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Effects of the amount of Au nanoparticles on the visible light response of TiO2 photocatalysts
Catalysis Today xxx (xxxx) xxx–xxx
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Effects of the amount of Au nanoparticles on the visible light response of TiO2 photocatalysts Tomoko Yoshidaa,*, Yuhei Misub, Muneaki Yamamotoa, Tetsuo Tanabea, Jun Kumagaic, Satoshi Ogawab, Shinya Yagic a
Advanced Research Institute for Natural Science and Technology, Osaka City University, Osaka, Japan Graduate School of Engineering, Nagoya University, Nagoya, Japan c Institute for Materials and Systems for Sustainability, Nagoya University, Nagoya, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Au nanoparticle TiO2 photocatalyst Visible light response Loading amount of Au nanopartilce
We have succeeded to prepare Au nanopareticle deposited TiO2 photocatalysts (Au/TiO2) with control of Au nanoparticle size to be around 8 nm and variation of number density using a colloid photodepostion method. The prepared Au/TiO2 exhibits activity on decomposition (oxidation) of formic acid by thermally activated and photo-activated catalytic reactions. The thermally activated catalytic decomposition gradually increases with increasing the number density of deposited Au NPs and saturated, suggesting that the decomposition occurs on Au NPs surface and/or near the interface of TiO2 and Au NPs. On the other hand, the photocatalytic decomposition is significantly improved with rather small number density deposition of Au NPs and disappeared with high number density deposition. ESR measurements of Au/TiO2 in the surrounding similar to the photocatalytic decomposition suggests that electrons excited by plasmon resonance absorption in the Au NPs transfer to TiO2 to promote the decomposition. However, high number density deposition enhances electron capture by neighboring Au NPs and reduces the photocatalytic activity. Thus there should be the optimum number density of Au NPs on TiO2 for photocatalytic decomposition of formic acid.
1. Introduction TiO2 has been applied to various fields such as solar cells, purifying water and air, and optical degradation of organic compounds due to its unique optical, electrical and chemical properties [1–4]. However, as a wide band-gap oxide semiconductor, TiO2 shows photocatalytic activity only under UV-light irradiation. Therefore, the development of TiO2 photocatalysts working under visible light irradiation is important in order to utilize solar energy efficiently, and many efforts have been devoted to the preparation of visible light response TiO2 photocatalysts especially for the degradation of organic compounds. Recently, localized surface plasmon resonance (LSPR) on Au nanoparticles has been applied to visible light response photocatalysis [5–11], and some research groups reported chemical reactions such as oxidation of aromatic alcohols [12], formic acid [13] and 2-propanaol [14,15] and hydrogen production from alcohols [16,17] proceeded over Au/TiO2 under visible light irradiation. As for alternative mechanisms for the plasmonic photocatalyses are proposed: one is that the LSPR in metal nanoparticles (NPs) can enhance the local electronic field of the ⁎
neighboring TiO2 under visible light irradiation [5–8,18]. Another is that electrons excited by the LSPR are partly transferred to the conduction band of TiO2 to induce photocatalytic reactions [19–27]. Thus visible light response of TiO2 seems to depend on both size and number density of Au nanoparticles (NPs) as pointed out by some researches [13,17]. Kominami et al. and Kowaiska et al. reported that action spectra in the degradation of formic acid in aqueous suspensions of Au supported on TiO2 or CeO2 photocatalysts were in good agreement with their photo-absorption spectra [23,24]. These results suggest that degradation of formic acid was induced by photo-absorption due to LSPR of the supported Au nanoparticles. In addition, degradation (oxidation) of formic acid in air can undergo the direct decomposition without the formation of any other stable intermediates. Thus the decomposition of formic acid is suitable for understanding the loading effects of Au nanoparticles on the visible light response of TiO2 photocatalysts. In this study, we have tested two different methods to deposit (load) Au NPs on TiO2 for easier control of sizes and numbers density of deposited Au NPs. Using thus prepared Au NPs loaded TiO2 as catalysts, decomposition of formic acid under visible light irradiation was conducted to
Corresponding author. E-mail address: [email protected] (T. Yoshida).
https://doi.org/10.1016/j.cattod.2019.12.035 Received 24 June 2019; Received in revised form 18 November 2019; Accepted 26 December 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Tomoko Yoshida, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.12.035
Catalysis Today xxx (xxxx) xxx–xxx
T. Yoshida, et al.
examine the effect of number density of deposited Au NPs.
apparatus. The spectrum of BaSO4 powder was used as the reference.
2. Experimental
2.5. XAFS measurement
2.1. Catalysts preparation
The XAFS measurements at the Au L3-edge were carried out at BL5S1 of Aichi Synchrotron Radiation Center with a Si(111) doublecrystal monochromator in both transmission and fluorescence modes at room temperature (201504073).
Two different methods were employed for deposition (loading) of Au NPs on TiO2 and thus prepared samples are refereed as Au/TiO2 hereafter. They are a conventional photodeposition method and a colloid photodeposition method operated in the presence of a hole scavenger (CPH).
2.6. ESR measurement ESR measurements of TiO2 and Au/TiO2 with and without visible light irradiation were carried out at 20 K. The quart tube including the sample after evacuation was set to an X-band ESR spectrometer (JEOL JES-RE1X) with liquid helium cryostat temperature control system (JANIS ES-CT470) and cooled to 20 K. Visible light was irradiated to the sample in the ESR cavity with the cryostat by gathering the light by quartz lens to the cavity. Typical ESR parameters for the measurements were microwave power of 1 μW, microwave frequency of 9.099 GHz and magnetic field of 324 −339 mT.
(1) Preparation by photodeposition method HAuCl4·4H2O powder was dissolved in a methanol aqueous solution of 60 mL (10 mL of methanol was dissolved in distilled water of 50 mL). 1.0 g of rutile TiO2 power (Toho chemical Industry Co., BET surface area was 6 m2/g) was suspended in the aqueous solution. The suspension was irradiated for 3 h with a 300 W Xe lamp equipped with a band path filter (ca. 340 ± 30 nm, 20 mW/cm2), and then filtered and washed with purified water. (2) Preparation by colloid photodeposition method Au colloid NPs were prepared by pH shift method described elsewhere [28]; 41.8 mL of distilled water and 1.25 mL of HAuCl4 solution (20 mM) were introduced in a beaker and heated at 373 K, resulting a precursor solution. 1.75 mL of citric acid (100 mM) was added to the precursor solution, then the solution was stirred and refluxed. After waiting 7 s, 5.2 mL of NaOH (100 mM) solution was introduced into the solution to shift the pH from acidic to neutral. Consequently, colloidal solution of Au nanoparticles (NPs) was obtained. The prepared Au NPs were loaded on TiO2 powders by colloid photodeposition with methanol as a hole scavenger (CPH) method reported by Tanaka et al. [13]; TiO2 powders (0.1−1 g) are suspended in 50 mL of Au colloidal solution (20 mM) and stirred under UV light irradiation while 5 mL of methanol as a hole scavenger is added to the suspension every hour up to 7 h, and then filtered and washed with purified water.
3. Results and discussion (1) Au/TiO2 prepared by photodeposition method Fig. 1 shows Au L3-edge XANES spectra of Au/TiO2 samples prepared by the photodeposition method together with Au foil. The XANES spectra of Au/TiO2 samples are similar to that of Au foil, suggesting that the chemical state of Au cocatalysts is metalic (Au(0)). In the measurement of UV–vis diffuse reflectance spectra of Au/TiO2 samples (Fig. 2), the broad Localized Surface Plasmon Resonance (LSPR) absorption bands were observed and the band became broader with increasing the loading amount of Au, suggesting that Au NPs with various sizes were deposited on TiO2 by the photodeposition method and the average size of Au NPs increased with loading amount of Au. Actually, as shown in TEM image of Au/TiO2 sample (Fig. 3), the metallic Au cocatalysts were deposited on TiO2 as Au NPs with various sizes of several to several tens nm. Unfortunately, the Au loading by a photodeposition method often complicates understanding the effects of Au
2.2. Reaction test Au/TiO2 (50 mg) was suspended in distilled water (3 mL) in a test tube and formic acid (26 μmol) was injected into the suspension and then irradiated with visible light from a 300 W Xe lamp with a cut of filter (λ > 480 nm) in air. The formic acid would be oxidized as HCOOH + 1/2O2 → CO2 + H2O. The amount of CO2 produced in the gas phase was measured using a gas chromatograph (TCD-GC). The reaction was also conducted without photoirradiation at 323 K (the same temperature under visible light irradiation). 2.3. TEM observation For TEM observation, Au colloidal NPs solution was mounted directly on a carbon covered copper mesh, while Au/TiO2 were dispersed in an ethanol solution and a drop of the dispersed solution was mounted on a holey carbon covered copper mesh. TEM images were recorded with a JEM-2100 (JEOL) electron microscope operated at 200 kV at the High Voltage Electron Microscope Laboratory in Nagoya University. 2.4. UV–vis optical absorption measurement UV-vis optical absorption spectra of Au colloidal solution were recorded at room temperature using a JASCO V-670 in a transmission mode. As for Au loaded TiO2 samples prepared by the photodeposition method, diffuse reflectance spectra were measured using the same
Fig. 1. Au L3-edge XANES spectra of Au/TiO2 samples prepared by photodeposition method and a Au foil. 2
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Fig. 2. Diffuse reflectance spectra of Au/TiO2 samples prepared by photodeposition method.
Fig. 4. TEM image and the size distribution histogram of the colloidal Au NPs.
Fig. 3. TEM image of 2 wt% Au/TiO2 sample prepared by photodeposition method.
loading amount on the photocatalytic activity of Au/TiO2 sample, since both the size and the number density of Au NPs simultaneously change with Au loading amounts. (2) Au/TiO2 colloidal photodeposition (CPD) method To understand the correlation between photocatalytic activity of Au/TiO2 and the loading amount of Au NPs, we tried to prepare TiO2 photocatalysts loaded with the same sized Au NPs. The size and morphology of synthesized colloidal Au NPs were determined using TEM. Fig. 4 shows a TEM image and the size distribution histogram of the colloidal Au NPs, which clearly indicates that Au NPs with the average size of 7.3 nm are uniformly distributed. The loading of Au NPs on TiO2 was conducted by the CPH method where TiO2 powders were suspended in the Au colloidal solution and stirred under UV light irradiation while 5 mL of methanol as a hole scavenger is added to the suspension every hour. Fig. 5 shows changes of the UV–vis absorption spectra of the solution by every hour. Before UV light irradiation, the solution gave LSPR absorptions attributed to the Au NPs of around 520 nm [17,29]. After 7 h of the light irradiation, the LSPR absorption disappeared, suggesting that all colloidal Au NPs in the solution were deposited on TiO2 samples. Fig. 6 shows TEM image and the size distribution histogram of Au NPs loaded on TiO2 samples. The average size of Au NPs in all Au/TiO2 samples was estimated as ca. 8.2 nm. It was thus clarified that the Au NPs could be loaded on TiO2 by
Fig. 5. Variation of UV–vis absorption spectra of Au colloid solution with time.
the CPH method without remarkable aggregation of the Au NPs. In addition, the size distribution histograms of the Au NPs were almost the same regardless of the loading amount of Au NPs (as shown in S1). Table 1 summarizes average particle sizes and number densities of thus prepared Au/TiO2 samples used as catalysts for the decomposition of formic acid. 3
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Fig. 8. Variation of photo- and thermo- catalytic reaction rates with the number of Au nanoparticles.
Fig. 7 shows time courses of the amount of CO2 evolved by the decomposition (oxidation) of from formic acid in aqueous suspensions of 0.2 wt % Au/TiO2 and TiO2. CO2 was evolved over Au/TiO2 with more than 0.2 wt % Au and the amount of evolved CO2 increased almost linearly with the irradiation time. It was confirmed that no CO2 was evolved in direct photo-decomposition of formic acid and photocatalytic reaction using bare TiO2 in Au colloid solution under visible light irradiation. It should be noted that without photoirradation, the CO2 evolution appeared for 0.2 wt % Au/TiO2. Haruta et al. have reported that Au/TiO2 prepared by an impregnation method promotes the oxidation of carbon monoxide by activating oxygen at the Au-TiO2 interface [30] Therefore, similar reaction seems to proceed as thermally activated catalytic reaction on 0.2 wt% Au/TiO2. However, the CO2 evolution rate without the light irradiation was smaller than that with the light irradiation. Thus, the oxidation of formic acid was promoted by either or both of thermally- and photo- activated catalytic reactions on Au/TiO2. Since the CPH method allow us to control the sizes and number density of deposited NPs, we could investigate the effects of the number density of Au NPs on TiO2. Results are given in Fig. 8 where the evolution rate of CO2 by the thermal-activated catalytic and photo-activated-catalytic reactions are plotted against the number density of the loaded Au NPs. The thermally activated-catalytic reaction was improved with increasing the number density of Au NPs and saturated over 4.6 × 1015/g (2.4 wt%), suggesting that the decomposition occurs on Au NPs surface and/or near the interface of TiO2 and Au NPs. It is considered that the reaction site increased with the increase of the number of Au NPs for lower number density deposition, while the reaction sites would overlap at a high number density to give saturation. Different from the thermally activated catalytic reaction, the photoactivated catalytic reaction significantly increased only for small amount of Au loading 2.9 × 1014/g (0.2 wt %) and disappeared over 8.4 × 1014/g (0.5 wt %). The cause of such appreciable reduction could be attributed to two causes. One is the enhanced recombination of photo-generated electrons and holes and the other is the decrease of geometrical coverage of active surfaces by loaded Au NPs. As for the plasmonic photocatalytic mechanism of Au/TiO2 sample, it has been reported that some electrons excited by the localized surface plasmon resonance (LSPR) of Au NPs under visible light irradiation are partly transferred to the conduction band of TiO2 and the separated electrons and holes would result in reductive and oxidative reactions respectively. It is also proposed that Ti4+ ions trap the excited electrons to be Ti3+ ions which is photocatalytic active species. To confirm such electron transfer from Au NPs to TiO2, ESR spectra of Au/TiO2 samples taken under visible light irradiation were compared with that of bare TiO2. Fig. 9 compares ESR spectra of bare TiO2 and 2 wt% Au/TiO2. Small Ti3+ ESR signals [31] were observed in both bare TiO2 and 2 wt% Au/TiO2 samples. Under visible light irradiation, however the signal intensities increased remarkably in the TiO2 while decreased slightly in Au/TiO2. Ti3+ ions might be formed by electron excitation of the impurity levels such as defects at TiO2 surface, while most of the electrons
Fig. 6. TEM image of 4.7 wt% Au/TiO2 and the size distribution histogram of Au NPs loaded on TiO2 sample. Table 1 Particle size and number of Au nanoparticles in Au/TiO2 samples with various loading amount of Au. Loading amount of Au (wt%Au)
Particle size (nm)
Standard deviation
Number of AuNPs (×1015/g)
0.1 0.2 0.3 0.5 1.6 2.4 4.7
8.3 8.8 8.3 8.4 8.5 8 8.2
2.1 2.7 2.8 2.1 2.9 2.9 2.2
0.17 0.29 0.52 0.84 2.58 4.64 8.44
Fig. 7. Time courses of evolution of CO2 from formic acid in aqueous suspensions of 0.2 wt % Au/TiO2 and TiO2 samples.
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Author contributions In this study, Tomoko Yoshidacarried out XAFS analysis and UV–vis measurement. Yuhei Misuprepared the samples and performed catalytic reactions. Muneaki Yamamoto carried outTEM measurement. Jun Kumagai measured ESR spectra. Tomoko Yoshidawrote manuscript with support from Tetsuo Tanabe,Satoshi Ogawaand Shinya Yagi. Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Fig. 9. ESR spectra of TiO2 and Au/TiO2 before (control) and under visible light irradiation.
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.12.035.
are likely captured by Au NPs on Au/TiO2. The similar ESR results were obtained for Au/TiO2 with different Au loading amount. Generally electrons generated by LSPR photoexcitation in Au NPs would diffuse to the active sites of TiO2 and cause a reduction reaction. However, if the number density of Au NPs was large enough, the excited electrons are easily transferred to neighboring Au NPs and recombined with holes there before contributing to the photocatalytic reaction. Additionally, the increase of geometrical coverage by Au NPs over TiO2 could also reduce the number of active sites. As shown the TEM image in Fig. 6, the average distance between Au NPs for 4.7 wt% Au/TiO2 is ca. 30 nm, and therefore those for 0.2 wt% Au/TiO2 (photocatalytic active) and 0.5 wt% Au/TiO2 (photocatalytic inactive) would be estimated as ca. 150 nm and 90 nm, respectively. These numbers are reasonable, suppose the mean free path of excited electrons would be around 100 nm. Since the size of Au Np was around 8 nm, nearly 10 % of the TiO2 should be covered by Au NPs for 0.5 wt% Au/TiO2. In this respect, it is quite important to determine the mean free path of the excited electrons, which is remained for future study.
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4. Conclusion With using a colloid photodepostion method, we have succeeded to prepare Au NPs deposited on TiO2 (Au/TiO2). The size of the Au NPs were controlled to be ca. 8 nm varying their deposited density up to full coverage of TiO2 surfaces. Using the Au/TiO2 as catalysts, we have conducted decomposition of formic acid under visible light irradiation focusing to understand the effects of the number density of deposited Au NPs. The CO2 evolution resulting from the decomposition of formic acid were observed with and without visible light irradiation, indicating the decomposition promoted both thermally- and photo-activated catalytic reactions over Au/TiO2. The activity of the thermallyactivated catalytic reaction increased with increasing the number density of Au NPs, but saturated at higher number density deposition indicating strong contribution of Au NPs on the thermal-activated catalytic oxidation of formic acid. The photo-activated catalytic reaction was appreciable for low number density deposition, and for higher number deposition it was significantly suppressed, most probably easier recombination of electrons and holes on Au NPs. These results indicate the existence of optimum in the number density of Au NPs on TiO2 for the photocatalytic decomposition of formic acid.
5
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Effects of the amount of Au nanoparticles on the visible light response of TiO2 photocatalysts Tomoko Yoshidaa,*, Yuhei Misub, Muneaki Yamamotoa, Tetsuo Tanabea, Jun Kumagaic, Satoshi Ogawab, Shinya Yagic a
Advanced Research Institute for Natural Science and Technology, Osaka City University, Osaka, Japan Graduate School of Engineering, Nagoya University, Nagoya, Japan c Institute for Materials and Systems for Sustainability, Nagoya University, Nagoya, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Au nanoparticle TiO2 photocatalyst Visible light response Loading amount of Au nanopartilce
We have succeeded to prepare Au nanopareticle deposited TiO2 photocatalysts (Au/TiO2) with control of Au nanoparticle size to be around 8 nm and variation of number density using a colloid photodepostion method. The prepared Au/TiO2 exhibits activity on decomposition (oxidation) of formic acid by thermally activated and photo-activated catalytic reactions. The thermally activated catalytic decomposition gradually increases with increasing the number density of deposited Au NPs and saturated, suggesting that the decomposition occurs on Au NPs surface and/or near the interface of TiO2 and Au NPs. On the other hand, the photocatalytic decomposition is significantly improved with rather small number density deposition of Au NPs and disappeared with high number density deposition. ESR measurements of Au/TiO2 in the surrounding similar to the photocatalytic decomposition suggests that electrons excited by plasmon resonance absorption in the Au NPs transfer to TiO2 to promote the decomposition. However, high number density deposition enhances electron capture by neighboring Au NPs and reduces the photocatalytic activity. Thus there should be the optimum number density of Au NPs on TiO2 for photocatalytic decomposition of formic acid.
1. Introduction TiO2 has been applied to various fields such as solar cells, purifying water and air, and optical degradation of organic compounds due to its unique optical, electrical and chemical properties [1–4]. However, as a wide band-gap oxide semiconductor, TiO2 shows photocatalytic activity only under UV-light irradiation. Therefore, the development of TiO2 photocatalysts working under visible light irradiation is important in order to utilize solar energy efficiently, and many efforts have been devoted to the preparation of visible light response TiO2 photocatalysts especially for the degradation of organic compounds. Recently, localized surface plasmon resonance (LSPR) on Au nanoparticles has been applied to visible light response photocatalysis [5–11], and some research groups reported chemical reactions such as oxidation of aromatic alcohols [12], formic acid [13] and 2-propanaol [14,15] and hydrogen production from alcohols [16,17] proceeded over Au/TiO2 under visible light irradiation. As for alternative mechanisms for the plasmonic photocatalyses are proposed: one is that the LSPR in metal nanoparticles (NPs) can enhance the local electronic field of the ⁎
neighboring TiO2 under visible light irradiation [5–8,18]. Another is that electrons excited by the LSPR are partly transferred to the conduction band of TiO2 to induce photocatalytic reactions [19–27]. Thus visible light response of TiO2 seems to depend on both size and number density of Au nanoparticles (NPs) as pointed out by some researches [13,17]. Kominami et al. and Kowaiska et al. reported that action spectra in the degradation of formic acid in aqueous suspensions of Au supported on TiO2 or CeO2 photocatalysts were in good agreement with their photo-absorption spectra [23,24]. These results suggest that degradation of formic acid was induced by photo-absorption due to LSPR of the supported Au nanoparticles. In addition, degradation (oxidation) of formic acid in air can undergo the direct decomposition without the formation of any other stable intermediates. Thus the decomposition of formic acid is suitable for understanding the loading effects of Au nanoparticles on the visible light response of TiO2 photocatalysts. In this study, we have tested two different methods to deposit (load) Au NPs on TiO2 for easier control of sizes and numbers density of deposited Au NPs. Using thus prepared Au NPs loaded TiO2 as catalysts, decomposition of formic acid under visible light irradiation was conducted to
Corresponding author. E-mail address: [email protected] (T. Yoshida).
https://doi.org/10.1016/j.cattod.2019.12.035 Received 24 June 2019; Received in revised form 18 November 2019; Accepted 26 December 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Tomoko Yoshida, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.12.035
Catalysis Today xxx (xxxx) xxx–xxx
T. Yoshida, et al.
examine the effect of number density of deposited Au NPs.
apparatus. The spectrum of BaSO4 powder was used as the reference.
2. Experimental
2.5. XAFS measurement
2.1. Catalysts preparation
The XAFS measurements at the Au L3-edge were carried out at BL5S1 of Aichi Synchrotron Radiation Center with a Si(111) doublecrystal monochromator in both transmission and fluorescence modes at room temperature (201504073).
Two different methods were employed for deposition (loading) of Au NPs on TiO2 and thus prepared samples are refereed as Au/TiO2 hereafter. They are a conventional photodeposition method and a colloid photodeposition method operated in the presence of a hole scavenger (CPH).
2.6. ESR measurement ESR measurements of TiO2 and Au/TiO2 with and without visible light irradiation were carried out at 20 K. The quart tube including the sample after evacuation was set to an X-band ESR spectrometer (JEOL JES-RE1X) with liquid helium cryostat temperature control system (JANIS ES-CT470) and cooled to 20 K. Visible light was irradiated to the sample in the ESR cavity with the cryostat by gathering the light by quartz lens to the cavity. Typical ESR parameters for the measurements were microwave power of 1 μW, microwave frequency of 9.099 GHz and magnetic field of 324 −339 mT.
(1) Preparation by photodeposition method HAuCl4·4H2O powder was dissolved in a methanol aqueous solution of 60 mL (10 mL of methanol was dissolved in distilled water of 50 mL). 1.0 g of rutile TiO2 power (Toho chemical Industry Co., BET surface area was 6 m2/g) was suspended in the aqueous solution. The suspension was irradiated for 3 h with a 300 W Xe lamp equipped with a band path filter (ca. 340 ± 30 nm, 20 mW/cm2), and then filtered and washed with purified water. (2) Preparation by colloid photodeposition method Au colloid NPs were prepared by pH shift method described elsewhere [28]; 41.8 mL of distilled water and 1.25 mL of HAuCl4 solution (20 mM) were introduced in a beaker and heated at 373 K, resulting a precursor solution. 1.75 mL of citric acid (100 mM) was added to the precursor solution, then the solution was stirred and refluxed. After waiting 7 s, 5.2 mL of NaOH (100 mM) solution was introduced into the solution to shift the pH from acidic to neutral. Consequently, colloidal solution of Au nanoparticles (NPs) was obtained. The prepared Au NPs were loaded on TiO2 powders by colloid photodeposition with methanol as a hole scavenger (CPH) method reported by Tanaka et al. [13]; TiO2 powders (0.1−1 g) are suspended in 50 mL of Au colloidal solution (20 mM) and stirred under UV light irradiation while 5 mL of methanol as a hole scavenger is added to the suspension every hour up to 7 h, and then filtered and washed with purified water.
3. Results and discussion (1) Au/TiO2 prepared by photodeposition method Fig. 1 shows Au L3-edge XANES spectra of Au/TiO2 samples prepared by the photodeposition method together with Au foil. The XANES spectra of Au/TiO2 samples are similar to that of Au foil, suggesting that the chemical state of Au cocatalysts is metalic (Au(0)). In the measurement of UV–vis diffuse reflectance spectra of Au/TiO2 samples (Fig. 2), the broad Localized Surface Plasmon Resonance (LSPR) absorption bands were observed and the band became broader with increasing the loading amount of Au, suggesting that Au NPs with various sizes were deposited on TiO2 by the photodeposition method and the average size of Au NPs increased with loading amount of Au. Actually, as shown in TEM image of Au/TiO2 sample (Fig. 3), the metallic Au cocatalysts were deposited on TiO2 as Au NPs with various sizes of several to several tens nm. Unfortunately, the Au loading by a photodeposition method often complicates understanding the effects of Au
2.2. Reaction test Au/TiO2 (50 mg) was suspended in distilled water (3 mL) in a test tube and formic acid (26 μmol) was injected into the suspension and then irradiated with visible light from a 300 W Xe lamp with a cut of filter (λ > 480 nm) in air. The formic acid would be oxidized as HCOOH + 1/2O2 → CO2 + H2O. The amount of CO2 produced in the gas phase was measured using a gas chromatograph (TCD-GC). The reaction was also conducted without photoirradiation at 323 K (the same temperature under visible light irradiation). 2.3. TEM observation For TEM observation, Au colloidal NPs solution was mounted directly on a carbon covered copper mesh, while Au/TiO2 were dispersed in an ethanol solution and a drop of the dispersed solution was mounted on a holey carbon covered copper mesh. TEM images were recorded with a JEM-2100 (JEOL) electron microscope operated at 200 kV at the High Voltage Electron Microscope Laboratory in Nagoya University. 2.4. UV–vis optical absorption measurement UV-vis optical absorption spectra of Au colloidal solution were recorded at room temperature using a JASCO V-670 in a transmission mode. As for Au loaded TiO2 samples prepared by the photodeposition method, diffuse reflectance spectra were measured using the same
Fig. 1. Au L3-edge XANES spectra of Au/TiO2 samples prepared by photodeposition method and a Au foil. 2
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Fig. 2. Diffuse reflectance spectra of Au/TiO2 samples prepared by photodeposition method.
Fig. 4. TEM image and the size distribution histogram of the colloidal Au NPs.
Fig. 3. TEM image of 2 wt% Au/TiO2 sample prepared by photodeposition method.
loading amount on the photocatalytic activity of Au/TiO2 sample, since both the size and the number density of Au NPs simultaneously change with Au loading amounts. (2) Au/TiO2 colloidal photodeposition (CPD) method To understand the correlation between photocatalytic activity of Au/TiO2 and the loading amount of Au NPs, we tried to prepare TiO2 photocatalysts loaded with the same sized Au NPs. The size and morphology of synthesized colloidal Au NPs were determined using TEM. Fig. 4 shows a TEM image and the size distribution histogram of the colloidal Au NPs, which clearly indicates that Au NPs with the average size of 7.3 nm are uniformly distributed. The loading of Au NPs on TiO2 was conducted by the CPH method where TiO2 powders were suspended in the Au colloidal solution and stirred under UV light irradiation while 5 mL of methanol as a hole scavenger is added to the suspension every hour. Fig. 5 shows changes of the UV–vis absorption spectra of the solution by every hour. Before UV light irradiation, the solution gave LSPR absorptions attributed to the Au NPs of around 520 nm [17,29]. After 7 h of the light irradiation, the LSPR absorption disappeared, suggesting that all colloidal Au NPs in the solution were deposited on TiO2 samples. Fig. 6 shows TEM image and the size distribution histogram of Au NPs loaded on TiO2 samples. The average size of Au NPs in all Au/TiO2 samples was estimated as ca. 8.2 nm. It was thus clarified that the Au NPs could be loaded on TiO2 by
Fig. 5. Variation of UV–vis absorption spectra of Au colloid solution with time.
the CPH method without remarkable aggregation of the Au NPs. In addition, the size distribution histograms of the Au NPs were almost the same regardless of the loading amount of Au NPs (as shown in S1). Table 1 summarizes average particle sizes and number densities of thus prepared Au/TiO2 samples used as catalysts for the decomposition of formic acid. 3
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Fig. 8. Variation of photo- and thermo- catalytic reaction rates with the number of Au nanoparticles.
Fig. 7 shows time courses of the amount of CO2 evolved by the decomposition (oxidation) of from formic acid in aqueous suspensions of 0.2 wt % Au/TiO2 and TiO2. CO2 was evolved over Au/TiO2 with more than 0.2 wt % Au and the amount of evolved CO2 increased almost linearly with the irradiation time. It was confirmed that no CO2 was evolved in direct photo-decomposition of formic acid and photocatalytic reaction using bare TiO2 in Au colloid solution under visible light irradiation. It should be noted that without photoirradation, the CO2 evolution appeared for 0.2 wt % Au/TiO2. Haruta et al. have reported that Au/TiO2 prepared by an impregnation method promotes the oxidation of carbon monoxide by activating oxygen at the Au-TiO2 interface [30] Therefore, similar reaction seems to proceed as thermally activated catalytic reaction on 0.2 wt% Au/TiO2. However, the CO2 evolution rate without the light irradiation was smaller than that with the light irradiation. Thus, the oxidation of formic acid was promoted by either or both of thermally- and photo- activated catalytic reactions on Au/TiO2. Since the CPH method allow us to control the sizes and number density of deposited NPs, we could investigate the effects of the number density of Au NPs on TiO2. Results are given in Fig. 8 where the evolution rate of CO2 by the thermal-activated catalytic and photo-activated-catalytic reactions are plotted against the number density of the loaded Au NPs. The thermally activated-catalytic reaction was improved with increasing the number density of Au NPs and saturated over 4.6 × 1015/g (2.4 wt%), suggesting that the decomposition occurs on Au NPs surface and/or near the interface of TiO2 and Au NPs. It is considered that the reaction site increased with the increase of the number of Au NPs for lower number density deposition, while the reaction sites would overlap at a high number density to give saturation. Different from the thermally activated catalytic reaction, the photoactivated catalytic reaction significantly increased only for small amount of Au loading 2.9 × 1014/g (0.2 wt %) and disappeared over 8.4 × 1014/g (0.5 wt %). The cause of such appreciable reduction could be attributed to two causes. One is the enhanced recombination of photo-generated electrons and holes and the other is the decrease of geometrical coverage of active surfaces by loaded Au NPs. As for the plasmonic photocatalytic mechanism of Au/TiO2 sample, it has been reported that some electrons excited by the localized surface plasmon resonance (LSPR) of Au NPs under visible light irradiation are partly transferred to the conduction band of TiO2 and the separated electrons and holes would result in reductive and oxidative reactions respectively. It is also proposed that Ti4+ ions trap the excited electrons to be Ti3+ ions which is photocatalytic active species. To confirm such electron transfer from Au NPs to TiO2, ESR spectra of Au/TiO2 samples taken under visible light irradiation were compared with that of bare TiO2. Fig. 9 compares ESR spectra of bare TiO2 and 2 wt% Au/TiO2. Small Ti3+ ESR signals [31] were observed in both bare TiO2 and 2 wt% Au/TiO2 samples. Under visible light irradiation, however the signal intensities increased remarkably in the TiO2 while decreased slightly in Au/TiO2. Ti3+ ions might be formed by electron excitation of the impurity levels such as defects at TiO2 surface, while most of the electrons
Fig. 6. TEM image of 4.7 wt% Au/TiO2 and the size distribution histogram of Au NPs loaded on TiO2 sample. Table 1 Particle size and number of Au nanoparticles in Au/TiO2 samples with various loading amount of Au. Loading amount of Au (wt%Au)
Particle size (nm)
Standard deviation
Number of AuNPs (×1015/g)
0.1 0.2 0.3 0.5 1.6 2.4 4.7
8.3 8.8 8.3 8.4 8.5 8 8.2
2.1 2.7 2.8 2.1 2.9 2.9 2.2
0.17 0.29 0.52 0.84 2.58 4.64 8.44
Fig. 7. Time courses of evolution of CO2 from formic acid in aqueous suspensions of 0.2 wt % Au/TiO2 and TiO2 samples.
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Author contributions In this study, Tomoko Yoshidacarried out XAFS analysis and UV–vis measurement. Yuhei Misuprepared the samples and performed catalytic reactions. Muneaki Yamamoto carried outTEM measurement. Jun Kumagai measured ESR spectra. Tomoko Yoshidawrote manuscript with support from Tetsuo Tanabe,Satoshi Ogawaand Shinya Yagi. Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Fig. 9. ESR spectra of TiO2 and Au/TiO2 before (control) and under visible light irradiation.
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.12.035.
are likely captured by Au NPs on Au/TiO2. The similar ESR results were obtained for Au/TiO2 with different Au loading amount. Generally electrons generated by LSPR photoexcitation in Au NPs would diffuse to the active sites of TiO2 and cause a reduction reaction. However, if the number density of Au NPs was large enough, the excited electrons are easily transferred to neighboring Au NPs and recombined with holes there before contributing to the photocatalytic reaction. Additionally, the increase of geometrical coverage by Au NPs over TiO2 could also reduce the number of active sites. As shown the TEM image in Fig. 6, the average distance between Au NPs for 4.7 wt% Au/TiO2 is ca. 30 nm, and therefore those for 0.2 wt% Au/TiO2 (photocatalytic active) and 0.5 wt% Au/TiO2 (photocatalytic inactive) would be estimated as ca. 150 nm and 90 nm, respectively. These numbers are reasonable, suppose the mean free path of excited electrons would be around 100 nm. Since the size of Au Np was around 8 nm, nearly 10 % of the TiO2 should be covered by Au NPs for 0.5 wt% Au/TiO2. In this respect, it is quite important to determine the mean free path of the excited electrons, which is remained for future study.
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4. Conclusion With using a colloid photodepostion method, we have succeeded to prepare Au NPs deposited on TiO2 (Au/TiO2). The size of the Au NPs were controlled to be ca. 8 nm varying their deposited density up to full coverage of TiO2 surfaces. Using the Au/TiO2 as catalysts, we have conducted decomposition of formic acid under visible light irradiation focusing to understand the effects of the number density of deposited Au NPs. The CO2 evolution resulting from the decomposition of formic acid were observed with and without visible light irradiation, indicating the decomposition promoted both thermally- and photo-activated catalytic reactions over Au/TiO2. The activity of the thermallyactivated catalytic reaction increased with increasing the number density of Au NPs, but saturated at higher number density deposition indicating strong contribution of Au NPs on the thermal-activated catalytic oxidation of formic acid. The photo-activated catalytic reaction was appreciable for low number density deposition, and for higher number deposition it was significantly suppressed, most probably easier recombination of electrons and holes on Au NPs. These results indicate the existence of optimum in the number density of Au NPs on TiO2 for the photocatalytic decomposition of formic acid.
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