Article pubs.acs.org/JPCC
CaIn2O4/Fe-TiO2 Composite Photocatalysts with Enhanced Visible Light Performance for Hydrogen Production Wenhao Yan,† Yi Zhang,† Wei Xie,† Song Sun,†,‡ Jianjun Ding,*,†,‡ Jun Bao,†,‡ and Chen Gao*,†,‡ †
National Synchrotron Radiation Laboratory and Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui 230029, China ‡ CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *
ABSTRACT: A series of CaIn2O4/Fe-TiO2 composite photocatalysts with tunable Fe-TiO2 contents were prepared in which Fe-TiO2 nanoparticles were uniformly deposited onto the surface of CaIn2O4 nanorods. The photocatalytic activities of Pt-loaded CaIn2O4/Fe-TiO2 composites were evaluated for H2 evolution from aqueous KI solution under visible light irradiation. It was found that the composites showed higher H2 evolution rates in comparison with pure CaIn2O4 or Fe-TiO2, which could be attributed to the increased surface area and enhanced visible light absorption. A high H2 evolution rate of 280 μmol h−1 g−1 was achieved when the mass ratio of Fe-TiO2 to CaIn2O4 was 0.5, which was 12.3 and 2.2 times higher than that of pure CaIn2O4 and Fe-TiO2, respectively. Furthermore, the interfaces between CaIn2O4 nanorods and Fe-TiO2 nanoparticles facilitated efficient charge separation that also led to the improved photocatalytic activity. This study may provide some inspiration for the fabrication of visible-light-driven photocatalysts with efficient and stable performance.
■
INTRODUCTION Since the pioneering work reported by Fujishima and Honda on photoelectron−chemical splitting of water on a TiO2 electrode,1 semiconductor-based photocatalysts have received a lot of attention due to their potential applications in environmental pollution remediation and renewable energy generation. To date, TiO2 is still the most studied photocatalyst due to its photocatalytic activity, stability, and nontoxicity.2,3 However, TiO2 is a wide band gap energy semiconductor (3.0 and 3.2 eV for the rutile and anatase, respectively), which is active only under UV light irradiation. Development of visiblelight-driven photocatalysts becomes critical in current photocatalysis research to effectively use solar energy because visible light accounts for about 43% radiation energy of the solar spectrum. Many strategies have been used to shift the absorption band of TiO2 into the visible light region, such as metal ion doping and non-metal ion doping.4−7 For example, Fe-TiO2 was proved as a highly efficient visible photocatalyst and was stable in the decomposition of dye and water pollutant.8−10 At the © 2014 American Chemical Society
same time, numerous efforts have also been carried out on the development of new materials with intrinsic visible photocatalytic activity, such as BiVO4,11,12 Ag3PO4,13,14 Ta3N5,15,16 CaIn2O4,17,18 etc. However, as a single component, the efficiencies of these photocatalysts are still far from satisfactory due to the high recombination rate of photogenerated electron−hole pairs. Composite photocatalyst with two or more components can improve the separation of photoinduced charges and enhance photocatalytic activity due to high efficiency of the interfacial charge transfer between components. During past decades, lots of works have been successfully done to design different and efficient composite photocatalysts, such as noble-metal-based composites,19,20 TiO2-based composites,21,22 graphene-based composites,23−25 and other portable composites.26−28 One of the successful examples is the well-known CdS/TiO2 composite Received: December 31, 2013 Revised: March 5, 2014 Published: March 7, 2014 6077
dx.doi.org/10.1021/jp412810n | J. Phys. Chem. C 2014, 118, 6077−6083
The Journal of Physical Chemistry C
Article
silver chloride electrode were used as the counter and reference electrode, respectively. Na2SO4 (1 M, pH = 7) solution was used as the electrolyte. Slurry of photocatalyst and ethanol was coated to transparent ITO electrodes and dried on a hot plate. The prepared electrodes were then heated at 473 K for 1 h. All the electrodes had similar film thickness by the control of slurry volume. The ac amplitude is 5 mV, and the frequency is 5000 Hz. The flatband potential at photocatalyst/electrolyte interface can be estimated by the Mott−Schottky equation34
photocatalyst, of which the photocatalytic activity was remarkably increased under visible light irradiation in comparison with that of pure CdS or TiO2.29,30 Yu et al. reported a novel composite material TiO2/MoS2/graphene as a high performance photocatalyst for H2 evolution.31 The enhanced photocatalytic activity may be ascribed to the positive synergetic effect between the MoS2 and graphene components, which can efficiently suppress charge recombination and improve interfacial charge transfer. In previous studies, we have successfully developed visiblelight-driven photocatalysts, Fe-doping TiO2 and CaIn2O4,32,33 for methylene blue degradation, VOCs oxidation, and hydrogen production. To the best of our knowledge, the photocatalytic activity of CaIn2O4/Fe-TiO2 composite as visible-light-driven photocatalyst has not been reported. In this paper, we first prepared CaIn2O4/Fe-TiO2 composite photocatalyst consisting of CaIn2O4 nanorods decorated with Fe-TiO2 nanoparticles (the content of Fe was fixed at 0.7 wt %). High photocatalytic efficiency for H2 production from water was achieved using the CaIn2O4/Fe-TiO2 composite as the photocatalyst under visible light irradiation. A mechanism for the enhanced photocatalytic activity was proposed.
⎛ 2 ⎞⎡ 1 kT ⎤ =⎜ ⎥ ⎟⎢(V − VFB) − 2 e0 ⎦ C ⎝ e0εε0Nd ⎠⎣
where C is the specific capacitance (F/cm2), e0 the electron charge, ε the dielectric constant of the photocatalyst, ε0 the permittivity of vacuum, Nd the carrier density, V the applied potential of the electrode, VFB the flatband potential, and kT/e0 the temperature-dependent correction term. Photocatalytic Experiments. The photocatalytic hydrogen production experiments were performed in a 330 mL topirradiation gas-closed circulation reactor. A 300 W Xe arc lamp (PLS-SXE 300, ChangTuo Ltd.) was used as the light source through infrared and UV cutoff filters to ensure visible illumination only (420 nm ≤ λ ≤ 750 nm). In a typical experiment, 20 mg of photocatalyst and a certain content of KI (0−3 mmol) were added into 100 mL of deionized water with constant stirring. Prior to irradiation, the system was bubbled with argon for 45 min to remove the dissolved oxygen. In order to eliminate any thermal effect, a water jacket filled with circulating water was used to keep the temperature of the solution constant. The hydrogen evolved was analyzed using an online TCD gas chromatograph (Shimadzu GC-14C, TCD sensitivity ≥5000 mV mL/mg (benzene), equipped with a TDX-01 column).
■
EXPERIMENTAL SECTION Fabrication of CaIn2O4/Fe-TiO2 Composites. All the chemicals were of analytical grade and used without further purification. CaIn2O4 (CIO) and Fe-TiO2 (FTO) were prepared according to our previous studies.32,33 CaIn2O4/Fe-TiO2 composites were synthesized by mixing 500 mg of CIO and different contents of FTO. The mixture was fully grinded in an agate mortar. To increase the contact between CIO and FTO at the interface, a postannealing treatment at 573 K for 3 h was implemented. The composites with different mass ratios of FTO to CIO (0.25, 0.5, 0.75, and 1.0) were labeled as CF0.25, CF0.5, CF0.75, and CF1.0, respectively. For control experiments, pure CIO and FTO were also treated by the procedure described above. In order to improve the photocatalytic H2 evolution activity, 0.5 wt % Pt-dispersed photocatalyst was prepared by an incipient impregnation method using chloroplatinic acid as metal precursor. After impregnation, the sample was dried at 383 K for 24 h and then reduced at 573 K in pure H2 for 2 h. Characterization. Powder X-ray diffraction patterns were performed on a Rigaku D/max-γA rotation anode diffractometer with Cu Kα radiation (λ = 0.151 48 nm) at a scan rate of 5° min−1. The BET surface areas were determined by an adsorption−desorption method (Micrometitics ASAP 2000) using BaSO4 as the reference. The morphology and microstructure of the samples were investigated by a scanning electron microscope (Sirion 200) and a transmission electron microscope (JEOL JEM-2100F). The UV−vis diffuse reflectance spectra were measured at room temperature by an UV− vis spectrometer (SolidSpec-3700, Shimadzu, Japan) using BaSO4 as the reference. The band gap energy was estimated according to the Tauc equation. The photoluminescence (PL) spectra were measured by JY Fluorolog-3-Tou, equipped with solid state accessories. The surface characterization was carried out using X-ray photoelectron spectroscope (Thermo ESCALAB 250) using monochromatized Al Kα at hν = 1486.6 eV. Electrochemical Analysis. The photoelectrochemical property was measured with a CHI-760D electrochemical analyzer. The electrochemical properties of the samples were all investigated in a three-electrode cell. A platinum wire and a
■
RESULTS AND DISCUSSION Structure and Morphology Characterizations. Figure 1 shows the XRD patterns of the as-prepared CIO, FTO, and
Figure 1. XRD patterns of the CaIn2O4/Fe-TiO2 samples with different CIO/FT mass ratios: (a) CIO, (b) CF0.25, (c) CF0.5, (d) CF0.75, (e) CF1.0, (f) CF0.5 after photocatalytic reaction, and (g) FTO.
CaIn2O4/Fe-TiO2 composites with different mass ratios. The baseline is offset, and the intensity has been normalized for ease of comparison. For pure CIO, all the peaks can be well indexed to the orthorhombic phase (PDF#17-0643). The main diffraction peaks at 18.2°, 18.4°, 31.7°, 32.0°, 33.4°, 46.8°, 47.1°, 57.9°, and 67.1° correspond to the (120), (200), (040), 6078
dx.doi.org/10.1021/jp412810n | J. Phys. Chem. C 2014, 118, 6077−6083
The Journal of Physical Chemistry C
Article
(320), (121), (241), (401), (170), and (621) planes, respectively. Diffraction peaks of pure FTO at 2θ of 25.4°, 37.1°, 37.9°, 38.7°, 48.2°, 54.1°, 55.2°, 62.8°, and 75.3° can be indexed as the (101), (103), (004), (112), (200), (105), (211), (204), and (215) planes of anatase TiO2 (PDF#73-1764), respectively. From these figures, the XRD patterns of CaIn2O4/ Fe-TiO2 composites exhibit characteristic diffraction peaks of both CIO and FTO phases. When the mass ratio of CaIn2O4/ Fe-TiO2 composites is increased from 0.25 to 1.0, the diffraction peaks of FTO increased while the peak intensities of CIO decrease gradually. No other characteristic peaks are detected, indicating that no appreciable chemical reaction occurred between CIO and FTO during the annealing process. The morphology of CIO, FTO, and CF0.5 composite (the highest H2 evolution activity) were investigated by SEM and TEM. It can be seen that a large number of rod-shaped CIO and a small amount of spherical-shaped CIO are displayed in Figures 2a,b. They are 0.15−1.0 μm in diameter and 0.45−2.0
Figure 3. TEM (a) and HRTEM (b) images of CF0.5 composite.
The UV−vis diffuse reflectance spectra of the CaIn2O4/FeTiO2 composites are shown in Figure 4. Though the absorption
Figure 2. SEM images of pure CIO (a, b) and CF0.5 composite (c, d).
μm in length, with smooth surfaces. The synthesized CF0.5 composite (Figures 2c,d) clearly shows that FTO nanoparticles deposited onto CIO nanorods uniformly, which causes an obvious rougher surface. A low magnification TEM image is shown in Figure 3a, from which we can see that the FTO nanoparticles with the size of about 30 nm were tightly attached onto the surface of CIO nanorod. It is noteworthy that there are still noticeable FTO nanoparticles adhered to CIO rod after ultrasonic treatment before TEM observation, which may suggest that a compact interface was established between FTO and CIO. However, the excess FTO nanoparticle can not deposit on CIO rods when the mass ratio is higher than 0.5 (Figure S1), and they are inclined to self-agglomerate. Highresolution TEM image (Figure 3b), obtained from the marked area in Figure 3a, provides further insight into the microstructure and interface of the composite. The clear lattice fringes indicate high crystallinity. The d spacing of (121) is 0.26 nm, which is in good agreement with that of orthorhombic CIO, while the fringe intervals of 0.24 and 0.35 nm correspond to the interplanar spacing of (004) and (101) crystal planes of FTO, respectively. The HRTEM image clearly reveals the formation of CaIn2O4/Fe-TiO2 composite with a firm interface, which could make the interfacial charge transfer spatially available and accordingly improve the photocatalytic activity.
Figure 4. UV−vis diffuse reflectance spectra of the CaIn2O4/Fe-TiO2 composites with different mass ratios.
of CIO drops sharply at 340 nm and the band gap (3.4 eV) is relatively large, there is still a weak absorption at the region of 340−430 nm (Figure S2) which makes CIO possible as a visible photocatalyst for the degradation of organic pollutants and hydrogen production.17,18,33 The visible absorption may be caused by impurities or defects which could be formed during the combustion process. Compared with TiO2, FTO shows a strong absorption in the visible region. The band gap energy estimated according to the Tauc equation is 2.92 eV for FTO, which is consistent with literature.35,36 Our previous XAFS experiment has proved that the doped Fe3+ ions substitute into the octahedrally coordinated Ti4+ sites,32 which is crucial for extending the absorption of TiO2 to visible light region.37 As seen in Figure 4, the composites’ curves exhibit a mixed property of both CIO and FTO. The absorption edge of the CaIn2O4/Fe-TiO2 composites shows a shift toward the visible region upon the loading of FTO onto CIO. Moreover, the visible light absorbance increases with the increasing of the content of FTO. 6079
dx.doi.org/10.1021/jp412810n | J. Phys. Chem. C 2014, 118, 6077−6083
The Journal of Physical Chemistry C
Article
of FTO is crucial for the optimization of the photocatalytic activity of the CaIn2O4/Fe-TiO2 composites. The effect of KI concentration on the photocatalytic activity was investigated. Figure 6 shows the H2 evolution rate over the
Since photocatalytic reaction occurs at the surface, surface area is an important parameter for photocatalytic activity. As listed in Table 1, CaIn2O4/Fe-TiO2 composites show higher Table 1. BET Surface Area and Photocatalytic H2 Evolution Rate under Visible Light Irradiation sample
mass ratio of FTO to CIO
surface area (m2 g−1)
H2 evolution rate (μmol g−1 h−1)
CIO CF0.25 CF0.5 CF0.75 CF1.0 FTO
0 0.25 0.5 0.75 1 ∞
2.74 4.32 4.93 7.19 8.33 47.66
23 163 280 179 155 128
BET surface area than that of pure CIO. It can be seen that the BET surface area of the composites increases gradually from 4.3 to 8.3 m2 g−1 with the increasing of the mass content of FTO. The larger surface area could be beneficial for enhancing the photocatalytic activity of the composites.38,39 Photocatalytic Performance. Photocatalytic H2 production activity of the Pt-loaded CaIn2O4/Fe-TiO2 composites was evaluated under visible light irradiation using KI as sacrificial reagent. Control experiments were carried out in the dark or in the absence of photocatalyst. As there was no appreciable H2 production was detected, we confirm that hydrogen was produced by photocatalytic reaction. Figure 5 shows the
Figure 6. Effect of KI concentration on the photocatalytic H2 evolution rate over the CF0.5 composite.
CF0.5 composite with different concentrations of KI. With the KI concentration varies from 0 to 3 mmol, the photocatalytic activity experiences an initial improvement followed by a rapid decrease. During the reaction, I− is photooxidated into IO3− by photogenerated holes, which makes the effective separation of the photogenerated carriers and enhances the photocatalytic activity. Excess KI has a negative effect on activity. Similar results were also reported in the literature.40,41 The observation confirms the importance of the sacrificial agent. Notably, the enhanced visible photocatalytic activity of the CaIn2O4/Fe-TiO2 composites could be ascribed to the following three mechanisms: (1) The CaIn2O4/Fe-TiO2 composites show larger surface area than pure CIO. The larger surface area resulted from the rough surface of the composites can provide more active adsorption and photocatalytic reaction sites. (2) Visible absorption is a necessary condition for a photocatalyst to function under visible light irradiation. As mentioned in Figure 4, the CaIn2O4/Fe-TiO2 composites show more absorption in visible region than pure CIO. The photons absorbed would generate more electron−hole pairs. (3) HRTEM image showed the compact contact between CIO and FTO in the composites. We think that the fast charge separation at the interface of the CaIn2O4/Fe-TiO2 composites is the most important factor for the enhanced photocatalytic activity. This can be confirmed by the photoluminescence (PL) spectra,42,43 as shown in Figure 7. The excitation wavelength is 420 nm. The PL emission peak for CIO and FTO is located at about 465 and 500 nm, respectively. In comparison with CIO, FTO, and other CaIn2O4/Fe-TiO2 composites, the CF0.5 composite shows the lowest PL intensity at either 465 or 500 nm, demonstrating the charge separation efficiency of the CF0.5 composite is more efficient than others. That is why the photocatalytic activity of CF0.5 composite is the best, although the absorption edge (or the surface area) of CF0.75, CF1.0, or FTO is more red-shifted (or higher) compared to CF0.5. In order to examine the direction of charge transfer in the CaIn2O4/Fe-TiO2 composites, the relative flatband potentials (VFB) of CIO and FTO were measured using an electrochemical method. As shown in Figure 8, we can deduce from Mott−Schottky plots that the VFB of CIO and FTO is −0.62 and −0.81 vs Ag/AgCl electrode, respectively. The Mott−
Figure 5. Photocatalytic activities of Pt-dispersed CaIn2O4/Fe-TiO2 composites for H2 production under visible-light irradiation.
average hydrogen production rates as a function of FTO content from the first 5 h of the reaction. The result is also listed in Table 1. For pure CIO and FTO, the H2 evolution rate is 22.8 and 127.6 μmol h−1 g−1, respectively. It can be seen that the CaIn2O4/Fe-TiO2 composite photocatalysts shows a better activity than that of pure CIO or FTO. As shown in Figure 5, the amount of hydrogen evolution can reach to about 1.4 mmol with a rate of 280 μmol h−1 g−1 when the mass ratio of FTO to CIO is increased to 0.5, which is 12.3 and 2.2 times higher than that of pure CIO and FTO, respectively. With the further increasing of the content of FTO nanoparticles, the photocatalytic activity of the composites decreases. But they still maintain better photocatalytic activities than that of pure CIO or FTO. The photocatalytic experiments without loading Pt cocatalyst and KI agent were also carried out and shown in Figure S3. The overall trend for H2 evolution is in agreement with that of Figure 5. These results show that a suitable content 6080
dx.doi.org/10.1021/jp412810n | J. Phys. Chem. C 2014, 118, 6077−6083
The Journal of Physical Chemistry C
Article
Figure 7. Photoluminescence spectra of CIO, FTO, and CaIn2O4/FeTiO2 composites. The excited wavelength is 420 nm. Figure 9. Schematic diagram of the charge separation and transportation over CaIn2O4/Fe-TiO2 composites under visible light irradiation.
Figure 8. Mott−Schottky plots for CIO and FTO at the frequency of 5000 Hz. Figure 10. Stability of photocatalytic H2 evolution over the CF0.5 composite under visible light irradiation.
Schottky plots also demonstrate that both CIO and FTO are ntype semiconductor. It is well-known that the conduction band potential of n-type semiconductor is very close to the flatband potential,44 so it can be deduced that the conduction band position of FTO is about 0.19 V negative to CIO. This difference of ECB between CIO and FTO allows the transfer of electron from the CB of FTO to that of CIO. Based on the above results and discussions, a schematic diagram is illustrated to explain the enhancement of the photocatalytic activity for the CaIn2O4/Fe-TiO2 composites, as shown in Figure 9. Under visible light irradiation, CIO defects/ impurities or FTO absorbs visible light to generate electron− hole pairs. Since the absorption in visible region of FTO is much higher than CIO, we speculate that most of the visible light are absorbed by FTO. As the CB of FTO is more negative than that of CIO, the electrons on the CB of FTO can readily transfer to the CB of CIO via interface, whereas the holes on the VB of CIO transfer to the VB of FTO. The photogenerated electrons participate in H2 evolution and holes are scavenged by the sacrificial regent (I−). As a result, the photogenerated charge carriers can be effectively separated at the interface of the composites due to their matching band positions.45,46 So, it is reasonable that the CaIn2O4/Fe-TiO2 composites show enhance photocatalytic performances for hydrogen production. The stability of a photocatalyst is important for its potential application. The used CF0.5 composite was filtered out from the reaction solution and employed again with fresh sacrificial reagent solution periodically replaced in each run. As shown in Figure 10, the CF0.5 composite photocatalyst exhibits stability toward H2 production because no obvious deactivation is observed after third runs. However, there is a slight drop in the
rate of H2 production as the reaction proceeds, which could be attributed to the competitive reduction between IO3− and H+ since IO3− is more susceptible to reduction than H+.47 The structure and composition of the CF0.5 composite after three cycles of reaction were also analyzed using XRD and XPS. XRD results (Figure 1f) demonstrate that the crystal structure does not change after the reactions. From XPS spectra (Figures S4− S6), all the Ca 2p, In 3d, Ti 2p, Fe 2p, and Pt 4f spectra show two peaks, which indicates that the oxidation states of element Ca, In, Ti, Fe, and Pt are +2, +3, +4, +3, and 0, respectively.48−50 The C 1s peak at binding energy of 285.1 eV is due to the presence of CO2 absorbed on the sample surface. These results indicate that the CF0.5 composite is a stable and effective photocatalyst.
■
CONCLUSIONS In summary, the CaIn2O4/Fe-TiO2 composite photocatalysts were successfully synthesized using a grinding−annealing method, by which Fe-TiO2 nanoparticles were uniformly deposited onto the surface of CaIn2O4 nanorods. The asprepared CaIn2O4/Fe-TiO2 composites showed better photocatalytic activity for H2 evolution under visible light irradiation than that of pure CaIn2O4 or Fe-TiO2. A high H2 evolution rate of 280 μmol h−1 g−1, which is 12.3 and 2.2 times higher than that of pure CaIn2O4 and Fe-TiO2, respectively, was achieved when the mass ratio of Fe-TiO2 to CaIn2O4 was 0.5. The enhanced photocatalytic performance could be attributed to increased surface area, enhanced visible light absorption, and 6081
dx.doi.org/10.1021/jp412810n | J. Phys. Chem. C 2014, 118, 6077−6083
The Journal of Physical Chemistry C
Article
Powder from Layered Vanadates at Room Temperature and Its Photocatalytic and Photophysical Properties. J. Am. Chem. Soc. 1999, 121, 11459−11467. (12) Jo, W. J.; Jang, J.; Kong, K.; Kang, H. J.; Kim, J. Y.; Jun, H.; Parmar, K. P. S.; Lee, J. S. Phosphate Doping into Monoclinic BiVO4 for Enhanced Photoelectrochemical Water Oxidation Activity. Angew. Chem., Int. Ed. 2012, 51, 3147−3151. (13) Yi, Z.; Ye, J.; Kikugawa, N.; Kako, T.; Ouyang, S.; StuartWilliams, H.; Yang, H.; Cao, J.; Luo, W.; Li, Z.; et al. An Orthophosphate Semiconductor with Photooxidation Properties under Visible-Light Irradiation. Nat. Mater. 2010, 9, 559−564. (14) Bi, Y.; Ouyang, S.; Umezawa, N.; Cao, J.; Ye, J. Facet Effect of Single-Crystalline Ag3PO4 Sub-Microcrystals on Photocatalytic Properties. J. Am. Chem. Soc. 2011, 133, 6490−6492. (15) Ma, S. S. K.; Hisatomi, T.; Maeda, K.; Moriya, Y.; Domen, K. Enhanced Water Oxidation on Ta3N5 Photocatalysts by Modification with Alkaline Metal Salts. J. Am. Chem. Soc. 2012, 134, 19993−19996. (16) Hitoki, G.; Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K. Ta3N5 as a Novel Visible Light-Driven Photocatalyst (λ
