C-, N-, S-, and Fe-Doped TiO₂ and SrTiO₃ Nanotubes for Visible-Light-Driven Photocatalytic Water Splitting: Prediction from First Principles

Abstract

The ground state electronic structure and the formation energies of both TiO 2 and SrTiO 3 nanotubes (NTs) containing C O , N O , S O , and Fe Ti substitutional impurities are studied using first-principles calculations. We observe that N and S dopants in TiO 2 NTs lead to an enhancement of their visible-light-driven photocatalytic response, thereby increasing their ability to split H 2 O molecules. The differences between the highest occupied and lowest unoccupied impurity levels inside the band gap (HOIL and LUIL, respectively) are reduced in these defective nanotubes down to 2.4 and 2.5 eV for N and S doping, respectively. The band gap of an N O +S O co-doped titania nanotube is narrowed down to 2.2 eV (while preserving the proper disposition of the gap edges relatively to the reduction and oxidation potentials, so that HOIL < O 2 /H 2 O < H + /H 2 < LUIL ), thus decreasing the photon energy required for splitting of H 2 O molecule. For C-and Fe-doped TiO 2 NTs, some impurity levels lie in the interval between both redox potentials, which would lead to electron-hole recombination. Our calculations also reveal in sulfur-doped SrTiO 3 NTs a suitable band distribution for the oxygen evolution reaction, although the splitting of water molecules would be hardly possible due to an unsuitable conduction band position for the hydrogen reduction reaction. KeywordsDensity Functional Theory, SrTiO 3 and TiO 2 nanotubes, single-and double-atom dopants, atomic structure, electronic structure, photocatalytic properties. the influence of solar light on semiconducting photoelectrodes in aqueous electrolyte is a potentially clean and renewable source for hydrogen fuel. The process is often considered as artificial photosynthesis, and as such is an attractive and challenging research topic in the field of chemistry and renewable energy. 1-3The efficiency of the water splitting reactions 4 depends on the relative position of the semiconductor band edges (hole and electron energies) with respect to the redox levels, which are defined as measure of the affinity of the semiconducting substance for electrons (its electronegativity) compared with hydrogen. Redox couples in electrochemical reactions are characterized by molecules or ions in a solution which can be reduced and oxidized by a pure electron transfer. 5 This requires the semiconductors to exhibit a proper band alignment relative to the water redox potentials, e.g., the conduction band minimum of the p-type photocathode should be higher than the water reduction potential H + /H 2 , while the valence band maximum of the n-type photoanode, should be lower than the water oxidation potential O 2 /H 2 O. 6 Major limitations for the solar light conversion by photocatalysis relate to the band gap position in the corresponding photocatalytic materials and their stability in an aqueous environment.A number of binary and ternary metal oxide semiconductors have been intensively studied so far. 3,4,6 SrTiO 3 and, especially, TiO 2 (which distinguishes itself due to its superior chemica...

Ab initio calculations of doped TiO2 anatase (101) nanotubes for photocatalytical water splitting applications

“…In general, (0,n) nanotubes are found to be more stable than NTs with (n,0) chirality indexes. Electronic structure (band edges positions) of 6-layer (101) anatase NT is similar to the band structure of anatase nanotube made of 9-layer (001) TiO 2 nanosheet studied by us recently [6]. Based on results of our calculations, we predict that lone S or N dopants introduced into the 6-layered TiO 2 (0,12) NT cannot result in a significant rise of photocatalytic response.…”

“…In this study we model N and S doped, as well as N þS co-doped anatase NT with (101) morphology, which is reported to be the most stable NT [5]. From our recent calculations on less stable, but more chemically active TiO 2 NT rolled up from the (001) nanosheet we know that Nþ S co-doping may result in its visible-light-driven photoresponse enhancement [6]. Therefore from the modeling of the NT rolled-up from the titania nanosheet cut parallel to its (101) surface, we have revealed a number of dopant sites, and we have observed general trends in the composition of NT's electronic structure as a function of defect concentration in NT's wall.…”

Doped 1D Nanostructures of Transition‐metal Oxides: First‐principles Evaluation of Photocatalytic Suitability

“…In this review, we analyse applicability of large‐scale first‐principles calculations for the description of metal‐oxide 1D nanostructures doped with cations and/or anions, thus evaluating their suitability to be used for photocatalytic water splitting. Here, we systematize our results obtained mainly for two classes of 1D nanostructures estimated as the effective photocatalysts during the last few years – anatase‐structured titania nanotubes (NTs) and wurtzite‐structured ZnO nanowires (NWs) . At the same time, results of our recently performed ab initio simulations of strontium titanate, including both nanotubes and nanowires with cubic morphology, clearly show that their efficiency for photocatalytic applications is markedly lower than in the case of TiO 2 and ZnO nanostructures.…”

“…A series of large‐scale ab initio calculations have been performed by us for the two classes of monoperiodic nanostructures found to be suitable for photocatalytic applications. Firstly, we have studied anatase‐structured TiO 2 (001) and (101) NTs, with a fixed number of atomic layers and chiral indexes: ( n ,0), vs. (− n , n ), respectively, monodoped by either C, N, S, or Fe atoms, as well as codoped by N and S atoms, simultaneously. Alternative nanomaterials potentially suitable for photocatalysis have been considered to be [0001]‐oriented and hollow‐centred wurtzite‐structured ZnO NWs monodoped by C, N, or Ag atoms …”

“…To provide a balanced summation in both direct and reciprocal lattices, the reciprocal space integration over the direct and reciprocal lattices of TiO 2 nanotubes has been done by sampling the Brillouin zone (BZ) of 2×2 titania supercells with 6×1×1 Pack‐Monkhorst k ‐mesh, which results in four evenly distributed k‐points over the segment of the irreducible BZ . Alternatively, the reciprocal space integration in ZnO NW calculations has been performed by sampling the nanowire's BZ of 1×1 ZnO unit cells with the 12×1×1 k ‐mesh that gives, in total, seven k‐points evenly distributed along the BZ, which is a line section for 1D nanowires.…”