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Synthesis and Photocatalytic Activities of NaNbO3 Rods Modified by In2O3 Nanoparticles Jun Lv,†,‡ Tetsuya Kako,† Zhaosheng Li,‡ Zhigang Zou,‡ and Jinhua Ye*,†,‡ Photocatalytic Materials Center (PMC), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, and Eco-materials and Renewable Energy Research Center (ERERC), National Laboratory of Solid State Microstructures, Nanjing UniVersity, Nanjing 210093, China ReceiVed: July 11, 2009; ReVised Manuscript ReceiVed: February 26, 2010
NaNbO3 rods modified by In2O3 nanoparticles (In2O3/NaNbO3) were successfully synthesized by an improved coprecipitation method, and they were found to be advantageous for photocatalytic H2 evolution under visible light irradiation and pure water splitting under ultraviolet light irradiation. The composites were characterized by X-ray diffraction, UV-vis diffuse reflectance spectrometry, Brunauer-Emmett-Teller measurement, scanning electron microscopy, energy-dispersive spectrometry, and transmission electron microscopy. With use of the electrochemical and valence band X-ray photoelectron spectroscopy analysis, the improvement of the photocatalytic activity was attributed to the promoted transportation of photoexcited holes in the composite. 1. Introduction With the background of energy crisis and environmental concerns, more and more attention has been paid to photocatalytic water splitting by direct use of solar energy.1-8 However, most of the effective photocatalysts such as TiO2,9,10 NaTaO3,11,12 and NaNbO313,14 could respond only to UV light, and the activities of most visible light-responsive photocatalysts are still quite low, such as In2O3,15-18 InTaO4,19,20 and InNbO4.21,22 For a single-phase oxide semiconductor under light irradiation, electrons are excited from the valence band (VB) to the conduction band (CB), resulting in the formation of electron/ hole pairs over the band gap. From redox reactions, H2 and O2 are produced from a photoreduction process initialized by the photogenerated electrons in the CB and from a photo-oxidation process induced by the photogenerated holes in the VB, respectively. However, rapid recombination of the photoexcited electron/hole pairs is likely to occur, which will restrict the photocatalytic activity.23 Thus, researchers consider developing composite photocatalysts for photocatalytic H2 evolution. With different components having ohmic contact, the intraparticle charge excitation, separation, and transportation can proceed as multiple heterojunctions conceptually.24 Redox reactions in the well-contacted composite semiconductor system can behave like a single-phase semiconductor if the band edges of individual semiconductors are suitable for H2 and O2 evolutions, respectively. According to such theory, Wang et al.17 developed an effective composite photocatalyst of Cr-Ba2In2O5/ In2O3 for water splitting. In this system, both Ba2In2O5 and In2O3 could respond to visible light. When the composite was synthesized, the photocatalytic activity improved greatly. To develop a new system for photocatalytic water splitting, we propose combining a visible light-driven material (such as In2O3) and a UV light-driven photocatalyst (such as NaNbO3). The visible light-responsive photocatalysts might play an important role as a sensitizer for generating photoexcited electron/hole pairs under visible light irradiation. The UV light* To whom correspondence should be addressed. E-mail: Jinhua.Ye@ nims.go.jp. † PMC, NIMS. ‡ ERERC, National Laboratory of Solid State Microstructures, Nanjing University.
driven photocatalyst could help to separate and transport the photoexcited charge, decrease the probability of recombination, and then improve the photocatalytic activity. In this work, we studied the composite of In2O3 and NaNbO3 in view of the superior photocatalysis of NaNbO3 under UV light and strong visible light absorbability of In2O3. The photocatalytic experiments indicate the In2O3/NaNbO3 composite photocatalyst exhibits good performance for photocatalytic H2 evolution under visible light irradiation and for pure water splitting under UV light irradiation. The possible mechanism of photocatalytic activity enhancement of the composite was discussed in view of the separation and transport of charge carriers. 2. Experimental Section 2.1. Synthesizing. In2O3/NaNbO3 (abbreviated INNO) composites were synthesized by a coprecipitation method, which is described as follows. An appropriate amount of InCl3 and Nb(OC2H5)5 were dissolved in anhydrous ethanol, and the solution was added to a 8.4 M sodium hydroxide solution dropwise. After filtration and drying at 80 °C, precursors of In2O3/NaNbO3 were obtained. Finally, all of the precursors were calcined at 500 °C for 12 h. Pure In2O3 and NaNbO3 were synthesized by the same method for comparison. Because of the volatilization of In2O3 during calcination, it is not accurate to evaluate the In2O3 concentration of the composite according to the concentration of indium in the precursor solution. Therefore, the samples were assigned according to the concentration of indium in the precursor solution directly. For example, the composite synthesized from the precursor solution with 25 at. % indium was named as 25In-INNO. 2.2. Characterization. Phase analysis was conducted by a powder X-ray diffraction method (λ ) 1.54178 Å, Ultram III, Rigaku Co., Japan). UV-vis diffuse reflectance spectra were measured by a UV-vis spectrophotometer (UV-2500PC, Shimadzu Co., Japan) with an integration sphere, and the diffuse reflectance spectra were converted to an absorption spectra by the Kubelka-Munk method. The specific surface areas were determined by a surface area and porosity analyzer (Tristar 3000, Micromeritics Inc., USA) with nitrogen absorption at 77 K. The morphological observation and chemical composition analysis
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were realized by a scanning electron microscope (SEM, JSM6701F, JEOL Co., Japan) with an X-ray energy-dispersion spectrum (EDS, EX-64165JMU, JEOL Co., Japan); the accelerating voltage was 15 kV. Some samples were observed by a high-resolution transmission electron microscope (JEM200CX, JEOL Co., Japan) operating at 200 kV. The density of states of the valence band was analyzed by valence band X-ray photoelectron spectroscopy (ESCALAB 250, Thermo-VG Scientific, UK). The Mott-Schottky curves were measured using an electrochemical analyzer (Princeton Applied Research, 2273) in a three-electrode cell. A Pt wire and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. KNO3 (1 M, pH 7) solution was used as an electrolyte. 2.3. Photocatalytic Reaction. Photocatalytic reactions of H2 evolution were carried out in a closed gas circulation system with an external-irradiation Pyrex cell. The light source was a 300 W xenon lamp with (visible light, λ > 420 nm) and without (full arc, λ > 300 nm) a UV-cutoff filter (L-42, Hoya). The cocatalyst Pt was loaded by an in situ photodeposition method. The 0.8 wt % Pt-loaded catalyst (0.3 g) was dispersed with a magnetic stirrer in an aqueous methanol solution (50 mL of CH3OH + 220 mL of H2O). To suppress elution of Na+ ions, NaOH solution was utilized to adjust the pH value of the aqueous methanol solution into the range of 9-9.5.25 The reaction of H2 production can be described as
Figure 1. XRD patterns of all samples with different indium concentrations: (a) 0 at.% In, (b) 12.5 at.% In, (c) 25 at.% In, (d) 37.5 at.% In, (e) 50% at.% In, (f) 75 at.% In, and (g) 100 at.% In.
hν
CH3OH 98 CH2O + H2
(1)
hν
H2O + CH2O 98 CH2O2 + H2
(2)
hν
H2O + CH2O2 98 CO2 + H2
(3)
The total reaction could be described as26 hν
H2O + CH3OH 98 CO2 + 3H2
(4)
Wavelength-dependence experiments were also conducted in the same system and same reaction cell. Different cutoff filters (L-42, Y-44, Y-46, Hoya) were utilized to get the light with different wavelength region. The overall water-splitting experiments were conducted under a mercury lamp. In the experiments, 0.5 wt % NiO was loaded as a cocatalyst. The loading method was as follows: First, Ni(NO3)2 was dissolved in an aqueous solution; second, the solution was added into the photocatalyst. After drying, the mixtures were heated in a muffle furnace at 400 °C for 4 h and then the NiO-loaded photocatalyst was obtained. The evolved gases including H2 were analyzed by an online gas chromatograph (GC-8AIT, Shimadzu) equipped with a thermal conductivity detector. 3. Results and Discussion 3.1. Characterization of In2O3/NaNbO3 Composite. XRD Patterns. Figure 1 shows the XRD patterns of all samples. As shown in Figure 1a, pure NaNbO3 is obtained by such coprecipitation method; no impurity could be detected in its
Figure 2. UV-vis diffuses reflectance spectra of all samples with different indium concentrations: (a) 0 at.% In, (b) 12.5 at.% In, (c) 25 at.% In, (d) 37.5 at.% In, (e) 50 at.% In, (f) 75 at.% In, and (g) 100 at.% In. Insert shows the plots of (Rhν)1/2 vs (hν) for NaNbO3 (0 at.% In) and In2O3 (100 at.% In).
XRD pattern. With increasing indium concentration in the precursor solution, the diffraction peaks of In2O3 are strengthened gradually, as revealed in Figure 1b-f. Without Nb5+ in the precursor solution, pure In2O3 could be synthesized (Figure 1g). Optical Absorption Properties. The UV-vis diffuse reflectance spectra of all prepared samples were measured and converted to the absorption spectra by the Kubelka-Munk method. As shown in Figure 2, it is clear that, with increasing In3+, the absorption edge red-shifts gradually. The band gap energy of a semiconductor could be calculated by eq 527,28
Rhν ) A(hν - Eg)n
(5)
where R, ν, A, and Eg are the absorption coefficient, light frequency, proportionality constant, and band gap, respectively. In the equation, n decides the characteristics of the transition in a semiconductor. The method to evaluate the values of n has been reported elsewhere.27 With use of this method, the value of n for pure In2O3 and NaNbO3 was determined as 2 from their absorption spectra. This means that the optical transitions for the oxides are indirectly allowed. As shown in the insert of Figure 2, the values of the band gap are 2.8 eV for In2O3 and 3.5 eV for NaNbO3. SEM Images. Figure 3 shows the SEM images of the pure In2O3, pure NaNbO3, and In2O3/NaNbO3 composites. The morphologies of pure NaNbO3 and In2O3 are quite different from each other. As shown in Figure 3a, pure NaNbO3 is rodlike. The width of the rods is ca. 100 nm, and the length is more
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Figure 3. SEM images of all samples with different indium concentrations: (a) 0 at.% In, (b) 12.5 at.% In, (c) 25 at.% In, (d) 37.5 at.% In, (e) 50 at.% In, and (f) 100 at.% In.
TABLE 1: Specific Surface Areas and Photocatalytic H2 Evolution Rates of All Prepared Samples -1 -1 surface area photocatalytic H2 evolution (µmol h g ) 2 -1 In (at. %) (m g ) full arc visible light
0 12.5 25.0 37.5 50.0 75.0 100.0
12.5 17.4 22.9 29.3 32.7 44.0 27.1
12.3 16.9 42.9 44.6 15.6 4.1 4.2
0.3 6.6 16.4 13.2 4.8 1.3 1.7
than 1 µm. As shown in Figure 3f, the morphology of pure In2O3 is nanoparticles. Both the rods and particles could be found in the composites with 12.5-37.5 at.% indium concentration. The particles look like they are growing on the surface of the rods. EDX analysis indicates that the particles are indium-rich and the rods are niobium-rich. The results indicate the compositions of the rods and nanoparticles in the composite are NaNbO3 and In2O3, respectively. As to the 50In-INNO and 75In-INNO samples, no obvious rod could be found. The NaNbO3 rods might be covered by those In2O3 particles and make it difficult to be found. Specific Surface Areas. The specific surface areas of those composites are much larger than that of pure NaNbO3, and the data are listed in Table 1. With increasing indium concentration, the specific surface area increases gradually. When the indium concentration is as much as 75 at.%, the surface area is larger than that of pure In2O3. 3.2. Photocatalytic Activities. The photocatalytic activities of H2 evolution for all samples were tested in an externalirradiation cell with a 300 W xenon lamp (CERMAX LX 300, USA) with or without a cutoff filter (filter: L-42, Hoya; with filter: visible light irradiation; without filter: full arc irradiation). The rates of H2 evolution under visible light and full arc irradiation are listed in Table 1. Under the full-arc irradiation, 25In-INNO and 37.5In-INNO samples exhibit good performances for H2 evolution. Figure 4 shows the rates of H2
Figure 4. Photocatalytic activities under visible light irradiation and specific surface areas of all samples with different indium concentrations. The inset is the long time photocatalytic reaction of H2 evolution of 25In-INNO and Degussa P25 under visible light irradiation.
evolution over all prepared samples under visible light irradiation. It is clear that pure NaNbO3 has no activity for H2 evolution. With increasing indium concentration, the activity increases and the 25In-INNO sample shows the highest activity. Then the activity decreased, although the surface area is still increasing. Thus, the improvement in photocatalytic activity could not be attributed to the increase of surface area. The real reason should be considered in view of the transportation and separation of charge carriers. To investigate the stability of photocatalytic activity, the photocatalytic H2 evolution experiment with 25In-INNO under visible light irradiation (λ > 420 nm) lasted for more than 24 h. As shown in the insert of Figure 4, the rate of H2 evolution is stable during three circles, which indicates the photocatalyst is stable under light irradiation. The photocatalytic activity of Degussa P25 was measured under the same condition and is shown in the insert of Figure 4. The wavelength-dependence experiments with the 25In-INNO sample was also conducted in an external-irradiation cell with the
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Figure 5. Wavelength dependence of photocatalytic activity and absorption spectrum of In2O3/NaNbO3 composite: (a) the absorption spectrum of NaNbO3, (b) the absorption spectrum of 25In-INNO, (c) the absorption spectrum of In2O3, and (d) the wavelength dependence of photocatalytic H2 evolution rate with 25In-INNO.
300 W xenon lamp (λ > 300 nm). Each experiment lasted for 3 h, and the total amount of evolved H2 is plotted in Figure 5. The 25In-INNO composite could respond to the light of λ e 460 nm, which is in good agreement with its absorption spectrum. To calculate the quantum yields of the 25In-INNO sample, a series of experiments were conducted under the irradiation of a mercury-xenon lamp. The reaction system was exactly the same as that in the wavelength-dependence experiments except for the light source. The detailed conditions are as follows: 0.3 g of photocatalyst and 0.8 wt % Pt were dispersed in the methanol solution (50 mL of methanol and 220 mL of pure water); the light source was a 200 W mercury-xenon lamp (C8849, Hamamatsu Photonics K.K., Japan); a 420 nm band-pass filter and a water filter were utilized. The light intensity was measured by a spectroradiometer (USR-40D-13, Minolta Co., Japan). The average light intensity was 269.03 µW/cm2 and the irradiated area was 19.36 cm2. The average rate of H2 evolution was 0.48 µmol h-1 for 6 h of light irradiation. The quantum yield was calculated by eq 6
QY )
2 × MH2NAhν Ne Ne ) ) × 100% Nc E/E0 I0Stλ
(6)
where Ne is the number of effective photons, Nc is the number of irradiated photons, E is the energy of general irradiated photons, E0 is the energy of a single photon, MH2 is the mole mass of evolved H2, NA is the Avogadro constant, I0 is the measured light intensity, S is the irradiated area, t is the irradiation time, h is the Planck constant, ν is the light speed in the vacuum, and λ is the wavelength. The quantum yield calculated by such equation was about 1.45%. The photocatalytic overall water-splitting experiments were also conducted under Hg lamp irradiation. The activity of In2O3, NaNbO3, and 25In-INNO for pure water splitting is shown in Figure 6. The results indicate that the 25In-INNO sample exhibits the highest activity. The H2 evolution rate for 25In-INNO is 2 times the rate for NaNbO3 and 7 times the rate for In2O3. Furthermore, the O2 could be detected for the 25In-INNO composite, while no O2 could be detected for In2O3 and NaNbO3. 3.3. Relative Band Position of In2O3/NaNbO3 Composite. The electrodes of In2O3 and NaNbO3 were prepared to investigate their flatband potentials. Since the flatband potentials of some polycrystallines are similar to those of the corresponding single-crystal samples,29-33 the polycrystalline is easy to prepare.
Figure 6. Photocatalytic water splitting for NiO-loaded NaNbO3, In2O3, and the In2O3/NaNbO3 composite with 25 at. % In under UV light irradiation. Light source: a 400 W high-pressure Hg lamp (RIKO-400A).
Figure 7. Mott-Schottky plots for NaNbO3 and In2O3 electrodes in 1 M KNO3 solution (pH 7). The ac amplitude is 10 mV and the frequency is in the range 100-1000 Hz.
The Mott-Schottky plots of the materials in 1 M KNO3 solution (pH 7) in the dark are shown in Figure 7. The ac amplitude is 10 mV and the frequency varies from 100 to 1000 Hz. The flatband potentials values are determined to be -0.43 V for In2O3 and -0.81 V for NaNbO3 from Figure 7. Namely, the flatband potential of In2O3 is more positive than that of NaNbO3. According to the report of McCann and Bockris,34 the flatband potentials of single-crystal n-type In2O3 were determined to be -0.72 and 0.22 V (vs SCE) in 1 M NaOH and 1 M H2SO4, respectively. It indicates the determined flatband potential of In2O3 in this study is reasonable. The value of NaNbO3 is rarely reported. Scaife35 reported the experimental flatband potential of KNbO3, of which physical and chemical properties are similar to those of NaNbO3. Scaife’s results indicate the flatband of KNbO3 is more negative than that of In2O3. Therefore, the flatband potential values of In2O3 and NaNbO3 in this study are reasonable. The flatband potential is different with conduction band bottom; the relationship could be described as eq 7:36,37
Ec ) -Vfb -
κT ln(N*/N c d) e
(7)
where Ec is the CB position, e is the electronic charge, Vfb is the flatband potential, κ is Boltzmann constant, Nd is the carriers concentration, and Nc* is the electron effective mass. Nd and
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Figure 8. VB XPS spectra of pure NaNbO3 and pure In2O3.
Nc* are unkonwn; therefore, it is difficult to calculate Ec accurately. In general, for the n-type semiconductor, the CB bottom is near the flatband potential.38 Therefore, it is deduced that the CB bottom of NaNbO3 was higher than that of In2O3. The CB bottom could not be determined directly by electrochemical analysis. To give direct analysis, the CB bottom of In2O3 and NaNbO3 was also estimated from the absolute electronegativity of the atoms and the band gap of the semiconductors by eq 8:28,39,40
Ec ) -(χ(A)a · χ(B)b · χ(C)c)1/(a+b+c) + 1/2Eg + E0
(8) where χ(A), χ(B), and χ(C) are the absolute electronegativity of the atoms A, B, and C, respectively; Ec and Eg are the position of the conduction band and the band gap of the semiconductor, respectively; E0 is the scale factor relating the reference electrode redox level to the AVS (E0 ) -4.5 eV for NHE). Thus, Ec of In2O3 and NaNbO3 could be calculated as -0.63 and -0.78 eV vs NHE, respectively. It indicates the conduction band bottom of In2O3 is more positive than that of NaNbO3 which is in agreement with the deduction according to the flatband potential. The electrochemical analysis could not measure the CB bottom directly and the error of the empirical equation was not clear. Thus, the value of CB bottom obtained by those methods might not be accurate. It might be incorrect to deduce the VB position according to the inaccurate CB bottom. The VB top of In2O3 and NaNbO3 could be investigated directly by valence band X-ray photoelectron spectroscopy (VB-XPS). VB-XPS could indicate the situation of the total density of states (DOS) of VB. Therefore, the relative position of VB top of In2O3 and NaNbO3 could be investigated by comparing their VB-XPS spectra. As shown in Figure 8, the VB XPS spectra of In2O3 and NaNbO3 indicate the valence band top of NaNbO3 is about 0.6 eV higher than that of In2O3. The instrumental error of XPS analysis is about (0.2 eV, the gap of VB top between In2O3 and NaNbO3 is larger than double the error. Thus, it could be qualitatively concluded that the VB top of NaNbO3 is higher than that of In2O3. 3.4. Photocatalytic Activity Enhancement by the In2O3/ NaNbO3 Composite. In2O3 nanoparticles and NaNbO3 rods are composited well, which could be confirmed by the SEM images (as shown in Figure 3) and TEM images (as shown in Figure 9a-c). According to analysis of the relative band position, the conduction band bottom of In2O3 is lower than that of NaNbO3, and the valence band top of NaNbO3 is higher than that of In2O3, as described in Figure 9d. The charge carries could transport
Figure 9. The band structure diagram and TEM images of In2O3/ NaNbO3 composite: (a, b) TEM images; (c) HRTEM image; (d) the diagram for the relative band position of the In2O3/NaNbO3 composite.
through their interface. Under visible light irradiation, the electrons in the valence band (VB) of In2O3 are excited to its conduction band and left holes in VB. The holes in the VB of In2O3 could transport to the VB of NaNbO3 through the interface between In2O3 and NaNbO3. Therefore, the excited electrons/ holes are separated, the recombination of them is restricted, and the photocatalytic activity of In2O3/NaNbO3 composite is improved. When the indium concentration increases, the In2O3 nanoparticles contacted with NaNbO3 are covered by other In2O3 particles and block the light absorption. The photoexcited electrons/holes generated in the external In2O3 cannot transport to NaNbO3. Therefore, the photocatalytic activity is restricted when In2O3 is in excess. Conclusions In summary, the In2O3/NaNbO3 composite was successfully synthesized by an improved coprecipitation method. The photocatalytic experiments indicated the In2O3/NaNbO3 composite was advantageous for photocatalytic H2 evolution and water splitting. The characterization results confirm that formation of composite could promote the transfer of photoexcited holes and thus restrict their recombination with electrons, leading to the enhancement of photocatalytic activity. Acknowledgment. This work was partially supported by the Strategic International Cooperative Program, Japan Science and Technology Agency (JST). The authors also would like to acknowledge the National Natural Science Foundation of China (No. 20528302), the National Basic Research Program of China (973 program, 2007CB613305), and the China-Japan cooperation project of science and technology (2009DFA61090). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fujishim, A.; Honda, K. Nature 1972, 238, 37. (2) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (3) Dismukes, G. C. Science 2001, 292, 447. (4) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2005, 127, 8286.
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