Appl Phys A (2012) 108:171–175DOI 10.1007/s00339-012-6867-9

Band-structure modulation of SrTiO3 by hydrogenationfor enhanced photoactivity

Tao Sun · Ming Lu

Received: 11 October 2011 / Accepted: 28 February 2012 / Published online: 23 March 2012© Springer-Verlag 2012

Abstract A systematic study on the band-structure modu-lation of SrTiO3 (STO) by hydrogenation has been done bymeans of ultraviolet-visible (UV-vis) absorption, X-ray pho-toelectron spectroscopy (XPS), electron paramagnetic reso-nance (EPR) and photoluminescence (PL). Hydrogenationof STO was performed by annealing STO in the forminggas of H2:N2 (5 %:95 %) at elevated temperatures (mostly at1000 °C). It is found that electron transfer to the defect stateswithin the band gap of STO due to hydrogenation brings toSTO visible absorption, decrease in intrinsic PL intensity,Fermi level shift, elimination of the EPR signals, and finally,an enhancement of photocatalytic ability.

1 Introduction

SrTiO3(STO), a prototype perovskite [1], has received con-siderable interest in the past decades [2–10]. STO is of im-portance in many areas of application; for instance, it is usedas a substrate for epitaxial growth of other perovskite com-pounds for integration of thin-film oxide electronic devices[11, 12], as a buffer material for micro/nanoelectronics [13],and as an excellent photocatalyst for environmental protec-tion and hydrogen energy [14–16]. Band structure engineer-ing is an effective methodology to modulate physical andchemical properties of semiconductors for desired applica-tion purposes [17, 18]. Hydrogenation of anatase TiO2, aphotocatalyst similar to STO in band location and band gap

T. Sun · M. Lu (�)Department of Optical Science and Engineering, and ShanghaiUltra-Precision Optical Manufacturing Engineering Center,Fudan University, Shanghai 200433, Chinae-mail: minglu@fudan.ac.cnFax: +86-21-65641344

width, has been conducted for an enhanced and broadbandphotoactivity. Liu et al. [19] reported an enhanced photo-catalysis of TiO2 powder under ultraviolet (UV) light af-ter hydrogenation at atmospheric pressure. Recently, Chenet al. [20] found that after hydrogenating nanocrystals ofTiO2 at 200 °C under high pressure of 20 bar, the photocat-alytic ability of TiO2 becomes significantly promoted due tohydrogen-incorporated and amorphized surface. However,reports on band-structure modulation of STO via hydro-genation, especially for application of photocatalysis, arerelatively rare, and the details of underlying physics havenot been fully understood yet [21, 22]. In this work, weinvestigate the band-structure modulation of STO via hy-drogenation under atmospheric pressure for photocatalysisenhancement. Our results of ultraviolet-visible (UV-vis) ab-sorption and X-ray photoelectron spectroscopy (XPS) sug-gest that hydrogenation does not induce changes in the bandgap width of STO but shifts its Fermi level upward by 1.6 eVdue to the band-filling of electrons. The results of pho-toluminescence (PL) and electron paramagnetic resonance(EPR) of STO before and after hydrogenation support theconclusions derived from UV-vis absorption and XPS data.Photodecomposition results show that such a band-structuremodulation by hydrogenation enhances the photocatalyticability of STO under UV light.

2 Experiments

The starting materials are transparent STO 〈100〉 crystalsfrom the Crystal Center of SIOM, Chinese Academy of Sci-ence, with size of 5 × 5 × 0.5 mm3. Before hydrogenation,the samples were cleaned ultrasonically in acetone and de-ionized water subsequently and then blown dry. Hydrogena-tion was performed by annealing STO in the flowing form-ing gas of H2:N2 = 5 %:95 % at various temperatures (from

172 T. Sun, M. Lu

Fig. 1 Emission spectrum of the light source for experiments

600 to 1000 °C, mostly at T = 1000 °C) with a flow of40 L/h for 3 h and then cooling down rapidly. For concisionand clarity, in the text below the hydrogenation treatmentmeans that under T = 1000 °C, unless otherwise stated. Theabsorption spectra were recorded at room temperature withan UV-vis photospectrometer (WGD-8/8A, Tianjin Gang-dong), and an incident-light scanning mode was adopted.XPS spectra were taken on an electron spectrometer (Shi-madzu/Kratos AXIS Ultra), with Mg Kα lines of 1253.6 eVas excitation beams. X-band EPR spectra were measured atroom temperature on Bruker ER-200D-SRC-10/12 with themagnetic field perpendicular to STO 〈100〉. The PL emissionof STO was taken on a photospectrometer (Hitachi, F-4500)at room temperature with the 350-nm wavelength of excita-tion beam. To evaluate the photocatalytic ability of STO, thechange of absorbance of a methylene blue (MB) solutionafter photodecomposition of MB by various STO sampleswas recorded. The concentration of MB was 0.01 mmol/L.Before irradiation, all the MB solutions were kept in darkfor 48 h and no change in absorbance was found. A 400-Wlamp (Heraeus, Model MDQ 401SE) with filters served asthe light source for the photocatalysis. The spectrum of thelight source is shown in Fig. 1. In this work, only the lightsin UV and blue regions (labeled “UV” and “blue” in Fig. 1)were used, which have been found not to decompose MBwithout loading STO. The light-irradiation time for photo-catalysis was 20 h for all the samples. Atomic force mi-croscopy (AFM, PSIA model XE-100) was used to test thechange in surface area.

3 Results and discussion

Figure 2 shows three typical UV-vis absorption spectra forSTO under various treatments. The absorption edges of allthe samples locate at 380 nm, which corresponds to the bandgap of 3.2 eV. So, hydrogenation does not narrow the bandgap of STO in contrast to the case of nitrogen-doped STO[23] or TiO2 [24]. However, after hydrogenation, two vis-ible absorptions peaks are found: at 420 and 515 nm, re-spectively. These absorption peaks were suggested to result

Fig. 2 Absorption spectra of various STOs: as-received (curve 1), hy-drogenated at T = 880 °C (curve 2) and at 1000 °C (curve 3). The insetshows a schematic diagram of the band structure

Fig. 3 PL of STOs for different treatments: as-received (curve 1), hy-drogenated at 880 °C (curve 2) and at 1000 °C (curve 3)

from intrinsic oxygen-vacancy related defect (OVRD) statesof STO [25–27], which act as acceptors located near the topof valence band of STO as illustrated in the inset of Fig. 2.Peaks at similar positions have also been reported in thereduced STO previously [6, 7]. Before hydrogenation, theOVRD states are empty of electrons, so no visible absorp-tions corresponding to the transitions between OVRD levelsand conduction band occur; but after hydrogenation, theseempty OVRD states could be occupied by electrons due tohydrogenation, therefore visible absorptions become avail-able. To support this scenario, PL measurements were per-formed. Figure 3 presents PL spectra of STO as-received andhydrogenated at 880 and 1000 °C. For the as-received STO(curve 1 in Fig. 3), the PL spectrum of as-received samplecan be classified as two components: one is a strong broadPL band A that peaks at ∼440 nm and actually includes twocomponents peaking at 420 and 460 nm, respectively; andthe other, B, is a weak and narrow one located at 530 nm.These PL bands are close to the absorption peaks at 420and 515 nm, respectively, in wavelength as shown in Fig. 2.The PL emission has been attributed to radiation transitionsfrom the occupied states at or close to the bottom of con-duction band of STO to the OVRD states [25–27]. Whenpart of the OVRD states are occupied by electrons due to

Band-structure modulation of SrTiO3 by hydrogenation for enhanced photoactivity 173

Fig. 4 XPS spectra of (a) O 1s,(b) Ti 2p and (c) Sr 3d ofas-received STO (upper) andhydrogenated STO (lower)

hydrogenation, the PL emission is expected to decrease un-til disappearing, just as shown by curves 2 and 3 in Fig. 3.The apparent enhancement of the overall absorption back-ground within the measured range is due to the tail effect ofthe strong near-infrared absorption arising from the increaseof electrons in the conduction band after hydrogenation [6].

Figure 4 gives XPS spectra for O, Ti and Sr of STO be-fore and after hydrogenation. By examining the peak po-sitions, it is found that with respect to the spectra of as-received sample, all the peaks shift by 1.6 eV in binding en-ergy after hydrogenation. The rigid shift of the whole XPSspectrum cannot be due to the surface charging since hydro-genated STO is more conductive than as-received one as wefound (the square resistance of hydrogenated STO is ∼10 �,while that of as-received STO is ∼100 K�), which has beenascribed to the increase of electrons in the conduction band[6, 7]. Actually, the rigid shift of all the binding energies isa characteristic of Fermi level shift [28, 29]. The dopantsby hydrogenation to STO increase the equilibrium electrondensity and push the Fermi level upwards and therefore in-crease the measured binding energies.

To further identify that hydrogenation does transfer elec-trons to STO, EPR study has been conducted. In Fig. 5we show the EPR results for STO samples under varioustreatments. It is known that Fe is a common residual el-ement within the commercial STO, with concentration at10–100 ppm [30]. In the as-received STO, the content ofFe impurity is no more than 30 ppm according to the sup-plier’s report. The structures of EPR of as-received STO aredue to residual Fe3+ ions within STO. The signals 1 to 5in Fig. 5(a) correspond to Fe3+ ions in cubic crystal field,that is, Fe3+ at the Ti site with six oxygen ions in the first

Fig. 5 X-band EPR spectra of various STOs at room temperature:(a) as-received, (b–f) hydrogenated at various temperatures as labeled

nearest shell [31–33]. The signal 6 in Fig. 5(a) correspondsto Fe3+ ions in axis crystal field, that is, Fe3+ at the Ti sitewith one of the six oxygen ions in the first nearest shell miss-ing, sometimes written as Fe3+–VO [31–33]. It is seen thatwith the increasing hydrogenation, the EPR signals diminishsteadily and finally disappear. This trend can be explained bythe electron transfer to Fe3+, which reduces it into Fe2+ thatis silent for regular EPR measurements [34].

We then examine how the band-structure modulation byhydrogenation affects the photoactivity of STO. Figure 6gives four typical absorption spectra of the MB solutionsafter different treatments. Comparing curves 1 with 2, it isseen that STO can decompose MB under UV lights as ex-pected. Then, comparing curves 3 and 2, it is clear that theUV photocatalytic activity of STO is enhanced after hydro-genation of STO. In the inset of Fig. 6, the intensity of ab-sorbance of MB, defined as the peak height at 660 nm after

174 T. Sun, M. Lu

Fig. 6 Absorption spectra for MB solutions. Curve 1 is for MB aloneafter UV irradiation, curve 2 for MB loaded with as-received STO afterUV irradiation, curve 3 for MB loaded with hydrogenated STO afterUV irradiation, and curve 4 is for MB loaded with very hydrogenatedSTO after blue-light irradiation. The inset gives the absorption inten-sity of MB versus hydrogenation temperature T

UV photodecomposition by hydrogenated STO, is plotted asa function of hydrogenation temperature T . It is clear thathydrogenation starts to take effect when T > ∼700 °C. Theoccupation of the acceptor states as illustrated in the insetof Fig. 2 due to hydrogenation could be responsible for theenhanced UV photoactivity since it cuts down the numberof recombination channels and prolongs the lifetimes of ex-cited electrons and holes. The diminishing PL emission asshown in Fig. 3 for hydrogenated STO reflects the decreasein radiative recombination. The prevention of recombinationof excited carriers enhances the probability of surface redoxreactions. From curve 4 in Fig. 6, it is seen that no visible-light (for λ = 435 and 405 nm) photocatalysis is found inhydrogenated STO. It indicates that although visible absorp-tions exist in hydrogenated STO as shown in Fig. 2, the bandoverlap between the acceptor defect level and the top of va-lence band of STO is rather limited, hence the holes left inthe defect states cannot migrate to the STO surface via thevalence band to fulfill a whole redox reaction. This limita-tion of band overlap also explains why no band gap narrow-ing occurs in hydrogenated STO.

Finally, in order to examine whether the enhanced pho-toactivity has something related to the change in surface areaof STO after hydrogenation, we measured the morphologiesof STO surfaces before and after hydrogenation, and calcu-lated the surface area of STO by PSIA software XEI (version1.7.6) attached to the AFM system. It is found that the sur-face area of as-received STO was 1.0011 times that of thegeometric surface, i.e. an ideally flat and plane surface, andit was just 1.0021 times that of the geometric one after hy-drogenation. The images are shown in Figs. 7(a) and 7(b),respectively. It is clear that the change in surface area is anegligible factor here in enhanced photocatalysis.

Fig. 7 AFM images of STO surface before (a) and after (b) hydro-genation

4 Conclusions

In this work, we reported a systematic study on the band-structure modulation of STO by hydrogenation by meansof UV-vis absorption, PL, XPS, and EPR and examined thephotoactivity of STO after hydrogenation. The underlyingphysics has been explored. The key point of hydrogenationof STO is the transfer of electrons to the defects of STO dueto hydrogenation, which causes visible absorption, decreasein intrinsic PL intensity, Fermi level shift, elimination of theEPR signals and finally an enhanced UV photoactivity ofSTO.

Acknowledgements This work was supported by the NSFC underGrant number 10974034.

Band-structure modulation of SrTiO3 by hydrogenation for enhanced photoactivity 175

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