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TiO2 Nanoflakes Modified with Gold Nanoparticles as Photocatalysts with High Activity and Durability under near UV Irradiation Yong Liu,† Lifang Chen,‡ Juncheng Hu,*,† Jinlin Li,† and Ryan Richards*,§ Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central UniVersity for Nationalities, Wuhan, 430074, P.R. China, State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, P.R. China, and Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401 ReceiVed: NoVember 3, 2009; ReVised Manuscript ReceiVed: December 17, 2009
Titania nanoflakes modified with gold nanoparticles (NPs) were successfully prepared by depositing of Au NPs onto TiO2 nanoflakes using a direct “green” process in the absence of organic capping agents. The photocatalyst has been characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and UV-vis diffuse reflectance spectroscopy. Results of these analyses revealed that the Au NPs (∼1 nm) are homogeneously dispersed on the surface of highly crystalline TiO2 (anatase) nanoflakes with a narrow size distribution. Further, XPS studies indicate a strong electronic interaction between Au (in metallic state) and TiO2. Inspiringly, the photocatalyst showed high-efficiency and ultrastability for the degradation of the nonbiodegradable azo dye brilliant red X-3B under near UV irradiation with activity 4.3 times higher than that exhibited by the commercial Degussa P25. Durability tests established that the catalyst remains intact even after 15 consecutive experiments. The ultrafine metal particle size and strong metal-support interaction were considered as the key factors for the overall photocatalytic activity of the metal-semiconductor composite system. Introduction Over the past few decades, there has been a growing interest in the development of high-efficiency photocatalytic materials based on anatase TiO2 due to its low cost, low toxicity, and chemical stability.1–5 To date, a number of TiO2 photocatalysts with various morphologies such as nanorods,6,7 nanowires,8,9 and core-shell microspheres10–12 have been intensively designed for complete destruction of organic compounds in polluted air and wastewater. In particular, flake-like metal oxide nanomaterials showed excellent catalytic performance because of their unique surface chemistry.13–16 However, low quantum efficiency and a high band gap (3.0-3.2 eV) limited their practical application.17 To date, many efforts (e.g., doping TiO2 with noble transition metals) have been frequently employed to improve the photocatalytic activity of TiO2, and especially, gold-doped TiO2 (Au/TiO2) has attracted increasing research attention during the past decades.18–21 However, utilizing titania materials as highly active photocatalysts still remains a challenge. Doping TiO2 with noble metals (e.g., Au and Ag) has been demonstrated to greatly enhance the photoactivity of TiO2 by reducing the fast recombination of the photogenerated charge carriers.22 Meanwhile, one factor that can potentially influence the intrinsic properties of a metal nanostructure is the size of the metal particle. When the size of metal particles is below 2 nm, the composites display exceptional catalytic behavior.23 Moreover, the groups of Haruta and others24–27 demonstrated that gold nanoparticles (NPs) in the 2-5 nm range also showed unusually high catalytic activities. Therefore, it is significant to prepare Au/TiO2 nanocomposites with homogenerous Au NPs * To whom correspondence should be addressed. E-mail: junchenghuhu@ hotmail.com;
[email protected]. † South-Central University for Nationalities. ‡ East China University of Science and Technology. § Colorado School of Mines.
dispersion, ultrasmall Au particle size, and narrow particle size distribution. Another important factor that affects the electronic properties of the nanocomposite is the interaction between the Au (NPs) and TiO2 matrix. Au NPs are however mobile on the surface of most supporting oxides, and at present, one of the key problematic issues hampering the application of Au catalysts is the stability of the particles against sintering or leaching under reaction conditions with high temperatures, pressures or harsh chemical environments.28 To overcome the aggregation, organic capping ligand with desired function moieties have been widely employed to modify the surface of the host materials in recent works.29,30 However, these capping ligands have a common serious drawback that they may poison the catalytically active sites of both metals and supports.32 Therefore, it is highly desirable to design a capping ligand-free strategy for synthesizing ultrastable Au supported nanocatalysts. Herein, we reported a novel and simple synthetic strategy to prepare highly efficient and ultrastable Au/TiO2 via consecutive ion adsorption, photoreduction, and finally calcination treatments. (see Scheme 1 for experimental details). For the catalyst, the size of Au NPs can be well-controlled. The Au NPs (around 1.0 nm) were homogeneously dispersed on the TiO2 with a strong electronic interaction between the guest Au NPs and the host TiO2 as demonstrated by XPS. The photocatalytic activity of the fabricated sample was tested by photodegradation of nonbiodegradable azo dyes X-3B under near UV irradiation, and the factors influencing the photoactivity such as the size, the oxidation state of the Au clusters and particularly the strong metal-support interaction were emphatically investigated in our work. Experimental Section Materials and Reagents. High surface area TiO2 (HSA, BET area, ca. 500 m2/g, From Nanoscale Inc.) was used as pure photocatalyst and as further starting materials for the synthesis
10.1021/jp910500c 2010 American Chemical Society Published on Web 01/05/2010
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SCHEME 1: Graphical Representation of the Preparation Processes for Au/TiO2 Catalysts
of Au modified photocatalyst. Ethanol (C2H5OH, 99.7%, Shanghai Chem. Co.) was used as the solvent for HAuCl4 · 3H2O (Sigma-Aldrich). Azo dye reactive brilliant red X-3B was supplied by the Shanghai Chemical Co. and was used without any further purification. Deionized and doubly distilled water was used throughout this work. Preparation of Au/TiO2 (2 wt %). The catalyst was prepared by consecutive ion adsorption, photoreduction and calcination treatments. In a typical synthesis, 1 g of TiO2 (HSA) was suspended in 50 mL of HAuCl4 · 3H2O (2.0 × 10-3 M) and stirred vigorously for 3 h in the dark to reach a complete absorption for Au ions onto the surface of the TiO2 nanoflakes. Then, the suspending solution was irradiated with a 300 W Mercury lamp (wavelength 365 nm) with magnetic stirring for 4 h. The obtained precipitate was filtered and washed thoroughly three times with absolute ethanol. Finally, the purple Au/TiO2 catalyst was obtained after being dried at 80 °C and calcined in air at 500 °C for 3 h. Characterization of the Photocatalyst. The crystalline structure of the catalysts was characterized by power X-ray diffraction (XRD) employing a scanning rate of 0.05°/s in a 2θ range from 10° to 80°, in a Bruker D8 Advance using monochromatized Cu KR radiation. The morphology and particle size of the catalysts were analyzed by the transmission electron microscope (TEM), which were taken on a Tecnai G20 (FEI Co., Holland) TEM using an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was recorded on an VG Multilab 2000 (VG Inc.) photoelectron spectrometer by using monochromatic Al KR radiation under vacuum at 2 × 10-6 Pa. All of the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. The UV-vis DRS were collected using a Shimadzu UV-2450 spectrophotometer from 200 to 800 nm using BaSO4 as background. Photodegradation Experiment. The photoactivity of the asprepared sample was tested by the degradation of reactive brilliant X-3B under near UV irradiation. The installation of the photoreactor is shown in Figure S1. There was a cooling system to make the photocatalytic reaction at ambient temperatures. In a typical experiment, 50 mg of photocatalyst and 50
mL of aqueous solution of X-3B (1.14 × 10-4 mol/L) were added to a flask, and then the mixed solution was oscillated in darkness overnight. After reaching adsorption equilibrium, the photocatalytic reaction was initiated by irradiating the system with a 300 W Mercury lamp (main wavelength 365 nm). At given time intervals, 4 mL aliquots were collected, centrifuged, and then filtered to remove the catalyst particles for analysis. The filtrates were finally analyzed by a UV-vis spectrophotometer (UV-2450). In order to determine the catalyst durability, the photocatalyst was separated from aqueous solution after each run of reactions. The filtered catalyst was reused without any treatment in the subsequent recycling experiment. Results and Discussion As shown in Figure 1, the TEM image clearly showed that both TiO2 (HSA) and Au/TiO2 (AT) composite exhibited a similar flake-like morphology, Au (NPs) were highly dispersed on the surface of TiO2 (HSA). More than 90 gold NPs were counted in the TEM images to calculate their average size (∼0.9 nm) as shown by the gold distribution histogram in Figure 2. Such small NPs plays an important role for the photocatalytic performance. Meanwhile, the small NPs can serve as an electron conductor, which facilitates photoelectron transfer and further reduce the probability of charge recombination. However, the
Figure 1. TEM images of (a) TiO2 (HSA) nanoflakes and (b) Au/ TiO2 nanoflakes (calcined at 500 °C for 3 h).
TiO2 Nanoflakes
J. Phys. Chem. C, Vol. 114, No. 3, 2010 1643 Figure 5 shows the high-resolution XPS spectra of O1s region of TiO2 (HSA) and Au/TiO2 composite. The O1s region could be fitted into two peaks, the main peak at 529.8 eV is attributed to O1s of TiO2 since it is seen at the same position as that of O1s peak of many rutile and anatase TiO2 surfaces,34,35 while the minor peak was due to the hydroxyl group. As seen from Table S1 and Figure 5, the hydroxyl group content was obviously three times higher than that of TiO2 (HSA). The hydroxyl on the surface could be attributed to the Ti-OH on the surface of TiO2 nanoflakes.36 It is because so many hydroxyl groups exist on the TiO2 surface that they would be benificial in trapping photogenerated holes directly:
Figure 2. Particle size distribution histograms of Au NPs on TiO2 nanoflakes.
TiO2-OH + h+ f OH•
large NPs may act the centers of electron-hole recombination and reduce the quantum efficiency.33 Herein, as shown in Scheme 1, we believe the deposition of Au NPs on the surface of TiO2 may occur by the following steps:30
Therefore, recombination of the photogenerated electron-hole pairs is suppressed and the quantum efficiency of photocatalytic activity is improved. The X-ray diffraction (XRD) pattern (Figure 6) revealed that both TiO2 (HSA) and Au/TiO2 are present mainly in highly crystallized anatase, corresponding to (101), (004), (200), (105), and (211) diffractions around 2θ ) 25.3°, 37.8°, 48.1°, 53.9°, and 55.1° (JCPDS No. 86-1156). As compared to the raw TiO2 (HSA), all of the diffraction peaks of anatase become sharper and stronger, indicating that the formation of the strong electronic interaction between Au nanoclusters and host TiO2 greatly affects the crystallization process of TiO2 nanoflakes. It is well reported an increase in the crystallinity of anatase can usually lead to an enhancement of organic pollutants photodegration since the crystallinity would mean few defects acting as the recombination centers for photogenerated electrons and holes.37–39 It should be noted that the small peaks around 2θ ) 44.4° and 64.6° were clearly observed, which could be asssigned to the Au (200) and (220) diffractions (JCPDS.No.4-0784). The Au NPs crystallite size (1.5 nm) was calculated from the full width at half-maximum (fwhm) of Au (200) diffraction peak at the value of 2θ ) 44.4° by the Scherrer equation, in good accordance with the average particle size observed from the TEM image. The UV-vis DRS of TiO2 (HSA) and Au/TiO2 are compared in Figure 7. In contrast to TiO2 (HSA), Au/TiO2 has a significantly enhanced light adsorption in both UV and visible region, and the absorption edge shifts toward to longer wavelengths for the Au/TiO2 indicated a decrease in the band gap of TiO2 nanoflakes with doping of Au NPs. Herein, the corresponding band gap of TiO2 (HSA) and Au/TiO2 were found to be 3.2 ev and 2.8 ev, respectively. The obove results suggested a strong interaction between the Au metal and TiO2 semiconductor. As the previous characterization results of Au4f5/2 XPS spectra, the Au NPs, which is an excellent electronic conductor, could facilitate the rapid transfer of photoelectrons from the TiO2 and thus accelerate their separation from photoinduced holes, leading to a decreased recombination rate of charge carriers. The enhanced light absorption can therefore provide more photocharges needed for the photocatalytic reactions. Based on the above characterization analysis, we believe the ultrafine metal particle size and strong metal-support interaction play the main role in enhancing the photoactivity of the catalyst. First, the small Au NPs can serve as an electron conductor and further enhance the light adsorption which can therefore provide more photocharges needed for the photocatalytic reactions. Second, the strong interaction between the particles and support hinders the aggregation of neighboring particles, and the ultrafine Au NPs are also not easy to fall off even under relatively harsh reaction conditions. In addition, the hydroxyl group content is also an important factor for the photoactivity of the catalyst.
hV
TiO2 98 e- + h+
(1)
AuCl4- + 3e- f Au0 + 4Cl-
(2)
nAu0 f Aun(nanoclusters)
(3)
2CH3CH2OH + 5.5O2 + 2h+ f 4CO2 + 5H2O + 2H+
(4) The solvent ethanol here was used as hole scavengers to prevent recombination of the electron-hole pairs. Based on the present consecutive photoreduction and calcination treatment, the formed Au NPs has a strong interaction with the host TiO2 nanoflakes, which could be further studied using X-ray photoelectron spectroscopy (XPS). Figure 3 shows the high resolution XPS spectra of Ti2p of TiO2 (HSA) and Au/TiO2 (AT). The spectra reveal biomodal peaks which correspond to Ti2p 1/2 and Ti2p 3/2, respectively. However, the Ti2p 3/2 binding energy of AT is shifted from 457.9 to 458.5 eV as compared to TiO2 (HSA). This approximate 0.6 eV shift can be attributed to the conduction band electron of TiO2 transfer to the Au NPs, resulting in a decrease in the outer electron cloud density of Ti ions and it indicates the strong interaction between Au NPs and TiO2 (HSA). In the meantime, the photoinduced electron-hole is well separated and thus the quantum efficiency of photocatalysis is enhanced. Electron transfer between photoexcited semiconductor and metal is an important phenomenon in the photocatalysis. The transfer of electrons to Au NPs has now been probed by exciting TiO2 NPs under steady-state and laser pulse excitation.20 Moreover, the binding energies of Au4f7/2 at 83.4 eV and Au4f5/2 at 87.0 eV (Figure 4) indicate the Au species are present in the metallic state on the surface of TiO2 (HSA). The atomic relative abundance (at. %, Au:TiO2) on the surface of Au/TiO2 was calculated to be about 0.5% according to the XPS analysis. Meanwhile, a slightly negative shift (0.6 eV) in comparison with bulk gold (normally at 84.0 eV) also suggests the strong metal-support interaction may have influenced the electronic properties of surface gold, possibly accounting for the negative shift in binding energy of the gold nanoclusters, consistent with the result of XPS spectra of Ti2p.
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Figure 3. High-resolution XPS spectra of Ti2p in (a) TiO2 (HSA), (b) Au/TiO2 nanoflakes (calcined at 500 °C for 3 h).
Figure 4. High-resolution XPS spectra of Au4f of Au/TiO2 nanoflakes calcined at 500 °C for 3 h).
Figure 6. Powder X-ray diffraction patterns of the two samples (a) TiO2 (HSA) nanoflakes and (b) Au/TiO2 nanoflakes (calcined at 500 °C for 3 h).
Figure 5. High-resolution XPS spectra of O1S of (a) TiO2 (HSA) nanoflakes and (b) Au/TiO2 nanoflakes (calcined at 500 °C for 3 h). Figure 7. UV-vis DRS of (a) TiO2 (HSA) nanoflakes and (b) Au/ TiO2 nanoflakes (calcined at 500 °C for 3 h).
Photocatalytic Degradation of Azo Dye X-3B. To evaluate the photocatalytic capability of the fabricated sample, we examined the decomposition of brilliant red X-3B azo dye under near UV irradiation (365 nm). For comparison, we also carried out decomposition of the X-3B in solution over the TiO2 (HSA), Degussa P25 and Au/Degussa P25. As shown in Figure 8, when the reactions were conducted without catalyst, no reactions were observed after 20 min (Figure 8e), indicating this is a real photocatalytic reaction. The pure TiO2 exhibited very low activity due to its large energy gap (3.2 eV), however, the decomposition over the Au/TiO2 catalyst is completed in 20 min under UV irradiation. The linear relationships of ln(ct/c0) versus time revealed
that the photodegradation reaction followed a pseudofirst-order reactions. The apparent first-order reaction rate constants for the cases of TiO2 (HSA), P25, Au/Degussa P25 and Au/TiO2 (AT) were 0.0161, 0.0607, 0.0937, and 0.258 min-1 (Table S2), respectively, indicating the photodegradation rate of Au/TiO2 (AT) is 2.75, 4.25, and 16.0 times higher than that by P25 and TiO2 (HSA), respectively. Herein, it is worth mentioning that the degradation efficiency of Au/TiO2 (AT) is higher than that of Au/ Degussa P25, indicating that the flake-like TiO2 is more conducive to the formation of ultrasmall gold NPs, and further to enhance the photocatalytic activity of the host TiO2.
TiO2 Nanoflakes
J. Phys. Chem. C, Vol. 114, No. 3, 2010 1645 Au/TiO2 composite nanoflakes, the installation diagram of photodegration reaction, the kinetic constants, regression coefficients and the kinetics equation for the photodegradation of X-3B over the three samples, the X-ray diffraction patterns of Au/TiO2 catalyst before photodegradation and after eight photocatalytic runs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 8. Ct/C0 vs time curves of X-3B photodegration over the five samples: (a) Au/TiO2 nanoflakes (calcined at 500 °C for 3 h), (b) Au/ Degussa P25, (c) Degussa P25 TiO2, (d) TiO2 (HSA) nanoflakes, and (e) without photocatalyst.
Figure 9. Ct/C0 vs time curves of X-3B in solution for reusing of Au/TiO2 nanoflakes (calcined at 500 °C for 3 h).
It is known that the photocorrosion or photodissolution of catalyst might occur on the photocatalyst surface in the photocatalytic reaction. To test the stability of X-3B photodegration on Au/TiO2, we reused the catalyst for 15 times. As shown in Figure 9, the Au/TiO2 catalyst could be reused more than 15 times without significant deactivation. Figure S2 shows the XRD patterns of Au/TiO2 catalyst before the photocatalytic reaction and after the catalytic runs. After eight catalytic runs, the position and the ratio of peaks were nearly the same to that of the fresh catalyst, clearly indicating that sintering and shedding of gold NPs had not taken place. The high efficiency and ultrastability could be mainly attributed to the strong interaction between Au NPs and TiO2 (HSA). In summary, this work has successfully developed a novel and simple synthetic strategy for preparing highly efficient and ultrastable Au/TiO2 catalyst. Herein, we believe that the ultrafine metal particle size and strong electronic interaction between Au NPs and host TiO2 (HSA) are the key factors for the high photoactivity. Other noble metal (e.g., Ag, Pt, and Pd) may also be deposited on the surface of the TiO2 (HSA), and such work is underway in our laboratory. Our work also provided a new pathway for designing highly efficient and ultrastable supported noble metal catalysts for practical applications. Acknowledgment. This work is supported by National Natural Science Foundation of China (20803096), South-Central University for Nationalities (YZZ08002, KYCX090004E), and Colorado School of Mines. Supporting Information Available: Results of curve-fitting of high-resolution XPS spectra for the O1s region of TiO2 and
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JP910500C
