Study on selective adsorption of deuterium on boron nitride using photon-stimulated ion-desorption

Applied Surface Science 258 (2011) 1561–1564

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Study on selective adsorption of deuterium on boron nitride using photon-stimulated ion-desorption Kaveenga Rasika Koswattage a,b,∗ , Iwao Shimoyama a , Yuji Baba a , Tetsuhiro Sekiguchi a , Kazumichi Nakagawa b a b

Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan Graduate School of Human Development and Environment, Kobe University, Nada-ku, Kobe 657-8501, Japan

a r t i c l e

i n f o

Article history: Received 11 July 2011 Received in revised form 22 September 2011 Accepted 29 September 2011 Available online 6 October 2011 Keywords: Site-selective adsorption Hexagonal boron nitride (h-BN) Deuterium Photon-stimulated ion desorption (PSID) Near edge X-ray absorption fine structure (NEXAFS)

a b s t r a c t Adsorption behavior of atomic deuterium on a hexagonal boron nitride (h-BN) thin film is studied by photon-stimulated ion desorption (PSID) of D+ and near edge X-ray absorption fine structure (NEXAFS) at the B and N K-edges. After the adsorption of atomic deuterium, D+ desorption yield (h) shows clear enhancement at the B K-edge and almost no enhancement at the N K-edge. NEXAFS spectra show a large change in the B K-edge and a small change in the N K-edge after the adsorption. We propose selective adsorption of atomic deuterium on the h-BN thin film based on the experimental results, and mention the effectiveness of applying the PSID method with X-ray to study hydrogen storage materials. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Boron nitride nanotubes (BNNT) have attracted much research attention as a promising candidate for hydrogen storage. Several studies have revealed that the BN materials may be the best media other than carbon materials for storing hydrogen [1–5]. Extensive theoretical research works have been dedicated to understanding the adsorption of atomic hydrogen on BN materials [6–12]. Despite the great attention paid to this subject, it is not yet clear how the adsorption mechanism works on BN materials. One of the most basic arguments of these theoretical studies is the site dependence of atomic hydrogen adsorption. However, there are a number of theoretical reports whose conclusions contradict each other [6–12]. It is difficult to apply experimental methods to study the interaction between hydrogen and BNNT or other BN nanomaterials, because synthesis and separation methods for these materials have not been established. Thus, in this research, a thin film of hexagonal boron nitride (h-BN) on a Ni(1 1 1) substrate is selected for the investigation. It has been reported that epitaxial and commensurate h-BN thin film is formed on Ni(1 1 1) by chemical vapor

∗ Corresponding author. Tel.: +81 292843929; fax: +81 292825832. E-mail addresses: [email protected], [email protected] (K.R. Koswattage). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.133

deposition (CVD) method [13]. Since defect-free and high quality thin film is obtained on Ni(1 1 1) [14], we can avoid influence by defect sites on adsorption behavior using this system. Chemisorption of atomic hydrogen has already been reported for graphite as a model system of carbon nanotubes with regard to the degree of hydrogenation and the preferable adsorption structure. The research works in which the chemisorptions was achieved used X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and thermal desorption spectroscopy (TDS) [15,16]. However, NEXAFS and XPS spectroscopic methods are not direct methods of detecting hydrogen from the surface. Even though hydrogen can be detected directly using TDS, it is hard to examine the site selectivity. In the present work, we suggest that photon-stimulated ion desorption (PSID) can be employed to study hydrogen adsorption sites on a BN film. Since core excitation by a photon causes Auger decay and subsequent ion desorption at the excited state [17], we believe that site-specific adsorption behavior can be investigated by observing the desorbed ions. A similar attempt was first reported by Petravic et al. [18]. They compared the PSID yield of H+ from a hydrogenated GaAs(1 0 0) surface as a function of photon energy around As 3d and Ga 3p core excitations and reported the site-selective adsorption of hydrogen. Here, 1 s core excitation is focused for both B and N atoms to study the site selectivity of BN. We also performed NEXAFS spectroscopy to study the modification

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of the electronic structure of the BN film by hydrogenation. We use deuterium instead of hydrogen, which allows us to distinguish the adsorbed deuterium from the hydrogen that comes from the water adsorbates. Finally, we report a comparison of PSID yields of D+ from the B and N sites for a deuterated BN film.

3. Results and discussion First, we confirmed the existence of deuterium from the TOF spectrum of the BN film after atomic deuterium exposure by irradiating with a soft X-ray of 192.1 eV, which corresponds to the excitation energy of the most intense ␲* peak in the B K-edge region. Both the dominant H+ signal and the relatively low D+ signal were obtained in the spectrum as shown in Fig. 1. The same deuterated film was annealed at 473 K, and the peak intensity of H+ was smaller than that of D+ after the annealing. Thus, the persistence of the D+ signal after the annealing can be attributed to a chemisorption-like interaction between the D atoms and the BN film. We believe the origin of the H+ signal is the water molecules that were physisorbed on the BN film. The top and bottom of Fig. 2 show the B and N K-edge NEXAFS spectra of the deuterated sample, respectively. Broken and solid curves show the results before and after the exposure of atomic

Intensity (arb. unit)

All the experiments were performed at the BL-11A beam line of the Photon Factory in the High Energy Accelerator Research Organization (KEK-PF). A h-BN thin film was prepared on Ni(1 1 1) by dehydrogenation of borazine in an ultra high vacuum (UHV) chamber [13]. The Ni substrate was heated at 1073 K and exposed to borazine gas with the pressure of 1.3 × 10−4 Pa and the exposure time of 100 s. Composition and thickness of the h-BN thin film ˚ respectively, from the meawere estimated to be 0.98 and 6.6 A, surement of X-ray photoelectron spectroscopy [19]. We exposed the h-BN thin film to atomic deuterium produced by the tungsten hot filament method [20]. The filament temperature was around 2100 K and the typical pressure of deuterium was 1 × 10−4 Pa. NEXAFS spectra were measured with the total electron yield method before and after the exposure of atomic deuterium. The X-ray incidence angle  was defined as the angle between the electric field vector of the linearly polarized X-ray and the surface normal of the film. The experimental details are described elsewhere [19]. We used the time-of-flight (TOF) method for PSID measurements, because the TOF method has very high detection efficiency for D+ ions and can easily distinguish H+ and D+ signals. All the TOF experiments were performed with single bunch operation, in which soft X-ray pulses were emitted with a period of 624 ns and a width of 100 ps. The UHV chamber was equipped with a TOF mass spectrometer (TOF-MS) with a fixed angle of 45◦ with respect to the X-ray beam. The details of the spectrometer are described elsewhere [21]. The TOF of an ion is proportional to (m/q)1/2 , where m and q are the mass and charge of the ion, respectively. A reference in the time scale was determined with a prompt peak corresponding to the scattered light from the sample surface. The TOF was measured using a time-to-amplitude converter (TAC, ORTEC Model 567). TOF spectra were measured with various photon energies in both the B and the N K-edge regions. The dwell time of one TOF spectrum measurement was 2500 s. Desorption yield (h) was estimated from the D+ peak intensities in the TOF spectra. The background signal from the noise of the TOF-MS was subtracted from the D+ peak in the TOF spectra to obtain (h). We normalized (h) by the detection yield of the TOF-MS and the intensity of the incident photon flux. The drift of the detection yield was estimated from the intensities of background signals in TOF spectra.

After annealing

+

D

250

300

350

400

Time of flight / ns Fig. 1. TOF-PSID spectra of the deuterated BN film before and after annealing at 473 K.

deuterium, respectively. All the spectra were measured at the magic angle ( = 54.7◦ ) in which polarization dependence is cancelled out [22]. While a clear change was observed in the B K-edge NEXAFS spectrum, only a slight change was observed in the N K-edge NEXAFS spectrum after the exposure. This means that the modification of the electronic structure was larger at B sites than at N sites. We analyzed these results with density functional theory (DFT) calculations and suggested that B sites were preferentially adsorbed by

Photon Energy / eV 184

188

192

196

Before After

2

1

NEXAFS Intensity (arb. units)

2. Experimental

Before annealing

+

H

0 2 Before After

1

0 395

400

405

410

Photon energy / eV Fig. 2. NEXAFS spectra in B K-edge (top) and N K-edge (bottom) before (solid line) and after (dotted line) atomic deuterium exposure.

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184

188

192

196

D+ desorption yield η(arb. unit)

160

120

80

40

0

395

400

405

410

Photon Energy / eV Fig. 3. Filled and open diamonds represent the PSID yield () spectra for D+ ions in the B and N K-edges after atomic deuterium exposure, respectively. Broken curves were obtained by smoothing of every eight points of (h).

atomic deuterium [19]. However, strictly speaking, these findings are indirect evidence of selective adsorption. Besides the deuterium adsorption, there have been reported that oxidation of BN can also influence the B K-edge NEXAFS spectra [23]. According to the report, when BN materials are oxidized BN materials show some discrete peaks assigned to B–O bonds above the dominant ␲* peak at 192 eV. Consequently, one might think to assign the spectral change in B K-edge NEXAFS spectra after atomic deuterium exposure due to the formation of B–O bonds when BN film was oxidized. However, with regard to the influence of oxidation, we always checked XPS after every after the deuterium exposure and could not observe O 1s photoelectron and O KVV Auger peaks. We also measured O K-edge NEXAFS, but we did not observe clear signal increase at the O K-edge. These results suggest amount of oxygen was negligible on our BN film. Furthermore, in here, B K-edge NEXAFS spectra showed only a broad shoulder and no additional discrete peaks around 194 eV. In addition, we have estimated the [B]/[N] ratio of the BN film before and after atomic deuterium exposure as 0.98 and 0.96, respectively. This discrepancy can be assigned due to estimation error. If the B K-edge NEXAFS showing B–O related peak above the dominant ␲* peak when BN film was oxidized, we should have observed a large discrepancy of the [B]/[N] ratio before and after the deuterium exposure. Thus, these points are strong enough to suggest that existence of B–O bonds was negligible in the NEXAFS of our BN film. Fig. 3 shows the (h) of D+ ions at the B and N K-edges by filled and white diamonds, respectively. Broken curves were obtained by smoothing of every eight points of (h). In Fig. 3, while (h) showed a clear increase at the B K-edge, (h) was flat at the N Kedge. To interpret these results, we discuss primary and secondary effects that cause PSID. With regard to the primary effect in which core excitation is directly concerned, the Auger stimulated desorption (ASD) model is universally recognized as the most popular model [24]. This model takes account of coulomb repulsion induced by Auger decay and the repulsive potential of the Auger final state. In the case in which the primary effect is dominant, (h) and NEXAFS spectra have different shapes, because (h) can be enhanced by excitations to an unoccupied state that has strong anti-bonding character [25–27]. On the other hand, ion desorption is also caused by the secondary effect that comes from excitations by secondary electrons, i.e., Auger and scattered electrons. In the case in which the secondary effect is dominant, (h) resembles NEXAFS, because the number of secondary electrons emitted from near the surface is proportional to the absorption intensity.

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According to our DFT calculations, multiple B–D anti-bonding states are located in the modified components observed in the B Kedge NEXAFS spectrum in Fig. 2 [19]. In that case, the enhancement of the (h) at the B K-edge may be partially caused by the primary effect at the B–D anti-bonding states. However, there is no B–D anti-bonding state at 192 eV in the B K-edge NEXAFS spectrum [19]. Nevertheless, (h) increased around 192 eV in the B K-edge. The spectrum of (h) basically follows the corresponding NEXAFS, and we could not observe a clear difference beyond the statistical error between the (h) and NEXAFS spectra. We believe that the primary effect was not large, if it occurred at all, and the secondary effect would be the main cause of the increase of (h) at the B K-edge. Fig. 3 clearly shows that (h) was larger in the B K-edge region than in the N K-edge region by ca. three times. This difference can be explained well by the difference in the number of secondary electrons emitted from ionization of valance bands of the BN film and Ni substrate in the energy ranges at the B and N K-edges. We estimated the relative amount of secondary electrons from the total electron yield intensity, I(h), before the subtraction of the background signal that mainly came from the Ni substrate, and obtained the ratio of I(190 eV)/I(400 eV) = 2.8. This result also supports the interpretation that the D+ ion desorption was mainly caused by the secondary effect. Since the secondary effect reflects the number of secondary electrons, the edge jump is an important factor for assessing the magnitude of the secondary effect. We compared I(h) below and above the absorption edge and obtained I(192 eV)/I(190 eV) = 1.2 in the B K-edge and I(406 eV)/I(400 eV) = 1.1 in the N K-edge. This means that a similar increase would appear for (h) at both the B and the N K-edges if N sites were deuterated as well as B sites. However, (h) did not show a clear increase at the N K-edge. This suggests that the number of N sites that adsorbed deuterium was smaller than that of B sites. At a B site that adsorbed deuterium, the influence of secondary electrons emitted from the B site would be larger than that of the other secondary electrons. A B–D bond dissociation and following D+ desorption could also be caused by the secondary electrons emitted from other N core excitations; nevertheless, the influence would be suppressed, because the excited N sites are away from the adsorption site. We think this is why enhancement of (h) was suppressed at the N K-edge. One might think the intense (h) at 403.0 eV in the N K-edge in Fig. 2 originated from the enhancement of (h) by the primary effect. However, our DFT calculations clarified that no N–D antibonding state exists around the energy [19]. Since the spectrum of (h) is quite different from the NEXAFS in the N K-edge region, it is hard to regard the intense datum at 403.0 eV as the result of the secondary effect. These results support our idea that B sites of BN are preferentially adsorbed by atomic deuterium. Since hydrogen (or deuterium) can be detected, the ion desorption method has the advantage of allowing direct evidence of hydrogen adsorption to be obtained. This method would be useful to study site dependence on hydrogen adsorption for complicated materials consisting of multiple elements. Actually, some theoretical groups have reported that B and N dopings in carbon nanotubes improve the hydrogen storage property [28]. As a future work, we want to apply this method to such B–C–N ternary systems.

4. Summary The deuteration properties of a h-BN thin film on Ni(1 1 1) substrate were experimentally investigated to examine theoretical suggestion of site selective chemisorption-based hydrogen adsorption of BN material. Results for the B and N sites implied that deuteration mainly occurs on B sites.

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Acknowledgements The authors wish to thank Dr. Y. Kitajima, Dr. Mase, and other members of the staff of the Photon Factory for their support. This work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2008G156). References [1] [2] [3] [4] [5] [6] [7] [8] [9]

[10] [11]

R.J. Baierle, P. Piquini, T.M. Schmidt, A. Fazzio, J. Phys. Chem. B 110 (2006) 21184. G. Mpourmpakis, E. Froudakis, Catal. Today 120 (2007) 341. S.-H. Jhi, Y.-K. Kwon, Phys. Rev. B 69 (2004) 245407. S.-H. Jhi, Phys. Rev. B 74 (2006) 155424. J. Zhou, Q. Wang, Q. Sun, P. Jena, X.S. Chen, Proc. Natl. Acad. Sci. U. S. A. 107 (2010) 2801. B. Mårlid, K. Larsson, J.-O. Carlsson, J. Phys. Chem. B 103 (1999) 7637. J.-F. Jia, H. Wang, X.-Q. Pei, H.-S. Wu, Appl. Surf. Sci. 253 (2007) 4485. Z. Zhou, J. Zhao, Z. Chen, X. Gao, T. Yan, B. Wen, P. Schleyer, J. Phys. Chem. B 110 (2006) 13363. Vl.A.A. Margulis, E.E. Muryumin, O.B. Tomilin, in: T.N. Vezinaichenko, S.Y. Zaginaichenko, D.V. Schur, B. Baranowski, A.P. Shpak, V.V. Skorokhod, A. Kale (Eds.), Proceedings of the NATO Advanced Research Workshop on Hydrogen Materials Science and Chemistry of Carbon Nanomaterials, Springer, 2007, pp. 275–278. N. Koi, T. Oku, Sci. Technol. Adv. Mater. 5 (2004) 625. X. Wu, J. Yang, J.G. Hou, Q. Zhu, J. Chem. Phys. 121 (2004) 8481.

[12] P.F. Weck, E. Kim, S.H. Lepp, B. Naduvalath, H.R. Sadeghpour, Phys. Chem. Chem. Phys. 10 (2008) 5184 (Communication). [13] A. Nagashima, N. Tejima, Y. Gamou, T. Kawai, C. Oshima, Phys. Rev. B 51 (1995) 4606. [14] W. Auwarter, T.J. Kreutz, T. Greber, J. Osterwalder, Surf. Sci. 429 (1999) 229. [15] A. Nikitin, L. Naslund, Z. Zhang, A. Nilsson, Surf. Sci. 602 (2008) 2575. [16] T. Zecho, A. Guttler, X. Sha, B. Jackson, J. Kuppers, J. Chem. Phys. 117 (2002) 8486. [17] L. Knotek, V. Jones, V. Rehn, J. Phys. Rev. Lett. 43 (1979) 300. [18] M. Petravic, P.N.K. Deenapanray, G. Comtet, L. Hellner, G. Dujardin, B.F. Usher, Phys. Rev. Lett. 84 (2000) 2255. [19] K.R. Koswattage, I. Shimoyama, Y. Baba, T. Sekiguchi, K. Nakagawa, J. Chem. Phys. 135 (2011) 014706. [20] C. Eibl, G. Lackner, A. Winkler, J. Vac. Sci. Technol. A 16 (1998) 2979. [21] T. Sekiguchi, H. Ikeura, K. Tanaka, K. Obi, N. Ueno, K. Honma, J. Chem. Phys. 102 (1995) 1422. [22] J. Stöhr, NEXAFS Spectroscopy, Springer, Berlin, 1996, p. 284. [23] M. Petravic, R. Peter, I. Kavre, L. Li, Y. Chen, L.-J. Fan, Y.-W. Yang, Phys. Chem. Chem. Phys. 12 (2010) 15349 (Communication). [24] D.E. Ramaker, C.T. White, J.S. Murday, J. Vac. Sci. Technol. 18 (1981) 748. [25] D. Menzel, G. Rocker, H.-P. Steinrück, D. Coulman, P.A. Heinmann, W. Huber, P. Zebisch, D.R. Lloyd, J. Chem. Phys. 96 (1992) 1724. [26] D. Coulman, A. Puschmann, U. Hofer, H.-P. Steinruck, W. Wurth, P. Feulner, D. Menzel, J. Chem. Phys. 93 (1990) 58. [27] M.C.K. Tinone, N. Ueno, J. Maruyama, K. Kamiya, Y. Harada, T. Sekiguchi, K. Tanaka, J. Electron Spectrosc. Relat. Phenom. 80 (1996) 117. [28] Z. Zhou, X. Gao, J. Yan, D. Song, Carbon 44 (2006) 939.