Built-In Potential in Fe2O3-Cr2O3 Superlattices for Improved Photoexcited Carrier Separation

Harvesting the infrared part of solar light to promote charge transfer in Bi2S3/WO3 photoanode for enhanced photoelectrochemical water splitting

Infrared light absorbed by semiconductors hardly contributes to the solar energy conversion due to its low photon energy. Herein, photothermal effect activated by infrared part of solar light is introduced to promote the photoelectrochemical (PEC) water splitting of photoanodes. Narrow band-gap semiconductor Bi₂S₃ is deposited on the surface of WO₃ nanosheets, exhibiting a broad-spectral response. In addition to the enhanced density of photo-generated electrons, significant temperature elevation is observed for the Bi₂S₃/WO₃ composite photoanode under the illumination of infrared part of solar light because of the photothermal conversion property of Bi₂S₃. The moderately enhanced temperature accelerates charge carrier migration and finally increases the efficiency of solar energy conversion. With the assistance of photothermal effect, a remarkable photocurrent density of 4.05 mA cm^-2 at 1.23 V vs. reversible reference electrode (V_RHE) is achieved by Bi₂S₃/WO₃ composite photoanode, over 880% higher than that of the pristine WO₃. The introduction of photothermal effect activated by infrared light provides general and robust strategy to promote the PEC performance of photoanodes.

Controllable design of defect-rich hybrid iron oxide nanostructures on mesoporous carbon-based scaffold for pseudocapacitive applications

The controllable design of functional nanostructures for energy and environmental applications represents a critical yet challenging technology. The existing fabrication strategies focus mainly on increasing the number of accessible active sites. However, these techniques generally necessitate complex chemical agents and suffer from limited experimental conditions delivering high costs, low yields, and poor reproducibility. The present work reports a new strategy for controllable synthesis of a hybrid system including mixed iron oxide nanostructures enriched with non-stoichiometric Fe21.34O32 and Fe3+[Fe5/33+□1/32+]O4 phases, which possess a high concentration of oxygen and Fe2+ vacancies, and a mesoporous carbon-based scaffold (MCS), which was dervied from coffee residues, with graphitic surface and perforated architecture. The nanoperforates acted as trapping sites to localise the FexOy nanoparticles, thereby boosting the density of accessible active sites. Additionally, at the interfacial regions between the FexOy crystallites, a high density of oxygen vacancies with an oriented pattern was shown to create superlattice structures. The energy storage functionality of the defect-rich MCS/FexOy nanostructure with nanoperforated architecture was investigated, where the results exhibited a high gravimetric capacitance of 540 F g-1 at a current density of 1 A g-1 with outstanding capacitance retention of 73.6% after 14 000 cycles.

Enhanced visible-light photocatalytic degradation by Mn3O4/CeO2 heterojunction: a Z-scheme system photocatalyst

A novel 1-D Mn₃O₄/CeO₂ heterojunction exhibited enhanced photocatalytic degradation via a Z-scheme system.

Non-classical behaviour of higher valence dopants in chromium (III) oxide by a Cr vacancy compensation mechanism

Modification of metal oxides with dopants that have a stable oxidation in their parent oxides which is higher than the host system is expected to introduce extra electrons into the material to improve carrier mobility. This is essential for applications in catalysis, SOFCs and solar energy materials. Density functional theory calculations are used to investigate the change in electronic and geometric structure of chromium (III) oxide by higher valence dopants, namely; Ce, Ti, V and Zr. For single metal doping, we find that the dopants with variable oxidation states, Ce, Ti and V, adopt a valence state of  +3, while Zr dopant has a  +4 oxidation state and reduces a neighbouring Cr cation. Chromium vacancy formation is greatly enhanced for all dopants, and favoured over oxygen vacancy formation. The Cr vacancies generate holes which oxidise Ce, Ti and V from  +3 to  +4, while also oxidising lattice oxygen sites. For Zr doping, the generated holes oxidise the reduced Cr²⁺ cation back to Cr³⁺ and also two lattice oxygen atoms. Three metal atoms in the bulk lattice facilitate spontaneous Cr vacancy from charge compensation. A non-classical compensation mechanism is observed for Ce, Ti and V; all three metals are oxidised from  +3 to  +4, which explains experimental observations that these metals have a  +4 oxidation state in Cr₂O₃. Charge compensation of the three Zr metals proceeds by a classical higher valence doping mechanism; the three dopants reduce three Cr cations, which are subsequently charge compensated by a Cr vacancy oxidising three Cr²⁺ to Cr³⁺. The compensated structures are the correct ground state electronic structure for these doped systems, and used as a platform to investigate cation/anion vacancy formation. Unlike the single metal doped bulks, preference is now given for oxygen vacancy formation over Cr vacancy formation, indicating that the dopants increase the reducibility of Cr₂O₃ with Ce doping showing the strongest enhancement. The importance of the correct ground state in determining the formation of defects is emphasised.

Effect of doping and chemical ordering on the optoelectronic properties of complex oxides: Fe2O3-V2O3 solid solutions and hetero-structures

The electronic and optical properties of α-(Fe_1-xV_x)₂O₃ at low (x = 0.04) and high (x = 0.5) doping levels are investigated using a combination of periodic and embedded cluster approaches, and time-dependent density functional theory. At low V concentrations the onset of the optical absorption is ∼0.5 eV (i.e., nearly 1.6 eV lower than that in pure α-Fe₂O₃) and corresponds to the electron transitions from V 3d to Fe 3d* orbitals. At high V concentrations, optical absorption energies and intensities are sensitive to specific arrangements of Fe and V atoms and their spin configuration that determine Fe-V hybridization. The onset of the lowest inter-vanadium absorption band in the case of Fe₂O₃/V₂O₃ hetero-structures is as low as ∼0.3 eV and the corresponding peak is at ∼0.7 eV. In contrast, in the case of solid solutions this peak has lower intensity and is shifted to higher energy (∼1.2 eV). Analysis of the orbital character of electronic excitation suggests that Fe₂O₃/V₂O₃ hetero-structures absorb light much more effectively than random alloys, thus promoting efficient photo-induced carrier generation. These predictions can be tested in α-(Fe_1-xV_x)₂O₃ thin films synthesized with well-controlled spatial distribution of Fe and V species.