“…Compared with the pristine Nb 2 O 5 , additional small peaks centering separately at 205.8 eV and 208.3 eV in the r-Nb 2 O 5 conform to the 3d 5/2 and 3d 3/2 peaks of Nb 4+ (NbO 2 ). 37,38 The emerging Nb 4+ signals suggest that oxygen vacancies are introduced into r-Nb 2 O 5 during the reduction, which is consistent with the abovementioned Raman analysis.…”
Black titania, with greatly improved solar absorption, has demonstrated its effectiveness in photocatalysis and photoelectrochemical cells (PEC), inspiring us to explore the blackening of other wide band-gap oxide materials for enhanced performance. Herein, we report the fabrication of black, reduced Nb2O5 nanorods (r-Nb2O5), with active exposed (001) surfaces, and their enhanced photocatalytic and PEC properties. Black r-Nb2O5 nanorods were obtained via reduction of pristine Nb2O5 by molten aluminum in a two-zone furnace. Unlike the black titania, r-Nb2O5 nanorods are well-crystallized, without a core-shell structure, which makes them outstanding in photocatalytic stability. Substantial Nb(4+) cation and oxygen vacancies (VO) were introduced into r-Nb2O5, resulting in the enhanced absorption in both the visible and near-infrared regions and improved charge separation and transport capability. The advantage of the r-Nb2O5 was also proved by its more efficient photoelectrochemical performance (138 times at 1.23 VRHE) and higher photocatalytic hydrogen-generation activity (13 times) than pristine Nb2O5. These results indicate that black r-Nb2O5 is a promising material for PEC application and photocatalysis.
“…Compared with the pristine Nb 2 O 5 , additional small peaks centering separately at 205.8 eV and 208.3 eV in the r-Nb 2 O 5 conform to the 3d 5/2 and 3d 3/2 peaks of Nb 4+ (NbO 2 ). 37,38 The emerging Nb 4+ signals suggest that oxygen vacancies are introduced into r-Nb 2 O 5 during the reduction, which is consistent with the abovementioned Raman analysis.…”
Black titania, with greatly improved solar absorption, has demonstrated its effectiveness in photocatalysis and photoelectrochemical cells (PEC), inspiring us to explore the blackening of other wide band-gap oxide materials for enhanced performance. Herein, we report the fabrication of black, reduced Nb2O5 nanorods (r-Nb2O5), with active exposed (001) surfaces, and their enhanced photocatalytic and PEC properties. Black r-Nb2O5 nanorods were obtained via reduction of pristine Nb2O5 by molten aluminum in a two-zone furnace. Unlike the black titania, r-Nb2O5 nanorods are well-crystallized, without a core-shell structure, which makes them outstanding in photocatalytic stability. Substantial Nb(4+) cation and oxygen vacancies (VO) were introduced into r-Nb2O5, resulting in the enhanced absorption in both the visible and near-infrared regions and improved charge separation and transport capability. The advantage of the r-Nb2O5 was also proved by its more efficient photoelectrochemical performance (138 times at 1.23 VRHE) and higher photocatalytic hydrogen-generation activity (13 times) than pristine Nb2O5. These results indicate that black r-Nb2O5 is a promising material for PEC application and photocatalysis.
“…It is evident that at increased heating temperatures a larger amount of Cu(II) is present at the surface. In addition to Cu oxidation near the surface, XPS observations provided in the Supporting Information show that heating Cu 5 Ta 11 O 30 in air results in a partial reduction of Ta(V) to suboxides, similar to results seen in annealing studies of Ta 2 O 5 . These changes are in part responsible for maintaining charge balance in the crystal during the surface reactions.…”
The p-type semiconductor Cu 5 Ta 11 O 30 has been investigated for the effect of Cu extrusion on its crystalline structure, surface chemistry, and photoelectrochemical properties. The Cu 5 Ta 11 O 30 phase was prepared in high purity using a CuCl-mediated flux synthesis route, followed by heating the products in air from 250 to 750 °C in order to investigate the effects of its reported film preparation conditions as a p-type photoelectrode. At 650 °C and higher temperatures, Cu 5 Ta 11 O 30 is found to decompose into CuTa 2 O 6 and Ta 2 O 5 . At lower temperatures of 250 to 550 °C, nanosized Cu II O surface islands and a Cu-deficient Cu 5−x Ta 11 O 30 crystalline structure (i.e., x ∼ 1.8(1) after 450 °C for 3 h in air) is found by electron microscopy and Rietveld structural refinement results, respectively. Its crystalline structure exhibits a decrease in the unit cell volume with increasing reaction temperature and time, owing to the increasing removal of Cu(I) ions from its structure. The parent structure of Cu 5 Ta 11 O 30 is conserved up to ∼50% Cu vacancies but with one notably shorter Cu−O distance (by ∼0.26 Å) and concomitant changes in the Ta−O distances within the pentagonal bipyramidal TaO 7 layers (by ∼0.29 Å to ∼0.36 Å). The extrusion and oxidation of Cu(I) to Cu(II) cations at its surfaces is found by X-ray photoelectron spectroscopy, while magnetic susceptibility data are consistent with the oxidation of Cu(I) within its structure, as given by Cu I(5−2x) Cu II x Ta 11 O 30 . Polycrystalline films of Cu 5−x Ta 11 O 30 were prepared under similar conditions by sintering, followed by heating in air at temperatures of 350 °C, 450 °C, and 550 °C, each for 15, 30, and 60 min. An increasing amount of copper deficiency in the Cu 5−x Ta 11 O 30 structure and Cu II O surface islands are found to result in significant increases in its p-type visiblelight photocurrent at up to −2.5 mA/cm 2 (radiant power density of ∼500 mW/cm 2 ). Similarly high p-type photocurrents are also observed for Cu 5 Ta 11 O 30 films with an increasing amount of CuO nanoparticles deposited onto their surfaces, showing that the enhancement primarily arises from the presence of the CuO nanoparticles which provide a favorable band-energy offset to drive electron−hole separation at the surfaces. By contrast, negligible photocurrents are observed for Cu-deficient Cu 5−x Ta 11 O 30 without the CuO nanoparticles. Electronic structure calculations show that an increase in Cu vacancies shifts the Fermi level to lower energies, resulting in the depopulation of primarily Cu 3d 10 -orbitals as well as O 2p orbitals. Thus, these findings help shed new light into the role of Cu-deficiency and Cu II O surface islands on the p-type photoelectrode films for solar energy conversion systems.
“…The ternary alloys also showed the presence of both Ta-C and Hf-C with different proportions according to the different contents of Ta or Hf. Unlike the ternary alloy films, both binary samples showed the presence of very small peaks attributed to Ta-O and Hf-O bonds, with the Ta and Hf 4f 7/2 peaks at around 26.9 26 and 17.3 eV 27 , respectively. The absence of oxides and the lower amount of oxygen in the ternary films, compared to the binary ones, suggest the higher stability of the ternary carbide and its lower reactivity to oxygen.Figure 3High resolution ( a ) C 1s and ( b ) Ta and Hf 4 f spectra for the different Ta-Hf-C alloy films showing the evolution of the samples as they change from pure TaC to HfC.
…”
In this work we report the hot corrosion properties of binary and ternary films of the Ta-Hf-C system in V2O5-Na2SO4 (50%wt.-50%wt.) molten salts at 700 °C deposited on AISI D3 steel substrates. Additionally, the mechanical and nanowear properties of the films were studied. The results show that the ternary alloys consist of solid solutions of the TaC and HfC binary carbides. The ternary alloy films have higher hardness and elastic recoveries, reaching 26.2 GPa and 87%, respectively, and lower nanowear when compared to the binary films. The corrosion rates of the ternary alloys have a superior behavior compared to the binary films, with corrosion rates as low as 0.058 μm/year. The combination and tunability of high hardness, elastic recovery, low nanowear and an excellent resistance to high temperature corrosion demonstrates the potential of the ternary Ta-Hf-C alloy films for applications in extreme conditions.
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