Optimal methodology for explicit solvation prediction of band edges of transition metal oxide photocatalysts

Surface Adsorption and Photoinduced Degradation: A Study of Spinel Ferrite Nanomaterials for Removal of a Model Organic Pollutant from Water

Spinel oxide nanocrystals are attractive materials for photoinduced advanced oxidation processes that degrade organic pollutants in water due to their chemical stability and tunability, visible light absorption, and magnetic recoverability. However, a systematic understanding of the structural and chemical factors that control the reactivity of specific spinel oxide nanocrystal materials toward photoinduced degradation processes is lacking. This Perspective illustrates these knowledge gaps through an investigation into the impacts of surface chemistry and composition of spinel ferrite nanocrystals of formula MFe2O4 (M = Mg, Fe, Co, Ni, Cu, Zn) on their ability to remove a model organic pollutant (methyl orange (MO)) from water. We identify two mechanisms by which the nanocrystals remove MO from water: (i) surface adsorption and (ii) photoinduced degradation under visible light irradiation in the presence of hydrogen peroxide via the photo-Fenton reaction. Nanocrystals that do not contain any surface ligands are more effective at removing MO from water than nanocrystals that contain surface ligands, despite our observation that the ligand-less nanocrystals do not form stable colloidal dispersions in water, while ligand-coated nanocrystals are colloidally stable. For many of the spinel ferrite compositions studied here, the fraction of methyl orange removal via adsorption to the nanocrystal surface in the absence of photoexcitation is larger than the fraction removed under irradiation. Our data indicate that the composition-dependent surface charge of the nanocrystals controls the degree of surface adsorption of the charged MO molecule. Overall, these results demonstrate that careful consideration of the impacts of surface chemistry on the behavior of spinel ferrite nanocrystals is required to accurately assess and subsequently understand their activity toward the photoinduced degradation of organic molecules.

Photoelectrochemical Methods for the Determination of the Flat-Band Potential in Semiconducting Photocatalysts: A Comparison Study

In addition to the band gap of a semiconducting photocatalyst, its band edges are important because they play a crucial role in the analysis of charge transfer and possible pathways of the photocatalytic reaction. The Mott-Schottky method using electrochemical impedance spectroscopy is the most common experimental technique for the determination of the electron potential in photocatalysts. This method is well suited for large crystals, but in the case of nanocatalysts, when the thickness of the charged layer is comparable with the size of the nanocrystals, the capacitance of the Helmholtz layer can substantially affect the measured potential. A contact between the electrolyte and the substrate, used for deposition of the photocatalyst, also affects the impedance. Application of other photoelectrochemical methods may help to avoid concerns in the interpretation of impedance data and improve the reliability of measurements. In this study, we have successfully prepared five visible-light active photocatalysts (i.e., N-doped TiO2, WO3, Bi2WO6, CoO, and g-C3N4) and measured their flat-band potentials using four (photo)electrochemical methods. The potentials are compared for all methods and discussed regarding the type of semiconducting material and its properties. The effect of methanol as a sacrificial agent for the enhanced transfer of charge carriers is studied and discussed for each method.

Understanding the electronic structure of Y2Ti2O5S2 for green hydrogen production: a hybrid-DFT and GW study

Utilising photocatalytic water splitting to produce green hydrogen is the key to reducing the carbon footprint of this crucial chemical feedstock. In this study, density functional theory (DFT) is employed to gain insights into the photocatalytic performance of an up-and-coming photocatalyst Y2Ti2O5S2 from first principles. Eleven non-polar clean surfaces are evaluated at the generalised gradient approximation level to obtain a plate-like Wulff shape that agrees well with the experimental data. The (001), (101) and (211) surfaces are considered further at hybrid-DFT level to determine their band alignments with respect to vacuum. The large band offset between the basal (001) and side (101) and (211) surfaces confirms experimentally observed spatial separation of hydrogen and oxygen evolution facets. Furthermore, relevant optoelectronic bulk properties were established using a combination of hybrid-DFT and many-body perturbation theory. The optical absorption of Y2Ti2O5S2 weakly onsets due to dipole-forbidden transitions, and hybrid Wannier-Mott/Frenkel excitonic behaviour is predicted to occur due to the two-dimensional electronic structure, with an exciton binding energy of 0.4 eV.

Potential Application of Perovskite Structure for Water Treatment: Effects of Band Gap, Band Edges, and Lifetime of Charge Carrier for Photocatalysis

Perovskite structures have attracted scientific interest as a promising alternative for water treatment due to their unique structural, high oxidation activity, electronic stability, and optical properties. In addition, the photocatalytic activity of perovskite structures is higher than that of many transition metal compounds. A critical property that determines the high-performance photocatalytic and optical properties is the band gap, lifetime of carrier charge, and band edges relative to the redox potential. Thus, the synthesis/processing and study of the effect on the band gap, lifetime of carrier charge, and band edges relative to the redox potential in the development of high-performance photocatalysts for water treatment are critical. This review presents the basic physical principles of optical band gaps, their band gap tunability, potentials, and limitations in the applications for the water treatment. Furthermore, it reports recent advances in the synthesis process and comparatively examines the band gap effect in the photocatalytic response. In addition to the synthesis, the physical mechanisms associated with the change in the band gap have been discussed. Finally, the conclusions of this review, along with the current challenges of perovskites for photocatalysis, are presented.

Understanding the Photocatalytic Activity of La5Ti2AgS5O7 and La5Ti2CuS5O7 for Green Hydrogen Production: Computational Insights

Green production of hydrogen is possible with photocatalytic water splitting, where hydrogen is produced while water is reduced by using energy derived from light. In this study, density functional theory (DFT) is employed to gain insights into the photocatalytic performance of La₅Ti₂AgS₅O₇ and La₅Ti₂CuS₅O₇-two emerging candidate materials for water splitting. The electronic structure of both bulk materials was calculated by using hybrid DFT, which indicated the band gaps and charge carrier effective masses are suitable for photocatalytic water splitting. Notably, the unique one-dimensional octahedral TiO _x S_6-x and tetragonal MS₄ channels formed provide a structural separation for photoexcited charge carriers which should inhibit charge recombination. Band alignments of surfaces that appear on the Wulff constructions of 12 nonpolar symmetric surface slabs were calculated by using hybrid DFT for each of the materials. All surfaces of La₅Ti₂AgS₅O₇ have band edge positions suitable for hydrogen evolution; however, the small overpotentials on the largest facets likely decrease the photocatalytic activity. In La₅Ti₂CuS₅O₇, 72% of the surface area can support oxygen evolution thermodynamically and kinetically. Based on their similar electronic structures, La₅Ti₂AgS₅O₇ and La₅Ti₂CuS₅O₇ could be effectively employed in Z-scheme photocatalytic water splitting.

Eliminating Delocalization Error to Improve Heterogeneous Catalysis Predictions with Molecular DFT + U

Approximate semilocal density functional theory (DFT) is known to underestimate surface formation energies yet paradoxically overbind adsorbates on catalytic transition-metal oxide surfaces due to delocalization error. The low-cost DFT + U approach only improves surface formation energies for early transition-metal oxides or adsorption energies for late transition-metal oxides. In this work, we demonstrate that this inefficacy arises due to the conventional usage of metal-centered atomic orbitals as projectors within DFT + U. We analyze electron density rearrangement during surface formation and O atom adsorption on rutile transition-metal oxides to highlight that a standard DFT + U correction fails to tune properties when the corresponding density rearrangement is highly delocalized across both metal and oxygen sites. To improve both surface properties simultaneously while retaining the simplicity of a single-site DFT + U correction, we systematically construct multi-atom-centered molecular-orbital-like projectors for DFT + U. We demonstrate this molecular DFT + U approach for tuning adsorption energies and surface formation energies of minimal two-dimensional models of representative early (i.e., TiO₂) and late (i.e., PtO₂) transition-metal oxides. Molecular DFT + U simultaneously corrects adsorption energies and surface formation energies of multilayer models of rutile TiO₂(110) and PtO₂(110) to resolve the paradoxical description of surface stability and surface reactivity of semilocal DFT.

Characterization of Cu2O/CuO heterostructure photocathode by tailoring CuO thickness for photoelectrochemical water splitting

Cu₂O/CuO heterostructure is a well-known strategy to improve the performance of Cu₂O photocathodes for photoelectrochemical (PEC) water splitting. The CuO thickness in the Cu₂O/CuO heterostructure is considered as a critical factor affecting the PEC performance because it is highly related to the light utilization and charge separation/transport. In this study, the Cu₂O/CuO photocathode tailoring the CuO thickness was investigated to examine the CuO thickness influence on the PEC performance. Cu₂O/CuO photocathodes were prepared by the electrodeposition and subsequent thermal annealing process and the Cu₂O/CuO heterostructure was controlled by the annealing temperature and time. It was demonstrated that the increased CuO thickness enhances the light absorption in the long wavelength region and improves the charge separation by the reinforced band bending. However, the thick CuO hinders the efficient charge transport in the Cu₂O/CuO heterostructure, resulting in the decreased PEC performance. Therefore, it is necessary to optimize the CuO thickness for the enhanced PEC performance of Cu₂O/CuO photocathodes. Consequently, the Cu₂O/CuO photocathode consisting of the similar CuO thickness with its minority carrier diffusion length (∼90 nm) was fabricated by annealing at 350 °C for 20 min, and it shows the optimal PEC performance (−1.2 mA cm⁻² at 0 V vs. RHE) in pH 6.5 aqueous solution, resulting from the enhanced light utilization and the reinforced band bending.

Optimization of CuO thickness in the Cu₂O/CuO photocathode by controlling the annealing time: optimal thickness of CuO induces the improved light utilization and band bending, resulting in the enhanced photoelectrochemical performance.

Evaluation of electrochemical properties of nanostructured metal oxide electrodes immersed in redox-inactive organic media

This paper describes analysis of dropcast nanocrystalline and electrochemically deposited films of NiO and α-Fe₂O₃ as model metal oxide semiconductors immersed in redox-inactive organic electrolyte solutions using electrochemical impedance spectroscopy (EIS). Although the data reported here fit a circuit commonly used to model EIS data of metal oxide electrodes, which comprises an RC circuit nested inside a second RC circuit that is in series with a resistor, our interpretation of the physical meaning of these circuit elements differs from that applied to EIS measurements of metal oxide electrodes immersed in redox-active media. The data presented here are most consistent with an interpretation in which the nested RC circuit represents charge transfer between the metal oxide film and the underlying metal electrode, and the non-nested RC circuit represents the resistance and capacitance associated with formation of a charge-compensating double-layer at the exposed interface between the metal electrode and electrolyte solution. Applying this interpretation to analysis of EIS data collected for metal oxide films in organic media enables the impact of film morphology on electrochemical behavior to be distinguished from the effects of the intrinsic electronic structure of the metal oxide. This distinction is crucial to the evaluation of nanostructured metal oxide electrodes for electrochemical energy storage and electrocatalysis applications.

Spontaneously formed gradient chemical compositional structures of niobium doped titanium dioxide nanoparticles enhance ultraviolet- and visible-light photocatalytic performance

Semiconductor photocatalysts showing excellent performance under irradiation of both ultraviolet (UV)- and visible (VIS)-light are highly demanded towards realization of sustainable energy systems. TiO2 is one of the most common photocatalysts and has been widely investigated as candidate showing UV/VIS responsive performance. In this study, we report synthesis of Nb doped TiO2 by environmentally benign mechanochemical reaction. Nb atoms were successfully incorporated into TiO2 lattice by applying mechanical energy. As synthesized Nb doped TiO2 were metastable phase and formed chemical compositional gradient structure of poorly Nb doped TiO2 core and highly Nb doped TiO2 surface after high temperature heat treatment. It was found that formed gradient chemical compositional heterojunctions effectively enhanced photocatalytic performance of Nb doped TiO2 under both of UV- and VIS-light irradiation, which is different trend compared with Nb doped TiO2 prepared through conventional methods. The approach shown here will be employed for versatile systems because of simple and environmentally benign process.

Reversible Rapid Hydrogen Doping of WO3 in Non-Acid Solution

Hydrogenation, an effective way to tune the properties of transition metal oxide (TMO) thin films, has been long awaited to be performed safely and without an external energy input. Recently, metal-acid-TMO has been reported to be an effective approach for hydrogenation, but the requirement of acid limits its application. In this work, the reversible and rapid hydrogen doping of WO₃ in NaOH(aq) | Al(s) | WO₃(s) is revealed by structural and electrical measurements. Accompanied by the structural phase transition identified by in situ X-ray diffraction, the electric resistance of the WO₃ film is found to be able to change by 5 orders of magnitude. A significant electrical response of touching, 8-fold in amplitude and 3 s in a cycle, can be achieved in the low-resistance state. These reactions are reversible at room temperature. This study unambiguously proves that the hydrogenation-driven dynamic phase transition of WO₃ in metal-solution-WO₃ systems could occur not only in acid solutions but also in some non-acid environments. Unlike the monotonic increase of resistance revealed during H_δWO₃ to WO₃ transition, an intriguing non-monotonic evolution was found for crystal lattice parameter c, indicating that the mechanism of WO₃ hydrogenation involves a series of metastable states, more comprehensive and reasonable. This work sheds light on the potential applications of metal-solution-TMO hydrogenation in touching sensors, circuits survey, and information storage.

Solid Solutions Rb0.95NbxMo2-xO6.475-0.5x (x = 1.31-1.625) with Orthorhombic β-Pyrochlore Structure: Thermal Behavior and Electronic Structure of β-Pyrochlores Compounds Based on [Nb(Ta)/Mo] Octahedral Framework

Solid solutions Rb_0.95Nb_xMo_2-xO_6.475-0.5x (x = 1.31-1.625) having a β-pyrochlore structure with an orthorhombic system were synthesized by solid-state reaction. The elemental composition was confirmed by X-ray microanalysis. The Rb_0.95Nb_1.375Mo_0.625O_5.79 structure refinement was performed using the Rietveld method. The crystal structure consists of ordered O-Mo-O chains partly occupied by Nb atoms. The oxygen vacancies are necessary to save the electroneutrality of the unit cell. It predominantly appears between Mo atoms that lead to form two disconnected defect octahedra [MoO₅□···MoO₅□]. The structural defects cause the low thermal stability; the compounds obtained decompose in the 748-758 °C temperature range. The high-temperature phase transition of the CsNbMoO₆ and CsTaMoO₆ nonlinear optical β-pyrochlores has been studied by differential thermal analysis, differential scanning calorimetric analysis, high-temperature X-ray diffraction analysis, and second harmonic generation analysis. At room temperature the compounds possess the cubic noncentrosymmetric F4̅3m cell. Under heating to 437 °C and 401 °C for CsNbMoO₆ and CsTaMoO₆, respectively, they undergo transition into centrosymmetric Fd3̅m modification. This is accompanied by the SHG signal disappearing, as well as the 402 reflection, which is characteristic of the F4̅3m space group. The positions of the valence and conduction bands were determined by reflectance spectra and XPS analysis for structure-related β-pyrochlores CsNbMoO₆, CsTaMoO₆, and Rb_0.95Nb_1.375Mo_0.625O_5.79.

Do HOMO-LUMO Energy Levels and Band Gaps Provide Sufficient Understanding of Dye-Sensitizer Activity Trends for Water Purification?

A dye-sensitized solar cell assembly can be used to harvest solar energy, while suitable dye sensitizers can be used to purify water. Here, we characterized the activity trends of four dye sensitizers, namely, PORPC-1, PORPC-2, PORPC-3, and PORPC-4, for water purification applications using density functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE), B3LYP, and PBE0 functionals, ΔSCF, time-dependent DFT (TD-DFT), and quasiparticle Green's function (GW) methods. The energy levels of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) were calculated using gas-phase and aqueous-phase methods in order to understand charge-injection abilities and the dye regeneration processes. PBE, B3LYP, PBE0, and TD-DFT methods failed to predict PORPC-4 to be the best sensitizer, while PORPC-2 and PORPC-4 were predicted to be the best sensitizers using ΔSCF coupled with the implicit solvation method, and HOMO-LUMO energies were corrected for the aqueous environment in the GW calculations. However, none of these methods accurately predicted the performance trend of all four dye sensitizers. Consequently, we used the aggregation assembly patterns of the dye molecules in an aqueous environment to further probe the activity trends and found that PORPC-3 and PORPC-4 prefer J-aggregated assembly patterns, whereas PROPC-1 and PORPC-2 prefer to be H-aggregated. Therefore, the performance of these dye molecules can be determined by combining HOMO-LUMO energy levels with aggregate-assembly patterns, with the activity trend predicted to be PORPC-4 > PORPC-2 > PORPC-3 > PORPC-1, which is in good agreement with experimental findings.