Role of Nd3+ Ions in Modifying the Band Structure and Photocatalytic Properties of Substituted Indium Titanates, In2(1–x)Nd2xTiO5 Oxides

Abstract

We report here the role of Nd3+ ions in modifying the band structure of indium titanate and enhancement of photocatalytic evolution of H2 from H2O–methanol mixtures in both sunlight and visible irradiation over Nd3+–In3+2Ti4+O5 oxides. A series of neodymium, isovalent to In3+ ion, were doped at the A-site in indium titanate (In2(1–x)Nd2xTiO5) samples by means of a solid-state reaction and characterized by different techniques. First-principles local-density band structure calculations were performed here for the first time on Nd-doped In2TiO5 to analyze the distribution of valence states of Nd, Ti, In, O atoms near the Fermi level. The photocatalytic activities have been investigated for all of the substituted and unsubstituted samples under sunlight, visible, and UV–visible irradiation. Isovalent doping of neodymium ions was attempted with an objective to suppress the formation of nonstoichiometric defects, which would otherwise work as nonradiative recombination centers between photogenerated electrons and holes. Raman spectra in agreement with FTIR, XRD, and theoretical calculations demonstrated that the neodymium ion has been successfully incorporated into the lattice of In2TiO5, accompanied by an increase in cell volume, while a secondary phase of Nd2Ti2O7 was segregated beyond 10% Nd substitution. An optimal dosage of 10% Nd3+ ion doping, with In1.8Nd0.2TiO5 abbreviated as ITNd(2), resulted in significant enhancement in photocatalytic H2 yield, while In2TiO5 has not shown any visible light photoactivity. The decreasing order of catalytic activity is as follows: ITNd(2) > ITNd(3) > ITNd(1) > ITNd(4) > In2TiO5, The effect of different experimental conditions, duration of irradiation, and the presence of cocatalyst on the yield of hydrogen was also monitored. Maximum apparent quantum efficiency (AQE) of ∼4.5% was achieved with Pt/ITNd(2). First-principles calculations reveal that Nd is one of the elements that are able to make a valence-band position higher than O 2p orbitals, thereby resulting in a narrowing of the band gap by 0.82 eV. We propose that the introduction of the Nd plays a crucial role in visible light photosensitization and enhancement of the electron–hole separation.