Effects of oxygen partial pressure on structural and gasochromic properties of sputtered VOx thin films

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Effects of oxygen partial pressure on structural and gasochromic properties of

sputtered VOx thin

ﬁlms

Wei-Luen Jang

a

, Yang-Ming Lu

b

, Ying-Rui Lu

a,c

, Chi-Liang Chen

d

, Chung-Li Dong

a,

⁎

, Wu-Ching Chou

e

,

Jeng-Lung Chen

a

, Ting-Shan Chan

a

, Jyh-Fu Lee

a

, Chih-Wen Pao

a

, Weng-Sing Hwang

f

a

National Synchrotron Radiation Research Center (NSRRC), Hsinchu 30076, Taiwan

b

Department of Electrical Engineering, National University of Tainan, Tainan 70005, Taiwan

c

Program for Science and Technology of Accelerator Light Source, National Chiao Tung University, Hsinchu 30010, Taiwan

d

Institute of Physics, Academia Sinica, Taipei 11529, Taiwan

e_{Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan} f

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan

a b s t r a c t

a r t i c l e i n f o

Available online 5 March 2013 Keywords:

VOxﬁlm Gasochromic H2sensor

VOxfilms were deposited by radio-frequency reactive magnetron sputtering from a vanadium target in an Ar–O2gas mixture and pure O2. For thefilms deposited in the gas mixture, the Ar flow rate was controlled at 20 sccm and the oxygenflow rate was controlled at 1, 3, and 5 sccm, respectively. A thin (~5 nm) Pt layer was deposited on the VOx thin films as a hydrogen catalyst. The long-range structural order, short-range atom arrangement, and gasochromic properties of the depositedfilms were studied. The grazing incidence X-ray diffraction (GIXRD) results indicate that the depositedfilms are amorphous. Lamellar struc-tures were found at oxygenflow rates of 3 sccm and above. The X-ray absorption spectroscopy (XAS) results show that the short-range atom arrangement of the lamellar VOx thinfilms is similar to that of crystal V2O5. The GIXRD and XAS results show that thefilm obtained with the gas mixture and at an oxygen flow rate of 1 sccm did not significantly change after exposure to hydrogen, whereas the other films exhibited decreased interlayer distance, oxidation state, and crystallinity. The color of thefilms changed from light or deep yellow to gray. The results suggest that the gasochromic properties of the VOx thinfilms are related to the V2O5-like atom arrangement and the interlayer distance of the lamellar structure. Thefilms deposited with an oxygen flow rate of 3 sccm and above can be applied to H2gas sensors.

1. Introduction

Hydrogen is considered as a candidate alternative energy source. However, hydrogen tends to explode in air when its concentration is above 4%[1]. The development of hydrogen sensors is thus impor-tant for hydrogen energy applications. Hydrogen sensors based on

the gasochromic properties of metal oxide thin _{ﬁlms have been}

proposed[1–5]. The ﬁlms change color and oxidation state when exposed to hydrogen. This type of sensor is safer than that based on the measurement of electrical conductivity since the sensor does not introduce any additional ignition source.

V2O5thinﬁlms have been studied for application in H2sensors

[1–5]. A thin layer of palladium (Pd) or platinum (Pt) is deposited on theﬁlm surface to act as a catalyst to dissociate hydrogen mole-cules. The hydrogen atoms diffuse into the V2O5layer and cause a

change in the optical transmittance. Although the related phenomena have been reported, the change in the electronic structure and atomic

structure during the coloration are not fully understood yet. In addi-tion, the long-range structural order and the short-range atom ar-rangement of amorphous vanadium oxides prepared by sputtering deposition have not been reported to date. In this study, VOx thin ﬁlms are prepared by radio-frequency (RF) reactive magnetron sputtering. The effects of oxygen partial pressure onﬁlm structure, oxidation state, and gasochromic properties are studied.

2. Experimental methods

VOxﬁlms were deposited on a Corning 1737 substrate using an RF reactive magnetron sputtering system with a vanadium (V) target. The working power wasﬁxed at 150 W and the sputtering deposition was performed at a gas pressure of 1.33 Pa in pure O2(sample sO)

and an Ar–O2gas mixture. For the Ar–O2gas mixture, the Arﬂow

rate was controlled at 20 sccm and the oxygenﬂow rate was

con-trolled at 1 (s1), 3 (s3), and 5 (s5) sccm, respectively. The_ﬁlm thick-ness of the depositedﬁlms was controlled at 200 nm. The sputtering time for s1, s3, s5, and sO were 15, 40, 62, and 125 min, respectively.

A thin (~5 nm) Pt layer was deposited on the VOx thinﬁlms as a

Thin Solid Films 544 (2013) 448–451

⁎ Corresponding author. Tel.: +886 3 5780281x7106; fax: +886 3 5789816. E-mail address:[email protected](C.-L. Dong).

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hydrogen catalyst. In the gasochromic reaction, theﬁlms were ex-posed to hydrogen at 1 atm and 25 °C for 2 h. The crystal structure

of the deposited _{ﬁlm was measured by grazing incidence X-ray}

diffraction (GIXRD). The oxidation state and the surroundings of the vanadium atoms were measured by X-ray absorption spectroscopy (XAS), including X-ray absorption near-edge structure (XANES) spectroscopy and extended X-ray absorptionﬁne structure (EXAFS) spectroscopy. The absolute energy positions were calibrated by V (5465 eV) metal foil. The V K-edge spectra were collected in the

ﬂuo-rescence mode and the angle between the X-ray beam and theﬁlms

was 45°. The XAS measurements were conducted at wiggler beamline BL17C at the National Synchrotron Radiation Research Center, Hsinchu, Taiwan.

3. Results and discussion

Fig. 1shows the GIXRD patterns of the as-deposited VOx thinfilms before (black lines) and after gasochromic reaction (red lines). No dif-fraction peaks appear in s1, whereas one broad peak at ~ 6° appears in the otherfilms. The broad peak slightly shifts to lower angles with in-creasing oxygenflow rate during deposition. After gasochromic reac-tion, the peak position shifts to a higher angle, and the peak intensity markedly decreases. The GIXRD patterns change most for thefilm de-posited in pure O2. The GIXRD patterns of s1 remain unchanged after

gasochromic reaction. The insets show photographs of theﬁlms be-fore and after gasochromic reaction. s1 is initially black, and almost unchanged after gasochromic reaction. s3 and s5 are initially light yellow, becoming gray after gasochromic reaction. sO changes from deep yellow to dark gray after gasochromic reaction.

The GIXRD results indicate that the structure of the depositedﬁlms is amorphous and that the broad peak for s3, s5, and sO is related to the (001) peak of lamellar materials[6]. According to the reported liter-ature[6], the XRD pattern of V2O5·1.6H2O xerogel shows (001), (003),

(004), and (005) peaks. The (00l) peaks indicate that theﬁlm is com-posed of ribbon-like species and has lamellar ordering. Also, it is consid-ered as an intercalation compound, in which there is a reversible intercalation of mobile guest species (atoms, molecules, or ions) into a crystalline host lattice. V2O5·1.6H2O disappears and V2O5·0.5H2O

forms when theﬁlm is heated. After this transformation, the (003), (004), and (005) peaks almost disappear and the intensity of (001) dramatically decreases. The diffraction pattern of V2O5·0.5H2O

also suggests a lamellar structure, in which the interlayer distance is shorter than that of V2O5·1.6H2O (dV2O5·1.6H2O= 11.57 Å;

dV2O5·0.5H2O= 8.75 Å). In this study, only the (001) peak was

found. The interlayer distances of s3, s5, and sO obtained from the Bragg's law are 14.2, 14.4, and 14.7 Å, respectively. The results indi-cate that the sputtered_{films exhibit a lamellar structure. The initial} color of the as-depositedfilms is determined by the oxygen content in thefilms which is controlled by the introduced oxygen flow rate during deposition. After gasochromic reaction, the changes in the GIXRD patterns indicate that the interdiffusion of hydrogen atoms reduces the interlayer distance and disorders the lamellar structure. According to the GIXRD patterns and photographs, s3, s5, and sO can be applied to H2gas sensors. Also, a greater interlayer distance

ben-eﬁts the gasochromic reaction.

It is known that GIXRD provides an average structure of the crystalline phase with a long-range structural order, whereas XAS is

atom-speciﬁc and capable of probing the short-range structure

around the targeted atom. Due to the amorphous nature of the ﬁlms, their structural information and oxidation state cannot be obtained fromFig. 1.Fig. 2shows the V K-edge spectra of the depos-ited VOx thinﬁlms and the standards (VO2and V2O5). Insets (a) and

(b) respectively show the pre-edge region and the main absorption edge area of the depositedﬁlms and the standards. The VO2standard

was obtained via the two-stage thermal annealing of sO. sO wasﬁrst annealed at 360 °C in a 90% Ar–10% H2mixture for 3 h. Theﬁlm was

then annealed at 500 °C in pure N2for 1 h. The V2O5standard was

obtained by annealing sO at 500 °C in air for 2 h. The crystalline

struc-tures of the standards were conﬁrmed by GIXRD. The results are

shown inFig. 3. The V K-edge features of the standards are consistent with those reported in the literature[7,8].

InFig. 2, pre-edge peak A is due to the formally forbidden 1s→ 3d electronic transition, which is dipole-allowed if the full local octahedral symmetry is decreased[7,8]. The crystal structure of VO2is monoclinic,

which can be regarded as a distorted rutile structure[8]. The vanadium atoms are six fold-coordinated by oxygens and are displaced from the center of the octahedron. In the V2O5crystalline, the vanadium atoms

are placed in skew pyramidal oxygen coordination with one additional

Fig. 1. GIXRD patterns of as-deposited VOx thinﬁlms before (black lines) and after (red lines) gasochromic reaction. The insets show photographs of theﬁlms before (left) and after (right) gasochromic reaction.

Fig. 2. V K-edge spectra of as-deposited VOx thinﬁlms and standards (VO2and V2O5).

The insets show (a) pre-edge region and (b) main absorption edge area of theﬁlms. 449 W.-L. Jang et al. / Thin Solid Films 544 (2013) 448–451

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axial oxygen atom at a much further distance[7]. The geometry leads to a higher distorted octahedral coordination sphere for the vanadi-um atoms. The pre-edge peak intensity is affected by the vertical symmetry of apical V\O bonds in V2O5ﬁlms[9]. Peak B corresponds

to the resonances of the photoelectrons which is due to transitions from core levels to non-binding levels in the continuum. The peak is greatly affected by the symmetry of the four basal oxygens sur-rounding the vanadium sites[9,10].

The oxidation state of vanadium can be determined from the XANES spectrum, including the energy of pre-edge peak and the main absorption edge[8]. As shown in inset (a) inFig. 2, the energy position of the pre-edge peak of s1 is close to that of the VO2standard

and those of the others are close to that of the V2O5standard.

Like-wise, the main absorption edge (inset (b)) of s1 almost overlaps that of the VO2standard and those of the otherﬁlms are shifted to

high photon energies close to the V2O5standard. The oxidation states

of the VO2and V2O5 are + 4, and + 5, respectively. Consequently,

both results indicate that the oxidation state of s1 can be determined as + 4, and those of the others are slightly lower than + 5.

Further observation of inset (a) indicates that the pre-edge inten-sities of s3, s5, sO, and V2O5standard are comparable and the overall

absorption spectrum appear like that of amorphous V2O5prepared by

rapid quenching[10]and the sol_{–gel method}[9]. Moreover, the local atomic structure of amorphous V2O5has a great resemblance to that

of crystal V2O5[9,10]. As a result, the short-range atom arrangements

of s3, s5, and sO are close to that of crystal V2O5. The difference in

pre-edge peak intensity and features of edge resonance between the depositedﬁlms (s3, s5, and sO) and crystal V2O5are related to the

less split apart of V\O apical bonds[9]and the less distorted arrange-ment of the four basal oxygens[10].

EXAFS is aﬁnal state interference effect involving the scattering of outgoing photoelectrons from neighboring atoms. The structure parameters can be obtained by converting the photon energy into photoelectron wavevectorκ[11]. Theκ3_{-weighted EXAFS were}

stud-ied to further investigate the short-range atom arrangements of the ﬁlms.Fig. 4shows theκ3_{-weighted EXAFS at the V K-edge of the}

films. The figure clearly shows that the EXAFS oscillation of s1 is far from those of the otherfilms and the standards, whereas those of s3, s5, sO, and the V2O5standard are similar. The results confirm the

similarity in the short-range atom arrangement of s3, s5, sO, and crystal V2O5. The difference found at κ = 7.5 Å−1 is due to the

modiﬁcation of the V\O bonds mentioned above and can be used

to distinguish amorphous and crystal V2O5. This result agrees with

the earlier reports[9,10].

Fig. 5shows the V K-edge spectra of the deposited VOx thinﬁlms be-fore and after gasochromic reaction. The inset shows theκ3_-weighted

EXAFS curves of sO before and after gasochromic reaction. The spectrum

is almost unchanged in s1 before and after gasochromic reaction. However, the shift of the main absorption edge to low photon energy accompanies the reduction of pre-edge intensity are observed in s3, s5 and sO after gasochromic reaction. The results indicate that s1 has weaker reaction with hydrogen atoms compared to other which corre-spond with the poor gasochromic coloration displayed by s1 (inset in Fig. 3. GIXRD patterns of (a) VO2and (b) V2O5.

Fig. 4.κ3

-weighted EXAFS curves at the V K-edge of as-depositedﬁlms and standards (VO2and V2O5).

Fig. 5. V K-edge spectra of as-deposited VOx thinﬁlms before (black) and after (red) gasochromic reaction. The inset shows theκ3

-weighted EXAFS curves of sO before (black) and after (red) gasochromic reaction.

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Fig. 1). Theﬁlms with a V2O5-like atom arrangement and lamellar

struc-ture beneﬁt the gasochromic reaction. The changes in K-edge absorp-tion spectra suggest that the gasochromic reacabsorp-tion is strongly related to a decrease in the oxidation state of vanadium and accompanied by an increase in octahedral symmetry.

As shown in the inset, the EXAFS oscillations of sO are disordered, becoming more coherent in theκ range of 4–10 Å−1_{. It is known that}

the EXAFS oscillations of a crystal V2O5are composed of many sine

waves and each of them corresponding to a V\O bond with the

same distance. The coherence of EXAFS oscillations implies an equal-ization of V\O distance after gasochromic reaction, which is consis-tent with the results reported for Li intercalated V2O5xerogel[12]

and LixV2O5bronzes[13]. The gasochromic reaction of VOxﬁlms in

this study can be pictured as intercalated H atoms reducing the oxidation state of vanadium and the discrepancy in V\O distances. The geometry change then reduces the interlayer distance and disor-ders the lamellar structure, as shown inFig. 1.

4. Conclusion

The local atomic structural and gasochromic properties of sputtered

VOx thinﬁlms were studied by GIXRD and XAS. Films deposited at

150 W on an unheated substrate were amorphous. Lamellar structures were found infilms deposited at oxygen flow rates of 3 sccm and above. The XAS results indicate that the short-range atom arrangements of the lamellar VOxfilms are similar to that of crystal V2O5. The gasochromic

reaction of VOxﬁlms results from the intercalation of hydrogen atoms reduces the oxidation state of vanadium and the discrepancy in V\O bond distances. The changes in the oxidation state and V\O bond dis-tance are responsible for the coloration and structural change shown in GIXRD, respectively. The gasochromic properties of the VOx thin

ﬁlms were found to be correlated with the short-range atom ar-rangement and the interlayer distance of the lamellar structure. The V2O5-like atomic arrangement and a great interlayer distance

beneﬁt the gasochromic properties of sputtered VOx.

Acknowledgments

This study wasﬁnancially supported by the National Science

Council of Taiwan under grants NSC 98-2112-M-213-006-MY3, 99-2112-M-001-036-MY3, 99-2221-E-024-003, and 97-2221-E-006-006-MY3.

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