Water splitting and the band edge positions of TiO2

Boosting Photocatalytic Water Splitting of Polymeric C60 by Reduced Dimensionality from Two-Dimensional Monolayer to One-Dimensional Chain

The recent synthesis of monolayer fullerene networks (Hou, L., et al. Nature 2022, 606, 507) provides new opportunities for photovoltaics and photocatalysis because of their versatile crystal structures for further tailoring of electronic, optical, and chemical function. To shed light on the structural aspects of the photocatalytic water splitting performance of fullerene nanomaterials, we compare the photocatalytic properties of individual polymeric fullerene chains and monolayer fullerene networks from first-principles calculations. We find that the photocatalytic efficiency can be further optimized by reducing the dimensionality from two-dimensional (2D) to one-dimensional (1D). The conduction band edge of the polymeric C60 chain provides an external potential for the hydrogen reduction reaction much higher than that of its monolayer counterparts over a wider range of pH values, and there are 2 times more surface active sites in the 1D chain than in the 2D networks from a thermodynamic perspective. These observations identify the 1D fullerene polymer as a more promising candidate as a photocatalyst for the hydrogen evolution reaction in comparison to monolayer fullerene networks.

Water effect on the band edges of anatase TiO2 surfaces: A theoretical study on charge migration across surface heterojunctions and facet-dependent photoactivity

The charge migration mechanism across the surface heterojunction constructed on an anatase TiO2 nanocrystal is still under debate. To solve this longstanding question, we present a systematic study of the band edges (vs. standard hydrogen electrode, SHE) of aqueous TiO2 interfaces with anatase (101), (100) and (001) surfaces, using a combination of density functional theory-based molecular dynamics (DFTMD) and efficient computational SHE (cSHE) methods. Our calculations show that the conduction band minimum (CBM) of the (101) surface is lower than that of (001) and (100) surfaces, which is thermodynamically favorable for electrons migrating to the (101) surface through the surface heterojunction, while the hole preferentially accumulates on the (100) surface due to its highest valence band minimum (VBM). In addition, we qualitatively explore the facet-dependent photocatalytic activity of anatase TiO2. Due to the possession of both the beneficial atomic structure (with 100% undercoordinated Ti5c atoms at the surface) and electronic structure (more strongly oxidizing holes in the VBM and efficient electron-hole spatial separation separation), the (001) surface exhibits the most efficient photocatalytic performance for water oxidation. Furthermore, it is confirmed that the use of simplified theoretical models neglecting the detailed atomic structures of water at the aqueous interface is inadequate to predict the band alignment of semiconductors relative to water redox potentials, so that it may result in substantial errors in evaluating the photocatalytic performance of materials to be used for water splitting.

Monolayer Fullerene Networks as Photocatalysts for Overall Water Splitting

Photocatalytic water splitting can produce hydrogen in an environmentally friendly way and provide alternative energy sources to reduce global carbon emissions. Recently, monolayer fullerene networks have been successfully synthesized [Hou et al. Nature2022, 606, 507], offering new material candidates for photocatalysis because of their large surface area with abundant active sites, feasibility to be combined with other 2D materials to form heterojunctions, and the C₆₀ cages for potential hydrogen storage. However, efficient photocatalysts need a combination of a suitable band gap and appropriate positions of the band edges with sufficient driving force for water splitting. In this study, I employ semilocal density functional theory and hybrid functional calculations to investigate the electronic structures of monolayer fullerene networks. I find that only the weakly screened hybrid functional, combined with time-dependent Hartree–Fock calculations to include the exciton binding energy, can reproduce the experimentally obtained optical band gap of monolayer C₆₀. All the phases of monolayer fullerene networks have suitable band gaps with high carrier mobility and appropriate band edges to thermodynamically drive overall water splitting. In addition, the optical properties of monolayer C₆₀ are studied, and different phases of fullerene networks exhibit distinct absorption and recombination behavior, providing unique advantages either as an electron acceptor or as an electron donor in photocatalysis.

Hydrothermal synthesis of novel heterostructured Ag/TiO2 /CuFe2 O4 nanocomposite: Characterization, enhanced photocatalytic degradation of methylene blue dye, and efficient antibacterial studies

In this work, we reported the successful synthesis of novel Ag/TiO₂ /CuFe₂ O₄ ternary nanocomposite by hydrothermal technique by using TiO₂ /CuFe₂ O₄ binary nanocomposite precursor that was also prepared by hydrothermal treatment by using TiO₂ nanoparticles and CuFe₂ O₄ nanoparticles synthesized via sol-gel method. The synthesized nanomaterials were accessed for their morphological, structural, and optical properties. X-ray diffraction (XRD) study reveals the formation of pure Ag/TiO₂ /CuFe₂ O₄ ternary nanocomposite in which the Ag, TiO₂ , and CuFe₂ O₄ are in anatase, spinal, and cubic crystal phases, respectively. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses of Ag/TiO₂ /CuFe₂ O₄ ternary nanocomposite indicated granule-shaped morphology with bright spots of silver. The existence of Ti, O, Cu, Fe, and Ag without any other elements in the energy-dispersive X-ray spectroscopy (EDS) spectra of the prepared ternary nanocomposite depict its purity and its polycrystalline nature was confirmed by its selected area electron diffraction (SAED) pattern. The ternary nanocomposite was utilized for the methylene blue dye degradation with an optimum dose of 1.00 g/100 ml under ultraviolet (UV) light; the enhanced photocatalytic activity of the composite is attributed mainly due to the appreciable magnitudinal difference of positive charge of the valence band and negative charge of the conduction band of TiO₂ and CuFe₂ O₄ ; meanwhile, the interfacially placed Ag acts as a sink for the elections. Also, the ternary nanocomposite showed satisfactory antibacterial activities. PRACTITIONER POINTS: The prepared ternary nanocomposite showed effective results in dye degradation and satisfactory antibacterial property. The concentration of methylene dye has decreased considerably in every degradation process which was accessed through UV-vis studies. The highest degradation by using the ternary nanocomposite archived at pH = 6 Appreciable antibacterial activity was achieved against a few Gram-positive strains and Gram-negative strains of bacteria. This research activity can open a broad area of research towards textile dye degradation and antibacterial studies.

Engineering 2D Materials for Photocatalytic Water-Splitting from a Theoretical Perspective

Splitting of water with the help of photocatalysts has gained a strong interest in the scientific community for producing clean energy, thus requiring novel semiconductor materials to achieve high-yield hydrogen production. The emergence of 2D nanoscale materials with remarkable electronic and optical properties has received much attention in this field. Owing to the recent developments in high-end computation and advanced electronic structure theories, first principles studies offer powerful tools to screen photocatalytic systems reliably and efficiently. This review is organized to highlight the essential properties of 2D photocatalysts and the recent advances in the theoretical engineering of 2D materials for the improvement in photocatalytic overall water-splitting. The advancement in the strategies including (i) single-atom catalysts, (ii) defect engineering, (iii) strain engineering, (iv) Janus structures, (v) type-II heterostructures (vi) Z-scheme heterostructures (vii) multilayer configurations (viii) edge-modification in nanoribbons and (ix) the effect of pH in overall water-splitting are summarized to improve the existing problems for a photocatalytic catalytic reaction such as overcoming large overpotential to trigger the water-splitting reactions without using cocatalysts. This review could serve as a bridge between theoretical and experimental research on next-generation 2D photocatalysts.

TiO2 as a Photocatalyst for Water Splitting-An Experimental and Theoretical Review

Hydrogen produced from water using photocatalysts driven by sunlight is a sustainable way to overcome the intermittency issues of solar power and provide a green alternative to fossil fuels. TiO₂ has been used as a photocatalyst since the 1970s due to its low cost, earth abundance, and stability. There has been a wide range of research activities in order to enhance the use of TiO₂ as a photocatalyst using dopants, modifying the surface, or depositing noble metals. However, the issues such as wide bandgap, high electron-hole recombination time, and a large overpotential for the hydrogen evolution reaction (HER) persist as a challenge. Here, we review state-of-the-art experimental and theoretical research on TiO₂ based photocatalysts and identify challenges that have to be focused on to drive the field further. We conclude with a discussion of four challenges for TiO₂ photocatalysts-non-standardized presentation of results, bandgap in the ultraviolet (UV) region, lack of collaboration between experimental and theoretical work, and lack of large/small scale production facilities. We also highlight the importance of combining computational modeling with experimental work to make further advances in this exciting field.

Self-Consistent Potential Correction for Charged Periodic Systems

Supercell models are often used to calculate the electronic structure of local deviations from the ideal periodicity in the bulk or on the surface of a crystal or in wires. When the defect or adsorbent is charged, a jellium counter charge is applied to maintain overall neutrality, but the interaction of the artificially repeated charges has to be corrected, both in the total energy and in the one-electron eigenvalues and eigenstates. This becomes paramount in slab or wire calculations, where the jellium counter charge may induce spurious states in the vacuum. We present here a self-consistent potential correction scheme and provide successful tests of it for bulk and slab calculations.

Material Design for Photocatalytic Water Splitting from a Theoretical Perspective

Currently, problems associated with energy and environment have become increasingly serious. Producing hydrogen, a clean and renewable resource, through photocatalytic water splitting using solar energy is a feasible and efficient route for resolving these problems, and great efforts have been devoted to improve the solar-to-hydrogen efficiency. Light harvesting and electron-hole separation are key in enhancing the efficiency of solar energy utilization, which stimulates the development of new photocatalytic materials. Here, recent advances in material design for photocatalytic water splitting are presented from a theoretical perspective. Specifically, aiming to enhance the photocatalytic performance, general strategies of materials design are discussed, including codoping and introducing a built-in electric field to improve the light harvesting of materials, reducing the dimension of materials to shorten the migration pathway of carriers to inhibit electron-hole recombination, and constructing heterojunctions to enhance light harvesting and electron-hole separation. Future opportunities and challenges in the theoretical design of photocatalytic materials toward water splitting are also included.

Intrinsically Activated SrTiO3: Photocatalytic H2 Evolution from Neutral Aqueous Methanol Solution in the Absence of Any Noble Metal Cocatalyst

Noble metal cocatalysts are conventionally a crucial factor in oxide-semiconductor-based photocatalytic hydrogen generation. In the present work, we show that optimized high-temperature hydrogenation of commercially available strontium titanate (SrTiO₃) powder can be used to engineer an intrinsic cocatalytic shell around nanoparticles that can create a photocatalyst that is highly effective without the use of any additional cocatalyst for hydrogen generation from neutral aqueous methanol solutions. This intrinsic activation effect can also be observed for SrTiO₃[100] single crystal as well as Nb-doped SrTiO₃[100] single crystal. For all types of SrTiO₃ samples (nanopowders and either of the single crystals), hydrogenation under optimum conditions leads to a surface-hydroxylated layer together with lattice defects visible by transmission electron microscopy, electron paramagnetic resonance (EPR), and photoluminescence (PL). Active samples provide specific defects identified by EPR, PL, and electron-energy loss spectroscopy as Ti³⁺ states in a defective matrix-this is in contrast to the inactive defects formed in other reductive atmospheres. In aqueous media, active SrTiO₃ samples show a significant negative shift of the flatband potential (in photoelectrochemical as well as in capacitance data) and a lower charge-transfer resistance for photoexcited electrons. We therefore ascribe the remarkable cocatalyst-free activation of the material to a synergy between thermodynamics (altered interface energetics induced by hydroxylation) and kinetics (charge transfer mediation by suitable Ti³⁺ states).

Black TiO2−x with stable surface oxygen vacancies as the support of efficient gold catalysts for water-gas shift reaction

H₂-etching engineered oxygen vacancies on black TiO_2−x to enhance the hot-electron flow and water-gas shift catalytic performance of Au catalysts.

Theoretical Determination of Band Edge Alignments at the Water-CuInS2(112) Semiconductor Interface

Knowledge of a semiconductor electrode's band edge alignment is essential for optimizing processes that occur at the semiconductor/electrolyte interface. Photocatalytic processes are particularly sensitive to such alignments, as they govern the transfer of photoexcited electrons or holes from the surface to reactants in the electrolyte solution. Reconstructions of a semiconductor surface during operation, as well as its interaction with the electrolyte solution, must be considered when determining band edge alignment. Here, we employ density functional theory + U theory to assess the stability of reconstructed CuInS₂ surfaces, a system which has shown promise for the active and selective photoelectrocatalytic reduction of CO₂ to CH₃OH. Using many-body Green's function theory combined with calculations of surface work functions, we determine band edge positions of explicitly solvated, reconstructed CuInS₂ surfaces. We find that there is a linear relationship between band edge position and net surface dipole, with the most stable solvent/surface structures tending to minimize this dipole because of generally weak interactions between the surface and solvating water molecules. We predict a conduction band minimum (CBM) of the solvated, reconstructed CuInS₂ surface of -2.44 eV vs vacuum at the zero-dipole intercept of the dipole/CBM trendline, in reasonable agreement with the experimentally reported CBM position at -2.64 eV vs vacuum. This methodology offers a simplified approach for approximating the band edge positions at complex semiconductor/electrolyte interfaces.

Role of Oxygen Deposition Pressure in the Formation of Ti Defect States in TiO2(001) Anatase Thin Films

We report the study of anatase TiO₂(001)-oriented thin films grown by pulsed laser deposition on LaAlO₃(001). A combination of in situ and ex situ methods has been used to address both the origin of the Ti³⁺-localized states and their relationship with the structural and electronic properties on the surface and the subsurface. Localized in-gap states are analyzed using resonant X-ray photoelectron spectroscopy and are related to the Ti³⁺ electronic configuration, homogeneously distributed over the entire film thickness. We find that an increase in the oxygen pressure corresponds to an increase in Ti³⁺ only in a well-defined range of deposition pressure; outside this range, Ti³⁺ and the strength of the in-gap states are reduced.

Black Magic in Gray Titania: Noble-Metal-Free Photocatalytic H2 Evolution from Hydrogenated Anatase

'Black' TiO₂ -in the widest sense, TiO₂ reduced by various treatments-has attracted tremendous scientific interest in recent years because of some outstanding properties; most remarkably in photocatalysis. While the material effects visible light absorption (the blacker, the better), black titania produced by high pressure hydrogenation was recently reported to show another highly interesting feature; noble-metal-free photocatalytic H₂ generation. In a systematic investigation of high-temperature hydrogen treatments of anatase nanoparticles, TEM, XRD, EPR, XPS, and photoelectrochemistry are used to characterize different degrees of surface hydrogenation, surface termination, electrical conductivity, and structural defects in the differently treated materials. The materials' intrinsic activity for photocatalytic hydrogen evolution is coupled neither with their visible light absorption behavior nor the formation of amorphous material, but rather must be ascribed to optimized and specific defect formation (gray is better than black). This finding is further confirmed by using a mesoporous anatase matrix as a hydrogenation precursor, which, after conversion to the gray state, even further enhances the overall photocatalytic hydrogen evolution activity.

Energy level alignment at semiconductor–water interfaces from atomistic and continuum solvation models

Efficient electronic energy level alignment at solid–liquid interfaces with continuum solvation models.

Photoanodes based on TiO2and α-Fe2O3for solar water splitting – superior role of 1D nanoarchitectures and of combined heterostructures

Solar driven photoelectrochemical water splitting represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as H₂.

Continuous flow gas phase photoreforming of methanol at elevated reaction temperatures sensitised by Pt/TiO2

Gas phase photoreforming of methanol using a Pt/TiO₂ photocatalyst has been performed under flow conditions at elevated temperatures.

Comparative density functional theory and density functional tight binding study of arginine and arginine-rich cell penetrating peptide TAT adsorption on anatase TiO2

A comparative DFT-DFTB study of geometries and electronic structures of arginine, arginine dipeptide, and arginine-rich cell penetrating peptide TAT on the surface of TiO₂.