Comparative study of conduction-band and valence-band edges of TiO2, SrTiO3, and BaTiO3 by ionization potential measurements

Chemical Physics Letters 685 (2017) 23–26

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Research paper

Comparative study of conduction-band and valence-band edges of TiO2, SrTiO3, and BaTiO3 by ionization potential measurements Jun-ichi Fujisawa ⇑, Takumi Eda, Minoru Hanaya Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan

a r t i c l e

i n f o

Article history: Received 8 March 2017 In final form 12 July 2017 Available online 13 July 2017

a b s t r a c t Here, we report the energy positions of the conduction-band and valence-band edges of anatase titanium dioxide (TiO2), strontium titanate (SrTiO3), and barium titanate (BaTiO3). From the photoelectron yield spectra, the ionization potentials of anatase TiO2, SrTiO3, and BaTiO3 were estimated to be ca. 7.25, 6.90, 7.05 eV, respectively, which correspond to the top of the valence band of each titanium oxide. From these data and their band gaps, it was found that the bottoms of the conduction bands of SrTiO3 and BaTiO3 are positioned above that of anatase TiO2 by 0.40 and 0.23 eV, respectively. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Titanium oxides such as titanium dioxide (TiO2) are low-cost and environmentally-friendly wide-band-gap semiconductors with wide applications from pigments to solar cells. Among them, anatase TiO2 has attracted much interest in the research fields of photocatalysis and photovoltaics. [1–4] In addition to anatase TiO2, perovskite-type titanium oxides such as strontium titanate (SrTiO3) and barium titanate (BaTiO3) have also been studied as potential photocatalytic and photovoltaic materials. [5,6] Particularly, SrTiO3 is expected to show higher photocatalytic ability due to the higher-energy conduction band than anatase TiO2. Recently, the combination of anatase TiO2 with SrTiO3 or BaTiO3 was reported to enhance photocatalytic and photovoltaic energy conversion efficiencies. [7–11] The photocatalytic and photovoltaic functions of those titanium oxides and the composites are governed by the energy positions of the bottom of the conduction band and the top of the valence band. The conduction-band edges of TiO2 and SrTiO3 were examined by Bolts et al. with electrochemical capacitance measurements. [12] They reported that the bottom (ECB) of the conduction band of SrTiO3 is located above that of TiO2 by ca. 0.35 eV. In the experiment, they used n-type doped TiO2 and SrTiO3 to validate the assumption that the estimated flat-band potentials coincide with the conduction-band edges of undoped TiO2 and SrTiO3. For BaTiO3, several electrochemical and photoelectrochemical measurements suggested qualitatively that the conduction-band edge of BaTiO3 lies above that of TiO2. [9– 11] However, this suggestion is inconsistent with the result reported several decades ago. [13] Considering this situation, com⇑ Corresponding author. E-mail address: [email protected] (J.-i. Fujisawa). http://dx.doi.org/10.1016/j.cplett.2017.07.031 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.

parative and reliable estimation of conduction-band and valenceband edges of the titanium oxides are necessary. In contrast to the electrochemical measurements, ionization potential measurements based on photoelectron yield spectroscopy are expected to give more direct information about the energies of the valence-band edges of the intrinsic titaniumoxide semiconductors and reliable estimation of their conduction-band edges. Recently, photoelectron yield measurements of TiO2 nanoparticles were reported by Toyoda et al. [14] However, to our best knowledge, there have been no reports of comparative ionization potential measurements of several titanium oxides. In this paper, we reveal the energy positions of the conduction-band and valence-band edges of anatase TiO2, SrTiO3, and BaTiO3 by means of ionization potential measurements for the first time. 2. Methods Anatase TiO2 nanoparticles (P90) were obtained from Aerosil. SrTiO3 nanoparticles and BaTiO3 nanoparticles were purchased from Sigma-Aldrich. Scanning electron microscope (SEM) images and X-ray diffraction (XRD) data of the titanium-oxide nanoparticles were measured by means of a field emission scanning electron microscope (JSM-6330 F, JEOL) and an XRD diffractometer (RINT 2200VF, Rigaku) using CuKa radiation, respectively. Diffuse reflectance spectra of the anatase TiO2, SrTiO3, and BaTiO3 nanoparticles were measured by means of a UV–VIS-NIR spectrophotometer (V-670, JASCO). The measured diffuse reflectance spectra were converted to the spectra of Kubelka-Munk function that is defined as the ratio (K/S) of the absorption coefficient to the scattering coefficient (S) by use of the equation of Kubelka-Munk function = (1 - Rdiff)2/(2Rdiff). Photoelectron yield measurements were

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performed by means of an ionization energy measurement system (BIP-KV201, Bunkoukeiki) with a deuterium lamp (30 W) as the light source. In the measurements, the titanium-oxide nanoparticles were attached to carbon tapes on a metal base plate in order to prevent charge up. Negative voltage of 150 V was applied to the base plate to prevent carrier recombination of photoelectrons emitted from the sample surface with photogenerated holes in the sample. All the measurements were performed in a vacuum chamber (ca. 4  10 3 Pa) at room temperature. 3. Results and discussion Fig. 1 shows SEM images and XRD patterns observed for anatase TiO2, SrTiO3 and BaTiO3 nanoparticles. From the SEM images, it is seen that the average particle size is ca. 20 nm for anatase TiO2, ca. 40 nm for SrTiO3, ca. 80 nm for BaTiO3. The XRD data of the anatase TiO2 indicate that the anatase TiO2 nanoparticles have a tetragonal crystal structure (a = 3.785, c = 9.514 Å) and the sample includes a very small amount of the rutile phase. From the ratio of the integrated intensity of the anatase (1 0 1) peak to that of the rutile (1 1 0), the fraction of the rutile phase was estimated to be ca. 6% by use of the reported equation.[15] From this small value, it is considered that the rutile phase can be regarded as an impurity. The SrTiO3 and BaTiO3 nanoparticles have a cubic perovskite structure (a = 3.905 and 4.006 Å, respectively).

Fig. 2(a) shows Kubelka-Munk function spectra of the anatase TiO2, SrTiO3, and BaTiO3 nanoparticles. The Kubelka-Munk function that is defined as the ratio (K/S) of the absorption coefficient (K) to the scattering coefficient (S) were calculated from the observed diffuse reflectance (Rdiff) by use of the conventional equation, Kubelka-Munk function = (1 – Rdiff)2/(2Rdiff). The anatase TiO2, SrTiO3, and BaTiO3 nanoparticles show light absorption due to inter-band electronic transitions in the near UV region with the onset at ca. 390 nm, reflecting their wide band gaps. The onset energy (3.2 eV) well corresponds to the reported band gaps of bulk crystals of anatase TiO2 (3.20 eV) [16,17], SrTiO3, (3.25 eV) [18] and BaTiO3 (3.23 eV) [19]. Fig. 2(b) shows the enlarged Kubelka-Munk function spectra around the absorption onsets. The onset wavelength of the absorption band is blue-shifted slightly in the order of anatase TiO2, BaTiO3, and SrTiO3, which is consistent with their reported band gaps, as shown by dashed arrows. This agreement also indicates that the titanium-oxide nanoparticles larger than ca. 15 nm exhibit almost no quantum size effects, as reported in the literature. [16,19]. Fig. 3 shows photoelectron yield spectra measured for the anatase TiO2, SrTiO3, and BaTiO3 nanoparticles. All the titanium oxides exhibit strong photoelectron signals above ca. 7.0 eV and relatively weak photoelectron signals between 6.0 and ca. 7.0 eV. For the photoelectron signals above ca. 7.0 eV, the onset energy is dependent on the kind of titanium oxides. This result indicates that the

Fig. 1. SEM images (left) and XRD data (right) of (a) anatase TiO2 (blue), (b) SrTiO3 (green), and (c) BaTiO3 nanoparticles (red) together with corresponding line spectra (black) of anatase tetragonal TiO2 (JCPDS No. 00-021-1272), cubic SrTiO3 (No. 00-005-0634), and cubic BaTiO3 crystals (No. 01-079-2263) taken from Power Diffraction Datafile (PDF) for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 3. Photoelectron yield spectra of anatase TiO2 (blue), SrTiO3 (green), and BaTiO3 nanoparticles (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. (a) Kubelka-Munk function spectra of anatase TiO2 (blue), SrTiO3 (green), and BaTiO3 (red) nanoparticles and (b) their enlarged spectra around 390 nm together with the reported band gaps (dashed arrows) of anatase TiO2, SrTiO3, and BaTiO3 crystals. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

strong signals come from the valence band of each the titanium oxide. On the other hand, the spectral shape of the weak signals between 6.0 and ca. 7.0 eV is independent of the kind of titanium oxides. The photoelectrons detected between 6.0 and ca. 7.0 eV are considered to come from surface hydroxy groups and/or surface water molecules, as reported by Yamashita et al. [20] The black dashed curve in Fig. 3 represents the presumed spectrum of the weak signals. The onset energy of the strong signals is estimated approximately to be 7.25 eV for anatase TiO2, 6.90 eV for SrTiO3, and 7.05 eV for BaTiO3. The estimated ionization potential of anatase TiO2 is close to the reported value. [14] From the ionization potentials, it is seen that the top of the valence band is located at 7.25 eV for anatase TiO2, 6.90 eV for SrTiO3, and 7.05 eV for BaTiO3 versus the vacuum level under the assumption that Coulomb repulsion between electrons is rather weak in these titanium oxides and the band-picture is appropriate to describe their electronic structures. From the above-mentioned band-gap energies reported for anatase TiO2, SrTiO3, and BaTiO3, the energy of the

bottom of the conduction band is estimated to be 4.05 eV for anatase TiO2, 3.65 eV for SrTiO3, and 3.82 eV for BaTiO3, as shown in Fig. 4. The estimated energy ( 4.05 eV) of the conductionband edge of anatase TiO2 agrees well with the widely-used value ( 4.0 to 4.3 eV) in the literature. [21–23] In addition, our study reveals that the conduction-band edge of SrTiO3 is above that of anatase TiO2 by 0.40 eV and that of BaTiO3 is above that of anatase TiO2 by 0.23 eV. The higher-energy conduction-band edge of SrTiO3 as compared to anatase TiO2 is consistent quantitatively with the reported result estimated by the electrochemical measurements. [12] Furthermore, our result supports the result of ECB(TiO2) < ECB (BaTiO3) reported in Ref. [9,11] rather than that of ECB(TiO2) > ECB (BaTiO3) reported in Ref. [13]. The entire elevation of the conduction band and valence band in SrTiO3 and BaTiO3 as compared to those of anatase TiO2 is explained reasonably by the total negative charge of the TiO3 framework on which the conduction band and valence band are distributed.

Fig. 4. Energy level diagram of anatase TiO2, SrTiO3, and BaTiO3 nanoparticles. CB and VB stand for conduction band and valence band, respectively.

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4. Conclusion We have clarified the energy positions of the conduction-band and valence-band edges of anatase TiO2, SrTiO3, and BaTiO3 by means of comparative photoelectron yield measurements. This knowledge is important for the design and development of titanium-oxide based solar cells and photocatalytic reaction systems. Acknowledgement We thank T. Amano and K. Nakajima of Bunkoukeiki for their kind cooperation in the photoelectron yield measurements. References [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010) 6595. [2] W. Zhang, G.E. Eperon, H.J. Snaith, Nat. Energy 1 (2016) 16048. [3] K. Hashimoto, H. Irie, A. Fujishima, Jpn J. Appl. Phys. 44 (2005) 8269. [4] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Chem. Rev. 114 (2014) 9919. [5] Y. Ham, T. Hisatomi, Y. Goto, Y. Moriya, Y. Sakata, A. Yamakata, J. Kubota, K. Domen, J. Mater. Chem. A 4 (2016) 3027.

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