Elements doping to expand the light response of SrTiO3

doi:10.1016/j.jphotochem.2013.10.014

Journal of Photochemistry and Photobiology A: Chemistry

Volume 275, 1 February 2014, Pages 65-71

https://doi.org/10.1016/j.jphotochem.2013.10.014 Get rights and content

Highlights

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Cr,B-codoped-SrTiO₃ was successfully synthesized by one-step hydrothermal method.
•
Cr,B-codoped-SrTiO₃ had stronger visible light absorption and smaller band gap (2.07 eV) than Cr-doped-SrTiO₃.
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The strong interaction of Cr 3d with substitutional B 2p levels lead to the stronger visible light absorption and smaller band gap.
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Cr,B-codoped SrTiO₃ photocatalyst showed higher photocatalytic activity for hydrogen evolution (15.4 μmol/h) than that over Cr-doped SrTiO₃ (9.3 μmol/h).

Abstract

Cr,B-codoped-SrTiO₃ was successfully synthesized by one-step hydrothermal method. XRD, FTIR, Raman and XPS results showed that B existed in the forms of both substitutional B for O and interstitial B in the bulk of SrTiO₃. UV-Vis absorption spectra showed that Cr,B-codoped-SrTiO₃ had strong visible light absorption and small band gap (2.07 eV). Theoretical calculation pointed out that the strong p–d repulsion of substitutional B 2p with Cr 3d was responsible for the narrowing of band gap and enhanced visible light absorption. Our results indicate that the codoping of Cr and B offers a novel route to modify the light response of SrTiO₃. The photocatalytic activity for hydrogen production over Cr,B-codoped SrTiO₃ photocatalyst is 15.4 μmol/h, which is higher than that over Cr-doped SrTiO₃ (9.3 μmol/h) synthesized by the similar method. The higher activity for Cr,B-codoped SrTiO₃ is attributed to its smaller band gap than Cr-doped SrTiO₃.

Graphical abstract

Introduction

Hydrogen, as an environmental friendly fuel, is attracting more and more attentions. Since the production of hydrogen through photoinduced water splitting on titanium oxide was discovered [1], semiconductor-based photocatalyst has prompted many investigations, due to its advantage of using the abundant, long lasting, and clean solar energy. In order to construct an efficient photocatalytic water splitting reaction, some photocatalysts, such as oxides, oxynitrides, and sulfides, have been explored [2]. SrTiO₃, as one of the promising photocatalytic candidates, has attracted many attentions due to its excellent photocatalytic performance in water splitting [3]. However, SrTiO₃ only responds to UV light, which occupies only 4% of the solar light, because of its relatively large band gap (3.2 eV). Many methods, such as dye sensitization [4], [5], quantum dot sensitization [6], [7], and element doping, have been reported to enhance the utilization of solar light for wide band gap semiconductors. The element doping received much more attentions owing to its process simplicity and low cost [8], [9].

For elements doped SrTiO₃, the change in electronic band structures can be divided into two kinds: (1) Forming a new mid gap state between the valence band (VB) and conduction band (CB), such as Rh, Cr, Ir, or Ru doped SrTiO₃ [10], [11], [12], [13]. (2) Changing the VB or CB through the mixing of energy level between dopant and host elements, such as N, F, C, or S doped SrTiO₃[14], [15], [16], [17], [18]. Summarizing the UV–Vis absorption spectra of elements doped SrTiO₃, we find that the red shift for most of these samples is slight. The origin of the slight change in absorption edge can be attributed to that the new mid gap state is most close to the VB/CB or the change of VB/CB is small.

Cr-doped SrTiO₃ is a promising photocatalyst for solar hydrogen production, which exhibited the highest performance in hydrogen evolution among the elements doped SrTiO₃ under visible light irradiation. For Cr³⁺ doped SrTiO₃, the visible light response is attributed to the formation of a new mid gap state, which is about 1.0 eV higher than the valence band top of SrTiO₃ formed by O 2p orbits [20]. The experimental band gap for Cr³⁺-doped SrTiO₃ is 2.48 eV [13]. It was well known that the p–d repulsion can reduce the band gap of semiconductors by raising the position of valance band top [19]. The p–d repulsion is proportional to 1/(ɛ_p–ɛ_d)(ɛ_p: the orbital energy of p orbit, ɛ_d: the orbital energy of d orbit). The p–d repulsion becomes strong when the energy difference between ɛ_p and ɛ_d becomes smaller, which significantly contributes to the band gap narrowing. The orbital energy of p orbit of B⁻ ions is close to that of Cr 3d, meaning that the strong p–d repulsion between B2p and C3d if compared to other typical nonmetal elements such as O 2p, S2p, and C2p (Fig. 1). Therefore, it may be a good choice to further broaden the optical absorption region of Cr³⁺-doped SrTiO₃ by enhancing the position of the mid gap state formed by Cr 3d through p–d repulsion between Cr 3d and the p orbits of B. Here, we successfully synthesized Cr,B-codoped-SrTiO₃ by one-step hydrothermal method using TiB₂ as the Ti precursor and discussed synergistic effects of Cr and B on the band gap narrowing (2.07 eV) of SrTiO₃.

Section snippets

Preparation of the photocatalysts

Analytical grade Sr(NO₃)₂, Cr(NO₃)₃·9H₂O, SrCO₃, TiB₂, TiO₂, and NaOH were used as raw materials without further purification. The Cr, B-codoped-SrTiO₃, B-doped-SrTiO₃ and Cr-doped SrTiO₃ powder were synthesized by hydrothermal method. In the synthesis process of Cr,B-codoped-SrTiO₃, firstly, Sr(NO₃)₂ (0.005 mol), Cr(NO₃)₃·9H₂O (0.00025 mol), and TiB₂ (0.00475 mol) were added to Teflon-lined stainless autoclave (60 ml). Then 50 ml water was added to the autoclave. After stirring 5 min, 6 g NaOH was

Crystal structures

Fig. 2 shows the XRD patterns of samples (Cr,B-codoped-SrTiO₃ and B-doped-SrTiO₃) synthesized under different conditions. For samples synthesized by hydrothermal treatment for 72 h, the patterns (Fig. 2b and c) can be indexed as the cubic SrTiO₃ (JCPDS No. 89-4934). But TiB₂ impurities were observed in the sample synthesized for 24 h (Fig. 2a), which indicating that TiB₂ can directly transfer into SrTiO₃ without processing TiO₂ as the intermediate phase. The crystal cell parameters for

Conclusions

Cr,B-codoped-SrTiO₃ was synthesized by one-step hydrothermal method using TiB₂ as the Ti precursor. Based on the results of XPS, FT-IR, and Raman, we proposed that B existed in the forms of both substitution to oxygen and interstitial atoms in the bulk of SrTiO₃. Due to strong p–d replusion of Cr 3d with 2p levels of substitutional B, which uplift the position of new mid gap state and induced more delocalized in-gap energy bands, the Cr,B-codoped SrTiO₃ had stronger visible light absorption and

Acknowledgements

The authors gratefully acknowledge financial support from the National Basic Research Program of China (973 Program, 2013CB632404), the National Natural Science Foundation of China (Nos. 51102132, 11174129, 51272101, and 51272102).

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Elements doping to expand the light response of SrTiO3

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of the photocatalysts

Crystal structures

Conclusions

Acknowledgements

Int. J. Hydrogen Energy

J. Photochem. Photobiol. A

Appl. Catal. A 288

Appl. Catal. B

J. Eur. Ceram. Soc.

J. Phys. Chem. Solids

J. Photochem. Photobiol. A

Int. J. Hydrogen Energy

J. Phys. Chem. Solids

Electrochemical photolysis of water at a semiconductor electrode

Nature

Semiconductor-based photocatalytic hydrogen generation

Chem. Rev.

Photocatalytic decomposition of water into hydrogen and oxygen over nickel(II) oxide-strontium titanate (SrTiO3) powder. 1. Structure of the catalysts

J. Phys. Chem.

An advanced visible-light-induced water reduction with dye-sensitized semiconductor powder catalyst

J. Am. Chem. Soc.

CdS Quantum dots sensitized TiO2 nanotube-array photoelectrodes

J. Am. Chem. Soc.

CdSe quantum dot-sensitized TiO2 electrodes: Effect of quantum dot coverage and mode of attachment

J. Phys. Chem. C

Geometric and electronic structures of the boron-doped photocatalyst TiO2

J. Phys.: Condens. Matter

Synergetic effects of thermal and photo-catalysis in purification of dye water over SrTi1-xMnxO3 solid solutions

J. Phys. Chem. C

Photocatalytic activities of noble metal ion doped SrTiO3 under visible light irradiation

J. Phys. Chem. B

Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium

J. Phys. Chem. B

Elements doping to expand the light response of SrTiO₃

Photocatalytic decomposition of water into hydrogen and oxygen over nickel(II) oxide-strontium titanate (SrTiO₃) powder. 1. Structure of the catalysts

CdSe quantum dot-sensitized TiO₂ electrodes: Effect of quantum dot coverage and mode of attachment

Geometric and electronic structures of the boron-doped photocatalyst TiO₂

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Visible-light-response and photocatalytic activities of TiO₂ and SrTiO₃ photocatalysts codoped with antimony and chromium