Bismuth vanadate (BiVO4) has a band structure that is well-suited for potential use as a photoanode in solar water splitting, but it suffers from poor electron-hole separation. Here, we demonstrate that a nanoporous morphology (specific surface area of 31.8 square meters per gram) effectively suppresses bulk carrier recombination without additional doping, manifesting an electron-hole separation yield of 0.90 at 1.23 volts (V) versus the reversible hydrogen electrode (RHE). We enhanced the propensity for surface-reaching holes to instigate water-splitting chemistry by serially applying two different oxygen evolution catalyst (OEC) layers, FeOOH and NiOOH, which reduces interface recombination at the BiVO4/OEC junction while creating a more favorable Helmholtz layer potential drop at the OEC/electrolyte junction. The resulting BiVO4/FeOOH/NiOOH photoanode achieves a photocurrent density of 2.73 milliamps per square centimenter at a potential as low as 0.6 V versus RHE.
were synthesized by a molten flux method. The black needles of compound I were formed at 600°C and crystallized in the monoclinic P2 1 /m space group (No. 11) with a ) 17.492(3) Å, b ) 4.205(1) Å, c ) 18.461(4) Å, ) 90.49(2)°. The final R/R w ) 6.7/5.7%. Compound II is isostructural to I. Both I and II are isostructural with K 2 Bi 8 S 13 which is composed of NaCl-, Bi 2 Te 3 -, and CdI 2 -type units connecting to form K + -filled channels. The thin black needles of III and IV obtained at 530°C crystallize in the same space group P2 1 /m with a ) 17.534 (4) Å, b ) 4.206(1) Å, c ) 21.387(5) Å, ) 109.65(2)°and a ) 17.265(3) Å, b ) 4.0801(9) Å, c ) 21.280(3) Å, ) 109.31 (1)°, respectively. The final R/R w ) 6.3/8.3% and 5.1/3.6%. Compounds III and IV are isostructural and potassium and bismuth/antimony atoms are disordered over two crystallographic sites. The structure type is very closely related to that of I. Electrical conductivity and thermopower measurements show semiconductor behavior with ∼250 S/cm and ∼-200 µV/K for a single crystal of I and ∼150 S/cm and ∼-100 µV/K for a polycrystalline ingot of III at room temperature. The effect of vaccum annealing on these compounds is explored. The optical bandgaps of all compounds were determined to be 0.59, 0.78, 0.56, and 0.82 eV, respectively. The thermal conductivities of melt-grown polycrystalline ingots of I and III are reported.
The two isostructural compounds, ATh2Se6 (A = K, Rb), adopt the orthorhombic space group Immm.
ATh2Se6 has a two-dimensional structure which is related to the ZrSe3-type structure with K+/Rb+ cations
stabilized between the layers. These compounds represent the intercalated form of ThSe3 with 0.5 equiv of
alkali metal ion. The stacking arrangement of the layers is slightly modified from that of ZrSe3 in order to
stabilize the newly introduced alkali metal ions between the layers. Electron diffraction studies reveal a static
charge density wave (CDW), due to electron localization, resulting in 4a × 4b superstructure. An atomic pair
distribution function analysis and spectroscopy confirmed the presence of diselenide groups in the ZrSe3-type
layer (invisible by the single-crystal structure analysis) and support the notion that these Se atoms in the
[Th2Se6] layers accept the extra electron from the alkali metal, and this results in breaking one out of four
diselenide bonds. The superstructure is due to ordering of the three Se2
2- and two Se2- species along both
directions. Optical absorption, Raman spectroscopy, and atomic force microscopy as well as magnetic
susceptibility measurements support these conclusions.
The quaternary
isostructural compounds KThSb2Se6 and
BaLaBi2Q6 (Q = S, Se) were prepared. The
structure type contains dichalcogenide ions,
Q2
2- (Q = S, Se). The
coordination geometry around the Th/La atoms is best described as a
tricapped trigonal prism. The Th/La atoms form one-dimensional
infinite double chains parallel to the [100] direction. The
dichalcogenide ligand plays an important role in the construction of
this double chain bridging two single chains composed of
ThSe6/LaS6
prisms.
The new compounds, RbU 2 SbS 8 and KU 2 SbSe 8 , were prepared as golden-black, blocklike crystals by the polychalcogenide molten flux method. RbU 2 SbS 8 crystallizes in the monoclinic space group Cm with a ) 7.9543(9) Å, b ) 11.0987(13) Å, c ) 7.2794(10) Å, β ) 106.030(2)°, and Z ) 2. The compound has a two-dimensional character with layers running perpendicular to the c-axis. The coordination geometry around the U 4+ atoms is best described as a bicapped trigonal prism. The trigonal prisms share triangular faces with neighboring prisms, forming one-dimensional columns along the a-axis. The columns are then joined to construct sheets by sharing capping S atoms. Sb 3+ ions are sitting at the center of a slightly distorted seesaw coordination environment (CN ) 4). Rb + ions are stabilized in 8-coordinate bicapped trigonal prismatic sites. KU 2 SbSe 8 crystallizes in the monoclinic space group Cm with a ) 11.5763-(2) Å, b ) 8.2033(1) Å, c ) 15.2742(1) Å, β ) 112.22(2)°, and Z ) 4. KU 2 SbSe 8 has essentially the same structure as RbU 2 SbS 8 . However, Sb 3+ and K + ions appear disordered in every other layer, resulting in a different unit cell. RbU 2 SbS 8 is a semiconductor with a band gap of 1.38 eV. The band gap of KU 2 SbSe 8 could not be determined precisely due to the presence of overlapping intense f-f transitions in the region (0.5-1.1 eV). The Raman spectra show the disulfide stretching vibration in RbU 2 SbS 8 at 479 cm -1 and the diselenide stretching vibration in KU 2 SbSe 8 at 252 cm -1 . Magnetic susceptibility measurements indicate the presence of U 4+ centers in the compounds. The compounds do not melt below 1000 °C under vacuum.
High real-space resolution atomic pair distribution functions ͑PDF's͒ have been obtained from ZnSe 1Ϫx Te x using neutron powder diffraction. Distinct Zn-Se and Zn-Te nearest-neighbor ͑nn͒ bonds, differing in length by ⌬rϭ0.14 Å, are resolved in the measured PDF's, allowing the evolution with composition of the individual bond lengths to be studied. The local bond lengths change much more slowly with doping than the average bond length obtained crystallographically. The nn bond-length distributions are constant with doping, but higher-neighbor pair distributions broaden significantly, indicating that most of the strain from the alloying is accommodated by bond-bending forces in the alloy. PDF's of alloys across the whole doping range are well fit using a model based on the Kirkwood potential. The resulting PDF's give excellent agreement with the measured PDF's over the entire alloy range with no adjustable parameters.
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