Enhancement of Visible-Light Photocatalytic Activity of Ag0.7Na0

Dec 4, 2008 - ... People's Republic of China, Photocatalytic Materials Center (PCMC), National Institute for Materials Science (NIMS), 1-2-1 Sengen, T...
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J. Phys. Chem. C 2008, 112, 20329–20333

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Enhancement of Visible-Light Photocatalytic Activity of Ag0.7Na0.3NbO3 Modified by a Platinum Complex Guoqiang Li,†,‡,§ Defa Wang,‡ Zhigang Zou,†,§ and Jinhua Ye*,†,‡ Department of Materials Science and Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China, Photocatalytic Materials Center (PCMC), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, and Ecomaterials and Renewable Energy Research Center (ERERC), Department of Physics, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: May 02, 2008; ReVised Manuscript ReceiVed: September 18, 2008

The visible-light-sensitive photocatalyst of Ag0.7Na0.3NbO3 is synthesized by a polymerized complex (PC) method, and the photocatalytic activity is evaluated by decomposition of gaseous 2-propanol under visiblelight irradiation. The PC samples have larger surface areas and exhibit higher photocatalytic activity than the sample prepared by a solid-state reaction method. In addition, the activity of the PC sample is improved significantly by the modification of H2PtCl6. The enhancement of photocatalytic activity is attributed to the increased visible-light absorbance and the promoted charge separation and transfer capability in the H2PtCl6/ PC800 hybrid system. 1. Introduction To utilize solar energy and indoor artificial illuminations effectively, the development of visible-light-active photocatalysts has become a hot topic in the field of photocatalysis.1-5 Recently, various modified TiO2 (e.g., nitrogen-doped TiO2)6 and new materials7-9 have been reported for photocatalytic decomposition of organic compounds under visible-light irradiation. So far, however, highly visible-light-active photocatalysts have rarely been reported. Many approaches have been adopted to develop efficient visible-light-active photocatalysts by improving the bulk and surface properties.10-15 It is known that, for a given photocatalyst, increasing the surface area usually plays an important role in improving its photocatalytic reaction.16,17 Moreover, photosensitizing with metal complex (e.g., H2PtCl6)14,15 has been found to be an effective method for enhancing the visible-light photocatalytic activity of TiO2 previously. Recently, the solid solutions of Ag1-xNaxNbO3 were found to be active for photocatalytic decomposition of gaseous 2-propanol (IPA) under weak visible-light irradiation, and Ag0.7Na0.3NbO3 showed the better activity.18 However, the surface area of the sample synthesized by a solid-state reaction method was small due to the high sintering temperature. Our preliminary research revealed that the surface area of AgNbO3 was hard to increase by wet chemical methods, whereas that of NaNbO3 was easy to increase by the polymerized complex (PC) method. In this paper, we synthesized the Ag0.7Na0.3NbO3 by a PC method and evaluated the photocatalytic activity for decomposition of gaseous IPA under visible-light irradiation. Moreover, the effect of H2PtCl6 on the photocatalytic activity of the PC sample was also investigated. The results showed that the activity of the hybrid H2PtCl6/PC sample was over 10fold higher than that of the naked sample. * Corresponding author. Tel.: +81-29-859-2646. Fax: +81-29-859-2301. E-mail: [email protected]. † Department of Materials Science and Engineering, Nanjing University. ‡ National Institute for Materials Science. § Department of Physics, Nanjing University.

2. Experimental Section 2.1. Catalyst Preparation. The Ag0.7Na0.3NbO3 photocatalyst was prepared by a PC method. Niobium ethoxide (Nb(EtO)5, 5 g) and citric acid (19.2 g) were dissolved in 100 and 40 mL of methanol, respectively. Under vigorous stirring, 20 mL of Nb(EtO)5 methanol solution was slowly added into the citric acid methanol solution, and then stoichiometric Na2CO3 and silver acetate powders were dissolved into the above solution at room temperature. After adding 2 mL of ethylene glycol, the mixture was preliminarily heated at 120-130 °C until it turned to solid. Then the solid precursor was heated at 400 °C for 1 h and finally at various temperatures from 700 to 800 °C for 1 h, obtaining the light yellow Ag0.7Na0.3NbO3 samples. For simplicity, the samples calcined at 700, 750, and 800 °C are denoted hereafter as PC700, PC750, and PC800, respectively. As a comparison, the Ag0.7Na0.3NbO3 sample (ANN(SSR)) was also synthesized through solid-state reaction of stoichiometric Na2CO3, Ag2O, and Nb2O5 at 970 °C for 7 h. The typical process for loading the metal complex H2PtCl6 on the catalyst surface was as follows: 0.2 g of PC800 powder sample was added into 0.55 mL of the H2PtCl6 solution (0.185 mol/L); after the solution was evaporated to dried solid, the hybrid photocatalyst H2PtCl6/PC800 was formed. For comparison, the H2PtCl6/SiO2, H2PtCl6/ANN(SSR), H2PtCl6/TiO2(A) (anatase; Wako), and H2PtCl6/N-TiO2 (type: TPS-201; lot. no.: ZB4302) were prepared following the similar procedure as mentioned above. Furthermore, the H2PtCl6/PC800 was calcined at 400 °C for 5 h to obtain the Pt/PC800(cal). The Pt/ PC800(photo) sample was also prepared by the in situ photodeposition method in a methanol solution (∼30 vol %) under UV-light irradiation. 2.2. Characterization. The crystal structures of the samples were determined by an X-ray diffractometer (XRD; RINT 2000; RIGAKU, Japan) operated at 30 kV and 40 mA using Cu KR radiation (λ ) 1.54178 Å). The scanned range was 2θ ) 20-70°, with a step of 2θ ) 0.02° and 0.5 s/step. The diffuse reflectance spectrum was recorded with a UV-vis spectrophotometer (UV-2500; Shimadzu, Japan) at room temperature and transformed to the absorption spectrum according to the

10.1021/jp803864j CCC: $40.75  2008 American Chemical Society Published on Web 12/04/2008

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Figure 1. XRD patterns of PC700, PC750, PC800, and ANN(SSR) samples. The peaks of impurities (Na2Nb4O11 or Ag2Nb4O11) are marked by the arrows (V).

Kubelka-Munk relationship, K/S ) (1 - R)2/2R, where R, K, and S are the value of reflectance measurements (relative value to the reference of BaSO4) and the absorption and scattering coefficients of the sample, respectively.19 The infrared transmittance spectrum was obtained on a Fourier transform infrared spectrophotometer (FTIR) (IRPrestige-21; Shimadzu, Japan). The surface area was analyzed using the surface area and porosity analyzer (Micromeritics, U.S.A.) by nitrogen adsorption at 77 K using the Brunauer-Emmett-Teller (BET) method. The X-ray photoelectron (XPS) spectra were obtained using a Thermo ESCALAB 250 physical electronics photoelectron spectrometer with monochromatized Al KR X-ray radiation (1486.6 eV). The binding energy was determined by reference to the C 1s line at 284.8 eV. 2.3. Evaluation of Photocatalytic Activity. The 0.2 g powder sample was evenly spread on the bottom of a vessel with 8.1 cm2 area, and the vessel was placed on the base of the 500 mL reactor. After the reactor was sealed with a quartz cover and the inside atmosphere was replaced by synthetic air, gaseous IPA with concentration of ca. 200 ppm was introduced into the reactor. The reactor was stored in the dark until the concentration of IPA became unchanged, i.e., the system reached the adsorption equilibrium. Then, the reactor was irradiated with visible light, which was emitted from blue light-emitting diodes (BLEDs). With the use of a spectroradiometer (USR-40D; Ushio, Japan), the light intensity was determined to be 0.01 mW · cm-2 and the light wavelength range was from 400 to 520 nm.18 The use of such a weak light could avoid the possible thermal effect. The concentrations of IPA, acetone, and CO2 were measured using a gas chromatograph (GC-14B; Shimadzu, Japan) equipped with a flame ionization detector (FID) and a methanizer. The gaseous IPA would first be oxidized to acetone as the intermediate product.20-22 Therefore, the photocatalytic activity was evaluated by the average rate of acetone evolution within 2 h. 3. Results and Discussion From the XRD patterns as shown in Figure 1, we can see that the PC samples mainly contain the phase Ag0.7Na0.3NbO3 with a very small amount of Na2Nb4O11 or Ag2Nb4O11 impurities. In comparison to the PC samples, the ANN(SSR) sample has a higher purity and crystallinity. Light absorption property was characterized by the UV-vis diffuse reflectance spectrometry, and the results are shown in Figure 2. Both PC800 and ANN(SSR) can absorb the visible light. It should be mentioned that the impurities of Na2Nb4O11 or Ag2Nb4O11 in the PC800 sample do not absorb the visible

Li et al.

Figure 2. UV-vis diffuse reflectance spectra of PC800 (dash) and ANN(SSR) (solid line). The inset is the plot of (Rhν)1/2 vs hν.

Figure 3. Acetone evolution rates and BET surface areas of the SSR, PC700, PC750, and PC800 samples.

light; thus, the possibility of visible-light activity from these impurities can be excluded. The spectrum of PC800 has a little difference from that of the SSR sample, probably caused by their different crystallinities.23 The band gap of the PC800 sample was estimated to be about 2.83 eV from the plot of (Rhν)1/2 versus hν as shown in Figure 2 (inset). Figure 3 shows the acetone evolution rates and the BET surface areas of the prepared PC samples under visible-light irradiation emitted from BLEDs. The surface areas for PC700, PC750, PC800, and ANN(SSR) are 7.1, 5.6, 2.8, and 1.0 m2 · g-1, respectively. As we expected, the BET surface areas of PC samples were larger than that of ANN(SSR) and the PC samples showed apparently higher activities than the SSR sample. Among all the PC samples, the PC750 exhibited the highest activity although it had not the largest surface area. A similar phenomenon was observed in other photocatalysts such as Pb3Nb4O13.24 This result indicates that, besides the BET surface area, the crystallinity also plays an important role in the enhancement of photocatalytic activity. The highest activity of a photocatalyst might be the result of competition between the crystallinity and the surface area. To further increase the activity, the PC800 sample was modified with the metal complex H2PtCl6. The PC800 was selected because its surface area was comparable to that of ANN(SSR). Figure 4 shows the photocatalytic decomposition of gaseous IPA over various samples under the same conditions. We can see that the rate of acetone evolution over H2PtCl6/ PC800 is 10-fold larger than that over naked PC800 and H2PtCl6/SiO2. The H2PtCl6/SiO2 sample also showed a low activity, which was from the oxidation property of H2PtCl6 under the visible-light irradiation. Nevertheless, the rate of acetone evolution over H2PtCl6/PC800 is much larger than the sum of those over PC800 and H2PtCl6, indicating the advantages of the hybrid H2PtCl6/PC800 as will be discussed below.

Photocatalytic Activity of Modified Ag0.7Na0.3NbO3

Figure 4. Acetone evolution rates over H2PtCl6/SiO2, PC800, H2PtCl6/ PC800, H2PtCl6/ANN(SSR), H2PtCl6/TiO2(A), N-TiO2, H2PtCl6/N-TiO2, Pt/PC800(cal), and Pt/PC800(photo).

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Figure 6. FTIR spectra of PC800 and H2PtCl6/PC800.

SCHEME 1: Primary Steps of the Charge Transfer for PC800 Modified by H2PtCl6 on the Surface

Figure 5. UV-vis diffuse reflectance spectra of PC800, H2PtCl6, and H2PtCl6/PC800. The spectrum of H2PtCl6 was measured using the transmittance mode, and the others were reflectance mode. The inset is the enlarged view of the spectra between 400 and 600 nm and the BLEDs spectrum.

The activity of the hybrid photocatalyst H2PtCl6/PC800 was nearly 18-fold higher than that of H2PtCl6/TiO2(A) and 6-fold higher than that of H2PtCl6/N-TiO2. The activity of H2PtCl6/ N-TiO2 was even lower than that of naked N-TiO2. These results suggest that the supporting material also plays an important role in the enhancement of photocatalytic activity. The activities of Pt/PC800(cal) and Pt/PC800(photo) were found to be similar to that of the naked ANN and PC800 samples, indicating that it was H2PtCl6 rather than Pt that accounted for the enhancement of photocatalytic activity in the hybrid photocatalyst H2PtCl6/ PC800. To understand the effect of H2PtCl6 on the enhancement in the photocatalytic activity of this hybrid photocatalyst, the UV-vis diffuse reflectance spectra of PC800, H2PtCl6, and H2PtCl6/PC800 were measured and are shown in Figure 5. It is clearly seen that the main absorption edge of the H2PtCl6/PC800 is same as that of PC800, indicating that the addition of H2PtCl6 did not change the absorption property of the bulk PC800. However, the onset of light absorption of H2PtCl6/PC800, which is the same as that of H2PtCl6, is red-shifted about 90 nm in comparison with that of naked PC800, as shown Figure 5 (inset). These results indicate that H2PtCl6 behaves as a photosensitizer to make PC800 absorb more visible light through the surface modification. To identify whether the H2PtCl6 characteristics were changed after interacting with the PC800, the FTIR spectra of PC800 and H2PtCl6/PC800 were measured, and the results are shown in Figure 6. We can see that, differing from the naked PC800, a negative peak appears at 1090.9 cm-1 after loading H2PtCl6 on the PC800 surface. Since the solid of H2PtCl6 usually contains six molecules of crystal water, to exclude the effect of crystal water, the FTIR spectra of SiO2 and H2PtCl6/SiO2 were measured instead of H2PtCl6. It was found that a similar negative

peak also appeared at 1090.9 cm-1 in the FTIR spectrum of H2PtCl6/SiO2. These findings revealed that this negative peak was simply caused by the addition of H2PtCl6 but was not a new vibration mode. We further carried out the following experiments to identify the states of the platinum complex on the photocatalyst. First, taking the difference in the stability of physisorption and chemisorption into account, an aqueous suspension of H2PtCl6/ PC800 was stirred in the dark for 2 days. After filtration through the Millipore filter, we obtained the filtrate. The physisorbed H2PtCl6 would dissolve in the filtrate due to the weak interaction between H2PtCl6 and PC800. The Pt concentrations of the original H2PtCl6 solution and filtrate were measured by the inductively coupled plasma atomic emission spectrometric method (ICP). The ICP result shows that in comparison with the original solution there is ∼3% of total H2PtCl6 amount in the filtrate. In addition, the UV-vis absorption spectrum was used to monitor the Pt complex in the liquid phase. From the spectra of the original solution and filtrate (see the Supporting Information, Figure S1), it can be seen that the absorbance at 259 nm (the strong absorption of platinum complex) decreases significantly in the spectrum of the filtrate, indicating that a small amount of H2PtCl6 was desorbed from the surface and implying that H2PtCl6/PC800 has a good stability. The good stability of H2PtCl6/PC800 implies that chemisorption has taken place in the sample.15 However, in the FTIR spectra, no evidence of a new support-complex interaction was detected. Maybe the interaction is too weak to be detected because of the low percentage of H2PtCl6 in the sample (0.1%, given in wt % of Pt). On the basis of above results, we ascribed the enhancement in the photocatalytic activity of H2PtCl6/PC800 to the increased capabilities of visible-light absorption and charge transfer between PC800 and H2PtCl6. As shown in the inset of Figure 5, the enhanced absorbance due to H2PtCl6 coincidently covers the main range from 450 to 500 nm in the BLEDs spectrum. Moreover, the PC800 in the hybrid H2PtCl6/PC800 works as not only a medium of electron transfer but also a generator of photoelectrons and photoholes under visible-light irradiation as

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Li et al. to be 0.165 and 0.027 h-1, respectively. It is obvious that the true rate constant of H2PtCl6/PC800 is about 6 times larger than that of ANN(SSR). To identify the stability, the XPS spectrum was used to investigate the chemical state of the platinum before and after irradiation for 50 h. The XPS results (see the Supporting Information, Figure S2) indicate that the chemical state of Pt was not changed obviously before and after irradiation for 50 h and was not affected by the O2 atmosphere. Furthermore, no peak of Pt0 (at 70.9 eV) appeared in the spectra; therefore, it was considered that the platinum complex in our experiment conditions was not changed. 4. Conclusions In summary, we have synthesized the Ag0.7Na0.3NbO3 by a PC method and investigated the photocatalytic activities of the PC samples with or without H2PtCl6 modification for gaseous IPA decomposition. The PC samples show higher activity and larger surface area in comparison with the sample prepared by a solid-state reaction method. The PC800 sample photosensitized by the H2PtCl6 exhibits 10-fold higher activity than that of naked PC800. The improvement of photocatalytic activity of the hybrid H2PtCl6/PC800 is attributed to the increased absorbance to visible light, the enhanced charge separation and transfer process, and the involvement of the photohole-based oxidation in this hybrid system.

Figure 7. (a) Variations in IPA, acetone, and CO2 concentration over H2PtCl6/PC800. (b) First-order plots for photocatalytic decomposition of IPA over H2PtCl6/PC800 (0.2 g) and ANN(SSR) (0.4 g).

denoted (1) in Scheme 1. Clearly, there is one more pathway of photoelectrons in the hybrid system than the naked PC800, as denoted (2) in Scheme 1, which is considered to be similar to the case of TiO2 modified by platinum halide.14,15 The CB electrons and VB holes will react with the adsorbed O2 and OH-, forming O2• - and · OH, respectively. These oxidation species, O2• - and · OH, will oxidize gaseous IPA to acetone.20-22 The photohole-based oxidation is thought to play an important role in the enhancement in photocatalytic activity. To evaluate the mineralization capacity of the hybrid photocatalyst H2PtCl6/PC800, a long-term experiment was carried out, and the result is shown in Figure 7a. The slightly increased concentration of CO2 in the dark was possibly caused by the desorption of CO2 adsorbed on the surface of the photocatalyst before the dark experiment, because during the dark experiment partial CO2 adsorbed on the surface will be desorbed into the gas phase and reach the equilibrium of adsorbed CO2 and CO2 in the gas phase. With increasing the irradiation time, the concentrations of acetone and CO2 increased while that of IPA decreased, indicating that IPA could be mineralized to CO2 by the H2PtCl6/PC800 photocatalysis eventually. The rate of CO2 evolution on H2PtCl6/PC800 is estimated to be ∼2.9 ppm · h-1, which is nearly 10-fold higher than that of ANN(SSR).18 The first-order linear relationship was revealed by the plots of the ln(C/C0) versus irradiation time, as shown in Figure 7b. In this system, it can be explained in a Langmuir-Hinshelwood model. When the initial gaseous IPA is dilute (C < 10-3 M), the reaction rate (r) can be expressed as r ) kKC, where, K refers to adsorption equilibrium constant, k is the true rate constant, and C is the instantaneous concentration of the reactant. According to the first-order linear relationship, the true rate constant k for H2PtCl6/PC800 and ANN(SSR) was estimated

Acknowledgment. The present research was partially supported by the Global Environment Research Fund of the Japanese Government. It was also partially supported by the National Natural Science Foundation of China (No. 20528302), the National Basic Research Program of China (973 program, 2007CB613301 and 2007CB613305), and the Jiangsu Provincial Natural Science Foundation (Nos. BK2006718 and BK2006127). Professor Z. G. Zou thanks the Jiangsu Provincial Talent Scholars Program. Supporting Information Available: UV-vis absorption spectra of the H2PtCl6 solution and Pt 4f XPS spectra before and after irradiation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69–96. (2) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735–758. (3) Herrmann, J. M. Top. Catal. 2005, 34, 49–65. (4) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625– 627. (5) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (6) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (7) Tang, J.; Zou, Z.; Ye, J. Angew. Chem., Int. Ed. 2004, 43 (34), 4463–4466. (8) Kim, H.; Hwang, D.; Lee, J. J. Am. Chem. Soc. 2004, 126, 8912– 8913. (9) Wang, D.; Kako, T.; Ye, J. J. Am. Chem. Soc. 2008, 130, 2724– 2725. (10) Wang, C. Y.; Bahnemann, D. W.; Dohrmann, J. K. Chem. Commun. 2000, 16, 1539–1540. (11) Miyauchi, M.; Takashio, M.; Tobimatsu, H. Langmuir 2004, 20, 232–236. (12) Zhang, F.; Zhao, J. C.; Shen, T.; Hidaka, H.; Pelizzetti, E.; Serpone, N. Appl. Catal., B 1998, 15 (1-2), 147–156. (13) Zhao, W.; Chen, C.; Ma, W.; Zhao, J. C.; Wang, D.; Hidaka, H.; Serpone, N. Chem. Eur. J. 2003, 9, 3292–3299. (14) Kisch, H.; Zang, L.; Lange, C.; Maier, W. F.; Antonius, C.; Meissner, D. Angew. Chem., Int. Ed. 1998, 37, 3034–3036.

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