12848
J. Phys. Chem. C 2007, 111, 12848-12854
Photophysical and Photocatalytic Properties of Three Isostructural Oxide Semiconductors In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 with Different 3d Transition Metals Defa Wang,† Jinhua Ye,*,† Hideaki Kitazawa,‡ and Takashi Kimura§ Photocatalytic Materials Center, Quantum Beam Center, and Materials Analysis Station, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ReceiVed: NoVember 27, 2006; In Final Form: June 21, 2007
Three oxide semiconductor photocatalysts, namely, In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42, have been investigated systematically to clarify the effects of transition metal cations (Ni and/or Cr) with partially filled 3d orbitals on the photophysical and photocatalytic properties. It was found that the three compounds all crystallized in a monoclinic crystal system with the same space group P21/a irrespective of the minor constituent cations Ni and/or Cr. However, their band-gap energies differed greatly depending on the minor cations: ∼2.48 eV for In6NiTi6O22, ∼2.0 eV for In3CrTi2O10, and ∼2.14 eV for In12NiCr2Ti10O42. In terms of photocatalytic H2 evolution, In12NiCr2Ti10O42, with both Ni and Cr as the minor cations, showed a much higher activity (∼8.2 µM/h) than either In6NiTi6O22 (∼0.3 µM/h) or In3CrTi2O10 (∼0.2 µM/h), with Ni or Cr, respectively, as the sole minor cation. In accordance with the photophysical and photocatalytic properties, we suggest that discontinuous interbands are formed by the split Ni 3d orbital in In6NiTi6O22 or Cr 3d orbital in In3CrTi2O10 individually, whereas continuous conduction and valence bands are formed in In12NiCr2Ti10O42 by the hybridization of split Ni 3d and Cr 3d orbitals with Ti 3d/In 5sp and O 2p orbitals, respectively. The improved photocatalytic activity of In12NiCr2Ti10O42 can be attributed to the enhanced mobility of photoexcited charge carriers.
1. Introduction Development of visible-light-driven photocatalysts for water splitting is of importance in view of efficient solar energy conversion because ∼43% of solar energy is visible light whereas only ∼4% is UV light.1-12 In principle, a visible-lightdriven semiconductor material capable of photocatalytic water splitting should have a small band gap able to absorb visible light as well as appropriate band edges suitable for the redox reaction of H2/O2 evolution. Undoubtedly, an ideal photocatalyst should be able to split pure water stoichiometricaly into H2 and O2 under visible light irradiation. To our best knowledge, however, only two single-phase photocatalysts, namely, In1-xNixTaO4,6 and (Ga1-xZnx)(N1-xOx),7 have been reported so far as visible-light-driven photocatalysts. On the other hand, the two-step photoexcitation (Z-scheme) system, in which two photocatalysts, one for H2 evolution and the other for O2 evolution, are essential, is also known to be a promising method for pure water splitting.8 However, even if the redox reaction is assisted with an electron donor (e.g., CH3OH) or an electron scavenger (e.g., AgNO3), the number of visible-light-driven photocatalysts for efficient H2 or O2 evolution is still very limited. In the past three decades, many efforts have been made toward the development of highly efficient visible-lightresponsive photocatalysts for H2 and/or O2 evolution by employing various strategies. For example, some titanium oxides such as the well-known TiO2,1-4 SrTiO3,13 and various indium * Corresponding author. Fax: +81-298-59-2301. E-mail: jinhua.ye@ nims.go.jp. † Photocatalytic Materials Center. ‡ Quantum Beam Center. § Materials Analysis Station.
oxides such as In2O3(ZnO)m,14 MIn2O4 (M ) Ca, Sr, Ba),15,16 and Sr1-xMxIn2O4 (M ) Ca, Ba)17 have previously been reported as photocatalysts for H2 evolution. However, most In3+- and/or Ti4+-containing oxides, except for those containing Ti cations with lower valences,18 absorb only UV light or little visible light, because the O 2p orbital is usually located at a deep level. One method for developing visible-light-driven photocatalysts is to move the top of the valence band of a semiconductor to a more negative position than O 2p by forming hybrid valence bands (e.g., Bi 6s + O 2p in BiVO4,19 Ag 4d + O 2p in AgNbO320). On the other hand, the doping of foreign elements, mostly 3d transition metals, into an active photocatalyst with a wide band gap to form a donor level in the forbidden band is also adopted for the development of visible-light-responsive photocatalysts, because the 3d orbitals of transition metals usually display versatile features depending on the crystal field and valence state. Many 3d-transition-metal-doped photocatalysts such as In1-xNixTaO4,6 Ti1-xCrxO2,21-24 SrTi1-x(Cr,M)xO3 (M ) Nb/ Ta),25 Sr1-xCrxTiO3,26 and La2(Ti1-xMx)2O7 (M ) Cr, Fe)27 have been reported for H2 evolution under visible light irradiation. However, it is usually hard to achieve a high photocatalytic activity from a doped material because of the small amount of visible light absorbed by a discrete doping level and the low mobility of photoinduced charge carriers (e-/h+). Generally speaking, continuous energy bands are indispensable to a highly active semiconductor photocatalyst in terms of photoinduced charge carrier mobility. The concept of making solid-solution photocatalysts with controlled electronic structures has been attracting increasing attention in recent years. Some solid solutions such as GaN-ZnO,7 ZnS-AgInS2,28 and CaMoO4-BiVO429 have been reported to exhibit enhanced photocatalytic activities under visible light irradiation. Considering stability upon photoreaction, oxides are undoubtedly ad-
10.1021/jp0678599 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007
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Figure 1. X-ray diffraction patterns and main indices of (a) In6NiTi6O22, (b) In3CrTi2O10, and (c) In12NiCr2Ti10O42 powder samples at room temperature.
vantageous over the other types of materials such as sulfides, oxysulfides, nitrides, and oxynitrides. Recently, we reported the photocatalytic H2 evolution over the new visible-light-driven photocatalyst In12NiCr2Ti10O42,30 which is actually a solid solution between the two isostructural compounds In6NiTi6O22 and In3CrTi2O10, assuming In12NiCr2Ti10O42 ) In6NiTi6O22 + 2In3CrTi2O10. We also notice that In12NiCr2Ti10O42 contains both Ni and Cr as minor constituents, whereas In6NiTi6O22 contains only Ni and In3CrTi2O10 contains only Cr as the minor constituent. In the present work, the effects of transition metal cations Ni and/or Cr with partially filled 3d orbitals on the electronic structures and photophysical and photocatalytic properties of the three isostructural compounds In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 were systematically studied. In particular, the valence states of the minor cations Ni and Cr were analyzed by magnetic susceptibility and X-ray photoelectron spectroscopy (XPS). Some useful guidelines are provided for designing visible-light-driven oxide semiconductor photocatalysts with tailored properties by means of transition-metalmediated band engineering. 2. Experimental Procedures 2.1. Sample Preparation. Polycrystalline powder samples of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 were synthesized by a conventional solid-state reaction method. Reagentgrade oxides In2O3, NiO, Cr2O3, and TiO2 (Wako, 99.9%) in the appropriate stoichiometric ratios for the desired compounds were carefully mixed with addition of ethanol in an agate mortar and then pressed into pellets. After being calcined in alumina crucibles at 1200 °C for 48 h with one intermittent regrinding, the pellets were rapidly cooled to room temperature in air and finally milled into powders for the experiments. 2.2. Sample Characterization. Crystal structures were determined with an X-ray diffractometer (XRD, JEOL JDX-
3500) using Cu KR radiation (λ ) 1.54178 Å). Raman spectroscopy was performed on a laser Raman spectrophotometer (NRS-1000, Jasco) at room temperature. The power of the incident laser beam with a monochromatic wavelength of 532 nm was 100 mW, and the exposure time was 1 s. The morphology was observed with a field-emission scanning electron microscope (FE-SEM, JEOL-JSM 6500F), and the composition was analyzed with an X-ray energy-dispersive spectrometer (EDS) equipped on the FE-SEM. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was also employed for precise analysis of chemical compositions. UV-vis diffuse reflectance spectra were measured at room temperature with a UV-vis spectrometer (UV-2500, Shimadzu) and were converted to absorption spectra by the Kubelka-Munk method. Surface areas were measured on a Gemini 2360 surface area analyzer (Micromeritics, Shimadzu) by nitrogen absorption at 77 K using the Brunauer-Emmett-Teller (BET) method. 2.3. Valence State Measurement by Magnetic Susceptibility and XPS. The static (dc) magnetic susceptibilities χ(T) of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 powder samples were measured on a superconducting quantum interference device (SQUID) magnetometer (MPMS5S, Quantum Design Ltd.) in the temperature range of 2-300 K under a magnetic field of 0.1 T. The zero-field-cooling (ZFC) process was applied in performing the measurements. The average valence states of the Ni and Cr cations in the bulk compounds were calculated from charge neutrality using the obtained magnetic parameters. In addition, an X-ray photoelectron spectrometer (XPS, PHI Quantera SXM, ULVAC PHI) using monochromatic Al KR irradiation (power, 25 W; beam diameter, 100 µm; incident angle, 45°) was employed for determining the valence states of the Ni and Cr cations on the surface of powder samples. 2.4. Evaluation of Photocatalytic Properties. Photocatalytic reactions were carried out in an outer irradiation Pyrex glass cell connected to a closed gas circulation system. For H2 evolution, the Pt-loaded (0.2 wt %) catalyst powders (0.5 g) were dispersed by a magnetic stirrer in aqueous methanol solution (50 mL CH3OH + 220 mL H2O) in the reaction cell. The cocatalyst Pt was loaded by an in situ photodeposition method: Under light irradiation, an equivalent molar amount of H2PtCl6 in solution was reduced to the metallic state and deposited onto the surface of catalyst. A 300-W xenon arc lamp (ILC Technology, CERMAX LX-300, operated at 200 W) was focused on the side window of the cell through a long-pass cutoff filter (λ g 420 nm; L42, HOYA). Upon irradiation, the evolved gas was analyzed in situ with an online TCD gas chromatograph (Shimadzu GC-8AIT, argon carrier).
TABLE 1: Crystal Structures and Photophysical and Photocatalytic Properties of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 crystal system
space group
lattice parameters
band-gap energy (eV)
surface area (m2/g)
H2 evolution rate (µM/h)
In6NiTi6O22
monoclinic
P21/a
2.48
0.25
0.3
In3CrTi2O10
monoclinic
P21/a
2.0
0.54
0.2
In12NiCr2Ti10O4 2
monoclinic
P21/a
a ) 5.937(3) Å b ) 10.128(1) Å c ) 6.363(3) Å β ) 108.11(3)° a ) 5.931(2) Å b ) 10.087(1) Å c ) 6.366(2) Å β ) 108.08(1)° a ) 5.932(2) Å b ) 10.098(1) Å c ) 6.358(2) Å β ) 108.12(2)°
2.14
0.44
8.2
photocatalyst
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Figure 3. Raman spectra of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 powder samples at room temperature.
Figure 2. Layered crystal structure of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42. An edge-shared M(1)O6 octahedral layer and a pseudo-octahedral M(2)O6 layer are alternately stacked along the c axis. The M(1) atoms are primarily In and very few Ti, and the M(2) atoms are (Ti/Ni) for In6NiTi6O22, (Ti/Cr) for In3CrTi2O10, and (Ti/Ni/Cr) for In12NiCr2Ti10O42.
3. Results and Discussion 3.1. Crystal Structure. Figure 1 shows the XRD patterns of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 polycrystalline powder samples, which are almost identical to each other. The indexed results verify that the three materials all crystallized in a monoclinic system with the same space group P21/a. The lattice constants calculated by the least-squares method are listed in Table 1. These values are in a good agreement with those reported by Brown et al.,31 who clarified that In6ATi6O22, In3BTi2O10, and their solid solutions In12AB2Ti10O42 (A ) divalent cations Mg, Co, Ni, Cu, Zn, etc.; B ) trivalent cations Al, Cr, Fe, Ga, etc.) have the same crystal structure. Referring to In3CrTi2O10 as reported by Michiue et al.,32 the pyrochlore-related layered structure of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 with a pseudo-rhombohedral symmetry is depicted in Figure 2. It shows that two layers are alternately stacked along the c axis: one is an edge-shared M(1)O6 octahedral sheet, and the other consists of the M(2)-O sheet surrounded by three or four oxygen ions on the plane and two additional oxygen ions along the c axis. For simplicity, M(2)-O can be regarded as a pseudo-octahedron M(2)O6 with oxygen vacancies. Two metal positions are involved in the whole structure: M(1), occupied primarily by In and very few Ti, and M(2), occupied mainly by Ti, Ni, and/or Cr depending on the composition. The contributions of Ti atoms coexisting with In at the M(1) site to both crystal and electronic structures are negligible. 3.2. Raman Spectroscopy. Raman spectroscopy is known to be more sensitive than XRD to local atomic arrangements in a crystal lattice.33 As shown in Figure 1, In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 have nearly the same XRD patterns; however, their Raman spectra are noticeably different (Figure 3). It is interesting to note that the Raman spectrum of In12NiCr2Ti10O42 seems to be a linear sum of those of In6NiTi6O22 and In3CrTi2O10, analogous to the linear relation among the molecular formulas of these three compounds. At present, the corresponding lattice vibration modes cannot be assigned to each Raman peak. Nevertheless, we can conclude that the different Raman spectra are undoubtedly caused by the different minor cations Ni and/or Cr locating in the pseudooctahedron M(2)O6. This indicates that the three compounds In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 might show different photophysical and photocatalytic properties associated with crystal lattice vibrations.
3.3. SEM Micrograph and BET Surface Area. Figure 4a-c shows typical SEM morphologies of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 powder samples. The particle sizes are in the order In6NiTi6O22 > In12NiCr2Ti10O42 > In3CrTi2O10. Consistently, the BET surface areas were measured to be 0.25 m2/g for In6NiTi6O22, 0.44 m2/g for In12NiCr2Ti10O42, and 0.54 m2/g for In3CrTi2O10. It seems that the involvement of Cr cations is favorable for the particle refinement. The EDS mapping results confirmed the homogeneous distribution of all constituent elements, including the minor elements Ni and/or Cr. 3.4. UV-Vis Diffuse Reflectance Spectra. Figure 5 shows absorption spectra of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 powder samples at room temperature. From the main absorption edges, the band-gap energies were estimated to be ∼2.48 eV for In6NiTi6O22, ∼2.0 eV for In3CrTi2O10, and ∼2.14 eV for In12NiCr2Ti10O42. Previous studies on Cr-doped TiO2 and SrTiO3 have shown that the occupied Cr 3d t2g state is usually located at a potential level ∼2.2 eV below the bottom of the conduction band formed by Ti 3d orbitals.24 Coincidently, the band-gap energies of In3CrTi2O10 (∼2.0 eV) and In12NiCr2Ti10O42 (∼2.14 eV) containing the minor cation Cr are very close to 2.2 eV. This means that the behavior of the Cr 3d orbitals in the M(2)O6 pseudo-octahedra of In3CrTi2O10 and In12NiCr2Ti10O42 is very similar to that in Cr-doped TiO2 and SrTiO3, even though their crystal structures are different. We also notice that, in addition to the main absorption edge, there is an apparent broad feature from 680 to 850 nm in the absorption spectrum of In6NiTi6O22 and a similar broad feature from 630 to 800 nm in the absorption spectrum of In3CrTi2O10. Preliminarily, these additional absorptions could be assigned to the d-d transition between the occupied and empty 3d orbitals of Ni or Cr, which are probably isolated between the major Ti 3d/In 5sp and O 2p bands. Compared to In6NiTi6O22 and In3CrTi2O10, In12NiCr2Ti10O42 does not show an obvious broad feature in its absorption spectrum, indicating that the split 3d orbitals of the Ni and Cr cations might be hybridized with the major Ti 3d/In 5sp and O 2p bands, respectively. The relatively high backgrounds in the absorption spectra of In3CrTi2O10 and In12NiCr2Ti10O42 are assumed to be related to the split empty Cr 3d orbitals scattering at different levels. Further details on the electronic structures associated with the minor cations Ni and/or Cr are discussed below in relation to the photocatalytic activities. 3.5. Photocatalytic Activity. Figure 6 shows the photocatalytic H2 evolution from aqueous methanol solution (50 mL CH3OH + 220 mL H2O) over Pt-loaded (0.2 wt %) In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 powder catalysts (0.5 g) under visible light irradiation (λ g 420 nm). As mentioned previously, some properties such as the Raman spectra, surface areas, and band-gap energies of the isostructural In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 compounds have a nearly
Properties of Three Isostructural Oxide Semiconductors
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12851
Figure 5. UV-vis absorption spectra of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 powder samples at room temperature. The dotted line shows the absorption spectrum of In12ZnGa2Ti10O42.
Figure 6. Photocatalytic H2 evolution from aqueous methanol solution (50 mL CH3OH + 220 mL H2O) over Pt-loaded (0.2 wt %) In6NiTi6O22, In3CrTi2O10, or In12NiCr2Ti10O42 catalysts (0.5 g) under visible light irradiation (λ g 420 nm). Light source, 300-W Xe lamp (operated at 200 W).
Figure 7. Wavelength dependence of H2 evolution from aqueous methanol solution (50 mL CH3OH + 220 mL H2O) suspended with the Pt (0.2 wt %)/In12NiCr2Ti10O42 powder catalyst (0.5 g), showing good consistency with the absorption spectrum (Figure 2). Light source: a 300-W Xe lamp (operated at 200 W). Reaction time, 1 h.
Figure 4. SEM micrographs of (a) In6NiTi6O22, (b) In3CrTi2O10, and (c) In12NiCr2Ti10O42 powder samples.
linear relationship. However, their photocatalytic activities are significantly different: In12NiCr2Ti10O42 . In6NiTi6O22 ≈ In3CrTi2O10. The total molar amount of H2 (∼295 µM) evolved over In12NiCr2Ti10O42 for ∼44 h was nearly 1.6 times that of the catalyst (∼186 µM) used in the catalytic reaction. A dark test with In12NiCr2Ti10O42 showed that no H2 was evolved when the light was turned off, excluding the possibility of H2 generation by a “mechanocatalytic mechanism”.34 A long-term course of photocatalytic H2 evolution was also performed over
Pt (0.2 wt %)/In12NiCr2Ti10O42. The reaction was repeated twice, and the total reaction time was more than 100 h. We found that the catalytic activity was repeated very well (see Figure S1, Supporting Information). As shown in Figure 7, the wavelength dependence of H2 evolution conducted on In12NiCr2Ti10O42 by using a series of cutoff filters was consistent with the UV-vis absorption spectrum (inset). These results confirm that the H2 evolution resulted inherently from the photocatalytic reaction. Oxygen evolution from an aqueous silver nitrate solution (5 mM AgNO3 + 270 mL H2O) was not observed over these three compounds under visible light irradiation, probably because the top of the valence band determined by the occupied Cr 3d or Ni 3d orbitals is not sufficiently positive to oxidize H2O into O2 (see the detailed discussion in section 3.7). Nevertheless, using the developed photocatalyst In12NiCr2Ti10O42 for H2 evolution and a suitable counterpart photocatalyst for O2 evolution, it might be possible to construct a Z-scheme system for stoichiometric pure water splitting under visible light
12852 J. Phys. Chem. C, Vol. 111, No. 34, 2007
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Figure 9. Schematic band structures of (a) In6NiTi6O22, (b) In3CrTi2O10, and (c) In12NiCr2Ti10O42. Depending on the type and amount of the minor cations Ni2+ and/or Cr3+ in the crystal field, discrete or continuous interbands formed by the 3d orbitals of Ni2+ and/or Cr3+ are scattered at different potential levels between the Ti 3d/In 5sp and O 2p orbitals, giving rise to different band structures of In12NiCr2Ti10O42. See text for details.
Figure 8. Temperature dependence of the inverse magnetic susceptibility, 1/[χ(T) - χ0(T)], for (a) In6NiTi6O22, (b) In3CrTi2O10, and (c) In12NiCr2Ti10O42 polycrystalline powder samples. The magnetic susceptibility χ0(T) of In3AlTi2O10 as the nonmagnetic susceptibility was subtracted from the total magnetic susceptibility χ(T). The insets in parts b and c show different Curie-Weiss fittings for In3CrTi2O10 and In12NiCr2Ti10O42, respectively, in the lower temperature range.
irradiation. In such a Z-scheme system, other factors such as the shuttle redox mediator (e.g., IO3-/I-, Fe2+/Fe3+) and the pH values of reaction solution should also be considered systematically.35 Concerning the stability of the photocatalysts upon photoirradiation, the samples after the H2 evolution reaction were checked by XRD. It was found that the XRD patterns before and after reaction were almost identical to each other. The samples after reaction were also analyzed with EDS, and no change in surface composition was found. After the catalyst powders had been filtered, the reaction solution was chemically analyzed using the ICP-AES method, and no constituent elements of the compounds were detected. All of these results confirm the stability in both bulk structure and surface composition of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 against photoirradiation. 3.6. Magnetic Susceptibility and XPS Measurements. The variable valence states of partially filled 3d transition metal cations are known to play a very important role in determining the properties of transition metal oxides. Thus, it is necessary to clarify the valence states of the Ni and Cr cations in In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42. Figure 8a-c shows the temperature dependence of the inverse magnetic susceptibilities, 1/[χ(T) - χ0(T)], for In6NiTi6O22, In3CrTi2O10,
and In12NiCr2Ti10O42 powder samples, respectively. The amounts of the samples used for susceptibility measurements were 38.6 mg for In6NiTi6O22, 23.8 mg for In3CrTi2O10, and 30.7 mg for In12NiCr2Ti10O42. The magnetic susceptibility χ0(T) of In3AlTi2O10 as the nonmagnetic susceptibility was subtracted from the total magnetic susceptibility χ(T). The magnetic parameters could be obtained from the CurieWeiss fitting: χ ) C/(T - θw) + χ0, where C, θw, and χ0 are the Curie constant, the Weiss temperature, and the temperatureindependent susceptibility, respectively. Here, C ) RNpeff2/3kB, where R is the concentration of Ni and/or Cr per formula unit, N is Avogadro’s number, peff is the effective magnetic moment, and kB is the Boltzmann constant. For In6NiTi6O22, the data were fitted by the Curie-Weiss law in throughout the entire temperature range examined (2-300 K): 1/[χ(T) - χ0(T)] ) 3.0439 + 0.73725T. For In3CrTi2O10, the Curie-Weiss fittings were 1/[χ(T) - χ0(T)] ) -7.17025 + 0.57022T for 100-300 K and 1/[χ(T) - χ0(T)] ) 0.28086 + 0.38699T for 2-30 K (see Figure 8b, inset). For In12NiCr2Ti10O42, the Curie-Weiss fittings were 1/[χ(T) - χ0(T)] ) 2.6543 + 0.20161T for 100300 K and 1/[χ(T) - χ0(T)] ) 0.357 + 0.22832T for 2-100 K (see Figure 8c inset). According to crystal field theory (CFT),36 the average valence states of Ni and Cr cations with partially filled 3d orbitals were calculated from charge neutrality using the obtained magnetic parameters, the spin states (S). As summarized in Table 2, the Ni cations in both In6NiTi6O22 and In12NiCr2Ti10O42 were Ni2+, and the Cr cations in In12NiCr2Ti10O42 were Cr3+. However, the average valence of Cr cations in In3CrTi2O10 was calculated to be 4+. XPS measurements (see Figures S2-S4, Supporting Information) showed that the valences of the Ni and Cr cations in In6NiTi6O22 and In12NiCr2Ti10O42 were 2+ and 3+, respectively, being consistent with the results obtained from the magnetic susceptibility measurements. For In3CrTi2O10, however, no higher valence state of Cr cations (e.g., Cr6+), but only Cr3+, was detected by the XPS, in contrast to the magnetic susceptibility results. It is known that XPS information is mainly from the sample surface whereas magnetic susceptibility is a bulk property. In the case of the XPS measurements, the possible Cr6+ cations on the sample surface were very likely to be reduced by the incident X-ray photoelectrons to lower valence states such as Cr3+. Probably, this is the most important reason that only Cr3+ was detected by XPS. In this sense, we think that the magnetic susceptibility measurements showed a more reasonable and thus more reliable result than the XPS measure-
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TABLE 2: Magnetic Parameters for In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 from Curie-Weiss Fitting χ ) C/(T - θw) + χ0a photocatalyst In6NiTi6O22 In3CrTi2O10 In12NiCr2Ti10O4 2
temperature range (K) 1.9-300 2-30 100-300 1.9-100 100-300
peff
θw (K)
χ0 (emu/mol)
S
3.29 4.55 3.74 35.0 39.7
-4.1b -0.73b 12.6c -1.6b -13.2b
-1.3 × 10-4 -1.3 × 10-4 -1.3 × 10-4 -1.3 × 10-4 -1.3 × 10-4
0.877 1.09 0.957 1.29 1.34
valence states Ni2+ Cr4+ (2Cr3+ + Cr6+) Ni2+, Cr3+
a In the equation χ ) C/(T - θw) + χ0, C, θw, and χ0 are the Curie constant, Weiss temperature, and temperature-independent susceptibility, respectively. The value of χ0 ) -1.3 × 10-4 emu/mol was obtained from the nonmagnetic In3AlTi2O10. Here, C ) RNpeff2/3kB, where R is the concentration of Ni and/or Cr per formula unit, N is Avogadro’s number, peff is the effective magnetic moment, and kB is the Boltzmann constant. Approximately, peff ) gxs(s+1), where g ≈ 2. From the calculated average spin value (S), the valence states of the Ni and Cr cations were deduced according to charge neutrality. b Antiferromagnetic. c Ferromagnetic.
ments. Considering the order of stability of different Cr cations, i.e., Cr3+ > Cr6+ > Cr4+, we speculate that Cr3+ and Cr6+ coexisted in a ratio of 2:1 in In3CrTi2O10. Both EDS and ICP-AES analyses revealed that all of the prepared samples of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 had almost the ideal chemical compositions. According to the rule of charge neutrality, the valence states of the partially filled 3d transition metal cations should be Ni2+ and Cr3+ in In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42. However, the magnetic susceptibility and XPS measurements showed that charge neutrality was satisfied only in In6NiTi6O22 and In12NiCr2Ti10O42, but not in In3CrTi2O10, indicating the different nonstoichiometric chemistries in these three compounds. As discussed below, the different valence states of Cr cations are believed to play an important role in the electronic structures and, thus, the photophysical and photocatalytic properties of the Cr-containing compounds In3CrTi2O10 and In12NiCr2Ti10O42. 3.7. Electronic Structure. The isostructural compounds In6ATi6O22, In3BTi2O10, and In12AB2Ti10O42 (A ) Mg, Zn; B ) Al, Ga)31 with empty or fully filled 3d orbitals of divalent cations A and trivalent cations B have essentially the same absorption edge around ∼350 nm (eg, ∼3.54 eV), which is clearly shown, for instance, by In12ZnGa2Ti10O42 in Figure 5. This means that neither the divalent cations A nor the trivalent cations B with empty or fully filled 3d orbitals determine the band gap. However, when the divalent cations A and the trivalent cations B are the transition metal cations Ni and/or Cr with partially filled 3d orbitals, the absorption edges of the obtained In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 compounds shifted dramatically to the visible light region. As shown in Figure 5, the different absorption spectra of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 imply that the electronic structures are closely associated with the 3d orbitals of the minor cations Ni and/or Cr in the pseudo-octahedra M(2)O6: (Ti/Ni)O6 for In6NiTi6O22, (Ti/Cr)O6 for In3CrTi2O10, and (Ti/Ni/Cr)O6 for In12NiCr2Ti10O42. The occupancy proportions of the minor cations Ni and/or Cr in the M(2)O6 pseudo-octahedral layer were calculated from the generalized formula to be ∼15.4% for In6NiTi6O22, ∼33.3% for In3CrTi2O10, and ∼24.0% for In12NiCr2Ti10O42 (see Table S1, Supporting Information), which are actually considerable values in contributing to the electronic structures. According to crystal field theory (CFT),36 the 5-folddegenerate 3d orbitals in the M(2)O6 pseudo-octahedral field usually splits into the t2g (dxy, dyz, dzx) state with a lower energy level and the eg (dz2, dx2-y2) state with a higher energy level. Because the M(2)O6 pseudo-octahedron is subjected to an extension distortion along the c axis,32 Coulomb interactions lead to further splitting of the t2g state into a low-energy level t2g(low) (dyz, dzx) and a high-energy level t2g(high) (dxy). Similarly, the eg state also splits into a low-energy level eg(low) (dz2) near
t2g and a high-energy level eg(high) (dx2-y2).37 Taking into account the above-mentioned photophysical and photocatalytic properties, the band structures of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 are schematically illustrated in Figure 9a-c. For In6NiTi6O22 with Ni2+ as the minor cation, the occupied Ni 3d state is presumed to be adjacent to O 2p, serving as the top of the valence band, whereas the empty Ni 3d state is scattered below Ti 3d/In 5sp orbital discretely. The excitation from the occupied Ni 3d orbital to the empty (Ti 3d + In 5sp) orbital accounts for the main absorption edge around ∼500 nm (∼2.48 eV). The broad absorption feature around ∼800 nm could be attributed to the d-d transition between the split Ni 3d orbitals. For In3CrTi2O10 containing the minor cation of mixed valence states (Cr3+ + Cr6+), the occupied Cr3+ 3d t2g(low) state is assumed to form a discrete interband above O 2p, judging from the separate broad feature between 650 and 820 nm in the absorption spectrum of In3CrTi2O10. Moreover, the empty Cr6+ 3d level below the critical Ti 3d orbital is usually lower than the potential of H+/H2, indicating that it can behave only as the trapping center for photoinduced electrons.25,26 This is probably one of the crucial reasons accounting for the low H2 evolution activity of In3CrTi2O10 under visible light irradiation. The transition of photoinduced electrons from the occupied Cr 3d t2g(low) orbital to the (Ti 3d + In 5sp) orbital accounts for the main absorption at ∼620 nm (∼2.0 eV), whereas the transition between the split Cr 3d orbitals corresponds to the broad absorption feature around ∼700 nm. For In12NiCr2Ti10O42 containing both Ni2+ and Cr3+ as the minor cations, the split 3d orbitals of Ni2+ and Cr3+ scatter, as discussed previously, at different potential levels between O 2p and (Ti 3d + In 5sp). It is known that, in the same crystal field, the energy level of 3d orbitals usually decreases with increasing number of 3d electrons. This means that the potential level of the Ni 3d orbital is lower than that of the Cr 3d orbital. We speculate that, in the pseudo-octahedron (Ti/Ni/Cr)O6, the occupied Ni 3d states probably hybridize with the occupied Cr 3d t2g(low) states. Moreover, if the occupied Ni 3d state is adjacent to the O 2p orbital, the occupied Cr 3d t2g(low) states and the O 2p orbital might be “bridged” together and thus form a continuous valence band. Similarly, the empty 3d orbitals of Ni2+ and Cr3+ are also likely to hybridize with each other. In this case, the top of the valence band is determined by the occupied Cr 3d t2g(low) orbital and the bottom of the conduction band is formed by the empty Cr 3d (t2g(high) + eg) orbital. In summary, the discrete interbands formed by the split Ni 3d orbitals alone in In6NiTi6O22 or by the split Cr 3d orbitals alone in In3CrTi2O10 undoubtedly lead to a low charge carrier mobility and, consequently, to a decreased photocatalytic activity. The presence of Cr6+ cations in In3CrTi2O10 is also an
12854 J. Phys. Chem. C, Vol. 111, No. 34, 2007 important reason accounting for its low photocatalytic activity. In contrast, the more broadly dispersed and continuous valence band and conduction band formed by the hybridization of the 3d orbitals of Ni2+ and Cr3+ with the Ti 3d/In 5sp and O 2p orbitals in In12NiCr2Ti10O42 are favorable for the mobility of photoexcited charge carriers (e-/h+), giving rise to an improved photocatalytic activity. From a systematic study of the photophysical and photocatalytic properties of the isostructural oxide semiconductors In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 with different 3d transition metal cations (Ni and/or Cr), we have shown that transition-metal-mediated band engineering is a feasible strategy for developing highly efficient visible-lightresponsive semiconductor photocatalysts. 4. Conclusions The three isostructural compounds In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42, crystallized in a monoclinic system in space group P21/a, have been found to show different electronic structures and photophysical and photocatalytic properties related to the different transition metals Ni and/or Cr with partially filled 3d orbitals. The split Ni 3d orbital in In6NiTi6O22 or the split Cr 3d orbital in In3CrTi2O10 forms only a discrete band, thus decreasing the charge carrier mobility. Moreover, the possible Cr6+ level in In3CrTi2O10 is speculated to be located below the H+/H2 potential, behaving as the trapping center for photoinduced electrons. As a consequence, both In6NiTi6O22 and In3CrTi2O10 exhibit low photocatalytic activities for H2 evolution. In contrast, the coexistence of both Ni2+ and Cr3+ in In12NiCr2Ti10O42 enables the formation of continuous conduction and valence bands by the hybridization of split Ni 3d and Cr 3d orbitals with Ti 3d/In 5sp and O 2p orbitals, accounting for the enhanced mobility of photoexcited charge carriers. Thus, the photocatalytic activity of In12NiCr2Ti10O42 was markedly improved. The present study suggests that properly incorporating appropriate transition metals into a structure with suitable band edges is a promising approach to the development of efficient visible-light-driven photocatalysts for water splitting. Acknowledgment. The authors thank Dr. Takehiko Matsumoto for stimulating discussions and valuable comments. Drs. Tetsuya Kako, Satoshi Ota, and Sei Fukushima are also acknowledged for their help in XPS measurements and discussions. This work was partially supported by the Global Environment Research Fund and a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. Supporting Information Available: General formulas and occupancy proportions of minor cations Ni and/or Cr in the M(2)O6 pseudo-octahedral layer of In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 (Table S1). Long-term course of photocatalytic H2 evolution from aqueous methanol solution (50 mL CH3OH + 220 mL H2O) over Pt (0.2 wt %)/In12NiCr2Ti10O42
Wang et al. catalyst (0.5 g) under visible light irradiation (λ g 420 nm) (Figure S1). XPS spectra of the Cr 2p, Ni 2p, Ti 2p, and In 3d regions in In6NiTi6O22, In3CrTi2O10, and In12NiCr2Ti10O42 (Figures S2-S4). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (4) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; BKC, Inc.: Tokyo, 1999. (5) Photocatalysis: Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley-Interscience: Amsterdam, 1989. (6) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (7) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (8) Sayama, K.; Mukasa, K.; Abe, R.; Abe, Y.; Arakawa, H. Chem. Commun. 2001, 2416. (9) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (10) Kudo, A.; Kato, H.; Tsuji, I. Chem. Lett. 2004, 33, 1534. (11) Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Int. J. Hydrogen Energy 2002, 27, 991. (12) Anpo, M. Bull. Chem. Soc. Jpn. 2004, 77, 1427. (13) Domem, K.; Kudo, A.; Onishi, T. J. Phys. Chem. 1986, 90, 292. (14) Kudo, A.; Mikami, I. Chem. Lett. 1998, 27, 1027. (15) Sato, J.; Saito, S.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. 2001, 105, 6061. (16) Yin, J.; Zou, Z.; Ye, J. J. Mater. Res. 2002, 17, 2201. (17) Sato, J.; Saito, S.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. B 2003, 107, 7975. (18) Thomas, K. J. Phys. Chem. Solids 1997, 58, 1587. (19) Kudo, A.; Omori, K.; Kato, H. J. Am. Chem. Soc. 1999, 121, 11459. (20) Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 12441. (21) Herrmann, J.-M.; Disdier, J.; Pichat, P. Chem. Phys. Lett. 1984, 108, 618. (22) Borgarello, E.; Kiwi, J.; Grantzel, M.; Pelizetti, E.; Visca, M. J. Am. Chem. Soc. 1982, 104, 2996. (23) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (24) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. J. Phys. Chem. Solids 2002, 63, 1909. (25) Kato, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 5029. (26) Wang, D.; Ye, J.; Kako, T.; Kimura, T. J. Phys. Chem. B 2006, 110, 15824. (27) Hwang, D. H.; Kim, H. G.; Lee, J. S.; Kim, J.; Li, W.; Oh, S. H. J. Phys. Chem. B 2005, 109, 2093. (28) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Am. Chem. Soc. 2004, 126, 13406. (29) Yao, W.; Ye, J. J. Phys. Chem. B 2006, 110, 11188. (30) Wang, D.; Zou, Z.; Ye, J. Chem. Phys. Lett. 2005, 411, 285. (31) Brown, F.; Kimizuka, N.; Michiue, Y.; Mohri, T.; Nakamura, M.; Orita, M.; Morita, K. J. Solid State Chem. 1999, 147, 438. (32) Michiue, Y.; Onoda, M.; Brown, F.; Kimizuka, N. J. Solid State Chem. 2004, 177, 2644. (33) Yin, J.; Zou, Z.; Ye, J. J. Phys. Chem. B 2004, 108, 8888. (34) Domen, K.; Kondo, J. N.; Hara, M.; Takata, T. Bull. Chem. Soc. Jpn. 2000, 73, 1307. (35) Abe, R.; Sayama, K.; Sugihara, H. J. Phys. Chem. B 2005, 109, 16052. (36) Schriver, D. F.; Atkins, P.; Langford, C. H. Inorganic Chemistry, 2nd ed.; Freeman: New York, 1994. (37) Donnerberg, H.-J. Phys. ReV. B 1994, 50, 9053.
