Oxygen-Impurity-Induced DirectâIndirect Band Gap in Perovskite

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Oxygen Impurity Induced Direct-Indirect Bandgap in Perovskite SrTaON Xin Wang, Huiting Huang, Mengting Zhao, Weichang Hao, Zhaosheng Li, and Zhigang Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01279 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 4, 2017

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The Journal of Physical Chemistry

Oxygen Impurity Induced Direct-Indirect Bandgap in Perovskite SrTaO2N Xin Wanga, Huiting Huanga, Mengting Zhaob, Weichang Haob, Zhaosheng Li a*, Zhigang Zoua

National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Ecomaterials and Renewable Energy Research Center, and Nanjing University, 22 Hankou Road, Nanjing 210093, China.a Department of Physics and Key Laboratory of Micro-nano Measurement, Manipulation and Physics, Ministry of Education, Beihang University, Beijing 100191, China. b AUTHOR INFORMATION Corresponding Author Zhaosheng Li* E-mail: [email protected]. Tel: +86-25-83686304(ext. 800)

ACS Paragon Plus Environment

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ABSTRACT: Oxynitride semiconductors are supposed to be promising candidates for solar water splitting. In this work, we show oxygen-rich SrTaO2N has direct-indirect bandgap character via twin-VBMs (valence-band maximums), resulting in good photoelectronic responses. Compared with the direct bandgap of ideal SrTaO2N, the additional indirect-VBM of the oxygen-rich solid solution is found to be due to strontium-oxygen hybridizations, using orbital projections based on hybrid/GW density functional theory (DFT). This twin-VBMs character is validated by Strontium K-edge absorption via extended X-ray absorption fine structure (EXAFS) analysis. The twin-VBMs character of the band structure could enhance the photoelectronic responses and hole transports. Our findings provide a viable strategy for enhancing the solar water splitting performance of oxynitrides.


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Solar hydrogen generation via water splitting is a promising strategy for clean and renewable energy sources1-6. For effective use of this technique, bandgaps of photocatalysts should be limited to 1.7-2.4 eV to achieve more than 10% solar-to-hydrogen efficiency (STH)

7-9

. One of

the promising candidate materials is (oxy)nitride, which has an appropriate band structure for solar water splitting10-17. Because of the similarity between oxygen and nitrogen, most oxynitrides are anionic solid solutions15. It remains unclear how the oxygen/nitrogen ratio affects the photoresponse of oxynitrides. Several works show that the oxygen impurities in oxynitrides are stable and inevitable15, 18. This deviation in the ideal crystal lattice results in abnormal physical properties. For instance, although ideal SrTaO2N has a direct bandgap of 2.1 eV12, 18,19, the synthesized sample shows ambiguous absorption of direct or indirect bandgap, which could be ascribed to a band structure with both direct and indirect bandgaps (see supplementary Fig. S1). Based on the previous studies, ionic defect would be main cause for the abnormal photoabsorption in synthesized SrTaO2N15-16. According to formation energy calculations of ionic defect structures, defects from nitrogen with oxygen are stable and inevitable in SrTaO2N (see supplementary Figs. S2-3)20-31. The result indicates oxygen impurities are easy to form in SrTaO2N, leading to changes to band structures. As shown in Figure 1 (a), ideal SrTaO2N shows a direct bandgap character, for both the valence-band maximum (VBM) and conduction-band minimum (CBM) located at Г-point. The band structure of oxygen-rich model has twin-VBMs with same energy at reciprocal points Г and M, while the CBM remains fixed at its original Гpoint (see Figure 1 (b)). The twin-VBMs result in both direct (Г→Г) and indirect (M→Г) bandgap characters in oxygen-rich models, leading to the absorption being ambiguous in terms of direct or indirect bandgap in synthesized SrTaO2N.


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Figure 1. Electronic band structures of SrTaO2N for the (a) ideal model and (b) oxygen-rich model (i.e., substituting nitrogen with oxygen), via HSE06 method with VASP package. The results are verified to not be an artifact of the calculations, as shown in in supplementary Figs. S4-5. More calculating details can be found in Supporting Information. Band structure with twin-VBMs has been observed in strain-forced MoSe2 and other twodimensional materials, which is caused by lattice distortions and dielectric relaxations32-34. Besides, several studies of indium-oxides and indium-tin-oxides (ITO) suggest that different bond hybridizations would result in direct or indirect bandgap35-38. These bond hybridizations may coexist in quaternary compounds to induce twin-VBMs. To ascertain the role of lattice distortions and dielectric relaxations to the abnormal band structure, two oxygen-rich models with asymmetrical and symmetrical substitutions are investigated, as shown in Figure 2 and supplementary Fig. S6. An asymmetrical substituted Ta-anion octahedron has 2.5° distortion while symmetrical substituted one remains almost the constant (see α in Figure 2 (a) and (b), respectively). The twin-VBMs are found in both two models, which indicates that the abnormal band structures could not be attributed to an effect related to the octahedral distortions. On the other hand, the substituted Ta-anion octahedrons have clockwise-rotations, relaxing the intrinsic counterclockwise torsions of ideal lattices (see ∆ in both Figure 2 (a) and (b), left-hand screw


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rule along c). These clockwise-rotations would change the electron distribution in the models, resulting in dielectric relaxations or bond reformations. If dielectric relaxations cause the twinVBMs, the plural dielectric constant of the two models should be on the same side of the ideal model, fitting with the rotation being. The calculated plural dielectric constants disagree with the pre-assumption, indicating the dielectric relaxations are independent of the abnormal band structures (see supplementary Fig. S7). These results point to a possible connection between twin-VBMs and bond reformation. In other words, the indirect-VBM in SrTaO2N would be formed by additional bond hybridizations.

Figure 2. Crystal lattice deviations and electron density differences of (a) asymmetrical and (b) symmetrical oxygen substitutions in SrTaO2N, with the ideal one being subtracted. More details can be found in supplementary Fig. S8. Direct characters of bandgap in both ideal and oxygen-rich SrTaO2N are made up of N-2p and Ta-5d orbitals, as shown in density of states (DOS) in supplementary Fig. S9. The indirect aspects of twin-VBMs are illustrated with the electron density differences in Figure 3 (a). The oxygen impurity induced electron density differences locate on the sites of strontium and oxygen, which indicates delocalized-like bonding outside of the Ta-anion octahedrons.


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Therefore, the hybridization of strontium and oxygen (Sr-O) forms the indirect-VBM naturally. Specifically, the electron density of strontium in the oxygen-rich model show an increase in the pz direction and a decrease in other directions (see Figure 3 (a)). Compared with spherical electron-distributed Sr orbitals in ideal SrTaO2N, the oxygen impurity induced polar Sr-pz orbitals would play a critical role in the Sr-O hybridizations. As for the O orbitals, O-pz orbitals are the main cause of dielectric relaxations as shown in Figure 2. In this sense, the anionic part of the Sr-O hybridizations should be limited to O-pxy orbitals rather than O-pz orbitals. In a word, the indirect-VBM is attributed to the hybridizations of Sr-pz and O-pxy orbitals. The partial orbitals of hybridizations (i.e., Sr-pz and O-pxy) cannot be obtained by DFT calculations, therefore the Wannier function projection techniques based on GW0 methods are employed to obtain the specific orbitals. As shown in Figure 3 (b), the valence band edges around M-point are made up of hybridized pz-2pxy orbitals in both ideal and oxygen-rich models. The oxygen impurities would lift the hybridized pz-2pxy orbitals at M-point to the same level of the direct-VBM, resulting in an indirect-VBM in SrTaO2N. To the best of our knowledge, it could be testified by K-edge absorption of strontium via EXAFS experiments that the hybridized Sr-O orbitals induce the twin-VBMs

37-39

. As shown in Figure 3 (c), the fitting curves of both

ideal and oxygen-rich models coincide with the observed L-edge absorption of tantalum, which indicate the conduction bands remain the same with/without oxygen impurities. The K-edge absorption of strontium shows a plateau within 2.58-2.78 Å (i.e., path L1-L2), as shown in Figure 3 (d). Compared with the fitting curve of ideal model between L1-L2, the absorption plateau is in qualitative agreement with the oxygen-rich one. This absorption plateau demonstrates there are isoenergetic shells around strontium in the synthesized SrTaO2N. The isoenergetic shells in oxygen-rich SrTaO2N lattice imply the character of twin-VBMs in the corresponding Brillouin


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zone.

In this sense, the absorption plateau provides a direct evidence for twin-VBMs in

synthesized SrTaO2N. To be more specific, there are three coordination shells of oxygen around strontium, Sr-OL1 (2.58 Å), Sr-OL2 (2.78 Å) and Sr-OL3 (3.09 Å), as shown in Figure 3(a). The absorption plateau occurs within L1-L2, thereby could not be attributed to the Sr-OL3 hybridizations. Besides, the OL1 shows decreased electron density adjacent to the strontium, weakening the hybridization of Sr-OL1. Hence, the absorption plateau would not be ascribed to weakened Sr-OL1 hybridizations (see Figure 3(a)). These results indicate that the absorption plateau is mainly caused by the SrOL2 hybridizations, which are made up of the introduced oxygen impurity at OL2 sites and strontium ions. In a word, the absorption plateau, namely a direct evidence of twin-VBMs, originates from the hybridization of strontium and oxygen impurities via the K-edge absorption of strontium.

Figure 3. (a) Illustrations of electron density differences of oxygen-rich model (i.e. O•N), with the ideal one being subtracted. (b) Projections of hybrid Sr-pz orbitals and O-pxy orbitals, via


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Wannier function projection techniques based on GW0 methods. (c) L-edge absorption of tantalum and (d) K-edge absorption of strontium via EXAFS analysis, and fitting curves of both ideal and oxygen-rich models. The X-ray diffraction pattern of synthesized SrTaO2N applied to EXAFS is shown in the supplementary Fig. S10. The twin-VBMs in oxygen-rich SrTaO2N alter electronic structure and would benefit the hole transports. As shown in Figure 4 (a) and (b), the hole effective masses of direct-VBMs in both models are almost the same (see ГI and ГO). At the additional indirect-VBM, the hole effective masses are smaller than those at both direct-VBMs, suggesting a possible improvement in hole transports (see MI and MO). The reciprocal distances of VBMs in oxygen-rich model (ГO-MO in Figure 4 (b)) are reduced to 70% of the ideal one (ГI-ГI in Figure 4 (a)), showing an enlargement of the hole free path in real space. Moreover, the indirect-VBM would benefit the photoexcitation dynamics, because of the enlarged density of VBM sites and producing a rise in capacity for photoexcited holes in the valence bands. The additional indirect bandgap slows the electron-hole recombination due to a phonon-electron coupling requiring a specific momentum4042

.

Figure 4. 2D-profiles of valence band edges of (a) ideal and (b) oxygen-rich SrTaO2N at Kz=0, together with the corresponding effective masses of positive holes in both the (1 0 0) and (1 1 0) directions.


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In summary, we present twin-VBMs and direct-indirect bandgap characters in oxygen-rich SrTaO2N via substituting nitrogen with oxygen. By means of hybrid-DFT/GW methods, the additional indirect-VBM is ascribed to the orbital hybridization of strontium and oxygen. EXAFS analysis verifies the existence of twin-VBMs. K-edge absorption of strontium confirms that the orbital hybridization of strontium and oxygen impurities makes up the additional indirect-VBM. This work shows the influence of oxygen impurities in oxynitride semiconductors in more details, and implies a viable strategy for enhancing solar water splitting performance of oxynitrides. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Detailed computational procedures and supplemental works with Figs. S1−S10, as referred to in the text. (PDF). AUTHOR INFORMATION Corresponding Author Zhaosheng Li* E-mail: [email protected]. Tel: +86-25-83686304(ext. 800) Present Address (Z.S. Li) Nanjing University, 22 Hankou Road, Nanjing 210093, People’s Republic of China. Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This work is supported by National Basic Research Program of China (973 Program, 2013CB632404), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, the National Natural Science Foundation of China (Nos. 51272102, 21103070, 51672018 and 51472016). The author would like to thank Professor Haijun Zhang for the help with the physics rational. The authors are also grateful to the High Performance Computing Center (HPCC) of Nanjing University for allowing the use of its IBM Blade cluster system for the numerical calculations discussed in this paper. REFERENCES (1) Lewis, N. S. Toward Cost-Effective Solar Energy Use. Science. 2007, 315, 798-801. (2) Gratzel, M. Photoelectrochemical Cells. Nature. 2001, 414, 338-344. (3) Li, Z. S.; Feng, J.; Yan, S.; Zou, Z. Solar Fuel Production: Strategies and New Opportunities with Nanostructures. Nano Today. 2015, 10, 468-486. (4) Li, Z. S.; Luo, W.; Zhang, M.; Feng, J.; Zou, Z. Photoelectrochemical Cells for Solar Hydrogen Production: Current State of Promising Photoelectrodes, Methods to Improve their Properties, and Outlook. Energy Environ. Sci. 2013, 6, 347-370. (5) Fujishima, A.; Honda, K. Electrochemical Photocatalysis of Water at Semiconductor Electrode. Nature. 1972, 238, 37-38. (6) Sivula, K.; Formal, F. L.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (αFe2O3) Photoelectrodes. ChemSusChem. 2011, 4, 432-449. (7) Bard, A. J.; Fox, M. A. Holy Grails in Chemistry. Acc. Chem. Res. 1995, 28, 141-145. (8) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473.


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Oxygen-Impurity-Induced DirectâIndirect Band Gap in Perovskite

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Oxygen-Impurity-Induced DirectâIndirect Band Gap in Perovskite