Article pubs.acs.org/JPCC
Time-Resolved Infrared Absorption Study of SrTiO3 Photocatalysts Codoped with Rhodium and Antimony Koji Furuhashi,† Qingxin Jia,‡ Akihiko Kudo,‡ and Hiroshi Onishi*,† †
Department of Chemistry, School of Science, Kobe University, Rokko-dai, Nada, Kobe, 657-8501 Japan Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka, Shinjuku, Tokyo, 162-8601 Japan
‡
ABSTRACT: In this study, SrTiO3 photocatalysts doped with Rh and Sb are prepared by hydrothermal synthesis. Doping Rh alone causes optical absorption centered at 580 and 420 nm, attributed to Rh4+ and Rh3+. By adding Sb, the oxidation state of Rh shifts to be 3+ and the absorption at 580 nm disappears. The doped and nondoped photocatalysts exhibit transient infrared absorption at 3000−1000 cm−1 due to excited electrons, when pumped by 355 or 532 nm light pulses. The absorbance decay is observed in a vacuum as a function of microsecond time delays to deduce the relative rate of electron−hole recombination. The active role of Rh4+ as a recombination center is evidenced by the photocatalyst doped with Rh alone having the fastest absorbance decay. The decay retards in the presence of Sb to show the limited role of Rh3+ in recombination. Steadystate H2 production in an aqueous methanol solution is examined in the presence of Pt cocatalyst on the doped SrTiO3 photocatalysts. By comparing the H2 production rate with the recombination rate, a common efficiency of electron-to-H2 conversion is suggested, regardless of different dopant compositions. On the basis of the O2 production rate observed in a AgNO3 solution, the hole-to-O2 conversion efficiency is suggested to be sensitive to the dopant compositions.
1. INTRODUCTION The photocatalytic water splitting reaction frees a large amount of hydrogen from fossil fuels when driven by solar light.1−3 One method used to increase sensitivity to visible light is to dope transition metals into a wide-bandgap photocatalyst. However, the recombination of photoexcited electrons and holes is often catalyzed by the dopant, which is an impurity in the host lattice, to reduce or even eliminate the desired photocatalytic activity. It is thus important to restrict the dopant-induced recombination. One promising idea is the charge compensation of doped cations. When TiO2 is doped with Cr and Sb of an equivalent molar concentration, the dopants are stored at the substitutional site of Ti4+ to be Cr3+ and Sb5+. The oxidative halfreaction of water splitting, O2 production from a AgNO3 solution, is driven by 420 nm light on the codoped TiO2.4 Restricting the electron−hole recombination with the dopants of the optimum molar ratio was proposed4 and confirmed5 using time-resolved infrared (IR) absorption spectroscopy. A density functional theory simulation6 showed that Cr3+ is stably doped in TiO2 by the presence of Sb5+. Creating Cr with higher oxidation states or oxygen vacancies is not required. Successful sensitization by charge-compensated dopants has been further reported in SrTiO3 doped with Cr and Sb,4 TiO2 doped with Rh and Sb,7 TiO2 and SrTiO3 doped with Ni and Ta or Nb,8 and SrTiO3 doped with Cr and Ta.9,10 This study prepared SrTiO3 doped with Rh and Sb to examine the rate of recombination and the steady-state rate of the reductive and oxidative half-reactions. SrTiO3 is a semiconductor with a bandgap of 3.2 eV. By doping Rh alone, this material is sensitized for visible-light-driven H2 © 2013 American Chemical Society
production. A quantum yield of 5% was achieved with 420 nm light aided by a Pt cocatalyst.11 Applications to photoelectrodes12,13 and Z-scheme water splitting14−17 are examined. A Rh-doped SrTiO3 prepared by calcination in air contains Rh4+ and photochemical reduction to Rh3+ was required to produce H2.11 Charge compensation by the Sb codopant is expected to stabilize Rh3+ in as-prepared photocatalysts without photochemical reduction. The rate of electron−hole recombination is measured using time-resolved IR absorption. This pump and probe method is appropriate for quantifying the relative amount of electrons excited in photocatalyst particles of submicrometer sizes, as it is free from the scattering problems of probe light.18,19
2. EXPERIMENTAL SECTION Rhodium and antimony were doped into SrTiO3 by hydrothermal synthesis; details of this method appear in a separate paper.20 In brief, Sr(OH)2·8H2O, Rh(NO3)3, and Sb2O5 were mixed with P25 TiO2. The mixture was hydrothermally treated at 433 K for 40 h, washed with water, dried, and calcined at 1273 K in air. The concentration of Rh was fixed at 2 mol % relative to the Ti atoms in the starting materials. The Sb/Rh molar ratio was tuned to be 0, 0.5, 1.0, and 1.5. The SrTiO3 doped with 2 mol % Sb alone was prepared for comparison. The single phase of the doped SrTiO3 was checked and confirmed by X-ray diffraction.20 Received: July 16, 2013 Revised: August 25, 2013 Published: August 28, 2013 19101
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STO:Rh was blue-purple in color and presented two absorption bands peaked at 420 and 580 nm. The absorption at 580 nm was reduced by adding Sb and disappeared at Sb/Rh = 1.0. The color of the photocatalysts changed to pale yellow as a result of the reduced 580-nm absorption. The presence of two absorption bands, one sensitive and the other insensitive to the Sb/Rh ratio, indicates two oxidation states of Rh dopants in SrTiO3. Earlier studies11,21 proposed and confirmed that STO:Rh contained Rh4+ substitutional to Ti4+ as prepared in oxidizing environments. When the asprepared STO:Rh was photochemically or thermally reduced, the oxidation state of substitutional Rh4+ changed to be 3+. The absorption at 580 nm was caused by the electron transition from the valence band of SrTiO3 to an unoccupied d state of Rh4+. The electron transition from an occupied d state of Rh3+ to the conduction band of SrTiO3 absorbed 420-nm light. A d− d transition assignable to Rh4+ presented absorption at around 1000 nm in oxidized purple STO:Sb/Rh films.13 In the STO:Sb/Rh examined here, the absorption at 580 nm was reduced by adding Sb and disappeared at Sb/Rh = 1.0. Thus, it is assumed that the addition of Sb caused the Rh4+ to be reduced to Rh3+, even in STO:Sb/Rh calcined in air. Antimony can be oxidized to the 5+ state and the SrTiO3 host includes the equivalent amounts of Rh3+ and Sb5+ with no need for Rh4+ creation, as expected in the charge compensation scheme. 3.2. IR Absorption Induced by Electronic Excitation. Infrared absorption was induced by irradiation of 355-nm light pulses. Figure 2 shows transient absorbance spectra of the six photocatalysts observed in the vacuum with microsecond time delays. The nondoped STO presented monotonically increased absorbance from 3000 to 1000 cm−1, as seen in Figure 2a. Peaks at 2400 cm−1 were due to CO2 in air. The absorbance as a function of frequency ν was proportional to ν−2 in 3000− 2000 cm−1 and to ν−1.5 in 2000−1000 cm−1. Similar monotonic spectra proportional to ν−n have been observed with TiO2,22−26 S-doped TiO2,27 Cr and Sb codoped TiO2,5 NaTaO3,28 K3Ta3B2O12,29 and assigned to the electronic transition of bandgap-excited electrons. Bandgap-excited electrons are quickly trapped in midgap states. Electrons in shallow traps are thermally excited to the conduction band, and electrons in the conduction band absorb IR light to make intraband transitions. The optical transition from the trapped states to the conduction band also possibly causes the monotonic IR absorption. Following the previous findings, the transient absorption observed in STO is assigned to bandgap-excited elections not yet recombined with holes in the valence band. STO:Rh presented less intense absorption by 1 order of magnitude, and the monotonic shape of the spectrum was similar to that of STO. The weak absorption suggested quick electron−hole recombination in STO:Rh. STO:Sb and three STO:Sb/Rh exhibited a broad absorption peak at 3000−1800 cm−1, in addition to the monotonic spectrum. Those photocatalysts included Sb, and thus some electronic transition from or to Sb-related states caused the broad absorption. Figure 3 presents the transient absorbance spectra of three STO:Sb/Rh photocatalysts when irradiated by 532-nm light pulses. Those spectra contained broad peaks at 3000−1800 cm−1 with the background absorption monotonically strengthened at low wavenumbers. The shapes of the spectra were insensitive to the excitation wavelengths. The transient
The UV−visible diffuse reflection spectrum of the calcined photocatalysts was determined using a spectrometer (JASCO, V-570) with an integration sphere. For the IR studies, each photocatalyst was fixed on a 1-mm-thick CaF2 plate with 1 mg cm−2 density, heated at 573 K in air for 3 h to remove organic contaminants, and then placed in a gas cell. The pressure in the cell was 10 Pa when evacuated. In the time-resolved IR absorption study, a Q-switched Nd:YAG laser source (LOTIS, LS-2139) was operated at a repetition rate of 100 Hz. The fundamental output had a 20 ns period, which was frequency-tripled (or doubled) to produce pump light pulses of 355 (or 532) nm wavelength. The pump light of 1.0 mJ pulse−1 was focused on the CaF2 plate to be 6 mm in diameter. Continuous-wave IR light from a ceramic source was focused on the plate, and the transmitted light was dispersed with a grating monochromator of 25 cm focal length (JASCO, CT25). A mercury cadmium telluride detector (Kolmar, KMPV11) received the monochromatized light. The detector signal voltage was amplified with an ACcoupled amplifier (NF circuit, NF 5307). The bandwidth of the amplifier was set at 1 MHz and the time resolution of the spectrometer was thus 1 μs. By accumulating 2000 responses in an oscilloscope (Lecroy, LT264M), an absorbance change as small as 10−4 could be measured as a function of the time delay from the pump pulse. The absence of IR light thermally emitted from irradiated photocatalysts was checked and confirmed. Transient absorption spectra at different time delays were reconstructed from the absorbance−delay curves observed at wavenumbers of 20 cm−1 intervals. For steady-state reaction rate measurements, each of the calcined photocatalysts (0.2 g) was irradiated with light of 420 nm or longer wavelengths provided by a filtered 300 W Xe lamp. Platinum was photodeposited at 0.3 wt % as a cocatalyst for examining the reductive half-reaction of water splitting, H2 production, in an aqueous methanol solution of 10 vol. %. The oxidative half-reaction, O2 production, was tested in an aqueous AgNO3 solution of 0.02 mol L−1, without the aid of the cocatalyst. The rate of H2 or O2 production was quantified with a gas chromatograph.
3. RESULTS AND DISCUSSION 3.1. Visible Light Absorption of the Doped Photocatalysts. Figure 1 shows diffuse reflection spectra of the nondoped SrTiO3 (STO), Rh-doped SrTiO3 (STO:Rh), Sbdoped SrTiO 3 (STO:Sb), and Rh-and-Sb-doped SrTiO 3 (STO:Sb/Rh = 0.5, 1.0, 1.5) photocatalysts. The apparent absorbance, not Kubelka−Munk transformed, is shown as a function of wavelength. The STO and STO:Sb were transparent to visible light of 400 nm or longer wavelengths. The
Figure 1. Diffuse reflection spectra of the six photocatalysts. The broken line shows the spectrum of STO:Sb/Rh = 1.5, which overlaps that of STO:Sb/Rh = 1.0. 19102
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Figure 3. Transient IR absorbance spectra of (a) STO:Sb/Rh = 0.5, (b) STO:Sb/Rh = 1.0, and (c) STO:Sb/Rh = 1.5. Spectra reconstructed at 0, 300, and 700 μs are shown from the top to the bottom in each panel. Pump light wavelength: 532 nm.
Figure 4. Infrared absorbance at 2000 cm−1 as a function of the time delay from 355-nm pump light pulses.
smallest absorbance in the observed photocatalysts. This indicates that the electron−hole recombination in STO:Rh is the most quick. By adding Sb to STO:Rh, the zero-delay absorbance increases to suggest that recombination is retarded in the presence of the Sb codopant. The extent of retardation was sensitive to the Sb/Rh ratio. STO:Sb and STO:Sb/Rh = 0.5 and 1.0 presented enhanced zero-delay absorbances three times higher than that of STO. As evidenced in the diffuse reflection spectra of Figure 1, STO:Rh included Rh4+ and the concentration of this species decreased with the Sb/Rh ratio. Hence, the electron−hole recombination is proposed to be quickly catalyzed on Rh4+. In STO:Sb/Rh = 1.0, the equivalent amounts of Rh3+ and Sb5+ are present without Rh4+ or oxygen vacancy. Furthermore, the ionic radii of Rh3+ (0.067 nm) and Sb5+ (0.060 nm) are close to that of Ti4+ (0.061 nm).30 Tensile stress in the host lattice is not expected to be large in the presence of the charge-compensated dopants. In STO:Sb/Rh = 1.5, the zero-delay absorbance was smaller than that of STO:Sb/Rh = 1.0. Excess Sb can be autocompensated to be Sb3+ and Sb5+. However, Sb3+, with a large ionic radius of 0.076 nm,30 induces tensile stress in the SrTiO3 host lattice. Dislocations or grain boundaries, both possible sites for recombination, are induced as a result. The
Figure 2. Transient IR absorbance spectra of (a) STO, (b) STO:Rh, (c) STO:Sb, (d) STO:Sb/Rh = 0.5, (e) STO:Sb/Rh = 1.0, and (f) STO:Sb/Rh = 1.5. Spectra reconstructed at 0, 3, and 7 μs are shown from the top to the bottom in panel a. Spectra reconstructed at 0, 30, and 70 μs are shown from the top to the bottom in panels b−f. Pump light wavelength: 355 nm.
absorption of STO:Rh was too weak to identify spectral features, and thus it is not shown here. 3.3. Electron−Hole Recombination Rate. The absorbance at 2000 cm−1 was tracked as a measure of the concentration of excited electrons not yet recombined with holes. Figure 4 shows the absorbance observed in the vacuum as a function of the time delay. The absolute absorbance at zero delay reflects the excited electron concentration averaged over the time resolution of the spectrometer (1 μs in this study). When the electrons quickly recombine with holes, the zerodelay absorbance reduces accordingly. STO:Rh presented the 19103
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Figure 5b shows the normalized absorbance decay at 2000 cm−1 when excited by 532-nm light pulses. The order of recombination rate from fastest to slowest was STO:Sb/Rh = 1.5, STO:Sb/Rh = 1.0, and STO:Sb/Rh = 0.5, identical to the observation with 355-nm light pulses. The recombination rate was insensitive to the electronic transition in the initial stage of photoexcitation. The band gap excitation from the valence band to the conduction band of SrTiO3 is expected with an excitation wavelength of 355 nm. The 532-nm light pulses caused the transition from the occupied d state of Rh3+ to the conduction band of SrTiO3. The initial-state insensitive rate of recombination is natural under the assumption that the holes created in the valence band were quickly accommodated in the d state of Rh3+ and then recombined with electrons in the conduction band. The wavelength of 532 nm was on the absorption edge of the Rh3+ species, as shown in Figure 1. Valence band electrons could be excited by the 532-nm light pulses to the unoccupied d state of Rh4+ remaining in the presence of the Sb codopant. A further transition from the Rh4+ d state to the conduction band requires a photon energy of about 1.5 eV.13 Hence, the excited electrons in the Rh4+ d state are invisible by IR absorption. Electrons cannot absorb 2000 cm−1 IR light when trapped in midgap states energetically far from the conduction band edge. The presence of those states has been examined in dyesensitized TiO2 electrodes using 7500 cm−1 near-IR absorption.32 A limited fraction, less than 40%, of excited electrons was temporary accommodated in those states and then backtransferred to dye cations. 3.4. Steady-State Rates of H2 or O2 Production under Visible Light Irradiation. The rates of H2 or O2 production in the presence of the sacrificial agents is listed in Table 1.
large zero-delay absorbance in STO:Sb was also attributed to autocompensated Sb3+ and Sb5+ cations. The Sb concentration was 2 mol % in STO:Sb and 3 mol % in STO:Sb/Rh = 1.5. The Sb3+-induced stress should have been large in STO:Sb/Rh = 1.5. One may wonder about the mechanism for the retarded recombination. Dopants always reduce the flawlessness of a host lattice and frequently lead to an enhanced recombination rate. However, retarded recombination has been found with selected dopants of optimum concentrations: STO:Sb/Rh in the current study, Cr and Sb codoped TiO2,5 and La-doped,28 Ca-doped, Sr-doped, and Ba-doped31 NaTaO3. It is an essential issue for photocatalyst development to know why some selected dopants retard recombination and the others cannot. It had been proposed that the restricted mobility of electrons or holes may play a role in the charge-compensated dopants in TiO2,5 but this has yet to be confirmed. To trace recombination for time delays of 1−900 μs, the absorbance curves shown in Figure 4 were normalized by the zero-delay absorbance of each curve. The normalized absorbance decay is presented in Figure 5a. The photocatalysts
Table 1. Rates of H2 or O2 Production Induced by Visible Light Irradiation
Figure 5. Normalized decay of IR absorbance at 2000 cm−1 as a function of the time delay from (a) 355-nm and (b) 532-nm pump light pulses. Curves 1, 2, 3, and 4 represent the decay with STO:Sb/Rh = 0.5, 1.0, 1.5, and STO:Sb, respectively.
photocatalysts
H2 production/μmol h−1
O2 production/μmol h−1
STO:Rh STO:Sb/Rh = 0.5 STO:Sb/Rh = 1.0 STO:Sb/Rh = 1.5
30 9 6 5
0 0 1 2
STO:Rh provided the highest rate of H2 production, with an absence of O2 production. Purple STO:Rh as it was prepared in the oxidizing environment was unable to produce H2. When Rh4+ was converted to Rh3+ by photoreduction in the methanol solution, H2 was photocatalytically produced on the photoreduced, yellow STO:Rh. Diffuse reflection spectra of STO:Sb/ Rh = 0, 1.0, and 1.5 were checked after H2 production and showed identical features in the wavelengths of 300−500 nm. The Rh4+-derived absorption at 580 nm reduced according to photoreduction of Rh4+ to Rh3+.20 The total H2 production was in the order of 10−3 mol, whereas the amount of Rh dopant was on the order of 10−5 mol. H2 production was hence photocatalytic not stoichiometric. Oxygen production was examined and found to be null on the as-prepared, purple STO:Rh immersed in the AgNO3 solution. Although examination of the reduced STO:Rh was desirable to find further evidence on the role of Rh3+, it was difficult to cause photoreduction in the sacrificial reagent for the oxidative half-reaction. The photochemically reduced STO:Rh was easily reoxidized by exposing it to air. These results reproduced the previous findings by Konta et al.11
in order of fastest to slowest decay rate of normalized absorbance are STO:Sb/Rh = 1.5, STO:Sb, STO:Sb/Rh = 1.0, and STO:Sb/Rh = 0.5. The fast decay in STO:Sb/Rh = 1.5 is consistent with the relatively small zero-delay absorbance found in Figure 4, which represents the fast recombination at 0−1 μs. For STO:Sb, STO:Sb/Rh = 1.0, and STO:Sb/Rh = 0.5, the recombination rate in 1−900 μs was sensitive to the dopant composition, whereas the zero-delay absorbances were not much different to each other. Recombination in the early time delays of 0−1 μs is controlled by the direct, geminate recombination of electrons and holes. Trap and release cycles determine the recombination rate in the late time delays. The different mechanisms of recombination can be the origin of the decay rate sensitivities in the early delays or the insensitivities in the late delays to the dopant compositions. 19104
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nondoped photocatalysts exhibited the transient infrared absorption due to photoexcited electrons. Doping Rh alone caused the fastest absorbance decay, supporting the role of Rh4+ as an active center for recombination. The absorbance decay was retarded in the presence of Sb to show the limited ability of Rh3+ in recombination. The reductive or oxidative half-reaction of water splitting was examined in a solution containing a sacrificial reagent. Faster H2 production appeared on the codoped photocatalysts with slower recombination. The common efficiency of electron-toH2 conversion on the Pt cocatalyst could be the origin of this simple relationship. On the other hand, the fastest O2 production was found on the photocatalyst of Sb/Rh = 1.5, despite the intermediate rate of recombination. The hole-to-O2 conversion efficiency was thus suggested to be sensitive to the dopant compositions.
The transient IR results showed that electrons and holes quickly recombined on the Rh4+ present in the purple STO:Rh. This is consistent with the null production of O2. On the other hand, the active H2 production apparently disagrees with the IR results. The reason for the disagreement should be in the different oxidation states of STO:Rh. Hydrogen production was examined on the reduced phase of the photocatalyst, whereas IR absorption was observed on the oxidized phase. Unfortunately, it was difficult to install the reduced STO:Rh in the IR cell due to reoxidation by air. Addition of Sb to STO:Rh caused finite rates of H2 production even on the as-prepared, oxidized phase of the photocatalysts. The IR results indicated in STO:Sb/Rh that excited electrons were still available in the microsecond time delays from pump light irradiation. The electrons were transferred to Pt cocatalyst particles and consumed for H2 production. Decay curves in Figure 5 showed that the number of electrons available in this time range from largest to smallest was STO:Sb/Rh = 0.5, STO:Sb/Rh = 1.0, and STO:Sb/Rh = 1.5. An identical order was found for the H2 production rate. More H2 was produced on STO:Sb/Rh with more electrons. This simple relation suggests a common efficiency of electronto-H2 conversion. The suggested common efficiency is natural when we determine that H2 is produced on the surface of the Pt cocatalyst loaded on STO:Sb/Rh with different Sb/Rh ratios. The steady-state rate of the oxidative half-reaction was also enhanced by adding Sb. The retarded recombination caused by Sb is again the key for the enhanced reaction rates. The null O2 production is reasonable on the as-prepared STO:Rh, where the recombination was catalyzed by Rh4+, and hence, holes were unavailable for the oxidative half reaction. On the other hand, the O2 production rate quantitatively disagreed with the number of holes estimated in the IR results. STO:Sb/Rh = 0.5 presented intense zero-delay absorbance in Figure 4 and a slow absorbance decay in Figure 5. Nevertheless, O2 production was still limited. STO:Sb/Rh = 1.5 was more active than STO:Sb/ Rh = 1.0, although the IR results indicated that there were more holes in STO:Sb/Rh = 1.0. These quantitative disagreements suggested that the efficiency of hole-to-O2 conversion was sensitive to the dopant composition. The O2 production rate in Table 1 was determined in the absence of the cocatalyst. The reaction centers to produce O2 should be on the surface of the doped SrTiO3. The different concentrations of Sb, 1−3 mol % to Ti, lead to different extents of dopant segregation at the surface. Reaction centers favorable for the four-electron oxidation to produce O2 are possibly available on STO:Sb/Rh = 1.0 and 1.5, but not on STO:Sb/Rh = 0.5. An atomic force microscope study33 showed surface reconstruction of rutile (110) by codoping with Cr and Sb. Similar reconstructions may occur on the surface of the doped SrTiO3. Indeed, the O2 production rate was enhanced further by 30−50 times on STO:Sb/Rh in the presence of an IrOx cocatalyst.20 This enhancement suggests that the O2 production rate was limited by hole transfer reactions at the surface of STO:Sb/Rh.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS H.O. was supported by the Nippon Sheet Glass Foundation for Materials Science and Engineering. A.K. was supported by a Grant-in-Aid (No. 24107004) for Scientific Research on Innovative Areas (No. 2406) from the Ministry of Education, Culture, Sports, Science and Technology in Japan.
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REFERENCES
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4. CONCLUSION In this research, SrTiO3 photocatalysts doped with Rh and Sb were prepared by hydrothermal synthesis. The UV−visible absorption results showed that doping Rh alone resulted in a mixture of Rh4+ and Rh3+. The oxidation state of Rh was fixed to be 3+ by adding the equivalent amount of Sb, as expected in the cation charge compensation scheme. The doped and 19105
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The Journal of Physical Chemistry C
Article
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