Photocatalysis Coupled with Thermal Effect Induced by SPR on Ag

Article pubs.acs.org/JPCC

Photocatalysis Coupled with Thermal Effect Induced by SPR on AgLoaded Bi2WO6 with Enhanced Photocatalytic Activity Zhijie Zhang, Wenzhong Wang,* Erping Gao, Songmei Sun, and Ling Zhang State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China ABSTRACT: The temperature dependence of the photocatalytic activity of Bi2WO6 was studied and the results showed that a higher reaction temperature can facilitate the photocatalytic process. On the other hand, it is well-known that the excitation of localized plasmon polaritons at well-defined wavelengths on the surface of silver (Ag) nanoparticles causes a tremendous thermal effect. Thus, it is expected that the photocatalytic activity of Bi2WO6 would be enhanced if it was coupled with the thermal effect induced by surface plasmon resonance (SPR) of Ag. Herein, this idea is realized. The measured photocatalytic activity under simulated solar light, monitored by the decomposition of phenol, demonstrated that Ag loading could significantly enhance the photocatalytic activity of Bi2WO6. The enhanced photoactivity can be ascribed to the synergistic effect brought by electronic effect of Ag nanoparticles and thermal effect induced by SPR. This work provides some insights into the rational design and development of high-performance photocatalysts.

1. INTRODUCTION Water pollution is one of the most serious environmental problems modern society faces today. One promising technique for wastewater treatment is to employ semiconductor-mediated photocatalytic degradation of toxic chemicals by solar energy.1−5 Photocatalytic reactions are classified into two categories, downhill and uphill reactions. Degradation, such as photo-oxidation, of organic compounds is generally a downhill reaction,6 in which a photocatalyst works as a trigger to produce O2•−, HO2, or •OH as active species at the initial stage (Figure 1). However, in order to cross the potential barrier,

consumption, here comes the question: is there a way that releases thermal energy under the irradiation of light? Fortunately, noble-metal nanoparticles are reported to be able to generate heat when irradiated by light due to the surface plasmon resonance (SPR) effect.7−10 As is well-known, surface plasmons exist on the surface of noble metals by a collective oscillation of free electrons. By this collective oscillation, transient local electromagnetic fields generated by forming electron/hole-rich areas near the surface. When an external electromagnetic field such as light is imposed, the surface plasmon could be excited if their frequencies are matching, which is denoted as SPR. The irradiation of incident light with a wavelength in the range of the SPR band may result in an elevated temperature of the reaction medium. In particular, silver nanoparticles show efficient plasmon resonance in the visible region.11−13 Besides the thermal effect, the Ag nanoparticles are also reported to have electronic effect under visible light.14−18 This synergistic effect is expected to play a positive role in enhancing the photocatalytic activity. As one of the simplest Aurivillius oxides, bismuth tungstate (Bi2WO6) has received much attention due to its good photocatalytic performance in water splitting and organic contaminant degradation under visible light irradiation.19−24 In a previous report, our group synthesized Ag-loaded Bi2WO6 photocatalyst and investigated its photocatalytic activity in inactivation of bacterium.25 More recently, Xue et al. reported Ag loading could greatly improve the photocatalytic activity of Bi2WO6 in both the decolorization of RhB and desulfurization

Figure 1. Illustration of temperature dependence of the photocatalytic degradation process.

certain energy is required. When the reaction temperature is raised, the potential of the initial stage can be elevated (as shown in Figure 1), which makes it easier to cross the potential barrier. Therefore, increasing the temperature of the reaction solution can speed up the photocatalytic degradation process. Since an external heating medium causes extra energy © 2012 American Chemical Society

Received: October 1, 2012 Revised: November 7, 2012 Published: November 17, 2012 25898

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of thiophene.26 However, the light-induced thermal effect of Ag on the photocatalytic activity of Bi2WO6 was not investigated before. In this study, Ag-loaded Bi 2WO 6 photocatalyst was synthesized and the temperature dependence of its photocatalytic activity was studied for the first time. The photoactivity evaluation, via the photocatalytic degradation of phenol under simulated solar light irradiation, demonstrated that a higher temperature is beneficial to the photocatalytic performance. At the same reaction temperature, Ag-loaded Bi2WO6 exhibited higher photoactivity than bare Bi2WO6, which could be attributed to the electronic effect of Ag nanoparticles. However, if the reaction temperature was not controlled during the photocatalytic process, the difference between the degradation rates of phenol by the two samples was even larger. The reaction temperature during the photocatalytic process was measured and the results showed that Ag loading could significantly raise the solution temperature. Consequently, Ag-loaded Bi2WO6 photocatalyst exhibited much enhanced photocatalytic activity than bare Bi2WO6.

with a water jacket was used as the photoreactor. The experiments were performed as follows: 0.1 g of photocatalyst was suspended in 100 mL of phenol solution (20 mg/L) by stirring magnetically. The suspensions were kept at constant temperature by circulating thermostatted water through the jacket. Prior to irradiation, the suspensions were magnetically stirred for an hour in the dark to ensure the adsorption/ desorption equilibrium between phenol and photocatalyst powders. At given time intervals, 3 mL of suspension was sampled and centrifuged to remove the photocatalyst particles. Then, the absorption spectrum of the centrifugated solution was recorded using a Hitachi U-3010 UV−vis spectrophotometer. The change of phenol concentration was determined by monitoring the optical intensity of absorption spectra at 270 nm.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure. The phase structure of the asobtained products was examined by XRD pattern, as shown in Figure 2. All the peaks can be indexed to the orthorhombic

2. EXPERIMENTAL SECTION 2.1. Preparation of Bi2WO6 Photocatalyst. The Bi2WO6 photocatalysts were prepared by a hydrothermal method according to our previous report.27 In a typical process, 2 mmol of Bi(NO3)3·5H2O and 1 mmol of Na2WO4·2H2O were dissolved in 2 M nitric acid solution and 30 mL of deionized water, respectively. After that, these two solutions were mixed together and a white precipitate was formed. The pH value of the last suspension was about 1. After being stirred for several hours, the suspension was added to a 50 mL Teflon-lined autoclave up to 80% of the total volume. Then the autoclave was sealed in a stainless steel tank and heated at 160 °C for 20 h. Subsequently, the autoclave was cooled to room temperature naturally. The products were collected by filtration, washed with distilled water for several times, and then dried at 60 °C in air for 12 h. 2.2. Preparation of Ag-Loaded Bi2WO6 Photocatalyst. Ag-loaded Bi2WO6 was prepared by a photoreduction process as follows: the obtained Bi2WO6 (1 mmol) was added into 50 mL AgNO3 solution (2 mmol/L) by magnetic stirring. Photoreduction was carried out under a 500 W Xe lamp for 1 h during which silver ions are reduced to form silver nanoparticles on the surface of Bi2WO6. The obtained Agloaded Bi2WO6 was then washed with deionized water, and dried in an oven at 60 °C for 12 h. The loading amount of Ag is 10 mol %. 2.3. Characterization. The phase and composition of the as-prepared samples were detected by X-ray diffraction (XRD) studies using an X-ray diffractometer with Cu Kα radiation under 40 kV and 100 mA and with the 2θ ranging from 20° to 70° (Rigaku, Japan). The morphologies and microstructures of the as-prepared samples were investigated by transmission electron microscopy (TEM, JEOL JEM−2100F). The energydispersive X-ray analysis (EDX) of the samples was also performed during the TEM measurements. UV−vis diffuse reflectance spectra (DRS) of the samples were recorded with an UV−vis spectrophotometer (Hitachi U-3010) using BaSO4 as reference. 2.4. Photocatalytic Test. Photocatalytic activity of the Agloaded Bi2WO6 photocatalyst was evaluated by photodegradation of a colorless pollutant, phenol, under simulated solar light irradiation provided by a 500 W Xe lamp. A 200 mL beaker

Figure 2. XRD patterns of the products.

phase of Bi2WO6 according to the JCPDS card no. 39-0256 (a = 5.457 Å, b = 16.435 Å, and c = 5.438 Å) and no other crystalline phase can be detected. Moreover, no signal about silver can be detected for the low content of Ag element. 3.2. TEM Observation and Energy-Dispersed X-ray Microanalysis. The TEM image shown in Figure 3A indicated that the Ag-loaded Bi2WO6 sample is composed of nanosheets with size of ca. 100 nm, and many small nanoparticles were dispersed on the surface of nanosheets. Close observation of Figure 3B indicated that the nanoparticles are monodisperse with size of about 4 nm. In order to further confirm the existence of Ag, the areas with the nanoparticles were selected for energy-dispersed X-ray (EDX) microanalysis. According to Figure 3C, signals corresponding to Bi, W, O, and Ag elements were detected, which indicated that the nanoparticles dispersed on the surface of nanosheets were silver. 3.3. UV−Vis Diffuse Reflectance Spectra. The UV−vis diffuse reflectance spectrum (DRS) of the Ag-loaded Bi2WO6 sample is compared to that of bare Bi2WO6, as shown in Figure 4. According to the spectrum, bare Bi2WO6 sample presents the photoresponse property from the UV light region to visible light until 450 nm, which is due to the intrinsic band gap transition. The absorption spectrum of Ag-loaded Bi2WO6 is obviously different from that of Bi2WO6. The Ag-loaded 25899

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Figure 3. (A,B) TEM images of the Ag-loaded Bi2WO6; (C) EDX microanalysis spectra of the Ag-loaded Bi2WO6.

region and there is no photosensitization. Figure 5A shows the degradation rate of phenol as a function of irradiation time in the presence of Bi2WO6 photocatalyst at different reaction temperatures. C was the absorption of phenol at the wavelength of 270 nm and C0 was the absorption after the adsorption equilibrium on Bi2WO6 powders before irradiation. The results indicated that the photocatalytic degradation rate of phenol increased with increasing reaction temperature. After 120 min of irradiation, the degradation rates of phenol were 58.6%, 61.8%, 83.7%, and 91.4% at 5, 25, 50, and 70 °C, respectively. In other words, the reaction temperature had a positive effect on the photocatalytic activity of Bi2WO6. In order to investigate the effect of Ag loading on the photocatalytic activity of Bi2WO6, photocatalytic degradation of phenol was also carried out at temperatures between 5 and 70 °C using Ag-loaded Bi2WO6, as shown in Figure 5B. Similarly, the photocatalytic activity of Ag-loaded Bi2WO6 was enhanced with increasing reaction temperature. In order to quantitatively compare the reaction kinetics of phenol degradation by bare Bi2WO6 and Ag-loaded Bi2WO6, the apparent first-order rate constant k at different temperatures in the presence of the two samples are calculated, as shown in Table 1. It can be seen that, at the same reaction temperature, the apparent first-order rate constant k in the presence of Ag-loaded Bi2WO6 was higher than that of bare Bi2WO6, which can be ascribed to the electronic effect of Ag nanoparticles. In order to investigate the thermal effect induced by SPR of Ag nanoparticles on the photocatalytic activity of Bi2WO6, photocatalytic degradation of phenol was conducted in the presence of bare Bi2WO6 and Ag-loaded Bi2WO6 without controlling the reaction temperature. The result is shown in Figure 6A. Obviously, Ag-loaded Bi2WO6 exhibited much

Figure 4. UV−vis diffuse reflectance spectra of the as-prepared samples.

Bi2WO6 sample shows a shallow peak with higher intensities observed at around 550 nm, which indicates that it is silver metal rather than other compounds containing silver that existed in the samples. The prominent absorption in the visible light region could be attributed to the surface plasmon resonance (SPR) effect of Ag nanoparticles.28,29 The SPR effect of Ag nanoparticles may have a partial contribution to the photocatalytic activity of Bi2WO6. 3.4. Photocatalytic Activity. In order to elucidate the role of reaction temperature in the photocatalytic process, the photodegradation experiments were conducted at temperatures between 5 and 70 °C. Phenol was selected as the model pollutant because it has no light absorption in the visible light 25900

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Figure 5. Photocatalytic degradation of phenol at different reaction temperatures in the presence of (A) Bi2WO6 and (B) Ag-loaded Bi2WO6.

Table 1. Comparison of the Apparent First-Order Rate Constant k by Bi2WO6 and Ag-Loaded Bi2WO6 at Different Reaction Temperatures 5 °C

25 °C

50 °C

70 °C

kBi2WO6 (min )

0.00729

0.00776

0.01437

0.01935

kAg/Bi2WO6 (min−1)

0.00777

0.01027

0.01666

0.02018

−1

enhanced photocatalytic activity than bare Bi2WO6. After 120 min of irradiation, about 72.4% and 100% of phenol were degraded in the presence of bare Bi2WO6 and Ag-loaded Bi2WO6, respectively. In order to demonstrate the influence of the reaction temperature more vividly, a comparison figure was plotted. As shown in Figure 6B, without controlling the reaction temperature, both of degradation rates in the presence of bare Bi2WO6 and Ag-loaded Bi2WO6 increased compared with that at ambient temperature, while in the presence of Agloaded Bi2WO6 the degradation rate increased by a larger margin. This result indicated that besides the electronic effect of Ag nanoparticles, thermal effect is another factor that contributed to the enhanced photocatalytic activity of Agloaded Bi2WO6. The temperature variation during the photocatalytic process was measured to investigate the thermal effect of Ag nanoparticles, and the result is shown in Figure 7. Obviously, the reaction temperature in the presence of Ag-loaded Bi2WO6 increased much faster than that of bare Bi2WO6. After only 30 min of irradiation, the solution temperature in the presence of Ag-loaded Bi2WO6 increased up to 62 °C from the initial 12 °C, which was 22 °C higher than that of in the presence of bare Bi2WO6. After 120 min of irradiation, the solution temperature in the presence of Ag-loaded Bi2WO6 increased to as high as 68 °C, while that in the presence of bare Bi2WO6 was only 48 °C.

Figure 7. Temperature variation during the photocatalytic process in the presence of Bi2WO6 and Ag-loaded Bi2WO6, respectively.

This result indicated that Ag loading could significantly raise the reaction temperature during the photocatalytic process. The stability of the photocatalyst is important for its application. To demonstrate the potential applicability of Agloaded Bi2WO6 photocatalyst, circulating runs in the photocatalytic degradation of phenol were carried out under simulated solar light. As shown in Figure 8, the Ag-loaded Bi2WO6 photocatalyst was stable under repeated use with constant photodecomposition rate of phenol in 2 h. After five recycles for the photodegradation of phenol, the photocatalyst did not exhibit any significant loss of activity, which indicates that the Ag-loaded Bi2WO6 photocatalyst has high stability and does not photocorrode during the photocatalytic oxidation of the model pollutant molecules. This is different from previous reported works about silver-containing photocatalysts,30,31 in which the photocatalysts are not stable. The reason can be explained as follows: in the reported works, silver is monovalent

Figure 6. (A) Photocatalytic degradation of phenol without controlling the reaction temperature by the as-prepared samples. (B) Comparison of degradation efficiency of phenol by Bi2WO6 and Ag-loaded Bi2WO6 at ambient temperature and without controlling the reaction temperature. 25901

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photocatalysis.32 Therefore, the high-efficiency photocatalytic activity of the as-prepared Ag-loaded Bi2WO6 can be ascribed to the synergistic effect of electronic effect of Ag nanoparticles and thermal effect induced by SPR.

4. CONCLUSIONS The SPR-induced thermal effect of Ag on the photocatalytic activity of Bi2WO6 was investigated for the first time. The photocatalytic degradation of phenol under simulated solar light was conducted to investigate the effect of Ag. At the same reaction temperature, Ag-loaded Bi2WO6 exhibited higher photoactivity than bare Bi2WO6, while the difference between the degradation rates of phenol by the two samples was even larger if the solution temperature was not controlled. Both the electronic effect and thermal effect of Ag nanoparticles contribute to the enhanced photocatalytic activity of Ag-loaded Bi2WO6. Our work suggests that the idea of noble metal loading can be a plausible strategy to develop efficient photocatalysts with high activity for environmental remediation.

Figure 8. Cycling runs in the photocatalytic degradation of phenol by Ag-loaded Bi2WO6.

(such as Ag2CO3, Ag3PO4). When absorbing a photon, the photocatalyst generates an electron and a hole, and subsequently the photogenerated electron combines with an Ag+ ion to form metallic silver. So the Ag-based compounds are not stable. However, in our work, elemental silver was used but not silver with monovalence. The elemental silver could not be reduced by the photogenerated electron and can exist stably. So the Ag-loaded Bi2WO6 photocatalyst has high stability and does not photocorrode. 3.5. Mechanism of Enhanced Photoactivities. On the basis of the above results, we attributed the enhanced photoactivity of Ag-loaded Bi2WO6 photocatalyst to the following factors: first, at the same reaction temperature, Agloaded Bi2WO6 photocatalyst exhibits higher photoactivity than bare Bi2WO6, which could be attributed to the electronic effect of Ag nanoparticles. That is, the Ag nanoparticles function as visible-light-harvesting and electron-generating centers owing to the strong surface plasmon resonance (SPR).14−18 Under visible light illumination, SPR-excited electrons would be generated and enriched on the surface of Ag nanoparticles. The increased electron density lifts the Fermi energy level of Ag, which makes the transfer of SPR electrons from Ag to the conduction band of Bi2WO6 energetically favorable. Moreover, the Ag nanoparticles can act as electron traps to suppress electron−hole recombination and promote interfacial charge transfer. As a result, more photoinduced holes will have opportunity to participate in the oxidation reactions on the surface. This explains why the Ag-loaded Bi2WO6 sample exhibits higher photoactivity than bare Bi2WO6 at the same reaction temperature. On the other hand, Ag-loaded Bi2WO6 exhibits much enhanced photoactivity than bare Bi2WO6 if the reaction temperature was not controlled, which could partially be ascribed to the thermal effect induced by the SPR effect of silver nanoparticles. The irradiation of incident light with a wavelength in the range of the SPR band may result in two consequences. The first is that light absorption by the silver nanoparticles could heat up the nanoparticles and the solution. The measurement of temperature variation during the photocatalytic process has proved this. The significant increase of reaction temperature is beneficial to the photodegradation rate of phenol. Besides the contribution of thermal effect, the SPR effect is associated with a considerable enhancement of the electric near-field in the vicinity of the Ag nanoparticles. This enhanced near-field could boost the excitation of electron hole pairs in Bi2WO6 and therefore increase the efficiency of the

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-5241-5295. Fax: +86-21-5241-3122. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (50972155, 50902144, 51102262), National Basic Research Program of China (2010CB933503), and Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (SKL 200904).

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