Environ. Sci. Technol. 2010, 44, 451–455
Structuring a TiO2-Based Photonic Crystal Photocatalyst with Schottky Junction for Efficient Photocatalysis HUAN CHEN, SHUO CHEN, XIE QUAN,* AND YAOBIN ZHANG School of Environmental and Biological Science and Technology, Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), Dalian University of Technology, Dalian 116024, China
Received September 7, 2009. Revised manuscript received November 13, 2009. Accepted November 13, 2009.
Facile and effective approaches were developed to fabricate the inverse TiO2/Pt opals Schottky structures on the Ti substrate. The as-prepared samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and diffuse reflectance UV-vis spectra (DRS), respectively. The results indicate that these samples were of ordered network, which was built by the Pt skeleton frame and the outer TiO2 layer. The TiO2 layer was identified as anatase with the preferential orientation of (101) plane. The experiments of short-circuit photocurrent (SCPC) and photocatalytic degradation of phenol were also conducted under the UV irradiation in order to evaluate the photoactivity of the samples. By tuning the red edge of photonic stop-band overlapping the absorption maximum of anatase (at 360 nm), both the UV absorption and the carrier separation of the samples were improved. The kinetic constant using the optimal inverse TiO2/Pt opals (0.992 h-1) was about 1.5 times as great as that of the disordered inverse TiO2/Pt opals (TiO2/Pt-mix) and was 3.3 times as great as that of pristine TiO2 nanocrystalline film (TiO2-nc) on Ti substrate.
Introduction Heterogeneous photocatalysis has been proven to be effective in removing organic contaminants (1), especially those priority pollutants, such as chlorophenols (CPs) (2, 3), polycyclic aromatic hydrocarbons (PAHs) (4), nitrobenzene (5), polychlorinated biphenyl (PCBs) (6), and so forth. The final products of this process are generally nontoxic carbon dioxide and water and less toxic oxidized forms of inorganic anions. So far, TiO2 is believed to be the most promising photocatalyst for its superiority in the oxidation capacity, nontoxicity, and long-term stability (7). However, the photocatalytic potential of TiO2 has not been comprehensively understood. The photocatalytic reaction can be considered as the process of the generation, transfer, and consumption of the photogenerated carriers, that is, conduction-band electrons and valence-band holes. These excited-state electrons or holes are prone to react with the pollutants adsorbed onto the photocatalyst surface, which act as the electron acceptors or donors (8). Since more holes are benefit for more rapid and more complete photocatalytic oxidation degradation, two ways can be used to enhance the “apparent” photo* Corresponding author phone: +86-411-84706140; fax: +86-41184706263; e-mail:
[email protected]. 10.1021/es902712j
2010 American Chemical Society
Published on Web 12/08/2009
oxidative ability of TiO2. One is based on the theory of quantum efficiency, which is to enhance the conversion efficiency from the light energy absorbed by TiO2 into chemical energy. The combination of TiO2 with noble metals (9, 10) or semiconductors (11) is able to suppress the recombination of photogenerated carriers and therefore improve the photo-oxidation. The other is based on the theory of photon-harvest, which is to increase the photons caught by TiO2 under the same incident intensity condition. Doping of TiO2 with elements such as B (12), N (13), Si (14), and so forth is able to extend its absorption spectra. Increasing the optical path length due to multiple scatter (15) or surface resonant modes (16) can improve the absorption intensity of TiO2. On the other hand, the photonic crystals are of certain periodic dielectric structures, which can prohibit the propagation of light in certain directions within a frequency rangestop-band (17). At the frequency edges of the stop-band, photons propagate with strongly reduced group velocity. The group velocity is defined as the speed at which the energy of the electromagnetic field propagates in matter; it can be used to describe the photon-energy-transport velocity. While ideal photonic crystals would support modes with a vanishing group velocity, state-of-the-art structures can still provide a slow-down by roughly 2 orders of magnitude (18). When the group velocity decreases, the interaction between the light and matter tends to be reinforced (19): the effective optical path length and the consequent probability of absorption are increased. Chen et al. demonstrates that this unique property of photonic crystals facilitates some photon driven processes (20-23). In our previous work, TiO2/Pt coaxial nanotube arrays on a Ti substrate were fabricated. The Schottky barrier effectively enhanced the separation of photogenerated carriers and consequently the photocatalysis efficiency (24). To our knowledge, few works have been published about photonic crystal Schottky structures, including the preparation and the photoelectrochemical property, especially their photocatalytic ability in pollution controlling. Herein, we structured a TiO2/Pt Schottky junction in the shape of inverse opals that is one of the photonic crystal structures. Both the optical reflectance spectra of the prepared material and its photocurrent properties were investigated. Its photocatalytic activity was evaluated by means of the degradation of aqueous phenol.
Materials and Methods Preparation of the Inverse TiO2/Pt Opals. The process to fabricate the inverse TiO2/Pt opals was shown in Scheme 1. The polystyrene opals template for the inverse opals was prepared using a solvent-evaporation method (25). 10-15 layers of polystyrene spheres were self-assembled on the Ti substrate. The inverse Pt opals were made by replicating the polystyrene opals templates. To infiltrate Pt nanoparticles into the voids of the template, a cyclic voltammetry (CV) electrodeposition process was used. In the end, a thin TiO2 film was deposited onto the Pt skeleton frame using a liquid-phase deposition (LPD) method (26) without destroying its periodicity. Monodisperse polystyrene spheres were purchased as a suspension from Shenzhen Nanomicro Tech and used without purification. The suspension was diluted with deionized water to a concentration of 0.1 wt % and dispersed ultrasonically in a glass vial for 30 min. Ti substrate was cleaned ultrasonically with deionized water and 2-propanol, then rinsed with deionized water, and finally dried in the air stream. The Ti substrate was immersed in the polystyrene spheres suspension vertically. The vial was kept in an oven at 55 °C overnight until the water in the suspension was fully VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SCHEME 1. Schematic Diagrams of the Process to Fabricate the Inverse TiO2/Pt Opals Photocatalystsa
a (1) Polystyrene opals prepared using solvent-evaporation method; (2) deposition of Pt nanoparticles by CV electrodepostion; (3) inverse Pt opals were obtained by removing the polystyrene opals template; (4) coating a TiO2 layer outside the Pt skeleton frame by the LPD method.
evaporated, leaving a polystyrene opals film on the surface. The polystyrene opals were first immersed in a solution containing 2 g L-1 H2PtCl4 (27) and 1.2 mM hydrochloric acid (28) for 15 min to ensure that the void of the templates was wetted. Pt was electrodeposited, and the potential was controlled using a cyclic voltammetry (CV) technique. It was carried out with a potential ranging from -0.4 V to +1.6 V (29) for 8 cycles at room temperature, in a three-electrode system with the polystyrene opals as the working electrode, a Pt foil as the counter electrode, and Ag/AgCl as the reference electrode. After calcination at 450 °C for 4 h, the polystyrene opals were removed, and a highly ordered replica consisting of a mesoporous Pt framework with air holes was obtained. To obtain the inverse TiO2/Pt opals, the as-prepared inverse Pt opals were first reacted with an ethanol solution of 0.15 wt % titanium isopropoxide and 0.015% HNO3 for 5 min and then dried vertically in the air. The obtained TiO2 precursors were uniformly adsorbed on the surface of the ordered inverse Pt opals. After that, the sample was further soaked in an aqueous solution of 0.2 M ammonium hexafluorotitanate ((NH4)2TiF6) and 0.3 M boric acid (H3BO3) for 15 min. The pH of the solution was adjusted to around 3 by adding hydrochloric acid. After coating a TiO2 layer, the sample was annealed in air at 450 °C for 2 h to convert the amorphous TiO2 to a crystalline one. Characterization of the Inverse TiO2/Pt Opals. The morphology of the samples was observed using a scanning electron microscopy (SEM; Quanta 200 FEG) and a transmission electron microscopy (TEM; FEI-Tecnai G2 F30 S-Twin). The phase of the samples was identified by an X-ray diffractometer (Shimadzu LabX XRD-6000) employing Cu KR radiation at 40 kV and 30 mA over the 2θ range of 20-80°. The optical property of the samples was characterized by a UV-vis spectroptometer (UV 2450 Shimadzu). The position of absorption maximum of the samples was investigated using a lockin-based surface photovoltage (SPV) measurement system, which consists of a monochromator (model Omni-λ3005) and a lock-in amplifier (model SR830-DSP) with an optical chopper (model SR540) running at a frequency of 20 Hz. Photocurrent Measurements. Photocurrent densities were measured using a CHI electrochemical analyzer (CH Instruments 650B, Shanghai Chenhua Instrument Co. Ltd.) in a standard three-electrode configuration with the TiO2-Pt/ Ti electrode as the photoanode, a Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. A 300 W high-pressure mercury lamp (Beijing Huiyixin Lighting Co.) was used as the UV light source with the principal wavelength of 365 nm and the power density of incident light I0 ) 0.75 mW cm-2. Photocatalytic Degradation of Aqueous Phenol. The photocatalytic degradation was performed in a 100 mL cubic quartz reactor with 40 mL reaction solution, and the effective area of the photoanode was 1 cm2. A 300 W high-pressure mercury lamp (Beijing Huiyixin Lighting Co.) was used as the UV light source (I0 ) 2.0 mW cm-2). Phenol was selected as the test pollutant, and its initial concentration was 10 mg L-1 using 0.01 M Na2SO4 as electrolyte. The concentration of 452
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FIGURE 1. XRD pattern of the inverse TiO2/Pt opals.
FIGURE 2. SEM images of the inverse Pt opals (a) and the inverse TiO2/Pt opals (b); TEM images of the inverse TiO2/Pt opals (c) and its amplification (d) obtained from 140 nm polystyrene spheres as a template. phenol was determined by high-performance liquid chromatography (HPLC; Waters 2695, Photodiode Array Detector 2996) with a SunFire C18 (5 µm) reverse-phase column at 30 °C. The mobile phase was methanol and water (v/v ) 0.6/ 0.4) at a flow rate of 1.0 mL min-1, and the detection wavelength was set at 280 nm.
Results and Discussion XRD. Figure 1 shows the XRD patterns of the inverse TiO2/Pt opals using 140 nm polystyrene spheres as a template. The dominant Pt peaks were indexed to the (111) and (200) plane; a peak was observed at 25.2, corresponding to anatase (101), and all of the other dominant peaks were responsible for the Ti substrate, whereas no rutile phase was observed. SEM and TEM. As shown in Figure 2a, the inverse Pt opals surface was of an interconnected network structure with air spheres surrounded by the Pt shells, inheriting the face-centered-cubic (fcc) order of the polystyrene opal template. The samples here were obtained from 140 nm polystyrene spheres as a template. The average diameters of the air spheres were around 125 nm, approximately 90% of the original size of the polystyrene spheres. The cracks were probably generated during the drying process. Figure 2b indicates that the structure of the inverse opals
TABLE 1. Position of Photonic Stop-Band and Red Edge for Inverse TiO2/Pt Opals sample template size (nm) stop-band maximum (nm) red edge (nm)
FIGURE 3. Reflectance spectra of the inverse TiO2/Pt opals (solid lines) and TiO2-nc (gray dashed line). The arrows indicate their stop-band maxima, which were at 280, 320, 450, and 525 nm.
maintained very well after the LPD process, which meant that the samples were rugged enough to withstand the LPD process without loosing their morphology. However, the air spheres decreased to around 100 nm due to the growth of the TiO2 layer on the inner wall of the air spheres, which was about 70% that of the original polystyrene spheres. Even though some disorders appear, these samples possess order in long scale. Figure 2c shows the TEM image of inverse TiO2/Pt opals, and its amplification was present in Figure 2d. The samples were obtained by scraping the membranes from the Ti substrates and ultrasonically dispersing them in ethanol for 15 s. It can be seen that the inverse TiO2/Pt opals contained highly ordered hexagonal air spheres with a diameter of around 100 nm. The thickness of the TiO2 layer on the Pt opals was 25-30 nm, and the growth of TiO2 did not result in any block of the Pt network. Reflectance Measurements. Generally speaking, the photonic crystals should be nonabsorbing at the stopband frequencies in order to avoid the complications arising from the electronic transitions. Moreover, the group velocity can only be properly defined for a nonabsorbing system (30). However, since the as-prepared inverse TiO2/ Pt opals were an absorbing binary system, their stop-bands were probed directly by means of the reflectance measurements. Considering the application of this material in the water pollution remediation, the reflectance of the inverse TiO2/Pt opals was investigated by filling the inverse opal structures with water. To tune the position of a stopband, the inverse opals having stop-bands centered at 280, 320, 450, and 525 nm were obtained by using template spheres of different sizes (herein, the suffix after TiO2/Pt denotes the stop-band maximum of the inverse opal). Disordered inverse TiO2/Pt opals were fabricated using a mixture of different size template spheres (denoted as TiO2/ Pt-mix), which was used as a comparison of photonic effect with the ordered ones. The TiO2 nanocrystalline film (TiO2nc) on the Ti substrate was prepared by the LPD method under the same conditions. The reflectance of all samples was characterized with the incident light normal to the fcc (111) plane (substrate surface), and their reflectance spectra are shown in Figure 3. It can be seen that all the samples exhibited a TiO2 absorption edge at 400 nm. When the size of the template increased from 120 to 362 nm, the stop-band maximum of the inverse TiO2/Pt opal correspondingly red-shifted from 280 to 525 nm, and its reflectance dip became broader. No stop-band was
TiO2/Pt-280 TiO2/Pt-320 TiO2/Pt-450 TiO2/Pt-525 120
140
193
225
280
320
450
525
320
360
500
600
exhibited for the TiO2-nc film, which indicated that it had no photonic effect. As expected, the TiO2/Pt-mix did not exhibit any stop-band; this is because its disordered structure completely diminished the photonic effect of the sample. The reflectance spectrum of TiO2/Pt-450 was only a shoulder rather than a well-defined peak due to the overlap of the absorption edge of the anatase. The position of the stop-band maximum and the red edge of the inverse TiO2/Pt opals prepared using template spheres of different sizes were presented in Table 1. Figure 4 shows the reflectance spectrum for TiO2/Pt-320 and the surface photovoltage spectrum for TiO2-nc. The gray line indicates that the absorption maximum of anatase was at 360 nm, which was accordant with the red edge of the stop-band for TiO2/Pt-320. Therefore, it is reasonable to expect that the as-prepared TiO2/Pt-320 is of high photoactivity as a result of its improved UV absorption and its increased separation of the photogenerated carriers. Photocurrent Measurements. As shown in Figure 5, all these samples had strong instant photoresponses to the UV light illumination. The short-circuit photocurrent densities (SCPD) of TiO2-nc were 0.02 mA cm-2 .The SCPD of TiO2/Pt-280, -450, -525, and -mix were ranging from 0.09 to 0.11 mA cm-2, which were at least 3.5 times higher than that of TiO2-nc. The SCPD of TiO2/Pt-320 was 0.14 mA cm-2, the highest among all the samples. It can be thought that the separation ability of the photogenerated carriers was enhanced for all TiO2/Pt samples due to the presence of the Schottky barrier inside, even if the sample has no photonic effect. In addition, with a red edge (at 360 nm) overlapping the maximum absorption region of anatase, TiO2/Pt-320 was able to harvest photons at energies near 360 nm by reducing their group velocity and resulted in a larger population of photogenerated electronhole pairs. Therefore, the probability for the electrons to get to the substrate was increased, which was expressed as a higher photocurrent. The SCPD of TiO2/Pt-280, -450,
FIGURE 4. Reflectance spectrum for TiO2/Pt-320 (red solid line) and surface photovoltage spectrum for TiO2-nc (gray dashed line). The shaded region indicates the overlapping wavelengths between the red edge of the stop-band for TiO2/Pt-320 and the absorption maximum for TiO2-nc. VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The kinetics of the photocatalytic degradation of phenol was able to be depicted by the pseudo-first-order kinetics equation
()
ln
FIGURE 5. Short-circuit photocurrent density vs time plotted for the inverse TiO2/Pt opals and TiO2-nc in 0.01 M Na2SO4 solution under the UV light illumination (I0 )0.75 mW cm-2).
FIGURE 6. Process of phenol photocatalytic degradation under UV light illumination (I0 ) 2.0 mW cm-2).
and -525 were almost of the same level as that of TiO2/ Pt-mix. It indicates that the photonic effect played no role in these samples because their red edges did not match the maximum absorption region of TiO2. Photocatalytic Degradation of Aqueous Phenol. Since the photonic effect increased the UV absorption and the presence of the Schottky barrier improved the separation of the photogenerated carriers, inverse TiO2/Pt opals can be used as superior photocatalysts. Their photocatalytic activities were evaluated by photocatalytic degradation of phenol under UV light illumination as shown in Figure 6. The photocatalytic degradation of phenol with TiO2-nc was conducted for comparison, and the direct photocatalysis of phenol without photocatalyst served as a control test. The adsorption of phenol for TiO2/Pt-320 in the dark was also investigated. The loss of phenol due to adsorption was around 4.2% after 4 h. The presence of TiO2-nc improved the photodegradation of phenol under the UV light illumination; the degradation efficiency was 69.7% in 4 h, almost one time higher than that without adding any photocatalysts. The photocatalytic degradation of phenol with the inverse TiO2/Pt opals was even better than that with TiO2-nc; over 90% phenol was degraded in 4 h under the same experimental conditions. Among them, TiO2/Pt-320 exhibited the most excellent photocatalytic activity; its degradation efficiency was 98.1%.
C0 ) kt Ct
(1)
where C0 is the initial concentration of phenol, Ct is the concentration of phenol at time t, and k is the kinetic constant. The kinetic constants were calculated from the slope of the linear correlation between ln(C0/Ct) and t. It can be seen in Table 2 under the same experimental conditions; the kinetic constant of TiO2/Pt-320 was 3.3 times as great as that of TiO2-nc. Without improving the UV absorption, TiO2/Pt-280, -450, -525, and -mix exhibited similar performance in the process of phenol photocatalytic degradation. The average value of their kinetic constants was 2.3 times as great as that of TiO2-nc. Compared with TiO2-nc, the overall additional enhancement of TiO2/Pt-320 was 230%, including two contributions: 130% additional enhancement was arising from “Schottky barrier” and the other was arising from “photonic effect”. Discussion on Photocatalytic Mechanism of the Inverse TiO2/Pt Opals. Theoretically, photons are primarily localized in the high dielectric part (TiO2/Pt in our case) of the photonic crystal at the red edge of the stop-band, whereas they are localized in the low dielectric part (water in our case) at the blue edge. In the case of TiO2/Pt-320, the position of the red edge of the photonic stop-band was at 360 nm, which was accordant with the absorption maximum of anatase. Therefore, TiO2/Pt-320 can harvest photons whose energy is at ∼360 nm by reducing their group velocity. In other words, because of a reasonable volume filling fractions (tuned by the size of the polystyrene template spheres), TiO2/Pt-320 possesses an improved UV absorption. Thus, more electrons and holes can be photogenerated. In the TiO2/Pt system, since Pt has a higher work function than TiO2 does, electrons tend to migrate from TiO2 to Pt. A Schottky barrier is formed at the interface of TiO2 and Pt when their Fermi levels are equal, and the barrier height is the energy difference between the work function of the Pt and the electron affinity of the TiO2. Driven by the Schottky barrier, photogenerated electrons transfer directionally, from the outer TiO2 layer to the inner Pt network (24). As a result, an enhanced separation of generated carriers can be achieved. In addition, the inner Pt network is electrically connected to the conductive Ti substrate. Therefore, the electrons can flow to the external circuit via the Ti substrate, keeping a stable Schottky barrier for carriers-separation. As a benefit from this, an increased number of holes are free to move to the surface of the outer TiO2 layer, participating in oxidative reactions with pollutants. This novel photocatalyst takes both the increase of “photogenerated carriers-income” and the decrease of “photogenerated carriers-expenditure” into consideration, which make it a strong “photogenerated holes-support” for photocatalysis process. We believe that introducing “photonic effect” into semiconductor heterostructures can open up new perspectives on development of highly efficient photocatalysts for environmental remediation technology.
TABLE 2. Kinetic Constants and Regression Coefficients of Phenol Photocatalytic Degradation Processes photocatalysis process -1
kinetic constant (k, h ) R2
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control
TiO2/Pt-280
TiO2/Pt-320
TiO2/Pt-450
TiO2/Pt-525
TiO2/Pt-mix
TiO2-nc
0.117 0.983
0.751 0.993
0.992 0.995
0.721 0.992
0.689 0.996
0.640 0.991
0.303 0.990
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Acknowledgments We would like to thank the National Nature Science Foundation of China (No. 20837001), the National Science Foundation of Distinguished Young Scholars of China (No. 20525723), and Program for Changjiang Scholars and Innovative Research Team in University (IRT0813).
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