Visible Light Induced Photocatalytic Degradation of Rhodamine B

J. Phys. Chem. C 2010, 114, 17051–17061

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Visible Light Induced Photocatalytic Degradation of Rhodamine B on One-Dimensional Iron Oxide Particles† Xuemei Zhou,‡ Hongchao Yang,§ Chenxuan Wang,‡ Xiaobo Mao,‡ Yinshu Wang,§ Yanlian Yang,‡ and Gang Liu*,‡ National Center for Nanoscience and Technology, Beijing, 100190, China, and Department of Physics, Beijing Normal UniVersity, Beijing, 100875, China ReceiVed: April 9, 2010; ReVised Manuscript ReceiVed: August 16, 2010

Visible light (λ > 420 nm) induced photocatalytic degradation of rhodamine B (RhB) in the presence of H2O2 by one-dimensional (1D) nanorods of goethite (R-FeOOH) and hematite (R-Fe2O3) has been investigated, and results were compared to those of micrometer-sized rods. R-FeOOH nanorods were self-assembled by oriented attachment of R-FeOOH primary nanoparticles, while porous R-Fe2O3 rods were prepared by thermal dehydration of respective R-FeOOH precursors via a topotactic transformation. The as-prepared samples were characterized by powder X-ray diffraction, micro-Raman spectroscopy, diffuse reflectance UV-visible spectroscopy, X-ray photoelectron spectroscopy, nitrogen adsorption-desorption, high-angle annular darkfield scanning transmission electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. Nanosized R-FeOOH and R-Fe2O3 particles appeared to be more active than microsized ones in terms of surface area normalized reaction rate, suggesting intrinsic photocatalytic properties of nanorods as compared to microrods in both R-FeOOH and R-Fe2O3. In addition, R-Fe2O3 nanorods exhibited the greatest activity among the as-prepared samples. The observed photocatalytic performance by iron oxide particles was attributed to the synergetic effects of the particle composition, size, porosity, and the variations of local structure. The results from current study will be potentially applicable to a range of naturally abundant semiconducting minerals and compounds (e.g., metal oxyhydroxides and metal oxides). 1. Introduction Nanomaterials are currently receiving much attention due to their unique size-, shape-, and crystallinity-dependent optical, electronic, magnetic, and chemical properties.1 Nanomaterials possess physical and chemical properties that may benefit electronic device development, medicine, renewable energy, and environmental remediation. A significant amount of research has focused on understanding the unique and useful properties of nanomaterials. In nanocatalysis, one prototypical example is gold.2,3 Unlike bulk gold that is the most catalytically inert metal, gold nanoparticles supported on a range of oxides (e.g., TiO2, MgO) were found to exhibit enhanced catalytic activity for CO oxidation at room temperature and below4,5 and shows prominent reactivity in many other reactions such as desulfurization (DeSOx)6,7 and the water-gas shift (WGS).8 Among nanomaterials, metal oxides represent one of the most diverse classes of materials with both fundamental and technological importance. One of metal oxides which occur ubiquitously in the environment is iron oxides.9-12 In particular, goethite (R-FeOOH) and hematite (R-Fe2O3) are extremely common in soils and sediments at and near the Earth’s surface. Apart from its traditional use as a pigment,9 R-FeOOH has found other technological applications as well, such as magnetic recording media precursor,13 mineral liquid crystal colloid,14 and environmental contaminant scavenger for wastewater treatment.12,15,16 Compared to other iron oxides, under ambient †

Part of the “D. Wayne Goodman Festschrift”. * To whom all correspondence should be addressed: e-mail, liug@ nanoctr.cn; fax, (+86) 10-62656765. ‡ National Center for Nanoscience and Technology. § Department of Physics, Beijing Normal University.

conditions R-Fe2O3 is most stable. R-Fe2O3 has many applications such as light-induced water splitting,17 catalysis,18 gas sensors,19 solar cells,20 field emission devices,21 and magnetic materials.22-27 Both R-FeOOH and R-Fe2O3 nanocrystallites exhibit unique reactivity at the nanoscale. For example, the rate of reductive dissolution of 64 nm (length) × 5 nm (width) R-FeOOH nanorods (NRs) by hydroquinone is two times faster than that of 367 nm (length) × 22 nm (width) ones.15 In the catalytic oxidation of aqueous Mn2+, the rate exhibited by 7 nm R-Fe2O3 nanocrystals is 1 to 2 orders of magnitude faster than that of 37 nm R-Fe2O3 nanocrystals.28 In recent years, significant attention has been directed toward abatement of noxious species in air and aqueous media.29-32 Organic dyes are often used in printing, textile, and photographic industries. A sizable fraction of dyes is lost in the dying process and released into the effluent water streams. In many situations, the degradation rate of dyes is slow by sunlight without catalytic assistance. Therefore, it is necessary to employ appropriate catalysts to degrade dyes in aqueous solution. Dye degradation by semiconductor-based photocatalysts, e.g., TiO2, has been investigated extensively.31,33 Nevertheless, TiO2 with an inherent band gap of 3.0-3.3 eV can only adsorb ultraviolet light (UV, with wavelength λ < 400 nm), which accounts for a small fraction of the solar spectrum. Driven by global energy and environmental challenge, considerable effort has been made to modify existing TiO2 photocatalysts or developing alternative ones, with a goal to extend the range of light absorption into the visible region.31,32,34 Narrower band gap semiconductors, such as CdS,35 Fe2O3,36-43 WO3,44-46 CaIn2O4,47 BaBiO3,48 and Bi2SbVO7,49 have been addressed. In particular, nanosized semiconductor particles with large surface areas and a variety

10.1021/jp103816e  2010 American Chemical Society Published on Web 09/07/2010

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of morphologies offer great opportunity. Compared to other potential photocatalysts, iron oxides are plentiful in nature and environmentally benign. Furthermore, iron oxides are semiconductors (2.150 and 2.2 eV51 for R-FeOOH and R-Fe2O3, respectively) that can adsorb light up to 600 nm. Zhou and Wong et al.20 reported that R-Fe2O3 nanotubes with length of 6 ( 3 µm and width of 260 ( 60 nm show better performance in the photodegradation of 4-chlorophenol than their bulk analogues. Du and Xu et al.40 studied degradation of Orange II on various iron-containing compounds, such as R-Fe2O3, γ-Fe2O3, Fe3O4, R-FeOOH, γ-FeOOH, and δ-FeOOH, in aqueous suspension by UV light, and found that anhydrous iron oxides are more active than hydrated iron oxides. In contrast, Feng and Hu et al.39 studied degradation of Orange II by iron-containing catalysts and suggested that both amorphous FeOOH and calcinated FeOOH display superior performance to R-Fe2O3. To date, systematic studies of the impact of particle size and shape on photocatalytic property by iron oxides are still lacking. Over the past decade, shape control of nanomaterials using selfand directed-assembly as a “bottom-up” route has seen rapid growth.52-61 Of particular interest is to assemble unique and useful nanomaterials with novel optical, electrical, magnetic, and catalytic properties. For example, tuning the direction and path of charge carriers through quantum confinement in 1D anisotropic nanostructures is possible and could be useful in photocatalysis.62-64 Our recent work65,66 demonstrated that through oriented attachment (OA), anisotropic R-FeOOH nanocrystallites could be synthesized at high pH without the assistance of organic additives, and R-Fe2O3 was obtained through thermal dehydration of a corresponding R-FeOOH precursor via a topotacic transformation, thus opening a route to tailor size and shape of R-FeOOH and R-Fe2O3 nanomaterials. Insights into the dye degradation on iron oxides not only are of fundamental importance for our understanding of photocatalysis but allow extending its general applicability to a range of naturally abundant semiconducting minerals and compounds such as metal oxides and metal oxyhydroxides. In the current study, visible light induced photocatalytic activity toward rhodamine B (RhB, a xanthene dye) by nano- and microsized iron oxides, including oxyhydroxide (R-FeOOH) and anhydrous ferric oxide (R-Fe2O3), has been investigated. RhB is one of major cationic dyes and not biodegradable in wastewater. We are aware that studies on the comparison of photocatalytic activity between nanosized photocatalysts and their microsized counterparts are very limited.67 Anisotropic shapes, such as 1D rods, offer tunable exposed crystallographic surfaces and/or structures, which play an indispensable role in photocatalysis. Our results shed new light on composition-, size-, porosity-, and local structure-dependent photocatalysis by iron oxide particles. 2. Experimental Section 2.1. Synthesis. The synthesis of ferrihydrite nanoparticles and goethite NRs has been published elsewhere.15,68-71 R-FeOOH microrods (MRs) were synthesized using a method developed by Cornell and Schwertmann.9 R-Fe2O3 MRs and NRs were obtained by heating corresponding R-FeOOH precursors at 300 °C in air for 1 h. 2.2. Characterization. The Brunauer-Emmett-Teller (BET) specific surface area and the Barrett-Joyner-Halenda (BJH) pore size distribution were measured using a Micromeritics ASAP 2020 apparatus. Powder X-ray diffraction (XRD) data were collected using a Shimadzu X-ray diffractometer (XRD6000) with Cu KR radiation (λ ) 0.154178 nm). Measurement was in the 2θ range of 15-85° with a scanning step of 0.68

Zhou et al. (deg/min). The diffraction patterns were compared to the reference powder diffraction files (PDF) for goethite (81-0463) and hematite (84-0307). Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 20 S-Twin microscope operating at 200 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements were performed on a Tecnai G2 F20 U-Twin microscope operating at 200 kV. Goethite and hematite as dry powders were dissolved into Milli-Q water and sonicated in a cool water bath until a homogeneous suspension was formed. Second, a drop of sample suspension was spread onto an amorphous holey-carbon film supported by a standard TEM grid (Beijing XinXingBaiRui Technology Co., Ltd.). Finally, all TEM samples were allowed to dry under ambient conditions. No sample damage was observed during high-resolution TEM studies. Micro-Raman spectroscopy measurement was conducted using a Renishaw Micro-Raman spectroscopy system (Renishaw in Via plus). A Renishaw red laser at 785 nm was employed as the excitation source. Diffuse reflectance ultraviolet and visible (DRUV-vis) spectra were obtained using a PerkinElmer Lambda 950 UV-vis spectrometer. Fine BaSO4 powder was used as a standard, and the spectra were recorded in a range of 200-800 nm. X-ray photoelectron spectroscopy (XPS) data were obtained using an ESCALab 250 electron spectrometer from Thermo Scientific Corp. For high-resolution spectra, monochromatic 150 W Al KR radiation was utilized and the pass energy is 30 eV. Low-energy electrons were used for charge compensation to neutralize the samples. The binding energies were referenced to the adventitious C 1s line at 284.8 eV. A Shirley-type background was subtracted from each spectrum and Avantage 4.15 software was used to process the data. Particle size distributions in suspensions prior to reaction were evaluated using a Zetasizer (model Nano ZS, Malvern Instruments) operating with a He-Ne laser at a wavelength of 633 nm. Suspensions with a complete adsorption-desorption equilibrium were analyzed. 2.3. Photocatalytic Evaluation. The photocatalytic activity of the as-prepared samples for the degradation of RhB (Alfa Aesar, CAS# 81-88-9) in aqueous solution was evaluated by measuring the adsorbance of the irradiated solution. Prior to irradiation, 10 mg of photocatalyst was mixed with RhB (50 mL, with a concentration of 0.02 mM) in a 100-mL roundbottom flask and then sonicated in a cool water bath for 10 min. Afterward, the suspension was magnetically stirred in the dark overnight to reach a complete adsorption-desorption equilibrium, followed by the addition of 0.255 mL of hydrogen peroxide solution (H2O2, 30 wt %). Then the beaker was exposed to visible light irradiation with maximum illumination time up to 180 min. During the irradiation, the suspension was magnetically stirred by using a magnetic stirrer and the reaction temperature was kept at room temperature by using a cooling fan. The suspension was subsequently illuminated by a 300 W xenon lamp at a ca. 40 cm distance. A wave filter plate (λ > 420 nm) was utilized to allow visible light to transmit. At different time intervals, about 4 mL aliquots were sampled, centrifuged, and filtered through a membrane (0.22 µm in diameter, Agela Technologies). The dye concentration in the filtrate was analyzed by measuring the absorption intensity of RhB at 554 nm. 3. Results and Discussion 3.1. Characterization. The initial identification of microsized and nanosized R-FeOOH and R-Fe2O3 particles was performed using experimental powder XRD patterns (Figure 1a). The XRD peaks for R-FeOOH samples are indexed to a predominant orthorhombic phase of goethite (space group Pbnm, No. 62)

Photocatalytic Degradation of Rhodamine B

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Figure 1. Characterization of R-FeOOH and R-Fe2O3 NRs and MRs, while R-Fe2O3 NRs and MRs were obtained by calcinating R-FeOOH precursors at 300 °C in air for 1 h. (a) Powder XRD patterns. (b) Micro-Raman spectra excited by a red-line laser (785 nm). (c) Absorption and (d) reflectance spectra of R-FeOOH and R-Fe2O3 NRs and MRs.

with lattice constants of a ) 0.4596, b ) 0.9957, and c ) 0.3021 nm. For R-Fe2O3 samples, the XRD peaks can be indexed to a rhombohedral hexagonal phase (space group R3jc) with lattice constants a ) 0.5035 and c ) 1.3740 nm. A possible cause of peak broadening in a powder diffractogram as seen in R-FeOOH NRs and R-Fe2O3 NRs compared to the microsized counterparts may be due to smaller crystalline sizes. The corresponding micro-Raman spectra of the as-prepared samples are displayed in panel b of Figure 1. The positions of major bands (240, 295, 384, 476, 546 cm-1) for two types of R-FeOOH particles look identical and are similar to the Raman signature of R-FeOOH reported in the literature.72 As to R-Fe2O3 MRs, the bands at 217 and 487 cm-1 and 237, 288, 402, and 604 cm-1 are ascribed to the A1 g and Eg modes of R-Fe2O3, respectively.73 The band at 1322 cm-1 (not shown) is the characteristic band assigned to a two-magnon scattering resulting from the interaction of two magnons created on antiparallel close spin sites.74 Nevertheless, major bands of nanosized R-Fe2O3 shift to higher wavenumbers (red-shifted) relative to microsized R-Fe2O3. For instance, the Fe-O (Eg) symmetric bending mode occurs at 402 and 406 cm-1 for R-Fe2O3 MRs and NRs, respectively. In addition, the Fe-O (A1g) symmetric stretching mode locates at 217 cm-1, 487 cm-1 and 220 cm-1, 492 cm-1 for R-Fe2O3 MRs and NRs, respectively. Compared to XRD, Raman spectroscopy is more sensitive to the slight change of metal-O bond lengths as well

as lattice perturbations in the local structure. It has been reported that a metal-O bond length can be correlated to its Raman stretching frequency, with higher stretching frequencies corresponding to the shorter metal-O bond lengths.75,76 The variations of local structure could be critical to the photocatalytic performance, as discussed in section 3.2. The optical properties of the as-prepared samples were investigated by DRUV-vis. Parts c and d of Figure 1 are the absorption and reflectance spectra for R-Fe2O3 and R-FeOOH MRs and NRs, respectively. Lian et al.77 reported that optical properties are shape- and size-dependent. Figure 1c,d shows that R-Fe2O3 MRs and NRs almost share the same features of absorption, and R-FeOOH MRs and NRs almost share the same absorption bands. The absorption features for R-Fe2O3 MRs and NRs are different from R-FeOOH MRs and NRs. For instance, R-Fe2O3 dispalys a broad band located at ca. 533 nm, while R-FeOOH exhibits a broad band around 490 nm. He et al.78 reported that three types of electronic transitions are present in the optical absorption for Fe3+ substances, such as the Fe3+ ligand field transition or the d-d transitions, the ligand to metal charge-transfer transitions, and the pair excitations resulting from the simultaneous excitations of two neighboring Fe3+ cations that are magnetically coupled. The variations in the electronic

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Figure 2. High-resolution XPS spectra: (a) Fe 2p core-level, (b) O 1s core-level for R-FeOOH NRs and MRs, and (c) O 1s core-level for R-Fe2O3 NRs and MRs. The O1 s spectra were curve-fitted following the procedure described elsewhere.66

structures could lead to enhanced delocalization of photogenerated electron and hole pairs and the mobility of holes in particular.75 The oxidation states of the as-prepared samples are revealed in high-resolution XPS (Figure 2). Figure 2a shows the Fe 2p core level spectra. The nature of Fe 2p peak broading is very complicated, resulting from electrostatic interactions between photoionized Fe 2p core hole and unpaired Fe 3d electrons, spin-orbit coupling, and crystal field interactions.79 Additional

Zhou et al. satellite peaks are attributed to charge transfer and/or to shakeup processes.79 For MRs and NRs, Fe 2p XPS spectra are very similar, independent of particle sizes and samples. The centroids of Fe 2p3/2 peaks are located at 710.8 (R-Fe2O3) and 711.2 eV (R-FeOOH), respectively, and the centroids of Fe 2p1/2 peaks are located at 724.4 (R-Fe2O3) and 724.8 eV (R-FeOOH), respectively. The binding energies for Fe 2p core levels agree well with the values reported in the literature.79,80 The line shape for O 1s is dramatically different among R-Fe2O3 and R-FeOOH particles. As shown in Figure 2b,c, at least four peaks are seen in the O 1s region. For R-Fe2O3 particles, the peak at 530.0 eV is ascribed to the lattice oxygen binding with Fe (denoted as Fe-O). The other three O 1s peaks located at higher binding energies can be assigned as lattice hydroxyl (Fe-OHlattice), adsorbed hydroxyl (Fe-OHad), and water (H2O). For R-FeOOH particles, these four peaks in the O 1s region are located at 530.1, 531.3, 532.2, and 533.4 eV, respectively. The binding energies for O 1s core levels for both R-Fe2O3 and R-FeOOH particles agree well with the values previously published elsewhere.79,80 The analyses from XRD, Raman, and XPS demonstrate that R-FeOOH was completely transformed to R-Fe2O3. No other iron oxides, such as magnetite or maghemite, were present. Nitrogen adsorption/desorption isotherms and BJH pore size distributions of the as-prepared porous R-Fe2O3 MRs and NRs are displayed in Figure 3. The isotherms of both R-Fe2O3 MRs and NRs display type IV curves with a hysteresis loop, indicating a mesoporous structure. The pore size distribution curves for R-Fe2O3 MRs and NRs exhibited a maxima at ca. 3.8 and 2.0 nm, respectively. However, the line shape of pore size distribution curve for R-Fe2O3 NRs is dramatically different from that of R-Fe2O3 MRs. The large pores with a maximum at ca. 23 nm for R-Fe2O3 NRs could be attributed to the interparticle space, which is often observed in other porous metal oxides.81 The porous structures possibly provide more accessible active sites and enhanced catalytic activity. The physical parameters, including the BET specific surface area of the asprepared samples are displayed in Table 1. Table 1 shows that the surface area is increased with decreasing particle size, and R-Fe2O3 particles with porosity have greater surface area than the corresponding nonporous R-FeOOH precursors. The morphology and crystallinity of the as-prepared samples were characterized using TEM and high-resolution TEM. For the as-prepared R-FeOOH MRs, the TEM image in Figure 4a shows that the majority of MRs exhibit faceted ends with a needle shape. Statistical analysis of rod dimensional distributions (shown in parts c and d of Figure 4) based on TEM indicates that the length and width are 1.55 ( 0.29 and 0.12 ( 0.023 µm, respectively. High-resolution TEM image (Figure 4b) and FFT analysis (inset of Figure 4b) indicate that the lattice fringes are 0.25, 0.50, and 0.25 nm, which is well ascribed to the R-FeOOH (101), (020), and (021) planes. For R-Fe2O3 MRs, the TEM image (Figure 4e) shows the general morphology and dimensions are similar to those of the R-FeOOH MRs precursor. A high-resolution TEM image (Figure 4f) displays variations in contrast that reveal the internal porosity.82 Nanoscale porosity is often generated in that the nanopore microstructures serve as diffusion paths for water in the process of dehydration.83 In addition, the high-resolution TEM image shows porous nanostructures with an average size of ca. 4.0 nm, consistent with the pore size distribution analysis. In addition, the FFT patterns (inset of Figure 4f) display lattice fringes of 0.25 nm, which are consistent with (110), (1j20), and (2j10) planes of R-Fe2O3. A close look into this particle reveals that lattice fringes continuously span the entire structure, indicating single-domain



Figure 3. (a) Nitrogen adsorption and desorption isotherms measured at 77 K for the as-prepared R-Fe2O3 MRs. (b) BJH pore size distribution for R-Fe2O3 MRs. (c) Nitrogen adsorption and desorption isotherms measured at 77 K for the as-prepared R-Fe2O3 NRs. (d) BJH pore size distribution for R-Fe2O3 NRs.

TABLE 1: Physical Parameters of the As-Prepared Iron Oxides and Observed Raman Stretching Frequencies for Fe-O or Fe-OH Bonds samples

average length average width pore size BET surface ν (nm) (nm) (nm) area (m2/g) (cm-1)

R-Fe2O3 MRs

1206 ( 466

90 ( 46

3.8

70

R-Fe2O3 NRs

40 ( 9

11 ( 6

2.2

93

1550 ( 288 48 ( 9

120 ( 23 12 ( 4

R-FeOOH MRs R-FeOOH NRs

42 85

217 487 220 492 384 384

crystallinity. The current results suggest that except the porosity incorporated into R-Fe2O3 crystalline regions, the shape anisotropy and single-domain crystalline nature of R-Fe2O3 nanostructures are reminiscent of 2D assembly of R-FeOOH NRs by the OA mechanism in aqueous media. The details of selfassembly from R-FeOOH nanoparticles to nanorods, including pH, temperature, and aging time-dependent oriented attachment process, have been published elsewhere.65 For the as-prepared R-FeOOH NRs, the TEM image in Figure 5a shows that the majority of NRs exhibit faceted ends with a needle shape. Statistical analysis of nanorod dimensional distributions (shown in parts c and d of Figure 5) based on TEM indicates that the length and width is 47.53 ( 0.93 and 12.28 ( 0.33 nm, respectively. The high-resolution TEM image (Figure 5b) and FFT analysis (inset of Figure 5b) indicate that the lattice fringe is 0.41 nm, which is well ascribed to the

R-FeOOH (110) plane. For R-Fe2O3 NRs, the TEM image (Figure 4e) shows, except for porosity, the general morphology and dimensions are similar to those of the R-FeOOH NRs precursor. In addition, the high-resolution TEM image (Figure 5f) shows porous nanostructures with an average size of ca. 2.0 nm, consistent with the pore size distribution analysis. More information, including the HAADF-STEM image, is shown in Figure S1 of the Supporting Information. The FFT patterns (inset of Figure 5f) display lattice fringes of 0.25 nm, which are consistent with (110), (1j20), and (2j10) planes of R-Fe2O3. A close look at this particle reveals that lattice fringes continuously span the entire particle, indicating single-domain crystallinity. 3.2. Photocatalytic Properties. The photocatalytic activity of the as-prepared samples was evaluated for RhB degradation under visible light illumination in the presence of H2O2 additive. All experiments were conducted under the following conditions: RhB concentration 2 × 10-5 M, catalyst concentration 0.2 g/L, and H2O2 additive 50 mM. The temporal UV-vis spectral changes of RhB aqueous solutions in the process of photodegradation are displayed in Figure 6a-d. Figure 6 shows that with increasing irradiation time, the major absorbance in the visible and UV regions decreased and the positions of major absorbance were shifted to low wavenumbers, suggesting that both chromophores and aromatic rings of RhB have been destroyed, instead of being simply decolorized by adsorption.84,85 To evaluate the reactivity of iron oxide particles quantitatively, the

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Figure 4. Representative morphologies and structures of R-FeOOH MRs: (a) TEM image of R-FeOOH MRs; (b) high-resolution TEM image of a R-FeOOH MR (inset: FFT analysis); (c) length distribution; (d) width distribution; (e) TEM image of R-Fe2O3 MRs; (f) high-resolution TEM image of a R-Fe2O3 MR with apparent porosity (inset: FFT analysis).



Figure 5. Representative morphologies and structures of R-FeOOH NRs: (a) TEM image of R-FeOOH NRs; (b) high-resolution TEM image of a R-FeOOH NR (inset: FFT analysis); (c) length distribution histograms; (d) width distribution histograms; (e) TEM image of R-Fe2O3 NRs; (f) high-resolution TEM image of a R-Fe2O3 NR with apparent porosity (inset: FFT analysis).

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Figure 6. Photocatalytic degradation of RhB on iron oxides under visible-light illumination in the presence of H2O2. (a-d) UV-vis spectral changes of RhB aqueous solutions as a function of irradiation time in the presence of iron oxides and H2O2 additive. (e) The change of RhB concentration over various iron oxides as a function of irradiation time. The blank one is photolysis in the presence of H2O2 only. Reaction conditions: RhB concentration 2 × 10-5 M; catalyst concentration 0.2 g/L; H2O2 molar concentration 50 mM; 300 W Xe lamp (λ > 420 nm).

apparent reaction rates of RhB degradation were calculated and the results are summarized in Table 2. For the blank experiment, in the presence of H2O2 without catalysts, RhB degradation under visible light illumination is relatively slow, with an apparent reaction rate k ) 1.67 × 10-3 min-1. With both catalysts and H2O2 additive under the same experimental conditions, the rate of RhB degradation is significantly increased. To compare the intrinsic catalytic activity, k was normalized to

the specific surface area, referred to ks.86 Figure 6e and Table 2 illustrate that at equivalent catalyst mass loadings, nanoscale iron oxides display a greater rate than microscale iron oxides, and R-Fe2O3 NRs exhibit the greatest photoactivity (with apparent reaction rate k ) 1.48 × 10-2 min-1). When normalized to the BET specific surface area, the reaction rate (ks ) 7.96 × 10-3 min-1 L m-2) exhibited by R-Fe2O3 NRs is about 1 order of magnitude faster than that of R-Fe2O3 MRs (ks ) 6.97 ×



TABLE 2: Rate Constants of RhB Degradation on Iron Oxides Based on Pseudo-First-Order Kinetic Model and Corresponding Regression Coefficients r2 and Rate Constants Normalized to BET Specific Surface Area (SSA) reaction rate k (min-1)

samples photolysis without catalyst R-Fe2O3 MRs R-Fe2O3 NRs R-FeOOH MRs R-FeOOH NRs a

km ks (min-1 L g-1) (min-1 L m-2)

0.00167 (0.99) 0.00977 (0.98) 0.148 (0.98) 0.0195 (0.98) 0.0761 (0.99)

0.0488 0.740 0.0975 0.380

6.97 × 10-4 7.96 × 10-3 2.32 × 10-3 4.47 × 10-3

ks ) km/SSA, where km ) k/catalyst concentration.

10-4 min-1 L m-2), implying the difference in reactivity far exceeds that expected from simple consideration of surface area, as reported in other systems.87 The structural stability of the as-prepared iron oxide photocatalysts before and after photocatalysis reaction was examined by micro-Raman spectroscopy (data shown in Figure S2 in the Supporting Information). Figure S2 shows that the structures of catalysts used in the current study are stable upon irradiation. It has been suggested that dye degradation by H2O2 promotion mainly occurs on the surface of iron oxide photocatalysts rather than in solution bulk.40,41 The dual effects by H2O2 in enhancing the photocatalytic performance of catalysts is ascribed to either conduction electron scavenging or the Fenton-like reaction.40 In general, minority charge carriers such as electrons and holes are generated upon visible light illumination. There exist two pathways for the annihilation of electrons involving various electron transfer processes. First, electrons are directly trapped by H2O2 to form OH• radicals R-Fe2O3 f R-Fe2O3(e-, h+);

R-FeOOH f R-FeOOH(e-, h+)

(1) H2O2 + e- f OH- + OH•

(2)

Alternatively, the electrons can be trapped by Fe3+ on the catalyst surface

Fe3+ + e- f Fe2+

(3)

Fe2+ + H2O2 f Fe3+ + OH- + OH•

(4)

As a result, OH• radicals resulting from the above two pathways lead to the photocatalytic reaction. Bahnemann88 proposed that physical properties of a photocatalyst such as crystal structure, surface area, size distribution, porosity, and band gap determine its reactivity. As to our current experimental results for the enhanced reactivity toward RhB by NRs relative to MRs in both R-FeOOH and R-Fe2O3, a number of explanations can be envisioned. First, differences in the photocatalytic performance are not primarily caused by differences in intrinsic band gaps, given the fact that the intrinsic band gaps for R-FeOOH and R-Fe2O3 are essentially very close. Also as evidenced by Figure 1c,d, for both R-FeOOH and R-Fe2O3, NRs and MRs share almost identical optical absorption bands. As shown in Figure 1b and Table 1, major Raman bands of nanosized R-Fe2O3 shift to higher frequencies (red shifts) compared to microsized R-Fe2O3, including Raman stretching

frequencies that sensitively reflect the metal-O bond lengths, with the higher stretching frequencies corresponding to the shorter metal-O bond lengths.75 The variations of local structure could influence the photocatalytic performance of iron oxide particles. Yu and Kudo75 studied the photocatalytic properties of BiVO3 for oxygen evolution from aqueous AgNO3 solution under visible light irradiation and found that the initial rate of O2 evolution is proportional to the Raman V-O bond stretching frequencies. On the basis of an empirical correlation developed by Hardcastle and Wachs,76 Yu and Kudo correlated higher V-O bond stretching frequencies with shorter V-O bond lengths and proposed that shorter V-O bond lengths lead to greater mobility of photogenerated holes as well as greater photocatalytic activity.75 Analogously, the observed Raman higher Fe-O stretching frequencies for R-Fe2O3 NRs relative to R-Fe2O3 MRs are in part responsible for the greater photocatalytic performance on R-Fe2O3 NRs. It is well-known that the metal-O vibrations could be influenced by many other factors, including the confinement of optical phonons. The phonon confinement can cause the shift of the Raman peaks. For instance, for CuO nanocrystals at room temperature, as the grain size decreases from 100 to 10 nm, the Raman peaks shift to lower frequencies (blue shifts).89 In the current study, to determine Raman peak shifts by the phonon confinement and/ or bond length changes quantitatively, further theoretical work is needed. Another aspect in different photocatalytic activity can be ascribed to the differences in the relative amount of crystal faces present on NRs and MRs. This is the inherent size effect for anisotropically shaped 1D rods. For example, high index planes such as {021} often are apex facets of needle-shaped R-FeOOH rods (Figure S3 in the Supporting Information).65 The relative proportions of high index facets vs low index facets vary from R-FeOOH NRs to MRs, with NRs presenting more high index {021} facets than MRs. Using atomic force microscopy, Gaboriaud et al.90 showed that the relative area of R-FeOOH crystal surfaces can change substantially as a result of decreasing particle size. Molecular simulations by Rustad et al.91 suggested that the degree of protonation of the {021} facets is influenced by R-FeOOH particle size. Thus, it is reasonable to hypothesize that for R-FeOOH, {021} facets play a key role in the photodegradation of RhB under visible light illumination. In this work, R-Fe2O3 rods were prepared by thermal dehydration of respective R-FeOOH precursors via a topotactic transformation.66 Except for porosity incorporated into R-Fe2O3 crystalline region, the shape anisotropy of R-Fe2O3 is reminiscent of R-FeOOH precursors. Variations of relative proportions of high index facets vs low index facets between R-FeOOH NRs and MRs inherently lead to differences in the relative amount of crystal faces present on R-Fe2O3 NRs and MRs and, consequently, result in different photocatalytic properties between R-Fe2O3 NRs and MRs. Moreover, the line shape of pore size distributions shown in parts b and d of Figure 3 is significantly different between R-Fe2O3 MRs and NRs and, in turn, could lead to different photoactivity. Ideal pore size distributions facilitate transport of catalytic reactants and products from the surface, and promote photocatalytic performance. In summary, the observed catalytic performance from the current study demonstrates the synergetic effects of particle composition, size, porosity, and local structure.81,92 It is well-known that in aqueous nanoparticulate suspensions, particle aggregates could influence the surface chemistry and/ or surface reactivity, and the specific surface area determined from dry samples could not exactly represent the real reactive

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Zhou et al. by iron oxide particles. In addition, our findings will be potentially applicable to many other important semiconducting minerals and compounds (e.g., metal oxyhydroxides and metal oxides). Acknowledgment. The authors gratefully acknowledges the financial support of this work from National Center for Nanoscience and Technology, Beijing, China, and the National Basic Research Program of China (Grant No. 2007CB936802). Note Added after ASAP Publication. This article was published ASAP on September 7, 2010. Content of the Abstract has been modified. The correct version was published on September 13, 2010. Supporting Information Available: High-resolution TEM image and corresponding HAADF-STEM image of porous R-Fe2O3 NRs, micro-Raman spectra for 1D iron oxide particles before and after photocatalytic reaction, and schematic diagram of a needle shaped goethite nanorod. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 7. Intensity-normalized particle size distribution obtained from DLS measurements performed on suspensions of iron oxide particles that have been equilibrated with RhB prior to illumination with light: (a) R-Fe2O3 MRs and NRs; (b) R-FeOOH MRs and NRs. Suspensions analyzed with RhB concentration 2 × 10-5 M and catalyst concentration 0.2 g/L.

surface area in suspensions.87,93 It has been suggested that the size of nonspherical particles is related to the hypothetical hard sphere that would behave the same as the particles measured by DLS.93 In the current study, DLS measurements were carried out immediately after iron oxide particles have been equilibrated with RhB in solutions. Particle size distributions displayed in Figure 7 show that in addition to larger aggregates, relatively smaller aggregates are retained for both R-FeOOH NRs and R-Fe2O3 NRs prior to light illumination, distinct from microscale rods that exhibit only larger aggregates. These intrinsic smaller aggregates could play an essential role in enhanced photocatalyic performance. The precise role by individual particles or aggregates as a whole needs further systematic investigation. 4. Conclusions Visible-light photocatalytic activity toward RhB in the presence of H2O2 by rodlike R-FeOOH and R-Fe2O3 is composition-, size-, porosity-, and local structure-dependent, following the order R-Fe2O3 NRs > R-FeOOH NRs > R-FeOOH MRs > R-Fe2O3 MRs. Nanoscale R-FeOOH and R-Fe2O3 catalysts appeared to be more active than the micrometer-sized ones in terms of surface areanormalized reaction rate, suggesting unique photocatalytic properties of nanorods as compared to microrods. The results from current study shed new light on synergetic effects of particle composition, size, porosity, and the variations of local structure on photocatalysis

(1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Goodman, D. W. J. Catal. 2003, 216, 213. (3) Goodman, D. W. Nature 2008, 454, 948. (4) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (5) Chen, M. S.; Goodman, D. W. Chem. Soc. ReV. 2008, 37, 1860. (6) Rodriguez, J. A.; Liu, G.; Jirsak, T.; Hrbek, J.; Chang, Z.; Dvorak, J.; Maiti, A. J. Am. Chem. Soc. 2002, 124, 5242. (7) Rodriguez, J. A.; Liu, P.; Takahashi, Y.; Nakamura, K.; Vines, F.; Illas, F. J. Am. Chem. Soc. 2009, 131, 8595. (8) Rodriguez, J. A.; Evans, J.; Graciani, J.; Park, J.; Liu, P.; Hrbek, J.; Sanz, J. F. J. Phys. Chem. C 2009, 113, 7364. (9) Cornell, R. M.; Schwertmann, U. The iron oxides: structure, properties, reactions, occurrences, and uses, 2nd ed.; Wiley-VCH: Weinheim, 2003. (10) Hochella, M. F.; Lower, S. K.; Maurice, P. A.; Penn, R. L.; Sahai, N.; Sparks, D. L.; Twining, B. S. Science 2008, 319, 1631. (11) Navrotsky, A.; Mazeina, L.; Majzlan, J. Science 2008, 319, 1635. (12) Waychunas, G. A.; Kim, C. S.; Banfield, J. F. J. Nanopart. Res. 2005, 7, 409. (13) Wirnsberger, G.; Gatterer, K.; Fritzer, H. P.; Grogger, W.; Pillep, B.; Behrens, P.; Hansen, M. F.; Koch, C. B. Chem. Mater. 2001, 13, 1453. (14) Lemaire, B. J.; Davidson, P.; Ferre´, J.; Jamet, J. P.; Panine, P.; Dozov, I.; Jolivet, J. P. Phys. ReV. Lett. 2002, 88, 125507. (15) Anschutz, A. J.; Penn, R. L. Geochem. Trans. 2005, 6, 60. (16) Chun, C. L.; Penn, R. L.; Arnold, W. A. EnViron. Sci. Technol. 2006, 40, 3299. (17) Cesar, I.; Kay, A.; Martinez, J. A. G.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 4582. (18) Ohmori, T.; Takahashi, H.; Mametsuka, H.; Suzuki, E. Phys. Chem. Chem. Phys. 2000, 2, 3519. (19) Gou, X.; Wang, G.; Park, J.; Liu, H.; Yang, J. Nanotechnology 2008, 19, 125606. (20) Zhou, H.; Wong, S. S. ACS Nano 2008, 2, 944. (21) Zitoun, D.; Pinna, N.; Frolet, N.; Belin, C. J. Am. Chem. Soc. 2005, 127, 15034. (22) Park, S. J.; Kim, S.; Lee, S.; Khim, Z. G.; Char, K.; Hyeon, T. J. Am. Chem. Soc. 2000, 122, 8581. (23) Tang, B.; Wang, G.; Zhuo, L.; Ge, J.; Cui, L. Inorg. Chem. 2006, 45, 5196. (24) Wu, J.; Lee, Y. L.; Chiang, H.-H.; Wong, D. K.-P. J. Phys. Chem. B 2006, 110, 18108. (25) Liu, L.; Kou, H. Z.; Mo, W.; Liu, H.; Wang, Y. J. Phys. Chem. B 2006, 110, 15218. (26) Gou, X.; Wang, G.; Kong, X.; Wexler, D.; Horvat, J.; Yang, J.; Park, J. Chem.sEur. J. 2008, 14, 5996. (27) Jagadeesan, D.; Mansoori, U.; Mandal, P.; Sundaresan, A.; Eswaramoorthy, M. Angew. Chem., Int. Ed. 2008, 47, 7685. (28) Madden, A. S.; M. F. Hochella, J. Geochim. Cosmochim. Acta 2005, 69, 389. (29) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (30) Cwiertny, D. M.; Young, M. A.; Grassian, V. H. Annu. ReV. Phys. Chem. 2008, 59, 27.

Photocatalytic Degradation of Rhodamine B (31) Rajeshwar, K.; Osugi, M. E.; Chanmanee, W.; Chenthamarakshan, C. R.; Zanoni, M. V. B.; Kajitvichyanukul, P.; Krishnan-Ayer, R. J. Photochem. Photobiol., C 2008, 9, 171. (32) Zhang, H.; Chen, G.; Bahnemann, D. W. J. Mater. Chem. 2009, 19, 5089. (33) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (34) Rajeshwar, K.; Tacconi, N. R. d. Chem. Soc. ReV. 2009, 38, 1984. (35) Wang, L.; Wei, H.; Fan, Y.; Gu, X.; Zhan, J. J. Phys. Chem. C 2009, 113, 14119. (36) Liu, C.; Li, F.; Li, X.; Zhang, G.; Kuang, Y. J. Mol. Catal. A: Chem. 2006, 252, 40. (37) Bandara, J.; Mielczarski, J. A.; Kiwi, J. Langmuir 1999, 15, 7670. (38) Bandara, J.; Mielczarski, J. A.; Kiwi, J. Langmuir 1999, 15, 7680. (39) Feng, J.; Hu, X.; Yue, P. L. EnViron. Sci. Technol. 2004, 38, 5773. (40) Du, W.; Xu, Y.; Wang, Y. Langmuir 2008, 24, 175. (41) Wang, Y.; Du, W.; Xu, Y. Langmuir 2009, 25, 2895. (42) Kontos, A. I.; Likodimos, V.; Stergiopoulos, T.; Tsoukleris, D. S.; Falaras, P.; Rabias, I.; Papavassiliou, G.; Kim, D.; Kunze, J.; Schmuki, P. Chem. Mater. 2009, 21, 662. (43) Buonsanti, R.; Grillo, V.; Carlino, E.; Giannini, C.; Gozzo, F.; Garcia-Hernandez, M.; Garcia, M. A.; Cingolani, R.; Cozzoli, P. D. J. Am. Chem. Soc. 2010, 132, 2437. (44) Morales, W.; Cason, M.; Aina, O.; Tacconi, N. R. d.; Rajeshwar, K. J. Am. Chem. Soc. 2008, 130, 6318. (45) Chen, D.; Ye, J. AdV. Funct. Mater. 2008, 18, 1922. (46) Watcharenwong, A.; Chanmanee, W.; Tacconi, N. R. d.; Chenthamarakshan, C. R.; Kajitvichyanukul, P.; Rajeshwar, K. J. Electrochem. Soc. 2008, 612, 112. (47) Tang, J.; Zou, Z.; Ye, J. Chem. Mater. 2004, 16, 1644. (48) Tang, J.; Zou, Z.; Ye, J. J. Phys. Chem. C 2007, 111, 12779. (49) Luan, J.; Pan, B.; Paz, Y.; Li, Y.; Wu, X.; Zou, Z. Phys. Chem. Chem. Phys. 2009, 11, 6289. (50) Leland, J. K.; Bard, A. J. J. Phys. Chem. 1987, 91, 5076. (51) Dieckmann, R. Philos. Mag. A 1993, 68, 725. (52) Glotzer, S. C.; Solomon, M. J. Nat. Mater. 2007, 6, 557. (53) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664. (54) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (55) Niederberger, M.; Co¨lfen, H. Phys. Chem. Chem. Phys. 2006, 8, 3271. (56) Ribeiro, C.; Lee, E. J. H.; Longo, E.; Leite, E. R. ChemPhysChem 2006, 7, 664. (57) Jun, Y.-W.; Choi, J.-S.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (58) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. ReV. 2006, 35, 1195. (59) Kumar, S.; Nann, T. Small 2006, 3, 316. (60) Meldrum, F. C.; Co¨lfen, H. Chem. ReV. 2008, 108, 4332. (61) Zhang, Q.; Liu, S.-J.; Yu, S.-H. J. Mater. Chem. 2009, 19, 191. (62) Vayssieres, L.; Sathe, C.; Butorin, S. M.; Shuh, D. K.; Nordgren, J.; Guo, J. AdV. Mater. 2007, 17, 2320. (63) Mohapatra, S. K.; Banerjee, S.; Misra, M. Nanotechnology 2008, 19 (7), 315601. (64) Mohapatra, S. K.; John, S. E.; Banerjee, S.; Misra, M. Chem. Mater. 2009, 21, 3048. (65) Mao, X. B.; Yang, H. C.; Zhou, X. M.; Wang, C. X.; Wang, Y. S.; Yang, Y. L.; Wang, C.; Liu, G. Cryst. Growth Des. 2010, 10, 504.

J. Phys. Chem. C, Vol. 114, No. 40, 2010 17061 (66) Yang, H. C.; Mao, X. B.; Guo, Y. J.; Wang, D. W.; Ge, G.; Yang, R.; Qiu, X.; Yang, Y. L.; Wang, C.; Wang, Y. S.; Liu, G. CrystEngComm 2010, 12, 1842. (67) Ke, T. Y.; Peng, C. W.; Lee, C. Y.; Chiu, H. T.; Sheu, H. S. CrystEngComm 2009, 11, 1691. (68) Burleson, D. J.; Penn, R. L. Langmuir 2006, 22, 402. (69) Michel, F. M.; Ehm, L.; Antao, S. M.; Lee, P. L.; Chupas, P. J.; Liu, G.; Strongin, D. R.; Schoonen, M. A. A.; Phiillips, B. L.; Parise, J. B. Science 2007, 316, 1726. (70) Michel, F. M.; Ehm, L.; Liu, G.; Han, W. Q.; Antao, S. M.; Chupas, P. J.; Lee, P. L.; Knorr, K.; Eulert, H.; Kim, J.; Grey, C. P.; Celestian, A. J.; Gillow, J.; Schoonen, M. A. A.; Strongin, D. R.; Parise, J. B. Chem. Mater. 2007, 19, 1489. (71) Liu, G.; Debnath, S.; Paul, K. W.; Han, W.; Hausner, D. B.; Hosein, H.-A.; Michel, F. M.; Parise, J. B.; Sparks, D. L.; Strongin, D. R. Langmuir 2006, 22, 9313. (72) Froment, F.; Tournie´, A.; Colomban, P. J. Raman Spectrosc. 2008, 39, 560. (73) Bersani, D.; Lottici, P. P.; Montenero, A. J. Raman Spectrosc. 1999, 30, 355. (74) Parras-Guijarro, D.; Montejo-Ga´mez, M.; Ramos-Martos, N.; Sa´nchez, A. Spectrochim. Acta, Part A 2006, 64, 1133. (75) Yu, J. Q.; Kudo, A. AdV. Funct. Mater. 2006, 16, 2163. (76) Hardcastle, F. D.; Wachs, I. E. J. Phys. Chem. 1991, 95, 5031. (77) Lian, J.; Duan, X.; Ma, J.; Peng, P.; Kim, T.; Zheng, W. ACS Nano 2009, 3, 3749. (78) He, Y. P.; Miao, Y. M.; Li, C. R.; Wang, S. Q.; Cao, L.; Xie, S. S.; Yang, G. Z.; Zou, B. S. Phys. ReV. B 2005, 71 (9), 125411. (79) Baltrusaitis, J.; Cwiertny, D. M.; Grassian, V. H. Phys. Chem. Chem. Phys. 2007, 9, 5542. (80) Hsu, L. C.; Li, Y. Y.; Lo, C. G.; Huang, C. W.; Chern, G. J. Phys. D: Appl. Phys. 2008, 41, 185003. (81) Yue, W.; Randorn, C.; Attidekou, P. S.; Su, Z.; Irvine, J. T. S.; Zhou, W. AdV. Funct. Mater. 2009, 19, 1. (82) Penn, R. L.; Oskam, G.; Strathmann, T. J.; Searson, P. C.; Stone, A. T.; Veblen, D. R. J. Phys. Chem. B 2001, 105, 2177. (83) Lo¨ffler, L.; Mader, W. J. Eur. Ceram. Soc. 2006, 26, 131. (84) Li, X.; Kikugawa, N.; Ye, J. Chem.sEur. J. 2009, 15, 3538. (85) Li, X.; Kikugawa, N.; Ye, J. AdV. Mater. 2008, 20, 3816. (86) Hanna, K.; Kone, T.; Medjahdi, G. Catal. Commun. 2008, 9, 955. (87) Cwiertny, D. M.; Hunter, G. J.; Pettibone, J. M.; Scherer, M. M.; Grassian, V. H. J. Phys. Chem. C 2009, 113, 2175. (88) Bahnemann, D. W. In Photochemical ConVersion and Storage of Solar Energy; Pelizzetti, E., Schiavello, M., Eds.; Kluwer Academic Publishers: Dordrecht, 1991; p 251. (89) Xu, J. F.; Ji, W.; Shen, Z. X.; Li, W. S.; Tang, S. H.; Ye, X. R.; Jia, D. Z.; Xin, X. Q. J. Raman Spectrosc. 1999, 30, 413. (90) Gaboriaud, F.; Ehrhardt, J.-J. Geochim. Cosmochim. Acta 2003, 67, 967. (91) Rustad, J. R.; Felmy, A. R. Geochim. Cosmochim. Acta 2005, 69, 1405. (92) Zhang, D.; Yang, D.; Zhang, H.; Lu, C.; Qi, L. Chem. Mater. 2006, 18, 3477. (93) Grassian, V. H. J. Phys. Chem. C 2008, 112, 18303.

JP103816E

Visible Light Induced Photocatalytic Degradation of Rhodamine B

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