NaCu(Ta_{1鈭?span style='font-style: italic'>y}Nb_y)₄O₁₁ solid solution: A tunable band gap spanning the visible-light wavelengths

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• 作者：Olena Palasyuk ; Paul A. Maggard ; ^{Paul_Maggard@ncsu.edu}
• 关键词：Semiconducting oxide ; Solid solution ; Bandgap size
• 刊名：Journal of Solid State Chemistry
• 出版年：2012
• 期刊代码：130_00224596
• 类别：ch
• 出版时间：July, 2012
• 卷：191
• 期：Complete
• 页码：263-270
• 文件大小：1149 K

摘要

The new solid-solution NaCu(Ta_1鈭?em>yNb_y)₄O₁₁ (0鈮?em>y鈮?.7) was synthesized by solid-state methods in the form of bulk powders that ranged from light-yellow to brown colored and were characterized by powder X-ray diffraction techniques (Space group R-3c (#167); Z=6; a=6.214(1)-6.218(1) 脜 and c=36.86(1)-36.94(1) 脜). Full-profile Rietveld refinements confirmed a site-differentiated ordering of the Cu(I) and Na cations, i.e., occupying the 12c (linear environment) and 18d (seven-coordinate environment) crystallographic sites respectively. Conversely, a statistical mixture of Ta(V) and Nb(V) cations occurred over the 6b (octahedral environment) or the 18e (pentagonal-bipyramidal environment) crystallographic sites, without any preferential segregation. The UV-Vis diffuse reflectance spectra showed a significant red-shift of the optical bandgap size (indirect) from 鈭?.70 eV to 鈭?.80 eV across the solid solution with increasing Nb(V) content. Electronic-structure calculations using the tight-binding linear-muffin-tin-orbital approach showed that the reduction in bandgap size arises from the introduction of the lower-energy Nb 4d⁰ orbitals into the conduction band and consequently a lower energy of the conduction band edge. The lowest-energy bandgap transitions were found to be derived from electronic transitions between the filled Cu(I) and the empty Nb(V) d-orbitals, with a small amount of mixing with the O 2p orbitals. The resulting conduction and valence band energies are found to approximately bracket the redox potentials for water reduction and oxidation, and meeting the thermodynamic requirements for photocatalytic water-splitting reactions.