Double-hole-mediated coupling of dopants and its impact on band gap engineering in TiO2

Two-dimensional tetragonal AlOX (X = Cl, Br, or I) monolayers with promising photocatalytic performance: first-principles investigations

It is of great significance to search for new two-dimensional materials with excellent photocatalytic water splitting properties. Here, the AlOX (X = Cl, Br, or I) monolayers were constructed to explore their electronic and optical properties as a potential photocatalyst and mechanism of high photocatalytic activity by first principles calculations, for the first time. The results show that the AlOX (X = Cl, Br, or I) monolayers are all dynamically and thermodynamically stable. It is found that the AlOI monolayer exhibits visible optical absorption with a 538 nm absorption band edge, due to its direct band gap of 2.22 eV. Moreover, an appropriate band edge potential ensures its excellent reduction-oxidation (redox) ability. The asymmetry of crystals along different directions results in a noncoplanar HOMO and LUMO as well as an anisotropy effective mass and favors the separation of photogenerated carriers. These findings present the potential of the AlOX (X = Cl, Br, or I) monolayers as photocatalysts.

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.

Engineering the Electronic Structure towards Visible Lights Photocatalysis of CaTiO3 Perovskites by Cation (La/Ce)-Anion (N/S) Co-Doping: A First-Principles Study

Cation-anion co-doping has proven to be an effective method of improving the photocatalytic performances of CaTiO3 perovskites. In this regard, (La/Ce-N/S) co-doped CaTiO3 models were investigated for the first time using first-principles calculations based on a supercell of 2 × 2 × 2 with La/Ce concentrations of 0.125, 0.25, and 0.375. The energy band structure, density of states, charge differential density, electron-hole effective masses, optical properties, and the water redox potential were calculated for various models. According to our results, (La-S)-doped CaTiO3 with a doping ratio of 0.25 (LCOS1-0.25) has superior photocatalytic hydrolysis properties due to the synergistic performances of its narrow band gap, fast carrier mobility, and superb ability to absorb visible light. Apart from the reduction of the band gap, the introduction of intermediate energy levels by La and Ce within the band gap also facilitates the transition of excited electrons from valence to the conduction band. Our calculations and findings provide theoretical insights and solid predictions for discovering CaTiO3 perovskites with excellent photocatalysis performances.

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.

Strain-Tunable Visible-Light-Responsive Photocatalytic Properties of Two-Dimensional CdS/g-C₃N₄: A Hybrid Density Functional Study

By means of a hybrid density functional, we comprehensively investigate the energetic, electronic, optical properties, and band edge alignments of two-dimensional (2D) CdS/g-C 3 N 4 heterostructures by considering the effect of biaxial strain and pH value, so as to improve the photocatalytic activity. The results reveal that a CdS monolayer weakly contacts with g-C 3 N 4 , forming a type II van der Waals (vdW) heterostructure. The narrow bandgap makes CdS/g-C 3 N 4 suitable for absorbing visible light and the induced built-in electric field between the interface promotes the effective separation of photogenerated carriers. Through applying the biaxial strain, the interface adhesion energy, bandgap, and band edge positions, in contrast with water, redox levels of CdS/g-C 3 N 4 can be obviously adjusted. Especially, the pH of electrolyte also significantly influences the photocatalytic performance of CdS/g-C 3 N 4 . When pH is smaller than 6.5, the band edge alignments of CdS/g-C 3 N 4 are thermodynamically beneficial for oxygen and hydrogen generation. Our findings offer a theoretical basis to develop g-C 3 N 4 -based water-splitting photocatalysts.

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.

Mediating both valence and conduction bands of TiO2 by anionic dopants for visible- and infrared-light photocatalysis

Doping is an effective way to extend the optical absorption of TiO2 to the visible range. Doping of TiO2 by carbon has been found to enhance the water splitting efficiency significantly in experiment. However, the mechanism behind this is elusive. Using the ab initio many-body Green's function theory, we find that the C2 dimer formed on the TiO2 surface produces a shallow delocalized occupied Ti 3d state just below the bottom of the conduction bands. Therefore, band-gap narrowing in carbon-doped TiO2 is caused by the opposite shifts of both valence and conduction bands simultaneously, which is in contrast to the generally accepted idea that anionic dopants can only affect the valence band of TiO2. Optical absorption in the infrared region is also increased compared to reduced TiO2. The spatially well-separated photogenerated electrons and holes might help to reduce the recombination rate of carriers, in favor of improvement in photocatalysis efficiency. This novel behavior of anionic dopants is distinct from previous understandings and may guide the engineering of TiO2.

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.

Band gap engineering of SnS2 nanosheets by anion–anion codoping for visible-light photocatalysis

SnS₂ nanosheets with three atom thickness have previously been synthesized and it has been shown that visible light absorption and hydrogen evolution through photocatalytic water splitting are restricted. In the present study, we have systematically investigated the electronic structures of anionic monodoped (N and P) and codoped (N–N, N–P, and P–P) SnS₂ nanosheets for the design of efficient water redox photocatalysts by adopting first principles calculations with the hybrid HSE06 functional. The results show that the defect formation energies of both the anionic monodoped and all the codoped systems decrease monotonically with the decrease of the chemical potential of S. The P–P codoped SnS₂ nanosheets are not only more favorable than other codoped systems under an S-poor condition, but they also reduce the band gap without introducing unoccupied impurity states above the Fermi level. Interestingly, although the P–P(ii) codoped system gives a band gap reduction, this system is only suitable for oxygen production and not for hydrogen evolution, which indicates that it may serve as a Z-scheme photocatalyst for water splitting. The P–P(i) codoped system may be a potential candidate for photocatalytic water splitting to generate hydrogen because of the appropriate band gap and band edge positions, which overcome the disadvantage that the pure SnS₂ nanosheet is not beneficial for hydrogen production. More importantly, the result of optical absorption spectral analysis shows that the P–P(i) codoped SnS₂ nanosheet absorbs a longer wavelength of the visible light spectrum as compared to the pristine SnS₂ nanosheet. The P–P(I) codoped system with a lower doping concentration also has an absorption shift towards the visible light region.

We have systematically investigated the electronic structures of anionic monodoped (N and P) and codoped (N–N, N–P, and P–P) SnS₂ nanosheets for the design of efficient water redox photocatalysts.

A first-principles study of anionic (S) and cationic (V/Nb) doped Sr2Ta2O7 for visible light photocatalysis

In order to utilize the visible light to catalyze water, UV-active Sr₂Ta₂O₇ is engineered via co-doping of S and V/Nb to shift the valence band maximum upward and conduction band minimum downward by approximately 1 eV, respectively.

Enhanced visible light harnessing and oxygen vacancy promoted N, S co-doped CeO2 nanoparticle: a challenging photocatalyst for Cr(vi) reduction

Enhanced photocatalytic activity of N-, S-doped ceria nanoparticles towards Cr(vi) reduction under visible light irradiation.

Theoretical calculation and experiment study on the electronic structure, microstructures and photocatalytic activity of N–Al codoped TiO2

To more accurately demonstrate how the electronic structure was changed and to explain the origin of the enhanced photocatalytic activity by N–Al codoped TiO₂, we investigated N, Al monodoped, and codoped TiO₂by theoretical calculation and experiments.

Cadmium and lithium doping in silver orthophosphate: An ab initio study

Using hybrid functional calculations, we investigate the effects of defects and defect complexes related with Cd, Li, and N impurities on the atomic and electronic properties of Ag3PO4. It was found that substitutional Cd on Ag lattice site (CdAg) contributes to the n-type conductivity of Ag3PO4. For substitutional Cd on P (or O) lattice site (CdP) (or CdO), it is not expected that Cd will incorporate into the P (or O) site due to the strong covalent interactions in the PO4 structural units. The interstitial Cd (Cdi) acts as a shallow donor, but its formation energy is relatively high compared with that of CdAg. For the (CdAg-2NO) complex, the formation of this inactive complex generates a fully occupied impurity band just above the valence band maximum of Ag3PO4, which significantly reduces the acceptor transition energy level. But the formation energy of the (CdAg-2NO) complex is even higher than that of the corresponding single point defect NO. Unlike LiP and LiO which has relatively high formation energy, interstitial Li (Lii or Lii(s)) with an appreciable solubility is likely to be the n-type dopant under O-poor condition.

Kinetics stabilized doping: computational optimization of carbon-doped anatase TiO2 for visible-light driven water splitting

Carbon dimers exhibiting strong C–C binding could be ideal kinetics-stabilized dopants enhancing the visible-light activity of TiO₂ for photocatalysis.

Band gap engineering of ZnO substituted with nitrogen and fluorine, ZnO1−3xN2xFx: a hybrid density functional study

A series of first principles calculations within density functional theory (DFT) have been performed for ZnO, co-doped with N and F with the aim of engineering the band gap and improving its application to photo-absorption activity.

BiOX/BiOY (X, Y = F, Cl, Br, I) superlattices for visible light photocatalysis applications

BiOF/BiOI, BiOCl/BiOBr, BiOCl/BiOI, and BiOBr/BiOI superlattices are suitable for visible light photocatalytic degradation of organic pollutants.

Double-hole codoped huge-gap semiconductor ZrO2 for visible-light photocatalysis

Anionic codoping pairs make the huge-gap semiconductor ZrO₂ a promising catalyst for solar-driven water splitting.

Bandgap engineering of Magnéli phase Ti(n)O(2n-1): Electron-hole self-compensation

An electron-hole self-compensation effect is revealed and confirmed in nitrogen doped Magnéli phase Ti(n)O(2n-1) (n = 7, 8, and 9) by using hybrid density functional theory calculations. We found that the self-compensation effect between the free electrons in Magnéli phase Ti(n)O(2n-1) (n = 7, 8, and 9) and the holes induced by p-type nitrogen doping could not only prevent the recombination of photo-generated electron-hole pairs, but also lead to an effective bandgap reduction. This novel electron-hole self-compensation effect may provide a new approach for bandgap engineering of Magnéli phase metal suboxides.

Origin of enhanced visible light driven water splitting by (Rh, Sb)-SrTiO3

The origin of enhancement of photoconversion efficiency of Rh-doped SrTiO₃ with the codoping of Sb has been investigated using the hybrid density functional theory. Partially occupied t_2g subset of Rh 4d orbitals is completely filled in the presence of Sb, resulting in the formation of a continuous band structure. This improves the mobility of charge carriers and reduces the electron–hole recombination rate.

Band structure engineering of monolayer MoS2: a charge compensated codoping strategy

The monolayer MoS₂, possessing an advantage over graphene in that it exhibits a band gap whose magnitude is appropriate for solar applications, has attracted increasing attention because of its possible use as a photocatalyst.

Modification mechanism of praseodymium doping for the photocatalytic performance of TiO2: a combined experimental and theoretical study

Pr-doped TiO₂ with a mono-phase crystal structure was prepared by a sol–gel method. Crystal structure, binding information, optical absorption, and photocatalytic activity were systematically investigated

Synergistic effect of C, Ag-codoped TiO2 photocatalyst within the GGA + U framework

The electronic structures and optical properties of single C doped, single Ag doped, and C, Ag-codoped TiO₂ were calculated based on the GGA + U method.

Tailoring the electronic structure of β-Ga2O3 by non-metal doping from hybrid density functional theory calculations

Se-doped and I-doped β-Ga₂O₃ are theoretically found to be promising photocatalysts for water splitting in the visible region.

SiH/TiO2 and GeH/TiO2 heterojunctions: promising TiO2-based photocatalysts under visible light

We use hybrid density functional calculations to find that the monolayer silicane (SiH) and the anatase TiO2(101) composite (i.e. the SiH/TiO2 heterojunction) is a promising TiO2-based photocatalyst under visible light. The band gap of the SiH/TiO2(101) heterojunction is 2.082 eV, which is an ideal material for the visible-light photoexcitation of electron-hole pairs. Furthermore, the SiH/TiO2(101) heterojunction has a favorable type-II band alignment and thus the photoexcited electron can be injected to the conduction band of anatase TiO2 from that of silicane. Finally, the proper interface charge distribution facilitates the carrier separation in the SiH/TiO2(101) interface region. The electron injection and carrier separation can prevent the recombination of electron-hole pairs. Our calculation results suggest that such electronic structure of SiH/TiO2(101) heterojunction has significant advantages over these of doped TiO2 systems for visible-light photocatalysis.

Origins of electronic band gap reduction in Cr/N codoped TiO2

Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase TiO2 significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large (∼1 eV) band gap reduction of noncompensated codoped rutile TiO2. It is shown that Cr/N codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr 3d3 levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr 3d and delocalized N 2p states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.

Electronic and optical properties of Cr and Cr-N doped anatase TiO2 from screened Coulomb hybrid calculations

We studied the electronic and atomic structures of anatase TiO2 codoped with Cr and N using hybrid density functional theory calculations. The nonlocal screened Hartree-Fock exchange energy is partially mixed with the traditional semilocal exchange energy. This not only patches the bandgap underestimation but also improves the description of the anion/cation-driven impurity states and the magnetization of the dopants. Cr and/or N doping modifies the valence and conduction band edges of TiO2, leading to significant bandgap reduction. Hence, Cr, N and Cr-N doped TiO2 are promising for enhanced visible light absorbance.