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Chromium-doped SrTiO 3 nanocrystals of perovskite structure type and 45 nm (±15 nm) edge lengths were obtained by hydrothermal synthesis in water from titanium oxide, strontium hydroxide, and chromium( iii ) nitrate. According to XPS, the majority of the surface chromium (68.3%) is present in the 3+ state and the remainder (32.2%) in the 6+ state. Optical spectroscopy confirms a broad absorption at 2.3–2.9 eV from Cr(3+) dopant states, in addition to the 3.2 eV band edge of the SrTiO 3 host. After modification with Pt nanoparticles, Cr-doped SrTiO 3 nanocrystals catalyze photochemical H 2 evolution from aqueous methanol under visible light illumination (>400 nm) and with an apparent quantum yield of 0.66% at 435 nm. According to surface photovoltage spectroscopy (SPS), Cr-doped SrTiO 3 nanocrystals deposited onto gold substrates are n-type and have an effective band gap of 1.75 eV. SPS and transient illumination experiments at 2.50 eV reveal an anomalous surface photovoltage that increases with prior light exposure to values of up to −6.3 V. This photovoltage is assigned to ferroelectric polarization of the material in the space charge layer at the Au/SrTiO 3 :Cr interface. The polarization is stable for 24 h in vacuum but disappears after 12 h when samples are stored in air. The electric polarizability of SrTiO 3 :Cr is confirmed when films are exposed to static electric fields (1.20 MV m −1 ) in a fixed capacitor configuration. The discovery of a ferroelectric effect in Cr-doped SrTiO 3 could be significant for the development of improved photocatalysts for the conversion of solar energy into fuel.
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On the formation of superoxide radicals on colloidal ATiO 3 (A = Sr and Ba) nanocrystal surfaces
Controlling the surface chemistry of colloidal semiconductor nanocrystals is critical to exploiting their rich electronic structures for various technologies. We recently demonstrated that the hydrothermal synthesis of colloidal nanocrystals of SrTiO 3 , a technologically-relevant electronic material, provided a strong negative correlation between the presence of an O 2 -related surface defect and hydrazine hydrate [W. L. Harrigan, S. E. Michaud, K. A. Lehuta, and K. R. Kittilstved, Chem. Mater. , 2016, 28 (2), 430]. When hydrazine hydrate is omitted during the aerobic hydrothermal synthesis, the surface defect is observed. However, it can be removed by either the addition of hydrazine hydrate or by purging the reaction solution with argon gas before the hydrothermal synthesis. We also propose that the formation of the O 2 -related defect is mediated by the reduction of dissolved O 2 by lactate anions that are present from the titanium precursor. This work helps elucidate the nature of the O 2 -related defect as a superoxide anion and presents a mechanism to explain its formation during the hydrothermal synthesis of SrTiO 3 and related BaTiO 3 nanocrystals.
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- PAR ID:
- 10181582
- Date Published:
- Journal Name:
- Nanoscale Advances
- Volume:
- 2
- Issue:
- 5
- ISSN:
- 2516-0230
- Page Range / eLocation ID:
- 1949 to 1955
- 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/10.1039/d0na00106f
