in the 460 550 nm region. On the basis of the present optical data, a visible band gap Eg = 2.38 +__0.07 eV has been obtained for V20~ at room temperature. This is in agreement with the value 2.30 eV obtained recently from optical absorption measurements 48 and a value of about 2.35 eV corresponding to a threshold wavelength 2 ~ 520 nm from photodesorption studies 56. It should be noted that the above variation in the band gap energy may be associated with the degree of non-stoichiometry in V20 5Optical properties o f V 2 0 5 at temperatures T > 25 C are of great interest for many practical applications including photocatalysis and utilization in photovolatic cells. The absorption coefficient and band gap energy of the V 2 0 5 are the key factors affecting these applications. The absorption coefficient K at the fundamental absorption edge (460-550 nm) of V205 behaves as an exponential dependent on both photon energy hv and reciprocal temperature I/T. Therefore. K obeys "Urbach's rule":

(3)

K = Ko exp([3hv)/kT)

where k is Boltzmann's constant, and K o and [] are experimentally determined constants. Equation (3) can be used to study the temperature shift of the band gap energy. 4.4. Electrical V2Os is a low mobility semiconductor with a typical charge carrier mobility of about 1 cm z V ~ s i and having predominantly an n-type conductivity. Electrons are the charge carriers, and an increase in the carrier density is accompanied by reduction in oxygen concentration in the lattice. This oxygen deficiency will determine the electron concentration in VzO~. According to Vinogradov and Shelykh ~7, the carrier concentration n in V20~ x for 0.012 < x < 0.093 corresponds to 1.34 x 10 z° cm 3 < n < 1.03 × 1021 c m 3. Several theories have been proposed to explain conduction in the low mobility transition metal oxides. These are (i) "hopping", (ii) polarons, and (iii) narrow-band conduction. On the basis of the generally acceptable "hopping" mechanism 46' ~o. 58, an electron in V205 migrating from a V 4+ center to an adjacent V 5+ site can be represented by the following scheme:

--V4+--O--V 5+-

~--VS+--O--V 4+-

(4)

THIN FILM V OXIDES

265

The V 4+ sites are assumed to form as a result of thermally excited electrons from the trapping center. A detailed study on the nature and location of V 4÷ ions together with transport properties in V2Os-related compounds is given by Goodenough 59. 5. ULTRAHIGH VACUUM THERMALLY INDUCED REDUCTION

OF V 2 0

5

XPS has been employed here to investigate thermal reduction of V205 under ultrahigh vacuum conditions. A V-20 type of film with known composition o f V 2 0 5 was thermally heated under ultrahigh vacuum conditions at about 700 K for 45 min. The V(2p3/2 ) core level BE was monitored (before and after heating) in order to determine the change in surface chemical composition of the film under this treatment. A decrease in the V(2p3/2 ) BE of about 0.6 eV and broadening of its F W H M by about 0.8 eV were observed after the heating conditions. Table V lists the C(ls), O(ls), and V(2p3/2 ) core level BEs and the F W H M of the V20 5 before and after heating. TABLE V X-RAY PHOTOELECTRON ANALYSISOF W205 BEFORE AND AFTER HEATING IN AN ULTRAHIGH VACUUM ENVIRONMENT

C( Is)

Before heating After heating

V(2p3/e)

O( Is)

BE (eV)

F W H M (eV)

BE (eV)

F W H M (eV)

BE (eV)

F W H M (eV)

285.0 285.0

3.9 4.1

516.8 516.2

2.2 3.0

529.7 529.5

2.2 2.4

On the basis of the XPS analysis as well as thermodynamic considerations, it was found that the vanadium atoms are not in the single highest oxidation state (VS+), but are in mixed oxidation states of vanadium (V s+ , V 4+) that probably result in the formation of the lower oxide(s) V409 and/or V6013 on the surface of V205. This is consistent with observations 2,~2 of a phase identified as "Q" with a composition of W409 or V6013.5 as a result of ultrahigh vacuum thermal decomposition of an air-cleaved V205(010 ) single crystal. It is worth noting that in addition to the observed chemical shift in the V(2p3/2 ) core level the color of the film was also changed from orange-yellow (before the heating) to brown (after the heating). 6.

THERMODYNAMIC PROPERTIES OF VANADIUM OXIDES

Heats of formation and entropies for some of the oxide phases of vanadium have been previously reported 6°,61. However, these values are widely varying. The formation of vanadium oxides from metallic vanadium can be expressed by a general formula: 2V(s) + 2 0 2

' V20 n

(n = 2, 3, 4, and 5)

(5)

where V(s) is solid phase vanadium. An accurate study of heats of formations for VO,

266

A. Z. MOSHFEGH, A. IGNATIEV

V 2 0 3 , V204, a n d VzO 5 has been u n d e r t a k e n by M a h a n d Kelley 62 w h o o b t a i n e d

the t e m p e r a t u r e d e p e n d e n c e of heats of f o r m a t i o n a n d of free energies of f o r m a t i o n for these v a n a d i u m oxides up to 2000 K. T a b l e VI shows the heats of f o r m a t i o n AH~98 a n d e n t r o p i e s S~98 at r o o m t e m p e r a t u r e for 14 solid oxide phases of v a n a d i u m in the c o m p o s i t i o n range V O - V 2 0 5. These d a t a have been c a l c u l a t e d on the basis of ref. 63. A c c o r d i n g to these results, b o t h AH~98 a n d S~98 v a r y in the s a m e m a n n e r with oxide c o m p o s i t i o n up to V8015. H o w e v e r , t h e r m o d y n a m i c p a r a m e t e r s for oxide phases V 2 O a - V 2 O s are c o m p l e x a n d c a n n o t be fixed a c c u r a t e l y as has been suggested recently 3. O n the basis of o u r AES, X P S a n d X R D results a n d available p h a s e d i a g r a m s , we p r o p o s e t h a t the phase f o r m a t i o n from p a r e n t V 2 0 5 within the c o m p o s i t i o n range V 2 O s - V z O 4 s h o u l d be as follows: V205

,V307

,V409

, V6013

,W204

(6)

This r e d u c t i o n sequence is consistent with the m o s t recent 3 t h e r m o d y n a m i c c a l c u l a t i o n s of v a n a d i u m oxides. TABLE VI HEATS OF FORMATION~H298 AND ENTROPIES$298 FOR DIFFERENTOXIDESOF VANADIUM

Compound V VO VO~.24 V20 3 V30 5 V40 7 VsO 9 V601 ~ V7013

VsO 15 V204 V6013 V30 7 V20 5

AH~98(kcalmo I 1) - 103.27_+ 1.10 125".18_+1.40 -291.55_+0.63 -464.31 _+0.90 - 636.62 _+1.30 -807.93_+ 1.70 979.15 _+2.00 -- 1149.96_+2.30 - 1320.78 _+2.60 341.22_+ 1.40 - 1061.30 _+3.01 -556.38 - 371.24 _+2.30

S298(calmo I t K i) 7.05 8.03_+0.10 9.39_+0.1/) 23.51 _+0.30 37.02_+ 1.00 50.53 _+0.20 61.84_+ 1.50 77.05 _+2.00 84.05_+4.00 96.06 _+4.00 23.11 _+0.30 69.24 _+4.00 31.32 _+0.30

7. CONCLUSIONS A series of thin film v a n a d i u m oxides have been f o r m e d at t e m p e r a t u r e s between 400 a n d 600 °C u n d e r a t m o s p h e r i c pressure of u t r a h i g h p u r i t y of o x y g e n for p e r i o d s of 3 - 2 4 h. C h e m i c a l c o m p o s i t i o n , phase identification a n d structure of oxides of v a n a d i u m in the range V O 2 - V 2 0 5 have been d e t e r m i n e d using AES, XPS, X R D , S E M , optical reflectance a n d t h e r m o d y n a m i c calculations. A t t e n t i o n has been focused on V 2 0 5 thin film g r o w t h because of its unique c a t a l y t i c a n d optical properties. An X R D m o r p h o l o g i c a l factor of 0.94 a n d the o b t a i n e d lattice p a r a m e t e r s (a = 11.527 ,~, b = 3.567/~, a n d c = 4.374/~) confirm t h a t single-crystal V 2 0 s film has been g r o w n on the v a n a d i u m substrate. U V - v i s i b l e reflectance s p e c t r a of the films have revealed a visible b a n d g a p of a b o u t 2.38_+0.07 eV for

THIN FILM V OXIDES

267

V2Os. Ultrahigh vacuum thermally induced reduction of V2Os to lower oxide(s) V6013 and/or V409 has also been observed. Room temperature heats of formation AH~98 and entropies S~gs of 14 solid oxide phases of vanadium in the compositional range VO-V205 were obtained. ACKNOWLEDGMENTS

The authors would like to thank Y. Y. Sun for carrying out the XRD measurements. Support for this work by the Department of Energy-Solar Energy Research Institute, the R. A. Welch Foundation and the National Aeronautics and Space Administration is gratefully acknowledged.

REFERENCES 1 K. Kosuge, J. Phys. Chem. Solids, 28 (1967) 1613. 2 L. Fiermans, P. Clauws, W. Lambrecht, L. Vandenbroucke and J. Vennik, Phys. Status Solidi A, 59 (1980) 485. 3 L. Brewer and B. Ebbinghaus, Thermochim. Acta, 129 (1988)49. 4 S. Berglund, Acta Chem. Scand. A, 34 (1980) 702. 5 H. Kuwamoto, N. Otsuka, and H. Sato, J. Solid State Chem., 36 (1981) 133. 6 J.S. Anderson and B. G. Hyde, J. Phys. Chem. Solids, 28 (1967) 1393. 7 A . K . Cheetham, in O. T. Sorensen (ed,), Nonstoichiometric Oxides, Academic Ptress, New York, 1981. 8 M. Akimoto, M. Usami and E. Echigoya, Bull Chem. Soc. Jpn., 51 (1978) 2195. 9 A.D. Wadsley, in L. Mandelcorn (ed.), Nonstoichiometric Compounds, Academic Press, New York, 1964. 10 R. Dziembaj, J. SolidState Chem., 26 (1978) 167. I 1 A. Miyamoto, A. Hattori and Y. Murakami, J. Solid State Chem., 47 (1983) 373. 12 M.N. Colpaert, P. Clauws, L. Fiermans and J. Vennik, Surf Sci., 36 (1973) 513. 13 H. Poelman, J. Vennick and G. Dalmai, Phys. Status Solidi A, 107 (1988) 731. 14 L. Fiermans, L. Vandenbroucke, R. V. Berghe and J. Vennik, J. Microsc. Spectrosc. Electron., 4 (1979) 543. 15 G.L. Simard, J. F. Steger, R. J. Arnott and L. A. Siegel, Ind. Eng. Chem., 47 (1955) 1424. 16 F. Roozeboom, A. J. Van Dillen, J. W. Geus and P. J. Gellings, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981)304. 17 A. Miyamoto, K. Mori, M. Miura and Y. Murakami, in M. Che and G. C. Bond (eds.), Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, p. 371. 18 T.M. Riceand D. B. McWhan, IBMJ. Res. Dev., 14(1970)251. 19 F. Nava, O. Bisi, P. Psaras, H. TakaiandK. N. Tu, ThinSolidFilms, 140(1986) 167. 20 A. Suli, M. I. Torok and 1. Hevesi, Thin Solid Films, 139 (1986) 233. 21 J.T. Hogan and J. F. Clarke, in H. Wiedersich (ed.), Surface Effects in Controlled Fusion, NorthHolland, Amsterdam, 1974. 22 A.C. Klein and D. K. Sze, Fusion Technol., 10 (1986) 747. 23 F.J. Szalkowski and G. A. Somorjai, J. Chem. Phys., 56 (1972) 6097. 24 C.R. Brundle, Surf Sci., 52 (1975) 426. 25 R.J. Colton, A. M. Guzman and J. W. Rabalais, J. Appl. Phys., 49 (1978) 409. 26 C . N . R . Rao and D. D. Sarma, J. Mol. Struct., 79 (1982) 177. 27 A. Andersson, in M. Che and G. C. Bond (eds.), Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, p. 381. 28 G . A . Somorjai, Chemistry in Two Dimensions: Surfaces, Cornell University Press, Ithaca, NY, 1981.

268

29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

A.z.

MOSHFEGH, A. IGNATIEV

D. Briggs and M. P. Seah, Practical SutJaee Analysis hv Auger and X-ray Photoelectron Spectroscopy, Wiley, New York, 1983. G. Ertl and J. Kuppers, Low Energy Electrons and Sur/aee Chemistry, Verlag Chemic, Weinheim, 2rid edn., 1985. L. FiermansandJ. Vennik, Su~ji S¢i.,35(1973)42. S. Mroczkowski and D. Lichtman, J. Vac. Sci. Tectmol. A, 3 (1985) 1860. L.E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. W. Weber, Handbook q/Auger Electron Speetroseopy, Physical Electronics Industries, Eden Prairie, M N, 1976. R. Larrson, B. Folkesson and G. Schon, Chem. Scr., 3 (19733 88. G . A . Sawatzky and D. Post, Phys. Rer. B, 20 (1979) 1546. D. Wruck, S. Ramamurthi and M. Rubin, Thin Solid Films, 182 ( 19893 79. J . H . Scofield, J. Electron. Speetrose., 8 ( 19763 129. C . D . Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and C. E. Muilenberg, Handbook o/X-ray Photoelectron Spectroscopy, Physical Electronics Industries. Eden Prairie, M N, 1976. Joint Committee on Powder Diffraction Standards, A S T M Powder D([lkaetion File, 1980, Card 9387. L. Feirmans and J. Vennik, SutTll Sci., 9( 19683 187. J. Ziolkowski and J. Janas, J. Calal., 81 (1983) 298. F. Vratny and F. Micale, Faraday Soc. "l'ran.~.,59 (1963) 273t,~. H . G . Bachmann, F. R. Ahmad and W. H. Barnes, Z. Kris'tallogr., 115 ( 1961 ) 110. L. Fiermans, R. Hoogewijs and J. Vennik, Sutjl Sci., 47 ( 197531. N. Kenny, C. R. Kannewurfand D. H. Whitmore, J. Phys. Chem. Soli~£', 27 t 19663 [ 237. V.G. Mokerov and B. L. Sigalov, Sot,. Phvs Solid State, 14 (1973) 2875. P. Clauws and J. Vennik, Phys. Status Solidi B, 66 (19743 553. S.F. Cogan, N . M . Nyugen, S.J. P e r r o t t i a n d R . D. Rauh, J. Appl. Phys.,66(I989) 1333. V.A. loffe and I. B. Patrina, Phys. Status Solidi, 40 ( 19703 389. D . K . Chakrabarty, D.. Guha and A. B. Biswas, J. Mater. Sei., l l ( 19763 1347. A.Z. Moshfegh and A. Ignatiev, Catal. Lett., 4 (1990) I 13. A.Z. Moshfegh and A. Ignatiev, J. Chem. Phys., in the press. P. Clauws, J. Broeckx and J. Vennik, Phys. Status Solidi B, 131 ( 19853 459. W. Lambrecht, B. Djafari- Rouhani, M. Lannoo and J. Vennik, J. Phys. C. 13 (1980) 2485, 2503. W. Lambrecht, B. Djafari-Rouhani and J. Vennik, Suljl Sei., 126 (1983) 558. N. Van Hieuand D. Lichtman, J. Vae. Sei. Teehnol., 18(1981)49. A.A. Vinogradov and A. I. Shelykh, Soy. Phys. Solidi State, 13 ( 19723 2781. J . H . Perlstein, J, Solid State CTwm., 3 ( 1971 ) 2 l 7. J.B. Goodenough, J. Solid State Chem., 1 ( 19703349. N.P. Allen, O. Kubaschewski and O. Goldbeck, J. Eleetrochem. Soe., 9~ ( 1951 ) 417. R . D . Rossini, D. D. Wagman, W. H. Evans, S. Levine and I. Jail'e, Natl. Bur. Stand ( U.S. ), Circ., 500 (1952) 1268. A.D. Mah and K. K. Kelley, Heats and Free Energies ~?lFormation olOxides ol Vanadium, I.,'.S. Bur. M#ws, Rep. hwest., Rep. No. 5858(I 9613 I. G.V. Samsonov, The O.vi~h, Handbook, I FI Plenum, New York, 2nd edn., 1982.