Band gap narrowing of titanium oxide semiconductors by noncompensated anion-cation codoping for enhanced visible-light photoactivity

Advanced photocatalysis as a viable and sustainable wastewater treatment process: A comprehensive review

In our era, water pollution not only poses a serious threat to human, animal, and biotic life but also causes serious damage to infrastructure and the ecosystem. A set of physical, chemical, and biological technologies have been exploited to decontaminate and/or disinfect water pollutants, toxins, microbes, and contaminants, but none of these could be ranked as sustainable and scalable wastewater technology. The photocatalytic process can harmonize the sunlight to degrade certain toxins, chemicals, microbes, and antibiotics, present in water. For example, transition metal oxides (ZnO, SnO2, TiO2, etc.), when integrated into an organic framework of graphene or nitrides, can bring about more than 90% removal of dyes, microbial load, pesticides, and antibiotics. Similarly, a modified network of graphitic carbon nitride can completely decontaminate petrochemicals. The present review will primarily highlight the mechanistic aspects for the removal and/or degradation of highly concerned contaminants, factors affecting photocatalysis, engineering designs of photoreactors, and pros and cons of various wastewater treatment technologies already in practice. The photocatalytic reactor can be a more viable and sustainable wastewater treatment opportunity. We hope the researcher will find a handful of information regarding the advanced oxidation process accomplished via photocatalysis and the benefits associated with the photocatalytic-type degradation of water pollutants and contaminants.

Effects of La-N Co-Doping of BaTiO3 on Its Electron-Optical Properties for Photocatalysis: A DFT Study

In cation-anion co-doping, rare earth elements excel at regulating the electronic structure of perovskites, leading to their improved photocatalytic performance. In this regard, the impact of co-doping rare earth elements at the Ba and Ti sites in BaTiO3 on its electronic and photocatalytic properties was thoroughly investigated based on 2 × 2 × 2 supercell structures of BaTiO3 with different La concentrations of 12.5% and 25% using DFT calculations. The band structure, density of states, charge density difference, optical properties, and the redox band edge of the co-doped models mentioned above were analyzed. The results indicated that the BaTiO3 structure co-doped with 25% La at the Ti site exhibited higher absorption in the visible range and displayed a remarkable photocatalytic water-splitting performance. The introduced La dopant at the Ti site effectively reduced the energy required for electronic transitions by introducing intermediate energy levels within the bandgap. Our calculations and findings of this study provide theoretical support and reliable predictions for the exploration of BaTiO3 perovskites with superior photocatalytic performances.

Sketching Precursor Evolution to Delineate Growth Pathways for Anatase (TiO2 ) Crystal Design

Engineering advanced functional materials such as Anatase crystals through the molecular tuning of crystal facets is the current enigma of interest pertinent to solving the structure-property-performance triad. Developing optimal shapes and sizes of crystallite necessitates exploring the nanoscopic growth mechanism via precursor tracking. Here, the tapestry of particles varying in dimensionality (0D-3D), sizes (8-3000 nm), and morphology (aggregated to highly faceted crystals) is generated. To decipher and subsequently modulate the crystallization pathways, high-resolution microscopy (high-resolution transmission electron microscopy(HRTEM) and field emission scanning electron microscopy(FESEM)) is used to sketch time-stamped particle evolution. Interestingly, the studies provide evidence for 4-distinct mechanisms where nanoparticles/nanosheets play direct and/or indirect roles in crystallization through multi-stage aggregation (primary, secondary, and tertiary) beginning with similar growth solutions. The four distinct pathways elucidate bulk particle formation via non-classical routes of crystallization including nanosheet alignment and aggregation, nanocrystallite formation and fusion, nanobeads formation and attachment, and direct nanosheet incorporation in bulk crystals. Notably, the direct evidence of flexible-partially-ordered nanosheets being subsumed along the contours of bulk crystals is captured. These novel syntheses generated uniquely faceted particles with high-indexed surface planes such as (004), (200), and (105), amenable to photocatalytic applications.

Positive Charge Holes Revealed by Energy Band Theory in Multiphase Tix O2x-1 and Exploration of its Microscopic Electromagnetic Loss Mechanism

Although numerous experimental investigations have been carried out on the problem of defect engineering in semiconductor absorbers, the relationship among charge carrier, defects, heterointerfaces, and electromagnetic (EM) wave absorption has not been established systematically. Herein, the new thermodynamic and kinetic control strategy is proposed to establish multiphase Tix O2 x -1 (1 ≤ x ≤ 6) through a hydrogenation calcination. The TiOC-900 composite shows the efficient EM wave absorption capability with a minimum reflection loss (RLmin ) of -69.6 dB at a thickness of 2.04 mm corresponding to an effective absorption bandwidth (EAB) of 4.0 GHz due to the holes induced conductance loss and heterointerfaces induced interfacial polarization. Benefiting from the controllable preparation of multiphase Tix O2 x -1 , a new pathway is proposed for designing high-efficiency EM wave absorbing semiconducting oxides. The validity of the method for adopting energy band theory to explore the underlying relations among charge carriers, defects, heterointerfaces, and EM properties in multiphase Tix O2 x -1 is demonstrated for the first time, which is of great importance in optimizing the EM wave absorption performance by electronic structure tailoring.

Multiple strategies of improving photocatalytic water splitting efficiency in 2D arsenic sesquichalcogenides

Improving the solar-to-hydrogen efficiency has always been a significant topic in the field of photocatalysis. Based on first-principles calculations, herein, we propose multiple strategies to improve the photocatalytic properties of 2D arsenic sesquichalcogenides for full water splitting. The new configurations As2STe2 and As2SeTe2 monolayers, derived from the As2Te3 monolayers by surface modification, are manifested to be typical infrared-light driven photocatalysts. Notably, under the built-in electric field, As2STe2 and As2SeTe2 monolayers can fulfil overall water splitting and the predicted solar-to-hydrogen efficiencies even reach up to 36.19% and 29.36%, respectively. The Gibbs free energy calculations indicate that the OER can be successfully driven under light irradiation. In addition, the overpotentials can provide most of the energy for HER when illuminated, especially for As2STe2 with the . In addition, both As2S3 and As2Se3 monolayers are capable of satisfying the conditions for photocatalytic water splitting. Furthermore, the band gaps of As2Se3 and As2S3 can dramatically be narrowed by increasing the number of layers and doping, respectively. These findings provide a theoretical basis for As2X3 monolayers to achieve efficient photocatalytic water splitting.

Increasing the Efficiency of Photocatalytic Water Splitting via Introducing Intermediate Bands

Photocatalytic water splitting is a potential way to utilize solar energy. To be practically useful, it is important to have a high solar-to-hydrogen (STH) efficiency. In this study, we propose a conceptually new photocatalytic water splitting model based on intermediate bands (IBs). In this new model, introducing IBs within the band gap can significantly increase the STH efficiency limit (from 30.7% to 48.1% without an overpotential and from 13.4% to 36.2% with overpotentials) compared to that in conventional single-band gap photocatalytic water splitting. First-principles calculations indicate that N-doped TiO₂, Bi-doped TiO₂, and P-doped ZnO have suitable IBs that can be used to construct IB photocatalytic water splitting systems. The STH efficiency limits of these three doped systems are 10.0%, 12.0%, and 19.0%, respectively, while those of pristine TiO₂ and ZnO without IB are only 0.9% and 1.6%, respectively. The IB photocatalytic water splitting model proposed in this study opens a new avenue for photocatalytic water splitting design.

Hybrid density functional study on band structure engineering of ZnS(110) surface by anion–cation codoping for overall water splitting

The (Ru + C)-codoped ZnS(110) surface is predicted to be a potential candidate for solar-driven water splitting.

Copper Modified Titania Nanocomposites with a High Photocatalytic Inactivation of Escherichia coli

As everyone knows, bacterial infectious diseases are serious hazards to human health in the world. Despite performing many methods towards the bacterial pollution, containing many detection and sterilization techniques, there is still a lack of effective means. Herein, a novel copper modified titanium (Cu@TiO₂) nanocomposites were resoundingly synthesized via the well-known sol-gel process, which revealed a significant antibacterial activity under the illumination of sunlight. The XRD, Raman spectroscopy and TEM images showed that the Cu@TiO₂ nanocomposites with a globular shape are anatase phase, Moreover, low temperature physical adsorption test and UV- visible spectrum indicate Cu_0.01 @TiO₂ owns a supernal specific area (80 m²/g) and the high visible light absorbing ability. Furthermore, the novel Cu@TiO₂ nanocomposites showed an unprecedented photocatalytic capacity towards Escherichia Coli (E. coli) bacteria. In vitro, Cu@TiO₂ nanocomposites can kill almost 98.7% E. coli under 60 min simulated solar light irradiation than that of TiO ₂(31.3 %). This study suggests that the Cu@TiO₂ will be as a potential material for ameliorating antibiotic-resistant bacteria in food detection.

Ab Initio Studies of Bimetallic-Doped {0001} Hematite Surface for Enhanced Photoelectrochemical Water Splitting

First-principles calculations based on density functional theory (DFT) were carried out to study the energetic stability and electronic properties of a bimetallic-doped α-Fe2O3 photoanode surface with (Zn, Ti) and (Zn, Zr) pairs for enhanced PEC water splitting. The doped systems showed negative formation energies under both O-rich and Fe-rich conditions which make them thermodynamically stable and possible to be synthesised. It is found that in a bimetallic (Zn, Ti)-doped system, at a doping concentration of 4.20% of Ti, the bandgap decreases from 2.1 eV to 1.80 eV without the formation of impurity states in the bandgap. This is favourable for increased photon absorption and efficient movement of charges from the valance band maximum (VBM) to the conduction band minimum (CBM). In addition, the CBM becomes wavy and delocalised, suggesting a decrease in the charge carrier mass, enabling electron–holes to successfully diffuse to the surface, where they are needed for water oxidation. Interestingly, with single doping of Zr at the third layer (L3) of Fe atoms of the {0001} α-Fe2O3 surface, impurity levels do not appear in the bandgap, at both concentrations of 2.10% and 4.20%. Furthermore, at 2.10% doping concentration of α-Fe2O3 with Zr, CBM becomes delocalised, suggesting improved carrier mobility, while the bandgap is altered from 2.1 eV to 1.73 eV, allowing more light absorption in the visible region. Moreover, the photocatalytic activities of Zr-doped hematite could be improved further by codoping it with Zn because Zr is capable of increasing the conductivity of hematite by the substitution of Fe3+ with Zr4+, while Zn can foster the surface reaction and reduce quick recombination of the electron–hole pairs.

Photonic crystal light trapping for photocatalysis

The Achilles heel of wide-band photocatalysts such as TiO₂ is the insufficient photogeneration in the visible range under sunlight. This has been a longstanding impediment to large-scale, real-world deployment of titania-based photocatalysis applications. Instead of traditional band engineering through heavy-doping, we suggest enhancing photocatalytic efficiency of lightly-doped TiO₂ using photonic crystal (PC) structures. This strongly increases solar photogeneration through novel wave-interference-based light trapping. Four photocatalyst structures - simple cubic woodpile (wdp), square lattice nanorod (nrPC), slanted conical-pore (scPore), and face-centered cubic inverse opal (invop) - are optimized and compared for light harvesting in the sub- and above-gap (282 to 550 nm) regions of weakly absorbing TiO₂, with the imaginary part of the dielectric constant 0.01 in the visible range. The optimized lattice constants for the first three, and opal center-to-center distance for invop, are ∼300 - 350 nm. For fixed PC thickness, the ranking of visible light harvesting capability is: scPore > wdp ∼ nrPC > invop. The scPore PC deposited on highly reflective substrate is ideal for photocatalysis given its combination of enhanced light trapping and superior charge transport.

Wavelength-Dependent Water Oxidation on Rutile TiO2(110)

Understanding the microscopic mechanism of water photocatalysis on TiO₂ is of great value in energy chemistry and catalysis. To date, it is still unclear how water photocatalysis occurs after the initial light absorption. Here we report the investigation of the photoinduced water dissociation and desorption on a R-TiO₂(110) surface, at different wavelengths (from 250 to 330 nm), using temperature-programmed desorption and time-of-flight techniques. Primary photooxidation products, gas phase OH radicals and surface H atoms, were clearly observed at wavelengths of ≤290 nm. As the laser wavelength decreases from 290 to 250 nm, the relative yield of H₂O oxidation increases significantly. Likewise, photoinduced H₂O desorption was also observed in the range of 320-250 nm, and the relative yield of H₂O desorption also increases with a decrease in wavelength. The strong wavelength-dependent H₂O photooxidation and photodesorption suggest that the energy of charge carriers is important in these two processes. More importantly, the result raises doubt about the widely accepted photocatalysis model of TiO₂ in which the excess energy of charge carriers is useless for photocatalysis. In addition, the H₂O photooxidation is more likely initiated by nonthermalized holes and is accomplished on the ground state potential energy surface via a non-adiabatic decay process.

Theoretical insights of codoping to modulate electronic structure of [Formula: see text] and [Formula: see text] for enhanced photocatalytic efficiency

[Formula: see text] and [Formula: see text] are well known materials in the field of photocatalysis due to their exceptional electronic structure, high chemical stability, non-toxicity and low cost. However, owing to the wide band gap, these can be utilized only in the UV region. Thus, it's necessary to expand their optical response in visible region by reducing their band gap through doping with metals, nonmetals or the combination of different elements, while retaining intact the photocatalytic efficiency. We report here, the codoping of a metal and a nonmetal in anatase [Formula: see text] and [Formula: see text] for efficient photocatalytic water splitting using hybrid density functional theory and ab initio atomistic thermodynamics. The latter ensures to capture the environmental effect to understand thermodynamic stability of the charged defects at a realistic condition. We have observed that the charged defects are stable in addition to neutral defects in anatase [Formula: see text] and the codopants act as donor as well as acceptor depending on the nature of doping (p-type or n-type). However, the most stable codopants in [Formula: see text] mostly act as donor. Our results reveal that despite the response in visible light region, the codoping in [Formula: see text] and [Formula: see text] cannot always enhance the photocatalytic activity due to either the formation of recombination centers or the large shift in the conduction band minimum or valence band maximum. Amongst various metal-nonmetal combinations, [Formula: see text] (i.e. Mn is substituted at Ti site and S is substituted at O site), [Formula: see text] in anatase [Formula: see text] and [Formula: see text], [Formula: see text] in [Formula: see text] are the most potent candidates to enhance the photocatalytic efficiency of anatase [Formula: see text] and [Formula: see text] under visible light irradiation.

Manganese and Graphene Included Titanium Dioxide Composite Nanowires: Fabrication, Characterization and Enhanced Photocatalytic Activities

We report the detailed microstructural, morphological, optical and photocatalytic studies of graphene (G) and manganese (Mn) co-doped titanium dioxide nanowires (TiO₂(G–Mn) NWs) prepared through facile combined electrospinning–hydrothermal processes. The as-prepared samples were thoroughly characterized using X-ray diffraction (XRD), transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and diffuse reflectance spectroscopy (DRS). XRD studies reveal the formation of mixed anatase-rutile phases or rutile phase depending on the dopant (Mn) precursor concentrations in the electrospinning dope and calcination temperature. The evaluation of lattice parameters revealed that the incorporation of Mn species and carbon atoms in to the lattice of anatase or rutile TiO₂ could occur through substituting the sites of oxygen atoms. XPS results confirm the existence of Mn²⁺/Mn³⁺ within the TiO₂ NW. Raman spectroscopy provides the evidence for structural modification because of the graphene inclusion in TiO₂ NW. The optical band gap of G–Mn including TiO₂ is much lower than pristine TiO₂ as confirmed through UV-vis DRS. The photocatalytic activities were evaluated by nitric oxide (NOx) degradation tests under visible light irradiation. Superior catalytic activity was witnessed for rutile G–Mn-co-doped TiO₂ NW over their anatase counterparts. The enhanced photocatalytic property was discussed based on the synergistic effects of doped G and Mn atoms and explained by plausible mechanisms.

N-Promoted Ru1/TiO2 single-atom catalysts for photocatalytic water splitting for hydrogen production: a density functional theory study

In our Ru₁–N₁/TiO₂ single-atom catalyst system, isolated Ru₁ atoms act as active sites for the reduction of protons, and the TiO₂ support offers the photogenerated carriers, allowing for a hydrogen evolution activity comparable to that of Pd.

Synchronously achieved surface Bi0 metallisation and incorporation of Bi3+/5+ ions into a W6+, N3− doped TiO2 lattice by the hydrothermal-reduction method: surface plasmonic resonance effect for efficient photocatalysis

Vectorial charge transfer mechanisms in a bicrystalline framework of Bi⁰ surface deposited Bi^3+/5+, W⁶⁺ and N³⁻ doped TiO₂ under solar light irradiation.

Bilayer MoSe2/HfS2 Nanocomposite as a Potential Visible-Light-Driven Z-Scheme Photocatalyst

Visible-light-driven photocatalytic overall water splitting is deemed to be an ideal way to generate clean and renewable energy. The direct Z-scheme photocatalytic systems, which can realize the effective separation of photoinduced carriers and possess outstanding redox ability, have attracted a huge amount of interest. In this work, we have studied the photocatalytic performance of the bilayer MoSe2/HfS2 van der Waals (vdW) heterojunction following the direct Z-scheme mechanism by employing the hybrid density functional theory. Our calculated results show that the HfS2 and MoSe2 single layers in this heterojunction are used for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. The charge transfer between the two layers brought about an internal electric field pointing from the MoSe2 layer to the HfS2 slab, which can accelerate the separation of the photoinduced electron-hole pairs and support the Z-scheme electron migration near the interface. Excitingly, the optical absorption intensity of the MoSe2/HfS2 heterojunction is enhanced in the visible and infrared region. As a result, these results reveal that the MoSe2/HfS2 heterojunction is a promising direct Z-scheme photocatalyst for photocatalytic overall water splitting.

Reduced titania nanosheets as an effective visible-light germicide

Two-dimensional reduced titania nanosheets (RTNs), synthesized by a solvothermal method, reveal significant visible light-activated germicidal activity. XRD, XPS, EPR, TEM and Raman show successful reduction of anatase TiO₂, resulting in Ti³⁺ formation as well as an increase in the concentration of {001} facets. The RTNs possess a bandgap energy of approximately 2.86 eV, and demonstrate strong absorption over the visible spectrum. Under simulated solar light (λ = 320-780 nm, 70 mW cm^-2) the RTNs are found to completely inactivate Escherichia coli bacteria within 1 h. Our study indicates the RTNs have significant potential for ameliorating antibiotic-resistant bacteria in clinical settings under ambient lighting conditions.

Switchable Magnetic Anisotropy of Ferromagnets by Dual-Ion-Manipulated Orbital Engineering

Tailoring magnetic anisotropy of ferromagnetic films is a critical issue in constructing energy-efficient and high-density magnetic memory devices. Presently, the effective tunability was focused on a single-ion-manipulated electronic structure evolution. Here, we reported a new strategy of dual-ion-tuned orbital structure and magnetic anisotropy of ferromagnetic films. N-doped Fe/MgO bilayer films were deposited on shape memory alloy substrates which can generate a significant lattice strain on the films. Before the N ions participate into the manipulation, the Fe/MgO film shows an in-plane magnetic anisotropy, which may be due to excessive Fe-O orbital hybridization. Interestingly, the N and O ions synergistically manipulate electronic coordination of the Fe layer, which can be further modified by the lattice strain through a charge transfer among N-Fe-O. Under such effect, the magnetic anisotropy of the film is switchable from in-plane to perpendicular magnetic anisotropy (PMA). The X-ray line dichroism (XLD) characterization reveals that the anisotropy regulation is related to Fe 3d orbital evolution: N-Fe orbital hybridization promotes the Fe d_z² orbital occupation effectively, which is beneficial in increasing PMA by strengthening Fe-O orbital hybridization along the out-of-plane direction. However, the compressive strain induces a N-Fe-O charge transfer and reduces the Fe d_z² electronic occupation, which weakens the PMA of films. These findings provide a new dimensionality for regulating orbital performance of ferromagnetic materials and developing strain-assisted memory devices.

Stability of non-metal dopants to tune the photo-absorption of TiO2 at realistic temperatures and oxygen partial pressures: A hybrid DFT study

TiO2 anatase is considered to play a significant importance in energy and environmental research. However, for developing artificial photosynthesis with TiO2, the major drawback is its large bandgap of 3.2 eV. Several non-metals have been used experimentally for extending the TiO2 photo-absorption to the visible region of the spectrum. It's therefore of paramount importance to provide theoretical guidance to experiment about the kind of defects that are thermodynamically stable at a realistic condition (e.g. Temperature (T), oxygen partial pressure ([Formula: see text]), doping). However, disentangling the relative stability of different types of defects (viz. substitution, interstitial, etc.) as a function of charge state and realistic T, [Formula: see text] is quite challenging. We report here using state-of-the-art first-principles based methodologies, the stability and meta-stability of different non-metal dopants X (X = N, C, S, Se) at various charge states and realistic conditions. The ground state electronic structure is very accurately calculated via density functional theory with hybrid functionals, whereas the finite T and [Formula: see text] effects are captured by ab initio atomistic thermodynamics under harmonic approximations. On comparing the defect formation energies at a given T and [Formula: see text] (relevant to the experiment), we have found that Se interstitial defect (with two hole trapped) is energetically most favored in the p-type region, whereas N substitution (with one electron trapped) is the most abundant defect in the n-type region to provide visible region photo-absorption in TiO2. Our finding validates that the most stable defects in X doped TiO2 are not the neutral defects but the charged defects. The extra stability of [Formula: see text] is carefully analyzed by comparing the individual effect of bond-making/breaking and the charge carrier trapping energies.

Niobium-Doped TiO2: Effect of an Interstitial Oxygen Atom on the Charge State of Niobium

The structural and electronic properties of Nb-doped rutile TiO₂ with several doping configurations were investigated by first-principles calculations based on the density functional theory. The calculations show that although the band gap in the Nb_Ti, 2Nb_Ti, and Nb_Ti + O_i systems is small, the intragap states would be the electron-hole recombination center, leading to low photocatalytic efficiency. However, for the 2Nb_Ti + O_i configuration, the impurity states are mainly located at the top of the valence band and the electron-hole recombination would be inhibited, indicating relatively higher photocatalytic efficiency. On the basis of the charge-compensated theory, two electrons on the transition-metal Nb atoms compensate for the same amount of holes on the acceptor level of a nonmetal interstitial O atom, in the model of 2Nb_Ti + O_i. Such donor-acceptor codoping may not only suppress the electron-hole recombination but also maintain a reduced band gap, suggesting that the doping models would exhibit higher photocatalytic activity than pure TiO₂. The calculation results show that the interstitial O atoms play an essential role in manipulating the valence state of impurity Nb. The role of the interstitial O atom in Nb-doped rutile TiO₂ suggests that it can give rise to beneficial charge-compensation effects.

Activation of Water on MnOx-Nanocluster-Modified Rutile (110) and Anatase (101) TiO2 and the Role of Cation Reduction

Surface modification of titania surfaces with dispersed metal oxide nanoclusters has the potential to enhance photocatalytic activity. These modifications can induce visible light absorption and suppress charge carrier recombination which are vital in improving the efficiency. We have studied heterostructures of Mn4O6 nanoclusters modifying the TiO2 rutile (110) and anatase (101) surfaces using density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U). Such studies typically focus on the pristine surface, free of the point defects and surface hydroxyls present in real surfaces. In our study we have considered partial hydroxylation of the rutile and anatase surfaces and the role of cation reduction, via oxygen vacancy formation, and how this impacts on a variety of properties governing the photocatalytic performance such as nanocluster adsorption, light absorption, charge separation, and reducibility. Our results indicate that the modifiers adsorb strongly at the surface and that modification extends light absorption into the visible range. MnOx-modified titania can show an off-stoichiometric ground state, through oxygen vacancy formation and cation reduction spontaneously, and both modified rutile and anatase are highly reducible with moderate energy costs. Manganese ions are therefore present in a mixture of oxidation states. Photoexcited electrons and holes localize at cluster metal and oxygen sites, respectively. The interaction of water at the modified surfaces depends on the stoichiometry and spontaneous dissociation to surface bound hydroxyls is favored in the presence of oxygen vacancies and reduced metal cations. Comparisons with bare TiO2 and other TiO2-based photocatalyst materials are presented throughout.

Recent advances and strategies to tailor the energy levels, active sites and electron mobility in titania and its doped/composite analogues for hydrogen evolution in sunlight

Hydrogen production through photocatalytic water reduction, a potential path for future renewable and sustainable energy generation.

Blue Ti3+ self-doped TiO2 nanosheets with rich {001} facets for photocatalytic performance

The as-prepared BT_1.5TNs exhibits superior photocatalytic performance for RhB degradation due to Ti³⁺ doping.

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.

Intrinsic Electric Fields in Two-dimensional Materials Boost the Solar-to-Hydrogen Efficiency for Photocatalytic Water Splitting

Two-dimensional (2D) materials with the vertical intrinsic electric fields show great promise in inhibiting the recombination of photogenerated carriers and widening light absorption region for the photocatalytic applications. For the first time, we investigated the potential feasibility of the experimentally attainable 2D M₂X₃ (M = Al, Ga, In; X = S, Se, Te) family featuring out-of-plane ferroelectricity used in photocatalytic water splitting. By using first-principles calculations, all the nine members of 2D M₂X₃ are verified to be available photocatalysts for overall water splitting. The predicted solar-to-hydrogen efficiency of Al₂Te₃, Ga₂Se₃, Ga₂Te₃, In₂S₃, In₂Se₃, and In₂Te₃ are larger than 10%. Excitingly, In₂Te₃ is manifested to be an infrared-light driven photocatalyst, and its solar-to-hydrogen efficiency limit using the full solar spectrum even reaches up to 32.1%, which breaks the conventional theoretical efficiency limit.

Universal features underlying the magnetism in diluted magnetic semiconductors

Investigation of a diverse variety of wide band gap semiconductors and metal oxides that exhibit magnetism on substitutional doping has revealed the existence of universal features that relate the magnetic moment of the dopant to a number of physical properties inherent to the dopants and the hosts. The investigated materials consist of ZnO, GaN, GaP, TiO₂, SnO₂, Sn₃N₄, MoS₂, ZnS and CdS doped with 3d-transition metal atoms. The primary physical properties contributing to magnetism include the orbital hybridization and charge distribution, the d-band filling, d-band center, crystal field splitting, electron pairing energy and electronegativity. These features specify the strength of the spin-polarization induced by the dopants on their first nearest neighboring anions which in turn specify the long range magnetic coupling among the dopants through successively induced spin polarizations (SSP) on neighboring dopants. The proposed local SSP process for the establishment of the magnetic coupling among the TM-dopants appears as a competitor to other classical processes (superexchange, double exchange, etc). Furthermore, these properties can be used as a set of descriptors suitable for developing statistical predictive theories for a much larger class of magnetic materials.

Delocalized Impurity Phonon Induced Electron-Hole Recombination in Doped Semiconductors

Semiconductor doping is often proposed as an effective route to improving the solar energy conversion efficiency by engineering the band gap; however, it may also introduce electron-hole (e-h) recombination centers, where the determining element for e-h recombination is still unclear. Taking doped TiO₂ as a prototype system and by using time domain ab initio nonadiabatic molecular dynamics, we find that the localization of impurity-phonon modes (IPMs) is the key parameter to determine the e-h recombination time scale. Noncompensated charge doping introduces delocalized impurity-phonon modes that induce ultrafast e-h recombination within several picoseconds. However, the recombination can be largely suppressed using charge-compensated light-mass dopants due to the localization of their IPMs. For different doping systems, the e-h recombination time is shown to depend exponentially on the IPM localization. We propose that the observation that delocalized IPMs can induce fast e-h recombination is broadly applicable and can be used in the design and synthesis of functional semiconductors with optimal dopant control.

DFT study of benzyl alcohol/TiO2 interfacial surface complex: reaction pathway and mechanism of visible light absorption

We propose a new pathway for the adsorption of benzyl alcohol on the surface of TiO₂ and the formation of interfacial surface complex (ISC). The reaction free energies and reaction kinetics were thoroughly investigated by density functional calculations. The TiO₂ surfaces were modeled by clusters consisting of 4 Ti atoms and 18 O atoms passivated by H, OH group and H₂O molecules. Compared with solid-state calculations utilizing the periodicity of the materials, such cluster modeling allows inclusion of the high-order correlation effects that seem to be essential for the adsorption of organic molecules onto solid surfaces. The effects of both acidity and solvation are included in our calculations, which demonstrate that the new pathway is competitive with a previous pathway. The electronic structure calculations based on the relaxed ISC structures reveal that the chemisorption of benzyl alcohol on the TiO₂ surface greatly alters the nature of the frontier molecular orbitals. The resulted reduced energy gap in ISC matches the energy of visible light, showing how the adsorption of benzyl alcohol sensitizes the TiO₂ surface. Graphical Abstract The chemisorption of benzyl alcohol on TiO₂ surface greatly alters the nature of the frontier molecular orbitals and the formed interfacial surface complex can be sensitized by visible light.

Facile Synthesis of Vanadium-Doped Ni3S2 Nanowire Arrays as Active Electrocatalyst for Hydrogen Evolution Reaction

Ni₃S₂ nanowire arrays doped with vanadium(V) are directly grown on nickel foam by a facile one-step hydrothermal method. It is found that the doping can promote the formation of Ni₃S₂ nanowires at a low temperature. The doped nanowires show excellent electrocatalytic performance toward hydrogen evolution reaction (HER), and outperform pure Ni₃S₂ and other Ni₃S₂-based compounds. The stability test shows that the performance of V-doped Ni₃S₂ nanowires is improved and stabilized after thousands of linear sweep voltammetry test. The onset potential of V-doped Ni₃S₂ nanowire can be as low as 39 mV, which is comparable to platinum. The nanowire has an overpotential of 68 mV at 10 mA cm^-2, a relatively low Tafel slope of 112 mV dec^-1, good stability and high Faradaic efficiency. First-principles calculations show that the V-doping in Ni₃S₂ extremely enhances the free carrier density near the Fermi level, resulting in much improved catalytic activities. We expect that the doping can be an effective way to enhance the catalytic performance of metal disulfides in hydrogen evolution reaction and V-doped Ni₃S₂ nanowire is one of the most promising electrocatalysts for hydrogen production.

One-pot photoassisted synthesis, in situ photocatalytic testing for hydrogen generation and the mechanism of binary nitrogen and copper promoted titanium dioxide

One-pot photosynthesis and in situ testing of binary codoped Cu(0)–N-TiO₂ yields hydrogen rate 675 μmol g⁻¹ h⁻¹; photoluminescence shows mechanism by excitation of N substitutional site.

Enhanced photoelectrochemical performance of anatase TiO2 for water splitting via surface codoping

We explore the (Rh + F) surface codoping effect on anatase TiO₂ (101) and (001) facets for solar water splitting by performing extensive density functional theory calculations.

Substrate co-doping modulates electronic metal-support interactions and significantly enhances single-atom catalysis

Transitional metal nanoparticles or atoms deposited on appropriate substrates can lead to highly economical, efficient, and selective catalysis. One of the greatest challenges is to control the electronic metal-support interactions (EMSI) between the supported metal atoms and the substrate so as to optimize their catalytic performance. Here, from first-principles calculations, we show that an otherwise inactive Pd single adatom on TiO₂(110) can be tuned into a highly effective catalyst, e.g. for O₂ adsorption and CO oxidation, by purposefully selected metal-nonmetal co-dopant pairs in the substrate. Such an effect is proved here to result unambiguously from a significantly enhanced EMSI. A nearly linear correlation is noted between the strength of the EMSI and the activation of the adsorbed O₂ molecule, as well as the energy barrier for CO oxidation. Particularly, the enhanced EMSI shifts the frontier orbital of the deposited Pd atom upward and largely enhances the hybridization and charge transfer between the O₂ molecule and the Pd atom. Upon co-doping, the activation barrier for CO oxidation on the Pd monomer is also reduced to a level comparable to that on the Pd dimer which was experimentally reported to be highly efficient for CO oxidation. The present findings provide new insights into the understanding of the EMSI in heterogeneous catalysis and can open new avenues to design and fabricate cost-effective single-atom-sized and/or nanometer-sized catalysts.

Solution Combustion Synthesis of Nanoscale Materials

Solution combustion is an exciting phenomenon, which involves propagation of self-sustained exothermic reactions along an aqueous or sol-gel media. This process allows for the synthesis of a variety of nanoscale materials, including oxides, metals, alloys, and sulfides. This Review focuses on the analysis of new approaches and results in the field of solution combustion synthesis (SCS) obtained during recent years. Thermodynamics and kinetics of reactive solutions used in different chemical routes are considered, and the role of process parameters is discussed, emphasizing the chemical mechanisms that are responsible for rapid self-sustained combustion reactions. The basic principles for controlling the composition, structure, and nanostructure of SCS products, and routes to regulate the size and morphology of the nanoscale materials are also reviewed. Recently developed systems that lead to the formation of novel materials and unique structures (e.g., thin films and two-dimensional crystals) with unusual properties are outlined. To demonstrate the versatility of the approach, several application categories of SCS produced materials, such as for energy conversion and storage, optical devices, catalysts, and various important nanoceramics (e.g., bio-, electro-, magnetic), are discussed.

High-Temperature Quantum Anomalous Hall Effect in n-p Codoped Topological Insulators

The quantum anomalous Hall effect (QAHE) is a fundamental quantum transport phenomenon that manifests as a quantized transverse conductance in response to a longitudinally applied electric field in the absence of an external magnetic field, and it promises to have immense application potential in future dissipationless quantum electronics. Here, we present a novel kinetic pathway to realize the QAHE at high temperatures by n-p codoping of three-dimensional topological insulators. We provide a proof-of-principle numerical demonstration of this approach using vanadium-iodine (V-I) codoped Sb_{2}Te_{3} and demonstrate that, strikingly, even at low concentrations of ∼2%  V and ∼1% I, the system exhibits a quantized Hall conductance, the telltale hallmark of QAHE, at temperatures of at least ∼50  K, which is 3 orders of magnitude higher than the typical temperatures at which it has been realized to date. The underlying physical factor enabling this dramatic improvement is tied to the largely preserved intrinsic band gap of the host system upon compensated n-p codoping. The proposed approach is conceptually general and may shed new light in experimental realization of high-temperature QAHE.

Photoelectrochemical Conversion from Graphitic C3N4 Quantum Dot Decorated Semiconductor Nanowires

Despite the recent progress of developing graphitic carbon nitride (g-C3N4) as a metal-free photocatalyst, the synthesis of nanostructured g-C3N4 has still remained a complicated and time-consuming approach from its bulk powder, which substantially limits its photoelectrochemical (PEC) applications as well as the potential to form composites with other semiconductors. Different from the labor-intensive methods used before, such as exfoliation or assistant templates, herein, we developed a facile method to synthesize graphitic C3N4 quantum dots (g-CNQDs) directly grown on TiO2 nanowire arrays via a one-step quasi-chemical vapor deposition (CVD) process in a homemade system. The as-synthesized g-CNQDs uniformly covered over the surface of TiO2 nanowires and exhibited attractive photoluminescence (PL) properties. In addition, compared to pristine TiO2, the heterojunction of g-CNQD-decorated TiO2 nanowires showed a substantially enhanced PEC photocurrent density of 3.40 mA/cm(2) at 0 V of applied potential vs Ag/AgCl under simulated solar light (300 mW/cm(2)) and excellent stability with ∼82% of the photocurrent retained after over 10 h of continuous testing, attributed to the quantum and sensitization effects of g-CNQDs. Density functional theory calculations were further carried out to illustrate the synergistic effect of TiO2 and g-CNQD. Our method suggests that a variety of g-CNQD-based composites with other semiconductor nanowires can be synthesized for energy applications.

Rational Hydrogenation for Enhanced Mobility and High Reliability on ZnO-based Thin Film Transistors: From Simulation to Experiment

Hydrogenation is one of the effective methods for improving the performance of ZnO thin film transistors (TFTs), which originate from the fact that hydrogen (H) acts as a defect passivator and a shallow n-type dopant in ZnO materials. However, passivation accompanied by an excessive H doping of the channel region of a ZnO TFT is undesirable because high carrier density leads to negative threshold voltages. Herein, we report that Mg/H codoping could overcome the trade-off between performance and reliability in the ZnO TFTs. The theoretical calculation suggests that the incorporation of Mg in hydrogenated ZnO decrease the formation energy of interstitial H and increase formation energy of O-vacancy (VO). The experimental results demonstrate that the existence of the diluted Mg in hydrogenated ZnO TFTs could be sufficient to boost up mobility from 10 to 32.2 cm(2)/(V s) at a low carrier density (∼2.0 × 10(18) cm(-3)), which can be attributed to the decreased electron effective mass by surface band bending. The all results verified that the Mg/H codoping can significantly passivate the VO to improve device reliability and enhance mobility. Thus, this finding clearly points the way to realize high-performance metal oxide TFTs for low-cost, large-volume, flexible electronics.