Nanocomposites with Enhanced Photocatalytic Activity - American

J. Phys. Chem. C 2008, 112, 11481–11489

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Silver and Indium Oxide Codoped TiO2 Nanocomposites with Enhanced Photocatalytic Activity Xia Yang, Yonghui Wang, Leilei Xu, Xiaodan Yu, and Yihang Guo* Faculty of Chemistry, Northeast Normal UniVersity, Changchun 130024, PR, China

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ReceiVed: April 23, 2008; ReVised Manuscript ReceiVed: June 6, 2008

Metallic silver and semiconductor indium oxide codoped titania nanocomposites were prepared by a one-step sol-gel-solvothermal method in the presence of a triblock copolymer surfactant (P123). The resulting Ag/ In2O3-TiO2 three-component systems mainly exhibited an anatase phase structure, high crystallinity, and extremely small particle sizes with metallic Ag particles well-distributed on the surface. Compared with pure anatase TiO2, the Ag/In2O3-TiO2 systems showed narrowing of the band gap due to the change in the band position caused by the contribution of the In 5s5p orbits to the conduction band. By tuning loadings of Ag or In2O3 and molar ratios of titanium source to the surfactant, size and dispersion of the product particles can be controlled. As-prepared Ag/In2O3-TiO2 nanocomposites were used as the photocatalysts to degrade dye rhodamine B (RB) and methyl ter-butyl ether (MTBE) in the liquid phase. At 2.0% Ag and 1.9% In2O3 doping, the Ag/In2O3-TiO2 system exhibited the highest UV-light photocatalytic activity, and nearly total degradation of dye RB (25 mg L-1) or MTBT (200 mg L-1) was obtained after 45 or 120 min UV-light irradiation. In addition, the UV-light photocatalytic activity of three-component systems exceeded that of the single (TiO2) and two-component (Ag/TiO2 or In2O3-TiO2) systems as well as the commercial photocatalyst, Degussa P25. Reasons for this enhanced photocatalytic activity were revealed. 1. Introduction Development of high-efficiency photocatalytic materials based on anatase TiO2 nanoparticles still offers unexplored opportunities for total destruction of organic compounds in polluted air and wastewater although other new unique heterogeneous photocatalysts have been emerging.1 Owing to unique ability of photocatalytic degradation of various organic contaminants and to its chemical inertness and nontoxicity, TiO2 is the most studied semiconductor for environmental cleanup applications.2 However, low quantum efficiency and high band gap of TiO2 limit its photocatalytic activity.3 So far, many efforts have been made to further improve the photocatalytic activity of TiO2, among which utilizing doping metals, nonmetals, and other semiconductors with wide or narrow band gaps, reducing the size and changing the morphology of TiO2, and adding hole scavengers are the most prevalent means.4–7 Thus the photocatalytic activity of TiO2 was enhanced owing to the improved properties of TiO2-based materials including decreased band gaps, nanometer sizes, new surface compositions and structures, enhanced quantum efficiency, and high crystallinity. More recently, codoping of two components with TiO2 to prepare three-component junction systems has attracted considerable interest since these junction systems can result in higher photocatalytic activity and peculiar characteristics compared with pure titania or two-component junction systems. For example, Au@CdS/TiO2 with a core-shell structure demonstrated much higher visible-light photocatalytic activity than CdS/TiO2 or Au/TiO2 for the reduction of methylviologen;8 N-Fe (III) codoped TiO2 nanocomposites exhibited higher UVand visible-light photocatalytic activity toward dye RB degradation in comparison with the pure titania;9 Ag-InVO4 codoped * Corresponding author. Telephone or Fax: +86-431-85098705. E-mail: [email protected].

TiO2 composite thin film with 1% Ag doping exhibited higher visible-light activity for decomposition of aqueous methyl orange.10 Our present work focuses on nanoscale Ag/In2O3-TiO2 threecomponent junction system. Nanoscale metal and semiconductor particles are of current interest because they constitute a material transition range between quantum and bulk properties.11,12 With decreasing particle size, bulk properties are lost as the continuum of electronic states becomes discrete (i.e., quantum size effect) and as the fraction of surface atoms becomes large.11,13,14 On the other hand, metal/semiconductor oxide composite systems are extremely attractive since they represent efficient bifunctional catalysts.2 In the nanosized regime, specific metal-oxide interactions can indeed be responsible for the detection of new physical properties or for enhanced catalytic activity.15 Coupling with noble metals has also been demonstrated to increase the photocatalytic and photoelectron chemical responses of semiconductor oxides by reducing the fast recombination of the photogenerated charge carriers. Silver is an extremely intriguing noble metal to be investigated at nanoscale due to its remarkable catalytic activity16–18 and to its size- and shape-dependent optical properties.19–22 As for the semiconductor In2O3, it can absorb light of wavelengths shorter than 480 nm with the band gap of ∼2.5 eV.23 Codoped Ag and In2O3 with TiO2 attempt to present a new three-component junction photocatalytic system, and the co-operation of Ag and In2O3 is expected to lead to enhanced photocatalytic activity compared with Ag/TiO2 or In2O3/TiO2 two-component junction systems. Several approaches such as photodeposition, ultrasonicassisted sol-gel, and chemical reduction have been reported for the preparation of Ag/TiO2 nanocomposites.24,25 Nevertheless, some major limitations including hard control of the size and morphology, and aggregation of the metal clusters highlight the urgency of the improved preparative route. In order to deposit silver particles on the surface of In2O3-TiO2 composites

10.1021/jp803559g CCC: $40.75  2008 American Chemical Society Published on Web 07/08/2008

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homogeneously, the current work utilized a multicomponent assembly approach, where the surfactant, titanium source, indium source, and silver source were cooperatively assembled in a one-step process. By carefully designing the preparation route, aggregation of silver particles was reduced due to the function of the nonionic surfactant, Pluronic P123 (EO20PO70EO20, where EO ) CH2CH2O and PO ) CH2(CH3)CHO). Generally, aggregation of silver particles will enrich more electrons and become new recombination centers of photogenerated electronhole, accordingly, quantum efficiency will be reduced. On the other hand, growth of particles was inhibited by controlling the molar ratio of titanium source to surfactant. By applying the current method, Ag or In2O3 dopings in as-prepared Ag/ In2O3-TiO2 composites are tunable. Interestingly, Ag+ could be reduced into metallic Ag during the multicomponent selfassembly process in a solvothermal environment although no reductant was added. Morphology, structure, optical absorption, and textural properties of Ag/In2O3-TiO2 composites were well characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), Raman scattering spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), and nitrogen porosimetry. Subsequently, their UV-light photocatalytic activity was evaluated by degradation of dye RB and MTBE (their structures are shown in Scheme 1), and the factors influencing the photocatalytic activity such as morphology, crystallinity, optical properties, and Ag or In2O3 loadings were also investigated.

amounts (0.5 to 4 g) or without P123 were also obtained by the similar process. The samples were denoted as Ag/In2O3-TiO2(x, y), where x and y represent the weight percentage or doping of Ag and In2O3 (wt %) in the products, respectively. 2.2. Catalyst Characterization. Dopings of Ag and In2O3 in the products were determined by a Leeman Prodigy Spec ICP-AES. UV-vis DRS were recorded on a Cary 500 UV-visNIR spectrophotometer. XRD patterns were recorded on a Rigaku D/max-3c X-ray diffractometer (Cu KR radiation, λ ) 0.154 nm). The crystallite size was estimated by applying the Scherrer equation to the fwhm of the (101) peak of anatase, with R-silicon (99.999%) as a standard for the instrumental line broading. XPS was performed on a VG-ADES 400 instrument with Mg KR-ADES source at a residual gas pressure of below 10-8 Pa. All the binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. TEM micrographs were obtained on a JEOL JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. Nitrogen porosimetry was performed on a Micromeritics ASAP 2010 instrument. Surface areas were calculated using the BET equation. Pore size distributions were estimated from BJH desorption determination. Raman scattering spectra were recorded on a Jobin-Yvon HR 800 instrument with an Ar+ laser source of 488 nm wavelength in a macroscopic configuration. FT-IR spectra were recorded on a Nicolet Magna 560 IR spectrophotometer. 2.3. Photocatalytic Test. The photoreactor was designed with an internal light source (a 50 W high-pressure mercury lamp with a main emission wavelength of 313 nm and an average light intensity of 2.85 mW cm-2) surrounded by a watercooling jacket (quartz) to cool the lamp. The suspension containing the solid catalyst (0.15 g) and an aqueous solution of MTBE (200 mg L-1, 90 mL) or dye RB (25 mg L-1, 90 mL) surrounded the light source. The suspension was ultrasonicated for 10 min and then stirred in the dark for 30 min to obtain a good dispersion and thus adsorption-desorption equilibrium between MTBE or RB molecules and the catalyst surface was established. The acidity of the suspension was neutral, and the system was open to air. At given intervals of illumination, fixed amounts of reaction solution were taken out, centrifuged and filtrated. Finally, the filtrates were analyzed. Decreases of the concentrations of RB were analyzed by a Cary 500 UV-vis-NIR spectrophotometer at λ ) 554 nm. Decreases of the concentrations of MTBE were analyzed by a Varian 3400 GC-FID equipped with a PEG 20 M column (30 m × 0.32 mm × 0.25 µm). Changes of total organic carbon (TOC) in the reaction system were monitored using a Shimadzu TOC-500 total organic carbon analysis system.

2. Experimental Section 2.1. Catalyst Preparation. P123 (2 g) was dissolved in isopropanol (i-PrOH, 10 mL) under vigorously stirring, and then ice-cooled titanium isopropoxide (TTIP, 2 mL) was added, followed by further stirring for 30 min (mixture A). A mixture of AgNO3, In(NO3)3 · 4.5H2O, i-PrOH (4 mL), and deionized water (0.6 mL) (mixture B) was then added dropwise to mixture A under vigorously stirring, and the resulting mixture was continuously stirred until the gel was formed. The gel was transferred into an autoclave and heated to 473 K with a heating rate of 2 K min-1, and then held at this temperature for 2 h. The compact gel was suffered following thermal treatment in vacuum: 24 h at 313 K, 12 h at 333 K, 2 h at 353 K, 2 h at 373 K, 0.5 h at 393 K, and 3 h at 723 K. Ag/TiO2, In2O3-TiO2, pure Ag, In2O3, and TiO2 were prepared similarly. For comparison, Ag/In2O3-TiO2 samples prepared with different P123

3. Results and Discussion 3.1. Catalyst Preparation. One-step sol-gel co-condensation and solvothermal treatment were used to prepare Ag/ In2O3-TiO2 composites, and the preparation procedure included three steps. At first, cohydrolysis of TTIP and In(NO3)3 resulted in the sol of (Ti, In)-O2 species with a large number of surface hydroxyl groups due to incomplete condensation. After evaporation of most solvent (i-PrOH) at room temperature the gel of (Ti, In)-O2 species was formed. At the same time, during the above sol-gel step, AgNO3 was adsorbed as Ag+ on the surface of (Ti, In)-O2 species. Second, the gel was treated solvothermally at 473 K with a heating rate of 2 K min-1. After this step, anatase phase of TiO2 was formed with high crystallinity (see XRD results in Figure 2b). The product particle size was reduced and the particles were well distributed. Importantly, Ag+ was reduced into metallic Ag at this step. Finally, the gel was

SCHEME 1: Chemical Structures of RB and MTBE

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Figure 1. TEM images of the composites. (a) Ag/In2O3-TiO2-(2.0, 1.9) without P123; Ag/In2O3-TiO2-(2.0, 1.9) with P123:TTIP molar ratios of (b) 1:80, (c) 1:20, and (d) 1:10; (e) In2O3-TiO2-(0, 1.9) with P123:TTIP ) 1:20; (f) Ag/In2O3-TiO2-(8.5, 1.3) with P123:TTIP ) 1:20; (g) highresolution TEM image and (h) the lattice fringe of Ag/In2O3-TiO2-(2.0, 1.9) with P123:TTIP ) 1:20.

calcined at 723 K to remove the surfactant P123, concomitant with further improvement of the product crystallinity. During the preparation procedure, we neither added any reductant (e.g., NaBH4) nor used UV-light irradiation. Formation of metallic Ag is attributed to the oxidation of i-PrOH under solvothermal conditions (eq 1). Similar results have been reported by Li’s group.25

Ag+ + CH3CHCH3OH f Ag0 + CH3COCH3 + H+ (1) The conclusion is supported by FT-IR results. That is, no Ag-O-Ti bonds (960 cm-1) were found for the product before calcinations at 723 K, implying that formation of metallic Ag occurred at the step of solvothermal treatment.26 Formation of the metallic Ag in the composite has been confirmed by XPS (see Figure 3). During the above preparation process, the amounts of i-PrOH and P123 are crucial to the formation of nanoscale and uniform product particles. The amount of i-PrOH can tune the hydrolysis rate of TTIP and the time of gel formation. With the same amount of P123, the molar ratio of i-PrOH to TTIP should be in the range of 19:1 to 38:1. Lower or higher molar ratios can induce the hydrolysis rate of TTIP too fast or too slow. As for the surfactant P123, it acts as a dispersion reagent rather than

a structure directing reagent since the present experimental condition is not suitable for the construction of ordered porous structure. The amount of P123 affects the morphology, size, dispersion, and crystallinity of the product, which has been evidenced by TEM observations (see Figure 1). Therefore, for subsequent preparation with different Ag or In2O3 dopings, the molar ratio of P123:TTIP:i-PrOH:H2O was controlled at 1:20: 530:97. 3.2. Catalyst Characterization. 3.2.1. TEM and HRTEM ObserWation. Morphology of the products was characterized by TEM observations (Figure 1). The obtained images reveal that as-prepared In2O3-TiO2 was square in shape with a size of 12 ( 2 nm (Figure 1e). In the case of Ag/In2O3-TiO2, sphereshaped Ag particles deposited on the surface of In2O3-TiO2 as an individual phase. Influences of the molar ratio of P123:TTIP on the size, morphology, and crystallinity of the products were observed from the TEM results. With the same amount of i-PrOH, heavy aggregation of Ag clusters with a comparably larger particle size (25 ( 5 nm) occurred for the sample prepared in the absence of P123 (Figure 1a). With a low P123 content (molar ratio of P123:TTIP ) 1:80) the particle size (20 ( 2 nm) was still relatively large (Figure 1b). Once the molar ratio of P123:TTIP was increased to 1:20 the particle size decreased

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Figure 2. XRD patterns of as-prepared (a) pure Ag, In2O3, TiO2 and Ag/In2O3-TiO2-(2.0, 1.9); (b) Ag/In2O3-TiO2-(2.0, 1.9) calcined at different temperatures; (c) Ag/In2O3-TiO2-(x, 1.3) with x ) 0.97, 4.0, 8.5, and 21.8%, respectively; and (d) Ag/In2O3-TiO2-(2.0, y) with y ) 0.43, 1.9, and 3.8%, respectively.

to 12 ( 2 nm (Figure 1c). Moreover, Ag clusters homogeneously dispersed on the surface of the product with high crystallinity. Further increasing this molar ratio to 1:10, however, caused the particle size to increase again with lower crystallinity (Figure 1d). In addition, too high P123 amount may affect the photocatalytic process of the products if trace of P123 exists in the products. TEM images indicate that aggregation among the Ag/In2O3-TiO2-(2.0, 1.9) particles with P123:TTIP ranging from 1:80 to 1:10 was significantly restrained in contrast with Ag/In2O3-TiO2 without P123 (Figures 1a-d). The selected area electron diffraction (SAED) patterns of the composites (inserted in Figures 1a-d) further confirm the anatase structure of the Ag/In2O3-TiO2 (a set of concentric rings have been indexed to various planes of anatase TiO2). Among the tested Ag/ In2O3-TiO2-(2.0, 1.9) with a P123:TTIP molar ratio of 1:20 possessed the smallest particle size (12 ( 2 nm, Figure 1c), and its dispersion and crystallinity were also the highest. In the case of Ag/In2O3-TiO2-(8.5, 1.3), higher Ag loading led to slight aggregation among the particles, and the particle size increased to 15 ( 2 nm (Figure 1f). The nanocrystalline nature of Ag/In2O3-TiO2-(2.0, 1.9) with P123:TTIP ) 1:20 can be visibly observed in its high-resolution TEM images (Figures 1g and h). The lattice fringe used for phase determination is 0.35 nm and this value corresponds to the lattice spacing of (101) plane in the anatase phase. 3.2.2. XRD Analysis. The crystalline phase of Ag/In2O3-TiO2(2.0, 1.9) was characterized by XRD measurements. For comparison, as-prepared pure Ag, In2O3, and TiO2 were also investigated. As shown in Figure 2a, pure Ag exhibits a cubic phase with 2θ at 38.21, 44.47, 64.47, and 77.48°, respectively

(JCPDS 03-0921); pure In2O3 displays a rhombic phase with 2θ at 30.97, 32.57, 45.57, 50.24, 57.35, and 58.20°, respectively (JCPDS 22-0336); and pure TiO2 shows an anatase phase with the peaks at 25.31, 37.90, 48.02, 54.64, and 62.83°, respectively (JCPDS 21-1272). As for the Ag/In2O3-TiO2-(2.0, 1.9) composite, the weak peak appearing at 32.57° (110) corresponds to the rhombic In2O3 and the other peaks to the anatase TiO2, whereas the diffraction peaks originating from the cubic Ag are hardly detected. Figure 2b shows the XRD patterns of the Ag/In2O3-TiO2(2.0, 1.9) composites obtained at different calcination temperatures. The major phase of these samples is originated from anatase TiO2, whereas the other minor phase is assigned to rhombic In2O3. When the samples were annealed at a temperature up to 873 K, neither rutile peak nor Ti-In mixed oxide phase was detected. These results indicate that doping with Ag and In2O3 can prevent the TiO2 phase transformation from anatase to rutile even if the calcination temperature reached 873 K. Further, the crystallinity increased with increasing the calcination temperature since higher ordering in the structure of titania particles makes XRD peaks sharper and narrower.27 The estimated crystallite size of the anatase phase by Scherrer formula increased from 9.7 to 22.1 nm as the calcination temperature increased from 573 to 873 K. This is due to the fact that higher temperature resulted in the sintering of smaller particles into bigger ones. Consequently, an appropriate calcination temperature is necessary to obtain an ideal and perfect TiO2 nanocrystal with the anatase phase structure. Thus it is concluded that 723 K is suitable for the calcination of this series of materials.

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Figure 3. XPS survey spectra for the Ag/In2O3-TiO2-(2.0, 1.9) in the (a) Ag 3d, (b) In 3d, (c) Ti 2p, and (d) O 1s binding energy regions.

The crystalline phases of as-prepared composites with various Ag dopings (0.97 to 21.8%) were further studied (Figure 2c). The major phase of all the tested samples was the anatase structure regardless of Ag loadings in the products. As for the Ag/In2O3-TiO2-(21.8, 1.3) sample, the cubic Ag phase was distinctly observed at 38.21° (111) and 44.47° (200), respectively. Moreover, the estimated anatase phase crystallite sizes of all tested samples increased from 10.8 to 15.9 nm with Ag loadings increasing from 0.97 to 21.8%. At the same Ag loading but different In2O3 loadings, the phase structure of the products was also tested (Figure 2d). The intensity of rhombic In2O3 at (110) plane increased with increasing In2O3 dopings from 0.43 to 3.8%. The estimated anatase phase crystallite sizes of three tested samples are 10 ( 1 nm, and changing In2O3 dopings from 0.43 to 3.8% led to no obvious changes of the crystallite sizes of the products. 3.2.3. XPS Analysis. The chemical states of Ag, In, Ti, and O species in the Ag/In2O3-TiO2-(2.0, 1.9) composite were examined by XPS measurement. Figure 3a shows the XPS of this composite in the Ag 3d5/2 and Ag 3d3/2 binding energy regions. The determined binding energies of Ag 3d5/2 and Ag 3d3/2 are 367.8 and 373.8 eV, respectively, and the spin energy separation is 6.0 eV. This is the characteristic of metallic silver,28 indicating that the silver species existing in the product is metallic Ag (Ag0) rather than Ag+ ion. Further, Figure 3b displays the XPS of this composite in the In 3d5/2 and In 3d3/2 binding energy regions, which are centered at 444.6 and 452.1 eV, respectively. This is the characteristic of In3+ species.29 In addition, the spin-orbit components (2p3/2 and 2p1/2) of Ti peaks are well deconvoluted by two curves at 458.5 and 464.3 eV (see Figure 3c), in good agreement with pure anatase TiO2.30 Similarly, the O 1s XPS spectrum (Figure 3d) shows a narrow peak with a binding energy of 529.0 eV and slight asymmetry,

Figure 4. Raman spectra of as-prepared (a) pure TiO2, (b) Ag/ In2O3-TiO2-(0.97, 1.3), (c) Ag/In2O3-TiO2-(4.0, 1.3), (d) Ag/ In2O3-TiO2-(8.5, 1.3), and (e) Ag/In2O3-TiO2-(11.8, 1.3).

which is attributed to lattice oxygen in anatase TiO2.31 Furthermore, a few hydroxyl oxygen and adsorbed oxygen species were also founded in this sample with band energies of 531.8 and 533.1 eV, respectively.32,33 3.2.4. Raman Analysis. Figure 4 shows the Raman spectra of Ag/In2O3-TiO2 nanocomposites with different Ag loadings. A well-resolved Raman scattering peak of anatase TiO2 is observed at 141 cm-1 (Eg) for the pure TiO2. Moreover, other scattering peaks at 195.0 (Eg), 393.2 (B1g), 511.9 (A1g), and 637.0 cm-1 (Eg) are also present, indicating that the anatase phase is perfect.34 The strongest scattering peak (Raman shift at 141 cm-1) became broader and weaker and positively shifted by 12 cm-1 with Ag content increasing from 0.97 to 8.5%. The result suggests increasing crystalline defects within the framework, which could be ascribed to the distortion of anatase crystalline lattice induced by Ag-dopant.35,36 Further increasing Ag loadings from 8.5 to 11.8% did not affect the Raman shift

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Figure 6. Nitrogen adsorption-desorption isotherms and pore size distribution profiles (insert) according to BJH desorption dV/dD pore volume of as-prepared composites. (O) In2O3-TiO2-(0, 1.9); (b) Ag/ In2O3-TiO2-(2.0, 1.9); (9) Ag/In2O3-TiO2-(8.5, 1.3); (1) Ag/ In2O3-TiO2-(2.0, 1.9) without P123.

TABLE 1: Textural Parameters of As-Prepared Composites samples

Figure 5. UV-vis DRS of as-prepared (a) Ag/In2O3-TiO2-(x, 1.3) with x ) 0.97, 1.8, 4.0, 8.5, 11.8, and 21.8%, respectively; and (b) Ag/In2O3-TiO2-(2.0, y) with y ) 0.43, 0.99, 1.9, 2.8, and 3.8%, respectively.

significantly. This fact is consistent with TEM observation showing the formation of large nanoparticles on the surface of In2O3-TiO2. 3.2.5. UV-Vis DRS Analysis. The optical absorption properties of as-prepared Ag/In2O3-TiO2 were investigated by UV-vis DRS, as shown in Figure 5. For comparison, as-prepare TiO2, In2O3, and In2O3-TiO2 were also tested under the same conditions. At first, samples with the same Ag loading of 2.0% but different In2O3 loadings from 0.43 to 3.8% were studied (Figure 5a). The results show that the absorption edge of Ag/ In2O3-TiO2 samples was located between those of TiO2 and In2O3, indicating the redshift of the charge transfer (CT) band owing to In2O3 doping with TiO2. As for Ag/In2O3-TiO2-(2.0, 1.9) and Ag/In2O3-TiO2-(2.0, 3.8), the redshift of the CT band was more obvious with the absorption edge extending to 450 nm. This property may be very important to improve the photocatalyticactivityofsuchmaterials.Additionally,In2O3-TiO2(0, 1.9) and Ag/In2O3-TiO2-(2.0, 1.9) had the same absorption properties in the near-UV area, implying that doping Ag did not change the band gap of In2O3-TiO2. As mentioned above, redshift of the CT band is ascribed to the contribution of the In 5s5p orbits to the conduction band of TiO2, which results in decreased conduction band energy due to mixing of Ti 3d orbits with In 5s5p orbits. In order to further study the optical absorption properties of as-prepared Ag/In2O3-TiO2 composites, samples with the same In2O3 loading of 1.3% but various Ag loadings from 0.97 to 21.8% were also tested (Figure 5b). The results show that changing Ag loadings in these composites mainly affected the absorption of the composites in the visible-light region (400-800 nm). Such absorption originates from the surface plasmon resonance of metallic silver nanoparticles, further substantiating

In2O3-TiO2-(0, 1.3) Ag/In2O3-TiO2-(2.0, 1.9) Ag/In2O3-TiO2-(8.5, 1.3) Ag/In2O3-TiO2-(2.0, 1.9) without P123

SBET (m2 g-1)

Dp (nm)

Vp (cm3 g-1)

78.8 73.8 57.2 40.4

15.1 16.5 20.1 12.5

0.39 0.27 0.23 0.30

the formation of Ag0 in as-prepared Ag/In2O3-TiO2.37 With the increase of Ag content from 0.97 to 8.5%, the above absorption gradually increased. As the Ag content was further increased to 21.8%, this absorption sharply increased accompanying with redshift of the CT band. These facts are consistent with the formation of more and bigger particles. Similar results have been reported by Li and Lu’s group in the Au/TiO2 systems.35 3.2.6. Nitrogen Adsorption-desorption. Figure 6 shows nitrogen adsorption-desorption isotherms and pore size distribution curves of Ag and In2O3 codoped samples prepared with or without P123 as well as In2O3-TiO2 for comparison. Both single- and codoped TiO2 exhibit a type IV adsorption isotherm with a H3 hysteresis loop according to BDDT classification, which are typical characteristics of mesoporous materials.38 Moreover, formation of such mesoporous materials is attributed to the aggregation of the primary nanocrystallites. The textural parameters including surface areas, pore volumes, and median pore diameters derived from nitrogen adsorption-desorption isotherms are summarized in Table 1. From the above results it is found that doping Ag with In2O3-TiO2 resulted in slight decreases of BET surface areas and that higher Ag loadings led to smaller BET surface areas. In addition, the BET surface area of Ag/In2O3-TiO2-(2.0, 1.9) without P123 is obviously smaller than that of Ag/In2O3-TiO2-(2.0, 1.9) with P123. This result suggests that highly dispersed Ag/In2O3-TiO2-(2.0, 1.9) nanoparticles have been successfully fabricated by the P123 surfactant-assisted sol-gel process. 3.3. Photocatalytic Test. Dye RB and MTBE were selected as model molecules to evaluate the UV-light photocatalytic properties of as-prepared Ag/In2O3-TiO2. 3.3.1. UV-Light Photocatalytic Degradation of RB. The factors influencing the photocatalytic activity such as the kind of the photocatalyst, catalyst calcination temperature, Ag or In2O3 doping, and molar ratio of P123 to TTIP were studied via dye RB degradation reaction. In current photocatalytic

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Figure 7. Time courses of UV-light photodegradation of dye RB over (a) different photocatalysts; (b) Ag/In2O3-TiO2-(2.0, 1.9) with different calcination temperatures; (c) Ag/In2O3-TiO2-(x, 1.3) with x ) 0.97, 1.8, 4.0, 8.5, 11.8, and 21.8%, respectively; (d) Ag/In2O3-TiO2-(2.0, y) with y ) 0.43, 0.99, 1.9, 2.8, and 3.8%, respectively. Initial concentration of RB 25 mg L-1. Catalyst: 0.15 g.

system, direct photolysis of RB was hardly observed after UVlight irradiation for 120 min in the absence of the photocatalyst. At first, photocatalytic activity of Ag/In2O3-TiO2-(2.0, 1.9) was compared with various reference materials including pure TiO2, single-doped TiO2, Deggussa P25, and Ag/In2O3-TiO2(2.0, 1.9) without P123 for degradation of dye RB. As can be seen in Figure 7a, it took 45 to 120 min of UV-light irradiation to decompose dye RB totally over the six tested materials. The photocatalytic activity is in the order of Ag/In2O3-TiO2-(2.0, 1.9) > Ag/TiO2-(2.0, 0) > In2O3-TiO2-(0, 1.9) > Deggussa P25 > Ag/In2O3-TiO2-(2.0, 1.9) without P123 > pure TiO2. These results indicate that (i) codoped TiO2 system is more photoactive than single-doped TiO2 system, (ii) single doped TiO2 system is more photoactive than pure TiO2, and (iii) Ag/ In2O3-TiO2-(2.0, 1.9) prepared with P123 is more photoactive than that prepared without P123. Second, the influence of the catalyst calcination temperature on the photoactivity of Ag/In2O3-TiO2-(2.0, 1.9) was studied (see Figure 7c). It can be observed that the UV-light photocatalytic activity was enhanced as the annealing temperature

increased from 573 to 723 K. Further increasing the temperature to 873 K resulted in the decreased activity. Third, the effects of different Ag or In2O3 dopings on the photocatalytic activity of Ag/In2O3-TiO2 were examined (see Figure 7, parts d and e). As shown in Figure 7d, at the same In2O3 doping (1.3%) and different Ag dopings (0.97-21.8%), the UV-light photocatalytic activity of Ag/In2O3-TiO2 to RB degradation are different. Either lower (0.97 and 1.8%) or higher (21.8%) Ag dopings led to moderate activity. The highest activity was achieved by using Ag/In2O3-TiO2 with Ag loadings in the range 4.0-8.5%. From Figure 7e it can be found that at the same Ag loading (2.0%) but different In2O3 dopings (0.43-3.8%), the photocatalytic activity increased with In2O3 doping increasing from 0.43 to 1.9%. Further increasing In2O3 doping to 3.8% resulted in gradual decreases of the activity. Finally, influence of the molar ratio of P123 to TTIP on the photocatalytic activity of Ag/In2O3-TiO2-(2.0, 1.9) was studied (Figure 7f). Obviously, the photocatalyst prepared with P123:

11488 J. Phys. Chem. C, Vol. 112, No. 30, 2008

Figure 8. Changes of TOC in the reaction systems during the process of UV-light photocatalytic degradation of dye RB over different photocatalytic materials.

Figure 9. Time courses of UV-light photodegradation of MTBE over different photocatalysts. Initial concentration of MTBE 200 mg L-1. Catalyst: 0.15 g.

TTIP ) 1:20 showed the highest activity. Both lower (1:80) and higher (1:10) molar ratios led to decreased photocatalytic activity. It is a crucial step to obtain total mineralization of dye during its photodecomposition process since the yielded intermediates may be more toxic than the dye molecule itself. In current reaction system, the mineralization ability of as-prepared Ag/ In2O3-TiO2 nanocomposites to dye RB was evaluated by monitoring the changes of TOC (Figure 8, 9). It took 105 min of UV-light irradiation to mineralize RB totally over the Ag/ In2O3-TiO2-(2.0, 1.9). With the same UV-light irradiation time, mineralization of RB reached 90.1, 85.9 and 27.8%, respectively, for Ag/In2O3-TiO2-(8.5, 1.3), Degussa P25, and as-prepared pure anatase TiO2. 3.3.2. UV-Light Photocatalytic Degradation of MTBE. The photocatalytic activity of as-prepared Ag/In2O3-TiO2 composite was further evaluated by degradation of an aqueous MTBE under UV-light irradiation of Ag/In2O3-TiO2-(2.0, 1.9). Unlike RB, MTBE has no absorption in the UV-visible area. In addition, MTBE is a volatile, flammable, and colorless liquid which is widely used as a gasoline additive to replace tetraethyl lead to increase its octane rating and to help prevent engine knocking. MTBE often ends up in drinking water, negatively affecting its taste and odor, even at very low concentration.39 MTBE is not easily biodegradable, nor does it adsorb appreciably to surfaces such as activated carbon.40 Thus, elimination of MTBE from water is very important. In current photocatalytic system, degradation of MTBE (200 mg L-1) by direct photolysis was hardly observed after UVlight irradiation for 120 min. However, degradation of MTBE

Yang et al. reached to 94.5% after 120 min UV-light irradiation of Ag/ In2O3-TiO2-(2.0, 1.9). Under the same conditions, degradation of MTBE reached to 82.5%, 78.1%, and 62.2%, respectively, with UV-light irradiation of Ag/TiO2-(2.0, 0), In2O3-TiO2-(0, 1.9), and pure TiO2. The above results are consistent with those obtained for RB degradation (see section 3.3.1). 3.3.3. Discussion. According to the above photocatalytic test results and the morphology, structure, and physicochemical properties of as-prepared composites, we attribute the enhanced UV-light photocatalytic activity of the Ag/In2O3-TiO2 to the following reasons. (i) Enhanced quantum efficiency of Ag and In2O3 codoped TiO2 systems, which are contributed by three properties of the composites. First, since the Ag Fermi level is lower than that of TiO2, the photogenerated electrons may transfer to the Ag particles deposited on the surface of TiO2.41 Afterward, the transferred electrons are trapped by metallic Ag due to its strong electron accepting ability, resulting in the effective separation of the electrons and holes. Accordingly, more photoinduced holes and electrons can be produced, leading to more O2species and •OH radicals to participate in RB or MTBE degradation reaction.41–43 However, at high Ag loadings (more than 8.5%), the number of active sites capturing the photoinduced electrons is decreased with an increase in the size of Ag particles. Moreover, excessive Ag can cover the surface of TiO2, resulting in the reduced number of photogenarated charge carriers. Further, the presence of a large number of Ag nanoparticles on the surface of In2O3-TiO2 may slow down mass transport and reduce the reaction rate. These factors may account for the decreased photocatalytic activity at higher Ag contents (11.8 to 21.8%). Second, crystalline defects within TiO2 framework due to doping Ag may favor capturing photoelectrons and inhibiting charge recombination.35 At last, the wellcrystallized anatase could facilitate the transfer of the photoinduced electrons from bulk to surface, which lowers the probability of recombination of photoinduced holes and electrons.44–46 Among all TEM tested Ag/In2O3-TiO2 nanocomposites, the nanocrystalline nature of the Ag/In2O3-TiO2(2.0, 1.9) calcined at 723 K is perfect. Hence its photocatalytic activity is the highest. (ii) Decreased band gap due to coupling of semiconductor In2O3 with TiO2, which results from formation of dopant energy level within the band gap of TiO2. Therefore, the energy of the electron transition from the valence band to the conduction band is decreased, leading to more photogenerated carriers to join in the degradation reaction. Consequently, the activity of Ag/ In2O3-TiO2-(2.0, 1.9) was higher than Ag/TiO2-(2.0, 0) (Figure 7a). Moreover, with In2O3 loadings increasing from 0.43 to 1.9%, the activity increased gradually. These results are in accordance with their corresponding absorption properties. When the In2O3 doping was further increased to 3.8%, the photocatalytic activity of the samples decreased. The reason is that the aggregation of In2O3 nanoparticles became serious when more In2O3 was introduced into the nanocomposite. (iii) Size and morphology of Ag/In2O3-TiO2 are also responsible for its enhanced photocatalytic activity. In the present work, size and morphology of the products were tuned by changing the molar ratio of P123 to TTIP. At an appropriate molar ratio (1:20), the crystallite size of the product was extremely small with well-dispersed Ag clusters on the surface. Such a small size leads to a high surface to volume ratio and can also make indirect band electron transition possible and increase the generation rate of photogenerated carriers.47 In addition, a good dispersion or reduced aggregation among

TiO2 Nanocomposites particles may increase the active site-reactant contact area and also facilitate the electron transport. As for Ag/In2O3-TiO2(2.0, 1.9) without P123, its particle size is larger than those prepared with P123. Moreover, obvious aggregation and heterogeneous dispersion of Ag nanoparticles occurred. Such bigger metal particles not only decrease the number of active sites trapping the electrons but also enrich much more photogenerated electrons and become new recombination centers of photogenerated carriers.41 Further, the smallest BET surface area of the above sample is also accountable for its lower photocatalytic activity. 4. Conclusions The present work demonstrated a novel and simple route to prepare metallic Ag and semiconductor In2O3 codoped TiO2 nanocomposites. The surfactant P123 played an important role in reducing the product particle sizes and well dispersing the Ag nanoparticles on the surface of the products. At appropriate Ag or In2O3 dopings, photocatalytic activity of this Ag/ In2O3-TiO2 three-component system to dye RB and gasoline additive MTBE degradation outperformed as-prepared singleand two-component systems as well as commercial TiO2. The co-operation of the electron trapping ability of Ag and narrowing band gap of In2O3 was the key factor in determining this enhanced photocatalytic activity. In addition, crystalline defect, well-crystallized anatase phase structure, nanosized and welldispersed Ag nanoparticls on the surface of the composites, and larger BET surface areas also played important roles in this enhanced photocatalytic activity. Acknowledgment. This work is supported by the Program of New Century Excellent Talents in University (NCET-040311), the Key Project of Chinese Ministry of Education (No. 308008), the Program for Changjiang Scholars and Innovative Research Team in University, and the Analysis and Testing Foundation of Northeast Normal University. References and Notes (1) Wang, X.; Yu, J. C.; Ho, C.; Mak, A. C. Chem. Commun. (Cambridge) 2005, 2262. (2) Cozzoli, P. D.; Comparelli, R.; Fanizza, E.; Curri, M. L.; Agostiano, A.; Laub, D. J. Am. Chem. Soc. 2004, 126, 3868. (3) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (4) In, S.; Orlov, A.; Berg, R.; Garcı´a, F.; Pedrosa-Jimenez, S.; Tikhov, M. S.; Wright, D. S.; Lambert, R. M. J. Am. Chem. Soc. 2007, 129, 13790. (5) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935. (6) Ozaki, H.; Iwamoto, S.; Inoue, M. J. Phys. Chem. C 2007, 111, 17061. (7) Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Phys. Chem. C 2007, 111, 5259. (8) Tada, H.; Mitsul, T.; Klyonaga, T.; Aklta, T.; Tanaka, K. Nat. Mater. 2006, 5, 782.

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