Band-structure modulation of SrTiO3 by hydrogenation for enhanced photoactivity

Tao Sun

Supervisor: Ming Lu

Dept. Optical Science and Engineering, Fudan University

Abstract

SrTiO₃ (STO), a prototype perovskite,¹ has received considerable interest in the past decades.^2-6 STO is of importance in many areas of application; for instance, it is used as a substrate for epitaxial growth of other perovskite compounds for integration of thin-film oxide electronic devices,⁷ as a buffer material for micro/nanoelectronics,⁸ and as an excellent photocatalyst for environmental protection and hydrogen energy.^9-11 Band structure engineering is an effective methodology to modulate physical and chemical properties of semiconductors for desired application purposes.¹² Hydrogenation of anatase TiO₂, a photocatalyst similar to STO in band location and band gap width, has been conducted for an enhanced and broadband photoactivity. Liu et al.¹³ reported an enhanced photocatalysis of TiO₂ powder under ultraviolet (UV) light after hydrogenation at atmospheric pressure. Recently, Chen et al.¹⁴ found that after hydrogenating nanocrystals of TiO₂ at 200 °C under high pressure of 20 bar, the photocatalytic ability of TiO₂ becomes significantly promoted due to hydrogen-incorporated and amorphized surface. However, reports on band-structure modulation of STO via hydrogenation, especially for application of photocatalysis, are relatively rare, and the details of underlying physics have not been fully understood yet.^{15, 16} In this work,¹⁷ we investigate the band-structure modulation of STO via hydrogenation under atmospheric pressure for photocatalysis enhancement. Hydrogenation of STO was performed by annealing STO in the forming gas of H₂:N₂ (5 %:95 %) at elevated temperatures (mostly at 1000 °C). Our results of ultraviolet-visible (UV-vis) absorption and X-ray photoelectron spectroscopy (XPS) suggest that hydrogenation does not induce changes in the band gap width of STO but shifts its Fermi level upward by 1.6 eV due to the band-filling of electrons. The results of photoluminescence (PL) and electron paramagnetic resonance (EPR) of STO before and after hydrogenation support the conclusions derived from UV-vis absorption and XPS data. Photodecomposition results show that such a band-structure modulation by hydrogenation enhances the photocatalytic ability of STO under UV light. The key point of hydrogenation of STO is the transfer of electrons to the intrinsic and extrinsic defects of STO, which causes visible absorption, decrease in intrinsic PL intensity, Fermi level shift, elimination of the EPR signals and finally an enhanced UV photoactivity of STO.

References

1  P. A. Cox, Transition Metal Oxides (Clarendon Press, Oxford, 1995).

2 K. A. Müller and H. Burard, Phys. Rev. B 19, 3593 (1979).

3  J. H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y. L. Li, S. Choudhury, W. Tian, M. E. Hawley, B. Craigo, A. K. Tagantsev, X. Q. Pan, S. K. Streiffer, L. Q. Chen, S. W. Kirchoefer, J. Levy, and D. G. Schlom, Nature 430, 758 (2004).

4   A. F. Santander-Syro, O. Copie, T. Kondo, F. Fortuna, S. Pailhes, R. Weht, X. G. Qiu, F. Bertran, A. Nicolaou, A. Taleb-Ibrahimi, P. Le Fevre, G. Herranz, M. Bibes, N. Reyren, Y. Apertet, P. Lecoeur, A. Barthelemy, and M. J. Rozenberg, Nature 469, 189 (2011).

5   A. M. Kaiser, A. X. Gray, G. Conti, J. Son, A. Greer, A. Perona, A. Rattanachata, A. Y. Saw, A. Bostwick, S. Yang, S. H. Yang, E. M. Gullikson, J. B. Kortright, S. Stemmer, and C. S. Fadley, Phys. Rev. Lett. 107, 116402 (2011).

6  B. Jalan, R. Engel-Herbert, T. E. Mates, and S. Stemmer, Appl. Phys. Lett. 93, 052907 (2008).

7  G. Koster, G. Rijnders, D. A. Blank, and H. Rogalla, Physica C: Supercond. 339, 215 (2000).

8 C. Cen, S. Thiel, J. Mannhart, and J. Levy, Science 323, 1026 (2009).

9  A. Kudo and Y. Miseki, Chem. Soc. Rev. 38, 253 (2009).

10  K. Iwashina and A. Kudo, J. Am. Chem. Soc. 133, 13272 (2011).

11  J. W. Liu, G. Chen, Z. H. Li, and Z. G. Zhang, J. Solid State Chem. 179, 3704 (2006).

12  F. Capasso, Science 235, 172 (1987).

13  H. Liu , H. T. Ma, X. Z. Li , W. Z. Li, M. Wu, and X. H. Bao, Chemosphere 50, 39 (2003).

14  X. Chen, L. Liu, P. Y. Yu, and S. S. Mao, Science 331, 746 (2011).

15  M. C. Tarun and M. D. McCluskey, J. Appl. Phys. 109, 063706 (2011).

16 N. Bork, N. Bonanos, J. Rossmeisl, and T. Vegge, J. Appl. Phys. 109, 033702 (2011).

17  T. Sun and M. Lu, Appl. Phys. A 108, 171 (2012).

Study of the Propagation Properties of Visible Light at
Ag_1-xIn_x/air Interface

Ertao Hu

（Department of Optical Science and Engineering, Shanghai, 200433）

Since the first experimental demonstration of negative refraction in microwave frequency range done by D.R. Smith et al. ^[1] by using composite medium with an array of metal posts to create a frequency region with e_eff <0, interspersed with an array of split ring resonators (SRRs) having a frequency region with m_eff <0, it has been paid much attention due to its potential applications such as: cloak, perfect lens, super resolution, near field storage. In order to get negative refraction with e_eff <0 and m_eff <0 simultaneously, many special artificial structures were made such as: split spring resonators, fish-net structures, array of nano-metal wires ^[2-4]. In order to find the physical origin of negative refraction, many models were proposed as well such as: negative permeability model, a negative Goos–Hänchen shift, plasma resonances model, the effective medium approximation (EMA), and discontinuity of phase model. But which one is right is still unclear. When light propagates in artificial microstructures, it is always accompanied with many side effects such as multiple reflection, refraction and diffraction, which is disadvantage to the physical interpretation of negative refraction. We also note that in most of the artificial structures, noble metals are used though the propagation properties of light in them are still unclear. Therefore, it’s meaningful to investigate the propagation properties of light in noble metals (gold, silver and copper) directly. In our lab, the propagation properties of visible light in noble metals such as gold, silver and copper has been studied, many interesting properties were found. From the experimental results, we thought that negative refraction was due to the negative group velocity in the corresponding photon energy ^[5-7]. However, because just four lasers were used by us, it is not enough to validate our conclusions. On the other way, it will be interesting to study whether other metals have such properties as noble metals. It has been demonstrated that silver is most likely to produce negative refraction ^{[5, 8]}. So we first investigated the propagation properties of light in indium/air interface, then Ag_(1-x)In_x alloy/air by varying the content of indium (x=0.1,0.3,0.5). The dielectric constants will vary with content of indium, while the group refractive index will be varied as well. It is meaningful to investigate how the refraction properties will be changed with the indium content.

In my work，wedge-shaped samples are deposited on K9 glass by using RF sputtering. The refractive angle was measured by optical path amplification method. First, the refractive properties of indium are studied. Then a series of Ag_1-xIn_x were made with x=0.1, 0.3, 0.5. The relations between incidence angles and refractive angles were measured. The dielectric constants spectrums of Ag_1-xIn_x were measured by spectroscopic ellipsometer in the energy range from 1.5 eV to 4.5 eV. Positive refraction was found for indium in the four single wavelength, but negative refractive were found for all the AgIn alloy. When x0.5, the refractive indexes increase with the increasing of the energy of the incident photon for each component. However, no obvious relations were found for x=1. For each incident light, the refractive indexes increase from negative to positive with x.

References:

[1]      D. R. Smith and N. Kroll, Phys. Rev. Lett. 85 (14), 2933-2936 (2000).

[2]      John Pendry, Nat. 423, 22 (2003).

[3]      V. M. Shalaev, Nat. Photon 1 (1), 41-48 (2007).

[4]      V. P. Drachev, W. Cai, U. Chettiar, H. K. Yuan, A. K. Sarychev, A. V. Kildishev, G. Klimeck and V. M. Shalaev, Las. Phys. Let. 3 (1), 49-55 (2006).

[5]      Y.H. Wu, W. Gu, Y. R. Chen, Z. H. Dai, W. X. Zhou, Yu Xiang Zheng and L. Y. Chen, Appl. Phys. Lett. 93, 071910 (2008).

[6]      Y.-H. Wu, W. Gu, Y.-R. Chen, X.-F. Li, X.-S. Zhu, P. Zhou, J. Li, Y.-X. Zheng and L.-Y. Chen, Phys. Rev. B 77 (3), 035134 (2008).

[7]      W.-X. Zhou, Z.-H. Dai, Y. Shen, J.-B. Chen, J. Li, Y.-X. Zheng, H.-B. Zhao, L.-Y. Chen, W. Li, X.-Y. Jiang and D. W. Lynch, J. Phys. Soc. Japan 80 (8), 084715 (2011).

[8]      N.-H. Shen, T. Koschny, M. Kafesaki and C. M. Soukoulis, Phys. Rev. B 85 (7), 075120 (2012).

Theoretical prediction of crystallization ability

Ximing Rong

Department of Optical Science and Engineering, Fudan University

Abstract

Materials with single-crystal structure exhibit greater advantages than those with other structure in almost all fields.  Great efforts have been made to fabricate single crystals with lager size and better crystallization.  However, the crystallization ability for different materials shows great differ from each other.  Therefore, besides investigating the physical and chemistry properties, we also have to predict the crystallization ability which the molecular configuration possesses in future new material design.  Currently, new materials with excellent property and certain configuration could be easily designed in computer via common ab initio calculation methods, but it is unable to predict the probability of forming this structure.  The binding energy concept might be successful in understanding some processes of crystal growth, for example, the growth of rare gas (such as Ar or Xe) crystals are much more difficult compared to growing metal crystal, and the graphite structure is much easier to get than diamond structure, however, it fails to understand why the growth tendency of (100) faces is considerably different from that of (111) faces for the same metal crystal.  Our group developed an accurate and simple method to predict the ability of materials to form single crystal.  First, we introduce a concept of “condensing potential” (CP) that is expected to be able to evaluate the ability, and then extensive molecular dynamics (MD) simulations of crystal growth were performed for different materials to prove the validity of CP.  By introducing a concept of “fault degree” (FD) to evaluate the quality of the newly grown crystals, we found that FD decreases monotonically with increasing CP.  More significantly, CP could be calculated conveniently and exactly via ab initio calculation, which would make this method more practical in new material designs.  Furthermore, we extended the CP model to two component systems, and particularly investigated the Al doping (0–6 wt.%) influence on the crystallizing ability of Ni-based crystal and the Au doping (6, 17, 30 wt.%) influence on the crystallizing ability of Cu-based crystal.  Extensive MD simulations were performed to examine the crystal growth and the results showed that the Al doping of only 6 wt.% will considerably decrease the crystallizing ability of pure Ni crystal, which is in contrast with the conventional solid solution point of view that the Al atoms substitute for Ni atoms in the Ni lattice so well that the doping will not affect the crystal growth.  Similar to single-component systems, the ability of the two-component materials to form single crystals also increases monotonically with increasing CP, exhibiting a prospect of CP model in predicting the ability of multi-component materials to form single crystals.  Since nickel-aluminum (Ni-Al) metallic systems have already got wide applications ranging from metallization of integrated circuits, catalytic materials, high temperature alloy, even to the metallurgical industry, Ni-Al single crystal alloy has especially great application in turbine vane due to its long creep life and thermal stability, It’s therefore necessary to characterize Ni-Al single crystal to investigate the creep and thermal property.  Our CP model gives a critical direction in Ni-base superalloy research. 

Reference

[1]. X.X. Ye, C. Ming, Y.C. Hu, and X.J. Ning, J. Chem. Phys. 130, 16 (2009)

[2]. K. Peng, C. Ming, X.X. Ye, W.X. Zhang, J. Zhuang and X.J. Ning, Chem. Phys. Letts. 501, 330 (2011).

[3]. 陈建亭, 有序金属间化合物镍铝合金 [M]: 北京：科学出版社. 708. (2003).

[4]. Y. D. Li, Y. T. Qian, H. W. Liao, Y. Ding, L. Yang, C. Y. Xu, F. Q. Li, and G. Zhou, Science 281, 246 (1998).

[5]. D. C. Look, Mater. Sci. Eng., B 80, 383 (2001).

[6]. 胡壮麟, 刘丽荣, 金涛, 孙晓峰, 航空发动机, 31(3): 1-7. (2005).

[7]. A. Y. Liu and M. L. Cohen, Science 245, 841 (1989).

[8]. K.S. Kang, Y.S. Meng, J. Breger, C.P. Grey, and G. Ceder, Science, 311, 977 (2006).

[9]. F. Perreault, B. Champagne, and A. Soldera, Chem. Phys. Letts. 440, 116 (2007).

[10] F. Lin, G. Zhou, Z.Y. Li, J. Li, J. Wu, W.H. Duan, Chem. Phys. Lett. 475, 82(2009).