UV- and Visible-Light Photocatalytic Activity of Simultaneously

Aug 2, 2012 - ... of Simultaneously Deposited and Doped Ag/Ag(I)-TiO2 Photocatalyst ... account for the different UV- and visible-light photocatalytic...
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UV- and Visible-Light Photocatalytic Activity of Simultaneously Deposited and Doped Ag/Ag(I)-TiO2 Photocatalyst Rui Liu,† Ping Wang,† Xuefei Wang,† Huogen Yu,*,† and Jiaguo Yu‡ †

Department of Chemistry, School of Science, Wuhan University of Technology, Wuhan 430070, PR China State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China

‡

S Supporting Information *

ABSTRACT: Ag modification has been demonstrated to be an efficient strategy to improve the photocatalytic performance of TiO2 photocatalysts. However, the previous studies about the Ag modification are only restricted to the surface loading of metallic Ag or Ag(I) doping, investigations have seldom been focused on the simultaneously deposited and doped Ag/ Ag(I)-TiO2 photocatalyst. In this study, Ag/Ag(I)-TiO2 photocatalyst was prepared by a facile impregnated method in combination with a calcination process (450 °C) and the photocatalytic activity was evaluated by the photocatalytic decomposition of methyl orange and phenol solutions under both UVand visible-light irradiation, respectively. It was found that Ag(I) doping resulted in the formation of an isolated energy level of Ag 4d in the band gap of TiO2. On the basis of band-structure analysis of Ag/Ag(I)-TiO2 photocatalyst, a possible photocatalytic mechanism was proposed to account for the different UV- and visible-light photocatalytic activities. Under visible-light irradiation, the isolated energy level of Ag 4d contributes to the visible-light absorption while the surface metallic Ag promotes the effective separation of the following photogenerated electrons and holes in the Ag/Ag(I)-TiO2 nanoparticles, resulting in a higher visible-light photocatalytic activity than the one-component Ag-modified TiO2 (such as Ag(I)-TiO2 and Ag/TiO2). Under UV-light irradiation, the doping energy level of Ag(I) ions in the band gap of TiO2 acts as the recombination center of photogenerated electrons and holes, leading to a lower photocatalytic performance of Ag-doped TiO2 (such as Ag/Ag(I)-TiO2 and Ag(I)-TiO2) than the corresponding undoped photocatalysts (such as Ag/TiO2 and TiO2). Considering the well controllable preparation of various Ag-modified TiO2 (such as TiO2, Ag/TiO2, Ag(I)-TiO2, and Ag/Ag(I)-TiO2), this work may provide some insight into the smart design of novel and high-efficiency photocatalytic materials.

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INTRODUCTION

The surface loading of metallic Ag nanoparticles on various semiconductors has been demonstrated to be an effective method for the improved photocatalytic performance.8c,10 It is generally accepted that metallic Ag nanoparticle functions as an electron sink to accept photogenerated electrons from excited semiconductor facilitating dioxygen reduction. In addition to the surface loading of metallic Ag, the doping of Ag(I) ions into the TiO2 phase structure was also investigated to improve their performance.11 It was found that the Ag(I)-doped TiO2 showed an enhanced UV-light photocatalytic activity for the decomposition of organic substances.11a,12 Therefore, it is clear that the previously reported studies about the Ag modification are only restricted to the surface loading of metallic Ag or Ag(I) doping, and investigations have seldom been focused on the synergy effect of surface loading of metallic Ag and bulk doping of Ag(I) ions on the UV- and visible-light photocatalytic activity of TiO2 photocatalyst.13

TiO2, one of the most popular photocatalysts, has been widely investigated for photocatalytic degradation of hazardous organic contaminants from wastewater.1 However, the TiO2 photocatalyst still cannot be widely used in practical applications due to its limited visible-light absorption and low photocatalytic efficiency.2 Therefore, the modification of TiO2 to make it sensitive to visible light and/or to improve the photocatalytic efficiency is one of the most important objectives in photocatalyst studies. Various modification methods of TiO2 photocatalyst have been widely investigated, such as doping various anions (N, S, C, etc.)3 and cations (Cr, V, Fe, Mn, Cu, Co, Ni, etc.)4 into the lattice of TiO2, sensitization of TiO2 with absorbed molecules,5 coupling with narrow band gap semiconductors,6 and grafting transition metal ions such as Fe(III) and Cu(II) (or their oxides).7 Among them, noble metalmodified TiO2 photocatalysts become one of the most important strategies to improve the photocatalytic performance.8 Compared with the expensive Pt, Pd, Rh, and Au noble metals, Ag-modified TiO2 photocatalyst is a more promising material in practical applications.9 © 2012 American Chemical Society

Received: June 13, 2012 Revised: July 24, 2012 Published: August 2, 2012 17721

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P5. Ag-doped TiO2 (referred to as P5) nanoparticles were prepared by using P4 nanoparticles as the precursor, namely, metallic Ag nanoparticles on the surface of the P4 nanoparticles were removed via a dissolution reaction of metallic Ag in nitric acid (Figure 1). Typically, 1 g of the P4 nanoparticles was dispersed into 60 mL of HNO3 (1 mol/L) aqueous solution under stirring. After reaction for 2 h, the product was filtrated and rinsed with purified water several times until the pH of the filtrated solution was about 6.5. Finally, the resulting product was dried at 60 °C for 2 h and was then ground into a fine powder to obtain the P5 nanoparticles. For comparison, the silver-deposited TiO2 photocatalyst was also prepared as follows. P2. Silver-deposited TiO2 (referred to as P2) nanoparticles were prepared by a well-known photoreduction method (Figure 1). Briefly, 1 g of the as-prepared TiO2 nanoparticles was dispersed into 9.26 mL of AgNO3 (0.1 mol/L) aqueous solution to form a suspension solution (Ag/TiO2 = 7.4 atom %), and 10 mL of MO aqueous solution (20 mg/L) was provided to act as the electron donor. After being stirred for 15 min, the above suspension was irradiated with a UV light for 2 h under magnetic stirring. The resulting P2 nanoparticles were recovered by filtration, rinsed with purified water, and finally dried at 60 °C for 2 h. P3. For comparison, the above P2 nanoparticles were also treated with nitric acid under the identical experimental conditions as the preparation of P5 photocatalyst. The resulting product was referred to as P3 with the aim of distinguishing it from precursor TiO2 nanoparticles. Characterization. X-ray diffraction (XRD) patterns were obtained on a Rigaku Ultima III X-ray Diffractometer (Japan), using Cu Kα radiation. Morphological analysis was performed by an S-4800 field-emission scanning electron microscope (FESEM, Hitachi, Japan) with an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) analyses were conducted with a JEM-2100F electron microscope (JEOL, Japan), using a 200 kV accelerating voltage. X-ray photoelectron spectroscopy (XPS) measurements were done on a VG ESCALAB 210 XPS spectrometer system with Mg Kα source. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. UV−vis absorption spectra were obtained by using a UV−visible spectrophotometer (UV-2550, SHIMADZU, Japan). Photocatalytic Activity. The UV- and visible-light photocatalytic activities of the prepared samples were evaluated by the photocatalytic decolorization of MO solution.14 Experimental details are as follows: A 0.05-g sample was dispersed into 10 mL of MO solution (20 mg/L) in a disk with a diameter of ca. 5 cm. The solution was allowed to reach an adsorption−desorption equilibrium among the photocatalyst, MO, and water before irradiation. For the evaluation of visiblelight photocatalytic activity, a 350 W xenon lamp equipped with a UV-cutoff filter (providing visible-light with ≥400 nm) was used as a visible-light source, and the average light intensity striking the surface of the reaction solution was about 80 mW cm2. For the evaluation of UV-light photocatalytic activity, integrated UV intensity in the range of 310−400 nm striking the coatings, measured with a UV radiometer (Model UV-A, made in Photoelectric Instrument Factory of Beijing Normal University), was 2.5 mW/cm2, while the peak wavelength of UV-light was 365 nm. The concentration of MO was determined by an UV−visible spectrophotometer (UV-2550, SHIMADZU, Japan). After visible-light irradiation for some

In this study, the simultaneously deposited and doped Ag/ Ag(I)-TiO2 photocatalyst was prepared by a facile impregnated method in combination with a calcination process (450 °C). For comparison, the bare TiO2, Ag-loaded TiO2 (Ag/TiO2), and Ag(I)-doped TiO2 (Ag(I)-TiO2) were also prepared by a simple controllable method. The photocatalytic activities of the Ag-modified TiO2 nanoparticles were evaluated by the photocatalytic decomposition of methyl orange (MO) and phenol solutions under both UV- and visible-light irradiation, respectively. On the basis of band-structure analysis of Ag/ Ag(I)-TiO2 photocatalyst, a possible photocatalytic mechanism was proposed to account for the different UV- and visible-light photocatalytic activities. In addition, it was found that the Agmodified TiO2 photocatalysts (such as TiO2, Ag/TiO2, Ag(I)TiO2, and Ag/Ag(I)-TiO2) could be easily controlled. To the best of our knowledge, this is the first report about the compared UV- and visible-light photocatalytic performance of the simultaneously deposited and doped Ag/Ag(I)-TiO2 photocatalyst. This work may provide new insights into the smart design and preparation of new high-efficiency photocatalytic materials.

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EXPERIMENTAL DETAILS Preparation. P1. Tetrabutyl titanate (Ti(OC4H9)4 or TBOT) was used as a titanium source for the preparation of TiO2 nanoparticles. Typically, 10 mL of TBOT was slowly added into 1000 mL of distilled water under vigorous stirring at room temperature. After continuous stirring for 2 h and aging for another 24 h, the gel precipitate was filtered, washed with distilled water, dried at 80 °C for 8 h, and then ground into fine powder. Finally, the fine powder was calcined at 500 °C for 2 h. The specific surface area of the TiO2 nanoparticles is 96.1 m2/g based on the analysis of nitrogen adsorption−desorption isotherms (ASAP 2020, Micromeritics Instruments, USA). The obtained TiO2 nanoparticles were used to prepare various Ag-modified TiO2 photocatalysts and the corresponding schematic diagram is shown in Figure 1.

Figure 1. Schematic diagram illustrating the controllable preparation of various Ag-based TiO2 photocatalysts.

P4. Simultaneously Ag-deposited and Ag-doped TiO 2 (referred to as P4) photocatalyst was prepared by a facile impregnated method in combination with a calcination process (450 °C). Briefly, 1 g of the TiO2 nanoparticles was dispersed into 9.26 mL of AgNO3 solution (0.1 mol/L) to form a suspension (Ag/TiO2 = 7.4 atom %). After being stirred for 2 h, the resulting suspension was dried at 100 °C and then calcined at 450 °C for 1 h. The resulting sample was ground into a fine powder to obtain P4 composite nanoparticles. 17722

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time, the reaction solution was centrifuged to measure the concentration of MO. As for the MO aqueous solution with low concentration, its photocatalytic decolorization is a pseudofirst-order reaction and its kinetics may be expressed as ln(c /c0) = −kt, where k is the apparent rate constant, and c0 and c are the MO concentrations at the initial state and after irradiation for t min, respectively.15 In addition, the colorless phenol (10 mg/L) was also used as the target organic substance to evaluate the photocatalytic performance of the Ag-modified TiO2 samples under the identical UV- and visible-light irradiation.

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RESULTS AND DISCUSSION Morphology and Microstructures of P4 Nanoparticles. Figure 2 shows the XRD patterns of TiO2 nanoparticles

Figure 3. SEM images of (a) P1 nanoparticles and (b) P4 composite nanoparticles and (c, d) TEM images of the P4 composite nanoparticles. Figure 2. XRD patterns of the various TiO2 photocatalysts: (a) P1, (b) P2, (c) P3, (d) P4, and (e) P5.

before and after modification by Ag element. It is clear that all the diffraction peaks of the samples can be indexed to the TiO2 anatase phase (JCPDS 84−1285). Moreover, all the Agmodified TiO2 photocatalysts show the diffraction peaks with a similar intensity and full width at half-maximum (ca. 0.50), suggesting that the crystallization and crystallite size of TiO2 are not affected by the Ag modification processes (including Ag doping and surface deposition). According to the Scherrer formula, the calculated average crystallite size of the TiO2 nanoparticles is ca. 18 nm. However, it should be noted that the metallic Ag phase cannot be detected in the Ag-modified TiO2 nanoparticles due to a low concentration.16 When a powder sample is composed of nanoscale particles, aggregated particles with a larger size are usually formed and can be easily observed by SEM image. Panels a and b of Figure 3 show the typical SEM images of the P1 and P4 powders, respectively. It is found that the particle size of aggregated TiO2 particles (Figure 3a) is in the range of 100−500 nm. After modification by Ag to form the P4 (Figure 3b), the surface morphology and size of the sample have no obvious change. However, according to the TEM images (Figure 3c,d), metallic Ag nanoparticles with a size of 5−10 nm are found to be deposited on the surface of nanoparticles. In addition, compared with the TiO2, a thin layer with a thickness of 3−5 nm is formed on the surface of TiO2. Considering a lower temperature (450 °C) for the preparation of P4 photocatalyst than the P1 precursor (500 °C), the thin layer can be attributed to the Ag(I)-doped TiO2 owing to a limited diffusion length of Ag(I) into TiO2 at 450 °C. To further demonstrate the successful synthesis of various Ag-modified TiO2 composites, their elemental compositions and chemical status were analyzed by XPS (Figure 4). Compared with the precursor TiO2, new XPS peaks of Ag

Figure 4. (A) XPS survey spectra of the various TiO2 photocatalysts and (B) their corresponding XPS spectra of Ag 3d: (a) P1, (b) P2, (c) P3, (d) P4, and (e) P5.

element are found in the P2 and P4 samples in addition to the Ti, O, and C elements based on XPS survey spectra (Figure 4A). The Ti and O elements are mainly from the TiO2 phase, while the Ag element results from the surface deposition of 17723

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considering a larger binding energy of metallic Ag (368.6 eV, Figure 3B-b) and a smaller value of Ag(I) ions (367.7 eV, Figure 3B-e), it is very clear that the XPS peak of Ag at 368.3 eV (Figure 3B-d) can be ascribed to a composite structure of metallic Ag and Ag(I) ions. Therefore, in this study, the P4 nanoparticles are successfully prepared by the impregnated method in combination with a calcination process (450 °C), while after treatment in nitric acid, the resulting sample can be attributed to P5 nanoparticles. According to XPS analysis (Table 1), the amount of Ag element in P4 and P5 is 7.9 and 0.4 atom %, respectively. This suggests that most of the Ag element existed as metallic Ag nanoparticles and only a small amount of Ag(I) is doped into the phase structure of TiO2 by using the present preparation method. The compositions and microstructures of the Ag-modified samples can be further demonstrated by UV−vis spectra, as shown in Figure 5. As for the P1 (Figure 5a), only the band gap

metallic Ag and/or Ag(I) doping in the TiO2 (see below). To further investigate the possible chemical status of Ag element (including metallic Ag or Ag(I)), their corresponding XPS peaks of Ag element were shown in Figure 4B. It is clear that there is no XPS peak of Ag 3d in the TiO2 precursor due to the absence of Ag element. As for the P2 sample prepared by a well-known photoreduction method (Figure 4B-b), a strong XPS peak of Ag 3d can be observed. The binding energy of Ag 3d at 368.6 eV can be assigned to metallic Ag phase,17 indicating the successful formation of P2 composite by the photoreduction technology. The amount of metallic Ag on the surface of P2 is ca. 8.3 atom % (Table 1). When the prepared Table 1. Composition (atom %) of the Various TiO2 Photocatalysts According to XPS Analysis samples

O (mol %)

Ti (mol %)

Ag (mol %)

P1 P2 P3 P4 P5

65.5 61.0 63.6 60.5 63.6

34.5 30.7 36.4 31.6 36.0

0 8.3 0 7.9 0.4

P2 powder was dispersed into HNO3 aqueous solution to prepare the P3 photocatalyst, the resulting XPS result (Figure 4A-c) shows a similar XPS survey spectrum as the P1 precursor (Figure 4A-a). Moreover, the XPS peak of Ag 3d at 368.6 eV disappears completely (Figure 4B-c) and the amount of Ag element is undetectable by XPS (Table 1), suggesting the effective removal of metallic Ag from the surface of TiO2 nanoparticles by HNO3 solution. Actually, the successful removal of metallic Ag from the TiO2 surface can be further confirmed by observing their color change during sample preparation. The TiO2 precursor shows a white color due to the absence of visible-light absorption. After coating with metallic Ag, the resulting P2 exhibits a brown color owing to the local surface plasmon resonance (LSPR) of Ag nanoparticles (see below). However, when the P2 powder was dispersed into HNO3 aqueous solution for 2 h, the color gradually turned from brown to white again. Therefore, the above experimental results strongly suggest that the treatment in HNO3 solution is an effective strategy to remove the metallic Ag phase from the TiO2 surface, and it is possible for us to prepare pure Ag(I)doped TiO2 (namely, P5) photocatalyst by using the P4 nanoparticles as the precursor (Figure 1). When the TiO2 nanoparticles are impregnated with Ag(I) ions and then calcined at 450 °C, it is expected that the obtained Ag-modified TiO2 sample is the P4 composite. In fact, the composite structure of P4 nanoparticles can be well demonstrated as follows. According to XPS result (Figure 3Bd), a strong XPS peak of Ag with a binding energy of 368.3 eV can be observed in the P4 samples. On the basis of the TEM image (Figure 2d), the metallic Ag nanoparticles are shown to coat on the surface of nanoparticles. Therefore, it can be deduced that the XPS peak of Ag at 368.3 eV (Figure 3B-d) can partially come from the metallic Ag phase. To exclude the effect of surface metallic Ag on the binding energy of Ag (Figure 3Bd), the above P4 sample was treated by nitric acid and the resulting P5 sample was further tested by XPS. It is clear that the intensity of the XPS Ag 3d peak of the P4 sample (Figure 3B-e) has an obvious decrease. Moreover, its binding energy shifts from 368.3 eV to a smaller value of 367.7 eV, which can be attributed to the Ag(I) ions.16b,17a,c As a consequence,

Figure 5. UV−vis spectra of the various TiO2 photocatalysts: (a) P1, (b) P2, (c) P3, (d) P4, and (e) P5. The inset shows the difference UV−vis spectrum between (e) P5 and (a) P1.

absorption with a steep absorption edge at the UV region can be observed. After the surface of TiO2 nanoparticles is coated with Ag, the obtained P2 composite sample (Figure 5b) exhibits a wide visible-light absorption in the range of 400−800 nm owing to the strong LSPR of Ag nanoparticles.14a,15a After a simple treatment in nitric acid solution, the resulting P3 sample (Figure 5c) shows a very similar absorption curve with the TiO2 precursor, indicating the effective removal of metallic Ag from the TiO2 surface. For the P4 nanoparticles (Figure 5d), there is also a wide absorption in the visible-light region (400− 800 nm) due to the coating of Ag nanoparticles and the Ag(I) doping. However, after treatment of the P4 nanoparticles in nitric acid solution, there is still an obvious absorption shoulder at 380−550 nm compared with the pure TiO2 nanoparticles. The absorption shoulder is caused by the doping energy level of Ag(I) ion in the P5 nanoparticles (Figure 5e). The present UV−vis results are in good agreement with the XPS results. Visible-Light Photocatalytic Activity of P4 Nanoparticles. The photocatalytic performance of P4 nanoparticles was evaluated by photocatalytic decolorization of MO aqueous solution under visible-light irradiation, as shown in Figure 6A,B. It is clear that the P1 and P3 nanoparticles show no photocatalytic performance even after coating with metallic Ag to form P2 composite photocatalyst. However, the P4 composite photocatalysts show a high photocatalytic activity and MO is quickly decomposed with increasing irradiation time (Figure 6A-d). After the removal of metallic Ag on the P4 nanoparticles, the resulting P5 nanoparticles still show an obvious visible-light photocatalytic activity (Figure 6A-e) 17724

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mechanism usually exists in the photocatalytic processes when colored dyes are used as the target organic substances to evaluate the photocatalytic performance of a photocatalyst.18 In this study, it is found that P1, P2, and P3 photocatalysts show no photocatalytic activities (Figure 7a,b) although MO can absorb visible light in the range of 400−570 nm with a peak at 460 nm. Therefore, the decomposition of MO via photosensitization mechanism can be ignored in this study owing to its strong photostability. As for the P5 nanoparticles, it shows an obvious visible-light photocatalytic activity (Figure 6B). In view of the visible-light absorption shoulder at 400−550 nm in the UV−vis spectrum (Figure 5e), it can be deduced that the visible-light photocatalytic performance of P5 nanoparticles is caused by the Ag(I) doping in TiO2. It is usually reported that the Ag-4d orbital can have a great effect on the valence band (VB) of TiO2.19 Considering that only a small number of Ag+ ions are impregnated into the lattices of TiO2, the absorption shoulder in Figure 5e can be attributed to an isolated energy level of Ag 4d formed in the band gap of TiO2. A similar absorption shoulder is usually found in the N-doped TiO2 system owing to the formation of an isolated energy level of N element above the valence band of TiO2.20 To further determine the real position of Ag 4d in the band gap of TiO2, the difference UV−vis spectrum between the P5 and P1 nanoparticles is shown in the inset of Figure 5. It is clear that there is an obviously increased absorption at ca. 450 nm. Similar to the estimation of band gap of a semiconductor, the isolated energy level of Ag doping can be calculated to be ca. 2.56 V, as shown in the Figure 7. Under visible-light irradiation, the photogenerated electrons are excited from the isolated energy level of Ag(I) doping to the conduction band of TiO2 to reduce oxygen, while the photogenerated holes remain on the Ag-doping energy level to oxidize organic substances, resulting in the visible-light photocatalytic activity (Figure 7d). Compared with the P5 photocatalyst, the P4 nanoparticles show an obvious enhanced photocatalytic activity (Figure 6). This phenomenon is well understood as Ag nanoparticles can work as an efficient cocatalyst to promote the electron−hole separation and interfacial charge transfer after band gap excitation (Figure 7c).21 UV-Light Photocatalytic Activity of P4 Nanoparticles. Figure 8 shows the UV-light photocatalytic performance of various Ag-modified TiO2 nanoparticles. It is clear that the TiO2 nanoparticles show a high photocatalytic performance (Figure 8A-a) under UV-light irradiation and the corresponding rate constant (k) is calculated to be 2.5 × 10−3 min−1 (Figure 8B-a). After coating metallic Ag nanoparticles to form P2

Figure 6. (A) Photocatalytic decolorization of MO solution for the various TiO2 photocatalysts and (B) their corresponding rate constant (k) under visible-light irradiation: (a) P1, (b) P2, (c) P3, (d) P4, and (e) P5.

although the decomposition rate of MO has an obvious decrease. Provided that the photocatalytic reaction follows a pseudo-first-order reaction, the rate constants (k) of the MO decomposition over various photocatalysts were estimated and the corresponding results are shown in the Figure 6B. It is clear that the P4 shows the highest photocatalytic activity with a rate constant of 1.0 × 10−2 min−1, a value larger than that of P5 (1.9 × 10−3 min−1) by a factor of 5.3. In addition, further experiments indicated that the P4 photocatalyst also shows an effective decomposition for the phenol solution under an identical visible-light irradiation (Figure S1, Supporting Information). To understand the visible-light photocatalytic activity of P4 nanoparticles, it is important to explore its photocatalytic mechanism. It is well-known that the photosensitization

Figure 7. Schematic diagrams illustrating the possible photocatalytic mechanism of the various TiO2 photocatalysts under visible-light irradiation. 17725

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energy level in the band gap contributes to the visible-light absorption and produces photocatalytic activity (Figure 7d). On the contrary, under UV-light irradiation it usually becomes the recombination centers of photogenerated electrons and holes owing to a slower transfer rate of the holes in the isolated energy level (Figure 9d) than in the concessive O2p VB (Figure 9a).22 In addition, compared with the VB holes in the pure TiO2, the holes formed in the Ag-4d isolated energy level have a lower oxidation power for the oxidation of organic substances. Therefore, it is not surprised that the P5 show a lower photocatalytic activity than the pure TiO2. In fact, this phenomenon is in good agreement with the N-doped TiO2 photocatalyst in which it is found that the quantum efficiency of N-doped TiO2 is obviously lower than the pure TiO2 owing to the formation of the N-2p isolated energy level.20 After coating metallic Ag nanoparticles, the resulting P2 and P4 samples show enhanced photocatalytic performance compared with their corresponding P1 and P5 nanoparticles, respectively. The general principle is that the formation of a space-charge layer at the cocatalyst−semiconductor interface improves the charge-separation rate after band gap excitation. Therefore, the P2 exhibits the highest UV-light photocatalytic performance due to an efficient separation of photogenerated electrons and holes. In addition, it should be mentioned that the photocatalytic performance of P3 nanoparticles is higher than that of TiO2 precursor under UV irradiation, which can be ascribed to the increasing amount of surface hydroxyl after treatment by acids.23 Photoinduced Stability of the Ag(I) in P4 Photocatalyst. To evaluate the photostability of Ag(I) in the P4, the photostability of the Ag(I) in the P5 was first investigated under UV-light irradiation. After UV-light irradiation for 1 h, it is found that the color of the P5 obviously changes from light yellow to brown (inset b in Figure 10). The corresponding UV−vis spectra suggest that there is an obviously enhanced absorption in the visible-light region owing to the formation of Ag nanoparticles (Figure 10b). Therefore, the above results indicate that the Ag(I) in the P5 can be reduced under UV-light irradiation, in good agreement with the previous studies.16b However, with further increase of the UV-light irradiation time to 2 h, the color and UV−vis spectrum of the P5 show no significant change (not shown here), suggesting the formation of a new and stable P4 photocatalyst (referred to as P6). To further investigate the partial reduction of Ag(I) in the P5 after UV-light irradiation, the resulting P6 photocatalyst was treated with nitric acid to remove the metallic Ag under the identical experimental conditions as the preparation of P5 from P4. It is

Figure 8. (A) Photocatalytic decolorization of MO solution for the various TiO2 photocatalysts and (B) their corresponding rate constant (k) under UV-light irradiation: (a) P1, (b) P2, (c) P3, (d) P4, and (e) P5.

composite photocatalyst, the composite nanoparticles show an enhanced photocatalytic activity with a rate constant of 5.2 × 10−3 min−1 (Figure 8B-b), which is even higher than that (4.3 × 10−3 min−1) of the P4 nanoparticles (Figure 8B-d). When the metallic Ag nanoparticles were removed by HNO3 solution, the resulting P3 and P5 photocatalysts showed obviously lower photocatalytic activities with a rate constant of 3.7 × 10−3 min−1 (Figure 8B-c) and 2.1 × 10−3 min−1 (Figure 8B-e), respectively. A similar photocatalytic performance can also be observed for the decomposition of phenol solution (Figure S1, Supporting Information). Compared with their visible-light performances, the Agmodified TiO2 samples show a conspicuous difference in the UV-light photocatalytic activities owing to a different photocatalytic mechanism. It is interesting to find that the P5 photocatalyst shows the lowest UV-light photocatalytic activity even lower than the pure TiO2 nanoparticles. The present results can be well understood according to its band-structure analysis as shown in Figure 9d. For the P5 photocatalyst, there is a completely different effect of the Ag-4d isolated energy level (ca. 2.56 V) on the UV- and visible-light photocatalytic performance. Under visible-light irradiation, the Ag-4d isolated

Figure 9. Schematic diagrams illustrating the possible photocatalytic mechanism of the various TiO2 photocatalysts under UV-light irradiation. 17726

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Figure 10. UV−vis spectra of the P5 photocatalyst before and after light irradiation: (a) P5, (b) P5 after UV-light irradiation for 1 h, (c) P5 after UV light reaction for 1 h and then treatment with nitric acid, and (d) P5 after visible-light irradiation for 1 h. The inset shows their corresponding photos. Figure 11. Schematic diagrams illustrating the controllable preparation of various Ag-modified TiO2 photocatalysts.

found that the obtained sample (referred to as P7) still shows a yellow color (inset c in Figure 10) and a similar UV−vis spectrum (Figure 10c) as the as-prepared P5 (Figure 10a). The above results strongly demonstrate the fact that the Ag(I) in P5 can be partially reduced under UV-light irradiation to form a new and stable P4 photocatalyst. The photostability of Ag(I) in the P5 was further investigated under visible-light irradiation. It is found that after visible-light irradiation for 1 h, the color of the P5 photocatalyst only shows a slight change (inset d in Figure 10). With further increase of the irradiation time to 2 h, no obvious change of the color of P5 can be observed. In addition, the UV−vis spectra of the P5 photocatalyst before and after visible-light irradiation show similar results (Figure 10d). Therefore, it is clear that the P5 photocatalyst can be regarded as a stable photocatalyst under visible light irradiation. When P4 photocatalyst was irradiated under UV or visible light, it shows a similar result as the P5. However, owing to the existence of metallic Ag nanoparticles on the surface of P4, the P4 only shows a limited color change of brown after UV-light irradiation. Controllable Transformation of Ag-Modified TiO2 Nanoparticles. It is demonstrated that Ag modification (including doping into the lattice and surface loading on materials) is an important technology for the enhanced photocatalytic performance in the decomposition of organic substances, water splitting, CO2 reduction, and dye-sensitized solar cell.8c,10,11a,12 Therefore, the precise synthesis of photocatalytic materials is highly required to investigate the real function of Ag element. In this study, the above results highlight a simple and efficient strategy for the controlled synthesis of Ag-modified TiO2 photocatalysts with high UV- or visible-light photocatalytic performance (Figure 11). The photoreduction method of Ag(I) is an effective method for the preparation of highly efficient UV-light P2 photocatalysts, while the impregnation technology combined with the following calcination process can be one of the simple strategies for the preparation of visible-light-responded P4 photocatalysts. In addition, the HNO3 treatment is an effective technology for the removal of metallic Ag to synthesize pure Ag(I)-doped TiO2 photocatalysts. The controlled preparation of Ag-based TiO2 photocatalysts can be further developed as follows (Figure 11). (1) For the formation of P4 nanoparticles, it is believed that the 450 °Ccalcination process causes the diffusion of Ag into the TiO2 phase structure, which can be demonstrated by the following

experiments. After the photoreduced P2 sample was also calcined at 450 °C for 2 h, it was interesting to find that the obtained sample showed an enhanced visible-light photocatalytic activity (1.1 × 10−3 min−1), although the value is lower than that of P4 (1.0 × 10−2 min−1). After the removal of metallic Ag by HNO3, the Ag(I) can be detected by XPS and the amount of Ag is ca. 0.08 atom %. Therefore, the P4 photocatalysts can also be prepared from the calcination of P2. (2) When the Ag nanoparticles were redeposited on the P5 nanoparticles by a photoreduction method to form new Agloaded P5 composites, the photocatalytic activity of the resulting photocatalyst can be well recovered under both UV(3.6 × 10−3 min−1) and visible-light irradiation (4.4 × 10−3 min−1) owing to an effective separation of photogenerated electrons and holes.

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CONCLUSION The simultaneously deposited and doped P4 photocatalyst was successfully prepared by a facile impregnated method in combination with a calcination process (450 °C). It was found that the P4 photocatalyst showed a completely different mechanism for UV- and visible-light photocatalytic performance owing to the formation of an isolated energy level of Ag 4d in the band gap of TiO2. Under visible-light irradiation, the isolated energy level of Ag 4d contributes to the visible-light absorption while the metallic Ag promotes the effective separation of the following photogenerated electrons and holes in the P4 nanoparticles, resulting in the high visible-light photocatalytic activity. As for the UV-light photocatalytic activity, the doping energy level of Ag(I) ions in the band gap of TiO2 acts as the recombination centers of photogenerated carriers, leading to a lower photocatalytic performance of Ag-doped TiO2 nanoparticles (such as P4 and P5) than the undoped TiO2 photocatalysts (such as P2 and P1). In addition, the Ag(I) in the P5 photocatalysts can be partially reduced under UV-light irradiation to form a new P4 photocatalyst, while it shows a more stable structure under visible light. Considering a well-controllable preparation of various Ag-modified TiO2 (such as P1, P2, P5, and P4), this work may provide some insight into the smart design of novel and high-efficiency photocatalytic materials. 17727

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The Journal of Physical Chemistry C

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Article

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 giving the photocatalytic decolorization of phenol solution for the various TiO2 photocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 0086-27-87871029. Fax: 0086-27-87879468. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (20877061 and 51072154), Natural Science Foundation of Hubei Province (2010CDA078), and the 863 Program (2012AA062701). This work was also financially supported by the Fundamental Research Funds for the Central Universities (grants 2011-1a039, 2011-1a-016, 2012-ZY-103).

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