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TiO2 Catalysts
ACTA PHYSICO-CHIMICA SINICA Volume 23, Issue 7, July 2007 Online English edition of the Chinese language journal ARTICLE
Cite this article as: Acta Phys. -Chim. Sin., 2007, 23(7): 978−982.
Effect of Water Washing Treatment on the Photocatalytic Activity of Au/TiO2 Catalysts Baozhu Tian,
Tianzhong Tong,
Feng Chen,
Jinlong Zhang*
Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, P. R. China
Abstract:
By using Au(S2O3)3− 2 as the gold precursor, Au/TiO2 photocatalysts were prepared by water washing (W) and rotary
evaporation (E) processes, respectively. The samples were characterized by UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic absorption flame emission spectroscopy (AAS). The photocatalytic activity of the samples was evaluated from the analysis of the photodegradation of methyl orange (MO). By using the water washing treatment, well-dispersed gold nanoparticles with the diameter of 2−5 nm were formed on the surface of TiO2, whereas only the gold coating was formed for rotary evaporation processes. The photocatalytic activity of Au/TiO2 photocatalysts is related to the preparation process. With a similar gold loading, the photocatalysts prepared by water washing showed higher photocatalytic activity compared to the catalysts prepared by rotary evaporation. Key Words:
Au/TiO2; Photocatalytic activity; Au loading; Water washing; Rotary evaporation
In recent years, applying semiconductor photocatalysts to treat environmental contaminants has become a new type of pollution treatment technology. Amongst the various semiconductor photocatalysts, TiO2 is considered to be one of the most promising photocatalysts due to its stability, non-toxicity, and low cost[1,2]. However, its practical application is limited owing to its low photo-quantum efficiency and low reaction rate. It has been confirmed by various studies that noble metal loaded on the surface of TiO2 (M/TiO2) can depress the electron-hole recombination and consequently improve the photocatalytic activity of TiO2 semiconductor[3,4]. In particular, Au/TiO2 shows surprisingly high photocatalytic activity in the selective oxidation of propene, nitrogen oxide reduction, low-temperature oxidation of CO, and photocatalytic oxidations used for environmental cleanup[5−9]. Furthermore, many studies revealed that the catalytic activity of Au/TiO2 catalysts is severely relative to the morphology and size of Au particles,
the interaction between Au and TiO2 support, and so on. It is generally believed that highly dispersed Au particles with diameters below 5 nm show high photocatalytic activity[5,8,10,11]. Therefore, many chemical and physical methods, such as ion implantation co-precipitation[12], sputtering[13,14], chemical vapor deposition (CVD)[15], sol-gel[9], and deposition-precipitation (DP)[16] have been developed for the preparation of Au/TiO2 catalysts. Amongst the wet-chemical methods, many factors, such as precursor type, preparation method, and preparation methodology can influence the Au loading and morphology of gold on the surface of TiO2, and finally influence the photocatalytic activity of TiO2. Therefore, it is very important to select the appropriate precursor, preparation method, and methodology for the preparation of highly photoactive Au/TiO2 catalysts. In this study, Au/TiO2 catalysts were prepared using Au(S2O3)3− 2 as precursor and the influence of washing treatment on state of gold on the surface of TiO2 was studied. The
Received: January 22, 2007; Revised: March 19, 2007. * Corresponding author. Email: [email protected]; Tel: +8621-64252062. The project was supported by the National Natural Science Foundation of China (20577009), the International Co-operation Project of the Minister of Science and Technology, China (2006DFA52710), the Program for New Century Excellent Talents in Universities, China (NCET-04-0414), and Shanghai Nanotechnology Promotion Center, China (0552nm019). Copyright © 2007, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Baozhu Tian et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 978−982
photocatalytic activities of Au/TiO2 catalysts prepared with different process were evaluated from an analysis of the photodegradation of methyl orange (MO).
1 1.1
Experimental Preparation of samples
Au(S2O3)3− 2 aqueous solution was prepared by slowly adding HAuCl4 aqueous solution to excess Na2S2O3 aqueous solution under stirring (the mole ratio of HAuCl4 to Na2S2O3 was 1:4.25)[17]. At first, the pH of 60 mL of Au(S2O3)3− 2 aqueous solution was adjusted to 2.0 with diluted hydrochloric acid solution. Subsequently, 1 g dried TiO2 (P25) was added to this solution, followed by ultrasound irradiation for 10 min in an ultrasonic cleaning bath (Elma, T660/H, 35 kHZ, 360 W). Then, the mixture was further sonicated for 4 h when the temperature was heated to 80 . The obtained slurry was divided equally into two parts. One was dried with a rotary evaporator, and the other was centrifuged, separated, and washed with distilled water (100 mL) four times to remove residual Cl− and Na+ ions, as well as, Au species not interacting with the support. Finally, the two samples were heated from room temperature to a certain temperature with a heating rate of 2 ·min−1 and calcined at this temperature for 4 h. The samples prepared by washing treatment and rotary evaporation were labeled as wAu-T(W) and wAu-T(E), respectively, in which the “w” refers to the nominal mass percentage content of Au in the Au/TiO2, and “T” refers heat-treating temperature (). 1.2
Characterization
X-ray diffraction (XRD) measurements were carried out with a Rigaku D/max 2550 VB/PC X-ray diffractometer using Cu Kα radiation (λ=0.15406 nm) and a graphite monochromator, operated at 40 kV and 100 mA. UV-Vis diffuse reflectance spectra (DRS) were obtained using a Scan UV-Vis-NIR spectrophotometer (Varian Cary500) equipped with an integrating sphere assembly, using polytetrafluoroethylence (PTFE) as a reference material. The measure ranged from 300 to 800 nm. The state of Au on the surface of TiO2 was observed with a JEOL JEM-100 CX II transmission electron microscopy (TEM), operated at an acceleration voltage of 100 kV. The content of Au in Au/TiO2 samples was determined by atomic absorption flame emission spectroscopy (AAS) (Shimadzu AA-6400F). 1.3
water in a quartz cylindrical jacket around the lamp. The distance between the light and the reaction tube was fixed at 24 cm. 0.05 g of catalyst sample was added into a 70 mL quartz tube containing 50 mL of 30 mg·L−1 methyl orange. Before photoreaction, the mixture was stirred in the dark for 30 min to attain the adsorption-desorption equilibrium for MO on the surface of TiO2. At given time intervals, the samples were taken from the suspension and the absorbance of MO was measured at 464 nm, which is the maximum absorption of MO. The degradation rate of methyl orange was calculated from the absorbance.
2 2.1
Results and discussion UV-Vis diffuse reflectance spectra
Fig.1 shows the UV-Vis diffuse reflectance spectra (UV-Vis DRS) of sample 4%Au-T(W) calcined at different temperatures. As shown in Fig.1, the absorption onset (absorption edge) of P25 is around 400 nm, which is between the absorption onsets of anatase (387 nm) and rutile (418 nm). This can be ascribed to that P25 comprises in anatase and rutile. Compared with P25, there is a red shift of 20 nm in the onset absorption and a increase of light absorption in the visible region for the samples of 4%Au-T(W), which is consistent with Li′s results[9]. In DRS, an absorbance band around 545 nm arises and the intensity of the absorbance band increases with increasing calcination temperature within 200−350 for samples 4%Au-T(W). According to the relevant references[9,18,19], the absorbance band should be ascribed to the plasmon resonance of metallic Au particles. For the metal nanoparticles of Au0, Ag0, and Cu0, the plasmon absorption arises from the collective oscillations of the free conduction band electrons that are induced by the incident electromagnetic radiation. For Au/TiO2 samples, the intensity of the absorbance band is related to the size of Au particles and the content of Au in Au/TiO2. There is not much difference in the intensity when the temperature of 4%Au-T(W) varies from 300 to 350 , suggesting that the size of Au has not much difference and gold exists in the state of metallic gold. The characteristic absorbance band of Au around 545 nm in the DSR of sample
Photocatalytic activity measurement
Methyl orange was used as a simulated contaminant to evaluate the photocatalytic activity of the Au/TiO2 samples. The photocatalytic reactions were carried out with a homemade photoreactor, in which a 300 W high-pressure Hg lamp (λmax=365 nm) was used. The lamp was cooled with flowing
Fig.1
UV-Vis DRS of P25 and 4%Au-T(W) samples calcined at different temperatures
Baozhu Tian et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 978−982
4%Au-300(E) indicated the existence of metallic Au (the spectrum was not listed). 2.2
XRD analysis
The XRD patterns of P25, 8%Au-300(W), 8%Au-500(W), and 8%Au-300(E) are shown in Fig.2(a–d), respectively. As shown in Fig.2, the shapes and intensities of the diffraction peaks for both anatase and rutile in the four samples are very similar, indicating that calcining at 300 or 500 for 4 h did not change the content and particle size of anatase and rutile in the samples. By careful observation, it can be found that new peaks emerge at 2θ=38.2º, 44.4º, 64.6º, and 77.6º, which can be attributed to the diffraction peaks of (111), (200), (220), and (311) planes of polycrystalline Au, respectively (overlap occurred between the (111) diffraction peak of Au and the (101) diffraction peak of rutile). The above-mentioned results confirmed the formation of metallic gold. At 2θ=77.6º, the diffraction peak of Au shows no overlap with those of anatase and rutile. The average crystallite size of Au can be calculated by applying the Debye-Scherrer formula[20], D= Kλ βcosθ where D is the average crystallite size, K is a constant which is taken as 0.89 here, λ is the wavelength of the X-ray radiation (Cu Kα, λ=0.15406 nm), β is the corrected band broadening (full width at half-maximum (FWHM)) after subtraction of equipment broadening, and θ is the diffraction angle. The average crystallite sizes of samples 8%Au-500(W) and 8%Au-300(E) were calculated to be 12.9 nm and 17.0 nm, respectively. Compared with samples of 8%Au-500(W) and 8%Au-300(E), the diffraction peak of sample 8%Au-300(W) was broader and lower, suggesting a smaller average crystallite size of Au for the sample 8%Au-300(W) (cannot be calculated, because of very low peak intensity) . 2.3
TEM analysis
The size and morphology of Au particles on the surface of TiO2 can be observed with TEM. The TEM images of the samples 4%Au-300(W), 8%Au-300(W), and 8%Au-300(E)
Fig.3 TEM images of Au/TiO2 samples (a) 4%Au-300(W); (b) 8%Au-300(W); (c) 8%Au-300(E)
are shown in Fig.3(a, b, c), respectively. As shown in Fig.3, a lot of well-dispersed and isolated Au particles (black dots have obvious contrast with support) with particle size ranging from 2 nm to 5 nm are formed on the surface of sample 4%Au300(W) (Fig.3a) and 8%Au-300(W) (Fig.3b). However, it seems that gold coatings were formed on the surface of sample 8%Au-300(E) instead of the formation of dispersed Au particles (Fig.3c). The particle size calculated by XRD corresponds to the size of Au packages. It is obvious that the state of gold is closely related to whether washing process was carried out. For the evaporated samples, several compounds, such as chlorides, sulfides, and thiosulfates may present on the surface of the samples acting as “solvent” and “protective agent”, which can form coatings together with gold species to slow down the decomposition rate of gold compounds, resulting in the formation of gold coatings. For the washed samples, gold species lose “the shelter” and decompose at a higher rate, resulting in the formation of isolated gold particles. Zanella et al.[16] found that washing treatment played a key role in the formation of highly dispersed gold particles when they prepared Au/TiO2 catalysts with deposition-precipitation (DP) method. 2.4
Au loading analysis and mechanism of Au loading
In the washing process of Au/TiO2 samples, gold species that are not closely interacted with TiO2 surface might be washed out and will eventually lead to the loss of gold. The actual Au loading of washed samples were determined by AAS and calculated by applying the following formula, wAu=
mAu ×100% mAu + mTiO 2
where wAu is the mass percentage of Au, mAu and mTiO2 are the mass of Au and TiO2 in samples, respectively. Table 1 shows Table 1 Au loading (wAu) determined by AAS and calculated Au loading ratio (x) Sample
Fig.2
XRD patterns of Au/TiO2 samples
(a) P25; (b) 8%Au-300(W); (c) 8%Au-500(W); (d) 8%Au-300(E)
wAu (%)
x (%)
theoretical
determined by AAS
0.1%Au-300(W)
0.1
0.10
100
1%Au-300(W)
1.0
0.95
95.0
4%Au-300(W)
4.0
3.54
88.5
8%Au-300(W)
8.0
7.38
92.3
Baozhu Tian et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 978−982
the theory Au loading, Au loading determined by AAS, and the calculated Au loading ratio. According to Table 1, high Au loading and loading ratio can be obtained by using Au(S2O3)23− as the gold precursor. Au species, in the form of Au(OH)Cl−3 or AuCl−4, electrostatically adsorbed on the surface of TiO2 when samples underwent the impregnation process of HAuCl4 precursor. Due to the low adsorption forces between the complex ions and TiO2 surface, part of Au species should be washed out, resulting in the decrease of Au loading[16]. If Au(S2O3)3− 2 was simply adsorbed on the surface of TiO2 by electrostatic forces, Au(S2O3)3− 2 can reach adsorption-desorption equilibrium on the surface of TiO2 in short time. Thus, the final Au loading should not severely depend on the impregnation temperature and the impregnation time. According to Table 2, impregnation temperature and impregnation time do not influence the Au loading of the samples impregnated with HAuCl4, but both low impregnation temperature and short impregnation time decrease the Au loading of the samples impregnated with Au(S2O3)3− 2 precursor. Therefore, Au species should not electrostatically be adsorbed on the surface of TiO2 after Au(S2O3)3− 2 impregnation at high temperature. Although Au(S2O3)3– 2 can steadily exist under alkaline and 2– excess S2O3 conditions, it can gradually decompose with increasing temperature under acidic condition. Losing protective agent, Au(S2O3)3– 2 might be adsorbed on the surface of TiO2 in the form of sulphides and oxides. Owing to the low reaction rate, both impregnation temperature and impregnation time influence Au loading. According to the relational references[21], both gold sulphides (Au2S and Au2S3) and gold oxides (AuO and Au2O3) decompose at 200−260 and form metallic gold. Therefore, it can be confirmed that gold species exist in the form of metallic gold after the catalysts calcined at 300 for 4 h. The XRD and DRS results are also consistent with abovementioned presumption. 2.5
Photocatalytic activity measurement
Fig.4 shows the degradation of MO with Au/TiO2 catalysts under UV irradiation for 20 min. For the evaporated samples wAu-300(E), the photocatalytic activity increases with the increase in Au loading at the beginning and shows a downtrend with optimum photocatalytic activity at the Au loading of 4%.
Fig.4
Degradation percentage of methyl orange with various
Au/TiO2 catalysts under UV light irradiation for 20 min
For the washed samples wAu-300(W), the photocatalytic activity increases with the increase in Au loading at the beginning and almost shows no changes when the Au loading exceeds 4%. With the same theory of Au loading, the photocatalytic activity of the washed samples is obviously higher than that of the evaporated samples, but the actual Au loading of the evaporated samples is higher than that of the washed samples. Therefore, the difference in photocatalytic activities of the two types of catalysts is relative to the structure of Au, which influences the light absorption and the photoelectron transfer of TiO2 catalysts. For the washed samples, gold exists in the form of isolated Au particles, little light is absorbed, and most light can reach TiO2 to induce photoreaction, generating a considerable number of photoelectrons. For the evaporated samples, gold exists in the form of gold coating, more light was absorbed by gold instead of TiO2, resulting in the decrease of photoelectron numbers. Moreover, the photocatalytic activity of catalysts is also relative to the photoelectron transfer. The effects of gold structure on the electron transfer were discussed hereunder. Fig.5 shows the energy levels of valence band and conduction band of TiO2, gold particles, and adsorbed O2 on the TiO2 surface, which can determine the direction of electronic flow[22,23]. The electrons cannot directly or indirectly flow from bulk gold to adsorbed O2, because the energy level of adsorbed O2 is higher than that of bulk gold. Based on the fact that the Fermi energy of metal particles increases with decreasing size, because of the quantum size effect, the Au parti-
Table 2 Effects of impregnation temperature (T) and impregnation time (t) on Au loading wAu (%) Precursor
T/
t/h
80 80 80 60 80 80 80 60
1 2 4 4 1 2 4 4
theoretical HAuCl4 HAuCl4 HAuCl4 HAuCl4 Au(S2O3)3− 2 Au(S2O3)3− 2 Au(S2O3)3− 2 Au(S2O3)3− 2
4 4 4 4 4 4 4 4
determined by AAS 1.19 1.15 1.21 1.13 2.70 3.31 3.63 2.28
Fig.5 Schematic diagram showing the electronic energy levels of Au nanoparticle, valence band (VB), and conduction band (CB) of TiO2 and adsorbed O2
Baozhu Tian et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 978−982
cles with an appropriate size can possess an energy level between the conduct band of TiO2 and the adsorbed O2. Thus, the photoelectrons can be captured by gold particles and be transferred to the adsorbed O2, leading to the effective separation of electrons and holes, and consequently increasing the photocatalytic activity of TiO2. When the size of gold particles is large, the Fermi energy of gold particles will be lower than that of adsorbed O2, and the photoelectrons cannot be transferred to adsorbed O2. Whereas the size of Au particles is too small, photoelectrons also cannot be transferred from the bottom of TiO2 conduction band to the gold particles, because the Fermi energy of the gold particles is higher than that of adsorbed O2. Therefore, gold particles with an appropriate size is effective on the improvement of photocatalytic activity, gold particles with too large or too small size have no benefits for the improvement of photocatalytic activity. In the experiments, for the washed samples, gold particles with an appropriate size were formed within 1%−8% Au loading. So, the photocatalytic activity of samples increases with the increase in Au loading. For the evaporated samples, under low Au loading, gold has an appropriate Fermi energy, because it partly covered the TiO2 surface, leading to the effective transfer of electrons, separation of electron-hole pairs, and the improvement of photocatalytic activity. With the increase in Au loading, the Fermi energy of gold is near to bulk gold, because of excess gold coating. Thus, the electrons cannot be transferred effectively and the photocatalytic activity also cannot be improved. The photocatalytic activity depends on both its light absorption and its electronic transfer properties.
3
Conclusions
Au/TiO2 catalysts were prepared using Au(S2O3)3− 2 as gold precursor. The results indicated that the structure of Au on the surface of TiO2 is severely relative to the preparation methodology. By washing treatment, Au can be anchored on the surface of TiO2 in the state of highly dispersed Au nanoparticles (2−5 nm). By rotary evaporation, coating structure of Au was formed on the surface of TiO2. The results of photodegradation of methyl orange indicated that the samples prepared by washing treatment showed higher photocatalytic activity when compared to the samples prepared by rotary evaporation. The photocatalytic activity of Au/TiO2 catalysts is severely relative to the state of gold on the surface of TiO2. Compared with coating structure, gold particles with highly dispersed state and appropriate size not only have little light absorption, but also can effectively capture and transfer photoelectrons, re-
sulting in the decrease of recombination of electrons and holes and consequently favoring the improvement of photocatalytic activity.
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Cite this article as: Acta Phys. -Chim. Sin., 2007, 23(7): 978−982.
Effect of Water Washing Treatment on the Photocatalytic Activity of Au/TiO2 Catalysts Baozhu Tian,
Tianzhong Tong,
Feng Chen,
Jinlong Zhang*
Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, P. R. China
Abstract:
By using Au(S2O3)3− 2 as the gold precursor, Au/TiO2 photocatalysts were prepared by water washing (W) and rotary
evaporation (E) processes, respectively. The samples were characterized by UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic absorption flame emission spectroscopy (AAS). The photocatalytic activity of the samples was evaluated from the analysis of the photodegradation of methyl orange (MO). By using the water washing treatment, well-dispersed gold nanoparticles with the diameter of 2−5 nm were formed on the surface of TiO2, whereas only the gold coating was formed for rotary evaporation processes. The photocatalytic activity of Au/TiO2 photocatalysts is related to the preparation process. With a similar gold loading, the photocatalysts prepared by water washing showed higher photocatalytic activity compared to the catalysts prepared by rotary evaporation. Key Words:
Au/TiO2; Photocatalytic activity; Au loading; Water washing; Rotary evaporation
In recent years, applying semiconductor photocatalysts to treat environmental contaminants has become a new type of pollution treatment technology. Amongst the various semiconductor photocatalysts, TiO2 is considered to be one of the most promising photocatalysts due to its stability, non-toxicity, and low cost[1,2]. However, its practical application is limited owing to its low photo-quantum efficiency and low reaction rate. It has been confirmed by various studies that noble metal loaded on the surface of TiO2 (M/TiO2) can depress the electron-hole recombination and consequently improve the photocatalytic activity of TiO2 semiconductor[3,4]. In particular, Au/TiO2 shows surprisingly high photocatalytic activity in the selective oxidation of propene, nitrogen oxide reduction, low-temperature oxidation of CO, and photocatalytic oxidations used for environmental cleanup[5−9]. Furthermore, many studies revealed that the catalytic activity of Au/TiO2 catalysts is severely relative to the morphology and size of Au particles,
the interaction between Au and TiO2 support, and so on. It is generally believed that highly dispersed Au particles with diameters below 5 nm show high photocatalytic activity[5,8,10,11]. Therefore, many chemical and physical methods, such as ion implantation co-precipitation[12], sputtering[13,14], chemical vapor deposition (CVD)[15], sol-gel[9], and deposition-precipitation (DP)[16] have been developed for the preparation of Au/TiO2 catalysts. Amongst the wet-chemical methods, many factors, such as precursor type, preparation method, and preparation methodology can influence the Au loading and morphology of gold on the surface of TiO2, and finally influence the photocatalytic activity of TiO2. Therefore, it is very important to select the appropriate precursor, preparation method, and methodology for the preparation of highly photoactive Au/TiO2 catalysts. In this study, Au/TiO2 catalysts were prepared using Au(S2O3)3− 2 as precursor and the influence of washing treatment on state of gold on the surface of TiO2 was studied. The
Received: January 22, 2007; Revised: March 19, 2007. * Corresponding author. Email: [email protected]; Tel: +8621-64252062. The project was supported by the National Natural Science Foundation of China (20577009), the International Co-operation Project of the Minister of Science and Technology, China (2006DFA52710), the Program for New Century Excellent Talents in Universities, China (NCET-04-0414), and Shanghai Nanotechnology Promotion Center, China (0552nm019). Copyright © 2007, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Baozhu Tian et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 978−982
photocatalytic activities of Au/TiO2 catalysts prepared with different process were evaluated from an analysis of the photodegradation of methyl orange (MO).
1 1.1
Experimental Preparation of samples
Au(S2O3)3− 2 aqueous solution was prepared by slowly adding HAuCl4 aqueous solution to excess Na2S2O3 aqueous solution under stirring (the mole ratio of HAuCl4 to Na2S2O3 was 1:4.25)[17]. At first, the pH of 60 mL of Au(S2O3)3− 2 aqueous solution was adjusted to 2.0 with diluted hydrochloric acid solution. Subsequently, 1 g dried TiO2 (P25) was added to this solution, followed by ultrasound irradiation for 10 min in an ultrasonic cleaning bath (Elma, T660/H, 35 kHZ, 360 W). Then, the mixture was further sonicated for 4 h when the temperature was heated to 80 . The obtained slurry was divided equally into two parts. One was dried with a rotary evaporator, and the other was centrifuged, separated, and washed with distilled water (100 mL) four times to remove residual Cl− and Na+ ions, as well as, Au species not interacting with the support. Finally, the two samples were heated from room temperature to a certain temperature with a heating rate of 2 ·min−1 and calcined at this temperature for 4 h. The samples prepared by washing treatment and rotary evaporation were labeled as wAu-T(W) and wAu-T(E), respectively, in which the “w” refers to the nominal mass percentage content of Au in the Au/TiO2, and “T” refers heat-treating temperature (). 1.2
Characterization
X-ray diffraction (XRD) measurements were carried out with a Rigaku D/max 2550 VB/PC X-ray diffractometer using Cu Kα radiation (λ=0.15406 nm) and a graphite monochromator, operated at 40 kV and 100 mA. UV-Vis diffuse reflectance spectra (DRS) were obtained using a Scan UV-Vis-NIR spectrophotometer (Varian Cary500) equipped with an integrating sphere assembly, using polytetrafluoroethylence (PTFE) as a reference material. The measure ranged from 300 to 800 nm. The state of Au on the surface of TiO2 was observed with a JEOL JEM-100 CX II transmission electron microscopy (TEM), operated at an acceleration voltage of 100 kV. The content of Au in Au/TiO2 samples was determined by atomic absorption flame emission spectroscopy (AAS) (Shimadzu AA-6400F). 1.3
water in a quartz cylindrical jacket around the lamp. The distance between the light and the reaction tube was fixed at 24 cm. 0.05 g of catalyst sample was added into a 70 mL quartz tube containing 50 mL of 30 mg·L−1 methyl orange. Before photoreaction, the mixture was stirred in the dark for 30 min to attain the adsorption-desorption equilibrium for MO on the surface of TiO2. At given time intervals, the samples were taken from the suspension and the absorbance of MO was measured at 464 nm, which is the maximum absorption of MO. The degradation rate of methyl orange was calculated from the absorbance.
2 2.1
Results and discussion UV-Vis diffuse reflectance spectra
Fig.1 shows the UV-Vis diffuse reflectance spectra (UV-Vis DRS) of sample 4%Au-T(W) calcined at different temperatures. As shown in Fig.1, the absorption onset (absorption edge) of P25 is around 400 nm, which is between the absorption onsets of anatase (387 nm) and rutile (418 nm). This can be ascribed to that P25 comprises in anatase and rutile. Compared with P25, there is a red shift of 20 nm in the onset absorption and a increase of light absorption in the visible region for the samples of 4%Au-T(W), which is consistent with Li′s results[9]. In DRS, an absorbance band around 545 nm arises and the intensity of the absorbance band increases with increasing calcination temperature within 200−350 for samples 4%Au-T(W). According to the relevant references[9,18,19], the absorbance band should be ascribed to the plasmon resonance of metallic Au particles. For the metal nanoparticles of Au0, Ag0, and Cu0, the plasmon absorption arises from the collective oscillations of the free conduction band electrons that are induced by the incident electromagnetic radiation. For Au/TiO2 samples, the intensity of the absorbance band is related to the size of Au particles and the content of Au in Au/TiO2. There is not much difference in the intensity when the temperature of 4%Au-T(W) varies from 300 to 350 , suggesting that the size of Au has not much difference and gold exists in the state of metallic gold. The characteristic absorbance band of Au around 545 nm in the DSR of sample
Photocatalytic activity measurement
Methyl orange was used as a simulated contaminant to evaluate the photocatalytic activity of the Au/TiO2 samples. The photocatalytic reactions were carried out with a homemade photoreactor, in which a 300 W high-pressure Hg lamp (λmax=365 nm) was used. The lamp was cooled with flowing
Fig.1
UV-Vis DRS of P25 and 4%Au-T(W) samples calcined at different temperatures
Baozhu Tian et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 978−982
4%Au-300(E) indicated the existence of metallic Au (the spectrum was not listed). 2.2
XRD analysis
The XRD patterns of P25, 8%Au-300(W), 8%Au-500(W), and 8%Au-300(E) are shown in Fig.2(a–d), respectively. As shown in Fig.2, the shapes and intensities of the diffraction peaks for both anatase and rutile in the four samples are very similar, indicating that calcining at 300 or 500 for 4 h did not change the content and particle size of anatase and rutile in the samples. By careful observation, it can be found that new peaks emerge at 2θ=38.2º, 44.4º, 64.6º, and 77.6º, which can be attributed to the diffraction peaks of (111), (200), (220), and (311) planes of polycrystalline Au, respectively (overlap occurred between the (111) diffraction peak of Au and the (101) diffraction peak of rutile). The above-mentioned results confirmed the formation of metallic gold. At 2θ=77.6º, the diffraction peak of Au shows no overlap with those of anatase and rutile. The average crystallite size of Au can be calculated by applying the Debye-Scherrer formula[20], D= Kλ βcosθ where D is the average crystallite size, K is a constant which is taken as 0.89 here, λ is the wavelength of the X-ray radiation (Cu Kα, λ=0.15406 nm), β is the corrected band broadening (full width at half-maximum (FWHM)) after subtraction of equipment broadening, and θ is the diffraction angle. The average crystallite sizes of samples 8%Au-500(W) and 8%Au-300(E) were calculated to be 12.9 nm and 17.0 nm, respectively. Compared with samples of 8%Au-500(W) and 8%Au-300(E), the diffraction peak of sample 8%Au-300(W) was broader and lower, suggesting a smaller average crystallite size of Au for the sample 8%Au-300(W) (cannot be calculated, because of very low peak intensity) . 2.3
TEM analysis
The size and morphology of Au particles on the surface of TiO2 can be observed with TEM. The TEM images of the samples 4%Au-300(W), 8%Au-300(W), and 8%Au-300(E)
Fig.3 TEM images of Au/TiO2 samples (a) 4%Au-300(W); (b) 8%Au-300(W); (c) 8%Au-300(E)
are shown in Fig.3(a, b, c), respectively. As shown in Fig.3, a lot of well-dispersed and isolated Au particles (black dots have obvious contrast with support) with particle size ranging from 2 nm to 5 nm are formed on the surface of sample 4%Au300(W) (Fig.3a) and 8%Au-300(W) (Fig.3b). However, it seems that gold coatings were formed on the surface of sample 8%Au-300(E) instead of the formation of dispersed Au particles (Fig.3c). The particle size calculated by XRD corresponds to the size of Au packages. It is obvious that the state of gold is closely related to whether washing process was carried out. For the evaporated samples, several compounds, such as chlorides, sulfides, and thiosulfates may present on the surface of the samples acting as “solvent” and “protective agent”, which can form coatings together with gold species to slow down the decomposition rate of gold compounds, resulting in the formation of gold coatings. For the washed samples, gold species lose “the shelter” and decompose at a higher rate, resulting in the formation of isolated gold particles. Zanella et al.[16] found that washing treatment played a key role in the formation of highly dispersed gold particles when they prepared Au/TiO2 catalysts with deposition-precipitation (DP) method. 2.4
Au loading analysis and mechanism of Au loading
In the washing process of Au/TiO2 samples, gold species that are not closely interacted with TiO2 surface might be washed out and will eventually lead to the loss of gold. The actual Au loading of washed samples were determined by AAS and calculated by applying the following formula, wAu=
mAu ×100% mAu + mTiO 2
where wAu is the mass percentage of Au, mAu and mTiO2 are the mass of Au and TiO2 in samples, respectively. Table 1 shows Table 1 Au loading (wAu) determined by AAS and calculated Au loading ratio (x) Sample
Fig.2
XRD patterns of Au/TiO2 samples
(a) P25; (b) 8%Au-300(W); (c) 8%Au-500(W); (d) 8%Au-300(E)
wAu (%)
x (%)
theoretical
determined by AAS
0.1%Au-300(W)
0.1
0.10
100
1%Au-300(W)
1.0
0.95
95.0
4%Au-300(W)
4.0
3.54
88.5
8%Au-300(W)
8.0
7.38
92.3
Baozhu Tian et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 978−982
the theory Au loading, Au loading determined by AAS, and the calculated Au loading ratio. According to Table 1, high Au loading and loading ratio can be obtained by using Au(S2O3)23− as the gold precursor. Au species, in the form of Au(OH)Cl−3 or AuCl−4, electrostatically adsorbed on the surface of TiO2 when samples underwent the impregnation process of HAuCl4 precursor. Due to the low adsorption forces between the complex ions and TiO2 surface, part of Au species should be washed out, resulting in the decrease of Au loading[16]. If Au(S2O3)3− 2 was simply adsorbed on the surface of TiO2 by electrostatic forces, Au(S2O3)3− 2 can reach adsorption-desorption equilibrium on the surface of TiO2 in short time. Thus, the final Au loading should not severely depend on the impregnation temperature and the impregnation time. According to Table 2, impregnation temperature and impregnation time do not influence the Au loading of the samples impregnated with HAuCl4, but both low impregnation temperature and short impregnation time decrease the Au loading of the samples impregnated with Au(S2O3)3− 2 precursor. Therefore, Au species should not electrostatically be adsorbed on the surface of TiO2 after Au(S2O3)3− 2 impregnation at high temperature. Although Au(S2O3)3– 2 can steadily exist under alkaline and 2– excess S2O3 conditions, it can gradually decompose with increasing temperature under acidic condition. Losing protective agent, Au(S2O3)3– 2 might be adsorbed on the surface of TiO2 in the form of sulphides and oxides. Owing to the low reaction rate, both impregnation temperature and impregnation time influence Au loading. According to the relational references[21], both gold sulphides (Au2S and Au2S3) and gold oxides (AuO and Au2O3) decompose at 200−260 and form metallic gold. Therefore, it can be confirmed that gold species exist in the form of metallic gold after the catalysts calcined at 300 for 4 h. The XRD and DRS results are also consistent with abovementioned presumption. 2.5
Photocatalytic activity measurement
Fig.4 shows the degradation of MO with Au/TiO2 catalysts under UV irradiation for 20 min. For the evaporated samples wAu-300(E), the photocatalytic activity increases with the increase in Au loading at the beginning and shows a downtrend with optimum photocatalytic activity at the Au loading of 4%.
Fig.4
Degradation percentage of methyl orange with various
Au/TiO2 catalysts under UV light irradiation for 20 min
For the washed samples wAu-300(W), the photocatalytic activity increases with the increase in Au loading at the beginning and almost shows no changes when the Au loading exceeds 4%. With the same theory of Au loading, the photocatalytic activity of the washed samples is obviously higher than that of the evaporated samples, but the actual Au loading of the evaporated samples is higher than that of the washed samples. Therefore, the difference in photocatalytic activities of the two types of catalysts is relative to the structure of Au, which influences the light absorption and the photoelectron transfer of TiO2 catalysts. For the washed samples, gold exists in the form of isolated Au particles, little light is absorbed, and most light can reach TiO2 to induce photoreaction, generating a considerable number of photoelectrons. For the evaporated samples, gold exists in the form of gold coating, more light was absorbed by gold instead of TiO2, resulting in the decrease of photoelectron numbers. Moreover, the photocatalytic activity of catalysts is also relative to the photoelectron transfer. The effects of gold structure on the electron transfer were discussed hereunder. Fig.5 shows the energy levels of valence band and conduction band of TiO2, gold particles, and adsorbed O2 on the TiO2 surface, which can determine the direction of electronic flow[22,23]. The electrons cannot directly or indirectly flow from bulk gold to adsorbed O2, because the energy level of adsorbed O2 is higher than that of bulk gold. Based on the fact that the Fermi energy of metal particles increases with decreasing size, because of the quantum size effect, the Au parti-
Table 2 Effects of impregnation temperature (T) and impregnation time (t) on Au loading wAu (%) Precursor
T/
t/h
80 80 80 60 80 80 80 60
1 2 4 4 1 2 4 4
theoretical HAuCl4 HAuCl4 HAuCl4 HAuCl4 Au(S2O3)3− 2 Au(S2O3)3− 2 Au(S2O3)3− 2 Au(S2O3)3− 2
4 4 4 4 4 4 4 4
determined by AAS 1.19 1.15 1.21 1.13 2.70 3.31 3.63 2.28
Fig.5 Schematic diagram showing the electronic energy levels of Au nanoparticle, valence band (VB), and conduction band (CB) of TiO2 and adsorbed O2
Baozhu Tian et al. / Acta Physico-Chimica Sinica, 2007, 23(7): 978−982
cles with an appropriate size can possess an energy level between the conduct band of TiO2 and the adsorbed O2. Thus, the photoelectrons can be captured by gold particles and be transferred to the adsorbed O2, leading to the effective separation of electrons and holes, and consequently increasing the photocatalytic activity of TiO2. When the size of gold particles is large, the Fermi energy of gold particles will be lower than that of adsorbed O2, and the photoelectrons cannot be transferred to adsorbed O2. Whereas the size of Au particles is too small, photoelectrons also cannot be transferred from the bottom of TiO2 conduction band to the gold particles, because the Fermi energy of the gold particles is higher than that of adsorbed O2. Therefore, gold particles with an appropriate size is effective on the improvement of photocatalytic activity, gold particles with too large or too small size have no benefits for the improvement of photocatalytic activity. In the experiments, for the washed samples, gold particles with an appropriate size were formed within 1%−8% Au loading. So, the photocatalytic activity of samples increases with the increase in Au loading. For the evaporated samples, under low Au loading, gold has an appropriate Fermi energy, because it partly covered the TiO2 surface, leading to the effective transfer of electrons, separation of electron-hole pairs, and the improvement of photocatalytic activity. With the increase in Au loading, the Fermi energy of gold is near to bulk gold, because of excess gold coating. Thus, the electrons cannot be transferred effectively and the photocatalytic activity also cannot be improved. The photocatalytic activity depends on both its light absorption and its electronic transfer properties.
3
Conclusions
Au/TiO2 catalysts were prepared using Au(S2O3)3− 2 as gold precursor. The results indicated that the structure of Au on the surface of TiO2 is severely relative to the preparation methodology. By washing treatment, Au can be anchored on the surface of TiO2 in the state of highly dispersed Au nanoparticles (2−5 nm). By rotary evaporation, coating structure of Au was formed on the surface of TiO2. The results of photodegradation of methyl orange indicated that the samples prepared by washing treatment showed higher photocatalytic activity when compared to the samples prepared by rotary evaporation. The photocatalytic activity of Au/TiO2 catalysts is severely relative to the state of gold on the surface of TiO2. Compared with coating structure, gold particles with highly dispersed state and appropriate size not only have little light absorption, but also can effectively capture and transfer photoelectrons, re-
sulting in the decrease of recombination of electrons and holes and consequently favoring the improvement of photocatalytic activity.
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