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Quaternary metal‐chalcogenides combining rare‐earth cations with late transition metal cations are attracting growing attention for their optical properties, such as for solar energy conversion or second harmonic generation. Synthetic explorations of theII3‐I2‐IV2‐Ch8family (II = Eu;I = Cu or Ag;IV = Si, Ch = S or Se) have yielded Eu3Ag2Si2S8(1) and Eu3Cu1.08(1)Si2.42(1)Se8(2). Their structures have been characterized by X‐ray diffraction to form in the noncentrosymmetric space groupI3dand to exhibit two distinct types of mixed‐site occupancies, for the Ag(I) cations in1and mixed Cu(I)/Si(IV) cations in2. In both, the cation disorder occurs to achieve charge balancing with the chalcogenide anions. A high yield of1can be achievedwith optical measurements showing indirect and direct band transitions of ≈2.2(1) and ≈2.4(1) eV, respectively. Its second harmonic generation response is found to be relatively strong, approximately 0.9 × AgGaS2, confirming its noncentrosymmetric structure. Band structure calculations reveal the valence and conduction band edges stem predominantly from the filled Ag(I)/Cu(I)‐based states and empty Si(IV)‐based states, respectively, with additional contributions from the chalcogenide anions. Calculation results also show that cation disorder facilitates a reduction in the antibonding interactions between the Ag(I)/Cu(I)d‐based and chalcogenidep‐based states.
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A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode
A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon heating in air, yielding CuO as nano-sized islands. However, the bulk crystalline structure was maintained, with up to 50% Cu(I)-vacancies and a concomitant oxidation of the Cu(I) to Cu(II) cations within the structure. Thermogravimetric and magnetic susceptibility measurements showed the formation of increasing amounts of Cu(II) cations, according to the following reaction: Cu3Ta7O19 + x/2 O2 → Cu(3−x)Ta7O19 + x CuO (surface) (x = 0 to ~0.8). With minor amounts of surface oxidation, the cathodic photocurrents of the polycrystalline films increase significantly, from <0.1 mA cm−2 up to >0.5 mA cm−2, under visible-light irradiation (pH = 6.3; irradiant powder density of ~500 mW cm−2) at an applied bias of −0.6 V vs. SCE. Electronic structure calculations revealed that its defect tolerance arises from the antibonding nature of its valence band edge, with the formation of defect states in resonance with the valence band, rather than as mid-gap states that function as recombination centers. Thus, the metastable Cu(I)-containing semiconductor was demonstrated to possess a high defect tolerance, which facilitates its high cathodic photocurrents.
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- Award ID(s):
- 2004455
- PAR ID:
- 10301958
- Date Published:
- Journal Name:
- Molecules
- Volume:
- 26
- Issue:
- 22
- ISSN:
- 1420-3049
- Page Range / eLocation ID:
- 6830
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/molecules26226830
