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Size Effect of Au Nanoparticles on TiO2 Crystalline Phase of Nanocomposite Thin Films and Their Photocatalytic Properties Chihiro Yogi,† Kazuo Kojima,*,† Takeshi Hashishin,† Noriyuki Wada,‡ Yasuhiro Inada,† Enrico Della Gaspera,§ Marco Bersani,§ Alessandro Martucci,§ Lijia Liu,^ and Tsun-Kong Sham^ †
Department of Applied Chemistry, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan Department of Materials Science and Engineering, Suzuka National College of Technology, Shiroko, Suzuka, Mie 510-0294, Japan § Dipartimento di Ingegneria Meccanica S. Materiali, Universita di Padova, Via Marzolo, 9, 35131 Padova, Italy ^ Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B7, Canada ‡
ABSTRACT: This work focuses on the size effects of Au nanoparticles (AuNPs) on the TiO2 crystalline phase of nanocomposite AuNPs-embedded TiO2 (Au-TiO2) thin films, their adsorption ability, and photocatalytic activity. Au-TiO2 films were synthesized through a solgel method using polyvinylpyrrolidone-protected AuNPs (AuNPs@PVP). The mean diameters of AuNPs@PVP dispersed in a sol solution were 2.0 ( 0.7 or 7.9 ( 3.1 nm and the heat-treatment temperature of the films was 400900 C. XRD and Ti L3,2-edge X-ray absorption nearedge structure (XANES) analysis revealed that AuNPs doping could suppress an anatase to rutile phase transformation. In addition, the larger size of AuNPs doped in TiO2 film tended to prevent the transformation more effectively. The film doped with the smaller AuNPs@PVP and annealed at 500 C showed the highest photocatalytic activity among the obtained films because it had the wellcrystallized anatase phase and the high adsorption ability, which was attributed to the existence of a five-coordinated Ti site that was revealed from Ti K-edge XANES measurements.
1. INTRODUCTION Improvement of the photocatalytic activity of TiO2 is one of the most important aspects of heterogeneous photocatalysis. Au nanoparticles (AuNPs)-doped TiO2 is well-known to possess a high photocatalytic activity due to its inhibition of a recombination of photogenerated electronhole pairs by AuNPs.110 Furthermore, Bannat et al. suggested that the embedding of AuNPs into TiO2 film also resulted in the formation of an interface between Au and TiO2 nanocrystals, creating a Schottky barrier that acted as a very effective trap for electrons. This, in turn, led to an enhanced electron transfer to the AuNPs, thus resulting in a further increase in photocatalytic activity.11 Although AuNPs have played a very important role in the enhancement of photocatalytic activity, a morphological change of crystalline TiO2 has also had a strong influence. Recently, Zhao et al. reported that Au113þ nanoclusters promoted the formation of the TiO2(B) phase in competition with the anatase phase, resulting in a high activity.12 He et al. investigated the effect of Ag doping on the microstructure and photocatalytic activity of TiO2 films prepared by a solgel method.13 They found that suitable Ag dopant was able to increase the activity by a mechanism attributed mainly to the change in anatase grain size. AuNPs show various changes in electronic, optical, and physical properties depending on their diameter.14 Although incorporation of AuNPs of different sizes into TiO2 film would be expected to affect the morphology of the resulting film, there r 2011 American Chemical Society
have been few reports considering the effect of size of the introduced AuNPs. In the present study, different size AuNPs were synthesized in advance, and then Au-TiO2 nanocomposites thin films were prepared mixing Au colloidal solutions with TiO2 solgel precursors.15 X-ray absorption near-edge structure (XANES) is a powerful technique to identify an electronic state of an absorbing atom and Ti K-,1625 Ti L3,2-,2631 and O K-edge3235 XANES spectra of TiO2 have been investigated. We here report the XANES of the obtained films and discuss both the influence of AuNPs doping in AuNPs-embedding TiO2 films and their photocatalytic activity.
2. EXPERIMENTAL METHODS 2-1. Preparation of AuNPs@PVP Ethanol Solution. All chemicals were purchased from Wako Pure Chemical Industries, Ltd. and used without further purification. AuNPs stabilized by PVP (K-25) were synthesized by chemically reducing AuCl4 with an NaBH4 solution in a PVP aqueous solution. NaAuCl4 3 2H2O (16 mM, 20 mL Milli-Q-water) was added to a 106.4 mg PVP aqueous solution (300 mL) with stirring for 30 min, producing a yellow solution. Au ethanolic solution of NaBH4 Received: November 5, 2010 Revised: February 22, 2011 Published: March 22, 2011 6554
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The Journal of Physical Chemistry C (0.01 mM, 4 mL) was added with vigorous stirring (1000 rpm). Rapid addition of the NaBH4 solution yielded smaller AuNPs (sAuNPs) with an average diameter of 2.0 ( 0.7 nm, but slower injection of the solution (100 μL/2 min) produced larger AuNPs (LAuNPs) of 7.9 ( 3.2 nm in diameter. After addition of the NaBH4 solution, the mixture was stirred for 3 h, followed by washing with acetone, dispersing in ethanol, and finally obtaining a PVP-stabilized AuNPs (AuNPs@PVP) ethanol solution (8.0 M). 2-2. Preparation of TiO2 film and AuNPs-Embedded TiO2 Films. TiO2 film and AuNPs-embedded TiO2 composite (AuTiO2) films were prepared by a solgel method. Chemicals with the mole ratio of Ti(O-i-C3H7)4/H2O (distilled water)/ C2H5OH/NH(C2H4OH)2 = 1:1:40:1 were used to prepare a sol solution. First, NH(C2H4OH)2 and a half amount of C2H5OH were mixed for 30 min, followed by adding Ti(O-iC3H7)4 with stirring for 30 min at room temperature (solution A). Another solution B prepared by mixing the remaining C2H5OH with H2O was dropped into solution A for 30 min with stirring, followed by stirring for a further 60 min. To prepare an Au-TiO2 composite film, the prepared AuNPs@PVP ethanol solution including H2O was used, leading to the sol solution with a final gold concentration of 5.5 mol %. A quartz glass substrate (9 70 1 mm3) was dipped into the resulting sol solution, and then drawn up at the rate of 0.5 mm/s, followed by drying at 100 C for 10 min and heating at 300 C for 10 min. This process was repeated 8 times and the resulting gel film was heated at various temperatures for 3 h. Finally, a total of 18 films of TiO2 (Tx), sAuNPs-embedded TiO2 (sATx), and LAuNPs-embedded TiO2 (LATx) were obtained by heat treatment at temperatures of x = 400, 500, 600, 700, 800, and 900 C. 2-3. Characterization and Photocatalytic Activity Evaluation. The AuNPs@PVP ethanol solution was characterized by UVvis absorption spectroscopy (UVvis-NIR spectrophotometer; Shimadzu, UV-3101PC) and TEM (JEOL, JEM-2100) observations. The structural characterization was carried out using the X-ray diffraction technique (Rigaku, RINT-2200) with Cu KR radiation. UVvis absorption spectra were recorded using a UVvis-NIR spectrophotometer. The surface morphology of the films was observed by FE-SEM (Hitachi, S-4800). XANES spectra of Ti L3,2-edge were obtained at the spherical grating monochromator (SGM) beamline at the Canadian Light Source (CLS), Canada. The spectra were recorded in the total electron yield (TEY) mode using specimen current and in the fluorescence yield (FY) mode using a multichannel plate detector.36 The data were normalized to the incident photon flux. After background correction, the spectral intensities were normalized at 475 eV for Ti and 560 eV for O. Ti K-edge XANES measurements were performed using a Si(111) monochromator at a BL-12C station at the Photon Factory, High Energy Accelerator Research Organization (KEK-PF), Japan. The spectra were collected in the FY mode using a 19-elements solid-state detector (SSD). The spectral intensity was normalized at 5500 eV. A methylene blue (MB) aqueous solution of 1.63 105 M was photocatalyzed in a quartz cell (10 10 65 mm3) at 25 C. The film immersed in the solution was irradiated by a 365 nm UV lamp (Spectronics, ENF-260C/J; 60 μW/cm2). A Shimadzu UV1700 UVvis spectrophotometer was used to measure absorption spectra of the MB aqueous solution as a function of the UV irradiation time. The MB aqueous solution was bubbled with O2 gas for 20 min prior to the irradiation and the bubbling was continued during irradiation. The adsorption ability of the films was also estimated using the same MB aqueous solution; the
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Figure 1. (Left) TEM images of (a) sAuNPs@PVP and (b) LAuNPs@PVP. (Center) Particle size distribution. (Right) Absorption spectra of the AuNPs solution.
films were immersed in the solution in the dark for 1 h, and then the decrease in absorbance was recorded.
3. RESULTS AND DISCUSSION TEM images of sAuNPs@PVP and LAuNPs@PVP are shown in Figure 1. The sAuNPs@PVP solution had a brownish color with a plasmon absorption peak at 511 nm and the mean diameter of the particle was 2.0 ( 0.7 nm. The pale-red solution of LAuNPs@PVP showed a plasmon absorption peak at 523 nm and the mean diameter was 7.9 ( 3.1 nm. XRD patterns of the films are shown in Figure 2. The peaks at 2θ values of around 25.0, 27.8, and 38.3 were attributed to anatase TiO2, rutile TiO2, and cubic Au, respectively. No peaks related to TiO2 reflections appeared in the patterns of T400, sAT400, and LAT400, indicating that these films were apparently composed of amorphous TiO2. However, because the Ti L3,2edge XANES spectra of these films (shown below in Figure 5 where the relative intensity of a20 and a200 are signatures of anatase and rutile phase) showed anatase phase, the films of T400, sAT400, and LAT400 were believed to be formed by nanocrystalline anatase TiO2 undetectable by XRD measurements. Although phase transformation from anatase to rutile was observed in all samples, the transformation behavior of sATx and LATx was different from Tx. In Tx, the rutile peak appeared in T600 and the anatase peak disappeared in T700, whereas sAT600 had no rutile peak. The anatase peak was not observed in either sAT700 or T700. Although the rutile peak in the LATx appeared at x = 700 as well as sATx, the anatase peak still remained at x = 700, in contrast to the tendency of sATx, and disappeared in sAT900. These results revealed that AuNPs doping suppressed phase transformation from anatase to rutile of TiO237,38 and larger AuNPs doping had more effect on this suppression. Figure 3 shows absorption spectra of the films annealed at 500 C. The surface plasmon absorption peaks resulting from the incorporation of AuNPs were observed in sAT500 and LAT500 and its peak positions were 621 and 646 nm respectively indicating that sAT500 had smaller size of AuNPs in the film. Although the plasmon absorption peak of AuNPs colloidal solutions were at 511 (sAuNPs@PVP) and 523 nm (LAuNPs@PVP) as shown in Figure 1, the peaks of Au-TiO2 nanosomposite thin films clearly red6555
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Figure 2. XRD patterns of the films. A: anatase, R: rutile.
Figure 4. (Left) Surface and (Right) cross-sectional morphologies of T500, sAT500, and LAT500.
Figure 3. Absorption spectra of the films annealed at 500 C.
shift because of the higher refractive index of the TiO2. The other absorption peaks in all films were resulted from optical interference. Figure 4 shows SEM images of the surface and cross-section of T500, sAT500, and LAT500. It can be seen that sAT500 and LAT500 had large spaces among the particles with those of sAT500 being larger than those of LAT500, indicating that the smaller AuNPs doping resulted in larger space among the anatase particles in the TiO2 film. In addition, TiO2 particle size was in the same order of the space area and the crystallite diameters of anatase phases in T500, sAT500, and LAT500 calculated from the XRD peaks at 25.0 of 2θ in Figure 2 using the Sherrer equation, the corresponding sizes were 14, 23, and 18 nm respectively showing that AuNPs doping promoted crystallization of anatase phase of sAT500 and LAT500. There have been some similar reports of AuNPs doping,1,4042 and the present results revealed that the smaller AuNPs crystallized further in the anatase phase. Here, the thickness of sAT500 (229.9 nm) and LAT500 (220.0 nm) were thicker than T500 (186.1 nm) since they had larger TiO2 particles and spaces. The thicknesses of films for each annealing temperature estimated from cross-sectional SEM images are plotted in Figure 5, showing the Au-TiO2 films annealed at other temperature also had thicker TiO2 film than Tx as well as x = 500, except x = 400 lying at almost the same thickness due to consisting of ungrown-nanocrystals as mentioned in XRD section (Figure 2). There was less change of the thickness of films (x g 600) with increasing the heating temperature. Ti L3,2-edge XANES spectra collected in TEY mode are shown in Figure 6. There are two pre-edge peaks s1 and s2, and peaks a1,
Figure 5. Thickness of the films estimated from cross-sectional SEM images.
a20 , a200 , b1, and b2 in all spectra. Here, peaks of a and b groups representing the L3 and L2 edges respectively corresponding to the 2p3/2 and 2p1/2 excitations of the 2p63d0 to 2p53d1 transition30 and were split into a1, a20 , a200 , b1, and b2 due to the crystal field splitting of the d orbitals into the t2g and eg states. Furthermore, the local symmetry at the Ti site of anatase and rutile was reduced to D2d and D4h respectively, which induced an a2 peak split into a20 and a200 . The relative intensities of the doublet a20 and a200 can be important features of TiO2 crystal phase: a20 > a200 in anatase and a20 < a200 in rutile.29,30 Figure 7 shows the intensity ratio of a20 peak relative to a200 peak (a20 /a200 ) in all XANES spectra. The vertical axis of a20 /a200 means that the larger the value of a20 /a200 , the more anatase phase the films include. The a20 /a200 ratio is ca. 1.20 for T400 ∼ T600 and ca. 0.75 for T700 ∼ T900, indicating that the peak intensity of a20 and a200 is reversed between T600 and T700; TiO2 phase of Tx is thus transformed from anatase to rutile at the heating temperature between 600 and 700 C. In sATx and LATx, however, the a20 /a200 ratio gradually decreased with increasing 6556
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Figure 6. Ti L3,2-edge XANES spectra of the films.
Figure 8. Ti L3,2- and O K-edge XANES spectra of the films heated at 500 C.
Figure 7. Plot of peak intensity ratio of a20 against a200 .
temperature from 500 to 800 C, reflecting that the anatase-to-rutile phase transformation did not occur, unlike Tx where rutile phase is formed at the boundaries of anatase particles. In the XANES spectra of x = 400, all a2 peaks were split into two peaks. If TiO2 is amorphous, the splitting of a2 peak is rarely observed.26 Thus, films of T400, sAT400, and LAT400, which had no peaks in XRD patterns as described above, are believed to be composed of nanocrystalline of anatase TiO2 undetectable by XRD measurements. A comparison of Ti L3,2-edge and O K-edge XANES spectra of the anatase TiO2 (x = 500) films measured by TEY and FY modes is shown in Figure 8. The O K-edge probes that the O 2p state projected unoccupied density of states in the conduction band, and the peaks assigned with AD are attributed to the following states; the region from 530 to 535 eV is associated to transitions to the O(2p)Ti(3d) band, where the d orbital is split into t2g and eg levels (labeled A and B) due to a crystal field, and the 540 to 550 eV region includes a transition band to the O 2p antibonding state (peak C) and the O(2p)Ti(4sp) band (peak D).35 FY spectra are more representative of the bulk sample and broadened by self-absorption effects, whereas TEY spectra are surface sensitive. In the Ti L3,2- O K-edge XANES spectra in both TEY and FY modes, no significant difference is observed, except for the minor distortion in the FY spectra due to self-absorption in all films of T500, sAT500, and LAT500. This indicates that the electronic structure and local geometry around Ti ions are the same at the surface and in the bulk if the AuNPs were incorporated into the films, although the doping affected the phase transformation and crystallization.
Figure 9. Deconvolution of Ti K-edge XANES spectra of the films heated at 500 C (pre-edge region).
The Ti K-edge XANES spectra (x = 500) of the pre-edge region measured in FY mode are shown in Figure 9. All spectral shapes confirmed that the anatase TiO2 had four peaks labeled A1A3 and B. A1 peak is attributed to the quadrupolar 1s f 3d (t2g) transition, and A3 peak is a dipolar transition of 1s f 3d (t2g)-4p hybridized states in nature but includes also a little 1s f 3d (eg) quadrupolar component. B is the pure dipolar transitions of 1s f 3d (eg)-4p hybridized states. As for A2 peak, Wu et al. gave A2 an 6557
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Figure 10. (a) Adsorption activity of the films. (b) The plot of adsorption ability as a functional of the A2/A3 ratio (x = 500).
origin similar to the other A series transitions (1s f 3d-4p hybridized states).33 However, it has been empirically attributed to five-coordinate Ti by several authors.22,25 It was further interpreted to be due to the large fraction of Ti atoms at the surface where a five-coordinated geometry is present accounting for distorted environments22 and the intensity ratio of the two peaks (A2/A3) was linearly related to the mean particle diameter of the TiO2, that is, that the A2/A3 ratio was increased with decreasing particle size.24 The A2/A3 ratio in the present study is also shown in Figure 9 and sAT500 (A2/A3 = 0.79) had the highest value, followed by LAT500 (0.65) and T500 (0.55), indicating the smaller AuNPs doping into TiO2 film had an increase in fivecoordinated Ti sites. However, the A2/A3 ratio anticorrelates with TiO2 particle diameter of the films (Figure 3). Whereas the particle diameter was decreased in the order of T500, LAT500, and sAT500, the A2/A3 ratio was in the opposite order. Additionally, whereas there was a variance in the electronic state by AuNPs doping as seen from the Ti K-edge XANES spectra, little difference was observed in Ti L3,2- and O K-edge XANES, this aspect has also been reported.21,27 This suggests that the peaks A2 and A3 may be associated with the surface and near surface area and Ti K-edge has a much deeper probing depth than Ti L and O K-edges. Part a of Figure 10 shows an adsorption ability of the films: an amount of adsorption of MB on the films in an aqueous solution under the dark condition. The adsorption ability of both sATx and LATx was enhanced since an MB capacity of the films was increased due to growing the film thickness and the space among TiO2 particles in Au-TiO2 was larger by the AuNPs doping as mentioned above in the FE-SEM section (Figure 4), resulting in the ease access of the MB aqueous solution into the film deeply. Furthermore, sATx was more readily for MB to adsorb than LATx. The smaller difference in the amount of adsorption at the fixed, the higher heating temperatures despite less change of the thickness of films (x g 600) was due to the decrease in surface area because of the growth of TiO2 particles. In the case of x = 500, it was found that the amount of adsorbed MB was proportional to the A2/A3 ratio (part b of Figure 10). The relation between the adsorption and the A2 peak intensity was also reported by Chen et al.,22 thus supporting that the five-coordinated Ti site, which is the origin of A2 pre-edge peak in the Ti K-edge XANES spectrum, works as an adsorption site in these series of films. Absorption spectral degradations of MB aqueous solution by T400, T500, sAT400, and sAT500 are shown in Figure 11. The initial decreases in the absorption peaks were mostly due to the adsorption of MB on the film surfaces, indicating sAT400 and sAT500 had high adsorption ability as shown in part a of Figure 10. The sAT400 apparently had high photocatalytic
Figure 11. Absorption spectral degradation of MB aqueous solutions by T400, T500, sAT400, and sAT500 under 365 nm UV irradiation.
Figure 12. Photocatalytic activity of the films.
activity since the absorption peak intensity of MB dramatically decreased. However, there was less blue-shift in the peak, the shift being the extent of MB photocatalyzation,1,43 and the spectral shapes were remained, even at a longer irradiation time, indicating that the peak decrease for sAT400 was also mostly due to the adsorption of MB. However, in sAT500 (and also T500), the peak was weakened with a blue-shift, so that sAT500 had both high photocatalytic activity and adsorption ability. Here, it is difficult to accurately estimate MB concentration changes from the obtained spectral changes because of the coexistence of demethylated species of MB, which have somewhat different absorption spectra from the spectrum of MB; we thus decomposed each spectrum into the component spectra of MB and its demethylated species, and estimated the MB concentration from the decomposed MB absorption spectrum, according to our previous report.1 The normalized reaction rate constants of photocatalytic degradation reaction, that is, that the photocatalytic activities of the films, calculated from the obtained MB concentration changes, are shown in Figure 12. From these 6558
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The Journal of Physical Chemistry C results, it was found that the films heated at 500 C had the highest activity in the variation of heating temperature because they were mostly composed of anatase phase without rutile phase of low activity and AuNPs doping considerably improved the photocatalytic activity as well as the adsorption ability. A significant improvement of the activity of sAT500 was observed, which could be interpreted as being due first to the well crystallized anatase phase without rutile phase as indicated in the XRD and Ti L3,2-edge XANES sections. Another factor could be the higher adsorption ability caused by sAT500 possessing a number of five-coordinated Ti sites considered to be adsorption sites as indicated in Ti-K edge XANES section (Figure 9). Although sAT400 had excellent adsorption ability, its photocatalytic activity was low because of their insufficiently crystallized anatase phase. The photocatalytic activity of films (x g 500) gradually decreased with increasing the heating temperature due to decrease of adsorption ability (part a of Figure 10) and crystallization of rutile phase (Figures 2 and 6). Furthermore, the photocatalytic activity of LATx (x g 600) were higher than that of sATx, which was not like the film of x = 500 since much more of the anatase phase existed in LATx than in sATx.
4. CONCLUSIONS Au-TiO2 photocatalytic films were prepared by a solgel method and an ex situ synthesis of AuNPs @PVP. The doping of large-size AuNPs@PVP more effectively retarded the anatase to rutile phase transformation, as revealed by the XRD and Ti L3,2edge XANES analysis. In the Ti L3,2-edge XANES, the a20 /a200 ratio changes in the Au-TiO2 heated films were different from the TiO2 film, implying that the rutile phase in the former was formed through a different transformation mechanism from that in the latter; that is that rutile phase in the Au-TiO2 film might be formed through a distortion of anatase phase, whereas phase transformation occurred with the formation of rutile phase at boundaries of anatase particles in TiO2 film. The sAT500 had the highest photocatalytic activity among the obtained films since it had the well-crystallized anatase phase without rutile phase and high adsorption ability. The doping of small-size AuNPs@PVP accelerated the crystallization of anatase phase and increased the A2 peak intensity of pre-edge peak in the Ti K-edge XANES spectra, which was attributed to the adsorption site, resulting in the improvement of adsorption ability. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT One of the authors (C.Y.) thanks Mr T. Regier, at CLS, for his technical assistance and Overseas Education Program supported by the Graduated School of Science and Engineering for a subsidized financial support. This work is also supported by Grant-in-Aid for Scientific Research (No. 20560632) from JSPS, Japan, and by the Nishio Scholarship and Aster Scholarship in Ritsumeikan University. The XAFS measurements in part conducted at the Canadian Light Source, which is supported by NSERC, NRC, CIHR, and the University of Saskatchewan; research carried out at UWO was supported by NSERC, CFI,
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and CRC (TKS) of Canada. The XAFS measurements at the Photon Factory were performed under the approval of the Photon Factory Advisory Committee (proposal No. 2010G035).
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dx.doi.org/10.1021/jp110581j |J. Phys. Chem. C 2011, 115, 6554–6560
