Effect of Fluorination on Photocatalytic Degradation of Rhodamine B

Dec 9, 2010 - Shunwei Hu, Jia Zhu, Ling Wu, Xuxu Wang, Ping Liu, Yongfan Zhang,* and Zhaohui Li*. Research Institute of Photocatalysis, Fuzhou ...
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J. Phys. Chem. C 2011, 115, 460–467

Effect of Fluorination on Photocatalytic Degradation of Rhodamine B over In(OH)ySz: Promotion or Suppression? Shunwei Hu, Jia Zhu, Ling Wu, Xuxu Wang, Ping Liu, Yongfan Zhang,* and Zhaohui Li* Research Institute of Photocatalysis, Fuzhou UniVersity, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou 350002, People’s Republic of China ReceiVed: October 6, 2010; ReVised Manuscript ReceiVed: NoVember 21, 2010

Fluorinated In(OH)ySz solid solutions were synthesized via a hydrothermal method. The photocatalytic degradation of rhodamine B (RhB) over the fluorinated In(OH)ySz solid solutions revealed that the process of the de-ethylation of RhB was enhanced, while in the meantime, the rate of the mineralization of RhB was suppressed when In(OH)ySz was fluorinated. To elucidate the opposite effects of fluorination on the two processes of photocatalytic degradation of RhB (de-ethylation and mineralization) over In(OH)ySz, the adsorption of RhB over In(OH)ySz with and without fluorination, the active species involved in the de-ethylation and the mineralization of RhB, were investigated, and the electronic structure of fluorinated In(OH)ySz was studied. The results indicated that the major active species involved in the de-ethylation and the mineralization of RhB are different, that is, the hole direct oxidation leads to the de-ethylation of RhB, while the major active species responsible for the mineralization of RhB is the O2-• radical. The fluorination on In(OH)ySz solid solutions on one hand can enhance the adsorption of RhB on the catalyst surface, leading to a higher rate in the de-ethylation of RhB. On the other hand, fluorination lowers the conduction band of In(OH)ySz, which leads to a lower capability of the photogenerated electrons in the reduction of O2 to give O2-• radicals. Our results clearly show that the fluorine modifications can have ambilateral effects on the photocatalytic performance of semiconductor photocatalysts depending on the specific photocatalytic reactions. 1. Introduction The effects of fluorine modifications on TiO2, including surface modification and lattice fluorine doping, on its photocatalytic performance has been well documented.1-4 Surface fluorination on TiO2 by a simple ligand exchange between Fions and surface hydroxyl groups is believed to enhance the photocatalytic activity due to the generation of the mobile free • OH radicals.5-11 A typical example of this reported by Xu et al. is that the photocatalytic activity for the degradation of phenol can be significantly improved when NaF is added into the suspension of TiO2.12 The promoting effects of the lattice Fdopants on the photocatalytic performance of TiO2 have also been reported but with more diverse interpretations. Yu et al. reported that doping TiO2 with F- ions can convert part of lattice Ti4+ to Ti3+ by charge compensation, and the existence of a certain amount of Ti3+ can suppress the electron-hole recombination rate, leading to enhanced photocatalytic activity for the degradation of acetone.13 Ohashi et al. demonstrated a high visible light photocatalytic activity for the decompositions of both acetaldehyde and trichloroethylene over F--doped TiO2 powders due to the creation of oxygen vacancies in TiO2 lattice upon fluorination.14 The influence of fluorination on the physicochemical properties of TiO2 has also been revealed. For example, Hattori et al. reported that the fluorination on TiO2 improved the crystallinity of anatase, and the resultant TiO2 showed a higher activity in the degradation of acetaldehyde.15 Vijayabalan et al. reported that the fluorination enhanced the absorbance of TiO2 in the UV-vis region, thus leading to enhanced photocatalytic activity for the degradation of reactive * To whom correspondence should be addressed. Tel/Fax: 86-59122866156. E-mail: [email protected] (Y.Z.). Tel/Fax: 86-591-83779105. E-mail: [email protected] (Z.L.).

orange.16 Yu et al. found that TiO2 obtained by adding HF during the preparation process showed enhanced photocatalytic activity due to the formation of TiO2 with dominated {001} facets.17 The fluorination was also reported to increase the surface acidity of TiO2.4 In addition to these, the effect of fluorination on the band structure of TiO2 has recently been revealed. Zhao et al. reported that the incorporation of fluorine into N-doped TiO2 enhanced the oxidative power of the photogenerated holes, while it lessened the reduction ability of the photogenerated electrons.18 The promising results obtained from the fluorine-modified TiO2 have also stimulated the studies on the fluorination of other non-TiO2 photocatalysts. F-doped SrTiO3 has been reported to show about three times the photocatalytic activity in the degradation of nitrogen monoxide as compared to that of undoped SrTiO3 due to the formation of oxygen vacancies.19 Zhu et al. also reported that the fluorine interstitially doped ZnWO4 showed enhanced photocatalytic activity for degradation of RhB and 4-CP. This promotion is due to the change of the coordination sphere around W atom in a WO6 octahedron in ZnWO4.20,21 Fluorination over ZnWO4 can induce the distortion of WO6 octahedron in ZnWO4 crystal, resulting in an increased transfer rate of photogenerated electrons to the photocatalyst surface. A similar promotion in the photocatalytic activity over F-doped Bi2WO6 was also reported recently.22 At all events, the promoting effect of fluorination on the photocatalytic performance seems to be well established. However, recently, we found that fluorination on semiconductor photocatalyst can have ambilateral effects on its photocatalytic performance. In(OH)ySz solid solution, a previously reported visible light-induced photocatalyst was modified via fluorination.23 The fluorinated In(OH)ySz solid solutions were found to enhance the process of de-ethylation of RhB, while suppressing the process of the destruction of the conjugated

10.1021/jp109578g  2011 American Chemical Society Published on Web 12/09/2010

Photocatalytic Degradation of RhB over In(OH)ySz structure in RhB (mineralization of RhB) during the photocatalytic degradation of RhB. The phenomenon that the fluorination on In(OH)ySz solid solution can have opposite effects on the two steps of a photocatalytic reaction is interesting. To explore the origins, the adsorption of RhB over fluorinated In(OH)ySz, the active species involved in the de-ethylation and the mineralization during photocatalytic degradation of RhB, was investigated, and the electronic structure of fluorinated In(OH)ySz solid solution was studied. The results indicated that the major active species involved in the de-ethylation and the mineralization of RhB during the degradation of RhB are different, that is, the direct hole oxidation leads to the de-ethylation of RhB, while the major active species responsible for the mineralization of RhB is the O2-• radical. The fluorination on In(OH)ySz on one hand can enhance the adsorption of RhB on the catalyst surface, leading to a higher rate in the de-ethylation of RhB. On the other hand, fluorination lowers the conduction band of In(OH)ySz, which leads to a lower capability of the photogenerated electrons in the reduction of O2 to give O2-• radicals. 2. Experimental Section 2.1. Materials and Reagents. Indium nitrate [In(NO3)3 · 4.5H2O], thiourea, ammomiun fluoride, disodium ethylene diaminetetraacetate (EDTANa2), isopropanol, and ethylenediamine were purchased from Shanghai Chemical Reagent Co. These reagents were analytical grade and used without further treatment. Rhodamine B (RhB) (laser grade) was purchased from Aladdin Reagent Co. 2.2. Preparations. Fluorinated In(OH)ySz solid solutions were prepared by a modified hydrothermal method following the previous report in the preparation of In(OH)ySz solid solution.23 A ratio of thiourea/In(NO3)3 was fixed at 2.0, and different molar ratios of NH4F/In(NO3)3 · 4.5H2O (RF) were chosen in the preparation of fluorinated In(OH)ySz since our preliminary study showed that In(OH)ySz solid solution prepared with a thiourea/In(NO3)3 · 4.5H2O ratio of 2.0 exhibited the highest visible light photocatalytic activity. In a typical preparation, In(NO3)3 · 4.5H2O (1.96 g, 5 mmol), thiourea (0.76 g, 10 mmol), and different amounts of ammomiun fluoride (0, 0.74, 1.48, and 3.7 g for RF ) 0, 2, 4, and 10, respectively) were dissolved in 60 mL of distilled water under continuous stirring at room temperature, 5 mL of ethylenediamine was added to the solutions, and the pH of the solution was adjusted to about 11. The solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and was heated at 180 °C for 24 h. The final product was collected by filtration and washed several times with distilled water and ethanol. Yellow powders were obtained after drying at 60 °C for 6 h. 2.3. Characterizations. X-ray diffraction (XRD) patterns were collected on a D8 Advance X-ray diffractometer (Bruker, Germany) with Cu KR radiation. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Data were recorded at a scanning rate of 0.004° 2θ s-1 in the 2θ range of 10-80°. It was used to identify the phase present and their crystallite size. The crystallite size was calculated from X-ray line broadening analysis by Scherer equation: D ) 0.89λ/β cos θ, where D is the crystal size in nm, λ is the CuKR wavelength (0.15406 nm), β is the half-width of the peak in rad, and θ is the corresponding diffraction angle. UV-visible absorption spectra (UV-DRS) of the powders were obtained for the dry-pressed disk samples using a Cary 500 Scan Spectrophotometer (Varian, United States). BaSO4 was used as a reflectance standard in the UV-visible diffuse reflectance experiment. BET surface area measurements were performed by a Quantachrome NOVA-4200E system (Micromeritics,

J. Phys. Chem. C, Vol. 115, No. 2, 2011 461 United States). After the samples were degassed in vacuum at 160 °C until the pressure lower than 10-6 Torr, the nitrogen adsorption and desorption isotherms were measured at -196 °C. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS system (PHI, United States) with a monochromatic Al KR source and a charge neutralizer. All of the binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. Electron spin resonance (ESR) spectra were obtained on a Bruker model ESP 300 E electron paramagnetic resonance spectrometer. Mott-Schottky plots were obtained with a ZENNIUM electrochemical workstation (Zahner, Germany) equipped with an impedance analyzer. 2.4. Adsorption of RhB. The adsorptions of RhB over different samples were measured in the dark by suspending the photocatalysts (50.0 mg) in 50.0 mL of aqueous solution of RhB (10-5 m, pH ) 6.3). During the period of adsorption, 3 mL aliquots were sampled every 5 min and centrifuged to remove the catalyst. The filtrates were analyzed on a Varian Cary 50 Scan UV-vis spectrophotometer (Varian) to record the concentration change of RhB. The wavelength for analysis is 554 nm. 2.5. Photocatalytic Degradation of RhB. The photocatalytic activity of fluorinated In(OH)ySz solid solutions was evaluated by the degradation of RhB under visible light irradiations. A 500 W tungsten halogen lamp was positioned inside a cylindrical Pyrex vessel and surrounded by a circulating water jacket (Pyrex) to cool the lamp. A cutoff filter was placed outside the Pyrex jacket to completely remove all wavelengths less than 420 nm to ensure the irradiations with visible light only. Eighty milligrams of photocatalyst was added into 80 mL of RhB solution (10-5 m). Prior to irradiations, the suspensions were magnetically stirred in the dark for ca. 4 h to ensure the establishment of an adsorption/desorption equilibrium. At given irradiation time intervals, 4 mL of the suspensions was collected, centrifuged, and filtered through a Millipore filter to separate the photocatalyst particles. The adsorption peaks at 554 nm for RhB and those at 498 nm for rhodamine were monitored during the de-ethylation and the mineralization process, respectively. 2.6. Theoretical Study. The structural optimizations for undoped and F-doped In(OH)ySz were performed by using the projector-augmented wave (PAW) formalism of density functional theory (DFT), as implemented in the Vienna ab initio simulations package (VASP),24-26 and the Perdew-BurkeErnzerhof (PBE) type of exchange-correlation was adopted.27 A supercell consisting of a 2 × 2 × 1 unit cell was employed, and for the undoped In(OH)3, there are 112 atoms in the system, which contains 16 In atoms and 48 OH groups. For the fluorinated In(OH)ySz, two OH groups in the supercell were replaced by one S and one F atom, respectively. During the structural optimizations, the cell shape was fixed since the concentration of dopant was very small (about 2%), and the positions of all atoms were allowed to be relaxed. The energy cutoff was set to 400 eV, and a 3 × 3 × 3 Monkhorst-Pack k-point grid was employed. Furthermore, the effects of spin polarization were considered in the calculations. 3. Results and Discussion 3.1. Characterizations. The XRD patterns of the samples prepared with fixed thiourea and different RF values are shown in Figure 1. All of the samples exhibit XRD patterns similar to cubic In(OH)3 (JCPDS76-1463) and indicate the formation of stable solid solutions upon the introduction of S and F. The average crystallite sizes calculated from the Scherrer equation are 25.5, 22.3, 25.2, and 21.4 nm for RF ) 0, 2, 4, and 10,

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Figure 1. XRD patterns of fluorinated In(OH)ySz prepared with different RF values (RF ) 0, 2, 4, and 10).

TABLE 1: Amount of S and F Substituted in Fluorinated In(OH)ySz Prepared with Different RF Values (RF ) 0, 2, 4, and 10) as Estimated from XPS Results sample

S/In

F/In

) ) ) )

0.16 0.16 0.16 0.16

0 0.041 0.055 0.09

RF RF RF RF

0 2 4 10

respectively. The XPS was carried out on In(OH)ySz solid solutions to estimate the amount of substituted S and F. With a S/In ratio fixed at 2 and F/In (denoted as RF values) in the synthetic solutions changed from 0, 2, 4, and 10, the S/In atomic in the solid solutions is almost fixed at 0.16, while the F/In in the solid solutions increases from 0, 0.04, 0.055, and 0.09, to a less degree than the F/In atomic ratio in the synthetic solution (Table 1). This indicates that the introduction of F- into the solid solution do not change the amount of the substituted S. The high-resolution XPS spectra of the S 2p region for In(OH)ySz, and fluorinated In(OH)ySz solid solutions show one peak around 161.3 eV and suggest that the substituted S atom is in the chemical states of S2- (Figure 2A). This result is similar to what we reported previously and indicates that S atoms are incorporated into the lattice and substitute •OH.23 The highresolution XPS spectra of F 1s region reveal the existence of two types of fluorine in the fluorinated In(OH)ySz solid solutions (Figure 2B). The peak at 684.2 eV can be assigned to fluorine substituting the surface OH group, while the peak at 686.8 eV can be assigned to F- substituting lattice OH group.20,21 Such a lattice substitution is probably formed by a nucleophilic substitution reaction of F- ions during the hydrothermal process.22 The nitrogen adsorption-desorption isotherms of fluorinated In(OH)ySz solid solutions with varying RF are shown in Figure 3. The specific surface areas are calculated to be 61.3, 57.9,

Figure 3. Nitrogen adsorption-desorption isotherms of fluorinated In(OH)ySz with different RF values (RF ) 0, 2, 4, and 10). Inset: Pore size distribution curve of the as-prepared samples calculated from the desorption branch of the nitrogen isotherm by the BJH method.

Figure 4. UV-vis diffuse reflectance spectra of In(OH)3 and fluorinated In(OH)ySz with different RF values (RF ) 0, 2, 4, and 10).

65.4, and 58.3 m2 g-1 for RF ) 0, 2, 4, and 10, respectively. The average pore sizes are 14.4, 14.6, 14.3, and 17.2 nm for fluorinated In(OH)ySz with RF ) 0, 2, 4, and 10, respectively. The formation of pores is due to the intra-agglomeration and interagglomeration of the particles.22 These results indicate that the introduction of fluorine does not significantly influence the BET surface area and the pore structure of the resultant fluorinated In(OH)ySz solid solutions. The UV-vis DRS of fluorinated In(OH)ySz solid solutions with different RF values are shown in Figure 4. It is observed that the substitution of OH- in In(OH)3 with S2- leads to a dramatic shift of the absorption edge into the visible region. The further introduction of fluorine into In(OH)ySz does not significantly influence the absorption edge of In(OH)ySz solid solutions. 3.2. Photocatalytic Degradation of RhB. In(OH)ySz solid solutions with different RF values show different photocatalytic performance for the degradation of RhB. The temporal UV-vis

Figure 2. S2p (A) and F1s (B) XPS spectra of fluorinated In(OH)ySz (RF ) 10) and In(OH)ySz without fluorination (RF ) 0).

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Figure 5. Temporal UV-vis absorption spectral changes during the photodegradation of RhB under visible light (λ g 420 nm) over In(OH)ySz prepared with different RF values (RF ) 0, 2, 4, and 10). Inset: the mineralization of RhB.

Figure 6. Temporal concentration changes of RhB (A) and rhodamine (B) as monitored by the absorbance at 554 and 498 nm, respectively, during the photocatalytic degradation reactions over In(OH)ySz with different RF values (RF ) 0, 2, 4, and 10).

spectral changes of RhB aqueous solution during the photocatalytic degradation reactions over In(OH)ySz solid solutions with RF values of 0, 2, 4, and 10 are shown in Figure 5A-D, respectively. It is observed that over all irradiated fluorinated In(OH)ySz solid solutions, the main absorbance of the solution shifts gradually from the initial 554 nm to shorter wavelength and finally reach at 498 nm. This hypsochromic shift in λmax corresponds to a step-by-step de-ethylation of RhB, and the peak centered at 498 nm is assigned to the absorbance of rhodamine, the completely de-ethylated product of RhB.28 Upon further irradiations, the intensity of the peak at 498 nm decreases and indicates a decomposition of the conjugated structure of rhodamine to small molecules, that is, the mineralization of rhodamine. Although the degradation of RhB over In(OH)ySz solid solutions with different RF can all be separated into two steps, that is, a very fast de-ethylation process followed by a relatively slow process for the destruction of the conjugated structure in rhodamine, the rates of these two steps over In(OH)ySz with different RF values are different. The temporal concentration changes of RhB and rhodamine as monitored by the absorbance

at 554 (Figure 6A) and 498 nm (Figure 6B), respectively, during the photocatalytic degradation reactions over In(OH)ySz with different RF values clearly show that both the rates for the deethylation of RhB and the destruction of rhodamine are dependent on the RF value but exhibit different trends. With RF increasing from 0 to 10, the rate for the de-ethylation of RhB enhances, but the rate for the decomposition of rhodamine decreases. About 60, 50, 40, and 30 min are required to achieve a complete transformation of RhB to the de-ethylated products over In(OH)ySz with RF values of 0, 2, 4, and 10, respectively. This indicates that the fluorination over In(OH)ySz solid solution is benificial to the de-ethylation of RhB. On the contrary, the decomposition of rhodamine follows a different trend over these solid solutions. After 7 h, the conversion ratio of rhodamine reaches 0.72, 0.68, 0.40, and 0.28 for In(OH)ySz with RF values of 0, 2, 4, and 10, respectively. This indicates that the fluorination was detrimental to the destruction of the conjugated structure in rhodamine. The phenomenon that the fluorination on In(OH)ySz can have opposite effects on the two processes of one photocatalytic reaction implies that the major active species involved in these

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Figure 7. Temporal concentration changes of RhB over fluorinated In(OH)ySz (RF ) 10) and In(OH)ySz without fluorination (RF ) 0) in the presence of EDTA-Na and IPA.

two processes, that is, de-ethylation and the destruction of the chromophores in the degradation of RhB, are different. To explore the origins of the promotion of the de-ethylation of RhB and the suppression of the mineralization of rhodamine over the fluorinated In(OH)ySz, the active species involved in these two processes should be identified. 3.3. Identifications of the Active Species. To identify the active species involved in the two processes during the degradation of RhB, controlled experiments using different scavenges have been carried out. 3.3.1. Effects of Holes and •OH Radicals ScaWenge. The photocatalytic de-ethylation of RhB over In(OH)ySz and fluorinated In(OH)ySz (RF ) 10) in the presence of EDTANa2 or isopropanol (IPA) were shown in Figure 7. EDTANa2 is a strong electron donor and has been widely used as a scavenger for photogenerated holes,21,29 while IPA is a generally accepted scavenger for •OH radicals.30,31 The de-ethylation rates over both In(OH)ySz and fluorinated In(OH)ySz (RF ) 10) were not influenced by the addition of IPA, indicating that •OH radical is not responsible for the de-ethylation on both In(OH)ySz and fluorinated In(OH)ySz. However, the addition of EDTANa2 can moderately suppress the de-ethylation of RhB on both In(OH)ySz and fluorinated In(OH)ySz (RF ) 10) solid solutions. Besides this, the suppression effect of EDTANa2 was larger over fluorinated In(OH)ySz (RF ) 10) than that over In(OH)ySz without fluorination. This implies that the holes contribute to the deethylation of RhB on both In(OH)ySz and fluorinated In(OH)ySz systems and the contributions of holes to this de-ethylation process is larger in fluorinated In(OH)ySz than in In(OH)ySz without fluorination. The influences of EDTANa2 and IPA on the destruction of the chromophores process are shown in Figure 8. It was found that EDTANa2 and IPA both do not affect the rate of the mineralization of rhodamine over In(OH)ySz solid solution, implying that both holes and •OH radicals do not contribute to the destruction of chromophores in rhodamine, that is, the mineralization of rhodamine, over In(OH)ySz solid solution. However, for fluorinated In(OH)ySz solid solution, IPA does not influence the rate, while EDTANa2 can moderately slow the rate. This indicates that •OH radicals are not an active species involved in the mineralization of rhodamine over fluorinated In(OH)ySz solid solution, while holes play a certain role in this process. The ESR results confirm this by showing no characteristic peaks corresponding to DMPO-•OH in both In(OH)ySz and fluorinated In(OH)ySz systems. 3.3.2. Effect of Reaction Atmosphere. The comparison of the rates of the de-ethylation over In(OH)ySz and fluorinated In(OH)ySz (RF ) 10) under O2 and N2 is shown in Figure 9. As compared to the de-ethylation rate in O2 aerated condition, the

Hu et al.

Figure 8. Temporal concentration changes of rhodamine over fluorinated In(OH)ySz (RF ) 10) and In(OH)ySz without fluorination (RF ) 0) in the presence of EDTA-Na and IPA.

Figure 9. Temporal concentration changes of RhB over fluorinated In(OH)ySz (RF ) 10) and In(OH)ySz without fluorination (RF ) 0) when oxygen or nitrogen was aerated.

Figure 10. Temporal concentration changes of rhodamine over fluorinated In(OH)ySz (RF ) 10) and In(OH)ySz without fluorination (RF ) 0) when oxygen or nitrogen was aerated.

de-ethylation rate in N2 on both In(OH)ySz and fluorinated In(OH)ySz was suppressed, but the suppression degree was larger over In(OH)ySz than that over fluorinated In(OH)ySz. The influence of atmosphere on the mineralization of rhodamine over In(OH)ySz and fluorinated In(OH)ySz (RF ) 10) is shown in Figure 10. The mineralization of rhodamine over In(OH)ySz is totally suppressed at the presence of N2. This implies that O2-• radical is the only active species involved in the mineralization of rhodamine. As come to the fluorinated In(OH)ySz solid solution, although the mineralization of rhodamine is significantly suppressed, a little amount of rhodamine can still be mineralized under this circumstance. Therefore, in the fluorinated In(OH)ySz system, O2-• radical is a major active species, while holes can be a minor active species based on the EDTANa2 result discussed above. The formation of O2-• radicals is confirmed by the ESR spin trap with the DMPO technique (Figure 11). Characteristic peaks of DMPO-O2-• are observed

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Figure 11. DMPO spin-trapping ESR spectra of In(OH)ySz (A) and fluorinated In(OH)ySz (B) in methanol dispersion for DMPO-O2-•.

Figure 12. Adsorptions of RhB over F-In(OH)ySz with different RF values (RF ) 0, 2, 4, 10).

Figure 13. N 1s XPS spectra of (A) RhB adsorbed on In(OH)ySz (RF ) 0); (B) RhB adsorbed on fluorinated In(OH)ySz (RF ) 10).

for both In(OH)ySz and fluorinated In(OH)ySz systems. Moreover, the intensity of the peak corresponding to DMPO-O2-• is stronger in In(OH)ySz system than in fluorinated In(OH)ySz system under similar reaction condition. This indicates that the photogenerated electrons in the CB of In(OH)ySz are more effective in the reduction of O2 to give O2-• than those in the fluorinated In(OH)ySz system. Although singlet oxygen can also oxidize rhodamine,32 no characteristic peaks corresponding to TEMPO are detected in the TEMP-trapped ESR spectra and indicate that no singlet oxygen is involved. To elucidate why the fluorination of In(OH)ySz solid solution can make such an effect, the adsorptions of RhB over both In(OH)ySz and fluorinated In(OH)ySz solid solution as well as the influence of fluorination on the electronic structure of In(OH)ySz were also studied. As we know, the direct hole oxidation requires the adsorption of RhB on the surface of catalyst, and the efficiency of the reduction of O2 by the conduction band electrons is related to the electronic band structure. 3.4. Adsorption of RhB. The adsorptions of RhB over In(OH)ySz and fluorinated In(OH)ySz with different RF values are shown in Figure 12. It is observed that the adsorption of RhB increases with the value of RF. As mentioned above, the specific surface area of In(OH)ySz solid solutions does not change obviously upon fluorinations. Therefore, the increased ability in the adsorption of RhB over fluorinated In(OH)ySz solid solutions must be induced by the change of the surface characteristics after surface fluorination. The substitution of surface OH group by F- or S2- in In(OH)ySz and fluorinated In(OH)ySz solid solutions can create anionic sites and is advantage for the adsorption of cationic dye like RhB on the catalyst surface. As compared to pure S2- substitution, the fluorinations on the In(OH)ySz solid solutions can create more anionic sites; therefore, the adsorption of RhB over fluorinated In(OH)ySz solid solution is stronger than that over In(OH)ySz

without fluorination. Besides this, we proposed that RhB should be adsorbed on the anionic sites of the catalyst surface via -N(Et)2 groups. This assumption is confirmed by the XPS result of the RhB adsorbed photocatalyst. The XPS spectrum in the N 1s region of RhB adsorbed In(OH)ySz shows two peaks, as shown in Figure 13A. The peak located at 399.05 eV should be assigned to the -N(Et)2 group of RhB, which has no interaction with the surface anionic site, while the peak at 400.03 eV should be assigned to the -N(Et)2 group of RhB binding to the S2- sites on the catalyst surface. Besides these two peaks, the XPS spectrum in the same region of the RhB adsorbed fluorinated In(OH)ySz shows another peak at 401.5 eV, which can be assigned to the -N(Et)2 group binding to the substituted F- on the surface of fluorinated In(OH)ySz (Figure 13B).33 The strong adsorption of RhB on the catalyst surface via -N(Et)2 groups makes the direct hole oxidation and the de-ethylation of RhB feasible. As more F- substituting the surface OH groups, more anionic sites are created, and more RhB is adsorbed. Therefore, the direct hole oxidation resulting in the de-ethylation of RhB is easier over In(OH)ySz solid solution with a higher RF value. 3.5. Electronic Structure of F-In(OH)ySz. Figure 14A displays the band structure of In(OH)ySz solid solution. The analysis of the components of wave functions shows that the energy bands that appear in the band gap of In(OH)ySz are dominated by S 3p states (namely denoted by crosses in Figure 14A). The energy positions of these impurity levels are 0.23 eV (at Γ point, similarly hereinafter, also see Figure 14D) above the valence band (VB) maximum of In(OH)3 and 0.86 eV below the bottom of conduction band (CB). The predicted band gap of In(OH)3 is about 2.27 eV, which is smaller than the experimental value of 5.17 eV due to the inherent deficiency of the DFT method. For the fluorinated In(OH)ySz, two typical doping models, M1 and M2 are considered in this work. In the M1 model, F

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Figure 14. (A) Band structure of undoped In(OH)ySz solid solution, (B) band structure of M1 doping model, (C) band structure of M2 doping model, and (D) schematic illustration for the comparisons of band structures between undoped and M1 doping model. The energy bands that contain obvious components of S and F atoms are labeled by blue crosses and green circles, respectively. In panels B and C, the new energy bands separated from original valence band are denoted by red lines. The Fermi level is taken as the energy zero, and the bands corresponding to spin-up and spin-down states are shown by solid and dot lines, respectively.

atom replaces the site far from S atom, and on the contrary in M2, the substituent position of F atom is close to the S atom. For the M1 model, according to the band structure shown in Figure 14B, the band gap states still originate from S 3p orbitals, and the states derived from F atom are mainly distributed in the region about -3.8 ∼ -2.1 eV. With respect to In(OH)ySz, the introduction of F has some effects on the electronic structure of In(OH)ySz, which can be seen clearly in Figure 14D by comparing band structures between undoped and M1 model. The first is that the splitting of those occupied band gap states becomes more obvious, and the width of these energy bands increases from 0.25 to 0.33 eV (Figure 14D). Correspondingly, now, the S 3p states are more close to VB. The second effect is found in the VB maximum region of In(OH)ySz. Because the doping results in the modification of the coordination environments, as denoted by red lines in Figure 14B, two energy bands (one spin-up plus one spin-down) dominated by O 2p states separate from original valence band. Finally, after F doping, the band gap of In(OH)ySz decreases from 2.27 to 2.20 eV. For another doping case, the M2 model shows similar changes of band structure; however, our calculated result indicates that M2 is about 1.6 eV higher in energy than M1 structure. Therefore, from a thermodynamical point of view, it seems that the F atom favors the doping site far from S atom.

Figure 15. Mott-Schottky plots of In(OH)3, In(OH)ySz (RF ) 0) and fluorinated In(OH)ySz (RF ) 10).

As displayed in Figure 14D, the theoretical calculations reveal that the change of the position of VB is almost negligible, while the introduction of F can lower (about 0.05 eV) the position of the CB of In(OH)ySz solid solution. The position of the flat band potential of In(OH)ySz and fluorinated In(OH)ySz determined by Mott-Schottky method confirms the theoretical result (Figure 15). The flat band potential of fluorinated In(OH)ySz shifts to -0.64 V (versus NHE), a little positive than -0.70 V (versus NHE) observed over In(OH)ySz. This implies that although the

Photocatalytic Degradation of RhB over In(OH)ySz photogenerated electrons in both In(OH)ySz and fluorinated In(OH)ySz can thermodynamically enable the formation of O2-• (O2/O2-•: -0.33 vs NHE),34 the photogenerated electrons in In(OH)ySz are more easily to be trapped by O2 due to a more negative conduction band. Moreover, the flat band potentials of both In(OH)3 and In(OH)ySz are determined to be -0.70 V (versus NHE), consistent with our previous result obtained from UPS measurements.23 The similar flat band potentials observed over In(OH)3 and In(OH)ySz again confirms that the introduction of S2- into In(OH)3 does not change the position of the conduction band of In(OH)3. 4. Conclusion Fluorinated In(OH)ySz shows an enhanced photocatalytic activity for the de-ethylation of RhB, while it suppresses the process of the mineralization of RhB during photocatalytic degradation of RhB. The promotion of the de-ethylation process can be attributed to the stronger adsorption of RhB on the catalyst surface, while the lowering of the conduction band upon fluorination over In(OH)ySz is responsible for the suppression of the mineralization of RhB. Our results clearly show that the fluorine modifications can have ambilateral effects on the photocatalytic performance of semiconductor photocatalysts depending on the specific photocatalytic reactions. Acknowledgment. This work was supported by the National Natural Science Foundation of China (21033003, 20977016, 90922022, 20773024, and U1033603), National Basic Research ProgramofChina(973Program:2007CB613306and2010CB234604), and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT0818). This work is also supported by fund of Fujian Provincial Key Laboratory of Nanomaterials (NM10-01). The NSF of Fujian province for a Distinguished Young Investigator Grant (2009J06004) for Z.L. is also acknowledged. References and Notes (1) Zhou, J. K.; Lv, L.; Yu, J. Q.; Li, H. L.; Guo, P. Z.; Sun, H.; Zhao, X. S. J. Phys. Chem. C 2008, 112, 5316. (2) Yu, J. G.; Liu, S. W.; Yu, H. G. J. Catal. 2007, 249, 59.

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