Article pubs.acs.org/est
Integrating Plasmonic Nanoparticles with TiO2 Photonic Crystal for Enhancement of Visible-Light-Driven Photocatalysis Ying Lu, Hongtao Yu, Shuo Chen, Xie Quan,* and Huimin Zhao Key Laboratory of Industrial Ecology and Environment Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, People's Republic of China S Supporting Information *
ABSTRACT: Aimed at enhancing photocatalysis through intensifying light harvesting, a new photocatalyst was fabricated by infiltrating Au nanoparticles into TiO2 photonic crystals (TiO2 PC/Au NPs). Scanning electron microscopy (SEM) and transmission electron microscope (TEM) images showed that the Au NPs with average diameter around 15 nm were dispersed uniformly into the porous TiO2 material. The results of the transmittance spectra demonstrated that the light absorption by Au NPs was amplified after they were infiltrated into TiO2 240, which was fabricated from 240 nm polystyrene spheres. In the photocatalytic experiments of 2,4-dichlorophenol degradation under visible light (λ > 420 nm) irradiation, the kinetic constant using TiO2 240/Au NPs was 2.3 fold larger than that using TiO2 nanocrystalline/Au NPs (TiO2 NC/Au NPs). The excellent photocatalysis benefited from the cooperatively enhanced light harvesting owing to the localized surface plasmon resonance of Au NPs, which extended the light response spectra and the photonic effect of the TiO2 240 which intensified the plasmonic absorption by Au NPs. The hydroxyl radicals originated from the electroreduction of dissolved oxygen with photogenerated electrons via chain reactions were the main reactive oxygen species responsible for the pollutant degradation.
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INTRODUCTION The light harvesting of semiconductor photocatalysts such as TiO2 is an important issue for good photocatalytic performance because it directly determines the generation of photocarriers responsible for the pollutants degradation; accordingly, endeavors have been devoted to design novel photocatalysts which can exhibit enhanced light harvesting.1,2 All of the efforts to improve the light harvesting can be categorized into two approaches. The first involves metal ion doping,3,4 nonmetal doping,5,6 dye sensitization,7,8 narrow band gap semiconductor coupling,9−11 and so forth, which can increase the quantity of photons absorbed by photocatalysts through enlarging the optical response region. The second is to enhance the interaction of light with semiconductors by using multiple scatter12 or surface resonant modes,13 which can increase the probability of the photons to be absorbed under the same incident intensity condition. A cooperative effect on the light harvesting enhancement may be accomplished by uniting the fundamentally different strategies, unfortunately, until now, few such studies has been reported. Noble-metal nanoparticles (NPs) such as Au and Ag can respond to visible light due to the localized surface plasmon resonance (LSPR), which is produced by the collective © 2012 American Chemical Society
oscillations of the surface electrons, exhibiting great potential for extending the light absorption range of wide band gap semiconductors.14−17 Some works have been reported that the immobilization of Au NPs on TiO2 led to visible-light-induced photocatalysis for the degradation of dyes,18,19 various volatile organic compounds,20 and toxic persistent organic pollutants.21,22 The photonic crystal (PC) is of certain optical material or structure, which is designed to confine, control and manipulate photons so as to intensify the light absorption.23−25 To be specific, the periodic modulation of PC leads to a photonic band gap (PBG). The light within the wavelength range of PBG could not propagate in the PC due to the Bragg diffraction and scattering (termed as band gap scattering effect);26,27 the light at the frequency edges of the PBG will propagate with strongly reduced group velocity (termed as slow photon effect).28,29 Both the band gap scattering effect and slow photon effect (generally named as photonic effect) could Received: Revised: Accepted: Published: 1724
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Figure 1. SEM images of TiO2 240/Au NPs: top view (a) and cross section view (b).
removed by calcining at 500 °C for 2 h with a heating rate of 2 °C/min, obtaining the highly ordered TiO2 PC. The TiO2 PC replicated from 193 and 240 nm polystyrene spheres were termed as TiO2 193 and TiO2 240, respectively. The TiO2 nanocrystalline (TiO2 NC) was prepared by the LPD method under the same condition, only without the polystyrene spheres template, and applied in the control experiments as a comparison of photonic effect with the TiO2 PC. Infiltration of Au NPs into the TiO2 PC. Au NPs were infiltrated into the TiO2 PC via in situ hydrothermal reduction approach. 0.6 mL HAuCl4 (0.01 M) was added in the mixture of 2 mL methanol and 40 mL deionized water, and the pH of the solution was adjusted to 7−8 by adding 0.01 M NaOH. The TiO2 sample was immersed in the mixture, and then sealed within a Teflon-lined autoclave (100 mL) and heated at 120 °C for 1 h. After the hydrothermal reduction process, the sample was cooled down to room temperature, washed with deionized water thoroughly, and dried in air. The TiO2 PC decorated with Au NPs was referred as TiO2 PC/Au NPs. Generally speaking, methanol can serve as an efficient reducing agent resulted in the generation of Au NPs. In this work, the application of elevated temperature and pressure under hydrothermal condition can not only intensify the reduction process and consequently shorten the reaction time, but also cause the sintering between gold and TiO2 interphase, therefore improving the gold catalysis performance and increasing the catalyst stability. Characterization. The scanning electron microscopy (SEM) images were taken by a Hitachi S-4800 operating at 3.0 KV. The transmission electron microscope (TEM) images were obtained on a Tecnai G2 20 operating at 200 KV. The Xray diffractometer (XRD) patterns were collected on a Shimadzu LabX XRD-6000 with Cu K radiation. The X-ray photoelectron spectroscopy (XPS) data were taken on an ESCALAB250 surface analysis system using monochromatic Al KR radiation (300 W, 20 mA, 15 kV) and low-energy electron flooding for charge compensation. The transmittance spectra were retrieved from a Shimadzu UV-2450 UV−vis spectrophotometer in the range of 200 to 800 nm. BaSO4 was used as the reflectance standard. The reactive oxygen species •OH and •O2− were detected using a Bruker Elexsys A200 electron spin resonance spectrometer (ESR, Bruker, Germany) with a 100W short arc mercury lamp (ER 203UV system) as the irradiation light source and a glass filter was added to allow visible light (λ > 420 nm) to pass through. Photocatalytic Experiment. The degradation of 2,4dichlorophenol (2,4-DCP) was carried out to evaluate the photocatalytic activity of the prepared catalysts. A high pressure xenon short arc lamp (CHFXM35−500W, Beijing Changtuo
enhance the interaction of light with photoresponsive material, amplifying the optical absorption and photochemical reaction. In this work, for the sake of achieving synergistically enhanced light harvesting arising from both the extent and intensity strategies, a new photocatalyst was fabricated by infiltrating Au NPs into TiO2 PC. This novel photocatalyst utilized the advantages of LSPR and PC, which can (i) extend the light absorption spectra to the visible region by using the LSPR of Au NPs for efficient utilization of the solar energy; (ii) strengthen the LSPR of Au NPs through the photonic effect of TiO2 PC and then increase the light harvesting; (iii) suppress the electron/hole recombination by forming the TiO2/Au heterojunction, thus improving the quantum efficiency of photocatalyst; and (iv) provide large surface area by using the ordered macropores of the PC structure for the photocatalysis. To the best of our knowledge, this is the first report about combining the LSPR with photonic effect for enhanced photocatalysis through intensifying light harvesting.
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EXPERIMENTAL SECTION Chemicals. Monodisperse polystyrene latex spheres (193 and 240 nm, 1 wt % in water) were purchased as a suspension from Shenzhen Nanomicro Tech. FTO glass with 2.2 mm thickness and 15 Ω/sq sheet resistance was obtained from Geao Co., China. All of the other reagents (analytical grade purity) were purchased from Tianjin Kermel Chemical Reagents Development Centre and were used without further purification. Fabrication of TiO2 PC. The FTO glass substrate was cleaned ultrasonically with dilute HCl and ethanol, respectively, then rinsed with deionized water, and finally dried in the air stream. Colloidal crystals polystyrene film was prepared via solvent evaporation method. The FTO substrate was immersed vertically in the polystyrene spheres suspension with a concentration of 0.1 wt % (sonicated for 40 min to break aggregation). The vial was kept in an oven at 55 °C overnight until the water in the suspension was fully evaporated, leaving a polystyrene opals film on the substrate. The polystyrene spheres template was replicated with TiO2 following a reported liquid-phase deposition (LPD) method. First, in order to form a TiO2 seed layer, the colloidal crystal film was immersed vertically in a solution of 0.15 wt % titanium isopropoxide and 0.015% HNO3 in ethanol for 5 min, then dried in air. Second, the sample was dipped in an aqueous solution of 0.1 M ammonium hexafluorotitanate and 0.3 M boric acid (adjusted the pH to 3 by 1 M HCl) at 60 °C for 15 min, then rinsed thoroughly with deionized water and dried in air at room temperature. At last, the polystyrene spheres template was 1725
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Figure 2. Transmittance spectra of TiO2 240/Au NPs (a) and TiO2 193/Au NPs (b). The shaded region indicates the overlapping wavelengths between the photonic band gap of TiO2 240 and the plasmonic absorption by Au NPs, and the circle represents the blue-edge of the photonic band gap.
Transmittance Measurements. In order to verify the enhancement of light harvesting in this 3D plasmonic photocatalyst, the transmittance spectra were characterized and shown in Figure 2. The peak positions of both PBG and plasmonic absorption are sensitive to the dielectric environment surrounding the material.29,30 Considering the application of this photocatalyst in the following water treatment experiments, we dipped it in deionized water for a few seconds and then investigated its optical properties in the presence of water. The optical absorption of TiO2 NC decreased sharply at wavelengths of incident light longer than 380 nm due to the limitation of wide band gap energy (3.2 eV). The TiO2 NC/Au NPs displayed obvious visible light absorption ranged from 480 to 590 nm because of the LSPR of Au NPs. In the transmittance spectra, the TiO2 240/Au NPs presented the most efficient light harvesting. The formation of high-quality PC structure can result in Bragg diffraction and slow light propagation at wavelengths with respect to the center and edges of the PBG, respectively. For the spectrum of TiO2 240, a Bragg diffraction peak centered at 560 nm was observed, and the position of the slow light propagation at the blue-edge of the PBG was in the vicinity of 493 nm, well within the absorption spectrum of Au NPs. Both the Bragg diffraction effect and slow photon effect of TiO2 240 increased the effective path length of light within this photocatalyst, reinforcing the interaction between photons and Au NPs. Consequently, the TiO2 240/Au NPs displayed the strongest plasmonic absorption. Additionally, compared to the TiO2 NC/ Au NPs, the optical absorption by Au NPs infiltrated into the TiO2 PC was red-shifted by approximately 30 nm, which could be attributed to the high refractive index of the TiO2 (n = 2.5 for anatase TiO2),30 suggesting the good contact between Au NPs and TiO2 PC. In order to approve the importance of matching between the PBG and the wavelength of the plasmonic absorption to the enhancement of light absorption by Au NPs, the transmittance spectrum of TiO2 193/Au NPs was also investigated and presented in Figure 2b. Because the PBG of TiO2 193 was centered at 443 nm, out of the absorption spectrum of Au NPs, the photons manipulated by the photonic effect of TiO2 193 cannot be utilized efficiently by Au NPs. Thus, the TiO2 193/Au NPs was not expected to exhibit efficient photocatalysis. Photocatalytic Performance. The degradation of 2,4dichlorophenol (2,4-DCP) under visible light (λ > 420 nm)
Co.) was served as the light source, a glass filter (ZUL0422, Asahi Spectra Co.) was added to allow visible light (λ > 420 nm) to pass through. The light intensity was 100 mW/cm2, which was measured using the radiometers of model FZ-A (Photoelectric Instrument Factory Beijing Normal University). The initial concentration of 2,4-DCP was 10 mg/L. During the photodegradation process, the 2,4-DCP solution was collected at 30 min intervals for analysis. The 2,4-DCP concentration was determined by HPLC (Waters-2695, photodiode array detector (PDA) −2996, Waters, U.S.).
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RESULTS AND DISCUSSION Morphology Examination. Figure 1 displays the SEM images of TiO2 240/Au NPs. It can be observed that the structure of the PC maintained well after the hydrothermal reduction process. The thickness of the TiO2 240 structure was about 2.5−3 μm, and the average diameter of the hollow spheres was around 223 nm, approximately 92% of the original size of the polystyrene spheres. The shrinkage may be due to the decomposition and vaporization of the colloidal crystal template during the calcination process. The Au NPs with the size around 15−20 nm were uniformly distributed and anchored along the heterogeneous macroporous surface of the 3D PC structure. The surface density of Au NPs was estimated as 7.5 × 108 cm−2 from the images. Figure S1 of the Supporting Information, SI, presents the TEM image of TiO2 240/Au NPs. The average diameter of the Au NPs obtained from the TEM image was approximately 15 nm, which was consistent with the data obtained from the SEM images. This homogeneous dispersion of Au NPs infiltrated into the TiO2 PC was desirable for efficient light harvesting, yielding high plasmon-induced activity for pollutant degradation. XRD and XPS. SI Figure S2 shows the XRD patterns of TiO2 240/Au NPs. The diffractive peak at 2θ = 25.2° was assigned to (101) crystal phase of anatase phase, and all of the other dominant peaks corresponded to the reflection of FTO substrate. No diffraction peaks of Au species were observed, which presumably was due to the low content incorporation of Au NPs. To affirm the metallic state of Au NPs, the sample TiO2 240/Au NPs was further characterized by XPS measurement, as presented in SI Figure S3. The peaks observed at 84 and 88 eV were ascribed to metallic Au 4f7/2 and Au 4f5/2, respectively, which confirmed that the Au species existed as Au0 in the PC. 1726
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absorption by Au NPs. Anyway, the pollutant degradation efficiency of all photocatalysts was in good agreement with the light harvesting efficiency presented in Figure 2. Mechanism. Generally, the photocatalytic reaction can be considered as the process of generation, transfer, and consumption of the photogenerated carriers.31 First, the photocatalyst absorbs the incident photons with energy above the semiconductor’s band gap, generating electrons and the same amount holes. The holes abstract electrons from absorbed pollutants or react with H2O to form hydroxyl radicals. However, the conduction band electrons reduce absorbed oxygen to form superoxide anions which can further disproportionate to form hydroxyl radicals via chain reactions. In addition, the electrons could react directly with the pollutant. A number of diagnostic experiments were conducted to probe the mechanism responsible for pollutant degradation using TiO2 PC/Au NPs, as shown in Figure 4. If the holes
irradiation was performed to evaluate the photocatalytic activity of the photocatalyst. First-order kinetics was observed and shown in Figure 3. The corresponding kinetic constant (k) and
Figure 3. The kinetics of 2,4-DCP degradation using various photocatalysts under visible light (λ > 420 nm) irradiation.
Table 1. Kinetic Constants and Regression Coefficients of 2,4-DCP Degradation under Visible Light (λ > 420 nm) Irradiation sample
kinetic constant (k, h−1)
R2
direct photolysis TiO2 NC TiO2 193 TiO2 240 TiO2 NC/Au NPs TiO2 193/Au NPs TiO2 240/Au NPs
0.08 0.12 0.18 0.19 0.25 0.40 0.57
0.993 0.993 0.995 0.996 0.998 0.998 0.996
Figure 4. The kinetics of 2,4-DCP degradation using TiO2 240/Au NPs with various additives under visible light (λ > 420 nm) irradiation.
played a crucial role in the pollutant degradation, the adding of formic acid (FA) which is a commonly used holes scavenger would inhibit significantly the pollutant degradation. In our experiments, the FA was added first and stirred magnetically in the dark for 30 min in order to achieve efficient holes capture, however, the degradation of 2,4-DCP was only slightly retarded, which indicated the minor role of holes either acting as the oxidizing agent or the origination of the •OH radicals in the pollutant degradation. Dissolved oxygen is highly electrophilic, and it reacts easily with the electron. In the O2-staturated aqueous solution, the degradation of 2,4-DCP was significantly enhanced. This observation implied that the pollutant degradation was not ascribed to direct reduction by electrons. The enhancement can be explained on the basis of the hypothesis that more O2 molecules scavenged more electrons with suppressing charge recombination and consequently producing more reactive oxygen species such as •OH radicals. In the presence of tert-butyl alcohol (TBA), an efficient •OH radicals quencher, the degradation of 2,4-DCP was markedly depressed. The inhibitory effect became more obvious as the concentration of the TBA increased, which suggested that the •OH radicals was the primary oxidant in TiO2 PC/Au NPs photocatalytic process under visible light irradiation. The collective observations suggested that the •OH radicals responsible for the pollutant degradation originated from the electroreduction of dissolved oxygen with electrons via chain reactions. The ESR spin-trap technique (with DMPO) was used to confirm the generation of both •OH and •O2− radicals, as presented in Figure 5. The obviously characteristic peaks of
regression coefficients (R2) were calculated and given in Table 1. The adsorption of 2,4-DCP using TiO2 240/Au NPs in dark and direct photolysis of 2,4-DCP served as control. During the 240 min illumination, the TiO2 NC showed the lowest photocatalytic activity because of no visible light response. The photocatalytic activity of TiO2 NC/Au NPs was higher than that of TiO2 NC, which was ascribed to the enlarged optical response due to the LSPR of Au NPs. Among these photocatalysts, the TiO2 240/Au NPs, which combined the advantages of LSPR and PC, displayed the highest photocatalytic activity. The kinetic constant using TiO2 240/Au NPs was 2.3 fold larger than that using TiO2 NC/Au NPs. This novel photocatalyst not only utilized the LSPR of Au NPs to extend the light absorption region, but also exploited the photonic effect of TiO2 240 whose PBG coincided with the wavelength of light absorption by Au NPs to amplify the plasmonic absorption, leading to cooperatively enhanced light harvesting and the consequently efficient pollutant degradation. In order to evaluate the reproducibility of both sample preparation and catalytic reaction, three TiO2 240/Au NPs were prepared and their photocatalytic activities were investigated. The results (SI Figure S4) confirmed the excellent photocatalytic activity was reproducible within acceptable statistical deviation (6%). The photocatalytic activity of TiO2 193/Au NPs was also investigated, whose kinetic constant was about one-half of that using TiO2 240/Au NPs, which was attributed to the inefficient light harvesting because of mismatching of the PBG with the wavelength of the plasmonic 1727
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Figure 5. DMPO spin-trapping ESR spectra recorded at ambient temperature in aqueous dispersion (for DMPO-•OH, (a) and methanol dispersion (for DMPO-•O2−, (b) under visible light (λ > 420 nm) irradiation. Peaks generated from the DMPO-•OH and DMPO-•O2− adducts are marked as (*) and (#), respectively.
DMPO-•OH and DMPO-•O2− were observed using TiO2 240/Au NPs under visible light irradiation, whereas weak or no such signals were detected using other photocatalysts. On the basis of the above experiments, a schematic mechanism for the pollutant degradation using TiO2 PC/Au NPs was illustrated in Scheme 1. The TiO2 PC/Au NPs not
fields enhancement, for example. Cronin et al. observed enhanced pollutant degradation using anodic TiO2 loaded with Au NPs.36 They attributed this improvement to the intense electric fields of Au NPs, which increased the electron− hole pair generation rate in the defect-rich TiO2. Similar noblemetal-loaded TiO2 photoreactions were also studied by Awazu and Duan et al.30,37 However, in this work, because of the mismatching of wavelength between TiO2 and plasmonic absorption, as shown in Figure 2, the influence of electric fields of Au NPs on the pristine TiO2 is negligible.36,37 More importantly, the anatase TiO2 could not be excited under visible light irradiation, no electrons and holes resulted in the pollutant degradation being generated. In summary, a 3D plasmonic photocatalyst TiO2 PC/Au NPs was prepared by infiltrating Au NPs into TiO2 PC. This novel photocatalyst showed synergistically enhanced light harvesting owing to the LSPR of Au NPs which extended the light response region and the photonic effect of TiO2 PC which intensified the LSPR by matching the PBG with the wavelength of plasmonic absorption. The hydroxyl radicals derived from the electroreduction of dissolved oxygen with electrons via chain reactions was the main reactive oxygen species and finally resulted in the efficient pollutant degradation. We believe that the strategy for photocatalyst preparation in this work can not only be significant in the field of photocatalysis to meet the requirements of future environmental and energy technologies, but also open up new prospects on the development of other fields such as nanodevices and photoelectronics.
Scheme 1. Schematic Illustration of the Pollutants Degradation Mechanism Using TiO2 PC/Au NPs under Visible Light (λ > 420 nm) Irradiationa
a
The blue lines represent the process of Bragg reflection, diffuse scattering and multiple internal scattering in TiO2 PC.
only used the LSPR of Au NPs to enlarge the light absorption range, but also exploited the photonic effect of TiO2 PC whose PBG coincided with the wavelength of light absorption by Au NPs to amplify the plasmonic absorption, increasing the yield of electrons under visible light irradiation. The electrons generated by exciting the Au NPs were injected into the conduction band of TiO2,32−34 and then reduced the dissolved oxygen absorbed on the surface of TiO2, forming •OH radicals via chain reactions. The •OH radicals originated from the electroreduction of dissolved oxygen with electrons was the major reactive oxygen species responsible for the pollutant degradation. Meanwhile, the oxidized Au NPs captured electrons from H2O or organic molecules adsorbed on them to neutralize the positive charges,18,31,34,35 accordingly the photocatalyst was regenerated. A specific process of generation, transfer, and consumption of the charge carriers during the LSPR-driven photocatalytic process were summarized in SI Table S1. Some other mechanisms may contribute to the improved plasmonic photocatalysis under visible light irradiation, electric
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ASSOCIATED CONTENT
S Supporting Information *
Detailed information includes TEM image of TiO2 240/Au NPs. The red circle displays the skeleton of TiO2 PC (Figure S1), XRD patterns of TiO2 240/Au NPs on FTO substrate (Figure S2), XPS spectra of TiO2 240/Au NPs (Figure S3), the kinetic constants of three TiO2 240/Au NPs (Figure S4), and specific process of generation, transfer, and consumption of the charge carriers during the LSPR-driven photocatalytic process (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-411-84706140; fax: +86-411-84706263; e-mail:
[email protected]. 1728
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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (No. 20837001), National Basic Research Program of China (2011CB936003) and Program for Changjiang Scholars and Innovative Research Team in University (IRT0813).
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